Indian Journal of Chemical Technology Vol. 7, September 2000 pp.216-222

Tin-Zinc alloy electrodeposit- Physicochemical characterisation

V S Vasantha, Malathy Pushpavanam & V S Muralidharan Central Electrochemcial Research Institute, Karaikudi-630006, India

Received 6 January 2000; accepted 14 June 2000

Efforts are being made globall y to replace the cadmium plating, which is being outlawed due to its toxic and carcino­genic nature. Tin- zinc alloy plating is one such approach to combine the good properties of the parent metals such as sol­derability and ductility with sacrificial corrosion resistance and hardness. A gluconate bath, identified in this work offers many advantages. Some additional information on their solderability, hardness, porosity, crystallography and surface topog­raphy are di scussed in this paper.

Electrodeposition of alloys with specific properties is one of the latest trends in plating ~ ndustry. Tin - zinc alloys are attractive because they combine the barrier property of tin with the cathodic protection offered by zinc. The coatings protect steel better than zinc under conditions of high humidity and they resist corrosion equally when compared with an equal thickness of cadmium in marine and industrial atmosphere. In addition, they offer better solderability and brightness , ductility and service li fe with Jess white corrosion product formation when compared to zinc coatings. Compared to cadmium coating the alloy has better hardness, low hydrogen uptake and is cost effective 1

-4

•

The developmental studies of the tin-zinc neutral gluconate bath 5

-9

, performance of different anode materials , effect of additives, the hydrogen embrittlement behaviour and the corrosion res istance of these deposits were reported earli e1. The present work deals with physicochemical prope1ties like solderability, porosity, crystallography and surface morphology of the deposits obtained from the neutral gluconate bath and compared with the cadmium coating obtained from the fluoborate bath.

Experimental Procedure

A 200 mL capacity glass cell with a PVC cover provided with holes for positioning the anode, cathode and thermometer was used. Large size specimens for testing and evaluation were produced from a rectangular cell of 20x5x 15 em size with a water jacket for hot water circulation to maintain the temperature of the plating bath. The inner electrode distance was maintained as 15 em. To enable uniform thickness and to avoid edge growth , the cathode was provided with a frame to shield the current

concentrating at the edges. Laboratory grade chemicals were dissolved in distilled water and the solutions were pretreated. Insoluble platinised titanium anodes were used having area two times that of the cathode.

Polished steel substrates of 7.5x2.5x0.1 em size was used for initial experiments and 15x5xO.lcm size was used for testing the corrosion resistance of the deposits . The substrates were polished mechanically, degreased with trichloroethylene, cathodically polarised for 3 min and then polarised anodically for 1 min at a current density of 1 Ndm2 in a conventional cleaning solution. The specimens were washed, treated in a 5% sulphuric acid solution, again washed, rinsed and taken to the plating bath.

The hardness was determined by measuring the indentation produced on the deposit by a diamond point applied under a suitable load in a metallurgical microscope using Knoop indentation method 10

. For deposits having thickness values - 25 f.!m, 50 g load was applied. The deposits were subjected to standard bend test for adhesion .. The appearance of solder and its spread area was calculated to evaluate the solderability. The larger the area, the higher the solderability. Spread factor= { (D-/1)/D} x 100 where D is the diameter of the sphere having a volume equal to that of the solder used and H is the height of the solder spot.

Electrographic method -It was used to evaluate porositi 1• A filter paper soaked in a solution containing sodium chloride, HCl, HN03 and 0.1 M pheanthroline hydrochloride was sandwiched between the specimen under test and a pl atinum anode. A current of 0.32 Ndm2 [+ 0.3 V vs SCEJ was passed for a minute. The steel substrate was attacked through

V ASANTHA et al. : TIN-ZINC ALLOY ELECTRODEPOSIT CHARACTERISATION 217

Table !--composition and conditions of different plating baths

S.No Electrodeposit Composition Current density Alloy composition(%)

gpl Nm2 Tin Zinc

Tin SnS04 32.25 10·2 100

Sodium gluconate 163.50

Sodium acetate 20.00

2 Alloy-A SnS04 32.25 10·2 97.7 0.30

ZnS04 28.80

Sodium gluconate 163. 50

Sodium acetate 20.00

3 Alloy-B SnS04 32.25 2x10·2 90.0 10.0

ZnS04 28.80

Sodium gluconate 163.50

Sodium acetate 20.00

4 Alloy-C SnS04 32.25 2x10·2 86.5 13.5

ZnS04 28.80

Sodium gluconate 163.50

Sodium acetate 20.00

5 Alloy-D SnS04 43.00 2x10·2 77.8 22.2

ZnS04 14.40

Sodium gluconate 163.50

Sodium acetate 20.00

6 Alloy-£ SnS04 21.50 2x10·2 62.8 37.2

ZnS04 44.65

Sodium gluconate 163.50

Sodium acetate 20.00

7 Alloy-F SnS04 10.75 2x10·2 62.8 37.2

ZnS04 57.60

Sodium gluconate 163.50

Sodium acetate 20.00

8 Zinc ZnS04 28.8 10·2 100

Sodium gluconate 163.50

9 Cadmium Cadmium fluoborate 222.00 10·2

Ammonium fluoborate 5.00

Boric acid 20.00

T = 333K for 1-7 & 303K for 8-9 pH = 7 for 1-7 & pH = 2.3 for 9; 1-7 contained 1.5gpl peptone in the bath

the pores and its corrosion products reacted with the indicator giving coloured spots. The pores indicated by such spots were counted with the help of a microscope. The results were expressed as the % defective area12

.

Corrosion resistance test/3

Salt spray test-The deposited panels were suspended in the salt spray cabinet "Heraeus Voltsch"

containing 5% NaCl solutions. Spraying was done for 8 hours and the thereafter, the panels were kept at rest for the remaining 16 hours of every 24 hours. Observations were made at the close of every day. The hours of spraying as well as the total hours spent in the cabinet were expressed. The corrosion resistance of the deposits were evaluated as per ASTM standard (B537-70) meant for coatings cathodic to the steel substrate.

218 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000

Polarisation experiments-lOG mL of the 5% sodium chloride solution was used in a three electrodes cell assembly. The electrodeposit was made a working electrode, a large platinum electrode as a counter and a saturated calomel electrode as reference respectively. Tafel polarisation experiment was carried out by scanning the electrode potential at a rate of 5 V/s 200 mV on either side of the corrosion potential.

Galvanic potential and current measurements--Measurements of galvanic potential and current between the electrodeposit and polished mild steel of 1 sq em area was based on zero resistance ammeter{ ZRA} technique. The given current was measured by an ammeter, by adjusting the voltage, E or resistance R so that the potential difference between the two electrodes was zero as indicated by the electrometer. Since short circuiting in

'B -·c: ~ ~

" ~ 0

] 100 -.... ~ ~ Ul ~ -1- :. ~ ~ z :::> :J._ 0 u

~0 30

Fig . 1-X ray diffraction pattern obtained on the deposit E

c

-0

L.. (II -(II ~ 5·584 .... 0 0..

(II

.~ 5·582 :::: E

•

0 0 0

•

a galvanic couple was indicated by zero potential drop, this current was the true short circuited current. The galvanic current was obtained from the known resistance, R and the potential, E.

Structure and surface morphology-The electrodeposits were examined under high magnification to assess the grain size, deposit nature, heterogeneties using a scanning electron microscope [JEOL, Model]. X-ray diffractometry with Cu-Ka radiation was used to understand the phases, their orientation and lattice parameters.

Results and Discussion

Table 1 presents the composition and conditions of zinc, tin, cadmium and tin-zinc alloy deposition. The adhesion of the alloy deposits to steel substrate was tested by subjecting them to standard bend test and the coating was found neither to crack nor peel. Ferroxyl test revealed that above 6 J.lm thickness, the electrodeposit was nonporous.

The Vicker's hardnesses of 78% Sn and 63% Sn alloys were found to be 26.5 and 28.5 kg/mm2

respectively (Table 2) compared to 12-16 kg/mm2 for tin deposit. A higher hardness of the alloy is likely to increase the wear resistance of the coating. Table 3 presents the spread factor for various deposits. On a porous electrodeposit through the pores, base metal iron atoms would reach the outer surface. However an increase of thickness of the deposit would decrease the pores and prevent oxide formation. The

r . -0 .... Q)

2·6476 C;j E 0 .... 0 0..

~

Weight of zinc and tin in the deposit

Zn- -Tin

Fig. 2 --Changes in the lattice parameter (A) of tin and zi nc with we ight % of alloying element in the depositi0-0-0 tin ; •-•-•-• zinc]

Y AS ANTHA eta/. : TIN-ZINC ALLOY ELECTRODEPOSIT CHARACTERISATION 219

Fig 3-Scanni ng electron microscopic pictures of alloy contai ning different % of Zinc (a) I 0 % (b) 22%, (c) 37%, (d) 67%

Table 2- Yicker' s hardness values fo r 6 11m deposits

Electrodeposit

Tin

Cadmium

78%Sn - 22% Zn

63%Sn - 37% Zn

Zinc

Hardness kg/mm2

14.5

21.0

26.5

28.5

34.5

Table 3-Soderability- Spread factor for the alloy deposits

Electrodeposit Thickness Spread Factor (±2%) Jlm

78% Sn - 22% Zn 1.5 87

78% Sn - 22% Zn 6.0 90

78% Sn- 22% Zn 12.0 93

63% Sn - 37% Zn 1.5 83

63% Sn- 37% Zn 3.0 85

63% Sn- 37% Zn 6.0 88

63% Sn - 37% Zn 12.0 90

solderability would increase and the spread factor is enhanced. The presence of zinc and an increase of zi nc content in the alloy deposit would hinder the solderability of the alloy. A value of percentage >90 was established to be excellent14

•

Structure of the deposits The X-ray diffraction patterns were obtained for

various alloy deposits and Fig. 1 presents the XRD pattern obtained for 63% tin alloy. When the tin content was 98% Sn (220) plane was predominant. From 63% to 90% tin content, Sn(lOl) plane was predominant. At 44% tin content, Sn(200) plane was predominant. The reflections from zinc(ll2) plane was seen on the deposits of 63% tin. The lattice parameters (A) for tin and zinc were found to be invariant with the weight % of zinc and tin respectively in the deposit (Fig. 2). The structure of the electrodeposited alloys can be derived from . a study of lattice parameters of their phases. When an alloying metal replaces the atoms of another metal in the space lattice, the cell parameter changes. In many

220 INDIAN J. CHEM. TECHNOL. , SEPTEMBER 2000

Fig. 4--Scanning electron microscopic pictures of 78%Sn alloy obtained at different condi tions (a) pH 6, (b) 40 ° C, (c) 50 o C, (d) 70 o C

400

1.8 96 TIME(h).

11.4

-<>Zn •Cd

•Etec~D -LJ-EiectrodepQtjt c

-o-sn

192

Fig. 5-Variation of galvanic current density with time • • • -Cadmium • • • • -Zinc; • • • - Electrodeposit C; .6..6..6. Electodeposit D; • • • - Tin

in stances it varies approximately linearly with the proportion of the atoms of the substituting metal. This

relation is known as Vegard ' s law 15• No change in the

lattice parameters of tin wi th the weight % of zinc in the deposit suggests that the Vegard ' s law does not hold good and the alloying e lement does not form solid solutions.

Surface morphology When viewed at a magnification of 5000, the

surface exhibited irregular shaped crystallites up to 37% of zinc in the deposit. Increase of zinc content favoured the growth of crystallites uniformly from 2 to 8 11m and undeposited area increased. However at 67% of zinc, needle shaped crystall ites with sizes of 4-5 11m were seen with increased surface coverage [Figs 3a-d]. On steel surface, tin would have been deposited initially more in concentration [tin-rich deposit] and with the passage of time the zinc content in the alloy would have been increased. This would have caused the growth of fewer crystallites. In zinc­rich { 67% } deposit, zinc might have been deposited on the exposed pores of the steel surface also.

F igs 4a-d show the effec t of operating conditions on the morphology of 22% zinc alloy. At pH 6,

V ASANTHA el a/. : TIN-ZINC ALLOY ELECTRODEPOSIT CHARACTERISATION 221

Table 4---(:orrosion behaviour of Sn and Sn - Zn alloys by electrography and polarisation methods

Thickness Porosity %

!lin Sn 22%Sn 37%Sn

alloy alloy

1.5 35 40 45

3.0 10 12 15

6.0 5 4 5

12.0 2

12.0*

*--(:admium deposit

Table 5--(:on·osion behaviour of Sn and Sn - Zn alloy coupled to steel (6 pm) at the end of 24 hours

Deposit Galvanic potenti al Galvanic cun·ent density

(mv vs SCE) 11A/cm2

Zinc -1058 34

37% Sn alloy -1018 134

22% Sn alloy -969 90

Tin --476 --40

Cadmium -779 60

irregular shaped crystals with varying sizes were observed. The deposit obtained at pH 8 did not show appreciable change in structure. With increase in temperature the crystal size became more regular and a large grain size was observed at 70°C.

Corrosion resistance tests Electrography-The porosity of tin, zinc, alloy of

different thicknesses are shown in Table 4. As the test was not suitable for coatings anodic to steel, comparison was made only with their corrosion current densities calculated from Tafel polarisation methods. Above 6 ~tm thickness, tin and the alloy had almost equal porosity.

Electrochemical behaviour-For the alloy deposit increase of thickness caused the corrosion potential to be shifted to the direction of noble metals. Zinc content in the deposit increased the corrosion current density. The cadmium deposit obtained from the fluoborate exhibited 10 times more corrosion compared to Sn- Zn allo/ 3

.

Galvanic behaviour-By coupling with highly polished mild steel, the galvanic behaviour of various 6 )..!In deposits the corrosion potential became noble with the addition of tin in the deposit at the end of 24 h. Zinc was anodic to steel and the galvanic current density of zinc deposit decreased with tin content

Corrosion current density (A/cm2)

Zn Sn 22%Sn 37%Sn

alloy alloy

5.75xl0'5

4. 10xl0·4 I.86x10·5 I. 86x 10'5 2.80x l0'5

8.30x l0·4 1.86x 10'5 1.45x 10'5 2.70xlo·5

8.30x!o·4 1.50x l0.5 1.20x I 0'5 2.50xl0.5

17xl0·5

(Table 5). Cadmium which exhibited very noble potential compared to zinc when coupled to steel gave a galvanic current density of 60 )..!A/cm2

. The tin deposit which is more noble to both zinc and cadmium13 exhibited 40)..! A/cm2

.

Increase of exposure time markedly influenced the galvanic current density. Tin content and time of exposure decreased the galvanic current density in the alloy [Fig. 5]. The corrosion product formed on these deposits might have covered the micropores of the deposit resulting in the decrease of galvanic current density. The time of exposure vi1tually had little influence on the galvanic current density for cadmium. With prolonged exposure tin content surface became relatively anodic and at the end of 96th hour, it virtually dropped to zero. After 144 hours exposure, tin-zinc alloy deposits exhibited nearly zero galvanic current similar to that of tin 13

.

Salt spray tests White rust formation-The initiation of white rust

occurred for 63% Sn alloy deposit at the end of 24 hours followed by 10% white rust at the end of 96 hours. A decrease of zinc content delayed the time of initiation of white rust and for cadmium, it appeared at the end of 80 hours. Though the time taken for the initiation of white rust was more for cadmium, 25% of the surface was covered at the same time for both cadmium and 78% tin deposit. 50% of the SUiface was covered at the end of 864 hours for cadmium while for the other two deposits it appeared relatively earlier13

.

Red rust formation-For 6 J..lm thick 78% Sn and 63% Sn deposits the initiation of red rust along with micro pits appeared earlier to cadmium. This suggests that for the alloy the dissolution of steel occurred via pores. For the 12 J..lm thick 78% Sn alloy deposit no red rust formation was observed -even after 1248

222 IN DIA N J. CHEM. TECHNOL., SEPTEMBER 2000

hours of exposure. For cadmium of the same thickness 67% Sn deposit it occurred at the end of 1008 hours and it was delayed by 72 hours for the 78% tin deposit. But for cadmiu m, though thickness prevented this to some extent , the 10% coverage of the surface by red rust occun-ed relati vely earlier compared to Sn-Zn alloy. At the end of 1080 hours , 70% and 60% of the cadmium surface was covered by red rust respectively, for 6 f.!m and 12 J..tm thicknesses. For the 6 f.!m 63% tin electrodeposit , after 1100 hours red rust covered 35 % of the surface while it took 1248 hours for the 78% tin deposit.

Conclusion The Sn-Zn elec trodeposits exhi bited more hardness

? values (27.5 ± 1 kg/mm-) when compared to the hardness values of ti n ( 14.5 kg/mm2

) and low hardness vaiues compared to zi nc (34.5 kg/mm\ The cadmi um electrodeposit obtai ned from fluoborate bath ex hibited a va lue of 21 kg/mm2

. For Sn-Zn electrodeposit the solderability as expressed by the spread factor was 85% and above and thickness increased the spread factor. XRD studies on the Sn-Zn deposit indicated the absence of solid solution formation. Scanning e lectron microscopic studies revealed that the zinc content in the plating ba th favoured the growth of crystallites.

Electrochemical studies on the coiTosion behaviour of Sn-Zn alloy revealed that zinc content increased the corrosion current density. Cadmium deposits exhi bited a corrosion current density of 17xl0·5

A/c m2. Sa lt spray exposure test carried out on the

alloy deposi ts revealed that a decrea se of zinc content delayed the time of initiation of whi te rust formation.

For cadmium deposits the initiation period for white rust formation was delayed.The red rust fo rmation with > 60 % coverage occurred ea rlier for cadmium deposit than for the alloy deposit of the same thickness.

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