Tin-Zinc Alloy Electrodeposit- Physicochemical Characterisation
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 globally to replace the cadmium plating, which is being outlawed due to its toxic and carcino genic nature. Tin- zinc alloy plating is one such approach to combine the good properties of the parent metals such as sol derability 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 topog raphy are di scussed in this paper.
Electrodeposition of alloys with specific properties is concentrating at the edges. Laboratory grade one of the latest trends in plating ~ ndustry. Tin - zinc chemicals were dissolved in distilled water and the alloys are attractive because they combine the barrier solutions were pretreated. Insoluble platinised property of tin with the cathodic protection offered by titanium anodes were used having area two times that zinc. The coatings protect steel better than zinc under of the cathode. conditions of high humidity and they resist corrosion Polished steel substrates of 7.5x2.5x0.1 em size equally when compared with an equal thickness of was used for initial experiments and 15x5xO.lcm size cadmium in marine and industrial atmosphere. In was used for testing the corrosion resistance of the addition, they offer better solderability and brightness, deposits. The substrates were polished mechanically, ductility and service li fe with Jess white corrosion degreased with trichloroethylene, cathodically product formation when compared to zinc coatings. polarised for 3 min and then polarised anodically for Compared to cadmium coating the alloy has better 1 min at a current density of 1 Ndm2 in a 1 4 hardness, low hydrogen uptake and is cost effective - • conventional cleaning solution. The specimens were The developmental studies of the tin-zinc neutral washed, treated in a 5% sulphuric acid solution, again 5 9 gluconate bath - , performance of different anode washed, rinsed and taken to the plating bath. materials, effect of additives, the hydrogen The hardness was determined by measuring the embrittlement behaviour and the corrosion resistance indentation produced on the deposit by a diamond of these deposits were reported earli e1. The present point applied under a suitable load in a metallurgical 10 work deals with physicochemical prope1ties like microscope using Knoop indentation method . For solderability, porosity, crystallography and surface deposits having thickness values - 25 f.!m, 50 g load morphology of the deposits obtained from the neutral was applied. The deposits were subjected to standard gluconate bath and compared with the cadmium bend test for adhesion .. The appearance of solder and coating obtained from the fluoborate bath. its spread area was calculated to evaluate the solderability. The larger the area, the hi gher the Experimental Procedure solderability. Spread factor= {(D -/1)/D} x 100 where A 200 mL capacity glass cell with a PVC cover D is the diameter of the sphere havi ng a volume equal provided with holes for positioning the anode, to that of the solder used and H is the height of the cathode and thermometer was used. Large size solder spot. specimens for testing and evaluation were produced Electrographic method -It was used to evaluate 1 from a rectangular cell of 20x5x 15 em size with a porositi • A filter paper soaked in a solution water jacket for hot water circulation to maintain the containing sodium chloride, HCl, HN03 and 0.1 M temperature of the plating bath. The inner electrode pheanthroline hydrochloride was sandwiched between distance was maintained as 15 em. To enable uniform the specimen under test and a pl atinum anode. A 2 thickness and to avoid edge growth , the cathode was current of 0.32 Ndm [+ 0.3 V vs SCEJ was passed provided with a frame to shield the current 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 10·2 Tin SnS04 32.25 100 Sodium gluconate 163 .50 Sodium acetate 20.00 10·2 2 Alloy-A SnS04 32.25 97.7 0.30
ZnS04 28 .80 Sodium gluconate 163. 50 Sodium acetate 20.00 2 3 Alloy-B SnS04 32.25 2x10· 90.0 10.0
ZnS04 28.80 Sodium gluconate 163.50 Sodium acetate 20.00 2 4 Alloy-C SnS04 32.25 2x10· 86.5 13.5
ZnS04 28.80 Sodium gluconate 163.50 Sodium acetate 20.00 2 5 Alloy-D SnS04 43.00 2x10· 77.8 22.2
ZnS04 14.40 Sodium gluconate 163.50 Sodium acetate 20.00 2 6 Alloy-£ SnS04 21.50 2x10· 62.8 37.2
ZnS04 44.65 Sodium gluconate 163.50 Sodium acetate 20.00 2 7 Alloy-F SnS04 10.75 2x10· 62.8 37.2
ZnS04 57.60 Sodium gluconate 163.50 Sodium acetate 20.00 10·2 8 Zinc ZnS04 28.8 100 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 containing 5% NaCl solutions. Spraying was done for indicator giving coloured spots. The pores indicated 8 hours and the thereafter, the panels were kept at rest by such spots were counted with the help of a for the remaining 16 hours of every 24 hours. microscope. The results were expressed as the % Observations were made at the close of every day. 12 defective area . The hours of spraying as well as the total hours spent in the cabinet were expressed. The corrosion Corrosion resistance test/3 resistance of the deposits were evaluated as per Salt spray test-The deposited panels were ASTM standard (B537-70) meant for coatings suspended in the salt spray cabinet "Heraeus Voltsch" cathodic to the steel substrate. 218 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000
Polarisation experiments-lOG mL of the 5% a galvanic couple was indicated by zero potential sodium chloride solution was used in a three drop, this current was the true short circuited current. electrodes cell assembly. The electrodeposit was The galvanic current was obtai ned from the known made a working electrode, a large platinum electrode resistance, R and the potential, E. as a counter and a saturated calomel electrode as Structure and surface morphology-The reference respectively. Tafel polarisation experiment electrodeposits were examined under high was carried out by scanning the electrode potential at magnification to assess the grain size, deposit nature, a rate of 5 V/s 200 mV on either side of the corrosion heterogeneties using a scanning electron microscope potential. [JEOL, Model]. X-ray diffractometry with Cu-Ka Galvanic potential and current radiation was used to understand the phases, their measurements--Measurements of galvanic potential orientation and lattice parameters. and current between the electrodeposit and polished mild steel of 1 sq em area was based on zero Results and Discussion resistance ammeter{ ZRA} technique. The given Table 1 presents the composition and conditions of current was measured by an ammeter, by adjusting the zinc, tin, cadmium and tin-zinc alloy deposition. The voltage, E or resistance R so that the potential adhesion of the alloy deposits to steel substrate was difference between the two electrodes was zero as tested by subjecting them to standard bend test and indicated by the electrometer. Since short circuiting in the coating was found neither to crack nor peel. Ferroxyl test revealed that above 6 J.lm thickness, the electrodeposit was nonporous. 'B - The Vicker's hardnesses of 78% Sn and 63 % Sn ·c: ~ " ~ alloys were found to be 26.5 and 28 .5 kg/mm2 ~ 2 0 respectively (Table 2) compared to 12-16 kg/mm for ] 100 .... - ~ tin deposit. A higher hardness of the alloy is likely to ~ Ul ~ 1- - increase the wear resistance of the coating. Table 3 z :. ~ ~ :::> presents the spread factor for various deposits. On a 0 :J._ u porous electrodeposit through the pores, base metal ~0 30 iron atoms would reach the outer surface. However an increase of thickness of the deposit would decrease Fig. 1-X ray diffraction pattern obtained on the deposit E the pores and prevent oxide formation. The
c • • r . -0 L.. (II 0 (II -.... - Q) ....~ 5·584 2·6476 C;j 0 E 0.. 0 0 0 0 .... (II 0 0.. .~:::: 5·582 E ~
Weight of zinc and tin in the deposit Zn- -Tin
Fig. 2 --Changes in the lattice parameter (A) of tin and zi nc with weight % 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 pictu res of alloy contai ning different % of Zin c (a) I 0 % (b) 22%, (c) 37%, (d) 67%
solderability would increase and the spread factor is Table 2- Yicker's hardness values fo r 6 11m deposits enhanced. The presence of zinc and an increase of Electrodeposit Hardness zi nc content in the alloy deposit would hinder the 2 kg/mm solderability of the alloy. A value of percentage >90 14 Tin 14.5 was established to be excellent • Cadmium 21.0 Structure of the deposits 78%Sn - 22% Zn 26.5 The X-ray diffraction patterns were obtained for 63%Sn - 37 % Zn 28.5 various alloy deposits and Fig. 1 presents the XRD Zinc 34.5 pattern obtained for 63% tin alloy. When the tin content was 98% Sn (220) plane was predominant. Tabl e 3-Soderability- Spread factor for the alloy deposits From 63 % to 90% tin content, Sn(lOl) plane was Electrodeposit Thickness Spread Factor (±2%) predominant. At 44% tin content, Sn(200) plane was Jlm predominant. The reflections from zinc(ll2) plane 78% Sn - 22% Zn 1.5 87 was seen on the deposits of 63% tin. The lattice parameters (A) for tin and zinc were found to be 78% Sn - 22% Zn 6.0 90 invariant with the weight % of zinc and tin 78% Sn- 22% Zn 12.0 93 respectively in the deposit (Fig. 2). The structure of 63 % Sn - 37% Zn 1.5 83 the electrodeposited alloys can be derived from . a 63 % Sn- 37% Zn 3.0 85 study of lattice parameters of their phases. When an 63 % Sn- 37% Zn 6.0 88 alloying metal replaces the atoms of another metal in 63 % Sn - 37% Zn 12.0 90 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
15 relation is known as Vegard' s law • No change in the 400 -<>Zn •Cd lattice parameters of tin wi th the weight % of zinc in •Etec~D the deposit suggests that the Vegard's law does not -LJ-EiectrodepQtjt c hold good and the alloying element does not form -o-sn 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 crystallites with sizes of 4-5 11m were seen with increased surface coverage [Figs 3a-d]. On steel surface, tin would have been
1.8 96 11.4 192 deposited initially more in concentration [tin-rich TIME(h). deposit] and with the passage of time the zinc content in the alloy would have been increased. This would Fig. 5-Variation of galvanic current density with time • • • - have caused the growth of fewer crystallites. In zinc Cadmium • • • • -Zinc; • • • - Electrodeposit C; .6..6..6. rich {67 % } deposit, zinc might have been deposited El ectodeposit D; • • • - Tin on the exposed pores of the steel surface also. in stances it varies approximately linearly with the Figs 4a-d show the effect of operating conditions proportion of the atoms of the substituting metal. This 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
2 Thickness Porosity % Corrosion current density (A/cm ) !lin Sn 22%Sn 37%Sn Zn Sn 22%Sn 37%Sn alloy alloy alloy alloy 1.5 35 40 45 5.75xl0'5 3.0 10 12 15 4. 10xl0·4 I.86x10·5 I. 86x 10'5 2.80x l0'5 6.0 5 4 5 8.30x l0·4 1. 86x 10'5 1.45x 10'5 2.70xlo·5 12.0 2 8.30x!o·4 1. 50x l0.5 1.20x I 0'5 2.50xl0.5 12.0* 17xl0·5 *--(:admium deposit
Table 5--(:on·osion behaviour of Sn and Sn - Zn alloy coupled to (Table 5). Cadmium which exhibited very noble steel (6 pm) at the end of 24 hours potential compared to zinc when coupled to steel gave 2 Deposit Galvanic potential Galvanic cun·ent density a galvanic current density of 60 )..!A/cm . The tin (mv vs SCE) 11A/cm2 deposit which is more noble to both zinc and 13 2 Zinc -1058 34 cadmium exhibited 40)..! A/cm . 37% Sn alloy -1018 134 Increase of exposure time markedly influenced the galvanic current density. Tin content and time of 22% Sn alloy -969 90 exposure decreased the galvanic current density in the Tin --476 --40 alloy [Fig. 5]. The corrosion product formed on these Cadmium -779 60 deposits might have covered the micropores of the deposit resulting in the decrease of galvanic current irregular shaped crystals with varying sizes were density. The time of exposure vi1tually had little observed. The deposit obtained at pH 8 did not show influence on the galvanic current density for appreciable change in structure. With increase in cadmium. With prolonged exposure tin content temperature the crystal size became more regular and surface became relatively anodic and at the end of 96th a large grain size was observed at 70°C. hour, it virtually dropped to zero. After 144 hours exposure, tin-zinc alloy deposits exhibited nearly zero Corrosion resistance tests 13 Electrography-The porosity of tin, zinc, alloy of galvanic current similar to that of tin . different thicknesses are shown in Table 4. As the test Salt spray tests was not suitable for coatings anodic to steel, White rust formation-The initiation of white rust comparison was made only with their corrosion occurred for 63% Sn alloy deposit at the end of 24 current densities calculated from Tafel polarisation hours followed by 10% white rust at the end of 96 methods. Above 6 ~tm thickness, tin and the alloy had hours. A decrease of zinc content delayed the time of almost equal porosity. initiation of white rust and for cadmium, it appeared Electrochemical behaviour-For the alloy deposit at the end of 80 hours. Though the time taken for the increase of thickness caused the corrosion potential to initiation of white rust was more for cadmium, 25% be shifted to the direction of noble metals. Zinc of the surface was covered at the same time for both content in the deposit increased the corrosion current cadmium and 78% tin deposit. 50% of the SUiface was density. The cadmium deposit obtained from the covered at the end of 864 hours for cadmium while fluoborate exhibited 10 times more corrosion for the other two deposits it appeared relatively 3 13 compared to Sn- Zn allo/ . earlier . Galvanic behaviour-By coupling with hi ghl y Red rust formation-For 6 J..lm thick 78% Sn and polished mild steel, the galvanic behaviour of various 63% Sn deposits the initiation of red rust along with 6 )..!In deposits the corrosion potential became noble micro pits appeared earlier to cadmium. This suggests with the addition of tin in the deposit at the end of that for the alloy the dissolution of steel occurred via 24 h. Zinc was anodic to steel and the galvanic current pores. For the 12 J..lm thick 78% Sn alloy deposit no density of zinc deposit decreased with tin content 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 For cadmium deposits the initiation period for white thickness 67% Sn deposit it occurred at the end of rust formation was delayed.The red rust fo rmation 1008 hours and it was delayed by 72 hours for the with > 60 % coverage occurred earlier for cadmiu m 78% tin deposit. But for cadmiu m, though thickness deposit than for the alloy deposit of the same prevented this to some extent, the 10% coverage of thickness. the surface by red rust occun-ed relati vely earlier compared to Sn-Zn alloy. At the end of 1080 hours, References 70% and 60% of the cadmium surface was covered by Edward budman & David Siever. Ami Corros Melhod. M(//er, red rust respectively, for 6 f.!m and 12 J..tm thicknesses. 45 (5) ( 1998 ) 327. For the 6 f.!m 63% tin electrodeposit, after 1100 hours 2 Vi tkova St, lvano va V & Reichc vsky G, Sw f Coal Techno!, red rust covered 35 % of the surface while it took 82 ( 1996) 226. 1248 hours for the 78% tin deposit. 3 Rai chevsky G, lvanova V, Vi tkova St & Nikolva M. Surf Coal Techno/, 82 ( 1996) 239. 4 Sziraki L, Cziraki A. Ve rtsy Z, Ki ss L, lvanova V, Reichevski Conclusion Z, Vitkova S & Marinova T S, Swf Coar Techno/, 29 (1999) The Sn-Zn electrodeposits exhi bited more hardness 927. ? values (27.5 ± 1 kg/mm-) when compared to the 5 Vasantha V S, Malathy Pushpavanam & Muraliuharan V S. 2 Pia ring Sw f Finish , June ( 1995) 82. hardness values of ti n ( 14.5 kg/mm ) and low 6 Vasantha V S, Mal ath y Pushpavanam & Muralidharan V S, hardness vaiues compared to zi nc (34.5 kg/mm\ The Trans In s/ Mel Finish, 74, I ( 1996) 28 cadmium electrodeposit obtained from fluoborate bath 7 Vasantha V S. Malathy Pushpavanarn & Muralidharan V S. 2 ex hibited a value of 21 kg/mm . For Sn-Zn Mel Finish. 94 ( 1996) 60. electrodeposit the solderability as expressed by the 8 Vasantha V S, Malathy Pushpavanam & Muralidharan V S, Met Finish , 93 ( 1995) 16. spread factor was 85% and above and thickness 9 Vasantha V S, Malath y Pu shpavanarn & Muralidharan V S. increased the spread factor. XRD studies on the Sn-Zn Bull Elec/rochem, I I ,8 ( 1995) 37 1. deposit indicated the absence of solid solution I 0 Sard R, Ogburn 0 & Leidheser S, Properties of Elec:trode formation. Scanning electron microscopic studies posils. Their Measurements and Significance, Symp Electro revealed that the zinc content in the plating bath chem Soc. USA. 1974. II Kutzelnig A,Tesling of Melallic Coatings (Robert Drapper. favoured the growth of crystallites. Miud1e sex, UK), 1963. Electrochemical studies on the coiTosion behaviour 12 Vasanth a V S, Elecrrodeposition of Tin Zin c and Tin -Zinc of Sn-Zn alloy revealed that zinc content increased Alloys from G/uconale Comple1es The is, Alagappa Univer the corrosion current density. Cadmium deposits si ty, 1994. 13 Vasa ntha V S, Malathy Pushpavanam & Muralidharan V S, exhi bited a corrosion current density of 17xl0·5 2 Trans Met Fin Association India, March 1997. 15. A/cm . Salt spray exposure test carried out on the 14 Lon g J B, J Elec/rochem Soc (1975)102. alloy deposits revealed that a decrease of zinc content 15 Brenner Abner, Electrodeposilion of Alloys (A cademic Press. delayed the time of initiation of white rust formation. London) 1963, 76.