Surface Technology, 22 (1984) 175 - 180 175

A STUDY OF THE EFFECTS OF AMINO ACIDS AS SURFACTANTS ON THE ELECTRODEPOSITION OF Sn-Fe ALLOYS

o. P. GUPTA, RAMA LOOMBA and MANJULA CHAUHAN Department of Chemistry, Lucknow University, Lucknow 226007 (India)

(Received October 14, 1983)

Summary

The effects of four different amino acids as surfactants on the electro- deposition of Sn-Fe alloys from chloride-sulphate baths were studied. Glycine and glutamic acid produce bright deposits of fine flakes in long chains, whereas alanine and aminobenzoic acid produce smaller needle-like flakes. The cathode potential shifts to more negative values on the addition of surfactants. The Tafel slope and the throwing number decrease in the order alanine > glycine > glutamic acid > aminobenzoic acid.

1. Introduct ion

For the simultaneous discharge of metal ions on a cathode, the standard potentials of the codeposited metals should not differ considerably and the metal ions should interact on the cathode surface. This can be achieved by the addition to the electrolyte of a small amount of colloidal matter or some organic compound (known as an addition agent, additive or surfactant) which has a specific action [1]. The presence of a surfactant in the electro- lyte can improve the quality of the electrodeposits and can affect the com- position of the deposited alloy. The addition of a surfactant to the bath solu- tion often results in the production of smooth fine-grained microcrystalline deposits [2].

The influence of surface-active colloidal additives on the cathodic electrodeposition of Sn-Fe alloys from a pyrophosphate bath has been studied by Kudryavtseva and Izmalov [3]. They observed that the presence of gelatin and glue in the pyrophosphate bath can modify the composition and characteristics of the Sn-Fe alloy electrodeposits.

In the present paper, studies of the effects of four different amino acids as surfactants on the electrodeposition of Sn-Fe alloys, their composition, the cathode current efficiency, the cathode potential, the overpotential, the Tafel slope and the throwing number in a chloride-sulphate bath are re- ported.

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2. The experiment

All the chemicals used were of analytical grade. The solutions were prepared in conductivity water. The electrolysis was carried out in a rect- angular cell [4] using a platinum electrode as the anode and a steel plate as the cathode.

The effects of four amino acids on the colour, morphology and com- position of Sn-Fe alloys deposited from a chloride-sulphate bath containing 50 g SnC12.2H20 1-1, 25 g FeSO4"7H20 1-1, 56 g (NH4)2SO4 1-1 and 1.5 g H3BO3 1-1 at pH 1 were studied at 45 °C for four different current densities. In addition, the effects of surfactants on the cathode potential, overpoten- tial, Tafel slope, resistivity and throwing number were also investigated.

The composition of the deposited Sn-Fe alloy was determined gravi- metrically. The efficiency was calculated from the formula used by Ray and Banerjee [ 5 ].

The cathode potential during electrodeposition was measured using a saturated calomel electrode and an agar-agar bridge drawn into a capillary approximately 1 mm in diameter and pressing tightly against the electrode to minimize the error due to the potential drop across the electrolyte [6]. To obtain results which were more reproducible the technique of Bockris [ 7 ] was used.

The Tafel slope was calculated from a plot of cathode potential v e r s u s

logarithm of the current density. The throwing number N was obtained from the slope b and the specific resistance p of the electrolyte using the formula

b g - - m 2p

The cathode overpotential was determined as the difference between the cathode potential and the corresponding open,circuit potential.

3. Results and discussion

The conditions for the electrodeposition of the Sn-Fe alloys in the presence of four different amino acids and the results obtained are given in Table 1.

As the current density was increased from 6.50 to 10.05 A dm -2 the percentage of tin in the deposited alloy decreased. This decrease in the tin content of the alloy occurred in an almost similar manner for the four dif- ferent amino acids, but the rate of decrease in the tin content is more pronounced for glutamic acid. The decrease in the percentage of tin in the alloy may be considered to be due to hydrolysis which results in the forma- tion of Fe(OH)2. The preferential adsorption of the Fe(OH)2 at the electrode prevents further discharge of Sn 2+ ions at the cathode [8, 9].

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TABLE 1

The effect of surfactants (amino acids) on the composition of the deposited alloy, the cathode current efficiency and the distribution of the total current for the discharge of tin, iron and hydrogen ions in the electrolyte (bath temperature, 45 °C; pH 1.0)

Surfactant Current A m o u n t o f Sn Cathode density in the alloy current (A dm -2) (%) efficiency

(%)

Percentage o f the current utilized for the discharge o f the following elements

Sn Fe H2

Glycine 6.50 91.91 14.89 12.55 2.34 85.11 8.00 87.82 17.84 13.78 4.06 82.16 9.25 79.86 19.98 13.01 6.97 80.02

10.05 75.29 21.44 12.63 8.81 78.56

Alanine 6.50 87.82 20.35 14.95 5.40 79.65 8.00 83.15 20.80 14.54 6.26 79.20 9.25 74.29 21.05 12.13 8.92 78.95

10.05 70.08 21.66 11.36 10.30 78.34

Glutamic 6.50 86.00 20.14 14.97 5.17 79.86 acid 8.00 81.21 20.84 14.63 6.21 79.16

9.25 72.35 22.48 12.39 10.09 77.52 10.05 67.97 23.88 11.93 11.95 76.12

Aminobenzoic 6.50 90.08 16.40 13.29 3.11 83.60 acid 8.00 86.93 17.76 12.43 5.33 82.24

9.25 78.16 20.19 11.79 8.40 79.81 10.05 73.22 25.20 10.18 15.02 74.80

Electrolyte composition: SnCI2-2H20 , 50.0 g I-I; FeSO4-7H20, 25.0 g I-I; (NH4)2SO4, 56.0 g I-I; H3BO3, 1.5 g l-l; surfactant, 5.0 g l -I.

The p r e sen t au t ho r s have in an e x p e r i m e n t a l s t u d y f o u n d t h a t in the absence o f a su r f ac t an t the c a t h o d e cu r r en t e f f i c iency decreased as the c u r r e n t dens i ty increased f r o m 6 .50 to 10 .05 A d m -2. In t he p re sen t s t u d y i t was f o u n d t h a t in t he p resence o f a m i n o acids as su r f ac t an t s in t he e lec t ro- ly te , t he re is a sharp increase in t he c a t h o d e c u r r en t e f f i c i ency f r o m 14 .89% to 21 .44% for g lycine and f r o m 16 .40% to 25 .20% fo r a m i n o b e n z o i c acid. Howeve r , fo r a lanine and g lu tamic acid t he c a t h o d e cu r r en t e f f i c iency in- creases very s lowly f r o m 20 .35% to 21 .66% and f r o m 20 .14% to 23 .88% respec t ive ly . These resul ts show t h a t a m i n o acids p r o m o t e ion discharge a t the ca thode .

The na tu r e o f the a l loy depos i t ed in t h e p resence o f su r fac t an t s is given in Tab le 2. With increases in t he c u r r e n t dens i ty , g iycine and g lu tamic acid i m p r o v e the qua l i ty o f t he depos i t s , giving br igh t f ine-grained long-chain depos i t s . In the p resence o f a lanine , smal le r f lakes are o b t a i n e d , and the n u m b e r o f f lakes is increased as t he cu r r en t dens i ty is increased f r o m 6 .50 to 9 .25 A d m -2. Howeve r , a t a c u r r e n t dens i ty o f 10 .05 A d m -2 the size o f t he f lakes is r e d u c e d so m u c h t h a t t h e y conve r t in to needles. In the p resence

178

TABLE 2 The effect of surfactants on the morphology and the external appearance of the Sn-Fe alloy electroplate at various current densities

Surfactant Current Amount of Sn density (%) (A dm -2)

Amount of Fe Nature of electroplate (%)

Glycine

Alanine

6.50 91.91 8.09 8.00 87.82 12.18 9.25 79.86 20.14

10.05 79.29 24.71

Bright thin flakes a Brighter thin flakes Smaller flakes in chains Many flakes in long chains

6.50 87.82 12.18 Bright thin flakes a 8.00 83.15 16.85 Brighter thin flakes in chains 9.25 74.29 25.71 Many thin flakes

10.05 70.88 29.92 Predominantly needle-like deposition

6.50 86.00 14.00 Bright thin flakes a 8.00 81.21 18.79 Increase in the number of

flakes

Glutamic 9.25 72.35 27.65 More flakes in chains acid 10.05 67.97 32.03 Many flakes in long chains

Aminobenzoic 6.50 90.08 9.92 Bright crystalline spots acid 8.00 86.93 13.07 Very small needles

9.25 78.16 21.84 Longer needles 10.05 73.22 26.78 Black porous muddy deposit

with a bright tinge

aThe flakes have an indented round shape similar to that of a coriander leaf with a diam- eter of about 1.5 - 2.0 cm.

o f aminobenzo ic acid the variat ion in the cur ren t densi ty p roduces a ra ther d i f ferent t y p e o f effect . Init ial ly, at a cur ren t densi ty o f 6.5 A d m -2 the depos i t exhibits bright crystal l ine spots. With an increase in the cur ren t densi ty to 8.0 A dm -2, very small needles are obta ined . At a cur ren t densi ty o f 9 .25 A dm -2 the size o f the needles in the depos i t increases. A cur ren t densi ty as high as 10 .05 A dm -2 p roduces a black m u d d y deposi t wi th a br ight tinge.

The variat ion in the ca thode potent ia l with cur ren t densi ty for an e lec t ro ly te with various amino acids as surfactants is given in Table 3. The ca thode potent ia l was observed to shift to more negative values at cons tan t cur ren t densities in the fo l lowing order : aminobenzo ic acid > glutamic acid ~ glycine > alanine. This decrease in the po ten t ia l is explained by the increase in the rate o f d i f fus ion o f meta l ions towards the ca thode wi th in- creases in the cur ren t densi ty and addi t ion o f surfactants .

In the e lec t rodepos i t ion o f S n - F e alloys f r o m a ch lo r ide - su lpha te bath, the ca thode potent ia l was f o u n d to vary linearly with the logar i thm o f the cur ren t densi ty (Fig. 1) in accordance with the fundamen ta l law of kinetics o f e lec t rode processes expressed by

TABLE 3

The influence of surfactants on the current density and cathode potential data

179

Current Logarithm o f the density current density (A dm -2)

Cathode potential (V) in the presence o f the following surfactants

Glycine Alanine Glutamic acid Aminobenzoic acid

0.00 1.248 1.248 1.498 1.498 4.00 0.6021 1.448 1.298 1.533 1.583 5.25 0.7202 1.498 1.374 1.563 1.623 6.50 0.8129 1.535 1.435 1.598 1.648 8.00 0.9031 1.573 1.493 1.623 1.673 9.25 0.9661 1.598 1.498 1.648 1.698

10.05 1.0021 1.613 1.558 1.658 1.703 10.15 1.5064 1.638 1.598 1.678 1.723

1-7

1"6

'5 4 ~ 4 4 e li tog of current der~ty(A/dn~} .

Fig. 1. The variation in the cathode potential with the logarithm of the current density for Sn-Fe alloy plating in the presence of various surfactants: /x, alanine; o, glycine; x, glutamic acid; D, aminobenzoic acid.

V = a - - b log ]

where V is the potent ia l , j is the cur ren t dens i ty and a and b are constants . Table 4 shows tha t the Tafel slopes and th rowing number s for the four

amino acids decrease in the order alanine > glycine > glutamic acid > amino- benzoic acid.

Table 5 indicates tha t the ca thode overpotent ia l increased wi th in- creases in the cur ren t densi ty . This increase in overpoten t ia l is due to polar- izat ion. The increase in the overpotent ia l values at cons tan t cur ren t densities is in the fol lowing order : glycine > alanine > aminobenzo i c acid ~> glutamic

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TABLE 4

The effect of surfactants on the resistivity, the Tafel slope and the throwing number

Surfactant Resistivity Tafel slope Throwing number (~ era) (V)

Glycine 4.690 0.4000 0.04364 Alanine 3.755 0.6522 0.08686 Glutamic acid 4.393 0.3333 0.03792 Aminobenzoic acid 4.291 0.3044 0.03538

TABLE 5

The influence of surfactants on the cathode overpotential

Current Cathode overpotential (V) in the presence o f the following surfactants density (A dm -2) Glycine Alanine Glutamic acid Aminobenzoic acid

4.00 0.200 0.050 0.035 0.085 5.25 0.250 0.126 0.065 0.125 6.50 0.287 0.187 0.100 0.150 8.00 0.325 0.245 0.125 0.175 9.25 0.350 0.250 0.150 0.200

10.05 0.365 0.310 0.160 0.205 10.15 0.390 0.350 0.180 0.225

acid with the exception of the values at a current density of 4.00 A dm -2. At this lowest current density the increase in the overpotential for the differ- ent amino acids is in the order glycine > aminobenzoic acid > alanine > glutamic acid.

References

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