The electrolytic deposition of lead - cadmium alloy was carried out on mild steel substrate, using fused salt bath, such as

sodium nitrite, containing lead nitrate and cadmium nitrate. The influence of electrolyte composition, current density and time

to get satisfactory deposit has been investigated. The physical and corrosion resistant properties of the coating were studied

and reported in this paper

Guruviah, S.

Mukherjee, D.

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0.  Electrochem 3(2) March-April 1987, pp 151-154 FUSED SALT PLATING OF LEAD-CADMIUM ALLOY ON MILD STEEL D MUKHERJEE and S CURUVIAH Eentral Electrochemical Research Institute, Karaikudi-623 006 The electrolytic deposition of lead - cadmium alloy was carried out on mild steel substrate, using fused salt bath, such as sodium nitrite, containing lead nitrate and cadmium nitrate. The influence of electrolyte composition, current density and time to get satisfactory deposit has been investigated. The physical and corrosion resistant properties of the coating were studied and reported in this paper. Key words: Lead-cadmium alloy, fused salt  plating INTRODUCTION T he electrodeposition of lead-cadmium alloy has not been widely reported in literature. Interest in the electrolytic deposition of metals from fused salts has been generated for a number of reasons. Metallic coatings from fused salt baths have certain unique properties, e.g. freedom from stress, high purity and ductility. Fused salt plating is a process by which a large number of common engineering metals may be deposited on various substrates at a temperature lower than the hot dipping temperature and with advantage of simultaneous deposition and diffusion. Electrolytic deposition of heavy metals like Cd, Pb, Sn and Th from fused salt has already been reported El-4. High deposition rate and high throwing power are possible to achieve in the fused salt medium due to the higher conductivity of the carrier salt. The use of overlay electrodeposits for bearing applications is generally limited to the soft metal such as lead and tin, which can satisfactorily meet a number of apparently conflicting requirements including load bearing fatigue and corrosion resistance. There is considerable interest in the development of new alloys to meet the requirements listed above. One such alloy is Pb-Cd and this study has been concerned with the development of suitable fused salt composition for Pb-Cd alloy deposition. These alloy coatings may be used for preventing bimetallic corrosion. The deposition of Pb-Cd alloy on mild steel from fused salt bath and its physical and corrosion resistance properties are presented here. EXPERIMENTAL (i) Preparation of mild steel plates Mild steel specimens of size 2.5 cm x 2.5 cm with a long stem were used for all the experiments. They were pickled in hydrochloric acid containing inhibitor and then polished with 120 emery wheel. The specimens were thoroughly degreased with acetone before electrodeposition. (ii) Cell set up A furnace with a provision to open from the top was constructed for this purpose with thermostat control. The cell consisted of a graphite crucible and a mild steel cathode placed in between two stainless steel anodes. Tungsten wires were used as current leads (Fig 1). C O P P E R  W I R E  C O N N E C T O R S  Fig 1:  Fused salt-plating set-up 1 .  Furnace 2. Graphite crucible 3. S .S .  anode 4.  Fused-salt-mixture 5. M . S .  cathode 6. Rectifier (iii) Thickness measurements The thickness of the electrodeposits mild steel plates was determined with a pencil type magnetic thickness meter at 3 or 4 places on the specimen and the average value was taken. (iv) Weight of the deposit Lead deposited specimens were initially weighed and then stripped in 20% nitric acid, containing 100 g/l ammonium nitrate at 60°C, till the deposit is completely removed, as indicated by the appearance of greenish black deposits. Similarly Pb-Cd plated specimens were stripped for Cd by using a mixture of Cr03 (200 g/l) and &So4 (27 ml/l) followed by stripping for Pb as already mentioned. (v) Electrochemical studies in NaCl Potential vs time study was conducted for mild steel, Pb plated mild steel and Pb-Cd plated mild steel (1 sq.cm) in 3% NaCl with reference to saturated calomel electrode (SCE). Potentiodynamic anodic and cathodic polarisations were conducted at a scan rate of lmV/sec in 3olrNaCI solution. These data were also supplemented by corrosion rate values, determined on 1 sq cm area specimen from the difference in weight before and after immersion of these specimens in 3% NaCl solution for a duration of 2 hours. Humidity tests were also conducted on these specimens at 100% R.H. at 48OC. RESULTS AND DISCUSSION Table I shows the conditions for the lead and lead-cadmium plating on mild steel substrate from NaN03 - NaN02 (1:l) eutectic salt mixture containing lead nitrate. Satisfactory coating upto 25 Y m  thickness could be obtained by a current density of 38A'dmz at a temperature of 400"C, for a plating duration of 45 minutes. Diffusion of Pb into the mild steel matrix has also been established from metallographic studies [a. The influence of adding cadmium nitrate in the bath of eutectic NaN03 : NaN02 (1:l) mixture, containing 10% Pb(N03)2 in the salt melt is given in Table I. Experiments were conducted at a current density of 38 A/dm2 by varying the concentration of cadmium nitrate from 3% to 25Vo in the melt. The appearance of the plated surface was lustrous greyish black. However, the coating thickness revealed a decreasing trend with increase in the concentration of cadmium nitrate in the meh. Therefore, an attempt was made to change the composition of the supporting salt-melt for better Pb-Cd deposit on mild steel substrate. 02 1.0 CURREN t ,p* 'w Fig.2:Potentiodynamic anodic and cathodic polarization for M.S., Pb-plated MS. and Pb-Cd plated M.S. Electrolyte = 3% NaCl solution. d6.M. S -0-UPb-plated M.S. -A-A-Pb-Cd plated M.S. Table I: Bath composition and operating condition for lead and lead-cadmium alloy deposits Temperature = 4OO0C S1. Lead Cadmium Current density Time Thickness No. nitrate (&) nitrate (%) (A/&$) ( ml) Nature of deposits Table I1 reveals the influence of flux composition on the nature and thickness of the deposit. It is seen that in all the concentrations of cadmium nitrate (3%-25Vo). satisfactory deposits were only obtained in tQc case of salt melt containing sodium nitrite as flux, while sodium nitrate alone resulted in unsatisfactory deposits. Thickness of the deposit increased from 4.1 p m  to 13 ,Um by increasing cadmium nitrate concentration from 3% to 25%. 25-27 Uniform lustrous grey deposits, edges cracked 7-10 Grey deposits 7-10 Uniform lustrous deposits without cracked edges 2-4 Uniform grey deposits 25-27 Thick uniform and lustrous deposition 9-10 Grey black uniform deposits 4 4  Grey black uniform deposits 4 4  Grey black uniform deposits 4-7 Grey black uniform deposits Table 111 shows the corrosion resistance properties of mild steel, Pb plated mild stel,  and Pb-Cd plated mild steel in 3% NaCl solution and in humidity chamber. Fig. 2 shows the anodic and cathodic polarization curves for mild steel. Pb plated mild steel and Pb-Cd plated mild 'steel in 3% NaCl solution. Tafel urtrapolation technique has been used to fmd the Lrn and Em, values. The values of i,, and Q, of mild steel, Pb plated mild 152 6. Electrochem 3(2) Mar-Apr 1987 Muk- ml G u m  - Fured rrlt pbtine of led-cadmium alloy on mild steel I Table II: Effect of flux composition on the deposition of Pb - Cd ailoy I Concentration Composition S1. of cadmium of the flux Thickness of the Appearance of the No. nitrate (%) (Salt mixture) deposit (Am) deposit 1. 3 Sodium nitrite 4.1 - 5 Uniform grey black 2. 10 -do- 4.1 - 6.8 -do- 3. 15 -do- 13 - 14 -do- 4. 20 -do- 13 - 14 -do- 5. 25 -do- 13 - 14 -do- 6. 20 Sodium nitrate 4.1 - 5 Nonuniform deposit 7. 25 -do- 4.1 - 5 -do- p b l e  Ill: Corrosion resistant properties of the coatings I 1 w r  E C O ~  Weight loss O.C.P. (mV) S1. Material (AA/cmz) (mV) corrosion rate (3% NaCl) 72 hours exposure studies No. (3070 NaCl) (3% NaCl) (3% NaCl) in humidity chamber (48°C) (g/cmz) Initial After 24 hrs 1. Mild steel 20 - 330 29 x 10 -4 - 500 - 650 Pitted and rusted within 24 hours 2. Pb-plated 0.6 - 368 4 x 10-4 - 300 - 450 Rusting observed after mild steel 72 hours 3. Pb-Cd plated 7 - 352 10 x 10-4 - 300 - 350 Rusting observed after mild steel 72 hours I able IV: Analysis of Pb-Cd deposit as  plated on mild steel Thickness of Pb-Cd Weight of Pb in the Pb-Cd Weight of Cd in Pb-Cd Percentage of Cd in Pb-Cd deposit ( p m )  alloy deposit (g/mz) alloy deposit (g/mZ) alloy deposit (070) I steel and Pb-Cd plated mild steel are respectively 20,UA/sq.cm, 0.6 ,UA/sq.cm, 7pA/sq. cm. and - 330 mV, - 368 mV and - 352 mV. It is already seen that Pb plated mild steel, shows lowest i, value, followed by that of Pb-Cd plated mild steel, and mild steel. The corrosion current of Pb-Cd plated mild steel specimen is one third of bare mild steel; Pb plated mild steel reveals ten times higher resistance than Pb-Cd plated mild steel. The weight loss values of these specimens are also in conformity with the values of &, obtained from potentmdynamic studies. Exposure test for a period of 72 hours at 48°C in humidity chamber revealed that at 100% R.H. pitting and rusting initiate within 24 hours for mild steel, while the Pb plated and Pb-Cd plated specimens resist upto 72 hours. O.C.P. values of Pb plated and Pb-Cd plated specimens were more positive than mild steel specimens. Moreover, the potential time studies for mild steel, Pb plated mild steel, andpb- Cd plated mild steel in 3% NaCl solution revealed that the plated Table IV shows the analysis of Pb-Cd deposit plated on mild steel substrate, for coating thickness from 2 pm to 10 pm. It is seen that percentage cadmium shows an increasing trend, as the coating thickness increases from 4 ,um to 10 p n .  Otherwise, the weight of Pb and Cd in the Pb-Cd coating increases with increase in coating thickness. CONCLUSION It was possible to deposit lead and lead-cadmium alloy on mild steel substrate to a thickness of 10 to 2 5 ~ m  from fused salt bath composition, based on an eutectic mixture (1: 1) of sodium nitrate and sodium nitrite and sodium nitrite alone respectively. The results show that lead plating is more corrosion resistant than lead cadmium alloy plating on mild steel for the same thickness (10 9 m). I specimens were more positive than the bare mild steel specimen Acknowledgements: The authors thank S/Sri C. Rajagopal and after 24 hours of immersion. R Argode for their help in the preparation of the plates. I 8. Electrochem 3(2) Mar-Apr 1967 1 53 Mukerjee and Guruviah - Fused salt plating of lead-cadmium alloy on mild steel REFERENCES 4. K Matiasovsky, S Lubyava and V Denck, Electrodeposition and surface treatment, 1 (1972) p 7 1. Yu K Delimarskii and B F Markov, Electrochemistry of fused salts, Sigma Press, New York (1961) p 1 2. S V Kirpachev and S Rampal, Zh Fiz Khimi, 11 (1%1) 144 5. D Mukherjee, C Rajagopal, R Argode and S Guruviah. Proc Symp Corros and its Control in Power and Chem Industries, 3. R Piontelli and G S Sternheim, J Chem Phys, 23, (1955) 1 M S University, Baroda, Dec. (1982) r./ 5th ASIAN - PACIFIC CORROSION CONTROL CONFERENCE in conjunction with 27th AUSTRALASIAN CORROSION ASSOCIATION CONFERENCE 22-28 November, 1987 Melbourne, Australia. The principal aim of the conference is to promote an exchange of information on all aspects of the corrosion of materials including methods of testing, prevention and mitigation. Emphasis will be placed on developments relevant t o  the Asian Pacific region. For further information contact David Whitb y, Convenor, 5th Asian-Pacific Corrosion Control Conference, Bloomsbury Conference Services, 319, Lennox Street, Richmond, Vic 3121, AUSTRALIA INTERNATIONAL CONFERENCES IN BOMBAY, INDIA Feb 11 - 12, 1988:- Environmental Pollution and Hazardous Wastes Nov 10 - 11, 1988:- Water Pollution and Environmental Control Paper may be sent to Dr. V .M .  Bhatnagar, Alena Enterprises of Canada P.O. Box 1779 Corn Wall, ONTARIO K6H5V7 Canada 154 B. Electrochem 3(2) Mar-Apr 1987 

Fused salt plating of lead-cadmium alloy on mild steel

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