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Tin-Zinc Alloy Electrodeposit- Physicochemical Characterisation

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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.
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