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PRQCESSES AND APPLICATIONS FOR TIN AND TIN-BASED ALLOY SURFACE COATING TECHNOLOGIES

A Technical Review and Assessment of Recent Developments

Compiled for Tin Technology

STAGE 1

ELECTROLYTIC DEPOSITION

PART 2

ELECTRODEPOSITION AND ELECTROPLATING OF TIN ALLOYS

CORROSION RESISTANT COATINGS

BY

L. M. Baugh, BSc, MSc, PhD, Chem, FRSC

Consultant

February 2005 C

CONTENTS

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1 . Introduction ........................................................................... 4

2 . Tin-Zinc Alloys ....................................................................... 4

2.1 Background .............. ...................................................... 4 2.2 EQCM Studies of Electrodeposition and Corrosion Behaviour ......... 5 2.3 Electrodeposition of Eutectic Alloy by Pulse Plating .................... 10 2.4 Electrodeposition from Sulphate-Gluconate Baths ....................... 13 2.5 Electrodeposition of Low Tin Content Alloys ............................ 14 2.6 Multilayered Alloy Films ..................................................... 14 2.7 Ternary Alloys ................................................................ 15 2.8 Role of Additives ............................................................. 15

3 .

4 .

5 .

6 .

Tin-Nickel Alloys ................................................................... 16

3.1 Background ..................................................................... 16 Electrodeposited Sn-Ni Alloys ............................................. 16 Electrodeposited Sn-Ni Alloy Films ....................................... 19 Sn/Ni Multilayer Composites ................................................ 20

3.2 3.3 3.4

Tin-Cobalt Alloys .................................................................. 23

4.1 Background .................................................................... 23 Electrodeposition from Neutral Gluconate Bath .......................... 24 Electrodeposition from Acid Gluconate Bath ............................. 25 Electrodeposition from Citrate Solutions ................................. 27 Electrodeposition from Pyrophosphate Solutions ..................... i .. 28 Binary and Ternary Alloys in Bearing Alloys ............................ 29

4.2 4.3 4.4 4.5 4.6

Tin-Copper Alloys .................................................................. 30

5.1 Background .................................................................... 30 Electrodeposition from Acid Gluconate Baths ........................... 30 Electrodeposition from Acid Tartrate Baths .............................. 32 Electrodeposition of CdCu-Sn Multilayers by Potential Pulse ......... 34

5.2 5.3 5.4

Electrolysis

Tin-Manganese Alloys ............................................................ 34

6.1 Background ..................................................................... 34 Sacrificial Tin Manganese Alloy Coatings ................................. 34 6.2

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Tin-Lead Alloys.. . . . .'. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . .... 43

7.1 7.2

7.3

Background.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 43 Sn-Pb Alloy Deposits from Electrochemically. . . . . . . . . . . . . . . . . . . . . . . . . . .43 Prepared Electrolytes Marine Corrosion Tests of Electroplates. . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . ..45

Tin-Antimony Alloys. .. .. . . ... . . .. . ... ... . .. . .... . .. . ... ... . .. . .. ... ... ... . .. .. ... 46

8.1 8.2 8.3

Background.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46 Sn-Sb Electrodeposits.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46 Sn-Sb-Cu Electrodeposits.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

Conclusions and Recommendations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 47

10. References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

11. Acknowledgements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 54

3

1. INTRODUCTION

About 20 different electrodeposited tin alloys are known. These can be roughly divided according to whether their primary application is in the field of corrosion protection or as solders and coatings in the field of electronics. The current review (Part 2) is concerned with recent developments in the application of electrodeposited tin alloys in the field of corrosion protection. Subsequent reviews will deal with the topic of the application of these alloys in electrochemical power sources (Part 3 ) or in solders and electronics (Part 4). An earlier review was concerned with the electrodeposition and electroplating of pure tin [ 11.

Several general review articles have been produced during the past few years describing the background to corrosion protection by metallic coatings and most of these include references to tin alloy systems [2-61. Amongst the important alloy systems to be considered are Sn-Zn, Sn-Cu, Sn-Ni, Sn-Co, Sn-Mn, Sn-Pb and Sn-Sb.

2. Tin-Zinc Alloys

' 2.1 Background

This has been discussed by Vitkova et al. [7].

Sn-Zn coatings are used both as functional and as protective-decorative plate. They offer improved corrosion resistance and good solderability. In some cases they can replace not only zinc but also cadmium coatings. It is well known that zinc coatings when exposed to adverse atmospheric conditions start to dissolve, leading to the formation of basic salts. Tin coatings resist atmospheric influence, but are quite porous and since tin is cathodic with respect to the steel substrate, the efficiency of corrosion protection depends on the porosity of the coating. Sn-Zn alloy coatings combine the advantages of both metals.

There are several compositions that are suggested to offer the best corrosion resistance and protective effect. Sn-Zn alloy coatings with high tin content are highly resistant, provided they have been deposited as a continuous layer. The addition of zinc to improve the resistance of the coating against tarnishing, enhances the solderability and elasticity and increases anti-frrction properties.

Comparison of tin, zinc and tin-zinc coatings is rather difficult. In the case ofpure tin and tin-rich coatings, uniform corrosion of the substrate is found, while the coating is attacked slightly. Pure zinc and zinc-rich alloys are subject to corrosion, but offer good protection of the substrate metal. After prolonged corrosion exposure, tin and tin-rich coatings are not heavily damaged, while zinc-rich coatings prevent further rusting of steel substrates. Previous investigations provide evidence that Sn-25%Zn plates are better than pure zinc or cadmium in salt spray fog corrosion tests, and equal to cadmium and superior to zinc in humid atmosphere tests. During atmospheric tests, the advantages of zinc, tin-zinc and cadmium coatings have been found to depend on the corrosion medium.

Several types of electrolytes are known including pyrophosphate, alkaline and acidic. Acidic electrolytes for the tin-zinc alloy are least investigated. The electrodeposition

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from boron silicon (borosilicate) and boron fluoride electrolytes has been studied. Electrolytes based on zinc and tin sulphates are also known. The plating regime is pH 4.1-4.7, cathodic current density 1-5 A dm-2. The zinc content in the alloy is within the range 10%-40%. The shortcomings of electrolytes is that they cannot support high current densities (up to 2 A dm-2). This in turn affects the alloy composition on the surface of complicated profile parts. In order to eliminate this disadvantage and to obtain bright, fine-grained deposits, various additives can be incorporated in the electrolyte.

2.2 EQCM Studies of Electrodeposition and Corrosion Behaviour

In a series of papers [8-101 Wang et al. have studied the electrodeposition and subsequent corrosion characteristics of a 70%Sn-30%Zn layer deposited from a neutral non-cyanide bath. It has been reported that alloys with this composition have the best comprehensive properties.

The above alloy was plated onto a steel substrate and the mechanism of corrosion was then investigated in a 0.1M NaS04 solution at pH 3.6. The same alloy was plated onto a gold electrode that formed part of an electrochemical quartz crystal microbalance (EQCM) in order to obtain the deposition current efficiency and the corrosion rate in situ. The electrolyte consisted of tin (1 0 gl), zinc (1 0 gl), stabiliser (1 30 gl), antioxidant (130 gl) , brightener (100 g/l), pH 5.5-7.0, and temperature 15-30 "C [ l l ] .

The composition of the Sn-Zn deposit changed with the plating current density. To determine the composition of the deposit, the whole steevdeposit sample was dissolved in nitric acid and the SdZn ratio measured using inductively coupled plasma spectroscopy. Fig. 1 shows the relationship between the plating current and the deposit composition. It can be seen that the higher the current density, the lower the tin content in the deposit. In order to make a 70%Sn-30%Zn alloy required a current density of 5 mA cm-*.

- 100

90

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Plating Current Density (mA cm-')

Fig. 1. Composition change with the plating current density [8].

The deposit layer morphology is shown in Fig. 2. According to the Sn-Zn phase diagram, Sn and Zn have a very low mutual solubility. They also do not form any compound at room temperature. X-ray diffraction confirmed that that the coating was composed of terminal Sn and Zn phases and EDS spot analysis failed to identify any

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composition differences at different locations, confirming that the Sn and Zn phases were finely mixed together.

Fig. 2. Scanning electron micrograph of the Sn-Zn deposit [8].

By’use of the EQCM, the mass increase of the deposit with time during plating was measured and the results are shown in Fig. 3. Over a wide frequency range, the relationship between the resonant frequency change, Af, of a quartz crystal and the additional mass, Am, that is rigidly attached to the crystal surface, is linear, i.e. Am = KAf, where K is a positive constant. The straight line in Fig. 3 indicates that the deposition rate was rather stable. Calculations showed that the plating efficiency was 71%.

Time (s)

Fig. 3. Mass change of the alloy deposit with time during electroplating [8].

After electroplating, the sample was immersed into a 0.1M Na2S04 solution of pH 3.6 and the open-circuit potential (OCP) was measured (Fig. 4). For the first 5000s (1.3h), the OCP of the sample was constant, then it showed a slow increase to more positive values. In order to make comparisons, the OCP’s of different metals in the same solution were measured. They were, vs. SCE: Ez,, -l.O5V, Esn -0.57V, and Estee, -0.70V. Hence, the starting OCP of the sample was identical with that of pure zinc in the same solution. This was interpreted in terms of the corrosion of zinc being the dominating process at the OCP. As expected, zinc acts as a sacrificial anode and by this action protects the tin and the steel substrate from corrosion.

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Fig. 4. Open-circuit potential of Sn-Zn deposit vs. time in 0.1M Na2S04 (pH 3.6) solution [8].

By use of the EQCM, the mass loss of the deposit with time was determined during the corrosion process at the OCP (Fig. 5). The mass loss was caused by the corrosion of the deposit, i.e. the dissolution of zinc. Thus, the slope of the curve gave the corrosion rate of the deposit. During the first period of about lh, a rather fast dissolution occurred. This was attributed to the dissolution of a hydroxide film that was formed on the top of the deposit after extended deposition times because of the simultaneous alkalisation of the surface caused by hydrogen evolution. Following this period, the rate of mass loss decreased significantly. From the data, a corrosion current in the range 7.1-8.2 pA cm-* was calculated.

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Fig. 5. Mass change of the.alloy deposit with time during immersion in 0.1M Na2S04 (pH 3.6) solution [8].

In order to measure the protective capability of the deposit layer, the plated sample was left in 300 ml of the 0.1M NaZS04 (pH 3.6) solution for 24h. After that, a small amount of potassium ferricyanide was added to the solution. No observable colour change occurred and therefore no Fe dissolution could be detected. Furthermore, ICP analysis of the solution showed that the Zn, Sn and Fe concentrations were 1.9, <O. 1 and < 0.1 ppm, respectively. This led to the conclusion that at the OCP, only Zn was dissolving.

Fig. 6 shows the polarisation curves of the different metals in the same corrosive medium. As the potential increased from the deposit's OCP, the anodic current density remained small until the potential caused dissolution of the tin. Then the current increased steeply with time. This was interpreted in terms of the fact that,

7

although the Zn gave cathodic protection to the steel matrix at the OCP, Sn contributes its bamer property for added protection in corrosive environments.

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Fig. 6 . (a) Polarisation curve of the Sn-Zn plated layer in 0.1M Na2S04 (pH 3.6), (b) comparison with the polarisation curves of other metals [8].

Fig. 7 shows the surface layer structure of the deposit after the polarisation experiments in Fig. 6. EDS analysis indicated that the composition of the deposit was now almost pure Sn.

Fig. 7. SEM image of Sn-Zn deposit after a potential scan from the OCP to -0.4V [SI.

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In separate work, Wang et al. deposited a 70% Sn-30% Zn alloy from the same non- cyanide bath and measured the corrosion rate by use of the EQCM technique [ 121. These measurements, together with observation by electron microscopy of the deposit’s cross-section, showed that without sufficient agitation in the plating bath, a zinc hydroxide is formed on top of the metal film. In order to obtain a uniform deposit layer without the formation of hydroxide the solution must be agitated during the plating process. These differences can be seen in Figs. 8 and 9.

Fig. 8. Duplex layer deposited without agitation [12].

Fig. 9. Single alloy layer produced with nitrogen agitation [ 121.

Additional work showed that 1,2,3-benzotriazole was an effective anodic corrosion inhibitor for the alloy layer.

In hrther studies, alloys having zinc contents in the range 18-38% were investigated [ 131. It was shown that the open-circuit potentials of the coatings immersed in the corrosive 0.1M Na2S04 @H 3.6) electrolyte increased with corrosion time. Also, the smaller the zinc content, the more rapidly did the potential increase. It was postulated that an iR drop mechanism might have been responsible. As zinc dissolved into the solution, cavities appeared on the deposit surface. Further zinc dissolution only occurred at the bottom of these pores, while the hydrogen evolution reaction mainly occurred on the outer surface. The separation of the anodic and cathodic sites caused an iR drop in the pore electrolyte and hence the change in potential was explained.

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2.3 Electrodeposition of Eutectic Alloy by Pulse Plating

Lin and Sun [ 141 have recently shown how a uniform deposit of a Sn-Zn alloy can be achieved using the technique of pulse plating. The relative composition of tin and zinc in the deposit was affected by the bath compositions and pulse condition. A pulse plating condition of 99.9 ms on-time and 1.0 ms off-time gave rise to a eutectic Sn-Zn deposit, with a eutectic temperature of 198.8 "C, as analysed by differential scanning calorimetry .

The cathode was a 7.5 x 5.5 cm copper plate. The electrolyte in this study was composed of 0.5-30 g/1 ZnS04.7H20; 3.7-7.5 g/1 SnC1,; 40-80 g/1 citric acid; 40 g/1 (NH&S04; and up to 500 ppm polyethylene glycol. The peak current density applied for pulse plating varied from 0.24 to 2.4 A dm-2. The deposits obtained at a peak current density of 0.24 A dm-2 with an on-time of 99.9 ms and an off-time of 0.1 ms exhibited relatively uniform distribution of Sn and Zn across the deposit (Fig. 10). The concentration of zinc increased as the concentration of ZnS04 rose from 1.5-30 g/1 at a fixed SnC12 concentration of 3.745 g/l. The highest concentration of Zn in the deposit was approximately 70-75%.

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Fig. 10. Elemental profiles for deposits obtained with pulse plating at 0.24 A dm-* peak current density, 99.9 ms on-time, 0.1 ms off-time, 3.75 g/1 SnC12 and ZnS04.7H20 of (a) 1.5 g/l, (b) 7.5 g/l, (c) 15 g/1 and (d) 30 g/l [14].

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An increase in current density was expected to increase the deposition speed. The peak current density was therefore raised to 0.72 A dm'2 when investigating the effect ofperiodicity, i.e. the ratio of on-time to total period (Fig. 11). Here the bath contained 1.5 gA ZnS04.7H20 and 5.617 gA SnC12. As can be seen, the variation in periodicity, on-time 99.9 ms and off-time 0.1-99.9 ms, generally did not affect the relative Sn:Zn composition of the deposit. All of the deposits obtained at different off- times exhibited uniform composition throughout, in contrast to the situation achieved in d.c. plating using a current density of 0.13 A dmm2 and similar electrolyte compositions.

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Fig. 1 1. Elemental profiles for deposits obtained with pulse plating at 0.72 A dm-2 peak current density and 99.9 ms on-time, in a bath containing 1.5 g/l ZnS04.7H20 and 5.617 g/1 SnC12.The off-times were (a) 0.1 ms, (b) 0.5 ms, (c) 1.0 ms and (d) 99.9 ms [14].

In addition to the electrochemical characteristics, the deposit microstructure was studied. The deposit produced with an off-time of 0.1 ms and an on-time of 99.9 ms (Fig. 12) exhibited a uniform thickness of approximately 9 pm. This deposit was produced at 0.72 A dm-2 for 2h using a bath containing 1.5 g/1 ZnS04.7H20 and 3.74 g/1 SnC12. The off-time duration also affected the surface morphology of the deposit. Fig. 13 shows that a nodular appearance was produced with a longer off-time (99.9 ms, Fig. 13 (b). A shorter off-time resulted in a deposit with a relatively smooth appearance (Fig. 13 (a), when the on-time was maintained at 99.9 ms.

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Fig. 12. SEM cross-section of a deposit obtained at 0.72 A dm-2 peak current density, an on-time of 99.9 ms, an off-time of 0.1 ms and a deposition time of 2h, from a bath containing 1.5 g/l ZnS04.7H20 and 3.74 g/1 SnC12 [14].

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Fig. 13. SEM surface morphology of deposits obtained at 0.72 A dm-2 peak current density, an off-time of (a) 0.1 ms and (b) 99.9 ms. The bath contained 1.5 g/l ZnS04.7H20 and 3.745 g/1 SnC12 [ 141.

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2.4 Electrodeposition from Sulphate-Gluconate Baths

The electrodeposition of tin-zinc alloys with 20-30% Zn by weight from sulphate gluconate baths at pH4 and different [ Sn(II)]: [Zn(II)] and [gluconate]/ [ Sn(II)]+[Zn(II)] ratios has recently been studies by Guaus and Torrent-Burgues using the technique of voltammetry and stripping voltammetry [ 151. The latter data was correlated with the morphology, composition and phase structure of the deposits. When [Zn(II)] was equal to or not much greater than [Sn(II)], a eutectic type Sn-Zn alloy was obtained, with a zinc content of around 20 % by weight. The morphology of the electrodeposited Sn-Zn alloy tended to disappear at high deposition charges and a second crystalline coating, richer in Sn, was obtained.

2.5 Electrodeposition of Low Tin Content Alloys

Vitkova et al. [ 161 have commented that within the Sn-Zn literature, mainly high tin content alloys have been studied. This was attributed to difficulties in depositing high quality coatings from known electrolytes with tin content less than 50 %. From a strictly economic viewpoint these authors claimed that it would be worth depositing alloys with tin content up to 20% and to evaluate their protective properties compared with zinc plates. With this objective, Vitkova et al. deposited Zn-Sn alloy coatings from a basic electrolyte consisting of 110 g/1 ZnS04.7H20, 1.5-1.7 g/1 SnS04, 80 g/1 NH4Cl, 20 g/1 Na-gluconate and 30 g/1 Na-citrate. To the basic electrolyte a brightener formulation composed of 350 g/1 ethoxylated nonyl-phenol with 15 ethylene oxide groups [C9H19-C6H4-0-(CH2CH2o)~~] and 30 g/1 benzyl acetone were added. The concentration of brightener in the electrolyte was within the range 1-15 mV1. The bath operating conditions were pH 4, room temperature and cathodic current density 7-30 mA cm-2. Copper or platinum cathodes were employed.

Zn-Sn alloy coatings containing 10-20 % tin with good appearance were deposited fkom the slightly acidic electrolytes. The effect of the concentration of tin sulphate and the cathodic current density on the composition and the structure of the alloy deposits was investigated. The partial deposition rates for Zn and Sn from electrolytes containing various brightener concentrations at a definite potential were also determined. It was shown that an increase in the concentration of the organic additive from 1-5 ml/l lead to an abrupt decrease in zinc deposition rate, while the tin deposition rate decreased slowly. A further increase in the concentration of the brightener did not significantly affect the deposition rate of either metal. The protective properties of the Sn-Zn coatings were investigated by means of a so-called ‘Praatsch’ test in a double electrochemical cell [ 171. In this test, the smaller the potential difference, AV, between an iron electrode and the test electrode, the better the protective properties of the test electrode. The results of this investigation are presented in Fig. 14. It can be seen that alloys containing 10-12% tin displayed a AV value of 290 mV, identical with that of pure zinc (AV = 291 mV). Coatings containing 14-1 5% Sn had protective properties equal to those of Zn-Co alloys with cobalt content 0.3%-1.3% and AV = 260 mV.

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. Fig. 14. The change in protective properties of Zn-Sn coatings vs. Sn content in the alloy [16].

2.6 Multilayering Technique for Producing Sn-Zn Alloys

Oki et al. [ 181 have devised a method of forming tin-zinc alloy films by sequentially stacking the respective metals by electrodeposition followed by irradiation with a laser beam to produce an alloy by inter-diffusion of the elements. These authors claim that their technique yields alloys in a short time without the creation of non-equilibrium phases and that the alloys remain stable for long periods of time, thus facilitating their use as economical corrosion resistant coatings.

The procedure for producing one type of film is as follows: A pure iron plate having a thickness of 2 mrn was employed as a substrate and then immersed into a fluoroboric acid bath having a total volume of 300 ml which included 18 ml of 42% boric hydrofluoric acidic bath, 2 ml of 44.6% fluoroboric tin and 15 mg of polyethylene glycol (MW = 2000). Then the fluoroboric acid bath was electrolysed by flowing a current of 1A dme2 for 5 mins to form a tin layer in a thickness of 30 pm on the iron plate. Then the iron plate having the tin layer thereon was immersed into a zinc plating bath having a total volume of 300 ml which included 137 g of zinc chloride, 10 g of boric acid, 5 g of sodium chloride and 10 g of alumiinium sulphate and whch was heated to 40 "C. Then, the zinc plating bath was electrolysed by flowing a current density of 20 A dm'2 for 2 mins to form a zinc layer 50 pm thick on the ti layer, to fabrcate a multilayered film composed of the tin layer and the zinc layer. During the formation of the zinc layer it was recognised that the thickness of the tin layer was reduced by several pm. Finally, a laser beam from a CO2 laser was focused onto the multilayer for 20-60s at an irradiation intensity of 100-300 W cm-2, to produce the tin- zinc alloy film.

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2.7 Ternary Alloys

The observation of developments in the field of corrosion protection of ferrous materials, as in the automobile industry, indicates that there will be higher requirements for anti-corrosion systems in the future which cannot be met with known processes. Such increased requirements are of the order of 3000 hours resistance in salt mist tests. Furthermore, such anti-corrosion coatings should have the highest possible hardness, be resistant to wear, and should also, as far as possible, be weldable. D u d e et al. [ 191 have very recently proposed that ternary tin-zinc alloys containing 30-65% by weight tin, 30-65% by weight zinc, and 0.1-15% by weight of a metal from the group iron, cobalt, or nickel, as the third alloying component, meet these requirements very well. The ternary tin-zinc alloys preferably contain cobalt as the third alloying component.

A very wide range of experimental conditions are claimed to be suitable for producing these alloys by electrodeposition, including acidic, neutral or alkaline solutions, together with a wide range of possible electrolyte salt compositions. As an example, a weakly acidic electrolyte for depositing an alloy consisting of 49.2% by weight Sn, 50.5% by weight of Zn and 0.3% by weight Ni has the composition: 5 g/1 SnS04; 6.8 g/1 ZnS04; 12 g/1 NiS04; 80 g/1 sodium citrate; 25 g/1 boric acid; 10 ml/l anionic surfactant; 1 ml/l beta naphthol ethoxylate; pH 4.5. The coating composition indicated can be produced with this electrolyte at a temperature of 40°C and a current density of 1.5 A dm-*. About 0.4 pm of the alloy is produced per min and the density of the layer is 7.2 g ~ m - ~ . Chromate treatment may also be given to provide additional corrosion protection to the ternary coatings.

2.8 Role of Additives

A variety of patents have been filed during the last few years relating to improved plating bath compositions for tin-zinc alloys. These compositions are designed to be either more environmentally friendly, replacing existing cyanide baths, or designed to produce smooth bright electrodeposits over a range of current densities.

Strube and Jordan [20] have described a plating solution comprising the following components: Zn(I1) ions; Sn(I1) ions; aliphatic carboxylic acids and/or their alkaline salts; anionic surfactants; non-ionogenic surfactants; and, optionally: aromatic aldehydes; aromatic ketones; aromatic carboxylic acids; and heterocyclic carboxylic acids, or their alkaline salts or conducting salts. It is claimed that the invention provides a means of electrodepositing unifom light coloured tin-zinc alloys without having to use cyanide ions, allowing low energy consumption and few requirements in terrns of the control of the bath.

Becking [Zl] has described an electroplating bath that is claimed to be particularly suited to the electrodeposition of smooth and bright tin-zinc alloys. The disclosed electroplaing bath comprises propanedioic acid; diethyl ester; polymer with N-(3- aminopropy1)- 1 ,3-propanediamine7 N-(2-carboxybenzoyl) as a brightener additive. In addition, the electroplating bath may also comprise carboxylic acids, ammonium salts, aldehyde compounds and a variety of co-brighteners.

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A plating bath suitable for the deposition of a bright and level deposit, which can be adapted to provide plated alloys having a high tin concentration over a wide current density range, has been described by Capper and Opaskar [22]. The bath comprises at least one bath-soluble stannous salt, at least one bath-soluble zinc salt and a quaternary ammonium polymer selected fiom a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer. The plating baths may also contain one or more of the following additives: hydroxy polycarboxylic acids or salts such as citric acid; ammonium salts; conducting salts; aromatic carbonyl containing compounds; polymers of aliphatic amines such as poly(alky1eneimine); and hydroxyalkyl substituted diamines as metal complexing agents.

3. Tin-Nickel Alloys

3.1 Background

Electrodeposited tin-nickel alloy is an intermetallic compound of approximately 65% tin and 35% nickel by weight, with a variation of 2-5% over a wide range of operating conditions and solution variables. They have been used for decorative and protective applications. When plated on a polished substrate, the alloy is fully bright and requires little or no buffing. Sn-Ni alloy is highly resistant to tarnish and corrosion in humid atmospheres. It also possesses good contact resistance and excellent frictional resistance. Because of these properties, the alloy is almost a substitute for decorative chromium in a vast number of industries such as metal furniture, hardware, domestic and light engineering, electrical appliances and other equipment that require strong resistance to corrosive chemicals [23].

Schwitzgebel and Mildenberger [24] have reported that the electrodeposited Sn-Ni alloy is composed of Ni3Sn2 with addition of Ni3S3 and Sn(II) compound, SnO or Sn(OH)2. The corrosion stability of Sn-Ni alloy has been attributed to the presence of stable, protective, self-limiting film of a glassy nickel poly-stannate on the alloy surface [25,26].

3.2 Electrodeposited Sn-Ni Alloys

Bapu and Ramesh [27] have recently studied Sn-Ni electrodeposits produced from an electrolyte consisting of ammonium-bifluoride, tin chloride and nickel chloride. Deposition parameters were optimised using Hull Cell experiments and the current eficiency and throwing power of the solution were determined. The hardness, wear resistance and structure of the deposited alloy were also examined.

Laboratory grade nickel chloride (300g) and stannous chloride (50g) were dissolved in 600 ml distilled water containing ammonium-bifluoride (55g). About 2g of activated charcoal was added to the solution and digested thoroughly. After 8 hours settling, it was decanted and pre-electrolysed at 0.3 A dnY2 for 24 hours. The pH was adjusted with dilute HC1 or NH4OH. Before plating, the copper or steel substrates were degreased with trichloroethylene, cathodically cleaned and etched.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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From the Hull Cell studies, the optimum conditions for obtaining smooth bright deposits from the chloride bath were found to be about 2-8 A dm-* at pH 2.5 and a temperature of 65°C.

Current Density 1 Adm-2

The effect of current density on current eficiency and alloy composition was significant and the weight percentages of tin in the Sn-Ni alloy at various current densities are shown in Table 1.

Current Efficiency %

Table 1. Effect of current density on current efficiency and alloy composition [27].

1 .o 1.5

59.6 73.6 26.4 65.5 69.8 31.2

% Tin

. 2.5 3.0 4.0

% Ni

80.3 65.2 34.8 94.8 63.6 36.4 95 .O 62.7 37.3

Sample

I 2.0 I 69.9 I 65.2 I 34.8

Hardness, VHNSo TWI (load 1OOOg) 1 Nickel

Sn-Ni (30%) Sn-Ni (35%) Sn-Ni (40%) Chromium

The current efficiency increased with current density and at 3-4 A dm-2 it remained at about 95 %. It was also observed that with an increase in current density, the tin content of the alloy had decreased from 74 to 63%. A bright alloy containing around 35% nickel was obtained over a wide current density range between 2-3 A dm-2.

190 21.1 625 25.2 662 23.7 685 23.5 920 12.8

The throwingpower of the chloride bath measured at various current densities was about 30% and vaned slightly in the range 29-32%.

The hardness of the Sn-Ni alloy with varying nickel content is given in Table 2 and a comparison is made with that of pure nickel and chromium deposits.

Table 2. Hardness and taber wear resistance data [27].

There appeared to be a close relationship between the structure and the hardness of the deposit, the harder deposits being fine grained, brighter and smoother. In this study, the electrodeposited Sn-Ni (30%) alloy had a hardness of 620 VHN and increased significantly with nickel content. The hardness was found to be between those reported [28] for nickel and chromium.

Bapu and Ramesh [27] commented that, in general, hardness is not necessarily a reliable criterion for wear behaviour of an electrolytic coating. The growth habit of

17

ad-atoms during electrocrystallisation is important for wear behaviour. The TWI of the alloy is given in Table 2 and compared with that of nickel and chromium deposits. The alloy had a TWI of about 25, which was found to decrease with the nickel content of the alloy. However, these values did not compare favourably with nickel and chromium deposits [28]. Stress and brittleness of the Sn-Ni alloy and the soft tin coatings may cause a considerable reduction in the wear-resistance, because under the influence of the applied load, the soft deposits are abraded easily, thereby increasing the wear loss [27].

Mild steel Sn-Ni(35%) 5 pm Sn-Ni(35%) 10 pm Sn-Ni(35%) 15 um

. Electroplated Sn-Ni alloy is resistant to corrosion at ordinary temperatures by alkalis, neutral solutions and nitric acid and by non-oxidising acids at pH values greater than 1.2 [29]. Typical galvanostatic polarisation behaviour of the Sn-Ni (35%) alloy in 1M HC1 is shown in Fig. 15. For comparison, the curve for mild steel is also shown and the corrosion data are given in Table 3.

120 -515 26 -516 24 -496 18 -474

W

LM; cu R R E N T I I PA /C m 2

Fig. 15. Galvanostatic polarisation curves of Sn-35%Ni alloy in 1M HCl solution [27].

Table 3. Corrosion data for Sn-Ni alloy in 1M HC1 [27].

Sample E,,, (mV vs. SCE)

It can be seen that the I,,, values of the alloy decreased with increase in thickness of the alloy coating. Compared with steel, the alloy displayed about 6-7 times more resistance to corrosion in chloride environments under identical experimental conditions.

Jalota [29] has recently described the electroplating of Sn65%-Ni35% alloys, corresponding to an equiatomic ratio of one part of tin to one part of nickel, from

18

0 0 0 0

0 0 0 0 a 0 0 0 0 0

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similar electrolyte solutions and under similar conditions of pH and temperature to those described above. In addition, this author claims that electroplating can also be accomplished from pyrophosphate baths containing glycine at near neutral 7.5-8.5 pH values.

The corrosion behaviour of Tin-Nickel electrodeposited alloy, having the approximate composition Tin66%-Ni34%, has recently been studied by Refaey et al. [30]. The alloy was deposited on a plane copper cathode of area 1 cm2 from a chloride-fluoride bath. The solution contained 250 g dm-3 NiC12.6H20; 33 g dm-3 SnC12.2H20; 20 g dm-3 NH4F and 10%(by volume) HC1, pH 2.5. The current density applied was 6 mA cm'2 for 10 mins and the bath temperature was 65°C. Potentiodynamic polarisation, cyclic voltammetry and a.c. impedance spectroscopy techniques, complemented by X-ray diffraction and SEM (EDX), were used to study the corrosion behaviour of the Sn-Ni alloy in Na2C03 solutions at pH 11.9. The effect of different factors, such as the concentration of carbonate ions and the progressive addition of halide ions on the electrochemical behaviour of the alloy, were discussed. The observed corrosion resistance of the electrodeposited Sn-Ni alloy was shown to be due to the formation of a passive film, which when examined by X-ray spectroscopy, was believed to be mainly nickel and tin oxides. The voltarnmograms revealed five anodic peaks. The first and second of these corresponded to the formation of SnO and Sn02 oxides. The remaining peaks corresponded to nickel oxides. The general corrosion rate and the pitting corrosion rate of the alloy increased with the addition of halide ions to the corrosive solution.

Riesenkampf et al. [31] have commented that the wider application of Sn-Ni coating s is hampered by the present technology involving the use of aggressive hot fluoride- chloride type plating baths. These authors have presented some results on the electrodeposition of Sn-Ni alloys with various Ni contents deposited from pyrophosphate solutions [ 321. The X-ray examination of these deposits revealed a set of lines that did not correspond to any known phases in the Sn-Ni system. It was therefore of interest to analyse both the morphology and crystal structure of this new phase in the Sn-Ni deposit and to determine its sensitivity to heat treatment [31]. Coatings containing from 50-100 at.% Sn of a nominal thickness 10 pm were electrodeposited from weak alkaline (PH 8.2) pyrophosphate solutions at ambient temperature on copper plates as described. It was demonstrated that multiphase Sn-Ni deposits with 3-34 at.% Ni contain a new phase of composition close to NiSng. The set of diffraction lines from this phase dominated for deposits containing from 14.8 to 19.9 % Ni. This new phase has a crystal lattice similar to Ni3 SQ. During heating, this new NiSng decomposed at 100°C to p tin and Ni3Sm.

3.3 Electrodeposited Tin-Nickel Alloy Films

The characteristics of Sn-Ni alloy films produced from stacked singZe Zayers by heat treatment has been investigated by Kanematsu et al. [33]. The technology has also been described in a patent [34].

Various electroplating solutions and conditions for the formation of the films on an iron plate have been described [34]. The following serves to illustrate the technique: Fluoroboric acid solutions in the presence of polyethylene glycol are used for plating

19

the tin layer, where a current of 1A dm-2’passed for 5 mins is sufficient to produce a coating of 30 pm. Next, the iron plate having the tin layer is immersed into a ‘Watts’ bath and electrolysed by passing a current of 5A dm-2 for 5 mins to form a nickel coating of thickness 30 pm. Finally, the iron plate having the multilayered film is set into a furnace at 200°C for several days. As a result, the tin layer disappears into the nickel layer by diffusion.

Higher m a c e temperatures can be employed [33]. Using 10 pm metal layers, experiments have been conducted at 350-550°C. NiSn was primarily formed at 350°C while Ni3Sm and Ni3Sn2were commonly formed at 450 and 550°C. The oxidation of the surface became significant with increasing heat treatment temperature. The hardness of the surface layers increased with temperature and reached a maximum of 700 VHN.

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3.4 Sn/Ni Multilayer Composites 0

Although not specifically corrosion resistant, these materials are gaining interest because of their unique mechanical properties. Multilayer composites can be prepared by electrodeposition and it is therefore appropriate to consider them in the current review. Wang and Singh [35] have discussed the background research on these materials and have investigated the influence of microstructure on the mechanical properties of Ni/Sn composites.

Metallic multilayer composites are composed of multiple alternate layers of at least two suitable metals. These composites have shown enhanced tensile strength and microhardness. The strengthening mechanism has been attributed to the bamer effect of the interface to dislocation glide, which is similar to the resistance offered by grain boundaries in monolithic metals. However, multilayer composites differ from fin- grained homogeneous materials because the composition and properties of their component layers can be adapted individually. The nature of the interfaces in the layered structure, which play an important role in determining the composite properties, can also be modified. Therefore, the properties, such as electromagnetic, mechanical and tribological properties of the composites can be controlled over a wide range.

Research interests in these materials were created by the early theoretical prediction of Koehler [36]. He proposed a multilayered structure consisting of two metals, A and B, in which metal A has a higher elastic modulus than metal B. In this case, the interface of the structure is expected to offer strong barriers to dislocation glide from B to A. To suppress the generation of new dislocations in the layers, the thickness of A and B must be as small as possible. Koehler predicted that this kind of ultra-fine multilayered metallic composite should exhibit a greater resistance to plastic deformation than the homogeneous alloys with the same component concentration.

A significant amount of work has been conducted to confirm the above theoretical predictions and Wang and Singh [35] provide references to the systems involved. These include A K u , N X u , Ag/A1, Fe/Cu and Cu/Ni. Although the mechanical properties of these metallic multilayers have been explored to some extent, the strengthening mechanisms were claimed to be poorly understood. Wang and Singh [ 3 51 therefore studied the mechanical properties, including tensile strength and

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microhardness, of Ni/Sn multilayer composites in order to identify the strengthening mechanisms. In addition, the dependence of mechanical properties on the layer thickness was analysed using theoretical models and the failures of the layered composites were studied by optical microscopy and SEM/EDS techniques.

The Ni/Sn multilayer composites were prepared by electrodeposition in which Sn and Ni layers were deposited alternately on a copper substrate from separate nickel and tin electrolytes [37]. To improve adhesion between the Ni and Sn layers, a thin (4 1 nm) copper film was deposited on the Sn layer surface.

The microstructures of the Ni/3O%Sn multilayers with a variable layer thickness are shown in Fig. 16. The dark layers are Sn which were etched in 2% HC1 in water solution and the light layers are Ni which are not etched by this solution. The copper layers are too thin to be observed in the micrograph. The deposit growth direction was from left to right. It can be seen that the multilayered composites have a controllable layer thickness, good adhesion and density, and an ‘even’ layer thickness.

Fig. 16. Optical micrographs of the cross-sections of Ni/30%Sn multilayers with 30 vol.% Sn but variable bilayer thickness of (a) 11.5pm (b) 5.0pm and (c) 1.7pm [35].

1

Fig. 17 shows the cross-section! 12 pm, but variable Sn content.

3 : from samples with a constant Ni layer thickness of

Fig. 17. Optical micrograph of Ni/Sn multilayer composites with a constant Ni layer thickness of 12 prn but with variable Sn content of (a) 16.1 (b) 45.2 and 61.7 vol.% Sn [35].

21

0

In samples of Ni/45.2%Sn and Ni/61.7%Sn, the Sn layers are thick enough to reveal the Sn grains. A bamboo type of structure is apparent in the tin layers between the Ni layers. The grain size is equal to the Sn layer thickness and it decreases with Sn layer spacing. Fig. 18 shows the SEM micrograph and X-ray analysis of Ni/45.2%Sn as-deposited sample. It indicates that the concentration profile near the interface is fairly sharp, with no evidence of any transition zone.

Fig. 18. Ni/45.2%Sn as-deposited sample (a) secondary electron image (b) X-ray analysis of Sn and (c) X-ray analysis of Ni [35].

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It was shown that the mechanical properties of these composites greatly depended on their composition (Sn vol.%) and layer thickness. The ultimate tensile strength (UTS), yield strength (YS) and Young’s modulus of the composites with variable Sn content, increased with decreasing Sn volume fraction. The measured UTS and YS values were generally found to be either equal to, or greater than, the values predicted by the ‘rule-of-mixtures’ because of the strengthening effect of the multilayered microstructure. A considerable enhancement in the UTS and YS was observed in composites with smaller layer spacing. This enhancement in strength was attributed to pinning of dislocations at the interfaces of the layered structure. The dependence of the YS on the layer spacing followed the Hall-Petch relation. The Hall-Petch constants, 0 and K in the multilayer composites, were also theoretically calculated and found to be comparable with the experimentally measured values.

4. Tin-Cobalt

4.1. Background

The commercial background to the use of this alloy has been discussed by Sujatha et al. [38]. Tin-cobalt alloy is considered as a viable substitute for chromium electrodeposits in applications where the high wear resistance is not required. Deposits with less than 2 pm thickness are bright with a colour very similar to that of chromium. The disadvantages inherent in chromium plating, viz. poor current efficiency, poor throwing power and covering power, high energy requirement, toxicity, a high polluting electrolyte and its unsuitability for barrel conditions, etc., have promoted the development of this alloy. Deposits of 0.5-1 .O pm thickness on a suitable nickel undercoat have a corrosion resistance adequate for indoor applications due to the presence of a passive film.

Survila et al. [39] have described the electrolyte solutions which have been used to deposit Sn-Co alloys with particular reference to the process of complexation and have provided a bibliography. A great variety of compositions have been tested. One group of solutions contained only simple inorganic salts such as sulphates, fluorides, chlorides, fluoroborates, pyrophosphates and cyanides, which may be treated as weak ligands with the exception of the last two. Although fluoride forms comparatively weak complexes with Sn2+, this effect inhibits the oxidation of Sn(n) to Sn(N). Stronger complexes give rise to some undesirable phenomena. The use of cyanides, for example, those that are able to form complexes with Co(II), leads to a significant decrease in the CO content of the coatings.

Survila et al. [39] also note that an array of organic complexing agents have been applied in the process of Sn-Co plating. These include gluconate, tartrate and some arnines that act as organic ligands. To make coatings brighter, various additives such as butynediol, sintanol and formaldehyde, have been used. Brighness can also be achieved even in the absense of special brighteners.

23

4.2 Electrodeposition from Neutral Gluconate Baths

Cobalt content Hardness Solderability % VHN %O

Sujatha et al. [38] have described the deposition of tin-cobalt alloy from a simple gluconate complex electrolyte which is claimed to be highly stable and easy to dispose of, besides yielding a bright, lustrous coating. These authors examined various properties of the deposit including hardness, porosity, solderability and corrosion resistance.

Porosity % defect area

The electrolyte consisted of cobalt sulphate, stannous sulphate, sodium gluconate and peptone. Fourteen different plating bath compositions containing different proportions of these constituents were investigated. In most of the experiments the applied current density was mainly 1A dm-2, the temperature 70°C and the pH 7, although slight variations to this norm were also investigated, (current densities 0.5-1.5 A dm-*, temperatures 30°C and 50°C and pH’s 5, 6 and 8). The alloy was plated onto copper cathodes.

5 12 20 29 35 45

At .a fixed stannous sulphate concentration of 25 g/1 and sodium gluconate concentration of 150 g/l, the cobalt content in the electrolyte was varied from 5 to 20 g/l. It was found that the cobalt content in the alloy deposit was always higher than its proportion in the electrolyte. The cobalt content in the alloy increased with a rise in concentration in the electrolyte and the deposit colour changed from silvery white to a finish having a blue hue resembling chromium deposits with 30-40% cobalt. At higher cobalt contents of around 50% the appearance changed again to a highly stressed nickel-like lustre. The cathode current efficiency of cobalt deposition increased correspodingly with a reduction in the tin efficiency.

28.16 33.48 7.65 59.46 32.1 1 2.94 72.80 23.30 1.17 187.48 - 1.37 224.15 22.57 1.28 225.13 - 1.57

An increase in the gluconate concentration from 120-1 80g/l increased the cobalt content of the alloy from 34-44%. The cobalt content in the alloy increased between pH 5-7 and thereafter at pH 7-8 it declined. At pH 7, lower operating temperatures of I

30-60°C resulted in an increase in cobalt codeposition with a grey colouration.

A gradual increase in the hardness of the alloy was obtained with increase in cobalt content up to 30%, but a sudden increase was observed above this limit attributed to the formation of an intermetallic compound, Table 4.

Table 4. Properties of Sn-Co alloy deposits [ 3 81.

The solderability of the alloy (estimated by the solder spread area method) diminished with increasing cobalt content. In almost all conditions, the alloy deposits at 5pm thickness were pore-free, as determined by an electrographic test.

24

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Cobalt content

5 12 29 35 45

Alloys of various compositions at 5pm thickness were evaluated for their corrosion resistance and passivity by a potentiodynamic polarisation method using an electrolyte consisting of 5% NaCl or 5% H2SO4. The data is shown in Table 5.

NaCl NaCl H2S04 H2S04

I,,, Ipass I,,, Ipass

1.54 x 10'6 - 5.09 x 104 1.88 x 104 6.11 1 0 - ~ - 9.19 1 0 - ~ 6.96 10 '~

A cm-2 A cm-2 A cm-2 A cm-2

1.34 x 10-6 2.51 1 0 - ~ 7.15 1 0 - ~ 1.54 x 104 1.11 x 10-6 3.16 1 0 - ~ 3.16 x 10-6 2.05 x 104 5.21 x.104 8.58 x 104 2.78 x 104 -

Table 5. Corrosion and passivity behaviour of Sn-Co alloys [ 3 81.

X-ray diffraction data were obtained on the deposits. With 5% CO, the alloy existed almost as Sn; with 10% CO as a mixture of Sn and CoSn phase; with 20% CO as a mixture of CoSn and CoSn2; with 29-35% CO mainly as CoSn phase; and above 50% as a mixture of Co3Sn2 phase in CoSn. Electron micrographs displayed a heterogeneous structure for the alloy containing 25-29% CO, a single phase fine grained structure for the alloy containing 35% CO and a cracked structure for an alloy containing more than 50% Co.

In conclusion, Sujatha et al. [38] claimed that alloys containing 30-40% CO exhibit the most desirable properties resulting from the alloy existing as a CoSn phase.

4.3 Electrodeposition fkom Acid Gluconate Baths

The electrodeposition of Sn-Co alloys from a slightly acidic sulphate-gluconate bath on both vitreous carbon and copper substrates has been studied by Gomez et al. [40] for different [Sn(II)]/[Co(II)] ratios in the bath, varying between 1/10 and 1/2. The baths contained SnS04, CoS04.7H20, 0.2M sodium gluconate (NaC6H1107) as chelating agent and 0.2M Na2S04 as supporting electrolyte. The pH was adjusted to 4. In all experiments the total metallic ion concentration was maintained around 0.1M. A relationship between the electrochemical stripping analysis and the morphology of the deposits was found.

Two different types of deposit were obtained. At low [Sn(II)]/[Co(II)] ratios and relatively high deposition rates a nodular, cobalt-rich, nanocrystalline coating was obtained, whereas at high [Sn(II)]/[Co(II)] ratios and low deposition rates a new, well defined tetragonaz SnCo phase was obtained. Stripping analysis was revealed as a useful tool for detecting the initial formation of each Sn + CO alloy type, since a direct correlation between the stripping curve and the morphology and structure of the deposits was found. Two major stripping peaks were observed, each one related to the formation of one kind of deposit. Peak I is related to a new SnCo tetragonal phase, whilst peak I1 corresponds to a CO-rich nanocrystalline deposits.

25

Fig. 19 illustrates data obtained with low [Sn(II)]/[Co(II)] ratios lower than 1/5 and for deposition potentials more negative than -1000 mV vs. Ag/AgCl. In this case the stripping peak I1 predominated. Fig. 19(a) shows stripping characteristics in which Peak I1 predominated and Fig. 19(b) shows the morphology of deposits yielding mainly peak 11. Homogeneous deposits with nodular morphology were obtained in the range of -980 mV to -1050 mV. These deposits were cobalt rich and in all cases the percentage of CO in the deposit increased with increasing negative potential. The diffractogram corresponding to these nodular deposits is shown in Fig. 19(c). Only one broad irregular peak around 28 = 4 4 O , next to the { 11 l} line of CO, was observed. This deposit appeared to correspond to a single phase with a preferred orientation, with a partially amorphous structure, or with a crystalline phase of nanometric crystal size. The latter hypothesis was confirmed by an estimation of the crystallite size domain from the broadening of the peaks, using Scherrer's equation.

60

40

20

0 -600 -400 -200 0 200 400

El mV (AglAgCI)

Intensity (c)

h

Fig. 19. (a) Stripping voltammogram of Sn-65%Co deposit obtained at -1050 mV (vs. Ag/AgCl) for 20s on a copper electrode rotating at 1000 rpm in a 0.1M CoS04 + 0.01M SnS04 + 0.2M NaC6H1107 + 0.2M Na2S04 solution. (b) SEM micrograph of deposit obtained under the same conditions as Fig. 19(a), but deposit obtained at 20 min at -1020 mV. (c) X-ray diffiactogram of the Sn + CO deposit of Fig. 19(b) [40].

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4.4 Electrodeposition from Citrate Solutions

Survila et al. [39] have commented that in most of the investigations involving Sn-Co alloys, attention has been focused on problems of an applied nature, underlining the influence of solution composition and electrolyte conditions on the phase and elemental composition of the coatings. However, little work has been conducted to explore the mechanism and kinetics of the electrode processes. Survila et al. have therefore focused some attention on these topics [39,41].

In a recent study, Survila et al. [39] investigated ‘optimal’ conditions for Sn and CO codeposition in slightly acidic citrate solutions containing no excess of ligand. Solutions were prepared from SnS04, CoS04.7H20, K$sH507.H20 and Na2S04 as supporting electrolyte. The working electrode in these studies was a platinum electrode plated with 2-3 pm of copper and then with a 5 p thick tin layer. Voltammetric characteristics were recorded at a potential sweep rate of 2 mV/s and both stationary and rotating electrode situations were considered.

Sn-Co coatings were deposited with amounts ranging from 15 to 86 mass %. Bright deposits were obtained when the CO content exceeded 76%. These coatings were considered as solid solutions of tin in a-CO and p-Co. The p-Sn phase was predominant in the case of coatings containing less Co. Fig. 20 shows an example of the ‘partial’ voltammograms obtained by X-ray analysis using photoelectron spectroscopy together with electrochemical data. Co(I1) reduction on a foreign (Sn) substrate started at around -0.62V (vs. SHE). Similar results were obtained for solutions containing lower amounts of Co(I1) (0.1M). They show that CO@) reduction on a foreign substrate occurs with large overvoltage.

i / m A cm-2

20

15

10

5

0

)#.-+J&LD+- Sn I I I I

I

-0.4 -0.6 -0.8 -1 .o E/V

Fig. 20. Net (full) and partial (symbols, dotted lines) voltammograms for the indicated solution compositions. Sn(fI) 0.05M; Co(1I) 0.3M; Citrate 0.1M; pH 4 [39].

27

A theoretical analysis was made involving a consideration of solution equilibrium characteristics and mass transport of Sn(II) and Co(I1) citrate complexes. This yielded valuable information on the distribution of complex species in the bulk and at the electrode surface. It enabled optimal solution compositions and electrolysis conditions to be selected for Sn and CO codeposition. Fig. 21 compares some of the simulated voltammetric data with experimentaz results. It can be seen that good agreement was obtained, for example, the theoretical data properly representing the sharp rise of the voltammogram at -0SV < E < -0.4V (vs. SHE) for the process of SnL-2 + 2e + Sn + L4-. (It should be noted that citric acid is represented by the LH4 and when dissociating it can release up to 4 protons). Similarly, the process of Co(n> reduction could be simulated, commencing at E - -0.6V.

i / m A cm-2 25

20

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CoLH-+2 e = C0+Lq+d-l'

0

ioea = 0.5 m~ cm-* a = 0.33

-0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0

E N

Fig. 2 1. Comparison of voltammograms simulated for the indicated partial processes (lines) and obtained experimentally (symbols) at 450 rpm. Solution compositions: Sn(II) 0.05My citrate 0.075M, pH 4 (left curve) and Sn(1I) OSM, Co(II) O.lM, citrate 0.075My pH 5 (right curve) [39].

4.5 Electrodeposition from Pyrophosphate Solutions

Wang has described an investigation on the corrosion resistance of electroplated Sn-Co alloy coatings [42] and has compared the results with those obtained for chromium coatings. The plating solution compositions and experimental conditions were as follows: potassium pyrophosphate (250-300g/l); stannous sulphate (10-25 g/l); cobalt sulphate (10-25 g/l); brightener (10-15 ml); pH 8.5-9.5; temperature 35-50°C; and d.c. current density 0.5-20 A dm-2. Experimental data from a neutral salt spray test (GB 5938-86) showed that the Sn-Co coating had a corrosion resistance comparable to that of electroplated chromium coatings. Their cobalt content varied from 10-50%. A lower content led to an obvious 'fogged' appearance and decreased corrosion resistance, whilst excessive cobalt concentrations produced dark surfaces and increased brittleness which resulted in microcracks in the coatings, thus reducing corrosion resistance.

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4.6 Binary and Ternary Alloys in Bearing Overlays

The ‘end of life’ vehicle regulations in Europe for 2003 aim to increase ‘recyclabilty’ of vehicles by removing toxic materials such as hexavalent chromium and lead. One common use of lead in automotive applications is in bearing overZays. Overlays are generally soft alloys deposited onto harder bearing alloys to produce a surface having compatibility and comformability with a cooperating shaft and also to provide a means of embedding debris particles to prevent damage to the shaft. More than 300,000 bearing shells are plated every year. The most commonly applied bearing overlay material is a Pb-Sn-Cu alloy containing at least 90% lead. In higher performance engines, Pb-In is commonly applied, where indium is plated on top of the bearing and diffuses into the Pb. Clearly, to comply with the new regulations, a replacement for lead must be found.

A suitable replacement alloy must be soft enough to allow the bearing to ‘bed in’ correctly and the melting point of the alloy must be higher than 250”C, because engine operating temperatures can approach this level. Tin-based alloys are an obvious choice as they have good lubrication properties and are soft enough. The low toxicity of tin is also an advantage. Tin cannot be used alone because the melting point is too low. The easiest alloy of tin to produce would be a Sn-Cu alloy. A Sn-Cu alloy containing 5% Cu would have the required melting point. However, tests have shown that Sn-Cu alloys do not have the necessary fatigue strength. Similarly, Sn-Zn alloys can readily be produced, but these alloys fail corrosion testing due the appearance of white corrosion products from the sacrificial corrosion of zinc in the alloy.

Pearson and Herdman [43] have recently described a method of depositing smooth, functional alloy coatings of tin and an alloying metal comprising nickel andor cobalt, the coating having a thickness of up to and in excess of 50 pm and a composition of about 1-25% (preferably 2-1 5% and especially 2-8%) of the alloying metal. The invention also related to tin alloys and bearing overlays produced by the method.

A considerable range of solution compositions andor electrochemical conditions are claimed to be effective. In general, a typical bath contained tin sulphate; nickel sulphate; a complexant comprising a hydroxycarboxylic acid or alkali metal salt such as sodium gluconate; boric acid; and a bath soluble substituted phenolic compound. The current regime applied to the plating bath could also include periods of direct current and pulsed current in order to selectively control the deposition of tin by activation or diffusion control.

The condition necessary for the production of either binary or ternary alloys are described [43]. For example, a bath containing the following constituents was claimed to produce a Sn-( 8-1 O%)Ni-( 1 -2%)Co alloy coating: sodium gluconate (1 50 g/l); boric acid (1 00 g/l); stannous sulphate anhydrous (30 gh); nickel sulphate hexahydrate (75 g/l); cobalt sulphate hexahydrate (7 g/l); Lugalvan BNO 24 (1 gA); and Empicol ESB 3 (1 g/l). Lugalvan BNO 24 is an ethoxylated beta-napthol manufacture by BASF. Empicol ESB 3 is an ionic surfactant manufactured by Albright and Wilson. A Hull cell test was performed at a temperature of 20°C. The deposit obtained was smooth and even. At a position on the panel corresponding to a primary current density of 4A dm‘2, the Sn-Ni-Co alloy having the above composition was obtained.

1 29

5. Tin-Copper

5.1 Background

0 0 0

The process of tin-copper (bronze) alloy electrodeposition has been widely applied in industry due to its excellent wear properties, attractive appearance, good corrosion resistance, excellent machinablity and solderability. El Rehim et al. [44] have listed some of the plating baths that have been used in the production of Sn-Cu alloy deposits and have provided a bibliography. These include sulphate, phosphate, fluoroborate, boron-fluoride, pyrophosphate and cyanide based baths.

Commercial electrodeposition of the alloys based on cyanide baths produces high quality deposits, but causes environmental problems in use and disposal. The following discussion focuses on recent attempts to develop non-cyanide replacement technology that is equally effective.

5.2 Electrodeposition from Acid Gluconate Baths

El Rehim et al. [44] have studied tin-copper electrodeposition on steel substrates from acidic sulphate solutions containing sodium gluconate complexing agent. The composition, structure and morphology of the alloys obtained from these baths were determined. The effects of various plating parameters on the potentiodynamic cathodic polarisation and cathodic current efficiency for the codeposition process were studied. The results were consistent with copper being the preferentially deposited metal. The copper content in the deposit increased with increase in the copper content of the bath and with temperature, but decreased with increase in sodium gluconate concentration, pH and current density. X-ray diffraction analysis revealed that the as-deposited alloy was composed of q, E and p phases as well as p tin. The morphology of the deposits was examined by scanning electron microscopy. A typical electrolyte composition consisted of 40 g/1 SnS04; 10 gA CuS04.5H20; 70 g/1 C6H7011)7Na; 10 g/1 K2S04; and pH 2.

Fig. 22 shows typical potentiodynamic polarisation curves for the electrodeposition of Cu, Sn and Sn-Cu alloy.

10

0

-02 6 4 -06 6 8 - 1 0 - 1 2 - 1 4 -16 - 1 8 - 2 0

E vs. SCE (V)

Fig. 22. Potentiodynamic cathodic polarisation curves during tin, copper and tin-copper alloy electrodeposition from gluconate baths at 16 "C, with a scan rate of 25 mV/s [44].

0 0 0 0 0 3 0 6 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0

30 0 0

e 0 0 0

0

0 0

e 0 0

0 0

0 0 0 0

e 0 a 0 0

0

0 0

0

0

0 0 0 0

0 0 0

0

The curves were swept from the rest (zero current potentials) in the more negative direction with a scan rate of 25 mV/s. It is known that in acid media (pH<4) both Cu2+ and Sn2+ ions form the cation complexs [cu C6H701 I]+ and [SnC6H701 I]+, respectively, via a ligand carboxyl group. An inspection of Fig. 22 reveals that each polarisation curve exhibited a limiting current plateau. Under the limiting conditions, alloy deposition proceeds via mass transport control. At high negative potentials, the current increased sharply due to hydrogen evolution as a side reaction. The tin polarisation curve lies at a more negative potential than that of copper demonstrating that copper is the nobler metal. It can be seen from the individual polarisation curves that the rate of hydrogen evolution in the copper bath was higher than in the tin bath, simply reflecting the difference in hydrogen overpotential on the two metals.

Fig. 23 illustrates the dependence of applied current density on the copper content in the deposit and the cathodic current efficiency (CCE%) for tin-copper codeposition from the same acidic gluconate bath as that used to obtain the data in Fig. 22. The composition reference line (CRL) in the figure represents the percentage of copper in the'bath. At the lowest current density (2 mA crn-*), the CCE% was high and approached 95%. However, the efficiency decreased significantly with further increase in the current density and reached about 15% at 14 mA cm-2. This feature was due to hydrogen evolving as a side reaction.

2 I ( m A c m )

Fig. 23. Effect of current density on the cathodic current efficiency (CCE%) for the electrodeposition of Sn-Cu alloy and its copper content [44].

The copper content in the alloy decreased markedly with the applied current density and then levelled off, approaching the composition reference line (CRL). Such a decrease in the rate of deposition of the more noble metal component is characteristic of a 'regular alloy plating system' which is characterised by the dposition being under diffusion control. It was observed that at sufficiently high current densities (> 4 mA cm-2) the deposit started to become powdery and non-adherent. Therefore, a current density of 4 mA cm-2 was chosen for further investigation.

The tin-copper alloy deposited from the gluconate bath under optimum conditions was adherent, smooth and bright in appearance. X-ray diffraction data for a 43% Cu alloy are shown in Fig. 24.

31

6

0 1 .o 0.8

0.G n m

U

W - .- 2 0.4 v) C

c ’ 2i 0.2 .-

0

0 0 0 0 0

40 50 60 70 80 0 10 20 30 0

28 angle (deg)

Fig. 24. X-ray diffkaction pattern of tin-copper alloy deposited from the same bath as that in Figs. 22 and 23 [44].

The data revealed that the alloy exhibited four intennetallic phases, q, E and p phases, as well as the p phase of tin. The q phase (Cu&n5) has an hexagonal structure. The E

phase (Cu3Sn) and p phase (CuSn) show orthorhombic crystal structures.

5.3 Electrodeposition from Acid Tartrate Baths

In two recent papers, Carlos et al. [45,46] have studied the effect of tartrate on the characteristics of copper-tin electrodeposits from non-cyanide type baths.

In the earlier investigation [45], copper and tin were electrodeposited on platinum substrates from a bath containing 0.12M CuSO4, 0.10M SnC12 and 1 .OM H2S04, in the presence and absence of 0.25M potassium sodium tartrate. Voltammetric curves indicated two deposition processes, at -0.3 1OV and -0.640V (vs. NCE), which did not shift on addition of tartrate to the plating bath. The presence of tartrate decreased the current density in the region of the more cathodic process. The metals were deposited at both deposition potentials and the deposits had the same proportions of copper and tin either in the presence or absense of tartrate in the plating bath, as observed by AAS. X-ray spectra suggested that a mixture of Cu and q phase (Cu&) alloy was deposited at the less cathodic potential SEM analysis showed that tartrate affected the morphology of the films. At -0.3 lOV, the grains became more uniform when tartrate was present and at -0.640V cracks and nodules in the deposit were removed when tartrate was present, suggesting a stress relieving function of the additive.

In later work [46], the effect of the presence of tartrate on the chemicaZ stability of the Sn-Cu acid electroplating bath was investigated. It was observed that the additive hinders decomposition of the bath with storage time, since the decrease in electrochemical efficiency was attenuated. In addition, it was observed that optimal galvanostatic deposition with or without tartrate occurred at approximately 11 mA cm-2. However, in the presence of tartrate the deposition charge was lower leading to lower energy consumption. SEM analysis revealed that the tartrate added to the plating bath caused a marked change in the morphology of the Sn-Cu films obtained galvanostically. Figs. 25 and 26 show SEM micrographs of the films formed at 11 mA cm-* and a charge density of 6.3 and 1.9 C cm-2 in the absense and presence of tartrate, respectively.

0 0 0 0 0 0 0 C 0 0 0 0 0 0 0 0 0 0 0 0 0

32

0 0 0

0 a a a a a a e a 0 0 a 0 0 a 0 a 0 a 0 0 a a a 0 0

0 0 0 0 0

0 0 0

Fig. 25. SEM micrographs of Sn-Cu films obtained at 11 mA cm12 and 6.3 C cm-2 fi-om 0.12M CuS04/0.10M SnC12/1 .OM H2S04 fieshly prepared solution in the absense of tartrate [46].

\

Fig. 26. As Fig. 25, but at 1.9 C cm-2 and with the addition of 0.25M tartrai te

33

5.4 Electrodeposition of Cu/Cu-Sn Multilayers by Potential Pulse Electrolysis

0

0

Shinohara et al. [47] have investigated the electrodeposition behaviour of copper and tin from sulphate solutions. The deposition potential of tin in the presence of copper(I1) ion was more positive by 0.25V compared with that in the absence of copper(I1). Multilayers were electrodeposited using a potential pulse technique. The positive potential of the pulse was -0.1 V (vs. SCE) and the negative potential was -0.35V. The retention times at each potential were varied in the range 2-8s. The multilayers consisted of sublayers with a thickness of 10 nm depending on the retention times. The sublayers deposited at -0.1V were pure copper and those deposited at -0.35V consisted of Cu, Cu3Sn and Cu6Sn~.

6. Tin-Manganese

6.1, Background

The background to this topic has recently been discussed by Gong and Zangari [48]. Due to their low redox potential, adequate tribology an suitable mechanical characteristics, electrodeposits of Mn and Mn alloys have been studied as potential sacrificial coatings for steel. However, pure Mn is highly chemically reactive and a coating of this material may provide sacrificial protection only for a limited time when immersed in electrolyte or exposed out of doors. Tin and tin alloys, on the other hand, are currently being actively researched as corrosion protective coatings. Electrodeposited tin-manganese coatings are thus of great interest, as they could potentially combine the barrier properties of tin with the sacrlficial protection afforded by manganese. Despite this appeal, few investigations have been concerned with a detailed study of the electrodeposition of Sn-Mn and the structure and properties of its deposits; the only report in the open literature being a review by Brenner [49]. This is because Mn is the most electronegative metal (EoMnZ+/Mn = -1.421V vs. SCE) that can be electrodeposited from aqueous solutions and only at H's above 2.0 [50,51], while tin has a much higher standard redox potential (E ~ n 2 + / ~ n = -0.377V vs. SCE) and tin coatings of good quality are difficult to grow from non-toxic, slightly acidic electrolytes without organic additives.

f

6.2 Sacrificial Tin Manganese Allov Coatings

Gong and Zangari [48] have conducted an extensive study of the electrodeposition of Sn-Mn coatings on steel substrates from simple ammonium sulphate baths with or without the addition of citrate, tartrate, EDTA or gluconate additives. The basic solution contained 0.01M SnS04, 0.59M MnSO4 and 1 .OM (NH4)2SO4. The additives were added separately in the form of sodium compounds and with variable concentration. The pH was adjusted between 2.5 and 3.0 by adding concentrated sulphuric acid or ammonium hydroxide. The effect of current density and the additives on coating composition, microstructure, crystallography and corrosion resistance, was investigated.

It was found that the (NH&SO4 brought the Sn2+ and Mn2+ discharging potentials closer, allowing codeposition of manganese and tin. As pointed out in the literature

0 0 0 01

0

0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

34 0 0

0

0 0

0 0

0

0

0 0

0

e 0

0 0

0 0

0

0

0

e 0 0 0 0 0

0

0 0

0 0

a a 0

[50,5 11, the addition of ammonium sulphate to electrolytes for manganese electrodeposition is essential to grow manganese coatings with good coverage. Ammonium sulphate prevents the precipitation of manganese hydroxide, improves the conductivity, increases the current efficiency and in general widens the operating window of manganease electrodeposition. Fig. 27 shows the cathodic potentiodynarnic behaviour of pure Sn, pure Mn and Sn-Mn alloy grown from simple ammonium sulphate solutions without additives and stirring compared with Sn/Mn solutions without ammonium sulphate.

Potential (V,,,>

Fig. 27. Potentiodynamic behaviour (SmV/s) of Sn, Mn and Sn-Mn solutions with and without (NH4)2S04 [48].

The discharging potential of Sn2+ in pure SnS04 solution (about -0.47VsCE) was close to the calculated Sn2+ reduction equilibrium potential (-0.436 VSCE, using the Nernst equation). The addition of ammonium sulphate polarised the Sn2+ discharging potential to a more negative value (about -0.6Vsc~) and shifted the H+ and Mn2+ reduction reactions to more positive potentials, thus bringing the reduction potential of Sn and Mn closer. This made codeposition of Sn and Mn possible, even if Sn still deposited preferentially and Mn codeposition was always accompanied by hydrogen evolution. Mn2+ reduction seemed to be enhanced by codeposition of tin, which meant that manganese could be deposited at a lower current density in alloy deposition than in pure manganese deposition. It was also shown that the reduction reaction of tin was already under diffusion control before manganese was electrodeposited. Codeposition of Sn and Mn, and the synthesis of Sn-Mn alloys of different composition by varying the current density, are therefore possible.

Figs. 28 and 29 illustrate the potentiodynamic behaviour of Sn, Mn and Sn-Mn electrolytes with the addition of one of various complexing agents without stirring. Citrate, tartrate, EDTA and gluconate were investigated at a concentration of 0.0 1 M, the same as [Sn2']. The various compexants shifted the potentiodynamic curves for Sn deposition in a negative direction to a relevant extent (Fig. 28a), with EDTA and citrate complexes inducing the strongest complexation. On the contrary, the polarising effect on Mn deposition was almost non-existent at low overpotentials (Fig. 28b), probably due to the high ratio [Mn2+]/[complexing agent].

35

n 0.03 pH=25 - 3.0, scan rats5mVls 1

-(a) SnSO,(O.OlM) + (NH,),SO,(lM) I

0

0

Potential OC,,,) pH=25 - 3.0, scan rate=5mV/s ; !I-

n 0.08

E 0.07; -(a) MnS04(0.59M) + (NHJ,SO,(lM) 0 3 0.06-

N ; !;

- - - (a) + Citrate ion (0.01 M) (a) + Tartrate ion (0.01M) - - - - -

-.-.- . (a) + EDTA (0.OlM) (a) + Gluconate ion (O.OlM)

1

- 0.04-

0.03

0.01 *,.A

-0.6 -0.8 -1'.O -1-.2 -1-.4 -1..6 -1.8 (b) Potential (VSCE)

Fig. 28. Potentiodynamic behaviour (SmV/s) of ammonium based SnS04 and MnS04 solutions with various additives [48].

The effect of the complexants was similar for the alloy deposition (Fig. 29), with the curves being shifted in both directions with the addition of different additives (Fig. 29b). In this case, the citrate addition was the only one shifting the potentiodynamic curve in the cathodic direction.

Sn-Mn deposits were grown galvanostatically from ammonium sulphate from ammonium sulphate solution with or without additives, with the current density varying in the range 10-600 mA cm-2 at pH 2.5-3.0. All electrodeposition processes were accompanied by hydrogen evolution, which was consistent with the potentiodynamic results and implied current efficiencies below 100%. Deposition was carried out in a quiescent solution, electrolyte stirring being provided by hydrogen evolution at the cathode. All coatings had a nominal thickness of 10 pm. After deposition, each sample was immediately removed from the bath, washed with distilled water and air dried, then characterised by SEM, EDX, X P S and XRD.

SEM micrographs of Sn-Mn coatings electroplated from simple ammonium sulphate electrolytes at different current densities are shown in Fig. 30 and the atomic composition of the coatings obtained at different current densities, as determined by EDX, is shown in Fig. 3 1.

0

0 0

0 0

0

0 0 0 0 0 0 0 G 0 0 0 0 0

36 0 0

0 0

0 0

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e 0 0

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rn 0

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0

0 0 0

0

0 0

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0 0

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0

0

0

0

(a) MnSOJ0.59M) + (NH,),SO,(IM) pH=2.5 - 3.0, scan rate=5mV/s -

+SnS04(0.01 M) - - - (a) + Citrate ion (0.0IM) ..... (a) + T a m t e ion (0.OlM) -_-_- (a) + EDTA (0.01 M) -_.-.. (a) + Gluconate ion (0.0IM)

0 -0.4 -0.8 -1.2 -1.6

(a) PO te n t i a I (Vs,,>

* . . pH=25 - 3.0, Scan rate=5mV/s I . I

+SnS04(0.01 M) !: ; (a) MnSO,(OSSM) + (NH,),SO,(lM) ,.' 0 3 0.00'- - - - (a) + citrate ion (0.01M)

0.004-

-0.4 -0.8 -1.2 Potential (VsE)

Fig. 29. (a) Potentiodynamic behaviour (SmV/s) of Sn-Mn ammonium solutions with various additives; (b) area circled in (a) is enlarged [48].

Fig. 30. SEM micrographs of Sn-Mn electrodeposited from base solution at different current densities and a pH 2.5-3.0 [48].

37

w

00

...a

3 A

tom

ic F

ract

ion

(%)

P

W

0

0

0

W 0

0

c 1

i3

3

Com

posi

tion

(at%

) P

03

0

0

0

w

0 0

0 0

0

0 0

0 0

0 0

a 0

0

0

Citrate, 3 0 m ~ c m ~ Tartrate, 50mA/cm2 EDTA, 30mA/cm2 Gluconate, 50mA/cm2

Fig. 32. SEM micrographs of Sn-Mn coatings electrodeposited from the base solution containing different additives at a concentration of 0.0 1M [48].

a 0

0

a 0

0 a 0 e 0

0 0

e 0

e a 0

Addition of sodium citrate to the base electrolyte in the concentration of 0.01M or 0.05M yielded coatings with a high oxygen content of 50-60 at.%. An X-ray analysis of these films showed a relevant percentage of Sn02 and SnO. Coatings grown at 20-330 mA cm-* appeared dark, spongy and loosely attached to the substrate. Therefore it was concluded citrate ligands do not improve the quality of Sn-Mn coatings. However, this was not the case for tartrate, EDTA and gluconate ligands which were shown to produce more compact and uniformcoatings. The bath with gluconate, in particular, was found to produce silvery, matte deposits comprised of small crystallites (Fig. 32 ). EDX showed that by the addition of complexing agents, the oxygen content in the Sn-Mn coatings could be reduced to 20-30 at.% for current densities below 50 mA cm-2.

Fig. 33 shows the relative (to tin) manganese content in the Sn-Mn alloy coatings with the various ligands/complexing agents.

I1 I I

80- /-

40-

0 40 330 Current Density (m~/crn*)

Fig. 33. Dependence of relative (to Sn) Mn content on the current density for Sn-Mn coatings obtained from the base solution with various additives [48].

39

The manganese content in general increased with current density and the addition of any of the ligands reduced the manganese content in the coatings with respect to the simple base electrolyte, to varying extents. This conclusion would be expected because, in the current range applied, Sn deposition was controlled by Sn2+ diffusion, while manganese deposition was not. Increasing the current density therefore led to an increase in both Mn deposition and H2 evolution rates. The decrease in Mn content upon addition of the complexing agents was attributed mainly to adsorption on the cathode surface leading to a suppression of Mn2+ discharge. While the additives also form ligands with Mn2+, their stability is lower and their relative concentration smaller, so that their influence on the potentiodynamic behavior of the electrolytes was considered to be very limited.

In order to evaluate the corrosion resistance and possible passivation behaviour of the coatings, anodic potentiodynamic curves were measured in ‘artificial sea water’. No attempt was made to purge the solutions with an inert gas and the solutions were not stirred. During these anodic potentiodynamic experiments, the samples were first immersed in a 3% NaCl solution at pH 3.0 for 10 mins to stabilise the open-circuit corrosion potential E,,,. Subsequently, potentiodynamic curves were recorded from -250 mV (with respect to E,,,, to +1200mV at a rate of 2mV/s. The experimentally determined potentiodynamic curves were fitted using the Stern-Geary equation to give values of the corrosion potential E,,, and the corrosion current Lo,.

A typical anodic potentiodynamic curve for a Sn47-Mn53 coating (from an EDTA- containing solution, 50 mA cm-2) is compared with that of electrodeposited pure manganese and pure tin in Fig. 34.

1E-8 1E-6 1E-4 0.01 1 Current Density (A/cm2)

Fig. 34. Typical anodic potentiodynamic behaviour of Sn-Mn, pure Mn and pure Sn coatings in 3% NaCl solution (pH 3.0) [48].

For pure manganese, the corrosion potential was about -1.35vSCE and the corrosion current 120 pA cm-2 for the crystalline and 37 pA cm-2 for the amorphous deposits.

40

0 0

0 0

0

0 0

0 0

0 e e 0

0

0

e m a a 0 a 0 a 0 a 0

0 0

0

0

e 0

J

A maximum current region (10-100 mA cm-2) extends from -1.2 to -0. lVscE for both manganese coatings. Above -0.1 V, a decrease in current density of about one order was observed, attributed to the formation of adherent corrosion products. However, this is not a ‘true’ passive region since the anodic currents were still high (-10 mA cm-2) and cannot therefore be compared with the ‘active-passive transition’ observed in other metals. The high anodic current density in this large potential range indicated that the dissolution rate of pure manganese is in most environments and the use of pur manganese as a sacrificial protective coating may not be economical.

According to the Pourbaix diagram, tin has a large tendency to passivate and can also be passivated with or without an applied potential. For pure tin the corrosion potential was about -0.505Vsc~ and the corrosion current about 213 pA cm-2. A ‘passive’ region extended from -0.25 to 0.25V, with the passive current density, Ipass, of about 100 PA cm-2.

The electrodeposited Sn-Mn alloy coatings generally had electrochemical properties intermediate between those of pure tin and pure manganese. In some cases, Ipass was even lower on the alloy than for pure tin (Fig. 34). E,,, for the Sn-Mn coatings was between -1.35 and -0.505V, depending on the manganese content, crystal structure and microstructure of the coatings. Icon of most of the Sn-Mn coatings, however, varied to a large extent depending on the composition and microstructure. All Sn-Mn coatings, even with Sn content as low as 0.66 at.% (600 mA cm-2), exhibited passive behaviour and their range of passivity and passive current density also depended on tin content and microstructure.

The anodic potentiodynamic characteristics of the Sn-Mn coatings obtained from the simple ammonium sulphate solutions at different current densities (corresponding to different composition, crystal structure and microstructure) are shown in Fig. 35 and the extracted E,,, and L,, values are shown in Fig. 36 as a function of Mn at.%, deposition current and crystal structure.

1E-9 1E-7 1E-5 1E-3 0.1 Current Density (Alcm’)

Fig. 35. Anodic potentiodynamic behaviour of Sn-Mn coatings obtained from simple I ammonium sulphate electrolytes in 3% NaCl solution (pH 3.0) [48].

41

When the current density was lower than 100 mA cm-2 (designated as Type I coatings), the relative Mn at.% in the coatings was lower than 86% (Fig. 31b).

Fig. 36. The relationship between relative Mn content, E,,, and Icon, of the coatings electrodeposited from a simple sulphate bath at different current densities [48].

For the Sn-Mn obtained from a bath with the additives, most of the alloys exhibited an anodic behaviour falling into the Type I category, despite the improvement in coating quality. This was attributed to the inhibiting effect of the additives on Mn2+ discharge, even at current densities as high as 330 mA cm-2. The resulting deficiency of Mn-rich intermetallics , y-Mn or Mn(O), in the Sn-Mn coatings prevented E,,, fiom shifting to a more negative values, although L,, could be low. For the purpose of the sacrificial protection of steels, coatings with a sufficiently negative E,,, and low Lo, are preferred, because they can be cathodic with respect to steel whilst exhibiting a low corrosion rate. From this point of view, both Type I (Eco, too high) and Type I1 (L,, too high) coatings are not ideal for the sacrificial protection of steels. However, it was found that a few samples, for example those obtained at 50 mA cm-2 from a tartrate bath exhibited a behaviour intermediate between the two types and thus gave the best results to date (Fig. 37).

1 1.0 A . ."'"'I ' """1 '""..V "'*".I ''W

nu 0.5

0.0 a C -0.5 Q)

0

0

>" U

m

.a w

c,

e -1.0 I . , , . , . .....I . . . . . , .....I . . . .....I . . rrm

I E-5 1 E-3 0.1 Current Density (Ncm')

Fig. 37. Anodic potentiodynamic behaviour of Sn-Mn coatings obtained from electrolytes containing various additives at 50 mA cm-* in 3% NaCl solution (pH 3.0) [48].

0 0 0 0 0 0

0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0

42 0 0

0 0 0 0

0

0

0

0 0

0

0

0

0

These coatings had E,,, about -1 .054V, Icon about 2.6pA cm‘2 and a wide passive region ranging from -0.5 to OSV, while the passive current was about 4 x 104 A cm-2. These coatings also had a Mn : Sn ratio of about 60 : 40, and were mainly comprised of a Mn1.77 Sn phase , with a little fraction of MnSn2 and p-Sn. The authors concluded that a high volume fraction of Mn1.77 Sn was probably the main reason for the good electrochemical properties. Therefore, manipulating electrochemical parameters to achieve a high percentage of Mnl.77 Sn was desirable for the synthesis of good quality Sn-Mn sacrificial coatings.

Gong and Zangan have further discussed the increased metallic character of electrodeposited Mn coatings by incorporation of metal ion additives including tin [521.

7. Tin-Lead Alloys

7.1 Background

Tin-lead alloys are plated on wire to protect the base metal from oxidising and to enhance solderability on printed circuit boards and on electrical contacts. Although the major uses for this alloy are for electronic applications, it is also used as an overlay on bearings because of its relative sofiness and lubricity. Steel sheet coated with Sn-Pb, known as ‘terneplate’ containing up to 20% tin, provides substantial corrosion protection. This application is important in the motor industry where the alloy has been used for fuel tanks and also in the construction industry for roofing and cladding .

7.2 Sn-Pb Alloy Deposits from Electrochemicallv Prepared Electrolytes

Silaimani et al. [53] have used an electrochemical method to produce high purity lead and tin fluoroborate electrolytes suitable for the production of Sn-Pb electrodeposits for a variety of applications. The impurities present in commercially available fluoroborates (Fe, Zn, Cu and Ni) impair many of the desirable qualities of the alloy deposits by adversely affecting its microstructure. Silaimani et al. used a method of electrolyte generation involving the direct eZectrochemicaZ dissolution of the respective metals in fluoroboric acid. A cornpartmental cell was employed containing an anion exchange membrane [ 541.

The solutions were suitably diluted so as to have the compositions: tin fluoroborate (52g/l); lead fluoroborate (40g/l); and free fluoroboric acid (100 g/l). The lead content of the alloy bath was varied from 30-40 g/1 and the free acid content from 100-150 g/l. To improve the quality of the deposits, 0.2 g/1 of glue was added as an addition agent. Deposition was carried out on mild steel panels having a plating area of 5 x 2.5 cm2. The electrolyte baths tested were designated as type ‘A’ (electrochemically prepared electrolyte) or type ‘ B’ (commercial grade electrolyte). Porosity, soZderabiZity and reflowing tests were performed on the coatings, in addition to corrosion tests. The corrosion resistances of coatings, having a thickness of 2.5, 3.5, 5.0 and 10.0 pm, were estimated by the potentiodynamic method (measurements made at k 150 mV from the open-circuit potential), from the extrapolation of the ‘Tafel’ lines.

43

The corrosion resistance of the deposits with thicknesses of 5.0 and 10.0 pm were also assessed by conducting salt spray experiments in 5% NaCl, spraying for 8 hours and resting for 16 hours. The total hours of spraying, together with the total hours spent in the cabinet until the onset of failure (indicated by red rust formation), were recorded. The corrosion resistance was evaluated according to ASTM standard B 545, (1985).

Defective area Hot water method

%

19 11 6

32 24 12

SEM revealed that the alloy deposits from bath ‘A’ were more uniform with spherical grains. The differences in the porosities shown in Table 6 were largely attributed to these differences in morphological charactristics.

Defective area Elec trographic

method %

15 10 4

29 21 10

Table 6. Porosity tests for lead-tin alloy [53].

Sample Coating Defective area thickness Ferroxyl

method pm %

Bath A 1 2 I 3.5 I 8

2.5 11

3 I 5 .O I 3

1 2 3

Bath B I I 2.5 25 3.5 13 5 .O 7

Bath A 1 2 3 4

2.5 -5 80 3.31 x 10-6 3.5 -569 1.96 x 10-6 5 .O -559 5.83 x10-7 10.0 -550 5.82 10-~

The results of all three tests given in Table 6 indicated that the porosity of the deposits decreased as the thickness increased from 2.5 to 5.0 pm.

Bath B 1 2 3

The results of the potentiodynamic corrosion tests are shown in Table 7.

2.5 -588 3.80 x 10-6 3.5 -575 2.46 x 10-6 5 .O -561 7.91 io-’

Table 7. Corrosion data fiom potentiodynamic polarisation measurements on [53]

4 10.0 -554

Sample

7.39 1 0 - ~

Coating thickness pm

Econ mV

lcorr A cm-2

0 0 0 0 0 0 0

0 0 0 0 0

0 0 0 0

0‘ 0

44

0

0 0

0

0

0

0 0 0

a e 0 0 0

0

0

a 0

0 0

0 0

0 0

e 0 0

a 0

e 0 0

Sample Coating thickness Protective rating Pm number

As the coating thickness was increased from 2.5 to 10.0 pm the corrosion current i,,, decreased up to 5.0 pm and then remained constant, in both baths. These results were consistent with the porosity results. The corrosion current from bath ‘B’ was always higher’than that from bath ‘A7.

Appearance rating number

The results of salt spray tests conducted for 1680 hours are shown in Table 8.

1 2

1 Bath B

Table 8. Salt spray data for tin-lead alloy [53].

5.0 5.8 5.1 10.0 6.6 6.0

5.0 4.6 4.1

I BathA I I I I

I 2 I 10.0 I 5.4 I 5 .O I

The appearance rating of the deposit obtained from the bath ‘B’ at a thickness of 5.0 pm was 4.1 while the deposit obtained from bath ‘A’ was 5.1. According to the ASTM specification, for a thickness of 10.0 pn, the recommended rating to pass he test is 4. The deposit obtained from the bath ‘A’ passed this test even at a thickness of 5.0 pm. A similar trend was obtained with the 10.0 pm thickness. These results confirmed that the deposits obtained from bath ‘A’ were superior to those from bath ‘B7. These results also confirm those from the porosity tests.

Silaimani et al. [53] concluded that the electrochemically prepared lead and tin fluoroborates are more suited for the Sn-Pb alloy deposition with respect to their porosity, corrosion resistance, solderability and reflowing ability. All the other conditions being common, the superiority of the electrochemically prepared electrolyte could only be attributed to the purity of the electrolyte, which in turn altered the structure of the deposits.

7.3 Marine Corrosion Tests of Electroplates

Marine corrosion tests on Sn-Pb (and Sn-Bi) electroplates for ships instruments have been conducted by Panchenko and Strekalov [54] as part of a survey of potentially useful alloys. The 9 pm and 18pm thick Sn-40Pb and Sn-1Bi alloy coatings applied to brass (D62) were tested for 14-20 years in containers simulating watertight and splashproof instrument casings. The simulated conditions included coastal regions with cold, temperate humid and warm humid climates. The protective and decorative properties of the coatings were estimated visually and, at the end of the tests, by gravimetry. Over a period of 20 years the corrosion depth amounted to 0.17-O.80pm7 or an average of 0.01-0.04 pm per year. The corrosion resistance of the Sn-Bi coating depended more strongly on the air salinity and it corroded 1.1 - 1.5 times faster than the Sn-Pb coating. From this data the life expectancy of tin coatings for ships instruments was estimated.

45

8. Tin-Antimony Alloys

8.1 Backmound

‘ miternetals’ (also known as ‘Babbitt metals) are a class of tin alloy incorporating antimony, lead, or other elements such as copper. These are used as bearing surfaces in automobile and diesel engines, railway stock, electric motors and generators, and many other types of industrial applications. For large bearings in heavy duty equipment, the white metal alloy is employed as a thick coating (3-6mm) on a steel or cast iron backing.

The following discussion concerns recent work on the electrodeposition of Sn-Sb and babbite alloys.

8.2 Sn- S b Elec trodepo sit s

In two papers, Medvedev et al. [55,56] have studied the electrodeposition of Sn-Sb in the presence of organic additives.

A composition of electrolytes and electrodeposition conditions were determined which enabled the production of lustrous coatings Sn-Sb [ 5 5 ] . The coatings were electrodeposited on copper or steel specimens at a temperature of 18-20 “C from an electrolyte containing SnS04, SbS03, H2SO4 and various organic additives such as syntanol, formalin and coumarin. The composition of the coatings could be regulated by changing the concentration of the components in the plating bath.

The effect of current density and organic additives on the levelling capabilty of the ,

electrolyte and microdistribution was investigated [ 561. Sulphate electrolytes with the above organic additives, plus butyne diol and benzyl alcohol, were studied.

8.3. Sn-Sb-Cu Electrodeposits

Electrolytic deposits of Sn-Sb-Cu alloy prepared on copper specimens using sulphate electrolyte in the presence of various additives, has been investigated by Medvedev and M a h s h i n [57]. The electrolyte consisted of SnS04 (20-30 g/l); SbS03 (0.5-0.8 gA); CuS04.5H20 (0.5-1 .O g/l); and H2S04 (4 g/l). The influence of current density and the chemical nature of the additives on the coating appearance, was determined. The conditions were optimised to prepare lustrous fine grained coatings. The use of the proposed electrolyte composition enabled ‘tin babbites’ (type B-83 and B-88) to be prepared for improving the anti-fiction properties of copper.

46

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0

0

0

0

0 0

0

0

0

0

0 0

0 0

0

0 0

0

0

0 0

0 e 0 0 0

0

0 0

0 0

9. Conclusions and Recommendations

The current review highlights aspects of the electrodeposition and electroplating of corrosion resistant tin alloys in which either progress in understanding or significant developments have been made during the past few years. As in the case of the electrodeposition of pure tin, the topic of tin alloy electrodeposition continues to receive much attention, particularly in the scientific literature. The following is a summary of general observations considered by the reviewer to have particular merit. Some recommendations for future research effort are also provided :-

The review contains references to the electrodeposition and electroplating of seven different tin alloys regarded as important engineering and corrosion resistant materials. Much of the recent work has been concerned with obtaining a better understanding of the electrochemical conditions and electrolyte compositions required to produce alloys having coherent and uniform structures. It is interesting to note that, even for a single alloy system, a significant variety of electrochemical techniques and electrolyte formulations can often be used to fulfil these objectives. These include d.c. and pulse methods, together with electrolytes based on several types of metal salts, solution pH's, complexing agents andor additives. The pulse plating technique has found particular application in the electrodeposition of a eutectic Sn-Zn alloy where a more uniform composition was claimed compared with more conventional methods.

Electrolyte composition effects are of paramount importance in the electrodeposition of alloys because of the need to bring the deposition pdtentials of the respective metals closer together' and hence facilitate codeposition. However, few basic studies, aimed at obtaining a better understanding of how the complexants or additives function, have been undertaken during the period covered by this review. As in the case of the electrodeposition of pure tin discussed previously, much of the work relating to alloy electrodeposition would appear to be of an empirical nature. One exception to the norm has been a study of the electrodeposition of Sn-Co coatings fiom citrate solutions in which a theoretical analysis of the process of

undertaken. This has allowed experimentally derived and theoretically predicted voltammograms for the deposition of the Sn-Co alloy to be compared.

complexation between Sn2+ and CO 2+ ions with citrate Zigands has been

Morphological and surface analytical data are clearly essential in characterising corrosion resistant deposits, and scanning electron microscopy and X-ray diffraction techniques have invariably been applied in most of the studies cited. In particular, the relationship between the applied current density in galvanostatically controlled deposition (or applied potential in potentiostatically controlled deposition) and the alloy composition is of paramount importance and has consequently received attention in the vast majority of the investigations conducted.

47

Whilst much emphasis has been placed on the processing conditions and electrolyte formulations necessary for the generation suitable electrodeposits, much less information has been accumulated concerning the corrosion testing of the deposits. Most studies have involved an assessment based on one particular technique for evaluating the corrosion rate and a single corrosive medium at a fixed pH value. Clearly, a more extensive range of test techniques and corrosive conditions are necessary. It is also important to establish the relationship between the alloy composition and corrosion resistance, but detailed information of this kind is often lacking. One recent exception has been a thorough investigation of the electrodeposition and sacrzjkial corrosion properties of Sn-Mn coatings on steel. The coatings were electrodeposited from simple ammonium sulphate baths, with or without a range of complexants including citrate, tartrate, EDTA and gluconate. Coatings with a high percentage of the intermetallic Mnl.77-Sn phase, produced in the presence of tartrate, exhibited good sacrificial protection for steel.

In addition to the limited corrosion data, there is also a need to compare the relative merits of different corrosion resistant alloys in order to rank their effectiveness. This approach, where several alloys are considered within the same investigation using common preparative methods and testing routines, has been particularly fmitfbl in the field of tin solder alloys and will be considered in a later review.

Multilayer tin alloy films and composites are gaining increased attention in the literature. Thus, a method has been devised of forming tin-zinc alloy films by sequentially stacking the respective metals by electrodeposition followed by irradiation with a laser beam to produce an alloy by inter-diffusion of the elements. The authors claim that their technique yields alloys in a short time without the creation of non-equilibrium phases and that the alloys remain stable for long periods of time, thus facilitating their use as economical corrosion resistant coatings. The electrodeposition of CdCu-Sn multiZayers by potential pulse electrolysis has also been investigated. Similarly, although not specifically corrosion resistant, Sn-Ni multilayer composites are gaining interest because of their unique mechanical properties. Multilayer composites can be prepared by electrodeposition and it has therefore been appropriate to consider them in the current review.

G) Developments in the field of the corrosion protection of ferrous materials, as for example in the automobile industry, indicate that there will be higher requirements for anti-corrosion coating systems in the future which cannot be met with known processes. Such increased requirements are of the order of 3000 hours resistance in salt mist tests. Furthermore, such anti-corrosion coatings should have the highest possible hardness, be resistant to wear, and should also, as far as possible, be weldable. Recently it has been proposed that electrodeposited ternary tin-zinc alloys containing 30-65% by weight tin, 30-65% by weight zinc, and 0.1-15% by weight of a metal from the group iron, cobalt, or nickel, as the third alloying component, could meet these requirements very well. The ternary tin-zinc alloys preferably contain cobalt as the third alloying component. A ternary alloy of composition

48

0

0 0 0

0

0

0

0 0

0

0

0 0

0

m e 0

0

a 0

0 0

0 a 0 e 0 0

0

0

0

a 0 0

Sn-( 8- 1 O%)Ni-( 1 -2%)Co produced by electrodeposition has also been proposed for use as a bearing overlay, together with other binary Sn-Ni or Sn- CO alloys. The deposits are claimed to be smooth and functional, having a thickness in excess of 50 pm. (

,

49

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0 0

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0 0

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Zangari, Materials Science and Engineering A344 (2003) pp 268-278. 0 0 0 0 52

0

0 e 0 0 e 0 0 e 0 e 0 0 0 e 0 0 0 0 0 0 0 0 0 a 0 0 0 0 a 0 0 a a

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5 5 . Tin-Antimony Alloy Electrodeposition from Sulphate Electrolytes with oganic Additives. G. I. Medvedev, N. A. Makrushin and N. Y. Fursova, Zhur. Prikl. Khim. 74 no. 9, (2001) pp 1424-1428.

56. Microdistribution in th Electrodeposition of Sn-Sb Alloy from Sulphate Electrolytes with Organic Additives. G. I. Medvedev, G. I. Kruglikov and N. Y. Fursova, Zhur. Prikl. Khim. 74, no.11 (2001) pp 1763-1766.

57. Electrodeposition of Sn-Sb-Cu Alloy from Sulphate Electrolytes with Organic Additives. G. I. Medvedev and N. A. Makrushin, Zhur. Prikl. Khim. 74, no.8, (2001) pp 1362-1364.

53

11. Acknowledgements

The following have kindly given their consent to reproduce the figures in this report :-

Figs. 1-7. Reproduced by permission of Elsevier.

Figs. 8 and 9. Reproduced by permission of The American Platers and Surface Finishers Association.

Figs. 10-1 3. Reproduced by permission of the Materials Research Society.

Fig. 14. Reproduced by permission of Elsevier.

Fig. 15. Reproduced by permission of Central Electrochemical Research Institute, India.

Figs. 16- 1 8. Reproduced by permission of Elsevier.

Figs. 19-2 1. Reproduced by permission of Springer Publications.

Figs. 22-24. Reproduced by permission of The Institute of Metal Finishing.

Figs. 25-37. Reproduced by permission of Elsevier.

54

0 0 0 0 0 0 0 0 0 -0 0 0 0 0

0 0 0 0

0 0 0 0 0 0 0 0 0