Electrochemical deposition of zinc-nickel alloys in .../67531/metadc12101/m2/1/high... · 1.4.2 Electrolytic Properties of the Alkaline Baths ... CV of nickel sulfate hexahydrate,

APPROVED: Teresa D. Golden, Major Professor William E. Acree, Jr., Committee Member and

Chair of the Department of Chemistry Michael Monticino, Dean of the Robert B.

Toulouse School of Graduate Studies

ELECTROCHEMICAL DEPOSITON OF ZINC-NICKEL ALLOYS IN ALKALINE

SOLUTION FOR INCREASED CORROSION RESISTANCE

Heidi A. Conrad

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

December 2009

Conrad, Heidi A. Electrochemical deposition of zinc-nickel alloys in alkaline

solution for increased corrosion resistance.

The optimal conditions for deposition of zinc-nickel alloys onto stainless steel

discs in alkaline solutions have been examined. In the past cadmium has been used

because it shows good corrosion protection, but other methods are being examined due to

the high toxicity and environmental threats posed by its use. Zinc has been found to

provide good corrosion resistance, but the corrosion resistance is greatly increased when

alloyed with nickel. The concentration of nickel in the deposit has long been a debated

issue, but for basic solutions a nickel concentration of 8-15% appears optimal. However,

deposition of zinc-nickel alloys from acidic solutions has average nickel concentrations

of 12-15%. Alkaline conditions give a more uniform deposition layer, or better metal

distribution, thereby a better corrosion resistance. Although TEA (triethanolamine) is

most commonly used to complex the metals in solution, in this work I examined TEA

along with other complexing agents. Although alkaline solutions have been examined,

most research has been done in pH ≥ 12 solutions. However, there has been some work

performed in the pH 9.3-9.5 range. This work examines different ligands in a pH 9.3-9.4

range. Direct potential plating and pulse potential plating methods are examined for

optimal platings. The deposits were examined and characterized by XRD.

Master of Science (Chemistry), December

2009, 128 pp, 5 tables, 77 illustrations, references, 36 titles.

ii

Copyright 2009

by

Heidi A. Conrad

ACKNOWLEDGEMENTS

I would like to thank my research advisor, Dr. Teresa D. Golden for her continued

support, guidance and encouragement while working on this research project.

I would like to thank Dr. William E. Acree for taking time to be a part of my

committee, and his guidance in completing my research and thesis.

I would like to give a special thank you to Dr. Jose Calderon for all of his help

with the use of different instruments needed to complete my research.

I would like to give a special thank you to John R. Corbett, who worked for me as

a TAMS (Texas Academy of Math and Science) student. John was a great help in the lab

running experiments and giving suggestions as needed.

I would like to thank all of my group members for their great advice and ideas in

completing my research.

I would also like to thank my daughter Samantha for being so patient as I try to

finish school, and being so helpful when needed and to my mom, Gayle, for being there

and believing in me, I really appreciate all you do for me.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS………………………………………………………………iii

LIST OF TABLES…………………………………………………………..………….viii

LIST OF ILLUSTRATIONS……………………………………………………………..ix

CHAPTER 1. INTRODUCTION AND LITERATURE REVIEW………………….…...1

1.1Electrodeposition of Alloys and Applications…………………………………1

1.2 Zinc Alloys…………………………………………………………………….2

1.2.1 Alloy Phases of Zinc-Nickel………………………………………...4

1.2.2 Temperature Dependence of Alloys………………………………...5

1.2.3 Nickel Content………………………………………………………6

1.3 Acid Bath Deposition………………………………………………………….6

1.3.1 Current Density and Effect of Deposition Potential in Acidic

Conditions…………………………………………………………………6

1.3.2 Pulse Plated Nickel in Acidic Conditions…………………………...9

1.3.3 Cyclic Voltammetric Study of Zinc Nickel Alloy Deposition in

Acidic Conditions………………………………………………………..11

1.3.4 Acidic Deposition Conditions……………………………………...15

1.3.5 X-Ray Diffraction Data for Acidic Depositions…………………...18

1.3.6 Acidic Deposition Mechanism……………………………………..20

1.3.7 Initial Deposition Studies for Acidic Conditions…………………..22

1.3.8 Effects of Morphology for Acidic Deposits………………………..26

1.3.9 Acid Deposition Conclusions……………………………………...27

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1.4 Alkaline Bath Deposition…………………………………………………….28

1.4.1 Hydrogen Embrittlement of Deposits……………………………...29

1.4.2 Electrolytic Properties of the Alkaline Baths………………………30

1.4.3 Complexing Agents for Alkaline Baths…………………………....34

1.5 Corrosion Protection from Alkaline Deposits………………………………..35

1.5.1 Salt Spray Testing………………………………………………….37

1.5.2 Sacrificial Electrodes………………………………………………39

1.5.3 Corrosion Phase………………………………………………….39

1.6 Summary……………………………………………………………………..40

1.6.1 This Thesis Work…………………………………………………..41

CHAPTER 2. DEPOSITION OF PURE METAL FILMS IN ALKALINE

SOLUTIONS…………………………………………………………………………….43

2.1 Introduction…………………………………………………………………..43

2.2 Experimental Parameters…………………………………………………….45

2.3 Zinc Sources for Deposition…………………………………………………48

2.3.1 Zinc Nitrate………………………………………………………...51

2.3.1.1 Zinc Nitrate Conclusions………………………………………………………...54

2.3.2 Zinc Sulfate Monohydrate…………………………………………54

2.3.2.1 Zinc Monosulfate Monohydrate

Conclusions………………………………………………..…......60

2.4 Borate Solutions for Zinc…………………………………………….………60

2.4.1 Zinc Sulfate Monohydrate in Borate……………………………….61

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2.4.1.1 Zinc Sulfate Monohydrate conclusions for borate solutions…….62

2.5 Nickel Sources for Deposition……………………………………………….63

2.5.1 Nickel Sulfate Hexahydrate………………………………………..64

2.5.1.1 Nickel Sulfate Hexahydrate conclusions………………………...70

2.5.2 Nickel Ammonium Sulfate Hexahydrate…………………………………………..71

2.5.2.1 Nickel Ammonium Sulfate Hexahydrate Conclusions……………….………….77

2.6 Borate Solutions for Nickel………………………………………………….77

2.6.1 Nickel Sulfate Hexahydrate in Borate……………………………..78

2.6.1.1 Nickel Sulfate Hexahydrate in borate Conclusions……………………………...79

2.6.2 Nickel Ammonium Sulfate Hexahydrate in Borate………………..80

2.6.2.1 Nickel Ammonium Sulfate Hexahydrate in Borate Conclusions………………..81

2.7 Nickel and Zinc Conclusions………………………………………………...81

2.8 Bath Conditions……………………………………………………………...82

2.9 Summary……………………………………………………………………..83

CHAPTER 3. ZINC AND NICKEL CO-DEPOSITION IN ALKALINE

SOLUTIONS…………………………………………………………………………….85

3.1 Introduction…………………………………………………………………..85

3.2 Initial Studies………………………………………………………………...87

3.3 Chronocoulometry…………………………………………………………...97

3.4 Linear Sweep Voltammetry…………………………………………..……...99

3.5 Atomic Absorption Analysis………………………………………………..101

3.6 Alkaline Metal Deposition from Water Solvent with Acetate Ligand……..101

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3.6.1 Zinc Sulfate Monohydrate and Nickel Sulfate Hexahydrate……..102

3.6.1.1 Zinc-Nickel 1:1 Molar Ratio……………………………102

3.6.1.2 Zinc-Nickel 3:1 and 4:1 Molar Ratios………………….104

3.6.2 Zinc Sulfate Monohydrate and Nickel Ammonium Sulfate

Hexahydrate…………………………………………………………….109



3.6.3 Conclusions for Alkaline Metal Deposition from Water Solvent with

Acetate Ligand………………………………………………………….113

3.7 Alkaline Metal Deposition from Borate Solvent…………………………...113

3.7.1 Zinc Sulfate Monohydrate and Nickel Sulfate Hexahydrate……..114

3.7.1.1 Zinc Nickel in a 1:1 Molar Ratio……………………….114

3.7.1.2 Zinc and Nickel in a 1:3 Molar Ratio…………………..116

3.7.2 Zinc Sulfate Monohydrate and Nickel Ammonium Sulfate Hexahydrate in

Borate………………………………………………….117


3.7.2.2 Zinc Nickel 2:1 Molar Ratio……………………………118

3.7.3 Conclusions for Alkaline Metal Deposition from Borate Solvent..120

3.8 Conclusions for Zinc-Nickel Co-Deposition in Alkaline Solutions………..120

3.9 Differences from Literature………………………………………………...123

3.10 Future Work……………………………………………………………….124

REFERENCES………………………………………………………………………...126

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LIST OF TABLES

Table 2.1: Possible ligands for zinc; zinc complex pKa’s…………….…………………48

Table 2.2: The powder diffraction file (PDF) data of XRD patterns of standard zinc metal

from the JCPDS Database (PDF #04-0831)……………………………………………..50

Table 2.3: The PDF data of XRD patterns of standard nickel metal from the JCPDS

Database (PDF#04-0850)………………………………………………………………...64

Table 2.4: Possible ligands for nickel; nickel complex pKa values……………………..65

Table 3.1: The PDF data of XRD patterns of standard zin-nickel alloy, gamma phase

metal from the JCPDS Database (PDF #06-0653)………………………………………86

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LIST OF ILLUSTRATIONS

Figure 1.1: Potential compared to hydrogen electrode…………………………………....3

Figure 1.2 Deposition ranges in acidic solutions………………………………………….7

Figure 1.3: E-I curve. Deposition of Zn, Ni and Zn-Ni alloys……………………………8

Figure 1.4: Cyclic voltammagram study of nickel deposition onto a glassy carbon

substrate………………………………………………………………………………….12

Figure 1.5: CVs of nickel deposition onto platinum and glassy carbon substrates……...13

Figure 1.6: CVs of Zn-Ni deposition from chloride plating solution……………………14

Figure 1.7: CV of Zn, Ni and Zn-Ni alloy on steel………………………………………16

Figure 1.8: Deposition of nickel, zinc and Zn-Ni alloy on steel………..………………..17

Figure 1.9: XRD pattern from 30.0°C bath, predominately δ phase…………………….19

Figure 1.10: XRD pattern from 50.0°C bath, predominately γ phase…………………...19

Figure 1.11: Analysis of zinc-nickel alloy deposit……….……………………………...23

Figure 1.12: Depth profiling of zinc-nickel coating, hydrogen in the deposit……….…..24

Figure 1.13: Model of deposit layer……………………………………………………...25

Figure 1.14: Pourbaix diagram of zinc species…….……………………………………32

Figure 1.15: Pourbaix diagram of nickel species……...…………………………………33

Figure 1.16: Salt spray testing of zinc-nickel coating on steel substrate……..………….36

Figure 1.17: Salt spray corrosion tests…………...………………………………………37

Figure 1.18: Salt spray corrosion resistance tests – chromated samples…………….…..38

Figure 2.1: Potential step method diagram…..…………………………………………..44

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Figure 2.2: Set up of electrochemical cell…………..…………………………………...45

Figure 2.3: Stainless steel disc background mounted in epoxy…...……………………..46

Figure 2.4: Stainless steel background disc, out of epoxy……………………………….47

Figure 2.5 Structure of zinc ammonia complex……………………...…………………..49

Figure 2.6 Structure of zinc tartaric acid\complex………....…………………………….49

Figure 2.7 Structure of zinc acetate complex……...……………………………………..49

Figure 2.8 Structure of zinc triethanolamine complex…...………………………………50

Figure 2.9 Zinc nitrate…………………………………………………………………....51

Figure 2.10: CV of zinc nitrate, pH=9.3 with 1M NH4OH………………………….…..52

Figure 2.11: CV of zinc nitrate, tartaric acid, and pH=9.3 with 1M NH4OH……………53

Figure 2.12 Zinc sulfate monohydrate…………….……………………………………..54

Figure 2.13: CV of zinc sulfate monohydrate, triethanolamine with pH=9.3 with 1M

NH4OH…………………………………………………………………………………...55

Figure 2.14: XRD Pattern of zinc sulfate monohydrate, triethanolamine, and pH=9.3 with

1M NH4OH………………………………………………………………………………56

Figure 2.15: CV of zinc sulfate monohydrate and triethanolamine, pH=11.04 with 1.5M

NH4OH…………………………………………………………………………………...57

Figure 2.16: CV of zinc sulfate monohydrate and sodium acetate, pH=9.3 with 1M

NH4OH…………………………………………………………………………………...58

Figure 2.17: XRD Pattern of zinc sulfate monohydrate with sodium acetate, pH=9.32

with 1M NH4OH…………………………………………………………………………59

Figure 2.18: CV of 0.5M Zn in 0.1M borate, pH=9.3 with 1M NH4OH………………...61

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Figure 2.19: XRD pattern of zinc deposited from 0.1M borate solution, pH=9.3 with 1M

NH4OH……………………………………………………………...……………………62

Figure 2.20: Nickel sulfate hexahydrate structure………………..……………………...64

Figure 2.21 Nickel ammonia……………..………………………………………………65

Figure 2.22 Nickel acetate .……….……………………………………………………..66

Figure 2.23 Nickel triethanolamine ….……………………….…………………………66

Figure 2.24: CV of nickel sulfate hexahydrate, sodium acetate, and pH=9.32 with 1M

NH4OH…………………………………………………………………………………...68

Figure 2.25: XRD pattern of nickel sulfate hexahydrate, sodium acetate, and pH=9.32

with 1M NH4OH…………………………………………………………………………70

Figure 2.26: Nickel ammonium sulfate structure……..…………………………………71

Figure 2.27: CV of nickel ammonium sulfate hexahydrate, pH=9.3 with 1M

NH4OH…………………………………………………………………………………...73

Figure 2.28: CV of nickel ammonium sulfate hexahydrate, pH=9.3 with 1 M

NH4OH………………………………………………………………………...…………74

Figure 2.29: XRD pattern of nickel ammonium sulfate hexahydrate and 1M NH4OH, not

in epoxy and plated at E=-1.50V…………………………..…………………………….75

Figure 2.30: XRD pattern of nickel ammonium sulfate hexahydrate, pH=9.3 with 1M

NH4OH, not in epoxy, plated at E=-1.250V……………………………………………..76

Figure 2.31: CV of nickel in borate solution, pH=9.3 with 1M NH4OH………………...78

Figure 2.32: XRD pattern of nickel sulfate hexahydrate in 0.1M borate solution, pH=9.3

with 1M NH4OH…………………………………………………………………………79

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Figure 2.33: XRD pattern of nickel ammonium sulfate hexahydrate in 0.1M borate

solution, pH=9.3 with 1M NH4OH………………………………………………………81

Figure 3.1 XRD pattern gamma phase alloy…………...………………………………...87

Figure 3.2: CV of 0.5M ZnSO4.H2O, 0.5M NiSO4

.6H2O and 1.0M acetate in solution,

pH=9.3 with 1M NH4OH………………………………………………………………...88


.6H2O, 1.0M Na+CH3COO-, pH=9.32

with 1M NH4OH…………………………………………………………………………90


.6H2O, 0.25M Na+CH3COO-, pH=9.34

with 1M NH4OH…………………………………………………………………………91

Figure 3.5: XRD pattern of plating from 0.5M ZnSO4.H2O, 0.5M NiSO4

.6H2O, 0.25M

Na+CH3COO-, pH=9.34 with 1M NH4OH ……………………………………...............92

Figure 3.6: XRD pattern from 0.5M ZnSO4.H2O, 0.25M NiSO4

.6H2O, 0.25M

Na+CH3COO-, pH=9.3 with 1M NH4OH………………………..………………………93


.6H2O, 0.5M Na+CH3COO-, pH=9.32

with 1M NH4OH…………………………………………………..……………………..94

Figure 3.8: CV of 0.5M ZnSO4.H2O, 0.5M Ni(NH4)2(SO4)2

.6H2O, 0.5M Na+CH3COO-,

pH=9.3 with 1M NH4OH…………………………...……………………………………95

Figure 3.9: XRD pattern from solution of 0.5M ZnSO4.H2O, 0.5M

Ni(NH4)2(SO4)2.6H2O, 0.5M Na+CH3COO-, pH=9.3 with 1M NH4OH……….………...96

Figure 3.10: Chronoucoulometry diagram………..………………………….…………..97

Figure 3.11 Anson plot diagram………………….……………………………………...98

Figure 3.12 LSV of zinc, nickel and zinc-nickel alloy…………………………………100

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Figure 3.13: CV of 1:1 ratio of zinc-nickel, with acetate, pH=9.39 with 1M

NH4OH………………………………………………………………………………….103

Figure 3.14: XRD Pattern, 1:1 Zn-Ni ratio with acetate ligand, pH=9.3 with 1M

NH4OH………………………………………………………………………………….104

Figure 3.15- 3:1 Molar ratio of ZnSO4.H2O, NiSO4

.H2O, pH=9.37 with 1M

NH4OH….........................................................................................................................105

Figure 3.16- XRD pattern of gamma phase alloy deposited from 2:1 ZnSO4.H2O,

Ni(NH4)2(SO4)2.6H2O………...………………………………………………………...106

Figure 3.17: AAS standard addition method- zinc concentration determination……....107

Figure 3.18: AAS standard addition method- nickel concentration determination…….107

Figure 3.19 XRD pattern 4:1 ratio zinc to nickel……………………...………………..109

Figure 3.20: XRD pattern, 1:1 Zn-Ni ratio with acetate ligand, pH=9.3 with 1M

NH4OH………………………………………………………………………………….110


NH4OH………………………………………………………………………………….112

Figure 3.22 CV of 0.1M ZnSO4.H2O, 0.1M NiSO4

.6H2O, 0.1M borate and a pH of 9.41

with 1M NH4OH………………………………………………………………………..114

Figure 3.23: XRD pattern, 1:1 Zn-Ni ratio in 0.1M borate, pH=9.41 with 1M

NH4OH………………………………………………………………………………….115

Figure 3.24: XRD pattern, 1:3 Zn-Ni ratio in 0.1M borate, pH=9.41 with 1M

NH4OH………………………………………………………………………………….116

Figure 3.25: XRD pattern, 1:1 Zn-Ni ratio with acetate ligand, pH=9.3 with 1M

NH4OH………………………………………………………………………………….118

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NH4OH………………………………………………………………………………….119

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

1.1 Electrodeposition of Alloys and Applications

There is great interest in the electrodeposition of metallic alloys because of the

increase in mechanical and chemical properties of the metals involved. For example, the

mechanical properties of zinc are greatly increased when alloyed with nickel [1]. Zinc

alloys are of great interest in research because they offer a greater resistance to corrosion

then pure zinc [2]. Thus modifying the composition can significantly improve the

stability of the metal system against corrosion [3].

Due to the automotive industry, there has been a real push to improve the

corrosion resistance of stainless steel. To date, cadmium and zinc coatings have been

used for the corrosion protection of steel. Although these materials do protect the

underlying steel, the protection offered needs to be increased. There are some high

corrosion resistant materials, but they tend to be very costly and not widely available [4].

For many years zinc coated stainless steel has been used in this field as a

corrosive resistive material. The zinc coating sacrificially decays on the stainless steel,

thereby protecting it [4]. Other options are being examined to withstand harsher

conditions, longer life of resistance, and reducing the coating thickness required for a

specific resistance time frame.

Electrodeposited zinc-nickel alloys have been examined for increased corrosion

resistance in the automotive industry. The automotive industry needs to find some type

of coating that can withstand the high salt conditions automobiles are exposed to during

1

icy road conditions. There is also the need to find a replacement for cadmium coatings

due to the high toxicity of working with cadmium [5]. Historically cadmium has been

deposited out of a cyanide bath, the use of cyanide is becoming more regulated, as is the

use of cadmium, since cadmium metal and cyanide salts are toxic [6].

At this point, alloys are being examined as a solution to this problem. Alloys

have different corrosion potentials then their single elements; therefore by picking the

correct combination of alloys, one can greatly increase the corrosion resistance of the

material [4].

1.2 Zinc Alloys

As a result of the push for increased corrosion resistance, the study of alloying

zinc with other metals began [5]. Cadmium was another metal often used in the

corrosion protection of steel, because, like zinc, it corrodes preferentially to the steel,

thereby protecting the steel [4]. Traditionally, zinc was alloyed with cadmium, but due to

the harsh environmental conditions associated with cadmium, other metals have been

examined [5]. Cadmium has become very regulated, or in some countries banned all

together, and zinc-nickel is a cheap environmentally friendlier alternative.

The biggest advantage for zinc-nickel deposits is that they can replace cadmium

plating in most applications [4]. Alloys that are high in zinc content retain cathodic

potential to steel [7]. The alloys are more electrochemically noble than pure zinc, so

sacrificial protection to the steel substrate is maintained but they still corrode at a slower

rate. Zinc-nickel alloys have a cathodic potential compared to steel, which is controlled

by the nickel content in the deposit [4], as can be seen in Figure 1.1.

2

Figure 1.1: Potential compared to hydrogen electrode [7].

For zinc, if the alloy is especially high in zinc content it still presents cathodic

potential to steel, thereby being preferentially corroded. However, the alloy corrodes at a

much slower rate than pure zinc because the alloy is less active then pure zinc. Some

zinc alloys that have been examined include zinc-cobalt, zinc-iron and zinc-nickel. The

work has mainly focused on zinc-nickel because this alloy has the highest documented

corrosion resistance. As the nickel content increases, the corrosion potential increases

which is due to microcracking. Microcracking spreads out the corrosion cells being

formed on the deposit so that there are many weak cells but few strong cells which leads

to a prolonged life of the coating. The corrosion cells are what enable the corrosives to

burrow down to basis metal, causing red rust. White rust is due to corrosion of the zinc

alloy deposit and red rust is due to the corrosion of the underlying stainless steel substrate

[8]. When a corrosion cell forms on the surface of a pure zinc coating, it forms and

3

quickly digs in through the zinc coating to the underlying steel substrate. When alloyed

with nickel, the corrosion cell is blocked by nickel and cannot quickly burrow through

the zinc coating. Instead, due to the microcracking, many small corrosion cells are

formed on the surface of the alloy coating, eventually leading on to full corrosion, but

increasing the time frame for the corrosion of the underlying steel to begin [4].

Zinc alloy deposits on a microscopic level, are more granular in texture and

generally harder than cadmium or pure zinc coatings. The oxidation of zinc is slowed

down by the nickel in the deposit, as oxidation progresses, the nickel remains as a barrier

against further corrosion. Therefore, initially the corrosion protection is sacrificial, but as

the corrosion continues, a gradual switch to barrier protection is observed [8].

Zinc has a low standard electrode potential (E=-0.76V vs. SHE) and is a very

active metal that corrodes easily. This allows zinc to act as a suitable sacrificial coating

on many metals with higher standard electrode potentials. The driving force for the

corrosion of the zinc in corrosive environments is the difference in electronegativity of

the coating and the substrate. When alloyed with another metal, the potential of the alloy

can be brought much closer to that of the substrate metal, while still being on the cathodic

side, thereby acting as a good sacrificial coating [9].

1.2.1 Alloy Phases of Zinc-Nickel

The five zinc-nickel alloy phases that have been detected are the following: η- (1%

Ni), α and β (30% Ni, known as the nickel rich phases), and [10] and - γ (Ni5Zn21) and δ-

(Ni3Zn22) (known as the zinc rich phases) phases dependent upon the Zn/Ni ratio used [3,

10, 11]. The γ and δ phases are most commonly found in the alkaline bath deposits,

4

with the γ phase showing the strongest protection against corrosion. Nickel is able to

enhance the corrosive resistance properties of zinc. There are many techniques utilized

for this deposition, the most common being the use of a rotating disk electrode. The α

and β phases are known as the nickel rich phases having a nickel content around 30%.

The η phase has about 1% nickel content but is only found when electrodeposited in a

chloride bath. The γ and δ phase alloys would be the best at corrosion resistance since

the alloys with a nickel content of approximately 8-15% have been found to be optimal.

The phase of the alloy present is temperature dependent, so to obtain the γ phase alloy,

which has been determined to be the best resistance to corrosion, a higher temperature is

needed in acidic conditions. 50.0°C is predominately γ phase, making this the optimal

temperature at which to deposit the coating from acidic bath conditions [3].

1.2.2 Temperature Dependence of Alloys

There is also a need for increased corrosion resistance under different temperature

conditions, such as under the hood of a car. In such “hot zone” conditions, zinc alone

does not perform well and chromate has no effect at these temperatures. Zinc also

produces white oxide products. The production of these bi-products becomes an issue

when trying to dismantle and reassemble parts during their service life. Increased deposit

hardness is also being examined to extend the life of corrosive components of vehicles

[7].

5

1.2.3 Nickel Content

The nickel content in the deposit is very important since it determines the alloy

phase present, and therefore the corrosion resistance that will be provided. A number of

factors play into nickel deposition in the alloy such as complexing agents, current density

and temperature.

1.3 Acid Bath Deposition

Acidic deposition of zinc-nickel began back in the mid 1980’s in the UK. The

bath was chloride based and ammonia was used to complex the nickel in solution. This

electrolyte was able to produce high plating efficiency with good deposition rates, but

poor alloy current density distribution caused corrosion failure under low current

densities [7]. Acid baths have a higher current efficiency than alkaline baths because

they release less hydrogen gas during plating. Alkaline baths are 40-65% efficient, while

acid baths are 85-95% efficient [8].

1.3.1 Current Density and Effect of Deposition Potential in Acidic Solutions

The effects of current density on the deposits are more apparent in acidic

solutions. Acid baths form high nickel containing deposits at low current densities,

before leveling off at a higher range. Alkaline baths form fairly constant nickel content

deposits from low to high current densities, which result in overall better corrosion

resistance in low current density areas [4].

6

Figure 1.2: Deposition ranges in acidic solutions [12].

There are 3 ranges of deposition in acidic solutions as shown in Figure 1.2. The

first range containing small potentials (-0.700 to -0.800V) gives coatings containing ≥95

wt % Ni. In the second range, where there are intermediate potentials, the alloys are

approximately 75 wt% Ni and the cathodic current efficiency drops. The third range

includes high potentials (-1.04 to -1.02V) and the Ni quantity decreases from 45 to 15 wt.

%, but the potential increases and the γ-phase Zn/Ni alloy predominates [12]. At low

current densities, the alloy is almost pure nickel in content without the electrochemical

characteristics of nickel. When the zinc content in the deposit reaches 10%, the current

efficiency of the system begins to decrease. As the current efficiency drops, the zinc

greatly inhibits the deposition of nickel in the deposit, while the presence of nickel aids in

the deposition of zinc. As the potential is shifted more negatively, alpha phase deposition

is observed with high hydrogen evolution. As the potential is shifted even more in the

negative direction, gamma phase deposition is observed, with a sharp decrease in the

7

hydrogen evolution [11]. Low current densities can lead to normal deposition, where the

nickel deposits preferentially to the zinc. For improved corrosion resistance, anomalous

deposition must occur, with the zinc preferentially depositing to the nickel [3].

Figure 1.3 shows a study performed by Abou-Krisha where it is noted that the

deposition of zinc starts around E=-1.14V, and had a similar shape to the potential of Zn-

Ni codeposition (which occurs around E=-1.12V) [1].

Figure 1.3: E-I curve. Deposition of Zn, Ni and Zn-Ni alloys [1].

It was also observed that Ni deposition began around E=-0.85V and as the

potential was shifted to more negative values, the growth of the deposited layer greatly

increased. The polarization curve of the alloy is between those of Ni or Zn only,

suggesting the codeposition allows the Zn to deposit at a more positive potential and the

Ni to deposit at more negative potential, due to presence of Ni2+ which aids in Zn

8

deposition [1] so zinc can co-deposit with nickel at potentials that are too low for it to

deposit in pure form [13].

As Ni2+ concentration increases, the deposit potential positively increases. The

cathodic peak current initially starting at -0.5V (believed to be due to hydrogen

evolution) decreases as Ni2+ concentration is increased. The amounts of γ-phase and δ-

phase alloys are dependent upon the concentration of Ni2+ in solution. A higher

concentration of Ni2+ leads to a higher content of γ-phase which is zinc rich [1, 14].

Anomalous deposition occurs in the zinc overpotential deposition region. Above

potentials of -1.0V vs. Ag/AgCl alloy deposition is inhibited in relation to pure nickel,

but is more readily deposited compared to pure zinc [13]. As current density is increased,

a higher overpotential is required to create nucleation sites on the electrode and to deposit

zinc, but the deposit can still grow at lower potentials [11].

1.3.2 Pulse Plated Nickel in Acidic Conditions

There is a correlation between crystal orientation and the corrosion potential

during anodic polarization. There have also been many studies performed on the growth

of nickel crystals on different substrates.

Under acidic conditions, the deposition of nickel follows a number of

intermediate steps as indicated:

Ni2+ + H2O ↔ NiOH+ + H+ (Equation 1.1)

NiOH+ + e- → NiOHad (Equation 1.2)

NiOHad + H+ + e- ↔ Ni0 + H2O (Equation 1.3)

9

When a pulsed current plating method is employed, with short cathodic pulses (1

ms or less) and high current densities (16A/dm2 or greater) another mechanism is

followed. There is a considerable pH increase near the cathode surface during a pulse, so

a layer of colloidal nickel hydroxide is formed at this surface. Micelles then form by

attaching further nickel ions.

Ni2+ + 2OH- → [Ni(OH)2]colloidal (Equation 1.4)

{Ni2+[Ni(OH)2]} colloidal (Equation 1.5)

Nickel deposition from this second mechanism is semibright in appearance and

has a different texture then direct current plated nickel. With the use of pulse plating, one

can improve the overall material distribution while obtaining more resistant coatings [16].

During the pulse deposit, the shortest on-time results in a deposit surface that is smooth

with no apparent cavities. The longer the on-time, the greater number of cavities present

on the deposit surface. In nickel pulse plating, as you increase the pause to pulse ratio,

the nickel content in the alloy is decreased. The time it is pulsed to a higher potential has

little effect on the alloy composition [15]. The cavities are a result of hydrogen gas

bubbles that can linger on the cathode surface during deposition. The density of the

hydrogen gas bubbles at the cathode surface increases as the on-time is increased, which

represents a higher overpotential for the nickel deposition. The morphology present on

the deposits of the shortest on-time results in large pyramidal-shaped crystallites with

preferential growth exhibited [17]. Direct current plating produces coatings with a higher

amount of surface roughness than when obtained with a pulse plating [18].

10

In acidic conditions, the corrosion rate of the 220 crystallites, which is the

preferred orientation obtained from direct current plating, is commonly higher than the

corrosion rate of the 200 crystallites, which is the preferred orientation obtained from

pulse platings [16].

Pulse plating disturbs the adsorption-desorption processes on the electrode

surface, which in turn controls the microstructure of the deposit [17]. Pulse plating leads

to a better metal distribution present in the deposit [16].

1:3.3 Cyclic Voltammetric Study of Zinc Nickel Alloy Deposition in Acidic Conditions

Lin and Selman performed an extensive cyclic voltammetry (CV) study to

determine the phase formation of the zinc-nickel alloy [19].

First, a typical CV of a nickel chloride salt solution was examined with a working

electrode of glassy carbon (GC) (the solid line present in figure 1.5). It is observed that

with a GC substrate, the deposition occurs at a cathodic potential of -0.88V. This is

almost 500mV past the normal equilibrium potential of nickel which is -0.496V vs. SCE.

This is indicative of overcoming an energy barrier present from deposition onto a foreign

substrate. In the potential range of -1.1 to -0.88V, the anodic sweep is more cathodic

then during the cathodic sweep of the potential. This is caused by two factors. First,

nickel nucleation is occurring on the substrate and second, the substrate effect on the rate

of hydrogen evolution. Once the cathodic direction sweep is complete, nickel deposits

have covered the substrate surface, so hydrogen evolution occurs more readily then on

the pure substrate [19].

11

Figure 1.4: Cyclic voltammogram study of nickel deposition onto a glassy carbon

substrate [19].

It is clear that with convection in figure 1.4, the deposition potential shifts in a

negative direction (approximately E= -0.91V), compared to the CV without convection.

It was found that nickel that was deposited in the α phase dissolves around a

potential of -0.1V during a potential-sweep stripping method. β-phase nickel dissolves

around E=-0.2V and pure nickel dissolves around E= + 0.1V. The α phase nickel is

composed of a solid mixture of hydrogen atoms in nickel, in an H/Ni ratio of 0.03. The β

phase nickel is composed of interstitial hydrogen atoms with the nickel, in an H/Ni ratio

12

greater than 0.6. It is believed that the anodic current present in figure 1.4 is due to the

dissolution of nickel from the hydrogen-nickel solid solution and the hydrogen evolution

from the substrate surface. It was concluded that the deposit of nickel onto a glassy

carbon substrate was a combination of nickel and hydrogen [19].

Figure 1.5: CVs of nickel deposition onto platinum and glassy carbon substrates [19].

Figure 1.5, CVs of nickel deposition onto platinum and glassy carbon substrates;

clearly demonstrate the substrate effect on nickel deposition onto a platinum electrode.

Hydrogen evolution begins to appear at a potential of -0.386V on the platinum electrode

during the cathodic-sweep direction. During the anodic-sweep direction, the current is

below that seen during the cathodic-direction sweep, due to less of the platinum surface

being exposed for hydrogen evolution to occur [19].

13

It is determined from figure 1.5 that hydrogen and nickel are codeposited.

Hydrogen evolution is rapid at the platinum surface, so a cathodic current appears at a

potential of -0.39V. The cathodic potential is -0.81V which is same potential when

deposited onto glassy carbon that the nickel deposition rate increases sharply [19].

Figure 1.6: CVs of Zn-Ni deposition from chloride plating solution [19].

Figure 1.6, CVs of Zn-Ni alloy deposition from a chloride based solution, shows

CVs with and without convection of Zn-Ni depositions. Near E= +0.160V, a mass-

transfer limitation of hydrogen adsorption is observed in the cathodic-direction sweep.

The sharp increase near E= -0.38V is due to hydrogen evolution [19].

This study was able to determine that hydrogen evolution does play a key role in

the electrodeposition of Zn-Ni alloys and depending on the strength of the adsorption of

hydrogen atoms on the surface of the substrate, 3 different types of nickel solid can be

formed. After the initial deposit of nickel onto the substrate, the hydrogen evolution is

14

enhanced, thereby lowering the current efficiency of the system during the electroplating

of the zinc-nickel alloy. As expected, when alloyed, the electrochemical properties of the

metals in the alloy are changed, and for zinc, the deposition potential of the alloyed zinc

is about 0.05 V more positive in potential then pure zinc [19].

The β and α nickel phases are somewhat responsible for the overall characteristic

alloy structure. β phase nickel deposition is mass transfer controlled and deposits at a

more positive potential then α phase nickel. Therefore, a negative cathodic potential and

convection of the system will result in more β phase nickel in the alloy deposit [19].

1.3.4 Acidic Deposition Conditions

The voltammagrams for the plating are shown in figure 1.7, CV of Zn, Ni and Zn-

Ni alloy on steel. The zinc, when no nickel is present has one peak, which corresponds to

the anodic dissolution. The two peaks observed for the Zn-Ni alloy represent the two

phases present, δ and γ phases. The first peak (E= -0.97V) is due to dissolution of Zn

from δ (Ni3Zn22)-phase. The second peak (E= -0.64V) is due to dissolution of Zn from γ

(Ni5Zn21)-phase [1, 13]. The Zn-Ni codeposition began around E= -1.12V and no nickel

cathodic peak is present because the Zn2+ inhibits nickel deposition [1].

15

Figure 1.7: CV of Zn, Ni and Zn-Ni alloy on steel [1].

In figure 1.8, deposition of nickel, zinc and Zn-Ni alloy on steel, it is shown that

the nickel deposition needs a low overpotential to deposit since the deposit is able to

grow at low potentials. The zinc deposition needs a higher overpotential for the deposit

to grow. The Zn-Ni codeposition occurs at a moderate overpotential, because the

deposition of nickel is inhibited by Zn2+ but the deposition of zinc is induced by the

presence of Ni2+ [1].

16

Figure 1.8: Deposition of nickel, zinc and zinc-nickel alloy on steel [1].

The zinc-nickel alloy is deposited at a rate of 0.2 to 0.3 microns per minute with a

current density of 3A/dm2 for alkaline baths and at slightly higher rates in acidic

conditions. Deposits up to 12 microns in thickness are low in stress, after 12 microns the

stress increases with thickness. Superior performance is found to be in the 5-8 micron

range for thickness. Zinc-nickel alloys have a cathodic potential compared to steel,

17

which is controlled by the nickel content in the deposit. As the nickel content increases,

the corrosion potential increases. Once this potential reaches a maximum protection, the

deposit becomes cathodic with respect to steel. Once the deposit moves to cathodic

potential in relation to the steel, the corrosion accelerates at the pore sites. The maximum

corrosion resistance is found to be in the 8-15% nickel range.

1.3.5 X-Ray Diffraction data for Acidic Depositions

Under acidic plating conditions, x-ray diffraction confirms that the higher the

temperature bath, the better the deposit. There is more nickel content present. δ and γ

phases are present in the 30.0 and 50.0°C baths. The deposit made at 30.0°C is

predominately the δ phase (figure 1.9). The deposit made at 50.0°C is predominately the

γ phase which represents an increase in the corrosion resistance because the γ phase has

been determined to have a higher resistance (figure 1.10) [1].

18

Figure 1.9: XRD pattern from 30.0°C bath, predominately δ phase [1].

Figure 1.10: XRD pattern from 50.0°C bath, predominately γ phase [1].

19

When there is a constant ZnCl2 concentration in solution, the quantity of η-phase

alloy decreases with an increase in ic and Ec because the quantity of Ni in the alloy

increases. The η-phase alloy dominates when the alloy is ‹12wt% Ni. When the alloy is

›16% Ni, the α and γ-phases dominate [10]. Zinc-nickel alloy coatings of homogeneous

compositions show enhanced corrosion resistance [2].

The best corrosion protecting Zn-Ni alloy is obtained from a chloride solution by

deposition at 20mA/cm2. It is also observed that steel surface modification with this

above specified alloy improves the corrosion stability due to an epoxy coating, when

compared to an epoxy coating on steel. It is found that after prolonged exposure to a

corrosive agent, the Zn-Ni alloy is able to significantly improve the corrosion stability of

the protective system. This is due to the formation of the passive layer of corrosion

products which acts as a barrier to passing materials [3].

The δ phase alloy is readily deposited in acidic deposition, and this alloy is less

resistant to corrosion resistance then the γ phase.

1.3.6 Acidic Deposition Mechanism

The other biggest debate is the actual mechanism being followed for the deposition.

The mechanism of Zn-Ni deposition is not well understood. It is widely accepted that

nickel is predominately deposited first, with zinc then being intercalculated into the

structure. The zinc deposition depolarization is assisted by the existing nickel support

[20]. Lehmberg et. al. did extensive studies of zinc-nickel thin films to study the

mechanism of the deposition. It has been predicted that the deposition occurs in 3 main

steps which are the initial, intermediate and final steps. The initial stage is regulated by

20

the substrate cathode material; the final stage is controlled by bath composition and

operating conditions. The intermediate step was not discussed. Because the more base

metal is preferred over the noble metal, the mechanism is classified as anomalous

deposition [21].

There are 4 different possible mechanisms for the deposition of the zinc-nickel

alloy. The first theory involves the underpotential deposition of zinc [1]. Zinc deposition

can begin at a more positive potential, up to +0.55V more positive than the equilibrium

potential for Zn2+/Zn which represents a deposition occurring at this underpotential for

zinc. The thickness of this underpotentially deposited zinc only reaches a monolayer [22]

and then deposition switches over to anomalous deposition [1].

The second theory is due to the anomalous deposition that is known to occur; the

question being asked is what causes this anomalous deposition. It has been discussed that

a local pH increase, due to the formation of Zn(OH)2 at the electrode surface induces

precipitation of the zinc oxide, which can inhibit the deposition of nickel at the surface.

The major drawback to this theory is that anomalous deposition also occurs at low current

densities where hydrogen gas formation does not cause an increase in pH at the electrode

surface [1].

The third theory assumes a discharge of Ni2+ forms a thin layer with water to form

an absorbed nickel hydroxide species. This creates a thin layer of nickel on the surface of

the substrate, leading to preferential deposition which also explains the anomalous

deposition observed [1]. This theory is most likely to occur based on later research of the

substrate surface [23].

21

The last theory presented believes hydrogen adsorption has an effect on the initial

layer formation [1] which has also been confirmed by further research [23]. The

codeposition of the iron group metals causes a reduction rate of deposition for the more

noble metal (nickel), and an increase in deposition of the less noble metal (zinc)

compared to a pure metal deposition [1].

1.3.7 Initial Deposition Studies for Acidic Conditions

A deep study of the initial stages of deposition was performed in acidic conditions by

examining thin deposits formed on copper and iron substrates. Many factors can affect

the initial stages of deposition including bath composition, operating conditions and the

substrate material. Depending on the crystallography of the material, stress and strain

might be introduced into the system if the two preferred structures are not compatible.

The growth of the deposit can also be affected when the electrochemical properties of the

depositing metal differ from the electrochemical properties of the substrate [11].

22

Figure 1.11: Analysis of zinc-nickel alloy deposit [23].

Upon examination of the plating conditions for anomalous deposition, it was

observed that the deposit does not have a uniform composition throughout, with the

nickel content of the first layer being higher then throughout the rest of the deposit.

Initially the α phase and hydrogen experience a codeposition, which switches over to

γ phase deposition once the deposit thickness increases [24].

23

Figure 1.12: Depth profiling of zinc-nickel coating, hydrogen in the deposit [23].

It is also clear from the depth profiling of the zinc nickel coating in Figure 1.12, that

hydrogen does play a role in the deposition of the zinc-nickel alloy as there is an increase

in the concentration of hydrogen, along with the increase in nickel concentration in the

film in the initial layers of the deposition [23].

24

Figure 1.13: Model of deposit layer [23].

When deposition of the alloy occurs, a very thin layer of pure nickel is deposited

onto the stainless steel. This pure layer along with a layer that is switching over to

anomalous deposition with the zinc is about 70-90nm in thickness. The second layer is

very rich in nickel, with a gradual gradient switching over to zinc-nickel deposition,

which eventually leads to gamma phase deposition on the substrate [23].

As the nickel concentration increases, the gamma phase in the alloy increases.

Zinc-nickel deposition falls under the category of anomalous deposition. The less noble

25

metal, hereby zinc, deposits preferentially and the zinc concentration in the deposit is

higher then the ratio present in the plating bath [14].

1.3.8 Effects of Morphology for Acidic Deposits

Corrosion resistance is related to the morphology of the deposit more than the

composition. The best corrosion resistance was found for alloys with nodular grains of

measurable size. Alloys with elongated and non-measurable grains resulted in decreased

corrosion protection [5]. As grain size is reduced, corrosion protection is increased [25].

Although the corrosion resistance of zinc-nickel alloys is often believed to be strictly due

to the gamma phase alloy, it is also important to note the corrosion protection is also

dependent upon the structure of the deposit [26]. As a deposit grows in thickness, the

alpha phase begins to emerge, therefore thick deposits are not desirable [11].

The more compact the layers, the better corrosion protection they offer. Less

porous coatings offer greater corrosion resistance. Nickel being present in the coating

decreases the electrochemical activity of the coating, which extends its life [27].

Bajat et al. found that the nickel content of the alloy is not the only factor to

consider when studying good corrosion protection. The deposits with nickel content are

highest for direct current (DC) plating, and this leads to poor corrosion protection.

Corrosion protection is dependent upon differing surface morphology obtained by

different deposition parameters. For instance, the quicker the actual deposition the better

the deposit since irregularity of the deposits increases with time [15]. Introducing the

nickel into the system is advantageous because it makes the electrodeposited coatings

harder and less porous [27].

26

The deposits are characterized with a variety of techniques including SEM

(scanning electron microscopy), XRD (x-ray diffraction) and AAS (atomic absorption

spectroscopy). SEM is examined in many of the studies to determine surface

morphology and measure the thickness of the coatings [6, 11, 14, 28]. XRD is examined

to identify phases of the Zn-Ni alloys deposited. The composition of the electrodeposited

material is analyzed by dissolving the deposit in 3.0M HNO3, diluting to 100mL and

analyzed with atomic absorption spectroscopy [11, 27]. The Zn and Ni contents in the

deposit are confirmed by EDS (energy dispersive x-ray spectroscopy) [1, 14].

Scanning electron microscope (SEM) studies have demonstrated the temperature

dependence of the deposits from acidic bath conditions. A deposition performed at

25.0°C has non-uniform coatings and a large number of voids present in the film. At

35.0°C the deposit compactness has been increased and fewer voids are present. 40.0 °C

bath demonstrates a very uniform and compact deposit. The film has large grain size. A

deposit obtained from a 50.0°C temperature bath demonstrates a definite transition to a

fine grained structure and full surface coverage is also observed. It can be seen that

increasing the bath temperature from 25.0-50.0°C activates the nickel deposition which

leads to the alloy being deposited, a better surface coverage, and a better corrosion

resistive material [1].

1.3.9 Acid Deposition Conclusions

The shortcomings of acid bath deposition, most importantly the poor surface

coating lead the research to alkaline bath deposition. Acid baths are heavily dependent

upon bath conditions, such as pH, temperature and plating conditions. Acid bath

27

depositions readily produced both γ and δ phase alloys, while only γ phase alloy is

preferred for maximum corrosion protection.

1.4 Alkaline Bath Deposition

Alkaline bath deposition was examined once it was realized that acid baths do not

result in a uniform metal distribution. The first industrial alkaline system was developed

in 1992 and this deposit contained 5-7% nickel. This electrolyte solution contained

sodium hydroxide, zinc salts and amines acting as complexing agents. The advantage of

alkaline deposition was the deposit had superior alloy distribution compared to the acidic

deposition, but there was lower efficiency of the system which leads to longer deposition

times. The deposits appeared to be much duller in color, not the bright finishes obtained

during acidic deposition [7].

In 1995 the first high alloy (12-15% nickel) deposit process was patented. The

biggest advantage to this alloy was that it was much cheaper to process and easier to

control and had an increased corrosion resistance and enhanced wear [7].

Zinc-nickel alloys offer corrosion protection to stainless steel substrates because

they possess cathodic potential to the steel, controlled by the nickel in the deposit. An

increase in nickel concentration offers an increase in the corrosion potential. In alkaline

solutions the zinc-nickel has a corrosion potential of E=-1.0V versus SCE. Pure zinc has

a corrosion potential of -1.1V versus SCE and the corrosion potential of steel is E=-0.6V

versus SCE [4].

The nickel deposition is found to occur at the initial stages of deposition, within

the first 0.2 s of deposition compared to zinc. After this point, the deposition of nickel

28

greatly reduces, and after approximately 0.5 s the nickel content in the deposit is

constant, regardless of deposition time. This suggests that there is a pure nickel layer

between the alloy deposition and the stainless steel substrate [5]. Most work has been

done at pH › 12, with triethanolamine (TEA) acting as a complexing agent. Sodium

hydroxide is commonly used as the base. There is a zinc nickel ratio of approximately

6:1. Most work is done at room temperature [4].

1.4.1 Hydrogen Embrittlement of Deposits

Hydrogen is easily absorbed into metals during processing and once the finished

product is in use. Hydrogen negatively affects the ductility of metals and high levels of

hydrogen can cause metals to be brittle when subjected to constant stress [8].

When electroplating from aqueous solutions, hydrogen can be absorbed into the base

metal. Once a coating has been plated onto the metal, release of the hydrogen is difficult

because the coating is a barrier on the base metal. When the material is exposed to heat

the hydrogen escapes. This often destroys the coating in the process, due to fractures in

the material. The composition of the plating can be altered, resulting in a deposit that

allows for the escape of the hydrogen without destruction of the coating [29].

Alkaline bath deposits have been found to form open grained smooth deposits.

The porosity of the deposit is maximized which allows one to bake out the hydrogen

present in the metal, and diminish the post-plating embrittlement due to hydrogen. These

properties make this an optimal choice for both low and high strength steels [8].

Hydrogen gas evolution is very low when gamma phase has been deposited [30]. The

deposit is formed from more microstructure particles, so the hydrogen can easily escape

29

from the underlying metal without creating cracks and holes in the deposits, thereby

leading to better corrosion protection then with other alloy phases [25].

Pulse plating can lead to more of an opened grained structure, which is useful

because hydrogen can readily release out of the deposit without causing holes or pits in

the coating, so methods to obtain these types of structures are being examined [8]. Pulse

processes create more refined microstructures by inducing larger grain nucleation rates

therefore the corrosion properties of the zinc-nickel alloy can be improved by controlling

the nickel content in the pulse current process. With pulse plating, one is able to obtain

fine particles in a compact arrangement which gives a good appearance and high

hardness to the alloy [31].

1.4.2 Electrolytic Properties of the Alkaline Baths

In alkaline conditions, zinc is soluble as a hydroxyl complex (Na2Zn(OH)4).

Complexing agents are needed to keep the zinc and nickel in solution. Amines have been

found to be optimal ligands for nickel in alkaline solutions. The metal and amine

complex needs to result in a constant alloy composition independent of the current

density since this leads to good corrosion resistance [32]. As the zinc nickel ratio is

increased, the percentage of nickel in the coating decreases. With a ratio of greater than

7.5, a minor amount of nickel (<7%) is present in the alloy. With a ratio of 5 and less

nickel deposits of ≥10% are present. The optimal deposit is obtained with a zinc nickel

ratio of 5 to 7 [33]. Grain size, chromating ability and stability of the alloy composition

are all affected by the amine used. The working temperature of the cell has been a long

30

debated issue. As the temperature increases, the amount of nickel in the deposits

increases as well. Most cells are kept at or below a working temperature of 35°C [32].

Carbon dioxide gas can be readily absorbed into the system in alkaline solution

leading to carbonate contamination. To keep the amount of carbonate contamination at a

minimum, amine concentrations of less than 1 molar are used [32].

The Zn:Ni ratio has to be heavily controlled in the cell, or effects will be seen on

the composition of the alloy, the metal distribution, and the chromating effect of the

solution. The composition of the alloy is regulated by the Zn:Ni ratio, the total metal

content in the system, the base concentration and the bath temperature [32]. Composition

uniformity can be achieved when mass transfer is fast in relation to the electrode kinetics.

When these parameters are met, the surface concentrations of the reacting species in

solution remain essentially the same as in the bulk solution. In this case, the average

alloy composition is dependent upon the temperature and the bath composition due to

their effects on the electrode kinetics [5]. Higher nickel concentration in the bath

promotes anomalous codeposition [13].

At high overpotentials, and high current densities, zinc and zinc enriched phases

are obtained due to the high overpotential needed for zinc deposition [34].

31

Figure 1.14: Pourbaix diagram of zinc species.

Line 3’ in the Pourbaix diagram (figure 1.14) for zinc represents an equilibrium

reaction between Zn2+ and HZnO2- in solution, occurring at a pH value of 9.21. The 4’

line represents the equilibrium between HZnO2- and ZnO2

2- at a pH value of 13.11.

Carbon dioxide gas can be readily absorbed into the system in alkaline solution leading to

32

carbonate contamination [14], which could lower the pH of the solution enough to allow

the equilibrium of Zn2+ and HZnO2- to occur at a pH of 9.21 so the pH must be measured

regularly to remain slightly more alkaline than the equilibrium.

Figure 1.15: Pourbaix diagram of nickel species.

33

The Pourbaix diagram for nickel, as shown in figure 1.15, has one equilibrium

reaction at a pH value of 10.13 with Ni2+ in equilibrium with HNiO2-. This equilibrium

reaction should not be an issue because the pH range being examined is slightly less

alkaline. As you decrease the pH of the plating bath, the protection offered by the

coating is decreased because as the pH reaches more acidic conditions, the coating is not

as uniform, leading to areas of low to no protection on the underlying substrate from

corrosion [26] so it is important to still work in a higher pH range.

Ammonium hydroxide is often used as a complexing agent, because the ammonia

can easily complex the zinc and nickel in solution, and the hydroxide can offer the base

needed for alkaline deposition. The ammonia concentration needs to remain between

0.5-2M [31]. Below a concentration of 0.5M zinc oxides and zinc hydroxides precipitate

out immediately and above a concentration of 2M no deposition occurs [35].

1.4.3 Complexing Agents for Alkaline Baths

In the bath, zinc and nickel must be stabilized with some type of complexing

agent to prevent the precipitation of zinc and nickel hydroxides. Most alkaline bath work

has been done in alkaline solutions of pH›12, but some work has also been examined in

the 9.3-9.5 pH range. For a pH›12 range, triethanolamine (TEA) is commonly used as

the complexing agent. For the pH range of 9.3-9.5, ammonium hydroxide and sodium

citrate have been used. As the pH increases, the nickel content of the film decreases, as

more nickel hydroxide species are formed. The nickel content of the film also decreases

as the stirring speed is increased [9]. Deposits obtained from complex electrolytes have

finer grain deposits because of the higher overpotentials involved, which lead to more

34

nucleation then grain growth [36], so again a complexing agent is required to obtain a

better deposit.

1.5 Corrosion Protection from Alkaline Deposits

Zinc-nickel coatings have been studied because this alloy provides a strong

corrosion protection when plated onto steel. It has been shown that Zn-Ni alloys

containing 15-20% Ni possess up to four times more corrosion resistance then a

cadmium-titanium deposit on steel [6]. Many works have been found on the

electrodeposition of Zn-Ni onto steel to obtain improved resistance to corrosion [2, 3, 6].

It has been observed that when exposed to 400 hours of salt fog corrosion resistance,

zinc-nickel has outperformed corrosion resistance of pure zinc platings by 500% and

zinc-cadmium platings by 300%. The optimal Ni content has also been under a lot of

review, but throughout the papers it seems 8-15% Ni is the optimal range [27].

It has also been noted that the bath composition has an effect on the corrosion

resistance of the alloy, whereas the alkaline baths have a higher protection then acidic

baths. This is believed to be due to zinc oxide being infused into the coatings in the

alkaline systems [26].

35

Figure 1.16: Salt spray testing of zinc-nickel coating on steel substrate [26].

A: chromated Zn (barrel, acid bath) B: chromated Zn (rack, alkaline bath) C:

chromated Zn-Ni (barrel, acid bath) D: chromated Zn-Ni (rack, acid bath) E: chromated

Zn-Ni (rack, alkaline bath) F: chromated Zn-Ni (rack, alkaline bath).

Figure 1.16, salt spray testing of zinc-nickel coatings on steel substrates, shows

zinc corrosion products on the coatings of the plating as a function of time. The

chromated coatings show the best resistance overall, both in the time that the initial white

rust appears and in the rate of corrosion. It is also clear that the two types of alkaline bath

deposits are superior to the acid bath deposits. After exposure in the salt spray chamber,

the platings were x-rayed and the main corrosion product was found to be

ZnCl2.4Zn(OH)2 for all coatings [26].

36

1.5.1 Salt Spray Testing

The most common type of a corrosion resistance test is a salt spray test. The

plating is submerged in a salt solution, and the corrosion of the plating is measured over

time. 5% salt spray tests have been performed, which demonstrate the time elapsed for

white and red rust formation [33].

Figure 1.17: Salt spray corrosion tests [33].

37

Figure 1.18: Salt spray corrosion resistance tests – chromated samples [33].

Figure 1.17 demonstrates the salt spray corrosion of non-chromated zinc and zinc-

nickel coatings that were deposited onto stainless steel in alkaline baths. The corrosion

of the coating is plotted versus the thickness of the deposit. Red rust, corrosion of the

underlying stainless steel substrate, first appears on the non-chromated alkaline zinc after

100 to 200 hours. Alkaline zinc-nickel with a constant thickness has a higher corrosion

resistance of 500 to 1000 hours, in relation to the non-chromated alkaline zinc. In all

cases, white rust formation, corrosion of the zinc and zinc-nickel coatings, occurred after

50 hours of exposure. As expected, as thickness increases the corrosion resistance

increases. A nickel content of 8-10% nickel appears to delay the formation of red rust,

thereby prolonging the life of the plating. Figure 1.18 shows chromating the zinc deposit.

38

It is seen that chromating extends the corrosion life for pure zinc. On chromated zinc-

nickel red rust did not appear before 500 hours of exposure [33].

1.5.2 Sacrificial Electrodes

Electrodeposited zinc alloys act as sacrificial electrodes, which means they

corrode preferentially (galvanic corrosion), which in turn protects the steel it has been

coated onto from corroding [28]. An alloy with a high enough zinc content could be

more stable than a pure zinc coating if it has a more negative potential then steel [27, 28].

Therefore, the corrosion stability of the zinc-nickel alloys is mainly dependent upon the

amount of nickel in the alloy. The higher the nickel content, the faster the coating

corrodes. It has been found that a nickel content of 8-15% is optimal for corrosion

protection.

1.5.3 Corrosion Phase

During the corrosion phase, while the zinc phase is being preferentially corroded,

internal stresses in the deposit increase which creates cracks throughout the plating.

Once these cracks form, there is a significant increase in the pH within the cracks which

is caused by cathodic reduction of the oxygen. The pH increase results in precipitation of

corrosion products which in turn fill in these cracks, thereby protecting the steel [26].

The oxidation of the zinc in the coating is slowed down by the presence of the nickel, but

as the oxidation increases, the nickel is present as a barrier to further corrosion [8]. The

coating is then a mixture of corrosion products with a nickel enriched alloy layer which

acts as a protection barrier to the steel substrate. The γ phase nickel is believed to be

39

primarily responsible for the corrosion protection properties of the alloy but overall the

protection also depends on the structural homogeneity of the plating [26]. Initially the

corrosion is sacrificial; the zinc corroding preferentially, but there is a gradual switch to

barrier protection by the nickel [8].

A nickel content of less than 15% is critical for sacrificial protection of the steel

to occur. Below 15% nickel content, the alloy follows the same corrosion mechanism of

pure zinc, corroding preferentially to the steel. Above 15% nickel the mechanism

follows that of a nickel deposit. The potential difference results in cathodic protection,

which, if the deposit becomes scratched, the base metal corrodes preferentially to the

plated deposit [8].

1.6 Summary

Due to the automotive industry there is a great push to improve the corrosion

resistance of steel, and this can be accomplished with the electrochemical deposition of

zinc-nickel alloys onto steel. For many years zinc coated stainless steel has been used in

this field. Zinc sacrificially corrodes compared to the stainless steel, thereby protecting it

[1]. Zinc alloys are of great interest in research because they offer a greater resistance to

corrosion then pure zinc [2]. Zinc-cadmium alloys have been used for the corrosion

protection of steel to date but due to the harsh environmental conditions associated with

cadmium, other metals have been examined. The use of cadmium has become very

regulated, or in some countries banned all together. At this point, alloys were examined

as a solution to the problem. Alloys have different corrosion potentials then their single

elements; therefore by picking the correct combination of alloys, one can greatly increase

40

the corrosion resistance of the material [1] and zinc-nickel is a cheap environmentally

milder alternative.

It has been shown that Zn-Ni alloys containing 15-20% Ni possess up to four

times more corrosion resistance then a cadmium-titanium deposit on steel [3]. When

exposed to 400 hours of salt fog (a common method used to test corrosion protection),

Zn-Ni has outperformed zinc platings by 500% and zinc-cadmium platings by 300% [5].

Historically acidic depositions have been performed, with a zinc nickel alloy ratio

of 10-15% nickel in the deposit. Acid bath deposition had one major drawback, which

was the creation of non-uniform deposits [5]. The advantage of alkaline deposition is the

deposit has superior alloy distribution compared to the acidic deposition, but there is

lower efficiency of the system which leads to longer deposition times.

1.6.1 This Thesis Work

Alkaline electodeposition gives a more uniform deposit, which leads to better

corrosion protection of the underlying metal, so a new method will be developed for the

co-deposition of zinc-nickel alloys at lower pH values (9-9.5 range) and room

temperatures rather then the current elevated temperatures employed. To date, work has

been performed with sodium hydroxide at caustic pH values (≥12) and at higher then

room temperatures. There has been a push for the use of less caustic solutions towards a

more neutral pH. Our method will use ammonium hydroxide as a base source, which is

less caustic then sodium hydroxide, at a lower pH range of 9.3-9.5, and room

temperature. The deposits obtained will contain the same qualities of deposits from more

difficult working conditions. The conditions to be developed will be easier to perform

41

under normal laboratory conditions and this technique will easily be integrated into

industry to continue the battle against corrosion.

Due to the highly caustic properties of alkaline baths in the pH›12 range our

research work has focused on a lower but still alkaline pH range of 9.3-9.5. Based on the

Pourbaix diagrams, there are a few reactions that may affect us in this range.

Ammonium hydroxide will be examined for its use both as a base source and a

complexing agent for zinc and nickel ions in solution. In alkaline conditions, zinc is

soluble as a hydroxyl complex (Zn(OH)4) but will still readily precipitate out of solution

if not agitated during deposition. Nickel must also be complexed in solution to prevent

the precipitation of nickel hydroxides. Amines have been found to be optimal ligands for

nickel in alkaline solutions but this work will examine the use of other ligands, such as

acetate, which are more environmentally friendly. The use of common electrolytes in

solution to aid in deposition of the metal alloy will also be examined. The range of pH

values to be examined will be 9.3-9.5, which is still in the alkaline range, but offers

milder working conditions.

A deposition method for the co-deposition of zinc-nickel alloys onto stainless

steel will be developed. This technique will offer better corrosion protection and will be

easier to apply on the macro scale of industry. The optimal nickel percentage in the alloy

when deposited from a non-acidic pH bath will also be determined, since, as to date this

has not been discussed. The best bath conditions for optimal zinc-nickel deposition will

be determined.

42

CHAPTER 2

DEPOSITION OF PURE METAL FILMS IN ALKALINE SOLUTIONS

2.1 Introduction

The depositions of pure zinc metal and pure nickel metal in alkaline solutions

were examined to determine the optimal plating conditions for pure metal films. Pure

metal deposition was important because if optimal conditions could be determined for

pure metal deposition, these parameters could be applied to alloy deposition. Alloy

deposition is usually at a shifted potential in relation to the pure metals, but in the case of

zinc and nickel alloys, the deposition of the alloy occurs at potentials between the

deposition of pure nickel and pure zinc. The alloy shifts the deposition potential from the

original pure metal potential for the metals present in the alloy phase. Complexing

ligands were required to keep the metal ions in solution, because the metals readily

precipitated out as hydroxide species, so different ligands were examined based on their

pKa’s with the metal complex. Borate was also examined as a possible electrolytic

solvent since nickel is known to deposit well from borate systems.

All deposits were obtained by a potentiostatic method, both from a direct potential

method and from a potential step method. The deposits obtained through a potential step

method demonstrated more uniform and better adhering deposits.

43

Figure 2.1: Potential step method diagram.

Figure 2.1 represents the potential step method employed for deposition. The E1

value was determined based on where nucleation of the metal began to occur on the

substrate, by picking a value past the crossover point in the cyclic voltammogram (CV).

The E2 value was determined by picking a value more positive then E1, but still to the

cathodic side of the stripping peak present for the metal. The purpose of performing the

potential step method was to obtain a better surface plating of the metal. Once the

nucleation of the metal onto the substrate began, the system was quickly brought back

down to the lower potential. This brought the metal that was being plated onto steel as

M0, back into solution as M2+. The system was then brought back up to the higher

potential, thereby creating more metal nucleation sites on the already populated substrate

surface. It was found that this method did result in a better metal deposit for both zinc

and nickel depositions.

44

2.2 Experimental Parameters

All electrochemical work was performed on an EG&G PARC

Potentiostat/Galvanostat Model 273A. The cell set up was as follows:

Figure 2.2: Set up of electrochemical cell.

The working electrode used throughout all experiments was a stainless steel disc

mounted in epoxy. The working electrode was polished with grit paper, diamond and

alumina until the steel had a mirror finish. The disc was bound to copper wire through an

electroconducting silver epoxy, and the copper wire was connected to the lead. The disc

was set into epoxy and hardened for 24 hours. The counter electrode used throughout the

experiments was a chromel coiled wire and the reference electrode was a saturated

calomel electrode (SCE, +0.241V vs. SHE).

Cyclic voltammograms (CV) were run on all electrochemical solutions prepared

with an EG & G PARC Potentiostat/Galvanostat. Based on the CV’s, the deposits were

plated onto the stainless steel discs at set potentials. X-ray patterns of these deposits were

examined to determine the content of the deposit. The XRD data was obtained on a

Siemens D-500 Diffractometer using a Cu Kα radiation (λ=0.1541 Å, 35kV, 24mA). The

45

scans were run on a θ:2θ coupled experiment, from 10 to 100°, step size of 0.05 degrees

and dwell time of 1 second.

A great deal of work in the literature has been done on alkaline systems with a pH

> 12, with sodium hydroxide used as the base. Some work has been performed with

ammonium hydroxide acting as the base. This work focused on the use of ammonium

hydroxide as the base because once sodium hydroxide was added to the system, the

metals precipitated out almost immediately as metal hydroxides. A suitable complexing

agent was not found to work with sodium hydroxide. The ammonium hydroxide also

only requires a pH range of 9.3 to 9.5, so it was preferential to work in these less harsh

conditions.

Background XRD patterns of the stainless steel disc in and out of epoxy are

attached to demonstrate the background that is expected in the deposits.

40 60 80 100

0

200

400

600

800

1000

1200

20=8

2.29

91, S

tain

less

Ste

el 2

11 P

eak

20=6

4.78

61, S

tain

less

Ste

el 2

00 P

eak

20=4

4.64

99, S

tain

less

Ste

el 1

10 P

eak

Inte

nsity

(CP

S)

2 Theta (degrees)

Figure 2.3: Stainless steel disc background mounted in epoxy.

46

Figure 2.3 is a scan of this background peak, which shows the stainless steel

peaks of the substrate. From the peaks observed, it is clear there are three main peaks

present for stainless steel in the XRD pattern. A peak at approximately 98.879 degrees is

expected for the steel 220 plane but was not observed due to the epoxy background noise.

40 60 80 100

0

50

100

150

200

250

20=9

8.87

33, S

tain

less

Ste

el 2

20 P

eak

20=8

2.17

17, S

tain

less

Ste

el 2

11 P

eak

20=6

4.84

82, S

tain

less

Ste

el 2

00 P

eak

20=4

4.59

90, S

tain

less

Ste

el 1

10 P

eak

20=2

9.46

38, S

tain

less

Ste

el P

eak

Inte

nsity

(CP

S)

2 Theta (degrees)

Figure 2.4: Stainless steel background disc, out of epoxy.

This XRD pattern (figure 2.4) shows the stainless steel background disc out of the

epoxy setting. The disc had been removed from the epoxy prior to running the XRD

pattern. Figure 2.4 clearly illustrates the main peaks expected for stainless steel in the 10

to 100 degree 2 theta range. The 220 peak present at 98.8733 is now observed; in the

epoxy setting (figure 2.3) this peak was lost to background noise.

47

Based on the following x-ray diffraction patterns, it is clear that nickel and zinc

have been deposited from a variety of electrochemical bath conditions.

2.3 Zinc Sources for Deposition

Zinc nitrate and zinc sulfate monohydrate have been examined as depositing salts for

pure zinc deposition. Zinc sulfate monohydrate was favored in this work because it more

readily dissolves in aqueous solutions. Zinc nitrate dissolves nicely in aqueous solutions

but no deposition appears to occur during plating. Hubert et al. noted that for zinc

compounds, an ammonia concentration must be set between 0.5M and 2M for deposition

to occur. Below 0.5M the metal will precipitate out immediately as a hydroxide and

above 2M no deposition is observed [31]. The ammonium hydroxide concentration for

this work was set at 1M, but the zinc still precipitated out upon addition of ammonium

hydroxide. When hydroxide was added to the zinc solution, a white precipitate

immediately formed. This solid is believed to be a mixture of zinc oxide and zinc

hydroxide. Consequently, the following ligands were examined to complex the zinc in

solution to stabilize the Zn2+ ions. These ligands were examined with zinc nitrate and

zinc sulfate monohydrate.

Table 2.1: Possible ligands for zinc; zinc complex pKa’s. Ligand pKa1 pKa2 pKa3 pKa4 Ammonia 2.37 4.81 7.31 9.46 Acetate 1.5 Tartaric Acid 2.68 8.32 Triethanolamine 2.00

48

Figure 2.5: Structure of zinc ammonia complex.

Figure 2.6: Structure of zinc tartaric acid complex.

Figure 2.7: Structure of zinc acetate complex.

49

Figure 2.8: Structure of zinc triethanolamine complex.

Throughout the literature, zinc was complexed with triethanolamine to keep it in

solution. TEA was examined as a possible ligand for zinc along with tartaric acid, lactic

acid, and ammonia from ammonium hydroxide. Zinc sulfate heptahydrate was the most

commonly used zinc source for zinc-nickel codeposition in the literature, but zinc nitrate

and zinc sulfate monohydrate were examined as possible zinc sources in this work. Zinc

sulfate monohydrate dissolved more readily in water and therefore was an easier choice

in an aqueous solution.

Based on the JCPDS Database, the following peaks should be observed for zinc

metal, in a random pattern.

Table 2.2: The PDF data of XRD patterns of standard zinc metal from the JCPDS Database (PDF #04-0831). 2θ hkl I/I0 36.296 002 53 38.992 100 40 43.231 101 100 54.336 102 28 70.056 103 25 70.661 110 21 77.027 004 2 82.102 112 23 83.765 200 5 86.557 201 17 89.920 104 3 94.900 202 5

50

2.3.1 Zinc Nitrate

Figure 2.9: Zinc nitrate.

Zinc nitrate was examined as a possible zinc source because it readily dissolved in

aqueous solutions. Initially the zinc nitrate was dissolved in water and the solution was

brought to a pH of 9.3 with ammonium hydroxide as the base. When zinc nitrate was

combined with ammonium hydroxide, a white precipitate formed upon immediate

addition of the base. This precipitate was believed to be a form of zinc hydroxide. After

initial research with this molecule, it was clear a complexing ligand would be required to

stabilize the zinc in solution. Tartaric acid and lactic acid were examined as possible

ligands for this molecule. Initially zinc nitrate was complexed with tartaric acid. The

tartaric acid seemed to complex the zinc in solution, but once base was added a white

precipitate formed. Zinc nitrate was also studied with lactic acid but the lactic acid did

not appear to plate the zinc metal onto the stainless steel disc. Zinc nitrate was not

examined with triethanolamine as the ligand.

51

The following solutions were prepared and examined:

Solution 1: 0.5 M Zinc Nitrate and 1M NH4OH at pH=9.3

Solution 2: 0.5 M Zinc Nitrate, 0.5M Tartaric Acid, 1M NH4OH at pH=9.3

-1 0 1

0.012

0.010

0.008

0.006

0.004

0.002

0.000

-0.002

Cur

rent

(A)

Potential (V)

Zinc Stripping Peak E=-0.900V

Crossover E=-1.0945V

Figure 2.10: CV of zinc nitrate, pH=9.3 with 1M NH4OH.

Zinc Nitrate was made basic (pH=9.3) with NH4OH and a cyclic voltammogram

(CV) (figure 2.10) was ran on this solution. The Zinc nitrate does not remain in solution

upon the addition of base; a white precipitate immediately formed which appeared to be a

zinc oxide/zinc hydroxide mixture. There is a clear stripping peak at E= -0.900V and a

crossover at E=-1.0945V. The crossover represents metal deposition nucleation sites

52

being formed on the electrode surface and the stripping peak is due to zinc metal being

stripped off of the electrode surface, dissolving back into the solution as Zn2+ ions.

-1 0 10.020

0.015

0.010

0.005

0.000

-0.005

Cur

rent

(A)

Potential (V)

Zinc reduction Peak E=-1.207V


Figure 2.11: CV of Zinc nitrate, tartaric acid, and pH=9.3 with 1M NH4OH.

The CV (figure 2.11) of zinc nitrate complexed with tartaric acid has a reduction

peak present around E=-1.207V which is due to zinc reduction. The solution was plated

at E1=E3=-1.37V, E2=-1.00V, delay 1=60.0 sec, delay 2=20.0 sec. In a two hour

deposition time, a charge of + 33.80V was passed. Many tiny bubbles were present on

the electrode and no metal was plated onto the stainless steel disc. The solution turned

yellow in color by the end of the deposition. Tartaric acid appeared to complex the zinc

metal too strongly in solution. The zinc metal was not released from the complex to plate

53

the steel with a zinc coating. The second pKa of the zinc-tartaric acid complex is 8.32,

which is too strong of an attraction to release the zinc ion into solution for metal

deposition.

2.3.1.1 Zinc Nitrate Conclusions

Zinc nitrate does not work as a zinc source in basic solution. The zinc does not

plate out as pure metal, no platings were obtained from the baths with zinc nitrate. Zinc

nitrate readily precipitates out of solution when not complexed with a ligand, even when

using ammonium hydroxide as the base source. When using tartaric acid the solution

turned yellow in color, but no deposition was observed.

2.3.2 Zinc Sulfate Monohydrate

Figure 2.12: Zinc sulfate monohydrate.

Zinc sulfate monohydrate was also examined as a possible zinc source. Initially,

the zinc sulfate monohydrate was combined with pure sodium hydroxide as the base

source. However, the zinc precipitated out immediately as a white precipitate. The zinc

sulfate monohydrate was also combined with ammonium hydroxide without any

54

complexing ligand but, again, the zinc was immediately precipitated out as a white solid.

A complexing agent was then examined to combine with the zinc sulfate monohydrate to

keep it in solution. Triethanolamine and sodium acetate were used as possible

complexing ligands for the zinc sulfate monohydrate. When complexed with

triethanolamine during electrodeposition, big bubbles formed on the electrode surface

and no deposit appeared to form. Acetate appeared to be the best ligand to use based on

the appearance of the deposits. The zinc/acetate solution needed to be stirred during

plating or a white precipitate fell out of solution.


Solution 1: 0.5 M ZnSO4.H20, 0.5M Triethanolamine, 1M NH4OH at pH=9.3

Solution 2: 0.5 M ZnSO4.H20, 0.5M Sodium Acetate, 1M NH4OH at pH=9.3

-1.5 -1.0 -0.5 0.0 0.5 1.0

0.020

0.015

0.010

0.005

0.000

-0.005

Crossover occurs at E= -1.306V



Cur

rent

(A)

Potential (V)

Figure 2.13: CV of Zinc sulfate monohydrate, triethanolamine with pH=9.3 with 1M

NH4OH.

55

The CV (figure 2.13) curve of zinc sulfate monohydrate and

triethanolamine with a pH=9.3 with 1M NH4OH showed a strong zinc stripping peak at

E=-0.996V. There were two crossovers occurring, both just past the reduction peak of

the zinc metal in solution. The first crossover was present at E= -1.174V, the second

crossover was present at E= -1.306V. A plating of this solution was performed at E= -

1.200V, which was between the two crossover peaks present in the CV. The solution was

stirred during deposition. The deposition lasted 6 hours and +7.770 C was passed with

current of +0.820mA to +0.301mA at the end of the deposition. After plating, white dots

were present on the electrode surface. No actual metal deposit was formed which was

confirmed by XRD. Powder formed at the bottom of the beaker and a solid precipitate

formed on the top of the liquid.

40 60 80 100

0

200

400

600

800

1000

1200

20=8

2.54

97, S

tain

less

Ste

el 2

11 P

eak

20=6

5.09

49, S

tain

less

Ste

el 2

00 P

eak

20=4

4.92

90, S

tain

less

Ste

el 1

10 P

eak

Inte

nsity

(CP

S)

2 Theta (Degrees)

Figure 2.14: XRD Pattern of zinc sulfate monohydrate, triethanolamine, and pH=9.3 with

1M NH4OH.

56

The electrode was x-rayed and the XRD pattern (figure 2.14) of zinc

sulfate monohydrate with the complexing agent of triethanolamine did not show any zinc

present in the pattern. The disc after plating did not appear to have any metal deposited

onto the steel.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.00.010

0.008

0.006

0.004

0.002

0.000

-0.002

-0.004

-0.006

-0.008

Crossover occurs at E=-1.328V

Zinc Reduction Peak, E=-1.237V

Cur

rent

(A)

Potential (V)

Figure 2.15: CV of zinc sulfate monohydrate and triethanolamine, pH=11.04 with 1.5M

NH4OH.

The CV (figure 2.15) of zinc sulfate monohydrate when complexed with

triethanolamine, and brought to an overall pH of 11.04 with ammonium hydroxide

showed a zinc stripping peak at E= -1.237V. A crossover occurred at E= -1.328V. The

solution was greenish yellow in color, and a brown precipitate formed over time. The

57

solution was plated at a potential of E= -1.3V but nothing plated onto the electrode and

large bubbles covered the electrode surface during the deposition time.

-2.0 -1.5 -1.0 -0.5 0.0 0.5

0.04

0.03

0.02

0.01

0.00

-0.01Zinc Stripping peak E=-1.02V

Cur

rent

(A)

Potential (V)

Figure 2.16: CV of zinc sulfate monohydrate and sodium acetate, pH=9.3 with 1M

NH4OH.

The CV (figure 2.16) of zinc sulfate monohydrate, sodium acetate, pH=9.3 with

ammonium hydroxide had a stripping peak at E= 1.02V. This CV appeared to have a

crossover but upon closer examination, no crossover actually occurred. This solution was

plated at a potential of E= -1.450V. The solution was stirred. The charge passed was

+20.19C in 45 minutes, with the current beginning at +14.61mA and dropping to

+8.00mA at the end of the deposition.

58

The plating of zinc sulfate, sodium acetate had a very smooth, silver deposit on

the stainless steel. The metal was examined with x-ray diffraction (figure 2.15) to

confirm the presence of zinc metal on the electrode.

40 60 80 100

0

500

1000

1500

2000

20=9

5.14

14, Z

inc

202

Peak

20=8

6.47

96, Z

inc

201

Peak

20=8

2.17

17, Z

inc

112

Peak

Stai

nles

s St

eel 2

11 P

eak

20=7

0.16

96, Z

inc

103

and

110

Peak

s

20=6

4.84

82, S

tain

less

Ste

el 2

00 P

eak

20=5

4.43

56, Z

inc

102

Peak

20=4

4.59

90, S

tain

less

Ste

el 1

10 P

eak

20=4

3.42

41, Z

inc

101

Peak

20=3

9.09

32, Z

inc

100

Peak

20=3

6.32

88, Z

inc

002

Peak

20=2

9.44

5, S

S Pe

ak

Inte

nsity

(CP

S)

2 Theta (Degrees)

Figure 2.17: XRD pattern of zinc sulfate monohydrate with sodium acetate, pH=9.32

with 1M NH4OH.

It was clear from the XRD pattern (figure 2.17) of the zinc/acetate deposit that

zinc metal had been deposited onto the steel electrode. The XRD pattern was a plating of

zinc sulfate monohydrate, complexed with sodium acetate and brought to a pH=9.32 with

ammonium hydroxide, plated onto a stainless steel disc. The x-ray pattern of the zinc

sulfate monohydrate was from a plating obtained at a potential of E= -1.450V. The x-ray

pattern was of the deposit still adhered to the stainless steel substrate, but had been

removed from the epoxy.

59

Zinc deposition was confirmed in the deposit. Based on JCPDS database, powder

diffraction file (PDF) 04-0831, zinc was deposited in a random pattern. The stainless

steel peaks have minimal intensities in the x-ray pattern, thereby demonstrating a thick

zinc deposit on the steel disc.

2.3.2.1 Zinc Sulfate Monohydrate Conclusions

Zinc sulfate monohydrate is a great source for zinc deposition from alkaline baths.

Zinc sulfate monohydrate readily plated out of solution with the addition of an acetate

complexing ligand in solution. Zinc sulfate monohydrate will be examined further as a

possible zinc source in the deposition bath.

2.4 Borate Solutions for Zinc

Borate was examined as a possible electrolyte for the deposition solutions due to

the advantages of using borate when depositing pure nickel metal. A solution of 0.1M

borate was used consistently throughout the following experiments. An advantage to

using borate is no complexing ligand is required for deposition, past the ammonium

ligand provided by the base source.

60

2.4.1 Zinc Sulfate Monohydrate in Borate

-2 -1 0 1

0.01

0.00

Cur

rent

(A)

Potential (V)


Zinc Reduction Peak E=-0.8055V

Figure 2.18: CV of 0.5M Zn in 0.1M borate, pH=9.3 with 1M NH4OH.

Based on the CV (figure 2.18) of zinc in borate, there is a zinc stripping peak

present at E=-0.8055V and a crossover at E=-1.0700V. The solution was deposited based

on previous experiments with E applied=-1.450V and the solution was continuously

stirred slowly to keep precipitate from collecting. Metal was deposited but had a very

weak adhesion to the metal; flakes were readily falling off the electrode substrate. A

charge of + 111.3C was passed with a current range of +5.350mA to + 4.570mA.

61

30 35 40 45 50 55 60 65 70 75 80 85 90 95 10050

100

150

200

250

300

350

400

450

500

550

600In

tens

ity (C

PS)

2 Theta (degrees)

36.4

Zn

002

39.2

Zn

100

43.3

Zn

101

54.5

Zn

102

82.1

Zn

112,

SS

211

70.3

Zn

103,

Zn

110

Figure 2.19: XRD pattern of zinc deposited from 0.1M borate solution, pH=9.3 with 1M

NH4OH.

Zinc is confirmed in the deposit from the zinc borate solution (figure 2.19); the

zinc has been deposited in a random orientation. There is not a lot of zinc present in the

deposit, because the metal flakes were readily falling off the electrode. A pulse plating

method was used with the zinc borate solution and the deposit had much better adhesion

to the stainless steel substrate.

2.4.1.1 Zinc Sulfate Monohydrate conclusions for borate solutions

Zinc sulfate monohydrate was readily deposited as a pure zinc coating onto

stainless steel from a borate electrolytic solution. When using a borate solution, the zinc

metal initially fell off the stainless steel substrate, obtained from a direct potential

62

method. A step potential method was used and a thick zinc coating with a strong

adhesion to the underlying stainless steel substrate was obtained.

2.5 Nickel Sources for Deposition

Nickel sulfate hexahydrate and nickel ammonium sulfate hexahydrate were both

examined as possible nickel sources for the alloy. Nickel is able to deposit at a lower

overpotential than zinc, so the parameters used for zinc deposition were also employed

for the deposition of nickel.

Nickel sulfate hexahydrate is commonly used as the nickel source for zinc-nickel

codeposition. Initially, aqueous solutions of nickel sulfate hexahydrate were prepared

and combined with sodium hydroxide and ammonium hydroxide respectively. Nickel

hydroxide precipitated out immediately when combined with sodium hydroxide or

ammonium hydroxide. A complexing agent was required to keep the nickel from

precipitating out as nickel hydroxide in basic solution. The most commonly used

complexing agent in alkaline solutions is triethanolamine (TEA). TEA was examined as

a possible ligand along with sodium acetate and lactic acid. Nickel sulfate hexahydrate

was complexed with sodium acetate to keep it in solution in order to plate the nickel

metal onto the stainless steel disc.

Sodium acetate was used as the complexing ligand, because of the ligands

examined for zinc, it was the only one to allow for deposition of zinc, and it is a common

ligand between the 2 metal systems, which will simplify the system when the metals are

combined for deposition. The nickel complex solution appeared green in color. Upon

addition of ammonium hydroxide; light green precipitate slowly began to form and the

63

solution turned blue in color. The solutions were stirred slowly during deposition to keep

the nickel in solution. If the solution was left over time before deposition, the solution

would turn back to green in color with a decrease in the pH indicative of carbon dioxide

absorbing into the solution.

Based on the JCPDS Database, the following peaks should be observed for nickel

metal, in a random pattern.

Table 2.3: The PDF data of XRD patterns of standard nickel metal from the JCPDS Database (PDF#04-0850) PDF#04-0850- JCPDS Database. 2θ hkl I/I0 44.507 111 100 51.846 200 42 76.370 220 21 92.944 311 20 98.446 222 7

2.5.1 Nickel Sulfate Hexahydrate

In nickel sulfate hexahydrate, the water molecules are bound to the nickel and the

sulfate ion is loosely associated through hydrogen bonding to the nickel center.

Figure 2.20: Nickel Sulfate Hexahydrate Structure.

64

In most previous work, sodium hydroxide was used as the base source for alkaline

deposition with a pH›12. Some work has been examined in a pH range of 9.3-9.5; where

ammonium hydroxide was used as the base source for most experiments. For this work,

both pH ranges were examined. Nickel sulfate hexahydrate when combined with

ammonium hydroxide or sodium hydroxide formed nickel hydroxide upon being mixed.

The nickel hydroxide was a green powder. Nickel sulfate hexahydrate was complexed

with a number of ligands to withstand the basic environment required for deposition. The

main base used was ammonium hydroxide with a pH range of 9.3-9.5 to avoid the

harsher conditions associated with a pH›12.

Table 2.4: Possible ligands for nickel; nickel complex pKa values. Ligand pKa1 pKa2 pKa3 pKa4 pKa5 pKa6 Ammonia 2.80 5.04 6.77 7.96 8.71 8.74 Acetate 1.12 1.81 Triethanolamine 2.7

Figure 2.21: Nickel ammonia.

65

Figure 2.22: Nickel acetate.

Figure 2.23: Nickel triethanolamine.

In previous work, it was stated that ammonia was used as the complexing agent

for nickel, specifically in the form of ammonium hydroxide. The ammonium hydroxide

was used as the ligand source and the base source. Once the ammonium hydroxide was

added to the nickel solution, the nickel precipitated out immediately, so this was not

examined as a possible ligand source.

66


Solution 1: 0.5 M NiSO4.6H20, 0.5M Sodium Acetate, 1M NH4OH at pH=9.3

Solution 3: 0.5 M NiSO4.6H20, 0.5M Triethanolamine, 1M NH4OH at pH=9.3

Sodium acetate appears to be the best possible ligand of those examined. With

the acetate, the nickel remained in solution upon the addition of hydroxide, but the

acetate released the nickel when the potential reached deposition conditions.

Nickel sulfate hexahydrate, with sodium acetate acting as a complexing agent,

appeared to stay in basic solution (with ammonium hydroxide) well over time without

stirring. Although the acetate did appear to keep the nickel in solution, the deposit was

very rough and uneven. A pulse method was attempted to obtain a more uniform surface

morphology. Pulse current plating and rotating disc electrode experiments have been

found to give better deposits. Pulse current plating has been examined as a possible

technique to increase adhesion to the stainless steel substrate. In the literature review, it

was noted that nickel deposits tend to be uneven in morphology due to hydrogen

embitterment [8]. Pulse plating was used where the shorter on-time resulted in more

durable plating. A deposit needs to be formed that allows for the release of hydrogen gas

upon heating, and with the smaller grain sizes this was possible without destruction of the

coating.

67

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.00.05

0.04

0.03

0.02

0.01

0.00

-0.01


Stripping peak=-0.4542V

Cur

rent

(A)

Potential (V)

Figure 2.24: CV of nickel sulfate hexahydrate, sodium acetate, and pH=9.32 with 1M

NH4OH.

The stripping peak of nickel was observed at E= -0.4542V in figure 2.24 for the

CV of nickel sulfate hexahydrate with sodium acetate at a pH=9.32 with 1M NH4OH,

which is what is expected for nickel. A crossover occurred in the CV at E= -0.6715V.

The crossover represents the point at which the nickel began to form nucleation sites on

the stainless steel substrate. Upon running the CV, small amounts of nickel were plated

onto the working electrode once the scan was completed. Based on this CV (figure 2.24)

nickel sulfate hexahydrate/sodium acetate with pH=9.3 with 1M NH4OH was plated onto

a stainless steel disc at E1=-1.350V, E2=-0.275V using a pulse waveform. The delay time

was 1 second for each potential, and the solution was not stirred. A charge of +20.08C

was passed in 18 minutes with current ranging from +31.6 to +37.4mA.

68

The deposition potentials were determined based on the CV of the solution. E1

began just past where the lines appeared to crossover, meaning, where nucleation began

to occur. E2 was just before the reduction peak present for the nickel in the solution.

With this method, it was hoped that a uniform layer of nickel would be formed on the

stainless steel disc, and then when brought to the lower potential, some of the top layer of

this deposit would be removed. When the potential was then brought back to the higher

value, the nucleation would again commence on this smoother surface, and a smoother

deposit would be created.

The nickel sulfate hexahydrate, sodium acetate solution was plated at a high

potential to ensure a deposit was formed. At lower potentials, although the stripping peak

of nickel was observed around E=-0.45V, no deposit formed. Small amounts of

precipitate formed onto the steel disc of the working electrode, but upon closer

examination, it appeared to just be base precipitate formed on the electrode surface.

Nickel deposited nicely at the higher potentials (above + 1.1V approximately), but the

deposit did not adhere well to the steel disc. The surface of the deposit seemed very

uneven and the plating occurred very quickly, usually in 30 minutes or less.

69

40 60 80 100

0

200

400

600

800

1000

1200

1400

20=9

8.48

17, N

icke

l 222

Pea

kS

tain

less

Ste

el 2

20 P

eak

20=9

2.97

60, N

icke

l 311

Pea

k

20=8

2.17

17, S

tain

less

Ste

el 2

11 P

eak

20=7

6.45

86, N

cike

l 220

Pea

k

20=6

4.84

82, S

tain

less

Ste

el 2

00 P

eak

20=5

1.87

85, N

ciek

l 200

pea

k

20=4

4.59

90, N

icke

l 111

Pea

kSt

ainl

ess

Stee

l 110

Pea

k

Inte

nsity

(CP

S)

2 Theta (Degrees)

Figure 2.25: XRD Pattern of nickel sulfate hexahydrate, sodium acetate, and pH=9.32

with 1M NH4OH.

Based on the XRD pattern (figure 2.25) of the nickel sulfate hexahydrate plating,

complexed with sodium acetate, it was clear that nickel is deposited in a random

orientation, based on JCPDS Database PDF 04-0850, onto the stainless steel electrode

with acetate as the complexing ligand and water solvent.

Nickel sulfate hexahydrate was also complexed with triethanolamine but no

deposit appeared on the electrode from this plating.

2.5.1.1 Nickel Sulfate Hexahydrate conclusions

Nickel sulfate hexahydrate is readily plated as pure nickel metal onto a stainless

steel substrate from alkaline conditions, with an added complexing ligand to keep the

70

nickel in solution. The base source used was ammonium hydroxide and the complexing

ligand used was sodium acetate. Sodium acetate is able to stabilize the nickel ions in

solution to allow then to reach the electrode surface and deposit as pure nickel metal.

2.5.2 Nickel Ammonium Sulfate Hexahydrate

In the nickel ammonium sulfate hexahydrate molecule, the atoms are similarly

arranged as in nickel sulfate hexahydrate. The nickel is bonded to the water molecules,

and the sulfate is hydrogen bonded through the water molecules. The ammonium atom is

then bound through the sulfate atom.

Figure 2.26: Nickel ammonium sulfate structure.

Nickel ammonium sulfate hexahydrate has a solubility of 10.6g/100cc at 25°C.

Dissolution of the nickel ammonium sulfate hexahydrate was examined as it does not

readily dissolve in water. Initially the solution was stirred overnight and in the morning

the remaining crystals were filtered out and the remaining solution was used to run the

experiments. Heat was also examined as a possible solution to dissolve the crystals and it

was found that heating while stirring is the most effective (time wise) way to dissolve the

71

crystals. The crystals also dissolved better in basic solution, so a little 1M NH4OH was

added to the crystal/water mixture before heating and stirring. Once dissolved, the nickel

in this solution plated well onto the stainless steel discs.

Nickel ammonium sulfate hexahydrate, when combined with ammonium

hydroxide did not precipitate out immediately. Therefore, no complexing agent was

examined.

The solution examined for nickel ammonium sulfate hexahydrate was as follows:

Solution 1: 0.5 M Ni(SO4)2(NH4)2.6H20 and 1M NH4OH at pH=9.3

Nickel ammonium sulfate hexahydrate did not require a complexing agent to

remain in solution. Although nickel ammonium sulfate hexahydrate did not readily

dissolve in water, with heating and stirring it easily entered solution. The solution

appeared green in color, but upon the addition of ammonium hydroxide the solution

turned dark blue in color.

72

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.03

0.02

0.01

0.00

-0.01

-0.02

-0.03



Nickel Reduction Peak, E=-0.4648V

Cur

rent

(A)

Potential (V)

Figure 2.27: CV of nickel ammonium sulfate hexahydrate, pH=9.3 with 1M NH4OH.

The CV of nickel ammonium sulfate hexahydrate (figure 2.27) showed a nickel

stripping peak at E= -0.4648V. This is around where the nickel reduction peak was

expected vs. SCE. Upon close examination, there was a crossover detected in the CV at

E= -0.8940V, and there was a second crossover present at E= -1.2518V. This CV was

based on a solution prepared by stirring the nickel ammonium sulfate crystals in water

overnight on high, then removing the excess crystals that did not dissolve. The remaining

solution was brought to a pH of 9.3 with ammonium hydroxide and the CV was run on

the solution.

A plating of this solution was attempted. After 4 hours of deposition time, +2.003

coulombs had passed the system and no deposit appeared on the substrate.

73

Heating and stirring the nickel ammonium sulfate hexahydrate crystals in water

and addition of ammonium hydroxide was then examined as a possible method to

dissolve the crystals. This proved to be a much more effective method and the crystals

were dissolved within a matter of minutes. The CV of the heated and stirred solution is

in figure 2.28.

-2 -1 0 1

0.02

0.01

0.00

-0.01

Cur

rent

(A)

Potential (V)

Crossover E=-0.904V

Nickel Stripping Peak E=-0.4835V

Figure 2.28: CV of nickel ammonium sulfate hexahydrate, pH=9.3 with 1 M NH4OH.

The CV (figure 2.28) of nickel ammonium sulfate hexahydrate has a stripping

peak of nickel present at E= -0.4835V. There was now only one crossover occurring at a

potential of E= -0.904V. Two depositions of this solution were performed, one a

potential of E= -1.150V and a second above the crossover at E= -1.250V. At a potential

74

of E=-1.150V, a charge of +50.19C was passed in 1 hour 25 minutes, with a current

ranging from + 8.760mA to +12.04mA. The solution was not stirred.

The x-ray pattern of the nickel ammonium sulfate hexahydrate deposit plated at

the potential E=-1.150V was examined to determine the nickel content. The steel disc

was removed from the epoxy prior to running the x-ray. The deposit was still adhered to

the stainless steel disc; therefore stainless steel peaks were present in the x-ray pattern.

40 60 80 100

0

200

400

600

800

1000

Stai

nles

s St

eel 2

11 p

eak

Stai

nles

s St

eel 2

00 p

eak

Nic

kel 2

22 p

eak

Nic

kel 3

11 p

eak

Nic

kel 2

20 p

eak

Nic

kel 2

00 p

eak

Nic

kel 1

11 p

eak

and

Stai

nles

s St

eel 1

10 p

eak

2The

ta=9

8.48

17

2The

ta=9

2.92

76

2The

ta=8

1.96

44

2The

ta=7

6.66

60

2The

ta=6

4.66

39

2The

ta=5

1.87

53

2The

ta=4

4.59

89

Inte

nsity

(CP

S)

2 Theta (Degrees)

Figure 2.29: XRD pattern of nickel ammonium sulfate hexahydrate and 1M NH4OH, not

in epoxy and plated at E=-1.50V.

The x-ray pattern (figure 2.29) of nickel ammonium sulfate hexahydrate of the

plating at a potential of E = -1.150V clearly had nickel in the deposit. The x-ray pattern

was of the deposit still adhered to the stainless steel substrate. From the x-ray pattern it

was clear nickel has been deposited in a random orientation, based on JCPDS Database

75

PDF 04-0850, onto the stainless steel substrate. Overall the stainless steel peaks

appeared to be minimal to the nickel peaks, thereby suggesting a strong nickel deposit.

The second plating was performed at a potential of E=-1.250V and passed a total

charge of +10.12C in 18 minutes, with a current ranging from +2.16mA to +9.96mA.

An XRD pattern of the nickel ammonium sulfate hexahydrate deposit plated at

E=-1.250V was performed. The nickel metal was still adhered to the stainless steel disc

but this disc had been removed from the epoxy background prior to running XRD.

40 60 80 100

0

100

200

300

400

500

600

700

20=9

8.20

74, N

icke

l 222

Pea

kSt

ainl

ess

Stee

l 220

Pea

k

20=9

3.18

32, N

icke

l 311

Pea

k

20=8

2.17

17, S

tain

less

Ste

el 2

11 P

eak

20=7

6.66

60, N

icke

l 220

Pea

k

20=6

4.66

39, S

tain

less

Ste

el 2

00 P

eak

20=5

1.67

12, N

icke

l 200

Pea

k

20=4

4.59

90, N

icke

l 111

Pea

kSt

ainl

ess

Stee

l 110

Pea

k

Inte

nsity

(CP

S)

2 Theta (Degrees)

Figure 2.30: XRD pattern of nickel ammonium sulfate hexahydrate, pH=9.3 with 1M

NH4OH, not in epoxy, plated at E=-1.250V.

The XRD pattern (figure 2.30) was a plating of nickel ammonium sulfate

hexahydrate, brought to a pH=9.32 with ammonium hydroxide, plated onto a stainless

steel disc. The x-ray pattern of nickel ammonium sulfate hexahydrate of the plating at E

= -1.250V also has nickel in the deposit. The x-ray pattern was of the deposit still

adhered to the stainless steel substrate, but had been removed from the epoxy. From the

76

x-ray pattern it is clear that nickel was deposited in a random orientation based on JCPDS

Database (PDF 04-0850) onto the stainless steel substrate. For the plating at a potential

of E=-1.50V the deposit appeared to possess more nickel in the plating, a charge of +

50.19 coulombs passed in a time frame of about one and a half hours. For the plating

performed above the crossover, a charge of +10.12 coulombs was passed in about 20

minutes. The longer deposition time of the plating before the crossover would explain

the higher nickel content of the deposit.

2.5.2.1 Nickel Ammonium sulfate hexahydrate conclusions

Nickel ammonium sulfate hexahydrate is an excellent nickel source for alkaline

deposition of pure nickel platings. Beyond the ligand provided by the base source,

ammonium hydroxide, nickel ammonium sulfate hexahydrate does not require an

additional complexing ligand to remain in solution. Upon the addition of base, the

solution turns from green to blue in color but a precipitate is not formed. If left overnight

a small amount of green precipitate is formed. Nickel ammonium sulfate hexahydrate

provides thick quality nickel coatings from alkaline solutions.

2.6 Borate Solutions for Nickel

Borate was examined as a possible electrolyte for the deposition solutions due to

the advantages of using borate when depositing pure nickel metal. A solution of 0.1M

borate was used consistently throughout the following experiments. An advantage to

using borate is no complexing ligand is required for deposition, past the ammonium

ligand provided by the base source.

77

2.6.1 Nickel Sulfate Hexahydrate in Borate

-1 0 10.020

0.015

0.010

0.005

0.000

-0.005

Cur

rent

(A)

Potential (V)



Figure 2.31: CV of nickel in borate solution, pH=9.3 with 1M NH4OH.

The CV (figure 2.31) of nickel sulfate hexahydrate in borate solution, at a pH of

9.3 with 1M NH4OH has a nickel stripping peak present at E=-0.4588V, which is

expected for nickel based on our previous experiments. The following parameters were

followed for deposition: E1= E3= -1.350V, E2=- 0.600V, delay 1= 10.0 sec, delay 2= 1.0

sec and the solution was continuously stirred. After a charge of + 11.76C had passed the

electrode was checked for deposition and silver colored metal was present on the

electrode. The electrode was replaced in solution and deposition continued at the above

parameters until a charge of +56.36C was passed. The deposit was now black in color.

78

This black color is a common effect of a nickel deposition being placed back into a

plating solution and more potential being passed through the cell.

40 45 50 55 60 65 70 75 80 85 90 95 1000

200

400

600

Inte

ntsi

ty (C

PS)

2 Theta (Degrees)

44.7

SS

110

51.9

Ni 2

00

76.7

Ni 2

20

93.0

Ni 3

11

97.9

SS

220

Figure 2.32: XRD pattern of nickel sulfate hexahydrate in 0.1M borate solution, pH=9.3

with 1M NH4OH.

Based on figure 2.32, nickel deposits out of borate solution in a random

orientation, based on JCPDS Database PDF 04-0850. Nickel gives an even deposit in

borate solution with a strong adhesion to the underlying substrate. A pulse plating

method had previously been determined to be the best deposition method for nickel, so

that method was also employed in the borate solutions.

2.6.1.1 Nickel sulfate hexahydrate in borate conclusions

Nickel sulfate hexahydrate readily deposits out as pure metal from borate

solutions. A complexing ligand, aside from the ligand provided by the ammonium

79

hydroxide base source, is not needed to keep the nickel in solution. The nickel sulfate

hexahydrate forms thick pure metal deposits onto stainless steel when deposited from a

borate solution. A step potential method shows better results, such as a more even

coating, compared to direct potential methods.

2.6.2 Nickel Ammonium Sulfate Hexahydrate in Borate

Nickel ammonium sulfate hexahydrate in borate solution was plated based on

previous nickel experiments with the following parameters: E1=E3=-1.350V, E2=-

0.600V, delay 1=+10.0sec, delay 2=+1.0 sec, while continuously stirring the solution. A

charge of +95.07C was passed in 2.5 hours. The average current was 18.0mA. There

was a strong metal deposition on the electrode and when x-rayed for confirmation (figure

2.33) and the presence of nickel deposited in a random orientation, based on JCPDS

Database PDF 04-0850, was confirmed. Nickel from the nickel ammonium sulfate

hexahydrate borate solution gave a very even deposit, when a pulse plating method was

used.

80

40 45 50 55 60 65 70 75 80 85 90 95 1000

200

400

600

Inte

nsity

(CPS

)

2 Theta (Degrees)

44.7

SS

110

52.0

Ni 2

00

76.9

Ni 2

20

82.2

SS

211

93.2

Ni 3

11

98.6

SS

220

Figure 2.33: XRD pattern of nickel ammonium sulfate hexahydrate in 0.1M borate

solution, pH=9.3 with 1M NH4OH.

2.6.2.1 Nickel Ammonium Sulfate Hexahydrate conclusions in borate solutions

Nickel ammonium sulfate hexahydrate also provides quality pure nickel platings

from borate electrolytic solutions. As examined previously, a step potential method

results in a more uniform, and smoother deposit then a direct potential method.

2.7 Nickel and Zinc Conclusions

From the x-ray patterns, it has been determined that the best possible complexing

agent examined was sodium acetate. The sodium acetate kept the metals in solution for

extended times, and along with stirring of the solution, nice deposits were obtained for

81

both nickel and zinc metals. From a direct potential method, the zinc deposit was smooth

and even and exhibited great adhesion to the steel surface. The nickel deposit appeared

to be very uneven and did not show great adhesion to the steel surface. When plated in a

step potential method, both the zinc and nickel metals showed much better adhesion to

the underlying stainless steel substrate, and more even, smooth deposits. Borate

electrolytic solutions also offered an alternative to a complexing ligand for the zinc and

nickel, as both zinc and nickel were deposited out of borate baths, again with a step

potential method being ideal.

2.8 Bath Conditions

Ammonium hydroxide was used as the primary base in this work. A pH of 9.3

was found to be optimal for the plating conditions. The metal and complexing agent

concentrations were 0.5 molar, in a 1:1 ratio of metal: ligand concentration. A plating at

or around the crossover which is present in the CV was optimal for the metal to plate.

Below the crossover point seen in the CV pattern, no deposition was observed. The

deposition time averaged between 20-60 minutes depending on the potential used.

Sodium acetate was an optimal complexing agent for both nickel and zinc.

Borate was found to be extremely useful in this deposition. Ammonium

hydroxide is used as the base source for this work and the ammonia is able to complex

the nickel and zinc in solution to stabilize the metal cations before deposition, but

working in alkaline solutions offers up an influx of hydroxide ions that readily combine

with the metal cations to form metal hydroxides. In aqueous solution, there is not a lot to

stabilize these metal cations, even with the complexing agent of ammonia and the metals

82

readily precipitated out of solution. With the addition of borate, the metals remain in

solution for extended periods of time, thereby making deposition easier.

Pulse plating was also found favorable in relation to borate solutions. When

applying a constant potential the metal deposits tended to adhere very weakly to the

stainless steel substrate, forming many flakes that fell into the solution. When pulsed

between 2 potentials, the deposit adhesion became much stronger and was difficult to

remove from the stainless steel substrate.

2.9 Summary

Zinc and nickel metals were readily deposited out of the electrolytic baths

employed. Zinc sulfate monohydrate is the zinc salt of choice for deposition because it

readily dissolves in aqueous solutions and zinc metal is easily deposited out of solution

onto a stainless steel electrode. Nickel sulfate hexahydrate and nickel ammonium sulfate

hexahydrate are sources for nickel metal in solution, both providing clear nickel

depositions onto a stainless steel electrode. The one advantage to nickel ammonium

sulfate hexahydrate is that a ligand is not required to keep the nickel ions in solution; it is

stabilized by the ammonia in the complex. Clear deposits have been obtained at a pH

range of 9.3, which is less caustic then working at a pH range ≥12 as seen throughout the

literature. Acetate is found to be the optimal ligand for zinc and nickel deposition

because pure metal deposits are easily obtained with this ligand, and can easily be

employed as a common ligand in a bath of both metal species to obtain the alloy phase

desired for the optimal corrosion protection. A borate solution also offers strong metal

deposits for both zinc and nickel metals, and borate can easily be employed for a bath

83

containing both metal species for alloy deposition. Pulse potential plating is best for

nickel deposition, resulting in a more uniform, smoother deposit. A smooth deposit is

important for corrosion resistance because corrosion cells are more readily formed on

uneven deposits, the smoother and more uniform the deposit, the longer the coating can

offer protection to the underlying substrate.

84

CHAPTER 3

ZINC AND NICKEL CO-DEPOSITION IN ALKALINE SOLUTIONS

3.1 Introduction

The deposition of zinc-nickel alloys onto stainless steel is a very important

application in the field of corrosion resistance. In this study, we have determined ways to

deposit the alloy in alkaline conditions, under milder working conditions. Previously,

zinc nickel platings from alkaline baths have been performed in a pH≥12 range, which

involves very caustic working conditions. Zinc-nickel platings deposited in acidic baths

do not exhibit the same metal uniformity in the deposits as exhibited with alkaline baths

so the platings from acid bath deposition do not protect the underlying substrate as well

as from an alkaline bath. My work focused on depositions in the alkaline range, but in a

less caustic pH range, which can be easily applied to larger production of these alloys.

Ammonium hydroxide was used as the base throughout all experiments to reach a pH

range of 9.3-9.4. The data obtained from the deposition of pure zinc and nickel metals

was applied here to determine the optimal plating conditions for zinc and nickel alloys

within the 9.3-9.4 pH range as exhibited with the pure metals. Direct potential plating

and step potential plating were both examined as plating techniques for deposition of the

alloy phase.

The gamma phase alloy (Zn5Ni21) has been found to be the best at corrosion

resistance in past studies, so deposition of this phase is examined. Based on the JCPDS

database, the gamma phase x-ray diffraction pattern has the following reflections:

85

Table 3.1: The PDF data of XRD patterns of standard zinc-nickel alloy, gamma phase metal from the JCPDS database (PDF #06-0653). 2θ hkl I/I0 24.640 211 60 37.425 321 40 42.844 330 100 47.621 332 60 49.843 422 60 51.973 510 60 56.102 521 40 62.258 600 80 64.078 611 60 69.997 622 60 71.588 631 80 73.261 444 80 75.442 550 80 78.688 552 100 80.269 642 40 85.572 651 60 89.358 741 100 90.673 820 60 92.537 653 40 94.261 660 100 95.940 750 80 97.695 662 80 99.397 752 60

86

Figure 3.1: XRD pattern gamma phase alloy.

In figure 3.1, for the XRD pattern of the gamma phase alloy, the 330 peak has

been clearly marked because in alkaline deposition, the gamma phase alloy deposits

preferentially on the stainless steel substrate as the 330 plane so this peak has been

monitored in this thesis work to confirm the presence of gamma phase. Other select

gamma phase peaks do appear when the deposit is mainly zinc metal in the deposit, with

some gamma phase contamination, but when pure gamma phase alloy is deposited, only

the 330 reflection is present.

3.2 Initial Studies

Initially zinc and nickel were combined in solution in equal molar quantities and

deposited based on the deposition potentials from pure zinc and nickel metals. Zinc

87

requires a large overpotential for deposition to occur. Direct potential deposition and step

potential plating were both examined as possible methods. In both the direct potential

deposition method and the step potential method, the potentials were determined based

on CV’s of the combined zinc and nickel electrolytic solutions.

Initially the metal salts were combined in equal molar concentrations and

depositions were performed but gamma phase was not obtained, at least not exclusively.

In a 1:1 molar ratio of zinc sulfate monohydrate, with nickel sulfate hexahydrate and

sodium acetate as the complexing ligand, the following CV (figure 3.2) was obtained.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.00.05

0.04

0.03

0.02

0.01

0.00

-0.01

-0.02

Cur

rent

(A)

Potential (V)

E=-0.872V

E=-0.570VE=-0.712V

Crossover E=-1.1V

Crossover E=-1.35V


.6H2O and 1.0M acetate in solution,

pH=9.3 with 1M NH4OH.

Figure 3.2 is a CV of 0.5M ZnSO4.H2O, 0.5M NiSO4

.6H2O, 1.0M Na+CH3COO-

at a pH=9.3 with 1M NH4OH. This CV has one sharp metal stripping peak at E=-

88

.0872V, believed to be due to zinc metal. In pure zinc solutions, the zinc stripping peaks

in solutions with acetate ligand were observed around E=-0.997V to E=-1.2V. The

stripping peak is shifted about 0.12V, which is due to the nickel in solution. The

potential a metal can reduce at in solution and then strip off of a substrate is dependent

upon what is in the solution. The only variable that has changed here is the presence of

nickel in the solution and the zinc peak has shifted due to the nickel presence. The nickel

stripping peak has also shifted. In aqueous solution with an acetate ligand the nickel

stripping peak is normally observed around a potential of E=-0.45V. Here the nickel

stripping peak is a very broad peak from E=-0.712V to E=-0.570V. Throughout the

literature it is noted that reduction peaks, and therefore anodic stripping peaks will shift

in solution, dependent upon other metals involved in the reduction. In consecutive CVs

of similar solutions, the nickel stripping peak is consistently present around E=-0.65V.

89

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

0.010

0.005

0.000

-0.005

-0.010

Nickel Stripping Peak E=-0.6478VZinc Stripping Peak E=-0.8878V

Cur

rent

(A)

Potential (V)

Crossover = -1.1013V


.6H2O, 1.0M Na+CH3COO-, pH=9.32

with 1M NH4OH.

In figure 3.3, the CV of 0.5M ZnSO4.H2O, 0.5M NiSO4

.6H2O with 1.0M

Na+CH3COO-, at a pH of 9.32 with 1M NH4OH, the zinc stripping peak is again present

at E=-0.8878V and the nickel stripping peak is present at E=-0.6478V, both shifted from

their original values in pure zinc and pure nickel deposition baths. A crossover is present

at E=-1.1005V. The nickel stripping peak has shifted from E=-0.570V to E=-0.6478V,

but based on consecutive CVs of zinc and nickel solutions, a nickel stripping peak around

E=-0.65V is most common.

90

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.00.03

0.02

0.01

0.00

-0.01

-0.02

Nickel Stripping Peak E=-0.6342VZinc Stripping Peak E=-0.8578V


Cur

rent

(A)

Potential (V)


.6H2O, 0.25M Na+CH3COO-, pH=9.34

with 1M NH4OH.

In figure 3.4 the nickel stripping peak is observed at E=-0.6342V and the zinc

stripping peak is observed at E=-0.8578V. A crossover is present at E=-1.0647V which

is consistent with previous CVs based on the same electrolytic deposition baths.

Based on the CV (figure 3.2), the solution was deposited at a potential E1=-

1.450V, Delay 1=100 mSec, E2=-0.4V, and the solution was stirred during deposition.

When deposited at less negative potentials, no deposition occurred. The electrode would

appear milky before rinsing from small amounts of solution on the working electrode, but

no actual metal was deposited. A large overpotential is required to deposit the zinc metal

onto the stainless steel electrode. The deposition time was 22 minutes and a charge of

91

+21.55C was passed with current ranging from +21.60mA at the beginning of the

deposition to +12.34mA at the end of deposition.

40 60 80 1000

200

Inte

nsity

(CP

S)

2 Theta (degrees)

43.5

Zn

101

44.5

SS

110

64.7

SS

200

82.1

SS

211

Figure 3.5: XRD pattern of plating from 0.5M ZnSO4.H2O, 0.5M NiSO4

.6H2O, 0.25M

Na+CH3COO-, pH=9.34 with 1M NH4OH.

Based on this XRD pattern (figure 3.5), it is clear the zinc is being preferentially

deposited (the 101 plane, pdf 04-0831) but the gamma phase alloy is not present. In

alkaline solutions the gamma phase preferentially deposits as the 330 plane which has a

reflection at 2θ=42.88◦ which is not observed in this x-ray pattern. A 2:1 zinc-nickel

molar ratio was also examined as a possible plating solution, which contained 0.25M Ni,

0.5M Zn, 0.25M acetate with 1M NH4OH, pH=9.3. Based on previous data this solution

was deposited at a potential of E=1.5V for 34 minutes with a total charge of +12.03C

92

being passed. The electrode appeared to have a dull metal plated onto it. Based on the

XRD pattern (figure 3.6), zinc metal is present in the deposit and small amounts of

gamma phase represented by the peaks at 2θ=62.4◦ (600 reflection) and 2θ=78.5◦ (552

reflection). When pure gamma phase is obtained from an alkaline bath, gamma phase is

preferentially deposited as the 330 reflection, as confirmed in my later work.

20 30 40 50 60 70 80 90 100 110

0

200

400

600

800

1000

1200

Inte

nsity

(CP

S)

2 Theta (degrees)

36.6

Zn

002

43 Z

n 10

144

.8 S

S 11

0

62.4

Gam

ma

Phas

e 60

065

.1 S

S 20

0

78.5

Gam

ma

Phas

e 55

282

.2 S

S 21

1

Figure 3.6: XRD pattern from 0.5M ZnSO4.H2O, 0.25M NiSO4

.6H2O, 0.25M

Na+CH3COO-, pH=9.3 with 1M NH4OH.

In figure 3.6 small amounts of gamma phase are present among a strong zinc

deposition present on the electrode. The gamma phase peaks are shifted from the

93

expected values by +0.142 degrees for the 600 reflection and -0.188 degrees for the 552

reflection.

The solution must be adjusted to allow for maximum gamma phase deposition.

Zinc and nickel salts with an acetate complexing ligand were dissolved and brought to a

pH of 9.3 with 1M ammonium hydroxide separately, then combined for deposition.

-2 -1 0 10.010

0.008

0.006

0.004

0.002

0.000

-0.002

-0.004

-0.006

-0.008

-0.010

Cur

rent

(A)

Potential (V)



Crossover E=-1.12V


.6H2O, 0.5M Na+CH3COO-, pH=9.32

with 1M NH4OH.

In the CV (figure 3.7) there are 2 separate stripping peaks present, E=-0.648V,

corresponding to nickel metal stripping off the electrode, and E=-0.878V, corresponding

to zinc metal stripping off the electrode, with a crossover present at E=-1.12V. The

94

crossover gives an indication of where nucleation sites begin to appear on the substrate.

Based on this CV, the solution was plated at E1=E3=-1.4V, E2=-1.0V and delay 1=delay

2=1 sec. The deposition lasted 3 hours 45 minutes and +20.04C of charge was passed.

The current ranged from +0.4780 mA to 5.906mA. No metal was present on the

electrode after the deposition.

Zinc sulfate monohydrate and nickel ammonium sulfate hexahydrate were

examined in equal molar concentrations.

-1.5 -1.0 -0.5 0.0 0.5 1.00.08

0.06

0.04

0.02

0.00

-0.02

Cur

rent

(A)

Potential (V)


E=+0.142V

Crossover E=-1.112VCrossover E=-1.28V


Figure 3.8: CV of 0.5M ZnSO4.H2O, 0.5M Ni(NH4)2(SO4)2

.6H2O, 0.5M Na+CH3COO-,

pH=9.3 with 1M NH4OH.

In figure 3.8, the CV shows a metal stripping peak at E=--0.936V which is

believed to be due to zinc metal, a stripping peak at E=-0.62 which is believed to be due

to nickel metal and a small stripping peak at E=+0.142V whose source is unknown.

95

There are 2 crossovers present at E=-1.112V and E=-1.28V. The solution was deposited

at E1= E2 =-1.350V, E2=-0.400V, delay 1=delay 2= 1 sec, and the solution was stirred

throughout the deposition. A total of +13.00C of charge was passed, with the current

ranging from +10.28mA to 11.93mA. The deposition lasted 23 minutes.

20 30 40 50 60 70 80 90 100 110

0

100

200

300

400

500

600

Inte

nsity

(CP

S)

2 Theta (degrees)

35.1

Zn

002

43.1

Zn

101

44.6

SS

110

62.6

Gam

ma

Phas

e 60

064

.8 S

S 20

0

73.7

Gam

ma

Phas

e 44

4

79 G

amm

a Ph

ase

552

82.1

SS

211

98.7

SS

220

Figure 3.9: XRD pattern from solution of 0.5M ZnSO4.H2O, 0.5M Ni(NH4)2(SO4)2

.6H2O,

0.5M Na+CH3COO-, pH=9.3 with 1M NH4OH.

Figure 3.9 shows there is again strong zinc deposition present on the electrode

with small amounts of gamma phase being deposited with the zinc metal. The gamma

phase peaks are again shifted from the expected values by 0.342 degrees for the 600

reflection, 0.439 degrees for the 444 reflection and 0.312 degrees for the 552 reflection.

96

3.3 Chronocoulometry

Based on the pure zinc and nickel depositions from baths containing both metals,

chronocoulometry was employed to determine the diffusion coefficients of zinc and

nickel metals in the electrolytic baths, to better determine the zinc:nickel ratio needed for

alloy deposition to occur. Chronocoulometry (CC) is a technique where a potential step

is applied to the system and the charge is measured versus time.

Figure 3.10: Chronocoulometry diagram.

97

CC can be used to accurately determine the kinetic rate constant of a depositing

species. The Cottrell equation is used to determine the diffusion coefficient of the

absorbing species.

i=(nFAC0D01/2)/(π1/2t) Eq. 3.1

Where: i= Current, A n= Number of electrons transferred A= Area of electrode, cm2 C0= Concentration of species of interest, mol/cm3 D0= Diffusion Coefficient, cm2/sec t= time, seconds

When integrated, the Cottrell equation can be plotted as the Anson plot, with the charge

(Q) as a function of t½.

Figure 3.11: Anson plot diagram.

98

The electrode area can be measured physically or electrochemically.

Electrochemical area can differ from physical area of an electrode dependent on the

roughness of the electrode surface. To determine electrode area, a chronocoulometry

experiment is performed on ferricyanide, which has a known diffusion coefficient of 7.6

x 10-6 cm2/sec. For ferricyande, n is 1, because one electrode is transferred. The

concentration used in the experiments was 1.1 x 10-6 mol/cm3. The electrochemically

active area was determined to be 0.80 ± 0.18 cm2. The physical area of the electrode was

0.7932cm2.

The diffusion coefficients of zinc and nickel in alkaline solution with acetate

ligand were determined to be 2.86 x 10-4 ± 8.33 x 10-5 for nickel and 1.13 x 10-4 ± 6.73 x

10-5 for zinc. The diffusion coefficient for nickel is slightly larger than the diffusion

coefficient of zinc, demonstrating the nickel concentration might need to be controlled in

this solution. Dependent upon the solution used the diffusion coefficient data can be used

to determine the optimal concentrations in the bath.

3.4 Linear Sweep Voltammetry

Linear sweep voltammetry (LSV) is a technique used to determine the deposition

current of species in solution, and the effects of secondary metals in a solution can be

examined. In LSV the electrode potential is varied at a constant rate.

99

-1.5 -1.0 -0.50.04

0.03

0.02

0.01

0.00

Cur

rent

(A)

Potential (V)

Nickel

Zinc Zinc-Nickel Alloy

Figure 3.12: LSV of zinc, nickel and zinc-nickel alloy.

Figure 3.12 is the linear sweep voltammetry plot of zinc, nickel and zinc-nickel

combined as a plot of the current versus potential. All solutions were swept from

potentials of E=-1.5V to open circuit potential (OCP, E=+0.060V). Individually zinc has

an anodic stripping peak present at E=-1.1052V and nickel has an anodic stripping peak

present at E=-0.4804V. When combined in solution, the zinc and nickel anodic stripping

peaks are clearly shifted from the original values. In the combined solution the zinc

anodic stripping peak is present at E-=0.679V and the nickel stripping peak is present at

E=-0.5444V. Clearly the anodic stripping potentials of the zinc and nickel metals have

100

been shifted in the combined solution, showing the metals affect the deposition of one

another when combined.

3.5 Atomic Absorption Analysis

The depositions of interest were examined with atomic absorption spectroscopy

(AAS) to determine the zinc and nickel percentages present in the deposited films. The

film deposit was dissolved in 20mL of a 3M HNO3 solution, and once dissolved diluted

to 100 mL with DI H2O. One mL of this diluted metal solution was diluted to 50 mL of

solution with DI H2O, to make an appropriate concentration range for atomic absorption

analysis. A standard addition method was used to determine the concentrations of the

metals. In AAS interfering species can cause false measurements or mask ions of

interest, and since both zinc and nickel were present a standard addition method was used

to obtain more accurate results.

3.6 Alkaline Metal Deposition from Aqueous Solution with Acetate Ligand

Acetate was able to efficiently complex both zinc and nickel in their respective

solutions so this was examined as a ligand for both zinc and nickel present in the same

solution. The acetate concentration was minimized in most deposition solutions to a one

molar equivalent to the metal in solution because ammonia should also be complexing

the metals in solution, acetate just adds the added stabilization needed to keep the metals

in solution in time to deposit. All solutions were stirred during deposition to keep the

zinc sulfate monohydrate and the nickel sulfate hexahydrate in solution.

101

3.6.1 Zinc Sulfate Monohydrate and Nickel Sulfate Hexahydrate

Zinc sulfate monohydrate and nickel sulfate hexahydrate with acetate as a

complexing ligand was examined in an aqueous solution for the deposition of zinc-nickel

alloys films. The general solution composition was the following:

ZnSO4.H2O + NiSO4

.6H2O + Na+CH3COO- + 1M NH4OH

3.6.1.1 Zinc-Nickel 1:1 Molar Ratio

The first ratio examined was a 1 to 1 molar equivalent of zinc to nickel in solution.

The diffusion coefficients of zinc and nickel were fairly close, with nickel being slightly

larger than zinc so a 1:1 molar ratio was examined. The molar ratios examined for zinc

sulfate monohydrate and nickel sulfate monohydrate were 0.1M ZnSO4.H2O, 0.1M

NiSO4.6H2O, 0.1M Na+CH3COO- and a pH of 9.39 with 1M NH4OH.

102

-2 -1 0 10.02

0.00

Cur

rent

(A)

Potential (V)



Figure 3.13: CV of 1:1 ratio of zinc-nickel, with acetate, pH=9.39 with 1M NH4OH.

In the CV (figure 3.13) there is only one stripping peak present at E=-0.4194V,

which is due to nickel metal being stripped off the electrode. There is a crossover at E=-

0.9485V. This solution was plated based on this CV with a potential step method under

the following parameters: E1= -1.1V, E=-0.66V, delay 1=60.0 sec, delay 2=15.0 sec.

The deposition time was one hour 13 minutes and a total of +18.23C was passed. A

strong metal deposition was observed on the electrode so this was x-rayed for

confirmation.

103

35 40 45 50 55 60 65 70 75 80 85 90 95 1000

500

Inte

nsity

(CPS

)

2 Theta (Degrees)

44.4

56 N

i 111

Pea

k

51.7

49 N

i 200

Pea

k

76.0

05 N

i 220

Pea

k

92.9

90 N

i 311

Pea

k

98.4

03 N

i 222

Pea

k

Figure 3.14: XRD pattern, 1:1 Zn-Ni ratio with acetate ligand, pH=9.3 with 1M NH4OH.

It is clear from figure 3.14, the XRD pattern of the plating that only nickel is

present on the electrode. The nickel has been deposited in a random fashion, confirmed

by PDF#04-0850 (JCPDS Database).

Based on this XRD pattern, it was clear the nickel concentration in the bath

needed to be controlled, so deposition baths were altered accordingly.

3.6.1.2 Zinc-Nickel 3:1 and 4:1 Molar Ratios

A deposition bath containing 0.3M ZnSO4.H2O, 0.1M NiSO4

.6H2O, and 0.1M

Na+CH3COO- was examined. In the chronocoulometry results, nickel demonstrated a

slight preference to zinc to diffuse to the electrode surface, so based on this data and

previous deposits, the nickel concentration was lowered in the electrochemical bath. The

104

pH of this solution was 9.37 with 1M NH4OH. The CV of this solution is shown in

figure 3.15.

-1.5 -1.0 -0.5 0.00.030

0.025

0.020

0.015

0.010

0.005

0.000

-0.005

-0.010

Potential (V)

Zinc Stripping Peak E=-0.9574VNickel Stripping Peak E=-0.5823V

Cur

rent

(A)

Figure 3.15: 3:1 molar ratio of ZnSO4.H2O, NiSO4

.6H2O, pH=9.37 with 1M NH4OH

Based on this CV (figure 3.15) the deposition was performed with a potential step

method under these conditions: E1=-1.37V, E2=-1.00V, delay 1=60.0 sec, delay 2=20.0

sec. A total of +38.73C was passed. A small amount of metal deposition was observed

and was x-rayed for confirmation.

105

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 1050

100

200

300

400

500In

tens

ity (C

PS

)

2 Theta (Degrees)

42.9

53 G

amm

a ph

ase

330

peak

44.6

59 S

tain

less

Ste

el 1

10 P

eak

64.8

57 S

tain

less

Ste

el 2

00 P

eak

82.3

93 S

tain

less

Ste

el 2

11 P

eak

Figure 3.16: XRD pattern of gamma phase alloy deposited from 2:1 ZnSO4.H2O,

Ni(NH4)2(SO4)2.6H2O, pH adjusted with 1M NH4OH.

The XRD pattern (figure 3.16) confirms the presence of gamma phase Zn-Ni

preferentially deposited to the 330 plane (per pdf #06-0653). Previously, pure gamma

phase was not obtained, zinc metal deposition with gamma phase contamination was

observed. In this pattern, the gamma phase is preferentially deposited as the gamma 330

reflection which is expected in alkaline solution.

Atomic absorption spectroscopy was used to determine the metal concentrations

in the deposit.

106

y = 1.698x + 0.1518R2 = 0.9845

-1

-0.5

0

0.5

1

1.5

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Volume standard added (ppm)

Abs

orba

nce

Figure 3.17: AAS standard addition method- zinc concentration determination.

y = 0.3977x + 0.0073R2 = 0.9971

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

-1.5 -1 -0.5 0 0.5 1

Volume Standard Nickel Added (ppm)

Abs

orba

nce

Figure 3.18: AAS standard addition method- nickel concentration determination.

107

The metal concentration in the deposit was back calculated based on the results

obtained from the following equation for standard addition method:

Eq. 3.1

The alloy was composed of 83% zinc and 17% nickel. The nickel percentage in

the deposit is a little high compared to what has been found to offer the best corrosion

resistance, so a deposition bath with a slightly higher zinc concentration was examined.

A deposition bath containing 0.4M ZnSO4.H2O, 0.1M NiSO4

.6H2O and 0.4 M

Na+CH3COO- was examined. The pH of this solution was 9.35 with 1M NH4OH. The

deposition was performed with a potential step method under these conditions: E1=-

1.37V, E2=-1.00V, delay 1=60.0 sec, delay 2=20.0 sec. A total of +27.86C was passed

in 41 minutes. A strong metal deposition was observed and was x-rayed for

confirmation.

108

35 40 45 50 55 60 65 70 75 80 85 90 95 100

100

200

300

Inte

nsity

(CPS

)

A

42.9

03 G

amm

a Ph

ase

330

Figure 3.19: XRD pattern 4:1 ratio zinc to nickel.

Figure 3.19 shows a deposition of gamma phase zinc-nickel alloy being

preferentially deposited as the gamma 330 plane (per pdf #06-0653- JCPDS Database).

The deposit was analyzed with AAS and the zinc concentration in the deposit was 92.4%

and the nickel concentration was 7.6%, which is in the range of interest for maximized

corrosion protection (8-15%).

3.6.2 Zinc Sulfate Monohydrate and Nickel Ammonium Sulfate Hexahydrate

Zinc sulfate monohydrate and nickel ammonium sulfate hexahydrate with acetate as a

complexing ligand for the zinc in solution was examined in an aqueous phase for the

deposition of zinc-nickel alloys films. The general solution composition was the

following:

109

ZnSO4.H2O + Ni(NH4)2(SO4)2

.6H2O + Na+CH3COO- + 1M NH4OH


The first ratio examined was a 2:1 molar equivalent of zinc to nickel in solution. The

molar ratios examined for zinc sulfate monohydrate and nickel sulfate monohydrate were

0.2M ZnSO4.H2O, 0.1M Ni(NH4)2(SO4)2

.6H2O, 0.1M Na+CH3COO- and a pH of 9.36

with 1M NH4OH.

This solution was plated based on previous deposition data with a potential step

method under the following parameters: E1= -1.45V, E=-0.V, delay 1=60.0 sec, delay

2=20.0 sec. The deposition time was 36 minutes and a total of +14.80C was passed. A


confirmation. Based on previous gamma phase deposition XRD patterns, it was clear the

gamma phase preferentially deposits to the 330 plane so the sample was measured in a 2θ

range value of 35 to 50 degrees.

40 45 50

200

400

600

800

Inte

nsity

(CP

S)

2 Theta (D egrees)

42.8

95 G

amm

a Ph

ase

330


110

It is clear from figure 3.20, the XRD pattern of the plating that gamma phase has been

preferentially deposited to the 330 plane, confirmed by PDF#06-0653 (JCPDS Database).

The deposit was analyzed with AAS and the zinc concentration in the deposit was

89.8% and the nickel concentration was 10.2%, which is in the range of interest for

maximized corrosion protection (8-15%).


The second ratio examined was a 1:2 molar equivalent of zinc to nickel in solution.

The molar ratios examined for zinc sulfate monohydrate and nickel sulfate monohydrate

were 0.1M ZnSO4.H2O, 0.2M Ni(NH4)2(SO4)2

.6H2O, 0.1M Na+CH3COO- and a pH of

9.34 with 1M NH4OH.



2=20.0 sec. The deposition time was two hours 13 minutes and a total of +141.62C was

passed. A strong metal deposition was observed on the electrode so this was x-rayed for

confirmation. Based on previous gamma phase deposition XRD patterns, it was clear the

gamma phase preferentially deposits to the 330 plane so the sample was measured in a 2θ

range value of 35 to 50 degrees.

111

35 40 45 50

100

200

300

Inte

nsity

(CP

S)

2 Theta(Degrees)

42.9

03 G

amm

a Ph

ase

330


It is clear from figure 3.21 the XRD pattern of the plating that gamma phase has

been preferentially deposited to the 330 plane, confirmed by PDF#06-0653 (JCPDS

Database).


74.5% and the nickel concentration was 25.5%, which is outside in the range of interest

for maximized corrosion protection (8-15%). There is too much nickel present in the

deposit, so the controlled nickel concentration of a 2:1 zinc nickel ratio in the bath is

needed for the correct metal percentage in the depositing film.

112

3.6.3 Conclusions for Alkaline Metal Deposition in Aqueous Solution with Acetate

Ligand

Acetate is clearly able to stabilize the metal species in solution prior to deposition.

When analyzed with atomic absorption spectroscopy the best plating conditions were a

4:1 molar equivalent of zinc to nickel, with a 4 molar equivalent of acetate present. This

deposit contained 92.4% zinc and 7.6% nickel which are just below the optimal range for

gamma phase deposition of 8% nickel. The deposition of a 3:1:1 molar equivalent of

zinc, nickel and acetate had a deposit with 83% zinc and 17% nickel, which is just past

the optimal range of 15% nickel in the deposit. A deposit of a 2:1 ratio of zinc-nickel

would better obtain the optimal nickel range in the deposit.

Nickel ammonium sulfate hexahydrate is a great nickel source for deposition in

the gamma phase alloy. A 2:1 molar ratio of zinc-nickel with the nickel ammonium

sulfate and acetate to complex the zinc in solution provided a deposit with 89.8% zinc

and 10.2% nickel, which is optimal for the gamma phase alloy.

3.7 Alkaline Metal Deposition from Borate Solvent

Borate was examined as a possible electrolyte for the deposition solutions due to the

advantages of using borate when depositing pure nickel metal. A solution of 0.1M borate

was used consistently throughout the following experiments. An advantage to using

borate is no complexing ligand is required for deposition, past the ammonium ligand

provided by the base source. The general solution composition was the following:

ZnSO4.H2O + NiSO4

.6H2O + 0.1M Na+ borate + 1M NH4OH

113

3.7.1 Zinc Sulfate Monohydrate and Nickel Sulfate Hexahydrate

Zinc sulfate monohydrate and nickel sulfate hexahydrate with 0.1M borate solution as

the aqueous phase was examined for the deposition of zinc-nickel alloys films. The

general solution composition was the following:

ZnSO4.H2O + NiSO4

.6H2O + Na+ borate + 1M NH4OH

3.7.1.1 Zinc Nickel in a 1:1 Molar Ratio

The first ratio examined was a 1 to 1 molar equivalent of zinc to nickel in solution.

The molar ratios examined for zinc sulfate monohydrate and nickel sulfate monohydrate

were 0.1M ZnSO4.H2O, 0.1M NiSO4

.6H2O, 0.1M borate and a pH of 9.41 with 1M

NH4OH.

-2 -1 0 1

0.016

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0.000

-0.002

Cur

rent

(A)

Potential (V)


Crossover E=-1.084V



with 1M NH4OH.

114

This solution was plated based on the CV (figure 3.22) with a potential step

method under the following parameters: E1= -1.30V, E2=-1.1V, delay 1=60.0 sec, delay

2=10.0 sec. The deposition time was one hour 23 minutes and a total of +8.705C was

passed. A strong metal deposition was observed on the electrode so this was x-rayed for

confirmation.

35 40 45 50 55 60 65 70 75 80 85 90 95 100

200

400

600

800

Inte

nsity

(CPS

)

2 Theta (degrees)

36.3

54 Z

n 00

239

.005

Zn

100

43.2

54 Z

n 10

1

54.4

39 Z

n 10

2

70.2

01 Z

n 10

370

.597

Zn

110

82.0

98 Z

n 11

2

86.8

74 Z

n 20

1

Figure 3.23: XRD pattern, 1:1 Zn-Ni ratio in 0.1M borate, pH=9.41 with 1M NH4OH.

It is clear from figure 3.23 the XRD pattern of the plating that only zinc is present

on the electrode. The zinc has been deposited in a random fashion, confirmed by

PDF#04-0831 (JCPDS Database).

Based on this XRD pattern, it was clear the zinc concentration in the bath needed

to be controlled, so deposition baths were altered accordingly.

115

3.7.1.2 Zinc and Nickel in a 1:3 Molar Ratio

The second ratio examined was a 1 to 3 molar equivalent of zinc to nickel in

solution. The molar ratios examined for zinc sulfate monohydrate and nickel sulfate

monohydrate were 0.1M ZnSO4.H2O, 0.3M NiSO4


with 1M NH4OH. This solution was plated based on previous deposition data with a

potential step method under the following parameters: E1= -1.37V, E2=-1.0V, delay

1=60.0 sec, delay 2=20.0 sec. The deposition time was 21 minutes and a total of

+34.18C were passed. A strong metal deposition was observed on the electrode so this

was x-rayed for confirmation.

35 40 45 50 55 60 65 70 75 80 85 90 95 100

200

400

Inte

nsity

(CPS

)

2 Theta (Degrees)

44.6

01 N

i 111

51.8

00 N

i 200

76.5

93 N

i 220

92.9

7 N

i 311


116

It is clear from figure 3.24, the XRD pattern of the plating that only nickel is

present on the electrode. The nickel has been deposited in a random fashion, confirmed

by PDF#04-0850 (JCPDS Database).

Based on this XRD pattern, it was clear both the zinc and nickel concentrations in

the bath needed to be carefully controlled.

3.7.2 Zinc Sulfate Monohydrate and Nickel Ammonium Sulfate Hexahydrate in Borate

Nickel ammonium sulfate hexahydrate was also examined as a nickel source for

gamma phase alloy deposition in borate solutions. The general solution composition was

the following:

ZnSO4.H2O + Ni(NH4)2(SO4)2

.6H2O + 1M NH4OH


The first ratio examined was a 1:1 molar equivalent of zinc to nickel in solution. The

molar ratios examined for zinc sulfate monohydrate and nickel ammonium sulfate

hexahydrate were 0.2M ZnSO4.H2O, 0.1M Ni(NH4)2(SO4)2

.6H2O and a pH of 9.35 with

1M NH4OH.


method under the following parameters: E1= -1.45V, E=-1.30V, delay 1=60.0 sec, delay



confirmation.

117

35 40 45 50 55 60 65 70 75 80 85 90 95 100

200

400

Inte

nsity

(CPS

)

2 Theta(Degrees)

44.1

51 N

i 111

51.6

45 N

i 200

75.9

56 N

i 220

92.4

08 N

i 311

Figure 3.25: XRD pattern, 1:1 Zn-Ni ratio, pH=9.3 with 1M NH4OH.

It is clear from figure 3.25, the XRD pattern of the plating that pure nickel metal

has been randomly deposited on the electrode, confirmed by PDF#04-0850 (JCPDS

Database). Based on this XRD pattern, it was clear the zinc concentration in the bath

needed to be controlled, so deposition baths were altered accordingly.

3.7.2.2 Zinc Nickel 2:1 Molar Ratio

The second ratio examined was a 2:1 molar equivalent of zinc to nickel in solution.

The molar ratios examined for zinc sulfate monohydrate and nickel ammonium sulfate

hexahydrate were 0.2M ZnSO4.H2O and 0.1M Ni(NH4)2(SO4)2

.6H2O in 0.1M borate

solution a pH of 9.34 with 1M NH4OH.

118





confirmation.

35 40 45 50 55 60 65 70 75 80 85 90 95 100

100

200

Inte

nsity

(CP

S)

2 Theta (Degrees)

43.9

01 G

amm

a Ph

ase

330


It is clear from figure 3.26, the XRD pattern of the plating that gamma phase has

been preferentially deposited as the 330 plane, confirmed by PDF#06-0653 (JCPDS

Database).

119


84.4% and the nickel concentration was 15.6%, which is in the range of interest for

maximized corrosion protection (8-15%).

3.7.3 Conclusions for Alkaline Metal Deposition from Borate Solution

Borate solutions can be used to deposit out the pure metals from solution but did

not demonstrate as well for deposition of the gamma phase alloy. When using nickel

sulfate hexahydrate with zinc a 1:1 equivalent of zinc-nickel resulted in a deposit of pure

zinc metal. When the nickel concentration was increased in the plating solution the result

was a deposit of pure nickel metal.

When using nickel ammonium sulfate hexahydrate as the nickel source with a 1:1

ratio of zinc-nickel only nickel was deposited out of solution. When a 2:1 ratio of zinc-

nickel was used, a gamma phase deposit was obtained, and upon analysis with atomic

absorption spectroscopy the deposit was 84.4% zinc and 15.6% nickel, which is in the

optimal range for gamma phase deposition.

From borate solutions, when nickel ammonium sulfate hexahydrate is used as the

nickel source, gamma phase alloys can be easily deposited in the ratios desired for

optimal corrosion protection.

3.8 Conclusions for Zinc-Nickel Co-Deposition in Alkaline Solutions

Ammonium hydroxide was found to be the optimal base source for this work. A

working pH of 9.3-9.4 was used throughout the depositions. The zinc-nickel alloy

requires a large over potential to deposit onto the stainless steel electrode. A plating at or

120

around the crossover which is present in the CV was optimal for the metal to plate.

Below the crossover point, no deposition was observed. Sodium acetate was found to be

an optimal complexing ligand for both nickel and zinc in alkaline solution.

Borate was found to be extremely useful in this deposition when using nickel

ammonium sulfate hexahydrate as the nickel source.

Ammonium hydroxide is used as the base source for this work and the ammonium

is able to complex the nickel and zinc in solution to stabilize the metal cations before

deposition, but working in alkaline solutions offers up an influx of hydroxide ions that

readily combine with the metal cations to form metal hydroxides. In aqueous solution,

there is not a lot to stabilize these metal cations, even with the compexing agent of

ammonium and the metals readily precipitated out of solution. With the addition of

borate, the metals remain in solution for extended periods of time, thereby making

deposition easier. Borate was used to deposit the pure zinc and nickel metals, but when

combined, a nickel source of nickel ammonium sulfate hexahydrate is best for use in

borate solutions, this will provide you with the optimal zinc and nickel percentages

needed for optimal protection by the gamma phase alloy. Pulse plating was also found

favorable in relation to borate solutions. When applying a constant potential the metal

deposits tended to adhere very weakly to the stainless steel substrate, causing many flakes

that fell into solution. When pulsed between 2 potentials, the deposit become much

stronger and was difficult to remove from the stainless steel substrate.

A potential step method was found to be optimal for deposition of the metal films.

With this method we are able to obtain smoother deposits in a shorter time frame, making

this method desirable for the wider scope of applications. A large overpotential is

121

required for deposition, and this causes the metal to precipitate onto the electrode

quickly, and often unevenly. At lower potentials no deposition is observed. With this

potential step method, we are able to obtain the deposits at the high overpotentials

required for deposition to occur, and the deposits are smooth and even over the electrode

substrate.

The gamma phase zinc-nickel alloy (Ni5Zn21) was readily deposited out of

alkaline solutions with a preferred orientation to the 330 reflection. Ammonium

hydroxide offers a base source that allows us to work in less caustic conditions while still

reaching the basic ranges needed for optimal deposition. In an aqueous solvent system,

acetate offers support as a complexing ligand to both zinc and nickel in solution, to keep

them from precipitating out as metal hydroxides. The acetate is able to keep the metal

ions in solution long enough for metal deposition to occur. Borate solutions also offer

gamma phase deposition, best with nickel ammonium sulfate hexahydrate as the nickel

source for deposition.

Based on atomic absorption data, there are 3 optimal baths for gamma phase

deposition. The ratio of 4:1:4 zinc, nickel sulfate hexahydrate and acetate with a zinc

percentage in the deposit of 92.4% and a nickel percentage of 7.56%. The ratio of 2:1:1

of zinc, nickel ammonium sulfate hexahydrate and acetate in solution had a zinc

percentage of 89.8% and a nickel percentage of 10.2%. In borate solutions, the optimal

deposit was obtained from a 2:1 ratio bath of zinc and nickel ammonium sulfate

hexahydrate with an overall zinc percentage of 84.4% and a nickel percentage of 15.6%

in the deposit.

122

It has been concluded that the zinc-nickel gamma phase alloy is readily deposited

from an aqueous solution with acetate, with the nickel concentration being controlled in

the bath. In borate solutions, the zinc-nickel gamma phase alloy is easily deposited if

using nickel ammonium sulfate hexahydrate as the nickel source in solution. Pulse

plating is the preferred method for all deposits because it offers better adhesion to the

substrate surface and a much smoother deposit.

Pure zinc and nickel metal films were easily deposited out of alkaline baths in the

pH=9.3 range. Acetate was found to be a good complexing ligand for both zinc and

nickel in alkaline baths with water as the aqueous phase; a borate solution as the aqueous

phase also provided quality films with nickel ammonium sulfate hexahydrate as the

nickel source.

3.9 Differences from Literature

In the literature, work has been performed at very caustic pH ranges of ≥12, with

very little work being performed in the pH range of 9.3-9.5. This work focuses on the pH

range of 9.3-9.4, which offers milder working conditions for the scale of industry.

Sodium acetate was found to be a good complexing ligand for both zinc and nickel ions

in solution, offering up quality gamma phase alloys without contamination often seen in

acidic baths. Previous work has required elevated working temperatures and this work

has been completed at room temperature, again lending to the simplicity of industrial

scale work.

The gamma phase alloy was readily deposited out of solution when using nickel

ammonium sulfate hexahydrate as the nickel source in a borate electrolytic solution,

123

which had not been examined previously. Nickel ammonium sulfate hexahydrate stays in

solution without the addition of a complexing ligand in alkaline conditions, and readily

deposits out as pure nickel metal, and as zinc-nickel gamma phase when in solution with

zinc.

The step potential method was also extremely useful in this work. With the step

potential method, one is able to obtain quality films that adhere very strongly to the

stainless steel electrode surface. Without this method the deposits readily came off of the

stainless steel electrode surface, both as metal flakes and as the whole deposit just being

removed. With this method, the deposit is very hard to remove from the stainless steel

electrode surface.

3.10 Future work

The next steps for this thesis work include obtaining scanning electron

microscope (SEM images) of the deposits to determine uniformity and grain structure. It

has been noted in the literature that more compact grains are preferred for optimal

corrosion resistance.

The acetate aqueous solution will be examined to determine how the acetate is

able to stabilize the metals in solution, along with ammonia being present due to

ammonium hydroxide being utilized as the base source.

Ammonia absorption on the electrode surface as a chemisorb will be examined, to

determine if this is playing a role in gamma phase deposition, and if this has an effect on

the gamma phase being preferentially deposited to the 330 reflection.

124

The borate electrolytic solution will be examined more extensively to determine if

the borate is complexing the metal ions in solution, and why an additional complexing

ligand is not required as in aqueous phase solutions.

Corrosion protection properties of these films will be studied using

electrochemical techniques.

125

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