The eledrodeposition of lead-tinqper alloy from fluoborate solution has been reported. The effects of

addition agent, armnt density, copper content and total metal content of the bath on the alloy

amposition have been discussed. The structure of the alloy obtained has been compared with that of

lead-tin alloy with the help of SEM

Kanakaraj, D.

Narasimhan, V.L.

Subramanian, R.

Venkatachalam, R.

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Bulletin of Eledrochemistry 2 (3) May - June. 1986, pp. 261 -263 LEAD-TIN-COPPER ALLOY DEPOSITION FROM FLUOBOMTE SOLUTIONS R VENKATACHAtAM, D K A N A C A W ,  V L  NARASIMHAN a d  R SUBRAMANIAN Central Electrochemical Research Institute, Karaikudi -623006 ABSTRACT The eledrodeposition of lead-tinqper alloy from fluoborate solution has been reported. The effects of addition agent, armnt density, copper content and total metal content of the bath on the alloy amposition have been discussed. The structure of the alloy obtained has been compared with that of lead-tin alloy with the help of SEM. Key words: W-SnCu alloy deposition, fluoborate bath I t is recopised in the p-t days that alloy plating has become a part of bearing mnaukturing technology. The electroplates in bearing induatry pre to be found in three main applications : (i) as bonding layer, e.g. copper and nickel pl&a on ateel which are sometimes used as preliminary step in bonding of bearing alloys such as I& bronze, copper-lead and aluminium-tin (ii) aa a protective and decorative plate and (iii) as overlays. In earlier days white metals were uaed as a bearing material, but they do m t  find much uae in p-t days because of their poor load-carrying uprity and unsuitability for use at high temperatures (above IOODC). They are replaced by steel backed copper-lead, I,&-bronze and aluminium-tin bearings because such bearings poacess good atrength and permit operations at high temperatures. However, these bearings have poor resistance, to hrtigue, conorion and seizure, beaidea high nte of erosion of journal. With the application of overlays on there bearings, the above mentioned drawbacks are overcome with the advantage of good embeddability and d u c e d  clearance between the journal and bearing surface. In the modern bearing technology, the recognised overlays are the systems of lead-tin, Id- indium and lead-tin-copper (copper 1-3% tin 9- 12% and the remainder I d ) .  From the view points of cost and plating, lead- indium is not widely us&. Though lead-tin possesses goal corrosion resistance and is plated with conatant composition, it loses to iead-tin-copper syatem from the point of view of fatigue resistance and hardneas. Tables I and I1 show the fatigue data [ I ]  of various bearing materials and their hardness 121. Many orgaoic additions such as reaorcinol, gelatin, 8-nnpthol, peptone, pymgdlol etc. have been reported in literature for binary alloys deposition (3-81. Also very little informption is found aa to the deposition of ternary alloys bued on lead-tin in literature. In the present ah~dy, electrodepoition of lead-tinsopper from fluoborpte solution haa been reported using hydroquinone and animal glue aa addition agents. The typical bath used in the present study is: Lead (aa metal) P ' l  Tin (as metal) 5 pm/l Coppr  (as metal) I - 4 p / l  (varied depending on the percentage of copper required in the alloy) Ruoboric acid 40-45 p / l  Hydroquinone 5 g d l  Animal glue 0.5 gm/l Temperature 30°-35OC Agitation Not used Anodes: Cast 90% Lead- 10% Tin Table I: Fatigue data of varioun beorlng materials Material-nominal Proceas Thickness Fatigue Average composition of bearing strength time to fati- material kg/mm2 gue (hn) (mm) Lead - 10% tin Plated 0.1 2.53 23 Lead-lO%tin , , ., 77 1% copper Lead - 10% tin ,, 2 1 13 3% copper Lead - 6% copper ., , 26 Tin base babit 3% copper - 7% anti- mon y Cast , - 30 Table I1 : Hardness date of various bearing materials Metal composition Hardneas (HV) Cast lead 4 Lead - 5% tin 8 Lead - 10% tin 10 Lead - 1031 tin - 2-3% copper 17- 19 (measured at 5 gm load) Venkatachalam el al- Pb-Sn-Cu depos~tion from fluoborate bath In all experiments a p.v.c. tank of one litre capacity. 5.0 mm (dia) stainless steel cathode were used, with two anodes. The anodes are wrapped with terylene cloth and are introdoced into the bath just before plating and removed after plating. The cathode is introduced into the bath with current- 'on'. Copper is added to the bath as copper fluoborate periodically to maintain the concentration In the bath as it is difficult to use lead-tin-copper ternary alloy anodes, which cause dissolution problems. Effect of copper and glue contents in the bath and of c.d. on the copper content and influence of lead-tin-ratio in the bath on the tin content of alloy deposit, were studied. RESULTS AND DISCUSSION From the unagitated solution it is possible to get uniformly smooth, fine grained and soft deposit free from nodules in the current density ranges (4-9 ~.dm-')'studied. The colour of the deposit gives an idea about the copper content in the deposit. If the deposit is slightly reddish in colour, copper content is around 3-4% in the deposit. Fig. 1 shows the effect of copper content in the bath on the percentage of copper in the deposit at three current densities. It is evident that the copper content in the alloy deposit is increased as the current density decreases. At 4 A dm-.' a higher percentage of copper in the deposit is obtained whereas a lesser percentage is obtained at 8 A /dm-', i.e. as the current density Increases, the noble metal in the bath is less in the alloy deposit as in the case J the deposition of Bi-Cu, Ag-Cd and Sb-Sn systems 101 which shows that t1,;. alloy behaves as a regular codeposition system. Also the copper content In the deposit increases linearly as the copper content in the bath, irrespective of the current density. p b (AS METAL)  = s n (AS METAL) = GLUE = HYOROWINNONE= ROOM TE UP. Fig. 1 : Effect of copper content In the bath on the percentage of copper in the deposit Fig. 2 gives the effect of current density on the percentage of copper in the deposit for different total metal contents, with copper content constantly kept at 4 g/l. The copper content in the alloy decreases with the increase in current density in all cases as well as with increase in total metal content. This is the case with all regular codeposition system, which is diffusion controlled. Fig. 3 shows the effect of glue content on the percentage of copper in the deposit for different amounts of copper in the bath. It is clear th t t  up to a concentration of 3 g/l copper in the solution, the copper in the deposit increases linearly and thereafter tends to level off. Lesser the glue content in the solution, higher is the percentagcn of copper in the* deposit, which indicates the inhibition of copper deposition in the presence of glue. This may probably be due to the increasing degree of polarisation of the noble metal in the presence of glue. However, in the case of lead-tin-antimony codeposition, a reverse effect has been reported [IO]. Fig 2 Ettect of current dens~ty on the percentage of copper In the depos~t at dtfllcult lotal metal content. Copper ftxed at 4 g l l  Fig. 4 shows the effect of lead-tin ratio in the bath on the percentage of tin in the alloy keeping total metal content at 114 g/l with constant copper content (4 g/l). In the current density ranges studied, as the lead-tin ratios increase the tin in the deposit decreases. However, the tin content in the deposit increases with an increase in current density. A similiar trend has been reported in the lead-tin binary system. It is also found that with higher ratios of lead and tin in the bath, the alloy deposit tends to deposit lead and tin in the same ratio as in the bath. BATH -~ ~ ( U E T A L )  = so 911 S ~ ( Y E T A L ) = I O  g/l  CLUE = @ S g / l  HYOROQUIWWONE r S g/1 HIF6 = 4 S d I  ROOM TEMP. C . 0  8.0 Fig. 3 : Effect of glue content on the percentage ~f copper in the deposit. This shows the metal depletion in the bath is not rapid and it can be adjusted periodically by adding copper fluoborate to the bath. Scanning electron micrographs have beeu taken at a magnification of 300 X for (a) P b S n  alloy (b) PbSn-Cu alloy (less than 2% of copper by wt.) and (c) Pb-Sn-Cu alloy (higher than 3% of copper by wt.) plated from fluoborate solution with a glue concentration of 0.5g/l at 5 A / d m  ' In the lead-tin alloy deposit, the crystallites are bigger and with the codeposition of copper in the alloy, lesser with be size of the crystallites, which increases the hardness of alloy. It has also been reported that the presence of copper in the Bulletin of Electrochemistry 2 (3) May - June. 1986 262 Venkatachalam et al- Pb-SnCu deposition from fluoborate bath bearing alloys adds to the hardness of the alloy 1141. Fig. 4 : Effect of lead-tin ratio on the percentage of tin in the deposit. Total metal content 1 14 g/l The codeposition of lead-tin is quite possible because their standard electrode potentials are very closer (for lead - 126 mV, for tin - 136 mV with reapect to normal hydrogen electrode). But the codeposition of copper ions with lead and tin seems to be unexpected from the point of view of its nobler potential (+ 334 mV), and a difference of 460-470 mV. Codeposition can also be done by the use of organic addition agents, particularly at low concentrations which usually increases considerably the polarization assaciated with deposition and their effect i i  specific. The codeposition of lead and copper from their sulphates or benzene sulphonates has been studied [I  11. using an addition agent. Also the codepoaiton of tin and copper from sulphate solutions in the presence of gelatin, @-naphthol and diphenylamine has been reported [12]. In the presence of an addition agent, the codepositon occun at very low current densities. The mechanism by which an addition agent works is different from complexation. These addition agents are effective only in solutions containing simpler ions. The composition of the deposit is affected by the addition aght.  If resorcinol is added to lead-tin bath, tin in the deposit would rise above 6% as compared to 1.6% with glue alone. The addition agents have practically no effect on the anode polarization. To put it in a simple way, the addition agent retards the deposition of nobler metal without affectins the depositon potentials of less noblermetals thereby enabling codeposition. Also as the current density increases the noble metal in the bath will be found less in the deposit (as in the case of Bi- Cu, Ag-Cd and Sb-Sn). In our experiments, it has been established that the addition of glue inhibits the codeposition of copper, a nobler metal. It has been shown that with the proper control of concentration of addition agents and total metal content, a ternary alloy deposit of lead and tin with copper content controlled at 23% can be deposited. The depletion of copper in the bath with time of deposition has also been investigated. It is now possible to replenish the copper in the bath by the additioh of copper fluoborate-stock solution at a controlled rate by incorporating an ampere-hour meter in the circuit. A simple volumetric procedure for estimation of the metals in the bath has also been reported to maintain the bath composition at the optimum level of metal contents [ 131. REFERENCES I ;  PG Forrester, Tram Institute of Metal Finsihing, 1 (1961) 52 2. J A Moms, Trans Institute of Metal Finishing, 51 (1973) 56 4. British Patent 1381426 5. U S S R  Patent 493945 6. A K Graham and H L Pinkerton, Plating, 52 (1965) 309 7. U S  Patent 2460252 8. Japan Patent 74034569 9. A Brenner, Ebchodeposition of al loy ,  Vol. I, Academic Press, New York and London (1963) p 124 10. R T  Putnam and EJ Roser, Phting, 42 (1955) 1133-36 1 1. a) P P Beliaev. C P Markov and C P Helman, Vcrtnik Metalloprom, 16 (1936) 90-94 b) P P  Beliaev, AG Valezheva and C P  Helman, Vestrik Mc*rlloprom 15 ( 1935) 1 1 7 12. MA Loshkarev and M P  Grechykhina, Zhur Fiz Khim, 24 (1956) 1502 13. D Kanagara', R Venkatachalam. VL Narasimhan and R Subramanian Metal Finishing 11983) Feb. 73 SEEMA Chemicals 17, MEGHAL SERVICE IND. ESTATE. 2ND. FLOOR. DEVIDAYAL ROAD. MULUND WEST, BOMBAY-400 080. PHONE: OFF: 561 32 7 2  & 5 6 0  2 6  9 6  Bulletin of Electrochemistry 2 (3) May - June. 1986 263 

Lead-tin-copper alloy deposition from fluoborate solutions

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