Electrodeposition of lead from a sulphamate solution

Electrodeposition of lead from a sulphamate solution

Surface Technology, 20 (1983) 139 - 147 139 ELECTRODEPOSITION OF LEAD FROM A SULPHAMATE SOLUTION M. A. F. SAMEL and D. R. GABE Department of Materi...

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Surface Technology, 20 (1983) 139 - 147

139

ELECTRODEPOSITION OF LEAD FROM A SULPHAMATE SOLUTION

M. A. F. SAMEL and D. R. GABE Department of Materials Engineering and Design, Loughborough University of Technology, Leics. LE11 3TU (Gt. Britain)

(Received March 31, 1983)

Summary The conditions for electrodeposition of lead from a sulphamate solution were studied using polarization, plating range and throwing power methods. The following opt!"mum parameters for the solution were proposed: 108 g Pb 1-1; 100 g of free sulphamic acid per litre; 3 g of gelatine additive per litre; current density, 1.2 - 3.5 A dm-2; pH 1.2 - 1.6; temperature, 55 °C. Anode and cathode efficiencies are both 100%.

1. Introduction Although the appearance and properties of lead severely limit its use as an electrodeposit in decorative metal finishing, its properties, combined with cheapness, are such that it is invaluable in some important functional applications including use in bearings and solderable electrical conductors and connectors. Because of its low melting point, lead can be applied by conventional h o t dipping or spraying for large values of thickness (greater than 20 #m), and for thinner deposits electrodeposition remains an important technique. Furthermore, a variety of lead alloys (Pb-Sn, P b - I n ) have also been electrodeposited commercially. Because lead forms a substantial number of salts which are either insoluble or sufficiently insoluble to make them impracticable for electroplating purposes, the number of electrolytes available for plating is limited and excludes m a n y c o m m o n cheaper salts. In two categories they include (a) acidic solutions including fluoborate, fluosilicate, perchlorate, nitrate and sulphamate and (b) alkaline solutions including pyrophosphate, tartrate and acetate. The acidic solutions are easily prepared, are generally corrosive and contain lead as an uncomplexed hydrated ion; consequently they can be operated at a high current efficiency with soluble anodes but may hydrolyse at higher temperatures and require additives to yield levelled deposits. The alkaline solutions are less c o m m o n although t h e y may have some advantages (e.g. better throwing power). In commercial practice, fluoborates are used for metal finishing applications and fluosilicates for electro0376-4583/83/$3.00

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140 refining b u t b o t h suffer f r o m disadvantages, n o t a b l y corrosivity and pollut i o n - e f f l u e n t c o n t r o l difficulties. A critical search f o r an alternative solution is t h e r e f o r e necessary and has i n d e e d b e e n carried o u t previously. T h e use o f sulphamates in electroplating can be a t t r i b u t e d largely to Piontelli and Cambi [1, 2] ; t h e process f o r nickel is t h e best k n o w n . Certain basic parameters for a lead s u l p h a m a t e have b e e n established [ 3 - 5 ] and include the n e e d f o r an excess o f sulphamic acid giving a p H o f less t h a n 2, a n e e d for organic levelling additives and a c u r r e n t density o f 1 - 5 A d m -2. T h e fact t h a t lead s u l p h a m a t e has a r e m a r k a b l e solubility (greater t h a n 2 0 0 0 g 1 1) [6] has as y e t n o t b e e n e x p l o i t e d . T h e lead s u l p h a m a t e s o l u t i o n is easily p r e p a r e d b y dissolving o n e o f t h e lead salts, e.g. basic lead c a r b o n a t e , in an excess o f sulphamic acid solution w h e n c e t h e main solution c o n s t i t u e n t is Pb(NH2SO3) 2 which at a pH o f less t h a n 1.5 is virtually stable, yielding simple lead ions with little or n o complexing. Several f o r m u l a t i o n s have b e e n studied and at t y p i c a l c u r r e n t densities t h e deposits have been claimed to be s m o o t h , finely crystalline, t o u g h and n o n - p o r o u s . T y p i c a l c o n d i t i o n s are given in Table 1. T h e main advantages c o m p a r e d with o t h e r solutions such as f l u o b o r a t e are their cheapness, lower t o x i c i t y , ease o f handling and t r a n s p o r t and simplicity to prepare. T h e main disadvantage is their t e n d e n c y t o h y d r o l y s e especially at high t e m p e r atures t o f o r m insoluble lead sulphate after s u l p h a m a t e d e c o m p o s i t i o n : NH2SO 3 + H a 0 --->NH4 + + S042 If c o n d i t i o n s are allowed t o stray, such r e a c t i o n s can effectively destabilize t h e s o l u t i o n b e y o n d use [ 7 ]. In this s t u d y t h e p o l a r i z a t i o n characteristics o f a s u l p h a m a t e solution, based o n t h a t used b y Mathers and F o r n e y [ 3 ], have b e e n investigated with a view t o o p t i m i z i n g t h e s o l u t i o n f o r some specific applications.

TABLE1 Sulphamatesolutionforleadplating Reference

Lead sulphamate (g 1 1) Free sulphamic acid (g 1-1) Additive pH Temperature (°C) Current density (A dm 2)

[31

[51

[21

54 (as Pb) 50

250 50 Binary phenol, 0.2 - 0.5 g l 1

80 (as Pb) 100 Yes

1 - 1.5 20 2.2

25 3-5

30 1 -1.2

141 2. Experimental details Three electrolytes containing 27 g Pb 1- I , 54 g Pb 1-1 and 108 g Pb 1-1 with 25 g NH2SOaH 1-~, 50 g NH2SO3H 1-1 and 100 g NH2SOaH 1-~ respectively were used f or the study. (Pure lead sulphamate contains 51.8% Pb.) Selected addition agents were later added. T h e y were prepared by completely neutralizing aqueous solutions containing 100 g NH2SO3H 1-~ with basic lead carbonate, filtering and t hen adding 100 g NH2SOaH 1-I to yield a stock solution containing 108 g Pb 1-1. The stock solution was diluted with distilled water as required. The pH was measured using a C o r n i n g - E e l instrum en t and adjusted with either a m m o n i u m sulphamate or sulphuric acid. Polarization characteristics were measured using a linear sweep potent i o d y n a m i c t e c h n i q u e used previously [8] whose features m ay be briefly described. Solutions were held in a 500 ml f l a t - b o t t o m e d glass vessel (250 ml per measurement) placed within a thermostatically controlled (+1 °C) water bath. The vessel was fitted with a five-necked lid in which were inserted three electrodes, a t h e r m o m e t e r and a gas {nitrogen) diffuser as required. Cathodes o f copper sheet (80 mm X 10 mm X 0.25 m m ) were prepared by pickling for I min in 50% HNO 3 followed by rinsing in distilled water; t h e y were then dried in acet one and stored in a desiccator before use. Soluble lead anodes o f size 210 mm X 60 mm X 2 mm f o r m e d into a cylinder within the containing vessel were used t oget her with a saturated calomel electrode (SCE) as a reference c o n n e c t e d via a fine Luggin capillary probe and agaragar gel bridge. Power was supplied by a p o t e n t i o s t a t driven by a linear sweep unit and t he current was pl ot t e d on a p o t e n t i o m e t r i c chart recorder. Plating range measurements were made using a standard 267 ml Hull cell and the throwing p o w e r was measured with a H a r i n g - B l u m cell and calculated using Field's formula. Cathode efficiencies were measured by weighing cathodes in the main polarization cell.

3. Results In all th e experimental work a solution containing 54 g Pb 1-1 and 50 g NH2SO3H 1-1 was taken to have the " r e gular" concentration. During all t he plating range (Hull cell) studies a strong t e n d e n c y to dendrite f o r m a t i o n was n o t e d after 2 min plating in all solutions. F o u r regions were distinguished: burned; bright; matte; no deposit. These were defined by photographing panels and tabulating range limits. The effects of additives were i m p o r t a n t as levelling and dendrite suppression agents. Typical sets o f panels are shown in Fig. 1, illustrating the effects o f using malic acid, resorcinol and p e p t o n e gelatine in the regular solution at 25 °C, pH 1.3 and a Hull cell current of 2 A. Malic acid was n o t very effective while resorcinol gave improved deposits with an o p t i m u m c o n c e n t r a t i o n o f a b o u t 3 g 1-1. F u r t h e r addition o f gelatine at a c o n c e n t r a t i o n of 1 - 2 g 1- I gave an improved range o f deposits which were not bright but acceptable for tribological applications.

142

(a)

(b)

(c) Fig. 1. Effect of (a) malic acid, (b) resorcinol and (c) p e p t o n e on Hull cell ranges for a sulphamate solution (pH 1.3 ; 25 °C). (Additive c o n c e n t r a t i o n s increase f r o m left to right and f r o m top to b o t t o m . )

143

An increase in the concentration of lead in solution increased the acceptable range width and an increased temperature improved the quality and flexibility. The acceptable ranges of current density are illustrated graphically in Figs. 2 - 4, showing the effect of metal concentration alone in the absence of additives (Fig. 2) and then the effect of additives in concentrations up to 4 g 1-~ for the regular solution (Figs. 3 and 4). Changes in temperature increase both the maximum acceptable current density and the current density range (Figs. 5 and 6) with an optimum value at about 55 °C.

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Fig. 2. E f f e c t o f lead s u l p h a m a t e c o n c e n t r a t i o n on the a c c e p t a b l e (Hull cell) plating range at 25 °C. Fig. 3. E f f e c t o f various additives on the acceptable plating ranges at 25 °C: curve 1, gelatine; curve 2, pyrogallol; curve 3, malic acid; curve 4, resorcinol; curve 5, p e p t o n e .

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Fig. 4. E f f e c t o f various additives o n the m e a n range for g o o d plating at 25 ~C: curve 1, gelatine; curve 2, pyrogallol; curve 3, malic acid; curve 4, resorcinol. Fig. 5. E f f e c t o f temperature o n the m a x i m u m current density for g o o d plating for various s o l u t i o n c o n c e n t r a t i o n s : curve 1, half the regular c o n c e n t r a t i o n ; curve 2, regular c o n c e n t r a t i o n ; curve 3, d o u b l e the regular c o n c e n t r a t i o n .

144 M e a s u r e m e n t s o f t h r o w i n g p o w e r are s u m m a r i z e d in Fig. 7 a n d t h e y s h o w t h e s a m e g e n e r a l t r e n d f o r all t h r e e s o l u t i o n s w i t h t h e t h r o w i n g p o w e r d e c r e a s i n g as t h e d i s t a n c e ratio i n c r e a s e s . T h e t h r o w i n g p o w e r w a s g r e a t e s t f o r h o t c o n c e n t r a t e d s o l u t i o n s b u t t h e t h r o w i n g p o w e r s w e r e in all c a s e s somewhat lower than those reported previously [3].

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Fig. 6. Effect of temperature on the mean range for good plating for various solution concentrations: curve 1, half the regular concentration; curve 2, regular concentration; curve 3, double the regular concentration. Fig. 7. Throwing power values at 25 °C for various solution concentrations (Haring-Blum cell; Field formula): curve 1, half the regular concentration; curve 2, regular concentration; curve 3, double the regular concentration. C a t h o d e c u r r e n t e f f i c i e n c y v a l u e s c o u l d a l w a y s b e m a i n t a i n e d near 1 0 0 % b y k e e p i n g t h e c u r r e n t d e n s i t y b e l o w 2 A d m 2 f o r t h e regular solut i o n at 2 5 °C (Fig. 8) a n d t h e p H b e l o w 1 . 6 f o r t h e s a m e s o l u t i o n at t h a t curr e n t d e n s i t y (Fig. 9). W h e n t h e p H r o s e a b o v e 1.6 t h e d e p o s i t s a p p e a r e d d a r k e r in c o l o u r a n d m o r e p o w d e r y . T h e e f f e c t o f t e m p e r a t u r e w a s s m a l l b u t

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EURRENT DENSITY, A/dm~

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pH

Fig. 8. Effect of current density on cathode current efficiency at 25 °C. Fig. 9. Effect of pH on cathode current efficiency at 25 °C and a current density of 2 A dm 2 in the regular solution.

145

real and revealed a maximum in efficiency at about 50 °C (Fig. 10). At all times the anode efficiency was n o t significantly less than 100%. Polarization data were obtained from p o t e n t i o d y n a m i c sweeps at rates of 15, 30 and 60 mV min-1; the differences were relatively smaller with faster sweep rates giving lower quasi-limiting currents. Cathodic polarization curves for three solution concentrations are given in Fig. 11 and the effect of gas agitation is illustrated for the regular solution in Fig. 12. Figure 13 shows the effect of gelatine as an additive o n the cathodic polarization behaviour for a solution containing a double concentration (108 g 1-1) of lead. The rest potential was f o u n d to be - - 4 1 4 (+ 3) mV (SCE). Anodic polarization curves were also obtained for the anode to ensure that no anode control difficulties might be encountered, and these are shown in Figs. 14 and 15. 400 >: -500 99 600 ~

96 700 800

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CURRENT OENSITY,Aldrn ~

Fig. I0. Effect o f temperature on c a t h o d e current efficiency at p H 1.3 and a current density o f 2 A dm 2 in the regular solution. Fig. 11. Effect of solution c o n c e n t r a t i o n on the cathodic polarization at 25 °C: curve 1, half the regular c o n c e n t r a t i o n ; curve 2, regular concentration; curve 3, double the regular concentration.

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Fig. 12. Effect o f agitation on the cathodic polarization at 25 °C and pH 1.3 in the regular solution: curve 1, no agitation; curve 2, w i t h agitation. Fig. 13. Effect of gelatine on the c a t h o d i c polarization of lead in a d o u b l e c o n c e n t r a t i o n solution: curve 1, copper substrate at 25 °C; curve 2, lead substrate at 25 °C; curve 3, lead substrate at 25 °C w i t h 3 g o f gelatine additive per litre; curve 4, lead substrate at 55 °C w i t h 3 g o f gelatine additive per litre.

146

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Fig. 14. E f f e c t o f c o n c e n t r a t i o n o n anodic p o l a r i z a t i o n at 25 °C: curve 1, half t h e regular c o n c e n t r a t i o n ; curve 2, regular c o n c e n t r a t i o n ; curve 3, d o u b l e t h e regular c o n c e n t r a t i o n . Fig. 15. E f f e c t o f agitation o n t h e a n o d i c p o l a r i z a t i o n at 25 °C and pH 1.3 in t h e regular s o l u t i o n : curve 1, no agitation; curve 2, w i t h agitation.

4. Discussion The main purpose of this investigation was to study afresh the operating characteristics of the lead sulphamate electrodeposition solutions before their use in certain functional applications where tribological properties will be exploited. The requirements for such a process, therefore, demand a relatively easy process to operate preferably at moderate levels of pH with good ranges of current density, yielding smooth level deposits which need not be bright. The results show clearly t h a t most of these criteria can be satisfied provided that addition agents are employed to suppress dendrite formation at the high ranges of current density, thereby extending the range and raising the m a x i m u m acceptable value of the current density. The additives studied include malic acid, peptone, pyrogallol, naphthol, resorcinol and gelatine used singly or in pairs at various concentrations. The best results were obtained by adding 3 g of gelatine per litre to the bath so that a dense finely crystalline compact deposit was obtained and a current density of 3.5 A dm -2 could be safely applied for 30 min. The use of a second additive with gelatine did not yield improved conditions. Inspection of these electroplated samples, carried out after storage for 18 months, revealed that a tarnish film had formed only on samples plated from solutions containing higher concentrations of gelatine. This tarnish is probably only harmful if appearance is important but for tribological applications it appeared to have little effect. In spite of the fact that solutions of high acidity were employed the pH was found to be fairly critical at that level and a pH range 1.2 - 1.6 at 25 °C and 2 A dm --2 was optimum. These conditions also gave the best cathode current efficiency. At low pH the efficiency decreased while at higher pH the deposits became rough

147 and p o w d e r y with a loss in adhesion. It was possible t h a t some c o d e p o s i t i o n o f a h y d r a t e d lead salt t o o k place. At low s o l u t i o n c o n c e n t r a t i o n s (27 g Pb 1 1; 25 g NH2SO3H 1-1) the a c c e p t a b l e range o f c u r r e n t d e n s i t y was very n a r r o w b u t at higher c o n c e n t r a tions (108 g Pb 1-1; 100 g NH2SO3H 1-1) t h e range increased m a r k e d l y while t h e t e m p e r a t u r e had a substantial e f f e c t which was o p t i m a l at a b o u t 55 °C. These c o n c e n t r a t i o n s were c o n s i d e r e d to be t h e e x t r e m e limits f o r practical plating because excessive heating led to a n o t i c e a b l e degree o f p r e c i p i t a t i o n in c o n c e n t r a t e d solutions owing t o h y d r o l y s i s o f lead s u l p h a m a t e and conseq u e n t loss f r o m s o l u t i o n as an insoluble basic lead sulphate. T h e r e f o r e , it is believed t h a t unless a high c u r r e n t d e n s i t y is a m a j o r r e q u i r e m e n t it m a y be best to use t h e regular s o l u t i o n c o n c e n t r a t i o n (54 g Pb 1-1; 50 g NH2SO3H 1-1 ) w h e n " d r a g - o u t " losses m a y also be lessened. P o l a r i z a t i o n m e a s u r e m e n t s s h o w t h a t t h e solution behaves essentially as a simple acid u n c o m p l e x e d s o l u t i o n with few p o l a r i z a t i o n losses and an early o n s e t o f c o n c e n t r a t i o n p o l a r i z a t i o n , t h e r e b y offering scope for increased plating rates t h r o u g h stirring. T h e same b e h a v i o u r m a y be e x p e c t e d for b o t h a n o d e and c a t h o d e r e a c t i o n s b o t h o f which can be m a i n t a i n e d a l m o s t at a 100% efficiency. No a t t e m p t has b e e n m a d e at this stage to o b t a i n precise data f o r a T a f e l - a c t i v a t i o n analysis o f the c a t h o d i c r e d u c t i o n r e a c t i o n f o r lead. In c o n c l u s i o n it m a y be r e p o r t e d t h a t a s u l p h a m a t e s o l u t i o n f o r t h e e l e c t r o d e p o s i t i o n o f s m o o t h lead deposits is feasible with o p t i m u m parameters as follows: lead ( a d d e d as s u l p h a m a t e ) , 108 g 1-1; free sulphamic acid, 100 g 1-'; gelatine, 3 g 1-1; c u r r e n t d e n s i t y , 1.2 - 3.5 A d m - 2 ; p H 1.2 - 1.6; t e m p e r a t u r e , 55 °C. F o r long-term stability a lower t e m p e r a t u r e m a y be p r e f e r r e d to m i n i m i z e h y d r o l y s i s o f lead s u l p h a m a t e in solution. Acknowledgments L a b o r a t o r y facilities were m a d e freely available b y Professor I. A. Menzies f o r this s t u d y carried o u t during a s e c o n d m e n t ( o f M.A.F.S.) f r o m the Syrian G o v e r n m e n t Service. The a u t h o r s express g r a t i t u d e f o r helpful discussions with Dr. D. E y r e and Dr. D. R. Eastham. References 1 2 3 4 5

R. Piontelli and L. Cambi, Ital. Patent 368,824, 1939. R. Piontelli, J. Electrochem. Soe., 94 (1948) 106. F. C. Mathers and F. Forney, J. Electrochem. Soc., 76 (1939) 371. E. Schweikher, Proc. Am. Electroplat. Soc., (1942) 90. A. I. Levin, V. F. Lazarev and V. A. Mukhin, J. Appl. Chem. U.S.S.R., 38 (1965) 1531. 6 B. A. Shenoi and K. S. Indira, Met. Finish., 21 (1969) 336. 7 M. Loshkarev, V. I. Cherneko and I. V. Gamali, J. Appl. Chem. U.S.S.R., 31 (1958) 248. 8 D. Eyre, Ph.D. Thesis, Loughborough University of Technology, Loughborough, 1982.