PERFORMANCE ANALYSIS OF 3D PRINTED 718 ALLOY

FOR CORROSIVE ENVIRONMENTS

A dissertation submitted to The University of Manchester for the degree of Master of Science (MSc.)

In the Faculty of Engineering and Physical Science

2015

DIEGO LANDETA

School of Materials

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List of Contents

LIST OF FIGURES ............................................................................................................................ 4

LIST OF TABLES.............................................................................................................................. 6

ABSTRACT ..................................................................................................................................... 7

DECLARATION ............................................................................................................................... 8

INTELLECTUAL PROPERTY STATEMENTS ...................................................................................... 9

ACKNOWLEDGEMENT ................................................................................................................ 10

1. CHAPTER 1 ......................................................................................................................... 11

1.1 INTRODUCTION ................................................................................................................. 11

1.2 SCOPE OF THE PRESENT WORK ......................................................................................... 13

1.3 OBJECTIVE ......................................................................................................................... 13

2 CHAPTER 2 ................................................................................................................................ 14

2.1 LITERATURE REVIEW ......................................................................................................... 14

2.1.1 Alloy 718 – General Information ................................................................................... 14

2.1.2 Applications of Alloy 718 .............................................................................................. 18

2.2 Microstructure .................................................................................................................. 19

2.2.1 Microstructural Analysis Techniques: SEM, EDX and XRD ............................................ 22

2.3 Electrochemical Principles ................................................................................................ 23

2.3.1 Corrosion Rate Fundaments ......................................................................................... 23

2.3.2 Electrochemical Measurements ................................................................................... 28

2.3.3 Regions of a Polarization Diagram ................................................................................ 31

2.3.3.1 Crevice Corrosion ...................................................................................................... 34

2.3.3.2 Pitting Corrosion ....................................................................................................... 34

2.4 Previous Experiments........................................................................................................ 36

3 CHAPTER 3 ................................................................................................................................ 39

3.1 EXPERIMENTAL METHODS AND MODES OF ANALYSIS ..................................................... 39

3.1.1 Background and Testing Plan ........................................................................................ 39

3.2 Sample Preparation and Surface Preparation .................................................................. 39

4 CHAPTER 4 ................................................................................................................................ 41

4.1 RESULTS AND DISCUSSIONS .............................................................................................. 41

4.1.1 Microstructure Analysis by SEM, EDX & XRD Methods. ............................................... 41

4.1.1.1 Scanning Electron Microscopy (SEM)........................................................................ 41

4.1.1.2 Energy Disperse X-ray Spectroscopy (EDX) ............................................................... 49

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4.1.1.3 X-ray Diffraction (XRD) .............................................................................................. 55

4.1.2 Electrochemical Tests.................................................................................................... 57

4.1.2.1 Open Circuit Potential Test (OCP) ............................................................................. 58

4.1.2.2 Linear Polarization Resistance (LPR) ......................................................................... 59

4.1.2.3 Full Polarization Tests ............................................................................................... 62

4.1.2.3.1 Full Polarization Under Condition 1 ...................................................................... 62

4.1.2.3.1.1 Corrosion Rate Evaluation via Tafel Extrapolation. .............................................. 64

4.1.2.3.2 Full Polarization Under Condition 2 ...................................................................... 66

4.1.2.3.3 Full Polarization Under Condition 3 ...................................................................... 67

4.1.2.3.4 Full Polarization Under Condition 4 ...................................................................... 69

4.1.2.4 Electrochemical Impedance Spectroscopy (EIS) ....................................................... 70

4.1.3 Devices Used for Microstructural Analysis and Electrochemical Experiments. ............ 74

5. CHAPTER 5 ......................................................................................................................... 78

5.1 CONCLUSIONS ................................................................................................................... 78

6. CHAPTER 6 ......................................................................................................................... 79

6.1 FUTURE WORKS ................................................................................................................ 79

7. CHAPTER 7 ......................................................................................................................... 80

7.1 REFERENCES ...................................................................................................................... 80

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LIST OF FIGURES Fig. 1 Scheme of the Process (Murr et al., 2012) ..................................................................... 12

Fig. 2 Micrograph of precipitates (30K magnification). The heat treatment applied was 1h

at 870oC. (Radavich, 1997) ....................................................................................................... 20

Fig. 3 Micrograph of and precipitate particles when electro-etched with CrO3 (10K

magnification). The heat treatment applied was 100h at 650oC. (Radavich, 1997) ............... 20

Fig. 4 Micrograph of depletion because of Phase (10K magnification). (Radavich, 1997)

.................................................................................................................................................. 21

Fig. 5 Anodic and Cathodic reactions on the surface of the alloy. .......................................... 24

Fig. 6 Evans Diagram showing the anodic and cathodic reactions. ......................................... 25

Fig. 7 Scheme of the Saturated Calomel Reference Electrode. ............................................... 27

Fig. 8 Electrochemical Cell with a working electrode (WE), a counter electrode (CE) and a

reference electrode (RE) coupled with a luggin probe. 7a a representative diagram from the

electrochemical cell, 7b the real electrochemical cell, and 7c internal view of the cell. ........ 28

Fig. 9 Open Circuit Potential plot. ............................................................................................ 29

Fig. 10 Equivalent circuit used to represent the Nyquist and Bode plots . ............................. 31

Fig. 11 Regions of a Polarization Diagram ............................................................................... 32

Fig. 12 Rest potential vs. time (up to 24hrs) curves recorded for IN718 with 0% (Re) in 1.0 M

H2SO4 solution at 25oC. (Amin et al., 2014) ............................................................................. 36

Fig. 13 Complex-plane impedance plots recorded for IN718 with 0% Re (black dots) in 1.0 M

H2SO4 solution at 25oC. (Amin et al., 2014) ............................................................................. 37

Fig. 14 Potentiodynamic anodic polarization curves recorded for the tested alloys in 1 M

H2SO4 + 0.6 M NaCl, at a scan rate of 1 mV/s at 25oC. (Amin et al., 2014) ............................. 38

Fig. 15 Testing Plan .................................................................................................................. 39

Fig. 16 Samples cut (red blocks) from the original chunk material (orange block). ................ 39

Fig. 17 Ceramic crucibles for the samples’ heat treatments. .................................................. 40

Fig. 18 BSE images of the “as received” as-AM sample. Note the complex grain structure

shown from the electron channelling contrast. ...................................................................... 43

Fig. 19 BSE images of the “solution annealed” sample showing the grain morphology due to

electron channelling contrast. ................................................................................................. 44

Fig. 20 BSE images of the “solution annealed plus age hardened” sample. ........................... 46

Fig. 21 Light optical micrographs of the “as received” (as-AM) sample prepared via

Electrolytic Etching (magnification 10x). a) Boundary between the immersed (etched) and

non-immersed regions, b) Central region from the immersed area. ...................................... 48

Fig. 22 Chemical composition for spectrum 2 in the “A.R.” sample. Note the high peak of Ni

denoting a high content of Ni as it can be anticipated for this alloy. ...................................... 49

Fig. 23 Chemical composition for spectrum 3 (23b), 4 (23c), 5 (23d) and 6 (23e) were

examined in the “S.A.” sample. ............................................................................................... 51

Fig. 24 Chemical composition for spectrum 8 (24b), 9 (24c), 10 (24d) and 11 (24e) were

examined in the “F.T.” sample. ................................................................................................ 53

Fig. 25 XRD pattern of the “A.R.” sample. ............................................................................... 55

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Fig. 26 XRD pattern of the “S.A.” sample. ................................................................................ 56

Fig. 27 XRD pattern of the “F.T.” sample. ................................................................................ 57

Fig. 28 OCP curves for “A.R.”, “S.A.” and “F.T.” samples. ........................................................ 58

Fig. 29 LPR curves for “A.R.”, “S.A.” and “F.T.” samples. 29a cyclic voltammetry graph and

29b cyclic voltammetry with log current. ................................................................................ 60

Fig. 30 LPR curve for “A.R. 1” sample with its trend line (black line) and the trend line

equation. .................................................................................................................................. 61

Fig. 31 Full Polarization for all the samples under Condition 1. .............................................. 63

Fig. 32 Tafel Extrapolation applied on sample AR1 in condition 1. ......................................... 65

Fig. 33 Full Polarization for all the samples under Condition 2. .............................................. 66

Fig. 34 Full Polarization for all the samples under Condition 3. .............................................. 68

Fig. 35 Crevice corrosion action on the samples. .................................................................... 68

Fig. 36 Full Polarization for all the samples under Condition 4. .............................................. 69

Fig. 37 Setup of the specimens inside the autoclave. ............................................................. 71

Fig. 38 Nyquist plot. ................................................................................................................. 72

Fig. 39 Bode Plot. ..................................................................................................................... 72

Fig. 40 Bode Plot. ..................................................................................................................... 73

Fig. 41 Potentiostat and PSTrace Software.............................................................................. 74

Fig. 42 Optical Microscope. ...................................................................................................... 75

Fig. 43 SEM - EDX microscope. ................................................................................................ 75

Fig. 44 Autoclave. ..................................................................................................................... 76

Fig. 45 XRD microscope. ........................................................................................................... 76

Fig. 46 Heater and Water Bath. ............................................................................................... 77

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LIST OF TABLES Table 1 – Limiting Chemical Composition, wt.% ...................................................................... 14

Table 2 – Physical Properties ................................................................................................... 17

Table 3 – Electrochemical Values for the alloy 718 in an aerated solution, 1 M H2SO4, at

room temperature. (Amin et al., 2014) ................................................................................... 38

Table 4 – Chemical Composition values for the “A.R.” sample. .............................................. 50

Table 5 – Chemical Composition values for the “S.A.” sample. .............................................. 52

Table 6 – Chemical Composition values for the “F.T.” sample. ............................................... 54

Table 7 – Phases revealed in the “A.R.” sample ...................................................................... 55

Table 8 – Phases revealed in the “S.A.” sample ...................................................................... 56

Table 9 – Phases revealed in the “F.T.” sample ....................................................................... 56

Table 10 – Corrosion Rate via LPR method. ............................................................................. 61

Table 11 – Conditions for Full Polarizations Tests. .................................................................. 62

Table 12 – Conditions for Full Polarizations Tests. .................................................................. 65

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ABSTRACT Alloy 718, a nickel-base alloy, is well-known for its excellent corrosion resistance to many

media due to its chemical composition and ability for precipitation strengthening.

Characteristics highly desired for industrial applications like Oil & Gas, Nuclear and

Aerospace. it is expected that Alloy 718 shows these good corrosion resistance

characteristics when manufacture by Selective Laser Sintering method. This project studied

the corrosion and metallurgical behaviour of Alloy 718 when it is manufactured by Selective

Laser Sintering method and when it was given conventional heat treatments which included

a solution anneal, and age-hardening treatments, 1100°C for 1 hour (SA) and 1100°C for 1

hour plus 720°C for 10 hours (SA + AH) respectively. They were compared to the as

manufactured material evaluating their microstructures, as well as their responses to

electrochemical tests in an aqueous solution which was aerated, unstirred, 0.1 M HCl, 1 M

NaCl at 25°C at room temperature. Throughout these experiments the evidence of a

corrosion resistance trend was observed. The SLS manufactured Alloy 718 exhibited better

corrosion resistance than when it is heat treated. More tests were subsequently performed

in the same solution with addition of 0.01 M NaSO3, as well as changes on the surface finish

(#320 versus #1200 carbide paper) which exhibited the same trend. The microstructure

analysis bore the evidence of elongated, contrasted and random-oriented grains showing

the effect of SLS manufacturing method as well as evidence of intermetallic phases proper

of Alloy 718.

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DECLARATION I hereby declare that “No portion of the work referred to in the dissertation has been

submitted in support of an application for another degree or qualification of this or any

other university or other institute of learning”.

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INTELLECTUAL PROPERTY STATEMENTS i. The author of this dissertation (including any appendices and/or schedules to this

dissertation) owns certain copyright or related rights in it (the “Copyright”) and s/he has

given The University of Manchester certain rights to use such Copyright, including for

administrative purposes.

ii. Copies of this dissertation, either in full or in extracts and whether in hard or electronic

copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as

amended) and regulations issued under it or, where appropriate, in accordance with

licensing agreements which the University has entered into. This page must form part of any

such copies made.

iii. The ownership of certain Copyright, patents, designs, trademarks and other intellectual

property (the “Intellectual Property”) and any reproductions of copyright works in the

dissertation, for example graphs and tables (“Reproductions”), which may be described in

this dissertation, may not be owned by the author and may be owned by third parties. Such

Intellectual Property and Reproductions cannot and must not be made available for use

without the prior written permission of the owner(s) of the relevant Intellectual Property

and/or Reproductions.

iv. Further information on the conditions under which disclosure, publication and

commercialisation of this dissertation, the Copyright and any Intellectual Property and/or

Reproductions described in it may take place is available in the University IP Policy (see

http://documents.manchester.ac.uk/display.aspx?DocID=487), in any relevant Dissertation

restriction declarations deposited in the University Library, The University Library’s

regulations (seehttp://www.manchester.ac.uk/library/aboutus/regulations) and in The

University’s Guidance for the Presentation of Dissertations.

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ACKNOWLEDGEMENT Firstly, I want to thank God for giving me the opportunity to study in this remarkable

university. I would like to express my whole gratitude to my supervisor Prof. Grace Burke

who guided me through all this project. As well as to PhD students like Giacomo Bertali, Alex

Carruthis, Sam Holdsworth, Elizabeth Hope for their kind support and encouragement which

was evident through their recommendations for my project.

It is important to mention the support from my Family who alwas was present during all this

project. And it was an enormous pleasure to work with my friends who shared with me the

bad and good times experienced during this project.

Thus, this work is dedicated to my family and friends.

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1. CHAPTER 1

1.1 INTRODUCTION

Ni-base alloys have major importance in industry due to the excellent corrosion resistance

and good mechanical properties they possess. These characteristics have been

demonstrated for a wide range of industrial applications including the chemical,

petrochemical processing, pollution control, oil and gas extraction, power generation

systems, paper industries. For these industries the material performance requirements are

more demanding; that is why the versatility and reliability of Ni-base alloys make them the

prime candidates for the fabrication or construction of vessels, piping systems, pumps,

valves and more applications intended to work in aqueous and high temperature

environments.

One of these alloys is the nickel-base Alloy 718, which is the material of interest in this

dissertation. It is well-known that this alloy has an excellent corrosion resistance to many

media due to its chemical composition and ability for precipitation strengthening. This alloy

is generally produced by casting and hot rolling, and is used in the wrought form. More

recently, new laser-processing developments have led to new methods of manufacturing

metal alloys enabling near-net shape fabrication of complex parts/shapes. So nowadays

Laser Metal “3D Printing” or Laser-Assisted Additive Manufacturing is being explored for the

manufacture of Alloy 718.

The first laser, developed in the 1960’s, was produced by shining a high-power flash lamp to

produce a white light directed to a ruby rod with silver-coated surfaces. Since then lasers

have gone through a path of evolution and improvements, and are now used in many

applications ranging from medical fields to military and research fields. Thus, the demand

for laser uses increases year to year.

That is also the case for manufacturing applications, where the use of the laser has emerged

as a new option for widely-used processes related to welding, surface treatment, drilling

and cutting. This has been accomplished thanks to the precision and controlled build-up of

material that can be achieved by computer control. Over the last 20 years, such

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development have opened the way for new fabrication technologies which are known

generically as Additive Manufacturing (AM) processes.

Most of the AM processes make use of a laser beam, which operates under a certain

frequency and power, to process different materials. In this case study, a metal alloy was

fabricated using a layer-by-layer laser melting technique using an Alloy 718 powder as the

starting material. This technique permits the reproduction of any shape programmed by

Computer Aided Design (CAD) software where the element to be fabricated is modelled.

The next step is where the metals (powder) fuse together layer-by-layer (solid powder

particles that have the desired material’s chemical elements in terms of composition), until

the final shape is obtained. (Medina and Ramses, 2013)

Nowadays a common name for the use of metal fabrication by laser is Metal 3D Printing,

which uses the direct metal Selective Laser Sintering (SLS) process to accomplish it (Murr et

al., 2012).

Fig. 1 Scheme of the Process (Murr et al., 2012)

From the figure 1, the SLS process basics can be explained, it consist on a high-powered

60kV electron laser beam generated by an electron gun (1) which is projected into a build

chamber. With the use of a CAD-driven mirror system (2) the laser is scanned and focused

with conventional glass lenses (3). Inside the build chamber area where the powder, which

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is stored in a container (6), is spread by a roll (4) on the dispensing platform (5) forming a

new single layer to be melt. Each time a new layer is selectively melted the platform climbs

down as shown on (5). So the designed element is built up additively layer by layer, typically

the build speed rages 7-8 mm/h. Any residual-excess powder is collected in a container (7).

Inside of the chamber the use of Nitrogen or Argon is used to have a pure and inert

environment. The advantage of using Nitrogen lies on its thermal conductivity, which is 40%

greater than that of Argon. Thus it allows a quicker cooling and solidification process. (Murr

et al., 2012)

1.2 SCOPE OF THE PRESENT WORK

The present work is conducted to study the corrosion and metallurgical behaviour of Alloy

718 when it is manufactured by Selective Laser Sintering method. In addition, the as-

manufactured material, was given conventional heat treatments which included a solution

anneal, and age-hardening treatments. Once the samples were heat-treated, at 1100°C for 1

hour (SA) and 1100°C for 1 hour plus 720°C for 10 hours (SA + AH), they are compared to the

as AM material to evaluate their microstructures using the Scanning Electron Microscope

(SEM) and Energy Disperse X-ray Spectroscopy (EDX). Furthermore, by performing

potentiodynamic polarization measurements on the samples, their electrochemical

responses were analysed. The aqueous solution for electrochemical measurements was

aerated, unstirred, 0.1 M HCl, 1 M NaCl at 25°C (room temperature) and 45°C. More tests

were subsequently performed in the same solution with addition of 0.01 M NaSO3. The

effect of surface finish (#320 versus #1200 carbide paper) on the electrochemical response

was also examined.

1.3 OBJECTIVE

The primary objective of this project is to observe the corrosion performance of this alloy

when it is fabricated through SLS and compare it with conventionally heat-treated SA and

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SA+AH materials. By analysing the results of the corrosion tests, along with the

microstructural analysis of each heat treatment the effect of microstructure in terms of

corrosion can be evaluated.

As secondary objectives are:

- To assess the effect of heat treating the AM samples on the resulting microstructure

and compare them with the as-AM microstructure.

- To understand the electrochemical behaviour of the samples by adding sodium

thiosulphate to the solution.

- To estimate the behaviour of the samples when the solution is heated at 45˚C.

- To evaluate the effect of surface finish (#1200 versus #320 carbide paper) on the

observed electrochemical response in the HCl-based solutions.

2 CHAPTER 2

2.1 LITERATURE REVIEW

2.1.1 Alloy 718 – General Information

Alloy 718 is a face-centred cubic (fcc) nickel-chromium-based super-alloy. It was

developed by INCO and originally known as INCONEL 718 because of the trademark and its

composition, although it is now known as Alloy 718. Under the unified numbering system

(UNS) it is UNS N07718. Alloy 718 can be readily manufactured and exhibits good tensile

strength, fatigue strength, creep strength, and rupture strength; thus, making it the choice

for a wide range of applications. Table 1 lists the typical chemical composition range for this

alloy in wt. %.

Table 1 – Limiting Chemical Composition, wt.%

ELEMENT COMPOSITION %

Nickel 50.00-55.00

Chromium 17.00-21.00

Iron Balance

Niobium (plus Tantalum) 4.75-5.50

Molybdenum 2.80-3.30

Titanium 0.65-1.15

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Aluminium 0.20-0.80

Cobalt 1.00 max

Carbon 0.08 max

Manganese 0.35 max

Silicon 0.35 max

Phosphorus 0.015 max

Sulphur 0.015 max

Boron 0.006 max

Copper 0.30 max

The definition of an alloy is “a metal made by combining two or more metallic elements,

especially to give greater strength or resistance to corrosion.” From the complex

composition listed in Table 1, Alloy 718 is an excellent example of this definition. The

relevant elements are described in the following section with respect to their beneficial

properties in terms of corrosion and mechanical properties.

Nickel retains an austenitic, face-centered-cubic (fcc) crystal structure up to its melting

point, providing freedom from ductile-to-brittle transitions and minimizing the fabrication

problems that can be encountered with other metals. In the electrochemical series, nickel is

more noble than iron but more active than copper. Thus, in reducing environments, nickel is

more corrosion resistant than iron, but not as resistant as copper. Alloying with chromium

provides resistance to oxidation thus providing a broad spectrum of alloys for optimum

corrosion resistance in both reducing and oxidizing environments.

Nickel – Besides being the base metal element, The role of Ni is to provide metallurgical

stability, to enhance thermal stability and weldability, to improve the resistance to reducing

acids and caustics, and to increase the resistance to stress corrosion cracking particularly in

chlorides and caustics.

Chromium – It increases the resistance to oxidizing corrosives and to high-temperature

oxidation and sulfidation, and to enhance the resistance to pitting and crevice corrosion.

Molybdenum – It helps to improve the resistance to reducing acids, and to pitting and

crevice corrosion in aqueous chloride containing environments. It contributes to increased

high-temperature strength.

Iron – Its presence improves the resistance to high-temperature carburizing

environments, it reduces the alloy costs, and controls thermal expansion.

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Copper – Its role is to improve the resistance to reducing acids (particularly non-aerated

sulphuric and hydrofluoric) and to salts. The effect of copper additions to nickel-chromium-

molybdenum-iron alloys is to improve the resistance to hydrochloric, phosphoric and

sulphuric acids.

Aluminium – It contributes to improve the resistance to oxidation at elevated

temperatures and it has a significant role to promote age hardening.

Titanium – It combines with carbon to reduce the susceptibility to intergranular corrosion

due to chromium carbide precipitation depletion resulting from heat treatments, and it has

a major effect to promote age hardening.

Niobium – It has a similar role like Titanium, because it combines as well with carbon to

reduce the susceptibility to intergranular corrosion due to chromium carbide precipitation

depletion resulting from heat treatments, it also improves the resistance to pitting and

crevice corrosion, and increases the high temperature strength. The dominant effect of Nb

is to promote the formation of Ni3Nb in Alloy 718.

Cobalt – Its function is to provide a higher temperature strength, and resistance to

carburization and sulfidation.

These alloying elements are combined with nickel in single phase solid solutions over a

broad composition range. With the purpose to provide the positive and useful corrosion –

mechanical characteristics to the alloy, so the alloy may have a better performance in a

wide variety of environments. As result the final alloy, in turn, provides useful engineering

properties that can be heat treated (solution annealing and age hardening) without any

problem related to deleterious metallurgical changes resulting from fabrication or thermal

processing.

Nitrogen – During manufactured products by SLS Nitrogen is used in order to have a pure

environment free of Oxygen. It enhances metallurgical stability, improves pitting and crevice

corrosion resistance. It also combines with Ti and Nb to form nitrides in Alloy 718.

Due to its characteristics the high-nickel alloys can have a higher strength. This is

achieved by performing a solution annealing treatment followed by an age-hardening

precipitation treatment. The melting temperature for this alloy ranges from 1280˚C –

1336˚C. (Metals, 2000)

The physical properties of this alloy are listed on the following table:

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Table 2 – Physical Properties

Property Room Temperature

Density 8193 (SA) and 8221 (AH) kg/m3

Tensile Strength 114 ksi (SA) and 195 ksi (AH)

Yield Strength 50 ksi (SA) and 165 ksi (AH)

Elongation in 2in 60% (SA) and 21 % (AH)

Hardness 87 Rb (SA) and 43 Rc (AH)

SA for Solution Annealed and AH for the Age-Hardened

Usually Alloy 718 is specified as: solution-annealed and age-hardened. Alloy 718 is

hardened by the precipitation of secondary phases (e.g. gamma prime and gamma double-

prime) in the nickel base matrix. (Cozar and Pineau, 1973). For other superalloys where Al

and Ti are present in a higher quantity, the Al + Ti content and Al/Ti ratio are determining

factors for the precipitation of second phases, while for Alloy 718, the Al + Ti content is

approximately 1.4 wt. %, which is too low to be the case. Different and diverse

experimentation has proved that these precipitates are made of nickel-(aluminium,

titanium, niobium) phases which are induced by the ageing treatment (Kotval, 1969). Since

this is a phase transformation (formation of new precipitates in the fcc matrix), it is

necessary that the aging elements (aluminium, titanium, niobium) are dissolved in the fcc

matrix prior to the ageing treatment. If they are precipitated as some other phase or are

combined in some other form, the precipitation treatment will not be the adequate. Thus,

the alloy’s strength might not be the desired one. These secondary phases will be explained

more in Section 4.2. (Metals, 2000).

There are different kinds of heat treatments commonly utilized for Alloy 718, which

provide specific characteristics to the alloy. For example NACE MR 0175 specifies a solution

anneal at 1010-1038°C followed by rapid cooling, usually in water, plus precipitation

hardening at 788°C for 6-8 hours, followed by air cooling (meeting a maximum of 40

Rockwell C). (Metals, 07-09-2007) In addition, some applications specify an additional

ageing treatment at 650°C for 8 hours.

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2.1.2 Applications of Alloy 718

As mentioned earlier, alloy 718 is used in Pressurized Water Reactors (PWRs) and

Boiling Water Reactors (BWRs), where there are some components that require to deal with

high stresses in a high-temperature water environment. As examples are bolts, pins, springs

and beams; for all these components is necessary to have high strength, relaxation

resistance, in addition to corrosion resistance. The frequency of failure of these alloys has

not been great, but some of the failures caused plant shutdown and major unscheduled

repairs or maintenance service. Also it is important to consider that when there is a failure,

pieces or parts from a failed component can travel through a reactor and cause internal

damages. (Miglin, 1989)

Another main area where alloy 718 is one of the preferred materials, is in the Oil and

Gas Industry. The manufacture of wellhead components, auxiliary and downhole tools for

oil and gas wells where the environment is a corrosive aqueous media due to the presence

of CO2, chlorides and H2S. Specific examples include subsurface safety valves, packers,

hangers, valve parts (gates seats, and stems), fire safe valves, blowout preventers, and

fasteners. All of these components must fulfil the requirements specified on the American

Petroleum Institute´s specifications. These components generally work in severe conditions

because they have to support high tensile stresses (loads up to 90000 psi), be in contact

with corrosive fluids and support the normal service temperatures which can vary from

ambient temperature to 177oC at normal production. Failure of such components can

results in substantial monetary risks, and release of deadly toxic gases. Thus such concerns

demand special material properties with tight process controls in manufacturing of the

alloy.

For applications in deep sour gas wells, the critical requirements for the alloy are:

high resistance to general and localized corrosion, high resistance to environmentally-

induced cracking for (sulphide and anodic) stress corrosion cracking, and high strength. With

these properties, the leak-break failures can be reduced. These requirements can be

obtained by controlling the alloy composition, as well as the melting and refining practices,

and the heat treatment. (Onyewuenyi, 1989)

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2.2 Microstructure

As mentioned previously, the complex composition of Alloy 718 can result in the

formation of the following intermetallic phases due to the role of the elements present in

the composition (Jena and Chaturvedi, 1984):

- Precipitates: It is an ordered crystalline lattice of type L12, body-centered cubic

structure and is generally is Ni3Al*. Atom probe analysis has shown that the

composition of these precipitates is complex, with measurable level of Ti and some Nb

in addition to Al (Burke and Miller, 1991). The morphology is roughly spherical and the

size can be 10 to 40 nm, the orientation relationship of precipitates in the matrix is

( ) II( ) , ⟨ ⟩ II⟨ ⟩ (Muralidharan et al., 1989, Miller and Babu, 2008).

The precipitates distribution is considerably lower in concentration than that for

secondary precipitates. These precipitates can appear where there is a free

precipitates zone adjacent to the precipitates. The precipitates contain important

quantities of Nb in addition to Al and Ti. The precipitates display greater Al content

and lower Nb content than the precipitates. (Burke and Miller, 1991, HANDBOOK,

1992, Li et al., 2002)

- Precipitates: They are ordered DO22, (body-centered tetragonal) whose composition

is Ni3(Nb,Ti,Al)*. They are disc-shaped with a diameter that ranges from 10 to 40 nm,

the orientation relationship of precipitates in the matrix is ( ) II( ) ,

⟨ ⟩ II (Muralidharan et al., 1989, Miller and Babu, 2008). Their occurrence

can be marked by the temperature set of the age hardening treatment. The DO22-

ordered structure may be described as an L12- ordered structure with an ɑ[ 1/2, 1/2, 0]

displacement every other( 0,0,1) plane thereby converting a cubic structure into a

tetragonal one. The crystal structures of both precipitates and are derivatives of

the face centered cubic structure and these precipitates have a cube-on-cube

orientation relationship with each other and the matrix. (Burke and Miller, 1991,

HANDBOOK, 1992, Goldstein et al., 2012)

If an elevated temperature (870 oC) ageing treatment is used, then a coarse

precipitate is formed, whose morphology is a coarse disc, having a diameter of

20 | P a g e

approximately of 0.3 µm. They have a random distribution throughout the matrix.

These precipitates are enriched in Nb and Ti and depleted in Al, Fe, Cr and Mo in

contrast to the matrix. (Burke and Miller, 1991, HANDBOOK, 1992)

Fig. 2 Micrograph of precipitates (30K magnification). The heat treatment applied

was 1h at 870oC. (Radavich, 1997)

Fig. 3 Micrograph of and precipitate particles when electro-etched with CrO3 (10K

magnification). The heat treatment applied was 100h at 650oC. (Radavich, 1997)

- Precipitates: It is an ordered orthorhombic crystalline lattice of type DOa, whose

composition is (Ni3Nb). The size can vary from 1 to 8 µm for the needle morphology and

1 µm for the globular morphology; the orientation relationship of precipitates in the

matrix is II ( ) , ⟨ ⟩ II (Muralidharan et al., 1989). These

precipitates can form at grain boundaries but may form intragranularlly as well. These

precipitates are enriched in Nb and Ti content and depleted in Fe and Cr in comparison

21 | P a g e

to the matrix composition. The surrounding matrix to the precipitates shows local

depletion of Nb.. (Burke and Miller, 1991, HANDBOOK, 1992, Li et al., 2002)

Fig. 4 Micrograph of depletion because of Phase (10K magnification). (Radavich,

1997)

- Laves Precipitates: They are coarse blocky precipitates with crystalline lattice of type

C14, whose composition is (NiFeCr)2(NbMoSi). Their composition is enriched in Nb, Fe,

Cr and Mo. These precipitates tend to form at high temperature after prolonged heat

treatment. (Burke and Miller, 1991, HANDBOOK, 1992)

- MC Carbides / MN Nitrides: They are another type of coarse blocky precipitates /

inclusions, whose composition is (TiNb)C or (TiNb)N. The size is approximately 0.5 µm or

larger. (Burke and Miller, 1991, HANDBOOK, 1992, Li et al., 2002)

- Matrix: The matrix is fcc. The composition is dependent also on the heat treatment,

this change is related to the presence of precipitates. The matrix content of Nb, Al and

Ti tend to decrease as the intermetallic phases precipitate and coarsen. In contrast the

matrix content of Fe, Cr, Mo and Si is notably higher than in the precipitates. (Burke and

Miller, 1991, HANDBOOK, 1992)

*The composition may vary according to the heat treatment applied to the material,

Based on information in the literature, the first phases that may form are the MC-

carbide (MN-nitride) and Laves phase. These phases may form during the solidification

process or during the annealing stage (recrystallizes the austenite matrix (ASTM, 2000)). As

22 | P a g e

temperature is reduced, and, depending on the temperature, the coarse precipitates

can form. The phase precipitates preferentially at grain boundaries and other interfaces

like inclusions and Laves phase whereas the coarse nucleates throughout the matrix.

The zones of precipitates where they are not surrounded by coarse , suggests that

prescipitates forms before coarse precipitates do. As the ageing temperature decreases

to (~700-760oC) fine precipitates are formed. Finally, at lower temperatures (~650-

720oC), the forms. Where the niobium content of the matrix is not enough for the

formation of precipitates it is possible that the superstaturation of solute is still sufficient

for precipitates to nucleate. The composition of these precipitates is also influenced by

the composition of the matrix at the time of precipitation. (Burke and Miller, 1991)

2.2.1 Microstructural Analysis Techniques: SEM, EDX and XRD

SEM: The Scanning Electron Microscope technique is non-destructive, versatile and

better for investigating the microstructure of metallic materials in comparison to the optical

(light) microscope. It has a better resolution, as well as an enhanced magnification for as-

polished, etched cross sections as well as rough surfaces. The electron beam is emitted from

a heated tungsten filament (or a field-emitter tip as in a field-emission gun high resolution

SEM) and focused by a system of magnetic lenses. Acceleration voltages range from ~500

eV to 40 keV. This beam of high-energy electrons generates many low energy secondary

electrons from its impact with the sample surface. The collected signals from the interaction

between the electron beam and the sample reveal the desired information on morphology

and crystalline structure and orientation of the material. The main signals that provides SEM

are the secondary electrons (SE), backscattered electrons (BSE), and X-rays. (HANDBOOK,

1992, Lloyd, 1987). For the BSE there are two important factors to consider which are: a) the

atomic number of the grains which shows a dependence of the BSE emission coefficient (q)

on target atomic number (Z). Thus, the contrast colour between phases in a specimens is

related to the atomic number of each phase. b) the electron channelling effect that is the

interaction between primary electrons and the crystal structure of the target, and can

provide information on the orientation of each grain. This effect is influenced by the grain

size and is structure. (Lloyd, 1987)

23 | P a g e

EDX: The Energy Dispersive X-ray Spectroscopy technique is a well-known for

material analysis. Its principles come from the SEM technique where the high energy

electrons after impacting the sample, generates X-rays from a volume of several cubic

microns. These X-rays are collected in an energy dispersive x-ray spectrometer (a Li-drifted

Si detector, which must be kept at -196 oC). The resulting data consist of a spectrum with

number of x-rays detected on the y-axis and X-ray energy (keV) on the x-axis; the various

peaks in the spectrum correspond to the elements present in the analysis volume. This can

be quantified to provide the composition of the sample. (HANDBOOK, 1992, Goldstein et al.,

2012)

XRD: The X-ray Diffraction technique is non-destructive which serves to characterize

crystalline materials, providing information on structures, phases, preferred crystal

orientations (texture), and other structural parameters, such as average grain size,

crystallinity, and crystal defects. X-ray diffraction peaks are produced by constructive

interference of a monochromatic beam of x-rays scattered at specific angles from each set

of lattice planes in a sample. The peak intensities are determined by the distribution of

atoms within the lattice. Consequently, the x-ray diffraction pattern is the fingerprint of

periodic atomic arrangements in a given material. A search of the ICDD (International Centre

for Diffraction Data) standard database of x-ray diffraction patterns enables quick phase

identification for a large variety of crystalline samples. (HANDBOOK, 1992, Moore and

Reynolds, 1989)

2.3 Electrochemical Principles

2.3.1 Corrosion Rate Fundaments

Firstly, it is necessary to discuss electrochemical reactions. Electrochemical

techniques are used to study these reactions. When electrochemical experiments are

performed in a lab under controlled conditions, the current-potential relations can be

measured as many times it may be needed. These experiments can provide information on

corrosion rates, passivity, pitting tendencies and other data that are needed to describe the

corrosion behaviour.

24 | P a g e

When a specimen is immersed in a corrosive medium, the specimen oxidizes

(corrodes) and the medium (solvent) is reduced. In acidic media, hydrogen ions are reduced.

As the reduction and oxidation processes take place on metal’s surface, the specimen works

as both anode and cathode, and both anodic and cathodic currents occur on the specimen

surface.

(1) (Cottis et al., 2010)

(2)

By algebraic addition of both elementary reactions gives the global anodic reaction:

(3) anodic reaction

(4) cathodic reaction

Fig. 5 Anodic and Cathodic reactions on the surface of the alloy.

One of these experiments is the potentiodynamic cathodic-anodic polarization,

which produces the characterization of the studied specimen by relating the current and

potential. The specimen potential is scanned starting from a potential below the rest

potential of the sample (OCP) and then continues slowly in the positive (anodic) direction. A

complete current-potential plot of a specimen can be measured in a couple hours or a few

minutes depending on the setting for the scan rate and potential step-up. These data can be

plotted that form curves I = f(E), where potential can be varied randomly both step by step

or continuously, and the current measured is the dependent variable. (PRINCETON

APPLIED RESEARCH, 2014)

25 | P a g e

Fig. 6 Evans Diagram showing the anodic and cathodic reactions.

The Evans diagram illustrates the situation when two different redox couples coexist;

the electrode potential always takes a single value, the corrosion potential Ecorr, which can

be determined by the intersection of two branches. (Cottis et al., 2010)

When a specimen is immersed into a corrosive liquid and it is not connected to any

instrumentation – as it would be “in service” – the specimen adopts a potential known as

the corrosion potential, Ecorr. When the specimen is situated at Ecorr, has both anodic and

cathodic currents present on its surface. Nonetheless, both currents are exactly equal in

magnitude so there is no net current to be measured. The specimen is at equilibrium with

the environment (even though it may be visibly corroding). Ecorr can be defined as the

potential at which the rate of oxidation is exactly equal to the rate of reduction. (RESEARCH,

2014

The corrosion current density can also be estimated by extrapolation of the Tafel

lines (red lines on fig. 6) from the Evans diagram where it projects a linear behaviour,

conventionally chosen +/- 50 mv from Ecorr. (Cottis et al., 2010). By using the Tafel

approximation which ends up with the Butler-Volmer equation:

( ( ) ( )) (5)

and

(6)

(

( )

) (

( )

) (7) (Frankel et al., 2010)

26 | P a g e

The Tafel anodic and cathodic ( ) can be obtained from the slopes of the

anodic and cathodic curve respectively. There are occasions where the Tafel extrapolation

cannot be performed on the anodic branch because it does not have a defined linear

log(icorr) – E characteristic. The existence of a single Tafel region (anodic or cathodic) is

sufficient to estimate the corrosion current by using a well-defined and wide Tafel

behaviour of the cathodic branch. This single Tafel region technique is quite useful, as the

anodic and cathodic branches are independent without any expected symmetry or even

equivalent shape. The negative side with the single Tafel region technique is an

overestimation on the corrosion current. But an overestimation is less harmful than a

subestimation, specially for industries where a failure on a component can cause a system

shutdown. (Cottis et al., 2010, McCafferty, 2005)

Once icorr (corrosion current density in A/cm2) is known, the corrosion rate in a

certain amount of time (usually expressed in millimetres per year) can be evaluated by using

Faraday’s law, which is expressed in the following equation:

(8)

Where CR=corrosion rate, A=surface area exposed, =density of the metal (for

Ni is 8.908 g/cm3), =time (1 year = 3.1536 E 7), M=molecular of mass of oxidizing

element (for Ni is 58.71 g/mol), i=current density = icorr, F=Faraday constant (96 485.3329 s

A / mol)

However, there is another option for estimating the corrosion rate. This method is

known as Linear Polarization Resistance. This option consists on using smaller

overpotentials, which are only close to Ecorr (generally in the range +/- 10 mV from Ecorr) the

Butler-Volmer equation becomes:

( )

( ) (9) (Frankel et al., 2010)

The constants βa and βc are proper of each material, and they are dependent on the

material surface activity. The activity is related to the present processes occurring on

sample’s surface, mechanism of the reaction, temperature and the controlling mechanism

of corrosion. (Cottis et al., 2010, Hamann et al., 1998). And there are several limitations

when the Tafel extrapolation is applied; the most important propose a well-defined cathodic

27 | P a g e

or anodic Tafel region at least taking place through one decay of current, using a slow scan

rate and at least one region (anodic or cathodic) is under activation control. (McCafferty,

2005)

If the constants are already collected ( ) then:

( ) (10)

The delta potential and delta current is small and can be considered “linear”, it obeys

the Ohm’s Law (V=I*R). Then, the process can be considered as:

(11)

Thus, the current density can be obtained (icorr) and so the corrosion rate can be

calculated from equation (8).

In order to obtain these data, the use of a three electrode electrochemical cell is

necessary. This cell is composed of a Reference Electrode, a Counter Electrode (or auxiliary

electrode) and a third electrode which is the Working Electrode. The reference electrode

(saturated calomel electrode (SCE) which basically is a reaction between mercury and

mercury chloride (Hg2Cl2 known as calomel) inside of a saturated solution of potassium

chloride in water) is set close to the working electrode; the reference electrode is connected

to a luggin capillary in order to reduce the IR drop. The working electrode is the Alloy 718

sample. The counter electrode is platinum.

Fig. 7 Scheme of the Saturated Calomel Reference Electrode.

28 | P a g e

a)

b)

c)

Fig. 8 Electrochemical Cell with a working electrode (WE), a counter electrode (CE) and

a reference electrode (RE) coupled with a luggin probe. 7a a representative diagram

from the electrochemical cell, 7b the real electrochemical cell, and 7c internal view of

the cell.

2.3.2 Electrochemical Measurements

OPEN CIRCUIT POTENTIAL: The open circuit potential (OCP) is the potential of the

working electrode relative to the reference electrode when no potential or current is being

applied to the cell. When a potential is applied relative to open circuit, the system

29 | P a g e

measures the OCP before turning on the cell, then applies the potential relative to that

measurement. By plotting these acquired data, a graph of potential versus exposure time

can be obtained.

The potential of the alloy may initially increase towards the more negative values

from the first moment of immersion as a result of the dissolution of an air oxide film was

formed on the electrode before its immersion in the solution. Then the potential should

start to have a rapid shift in the less negative direction. This positive potential change

exhibits the normal resistance of the alloy surface against corrosion thanks to the formation

of an oxide film and/or a corrosion product layer. (Chen et al., 2013, Amin et al., 2014). The

sample is exposed for 24 hours while immersed in the solution in order to reach a relatively

constant value.

Fig. 9 Open Circuit Potential plot.

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY: The electrochemical impedance

spectroscopy (EIS) serves to investigate the electrochemical behaviour of the sample by

applying a small AC potential to the electrochemical cell and then measuring the resulting

current. EIS is helpful to analyse oxide films as well as coatings on top of a metal (Kendig and

Scully, 1990).

The principle is related to Ohm’s law, where resistance is the ratio between voltage

and current (the ideal resistor that is independent of frequency, that follows all current and

voltage levels, and in AC current and voltage signals are in phase with each other). But in

real life the behaviour of circuit elements is more complex, so the use of impedance instead

of resistance is necessary. The application of a small excitation signal will result in a pseudo-

linear cell's response. Under a linear (or pseudo-linear) system, the current response to a

30 | P a g e

sinusoidal potential will have a sinusoidal behaviour and the same frequency, but shifted in

phase.

( ) (12)

Et=potential at time t, E0=amplitude of the signal, =radial frecuency

(13) =frecuency in Hz

( ) (14)

It=response signal shifted in phase , I0=amplitude

( )

( )

( )

( ) (15)

Using Euler’s relationship,

( ) (16)

Impedance can be expressed as a complex number,

( )

( ) ( ) (17)

Once the data are obtained, they can be plotted using the Nyquist and Bode plots.

The Nyquist plot shows the relationship between phase angle ( ) and Frequency (Hz), whilst

the Bode plot shows the relationship between Impedance (Ω∙cm2) and Frequency (Hz). With

the resulting spectra, the data can be converted to an equivalent circuit model. From the

equivalent circuit RS represents the solution resistance between the alloy surface and the

counter electrode. Q1 represents the first constant phase element (CPE). R1 is the

polarization resistance of the film layer formed on the surface of the alloy (working

electrode) and can be also considered as the charge transfer resistance. In other words R1

and Q1 show their similarity to the resistance and capacitance of the porous layer which is

responsible for the dissolution/precipitation processes through the passive film. Q2 is the

second constant phase element, and R2 is the polarization resistance of the oxide and/or

corrosion product layer formed on the alloy surface. A similar explanation is for R2 and Q2,

where in other words they denote the resistance and capacitance of the barrier layer that is

responsible for the good resistance of the alloy. All this corresponds to the behaviour of

nickel-base alloys’ passive films, which is described as a double layer structure of an n-type

outer region that contains Ni-Fe oxide and hydroxide and a p-type inner region containing Cr

oxide. (Chen et al., 2013, Rodríguez and Carranza, 2011)

31 | P a g e

Fig. 10 Equivalent circuit used to represent the Nyquist and Bode plots .

The impedance of the CPE is given by the following formula:

| ( ) | (18)

From the equation j2 = -1, is the angular frequency, and n is the empirical

exponent between 0 and 1. When n = 1, Q becomes equivalent to a true capacitance

associated to an ideal capacitor. When n = 0, Q becomes equivalent to a resistance. When n

= 0.5, Q becomes equivalent to Warburg impedance. (Chen et al., 2013)

POTENTIODYNAMIC POLARIZATION: This is a technique used to characterize a metal

specimen by its current - potential relationship. The specimen potential is usually scanned

slowly from a chosen cathodic potential towards an anodic potential (positive direction).

(PRINCETON APPLIED RESEARCH, 2014) The specimen acts as an anode so that it corrodes or

forms an oxide coating. These measurements are used to determine corrosion

characteristics of metal specimens in aqueous environments. Very important

electrochemical data can be obtained by studying a polarization graph, these data were

discussed in section 4.3.1. The result of a polarization curve is similar to figure 11. By doing

an analysis of this curve valuable information is obtained about the behaviour of the

specimen in a specific environment.

2.3.3 Regions of a Polarization Diagram

Normally a sample is polarized from a certain cathodic potential (more negative from

Ecorr) towards an anodic potential (more positive from Ecorr). These potentials can be

32 | P a g e

selected in order to set a study range where the material will be analysed. As an example

the next diagram will be explained on figure 11.

Fig. 11 Regions of a Polarization Diagram

Region A – B: This is the cathodic branch where the effect of hydrogen evolution or

oxygen reduction can be seen. However, since the media is acidic the expected

reaction will be hydrogen evolution, which obeys reaction (4).

Region B – C: This is the active region of the anodic branch, where the metal specimen

corrodes as the applied potential is made more positive. This region obeys reaction

(3). In this zone, General Corrosion occurs, which is a uniform attack over the surface

area. Hence, the alloy becomes thinner and eventually may fail.

At Point C: This is the critical current where corrosion rate doesn´t increase and starts

the onset of passivation.

Region C – D: This is the zone in which the current decreases rapidly while potential

increases, showing the generation of the passivating film on the surface of the

specimen.

Region D – E: This is the Passive zone, in which there is a small increasing variation in

current as potential is increased. The passive film that is formed makes the alloy

corrosion resistant. Passivation is considered to occur due to a solid-state mechanism.

As this alloy contains Chromium in its composition, it is responsible for the generation

of an oxide film. This film promoted by Nickel, is a hydrated chromium oxyhydroxide

[[CrOx(OH)3-2x]·nH2o] which is highly adherent and protective, besides preventing

33 | P a g e

further contact with the electrolyte. Chromium, contributes to diminish the critical

current for passivity, and enlarges the passivation potential region more than pure

nickel. (Turner et al., 1973, Singh and Gupta, 2004, Metals, 2000). But, there are other

researchers as (Li et al., 2011) who consider that the passive films on nickel-base alloys

can be more accurately described as a double layer structure composed of an n-type

outer region containing Ni/Fe oxide and hydroxide and a p-type inner region

containing Cr oxide. It is understood that Molybdenum and Chromium are resistant to

localized corrosion, specially pitting, as well as to uniform corrosion. By forming a

compact and protective film of Cr2O3 and MoO2, because Nickel by itself cannot resist

corrosion. (Liu et al., 2001)

Point E: Once the potential has continued to increase, it reaches a potential at which

the passive film begins to break down.

Region E – F: This is the transpassive region in which the evolution of oxygen or the

dissolution of the metal by breaking down the passive film can lead to localised

corrosion. The type of reaction which operates in a particular case is determined by

the kind of alloy/electrolyte interface and the potential at which the process takes

place. Once the passive film is broken down (dissolution of the passive film is a

synonym), a steep rise in the current density occurs as potential is increased. (Singh

and Gupta, 2004). The transpassive oxidation process is controlled by the solid state

oxidation of Cr(III) to Cr(VI) in the film. The reaction of Ni dissolution through the film

proceeds via two parallel reaction paths, due to two reaction intermediates. One of

these reactions, leads to a selfcatalytic dissolution of Ni through the passive film. For

alloys where the nickel content is higher, Ni is related with a lower Fe content, the

importance of the selfcatalytic path of Ni dissolution is greater. (Betova et al., 2002,

Bojinov et al., 2002)

It is not well-understood totally the mechanism for the formation of a secondary

passivation, but it can ascribed as the formation of a Ni – Fe(rich) film. Where Fe may

have a role on decelerating the transpassive dissolution. But the dissolution rate of Fe

is influenced by the presence of sulphates anions, hence, the stability of a secondary

passive state may be affected as well. (Bojinov et al., 2002). The dissolution rate is

weakly dependent on the solution pH as well as he influence of sulphate

34 | P a g e

concentration is also weak or negligible, although the anion plays a crucial role in the

very existence of transpassive dissolution. (Keddam et al., 1985)

Normally alloys which exhibit a passive region may suffer specific corrosion

problems. The Passive region can be interrupted by the activity of Localized Corrosion under

the action of Crevice Corrosion and Pitting Corrosion. When certain conditions are met

Localized Corrosion can occur.

2.3.3.1 Crevice Corrosion

Crevice Corrosion usually takes place in a very tight gap between two surfaces

(e.g. the mounting resin of the sample and the sample). This crevice creates a micro-

environment inside of the crevice which differs from the general medium. The oxygen

concentration difference between the outside and inside of the crevice, then there is an

alteration of the crevice solution chemistry, and depassivation. The size range of the crevice

may be between 0.1 and 100 µm.

The main theories that describe the generation of this kind of corrosion

according to (Frankel et al., 2010) are:

- The development of a critical internal crevice solution (usually low pH and high

halide concentrations) because of its geometry.

- The development of a critical ohmic potential difference (IR drop) between the

crevice interior and the exposed surrounding surfaces.

- The stabilization of metastable pits by the crevice geometry. The crevice

environment prevents repassivation of metastable pits, in a certain way similar

to the first theory.

2.3.3.2 Pitting Corrosion

Whilst Pitting Corrosion is a form of very localized attack that results in the

formation of pits or small “holes” in the alloy. The pits are the result of failure of the passive

film. The main theories that describe the generation of this kind of corrosion according to

(Frankel et al., 2010) are:

35 | P a g e

- Migration of aggressive ions into the passive film. Metal anion migration can

be stimulated by the high electric field in the film. As a result of changes of

the properties on the passive film, high currents could start to circulate

through the contaminated zones.

- The absorption of ions (e.g. Cl ions) exceeding its critical concentration is a

prerequisite for the break down of the passive film.

- Flaws, imperfection or inhomogenities which are always present in the oxide

film.

- Zones where the reparation of the passive film is not achieved completely

which leads to a continuous break down – repair cycle of the film.

Once the pit is formed, inside of it, a micro-environment different from the bulk

solution is created where the aggressive anions prevent repassivation of the local break

down sites. This environment is more aggressive and detrimental to the metal, that under

appropriate conditions the pit can survive, leading to a dangerous form of attack. A rise in

temperature promotes the action of this kind of corrosion.

The Pitting Resistance Number and the Critical Pitting Temperature can be

predicted according to the composition of the sample, the formulas are exposed in (Frankel

et al., 2010, ASTM, 2000) respectively. These values can be calculated when tests are

performed under conditions exposed in ASTM G48.

(19) for Alloy 718 is 22.5

( ) ( ) (20) for Alloy 718 is 45 oC

For the purpose of this project it is necessary to mention that Pitting Corrosion

can be enhanced by the presence of thiosulfate. The fundament is that thiosulfate prevents

passivation of an active alloy surface and helps to stabilize metastable pits initiated below

the actual pitting corrosion potential. Thiosulfate increases the possible potential range of

corrosion pits to grow by lowering the repassivation potential. Hydrogen sulphide

originating from thiosulfate is proposed to accelerate the anodic dissolution inside corrosion

pits and crevices by forming sparingly soluble metal sulphides and by acidifying the local

environment. (.Al-Taher et al., 2014). Na2S2O3 reacts with the HCl from the solution, where

36 | P a g e

the solution gets cloudy as yellow sulphur is formed and precipitated with time. The

reaction is: Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + SO2(g) + H2O(l).

2.4 Previous Experiments.

Potentiodynamic tests performed in an aqueous solution of 1 M H2SO4 and 0.6 M NaCl,

showed a potential values increment towards less negative values during the initial hours

after immersion for the Open Circuit Potential test by (Amin et al., 2014). This is due to the

initial formation and growth of the passive oxide film. In all the cases after the initial rise in

rest potential, a certain constant potential value (steady-state potential) was attained. This

value corresponds to the free corrosion potential (Ecorr) of the tested alloy. These findings

revealed the tested alloys tend to have a stable OCP value in H2SO4 solution after

approximately 12 hours of immersion. A similar behaviour was observed in the OCP tests

performed by (Chen et al., 2013). The work performed by (Priyantha et al., 2005) showed

the formation of the oxide film on the alloy surface for polished and unpolished samples,

based on the rise of rest potentials. But the polished surface exhibited more positive OCP

values than did unpolished one.

The curves shown in figure 12, 13 and 14 are marked with Re(0) are the ones that apply

for this case study, because they have no addition of Rhenium, and also due to the acidic pH

of the solution that is close to the proposed in this study.

Fig. 12 Rest potential vs. time (up to 24hrs) curves recorded for IN718 with 0% (Re) in 1.0 M

H2SO4 solution at 25oC. (Amin et al., 2014)

For the EIS measurements performed under the same conditions of pH and room

temperature as mentioned previously, the analysis of the impedance data with a suitable

37 | P a g e

semicircle showed that the value of impedance for the alloy 718 is approximately 24000 Ω

cm2. (Amin et al., 2014)

Fig. 13 Complex-plane impedance plots recorded for IN718 with 0% Re (black dots) in 1.0 M

H2SO4 solution at 25oC. (Amin et al., 2014)

For the polarization curves, which were obtained using Linear Sweep voltammetry

technique, showed that the potentials were swept starting from a cathodic potential of -1.5

V vs. Ag/AgCl in the anodic direction at a scan rate of 1.0 mV/s at 25oC. Before each

polarization experiment, the electrode was allowed to corrode freely for a period of 4 h.

After this time a steady state open circuit potential, OCP, corresponding to the corrosion

potential (Ecorr) of the working electrode, was obtained. It follows on the positive going

scan, the cathodic current density decreases gradually reaching the lowest value at the

corrosion potential, Ecorr, (close to the value for OCP measurement).

38 | P a g e

Fig. 14 Potentiodynamic anodic polarization curves recorded for the tested alloys in 1 M

H2SO4 + 0.6 M NaCl, at a scan rate of 1 mV/s at 25oC. (Amin et al., 2014)

An obtained resume from the electrochemical measurements performed by Amin et al,

is displayed on the next table. (Amin et al., 2014)

Table 3 – Electrochemical Values for the alloy 718 in an aerated solution, 1 M H2SO4, at

room temperature. (Amin et al., 2014)

Ecorr (V) Icorr (Tafel)

(mA/cm2)

βa

(V/dec)

-βc

(V/dec)

νTafel

(mpy)

Icorr (ICP)

(mA/cm2)

νICP

(mpy)

-0.01 2.19 0.123 0.093 876 2.04 815

Note.- The potential values are vs Ag/AgCl reference electrode.

It is observed from figure 9 that, as compared with the values of ipass obtained without

Cl, the ipass value increases, at any given potential, in presence of Cl, reflecting the

aggressiveness of Cl towards the passivity of the tested alloys. These findings could be

attributed to general weakness and thinning of the passive film as a result of Cl adsorption

(assisted by the applied electric field) on the passive oxide surface. This adsorption is

expected to increase as the potential (the applied electric field) made more positive, as

evidenced from the obvious increase in ipass with potential. (Amin et al., 2014)

39 | P a g e

3 CHAPTER 3

3.1 EXPERIMENTAL METHODS AND MODES OF ANALYSIS

3.1.1 Background and Testing Plan

Alloy 718 was received in the same conditions after it was manufactured by Selective

Laser Sintering. Samples were prepared by selecting 3 slices from the edge of the bulk

material which were cut into equal parts giving 9 samples. Figure 15 shows the test path for

the samples.

Fig. 15 Testing Plan

3.2 Sample Preparation and Surface Preparation

The as-received AM Alloy 718 material was cut by Electrical Discharge Machining (EDM)

due to the high hardness of this alloy in the as-AM condition. The main benefit of EDM,

besides the precision and promptness, is that there is minimal waste of material in contrast

with conventional machining techniques. As can be seen in the next figure 9 samples were

obtained.

Fig. 16 Samples cut (red blocks) from the original chunk material (orange block).

40 | P a g e

The samples were subsequently mechanically ground to remove the EDM surface layer.

Thereafter, 6 samples were placed into ceramic crucibles for being heat treated in the

furnace: solution annealing for 1 hour at 1100°C followed by water quenching. Next, the

surface oxide film formed on the samples during the solution-anneal was removed by

grinding. Afterwards, 3 samples were placed again into a ceramic crucible for the age-

hardening heat treatment for 10 hours at 720°C followed by water quenching. The surface

oxide film formed on the samples during the age-hardening treatment was removed by

grinding.

Fig. 17 Ceramic crucibles for the samples’ heat treatments.

The materials to be studied consisted of 3 groups. The first group of samples were the

“as received” as-AM (later on referred as AR) condition. The second group had the “solution

anneal treatment” (later on referred as SA) condition. The third group had the “solution

anneal plus age hardening treatment” (later on referred as FT = fully treated) condition. The

final dimensions for each sample were approximately: 15.7 mm length, 6 mm width and 3.5

mm depth.

Two (2) samples of each group were mounted in a transparent epoxy resin (araldite

resin 3138 mixed with its araldite hardener 3140, ratio 10:1) after which they were spot-

welded to a copper wire covered with plastic tubing. These samples were mounted in this

way to facilitate electrochemical testing. Specimens were ground using carbide paper #80

up to #1200 for the initial tests. For the later tests samples were ground using just carbide

paper #320.

One (1) sample from each group was mounted in a temporary black epoxy resin

(powder resin was heated, pressed and shaped into a mounting machine). The purpose of

using this resin is to have a larger area to hold during subsequent metallographic specimen

41 | P a g e

preparation. For the metallographic preparation, the samples were ground using SiC papers

from #80 up to #4200, followed by diamond polishing (from 3 µm up to 0.25 µm). These

samples were analysed by SEM, EDX and XRD.

4 CHAPTER 4

4.1 RESULTS AND DISCUSSIONS

4.1.1 Microstructure Analysis by SEM, EDX & XRD Methods.

4.1.1.1 Scanning Electron Microscopy (SEM)

The microstructural visualization of the samples via SEM was obtained by two

techniques for surface finish preparation.

- The first technique examined the microstructure of the samples after they were

ground and polished (using OPS). The polished surface was obtained by using a 0.25

µm diamond water base suspension, and subsequently with an oxide polishing

suspension (OPS) which works mechanically and chemically on the sample. The

oxide suspension reacts chemically with the surface of the specimen, resulting on

reaction product layer. This brittle layer is removed as a result of the small abrasive

particles in the suspension. Thus, it shows the “topography” of the sample.

Afterwards, the surface is washed thoroughly with soap and rinsed with water and

ethanol. Once the desired surface is achieved, the samples are placed into the SEM

chamber to be visualized. The SEM - EDX used was the Zeiss ∑IGMA HD VP-

equipped with an Oxford Instruments Xmax 150 Silicon Drift Detector and Aztec

analysis system.

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a)

b)

c)

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d)

Fig. 18 BSE images of the “as received” as-AM sample. Note the complex grain

structure shown from the electron channelling contrast.

a)

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b)

c)

d)

Fig. 19 BSE images of the “solution annealed” sample showing the grain morphology

due to electron channelling contrast.

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a)

b)

c)

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d)

Fig. 20 BSE images of the “solution annealed plus age hardened” sample.

From the BSE images, in figures 18, 19 and 20, the microstructural variation

from one sample to another was observed on the 3 samples. There was a marked contrast

between the “A.R.” sample and the “S.A.” sample, as well as with the “F.T.” sample. The

“A.R.” sample showed a compact, complex and random-oriented grain distribution; showing

a pattern in the laser melting deposition direction (figure 18a). The grains tended to have an

elongated shape for the “A.R.” sample (figures 18a and 18b), while for the “S.A.” and “F.T.”

samples the grains showed diverse shapes and were spread in a random distribution across

the samples (figures 19a, 19b, 20a and 20B). For the “S.A.” the grains exhibited sharper

boundaries in contrast to the “F.T.” sample. “F.T.” microstructure exhibited more

resemblance to the age-hardened microstructure obtained by Rao et al, (Rao et al., 2003)

showing a wider distribution of precipitates at the grain boundaries.

There were areas on the “A.R.” sample where the grain boundaries could not

be seen unless under high magnification, this may be related to sub-grains as can be seen in

figure 18c and 18d. The grain size for “A.R.” sample was smaller than “S.A.” and “F.T.”

samples respectively. Thus, grain size for the samples obeyed the trend “A.R.” < “S.A.” <

“F.T.”. The higher magnification images for “A.R.” sample allowed the observation of a clear

boundary area between neighbouring grains, as well as the grain direction (layer over layer

type). The presence of precipitates, white particles with blocky and globular shape, was

observed for all the samples. These precipitates exhibited a more uniform distribution

across the “S.A.” and “F.T.” samples in contrast to the “A.R.” sample that exhibited them at

47 | P a g e

the grain boundaries (figure 18d). Higher magnifications were necessary to observe these

precipitates for the “A.R.” sample in comparison to the other samples. The chemical

composition for these precipitates was analysed via EDX and can be observed in section

6.1.2. There was a greater number of observed holes on the surface for the heat treated

samples in comparison to the “A.R.” surface sample.

As can be regarded in figures 18, 19 and 20, they were BSE images, where a

contrast in grain colour was observed easily, this is related to the atomic number of each

grain, where grains with a higher atomic number exhibit generally a darker colour than

those with a lower atomic number, this feature is correlated with the EDX analysis on the

next section. Another important factor that was observed to influence the contrast colour

between grains is the Electron Channelling effect due to the interaction between the

primary electron and the crystal structure. as exposed by Lloyd (Lloyd, 1987).

Thus, the effect provoked by heat treating the samples was observed and

corroborated through the previous-exposed results. These results matched with the

recrystallization criteria after solution annealing a sample. Although by performing an age

hardened heat treatment the precipitates are likely to occur, they were not observed by

SEM. Thus, the use of a different technique is suggested.

- The second technique used was via Etching or Electrolytic Etching. Etching consists

on producing a microstructural contrast by means of an etchant that reacts with

the sample without applying an external current (with external current for the

Electrolytic Etching). The etchant performs a selective dissolution according to the

electrochemical characteristics of the sample microstructural constituents. In

Electrolytic Etching a potential is applied to the specimen as the anode and a

counter electrode as cathode (conventional platinum counter electrode) immersed

in the etchant (electrolyte). As consequence there is specimen dissolution which

allows the visualization of the microstructure. (HANDBOOK, 1992) For this

technique, the surface finish was polished up to 0.25 µm diamond water base

suspension.

Following the recommended “etching and electrolytic etching” techniques

exposed in the Metallography and Microstructures ASM Handbook (HANDBOOK, 1992), the

first attempt was via an Aqua Regia solution (20 mL HNO3 and 60 mL HCl) which didn’t

48 | P a g e

expose the microstructure of any of the samples. The second attempt was by using an

electrolytic etch (80 mL H3PO4 and 10 mL H2O), applying 3 V dc (closed-circuit), 0.11-0.12

A/cm2, and with an immersion time of 7-9 s per sample. This electrolytic etching only

exposed the microstructure of the “as received” sample, while on the others samples the

result was a black corrosion product on top of the metal surface. As results with this

technique were not satisfactory for the course of the project, the use of SEM technique was

adopted because the selective action on different phases in the microstructure is easier to

analyse. Nevertheless, it is important to mention that by selecting an appropriate

electrolytic technique, the microstructure can be evaluated for the remaining samples (S.A.

and F.T.) like the analysis performed by (Radavich, 1997, Ferrari et al., 1989).

a)

b)

Fig. 21 Light optical micrographs of the “as received” (as-AM) sample prepared via

Electrolytic Etching (magnification 10x). a) Boundary between the immersed (etched) and

non-immersed regions, b) Central region from the immersed area.

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4.1.1.2 Energy Disperse X-ray Spectroscopy (EDX)

The samples were analysed by EDX using the same SEM device which

accelerated the electrons at 20 kV, it worked up to an energy range of 20 keV; giving the

chemical composition of each sample.

For the “A.R.” sample, the area analysis enclosed in figure 22a) bore the next

data for chemical composition:

a)

b)

Fig. 22 Chemical composition for spectrum 2 in the “A.R.” sample. Note the high

peak of Ni denoting a high content of Ni as it can be anticipated for this alloy.

From spectrum 2 on the previous figure, the following table depicts the

results.

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Table 4 – Chemical Composition values for the “A.R.” sample.

SPECTRUM

2

Element Ni Cr Fe Nb Mo Ti Al

Wt.% 53.1 19.2 17.7 5.1 3.1 1.1 0.7

Specific and punctual EDX analysis were chosen for the “S.A.” sample,

because of the presence precipitates at the grain boundary as well as in the interior of the

grain. The spectra analysed from figure 23a bore the next data for chemical composition:

a)

b)

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c)

d)

e)

Fig. 23 Chemical composition for spectrum 3 (23b), 4 (23c), 5 (23d) and 6 (23e) were

examined in the “S.A.” sample.

From spectra 3, 4, 5 and 6 in the figure 23a, the following table depicts the

results.

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Table 5 – Chemical Composition values for the “S.A.” sample.

SPECTRUM 3 Element Ni Cr Fe Nb Mo Ti Al

Wt.% 21.3 9.8 8.3 52.7 1.8 5.9 0.3

SPECTRUM 4 Wt.% 14.2 6.4 5.3 37.8 - 4.4 -

SPECTRUM 5 Wt.% 53.3 19.3 17.7 5.1 2.8 1 0.8

SPECTRUM 6 Wt.% 54.6 19.8 18.1 4.8 - 1 0.7

Specific and punctual EDX analysis was chosen for the “F.T.” sample, due to

the presence of precipitates at the grain boundary as well as in the interior of the grain. The

spectra analysed from figure 24a bore the data for chemical composition:

a)

b)

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c)

d)

e)

Fig. 24 Chemical composition for spectrum 8 (24b), 9 (24c), 10 (24d) and 11 (24e) were

examined in the “F.T.” sample.

From spectra 8, 9, 10 and 11 in the figure 24a, the following table depicts the

results.

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Table 6 – Chemical Composition values for the “F.T.” sample.

SPECTRUM 8 Element Ni Cr Fe Nb Mo Ti Al

Wt.% 15.6 7.3 5.9 60.4 1.7 8.9 0.3

SPECTRUM 9 Wt.% 21.1 9.3 8.2 53.4 1.6 6.8 -

SPECTRUM

10 Wt.% 52.8 19.1 17.8 5.2 3.3 1.1 0.7

SPECTRUM

11 Wt.% 36.2 14.6 12.7 32.3 - 3.7 0.5

Comparing the chemical composition results obtained in Table 4 and Table 1,

the close resemblance between the usual composition of Alloy 718 (conventionally

manufactured) and the composition of Alloy 718 (SLS manufactured) could be observed.

Thus, the SLS manufacturing technique accomplishes to meet the desired composition for

Alloy 718, and this was the reason to do a general analysis for the “A.R.” sample.

Subsequently, a general analysis by EDX was not done for “S.A.” and “F.T.” because they

come from the same bulk material.

Results from Table 5 showed a slight difference between the chemical

composition for spectrum 5 and 6 where each spectrum corresponded to a different grain

as seen on figure 23a. But even with the slight difference in chemical composition their

atomic number is influenced. Their composition showed high similitude with the general

composition for Alloy 718. Hence, spectrum 5 and 6 belong to the matrix, which is a

similar result like the work of Burke and Miller (Burke and Miller, 1991). Spectrum 3 and 4

were chosen because they showed precipitates at the grain boundary and inside the grain

respectively. As it was expected, composition of spectrum 3 and spectrum 4 was

considerably different when compared with the matrix. The results for spectrum 3 and 4

confirmed the presence of precipitates as because their chemical compositions were

different from that of the matrix.

In Table 6, the results showed a slight difference between the chemical

composition for spectrum 10 and the general composition for Alloy 718, a similar behaviour

like spectra 5 and 6. Spectrum 9, 11 were precipitates at the grain boundary, spectrum 8

was a precipitate inside the grain boundary as can be seen on figure 24a. The obtained-

55 | P a g e

chemical composition for spectrum 8, 9 and 11 was significant, which denoted they are a

certain type of precipitate because they had a different chemical composition from the

matrix.

4.1.1.3 X-ray Diffraction (XRD)

XRD was used to analyse the samples further. The surface finish was the same

one for the SEM-EDX analysis. The main crystal structure in each sample was obtained. The

beam for XRD was focused in all the surface of the sample and not in a specific point. With

this technique the fcc matrix structure was attained. The XRD profiles of the samples were

recorded using a BRUKER D8 Discover X-ray diffractometer. The scan was performed in the

angular range from 20ᵒ to 129.98ᵒ, with a step size of 0.02ᵒ, with a scan step time of 120 s

and with X-ray generator power set at 1.4 kW (35 kV and 40 mA).

For the “A.R.” sample the Pattern List shows the presence of NiFe and Ni3Nb as

can be seen on the following table.

Table 7 – Phases revealed in the “A.R.” sample

Phase Structure Lattice constant, nm

Iron Nickel (Fe0.58Ni0.42) FCC a = 0.3597

Nickel Niobium (Ni3Nb) Tetragonal a = b = 0.362

c = 0.741

Fig. 25 XRD pattern of the “A.R.” sample.

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For the “S.A.” sample the Pattern List shows the presence of NiFe as can be

seen on the following table.

Table 8 – Phases revealed in the “S.A.” sample

Phase Structure Lattice constant, nm

Iron Nickel (Fe0.66Ni0.34) FCC a = 0.3597

Fig. 26 XRD pattern of the “S.A.” sample.

For the “F.T.” sample the Pattern List shows the presence of NiFe as can be

seen on the following table.

Table 9 – Phases revealed in the “F.T.” sample

Phase Structure Lattice constant, nm

Iron Nickel (Fe0.66Ni0.34) FCC a = 0.3597

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Fig. 27 XRD pattern of the “F.T.” sample.

The XRD results showed that a NiFe phase was present in all the samples, and

Ni3Nb was only observed in the “A.R.” sample. The Ni3Nb relates to the phase in

composition but not the structure type. Whilst for the “S.A.” and “F.T.” the NiFe phase

showed a slight difference in concentratrion for Fe and Ni. The XRD results shown in Table 7,

8 and 9 did not provide enough data as the work performed by Slama et al (Slama et al.,

1997) due to the reason exposed on the XRD analysis of intermetallic phases or XRD

performed by Li et al (Li et al., 2002) where was stated that these phases could not be

analysed by XRD unless they were extracted from the matrix. An example of extraction was

performed by Evers et al (Evers et al., 1978), where the analysis of second phases using an

electrolytic extraction technique was performed. But this technique was out of the scope of

this project so it was not performed.

4.1.2 Electrochemical Tests.

The surface preparation for all the samples was described in section 5.2. Once the

surface was ground-ready, beeswax resin was used to paint – cover the gap between the

alloy and the mounting araldite resin. Therefore, this gap which was present throughout the

perimeter of the sample was isolated from the aqueous solution, so the effect of crevice

corrosion due to that gap could be discarded. The aqueous solution for electrochemical

measurements was aerated, unstirred, 0.1 M HCl (pH = 1), 1 M NaCl at 25°C (room

58 | P a g e

temperature). Later on, more tests were subsequently performed in the same solution with

addition of 0.01 M NaSO3 and by increasing the temperature up to 45°C. Previous the start

of any test, all the potentiostat connections to the electrodes (working, reference and

counter) were checked as seen in figure 8b.

4.1.2.1 Open Circuit Potential Test (OCP)

In order to reach a stable OCP value, the samples were immersed in the

previously mentioned solution for 24 hours before being measured. Subsequently, they

were measured during 1 hour, and the time interval was 0.5 s. The next figure details the

results for each couple of samples of each group.

Fig. 28 OCP curves for “A.R.”, “S.A.” and “F.T.” samples.

OCP tests revealed a steady potential behaviour for all the analysed samples.

All the samples showed a slight increase in their potentials throughout the test. This

increase in the towards less negative potentials showed the normal resistance of the alloy

against the corrosive environment due to the formation of a protective film as proposed by

(Chen et al., 2013).

It was observed that “A.R.” samples showed a more noble potential in

comparison to “S.A.” samples, the same behaviour was seen between “S.A.” and “F.T.”.

Thus, the less noble samples were “F.T.”, followed by “S.A.” samples. The potential

59 | P a g e

differences between each couple of samples was relatively small. For the “A.R.” samples the

potential difference was approximately 10 mV, for the “S.A.” samples the potential

difference was approximately 24 mV, and for the “F.T.” samples the potential difference

was approximately 14 mV.

4.1.2.2 Linear Polarization Resistance (LPR)

After OCP test were performed on every sample, each sample was analysed

by Linear Polarization Resistance (LPR) in the same aqueous solution used for OCP tests. The

set up for these tests was: E1 vertex at -0.01 V from OCP, E2 vertex at +0.01 V from OCP, E

step at 0.5 mV, Scan rate at 0.2 mV/s, and cyclic voltammetry as software technique. The

area exposed of the samples was 0.5 cm2. The following figure displays the obtained curves

for all the samples.

a)

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b)

Fig. 29 LPR curves for “A.R.”, “S.A.” and “F.T.” samples. 29a cyclic voltammetry graph and

29b cyclic voltammetry with log current.

Every experiment was performed in a small lapse of time (approximately 15

min), due to the small delta potential applied (-10 mV from OCP to +10 mv from OCP)

together with the set up mentioned at the beginning of this section.

Figure 29b showed that “A.R.” samples are more noble than “S.A.” and ”F.T.”

samples, in the same ways as it was observed for the OCP test described in section 6.2.1.

With the use of Excel the trend line for each curve was obtained. An example

of this trend line step is detailed in the next figure. This step was performed in the same

manner for all the samples. The trend line is displayed in a black line with its corresponding

equation.

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Fig. 30 LPR curve for “A.R. 1” sample with its trend line (black line) and the trend line

equation.

From literature the values for βa and βc were known for the experiment

performed by Amin et al, where the solution used was 1M H2SO4 (pH = 0), aerated at room

temperature. These conditions are quite similar to the conditions in this project, the only

change is the value of pH (from 0 to 1) due to 1M H2SO4 in comparison to 0.1 M HCl used for

this project. This pH change is considered not to affect considerably the values for βa and

βc.

Consequently, these values were used for the calculation method via LPR.

Subsequently, using the Data Analysis tool on Excel, through a statistical regression (which

exhibits the relationship between the current factor and the potential factor) the Rp value

for each sample was obtained. Hence, by using equation (9) the calculation of icorr was

achieved. Afterwards, the corrosion rate was estimated by using equation (8). The values for

Rp and icorr for each sample are shown in the following table.

Table 10 – Corrosion Rate via LPR method.

Rp ( ) icorr (uA) Corr. Rate (mm/year) Corr. Rate Average (mm/year)

AR 1 0.094996384 0.312794 0.007383036 AR 0.004778

AR 2 0.322803136 0.092051 0.002172723

SA 1 0.101038924 0.294088 0.0069415 SA 0.010263

SA 2 0.051628109 0.575545 0.013584881

FT 1 0.041865573 0.709755 0.016752709 FT 0.015199

FT 2 0.051399689 0.578102 0.013645253

From these results, the exposed corrosion rate for the “A.R.” samples was

lower than the corrosion rate for the “S.A.” and “F.T.” samples. Thus, heat treating Alloy

718, which was manufactured by the SLS method, can increase the corrosion rate. However,

the corrosion rate for the heat treated samples was inside the range for similar corrosion

resistant alloys, C.R. ≤ 0.05 mm/year (Craig and Smith, 2011).

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4.1.2.3 Full Polarization Tests

After LPR tests were performed on every sample, the samples undergone by

full polarization analysis. In the same aqueous solution used for the previous tests. The set

up for these tests was: E start at -0.35 V from OCP, E1 vertex at -0.351 V from OCP, E2 vertex

at +2 V from OCP, E step at 0.5 mV, Scan rate at 0.5 mV/s, and cyclic voltammetry as

software technique. The area exposed of the samples was 0.5 cm2. Table 11 shows the set

of each condition used for this experimental stage.

Table 11 – Conditions for Full Polarizations Tests.

CONDITION BASE T oC SURFACE FINISH + EXTRA CONDITION

1 Aerated,

unstirred,

0.1 M HCl

and 1 M NaCl

25 Ground (#1200 carbide paper)

2 25 Ground (#1200 carbide paper) + 0.01 M Na2SO3

3 25 Ground (#320 carbide paper) + 0.01 M Na2SO3

4 45 Ground (#320 carbide paper)

4.1.2.3.1 Full Polarization Under Condition 1

The scan started from a cathodic potential towards an anodic potential.

The next figure displays the obtained curves for all the samples tested under Condition 1.

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Fig. 31 Full Polarization for all the samples under Condition 1.

Figure 31 showed the cathodic and anodic regions for each sample. But

for the concern of this project the analysis of the anodic region was detailed. The anodic

polarization curves revealed active, passive and transpassive behaviour for all the tested

samples. All the samples reached the transpassive region approximately at 0.9 V where

there was a significant rise in current, this point exhibited the dissolution of the metal

because the passive film was broken down due to oxidation of Cr(III) to Cr(VI) in the film as

reported by Betova et al. (Betova et al., 2002). The evolution of gas on the surface was not

seen in the transpassive region. The scan was reversed before reaching a high current in

order to avoid over loads on the potentiostat. During the reverse scan the curve showed a

path close to the initial scan line. All samples exhibited a marked wide passive region, free of

localized corrosion, where there was a minimum rise in current as potential was increased.

The mechanism for the formation of a passive film is considered to be via a solid-state

mechanism as detailed by Singh and Gupta (Singh and Gupta, 2004). The presence of well-

pronounced critical currents was evident for both “A.R.” samples as well as for SA1. While a

small critical current was exhibited for the rest of samples (SA2, FT1 and FT2). All the

samples demonstrated a narrow active region which is a desired characteristic for a

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corrosion resistant alloy. The surface of the samples was examined after the experiment and

it did not exhibit pits, just a general slight brownish layer on the surface.

It was visible that “A.R.” curves are shifted more to the left in contrast

to the “S.A.” curves, denoting a better corrosion resistance. The curve for sample FT2

exhibited a noble and unexpected behaviour which was reflected through its position close

the “A.R.” samples. While the curve for sample FT1 was coherently allocated on the right

side of the figure indicating a lower corrosion resistance than all the samples.

4.1.2.3.1.1 Corrosion Rate Evaluation via Tafel

Extrapolation.

The corrosion rate was estimated using displayed data on figure

31, by using the cathodic Tafel region of each curve. As detailed previously in section

6.2.3.1, the active region of the anodic branch was too narrow in order to apply an anodic

Tafel extrapolation. Thus, as described in the literature review, this led to achieve higher

corrosion rates than the values obtained in section 6.2.2 via LPR method. But it confirmed

the trend of corrosion resistance among the “A.R.”, “S.A.” and “F.T.” samples. This method

was applied for samples under condition 1 in order to have the testing conditions as the

used for the LPR method. Figure 32 is an example of the procedure followed to estimate the

corrosion rate via this method.

The procedure consisted on selecting a linear region, necessary

for Tafel Extrapolation, located at -50mV from Ecorr (cathodic region). A new curve (named

LINEAR PART CATHODIC, red line) was plotted on top of the initial curve (blue line). The

linear equation of the red line was achieved by using the trend line tool from Excel. This

equation served to extrapolate the cathodic Tafel region which was plotted as

EXTRAPOLATION CATHODE LINE (green line). The Ecorr potential was selected by filtering the

smallest value of “log i” from the initial data. Consequently, Ecorr line was plotted (purple

line). The intersection between purple and green line provided the icorr value. Subsequently,

the corrosion rate can be estimated by using icorr value in equation (8).

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Fig. 32 Tafel Extrapolation applied on sample AR1 in condition 1.

The same procedure was used to estimate the corrosion rate for the

other samples, and these data is displayed in Table 12. Sample SA2 exhibited a significant

low corrosion rate. This is because the cathodic branch for SA2 curve on figure 31 did not

follow the same behaviour as the other curves. The low corrosion rate obtained for FT2

sample has a connection with the position of this curve in figure 31.

Table 12 – Conditions for Full Polarizations Tests.

SAMPL

E

Icorr

(uA/cm2)

Corrosion Rate

(mm/year)

AVER

AGE

Average Corr.

Rate (mm/year)

AR1 44.28 1.04 AR 0.38

AR2 21.73 0.51

SA1 212.95 5.02 SA 2.57

SA2 4.46 0.11

FT1 286.89 6.77 FT 3.64

FT2 21.63 0.51

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Thus, the corrosion resistance of Alloy 718 (manufactured via SLS) when

it is subjected to heat treatments.

4.1.2.3.2 Full Polarization Under Condition 2

The scan started from a cathodic potential towards an anodic potential.

The next figure displays the obtained curves for all the samples tested under Condition 2.

Fig. 33 Full Polarization for all the samples under Condition 2.

Similarly as in Condition 1, figure 32 showed the cathodic and anodic

regions for each sample under Condition 2. But for the concern of this project the analysis of

the anodic region was detailed. The anodic polarization curves revealed active, passive and

transpassive behaviour for all the tested samples. All the samples reached the transpassive

region approximately at 0.9 V where there was a significant rise in current, this point

exhibited the dissolution of the metal because the passive film was broken down due to

oxidation of Cr(III) to Cr(VI) in the film as reported by Betova et al. (Betova et al., 2002).

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However, there was evidence of a secondary repassivation for FT2, SA1 and SA2; which was

marked for the action of a current plateau at 1.1 V approximately. Since all the samples

have the same chemical composition, the effect of Cr content was rejected for the

formation of the secondary passivation as well as the presence of sulphate anions which

influence the stability of the secondary repassivation. (Bojinov et al., 2002, Keddam et al.,

1985). As potential continued increasing, at 1.2 V approximately there was a rise in current,

and before reaching a high current value the scan was reversed. The evolution of gas on the

surface was not seen in the transpassive region. During the reverse scan the curve showed a

path close to the initial scan line. All the samples exhibited a small passive region followed

by the presence of a metastable pitting zone. The passive range in this condition was smaller

than that from Condition 1. Even though there was metastable pitting, no pitting corrosion

was observed. All the samples demonstrated a wider active region than those from

Condition 1. The surface of the samples was examined after the experiment and it did not

exhibit pits, just a general yellowish layer on the surface.

The position of the curves in figure 32 was aligned with the corrosion

resistance trend for these samples. “A.R.” are more corrosion resistant than “S.A.” samples

and than “F.T.” samples.

4.1.2.3.3 Full Polarization Under Condition 3

The scan started from an anodic potential, Figure 33 displays the

obtained curves for all the samples tested under Condition 3. Similarly as in Condition 1 and

2, figure 33 showed the cathodic and anodic regions for each sample under Condition 3. But,

in contrast to previous conditions, the anodic branch for all the curves revealed the

presence of localized corrosion because the passive region was interrupted by the presence

of Crevice corrosion. The range at which crevice corrosion was observed was from 0.25 V to

0.35 V approximately. The main outcome of this condition was the visualization of Crevice

corrosion on the surface of the samples. It was seen on the interface (gap) between the

beeswax resin and the metal surface, attributed to an error during the application of the

resin (resin was not heated until full liquid state) which did not allow the resin to adhere

normally to the sample surface, consequently a crevice was formed, where a more

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aggressive microenvironment was developed. An example is shown by red arrows in figure

34.

Fig. 34 Full Polarization for all the samples under Condition 3.

Fig. 35 Crevice corrosion action on the samples.

From figures 33 and 34 it was also evident a well uniform passive film

on the surface whithout being affected because of the surface finish (groun with #320

carbide paper).

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4.1.2.3.4 Full Polarization Under Condition 4

The scan started from an anodic potential, The next figure displays the

obtained curves for all the samples tested under Condition 4. Similarly as in Condition 1,

figure 35 showed the cathodic and anodic regions for each sample under Condition 4.

Where the main feature is the action of temperature which shifted all the curves towards

the right side by 1 decade in current.

Fig. 36 Full Polarization for all the samples under Condition 4.

The anodic polarization curves revealed active, passive and transpassive

behaviour for all the tested samples. All the samples reached the transpassive region

approximately at 0.9 V where there was a significant rise in current, this point exhibited the

dissolution of the metal because the passive film was broken down due to oxidation of

Cr(III) to Cr(VI) in the film as reported by Betova et al. (Betova et al., 2002). Only the SA1

sample showed a small secondary repassivation which lasted until 1.2 V, where there was

again an increase in current. No gas evolution was observed on the surface during the

transpassive region. In order to avoid over loads on the potentiostat the scan was reversed

before reaching a high current. During the reverse scan the curve showed a path close to

the initial scan line. All samples exhibited a clear wide passive region, free of localized

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corrosion, where there was a minimum rise in current as potential was increased. A well-

pronounced critical current was observed just for the FT2 sample, as well as a petite critical

current was exhibited for the AR1 sample, but the rest of the samples did not show this

characteristic. The active region for all the samples was wider than that for samples under

Condition 1. The surface of the samples was examined after the experiment and it did not

exhibit pits, just a general pale brownish layer on the surface.

The position of the curves in figure 35 was aligned with the corrosion

resistance trend for these samples. “A.R.” are more corrosion resistant than “S.A.” samples

and than “F.T.” samples. Where the FT1 sample exhibited the lowest corrosion resistance

because it was positioned more to the right on the sample. This behaviour observed in

figure 35 is very similar to the behaviour exposed for Condition 1.

4.1.2.4 Electrochemical Impedance Spectroscopy (EIS)

This experimental stage was performed in an aqueous solution with deionized

water, dearaeted with N2. The testing was performed in a static autoclave. The use of 2

coupon electrods acting as working (AR sample) and counter (FT sample) electrodes, 1

Platinum wire was used as pseudo reference electrode. The purpose to use an autoclave

was to estimate the effect on corrosion when the samples work at high temperature and

high pressure simulating Light Water Reactor conditions.

Test setup

The samples were spot welded on one of the top sides connecting a wire to

each sample. The wires should be covered by Teflon tube. The two coupon specimens shall

be mounted inside the autoclave parallel to each other with a distance of 5 mm (without

touching the Pt wire or each other).

Between the two specimens a Pt-wire shall be mounted in the centre of the

two electrodes, working as a pseudo reference electrode. In addition, if available, a high-

temperature reference electrode may also be mounted into the autoclave to allow

measurement of redox and corrosion potential (ECP), which can be compared between the

participants. Furthermore, the temperature inside the autoclave shall be monitored using a

thermocouple. Prior to start the testing all parts were cleaned using ethanol in order to

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remove any pollution (e.g. hand perspiration). The autoclave was cleaned and flushed with

high-purity (deionized) water prior to testing. Oxygen was removed by purging nitrogen for

30 minutes and to make an overpressure of approx. 5 bar nitrogen should be adjusted. After

nitrogen purging, the first EIS measurement shall be performed (cold condition = room

temperature) and then at 24 hours, 1 week and 2 weeks after reaching the testing

temperature of 288°C (related to 90 bar pressure). The set up for the samples inside the

autoclave are shown in figure 36.

Impedance measurements were carried out using AC signals of amplitude 10

mV RMS, in the frequency range from 100 kHz to 1.0 mHz. All these parameters was

established by the European Cooperative Group on Corrosion Monitoring of Nuclear

Materials.

a)

b)

Fig. 37 Setup of the specimens inside the autoclave.

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Fig. 38 Nyquist plot.

Fig. 39 Bode Plot.

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Fig. 40 Bode Plot.

Figure 37, 38 and 39 were acquired at room temperature. From these data it

was visible in figure 37 a smaller semicircle curve for the “AR” sample in comparison to the

“FT”. The transfer resistance, film resistance and the diffusion resistance of passive films on

both samples increase with increasing formation potential, prolonging formation time,

which indicates an enhanced film protection with increasing time.

The reduced impedance recorded for the “AR” exhibited faster charge-transfer

(corrosion) process that occurred on the surface of the sample. Thus, the passive film

formed for “FT” sample showed more protective characteristics.

The solution resistance exhibited the same value for both samples, as espected

and as can be observed in figure 39 on the onset of the curves which was the same for both.

The curves obtained at 288°C were highly distorted in order to be analysed.

This distortion may be attributed to the low conductivity of deionized water at high

-120

-100

-80

-60

-40

-20

0

20

1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

ph

i (d

eg)

log f (Hz)

Bode Plot

Bode AR

Bode FT

74 | P a g e

temperature and pressure, another reason was the distance between samples was not the

appropriate leading to a possible contact among them. The samples after being extracted

from the autoclave showed a opague and well uniform passive film on the surface of both

samples.

Unfortunately, the obtention of the equivalent electric circuit and more EIS

data was not done in this project due to the length of time required for the tests which were

up to 2 weeks. Consequently, further experiments and analysis need to be done at the high

temperature and pressure conditions, and if not an ex-situ (out of the autoclave)

experiment can be made, once the formation of the passive film due to high temperature

and pressure was formed.

4.1.3 Devices Used for Microstructural Analysis and

Electrochemical Experiments.

The following devices were used for the different experiments in this project.

A Potentiostat PalmSens3 was used to allow the application of the

potentiodynamic techniques as well as for the impedance analysis. All the data

from this device is linked to PSTrace electrochemistry software.

Fig. 41 Potentiostat and PSTrace Software.

An Optical Microscope Axio from Zeiss was used to visualize the surface

preparation or surface result after electrochemical tests.

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Fig. 42 Optical Microscope.

A Scanning Electron Microscope ∑IGMA HD VP from Zeiss was used for SEM and

EDX tests.

Fig. 43 SEM - EDX microscope.

An Autoclave was used for the high temperature and high pressure tests.

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Fig. 44 Autoclave.

An X-ray Diffraction D8 Discover from Bruker was used for the XRD test.

Fig. 45 XRD microscope.

A Heater and Water Bath Set were used for the potentiodynamic test at a higher

temperature (45°C) .

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Fig. 46 Heater and Water Bath.

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5. CHAPTER 5

5.1 CONCLUSIONS

Based on the results discussed under Chapter 4 of this work, the following conclusion are

drawn on the corrosion behaviour of Alloy 718 immersed in an aqueous solution (0.1 M HCl

plus 1 M NaCl) when Alloy 718 is manufactured by SLS and has undergone through a heat

treatment.

The microstructure of Alloy 718 manufactured via SLS showed a compact,

complex and random-oriented grain distribution. Under high magnification the

presence of subgrains was seen. The grain morphology was elongated and

exhibited contrast among neighbour grains. While the heat treated samples

exhibited grain morphology with diverse shapes, including sharp boundaries, and

were spread in a random distribution. All the samples exhibited the presence of

precipitates at the grain boundaries, and for the heat treated samples these

precipitates were present as well into the grains.

The contrast among grains was related to a different atomic number. Which was

correlated by EDX analysis. The XRD analysis exposed the crystal structure of the

matrix, but the structures and compositions of the different precipitates was not

observed because of the surface preparation.

The electrochemical tests showed the trend for corrosion resistance among the

samples. Where the more corrosion resistant samples were the samples which

did not undergo any heat treatment. The less corrosion resistant was the sample

which underwent for full heat treatment (solution annealed plus age-hardened).

The effect of increasing temperature from 25 to 45 oC was evident by increasing

the corrosion rate on all the samples, which was visible on the shifting of all the

samples towards the right side.

The effect of chloride anions did not expose any pitting behaviour on the

samples, while the addition of thiosulphate bore the presence of metastable

pitting. The addition of 0.01 M Na2SO3 did not induce any stable pit.

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The surface finish of the sample was independent for the formation of a well

uniform passive film on all the samples.

The corrosion rate determination via Tafel extrapolation bore coherent results as

the results estimated via LPR, but with a considerable rise in value.

6. CHAPTER 6

6.1 FUTURE WORKS

The following aspects of the above work are to be studied further in order to achieve a

better understanding of Alloy 718 (manufactured by SLS and that has been heat treated)

corrosion behaviour:

Analysis for Light Water Reactor conditions through EIS method in order to get more

details about the level of protection of the passive film.

XRD analysis on samples that have been properly extracted through an adequate

electrolytic etching.

Further characterization of the microstructure of the SLS manufactured Alloy 718

needs to be studied.

Analysis on variation on chloride concentration as well as for pH should be studied.

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7. CHAPTER 7

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