The Electrodeposition of Platinum and Platinum Alloys by M

Home , Chloroplatinic acid

The Electrodeposition of Platinum and Platinum Alloys By M. E. Baumgartner and Ch. J. Raub Forschungsinstitut fur Edelmetalle und Metallchemie, Schwabisch Gmund, West Germany

survey of hardness, ductility and the internal stresses in platinum layers deposited from Work on the electrolytic deposition of various electrolytes is given in (3). platinum and some platinum alloys is No literature has appeared on the production reviewed, and the results of our recent of platinum alloy films, especially the electro- work OR the deposition and the proper- deposition of alloys of platinum with the iron- ties of the layers produced from some group metals. The present paper reviews pre- promising electrolytes are briefly dis- vious work, tries to find guidelines for future cussed. In general, studies of plating research and in addition discusses recent results solutions are restricted by the economic obtained at the authors’ Institute on the electro- availability of the relevant platinum deposition of platinum and platinum-cobalt salts. On the other hand the hydrolysis alloys, and their properties. of the platinum salts in solution, and the incorporation of decomposition pro- Electrolytes for the Deposition ducts are also critical factors, especial- of Platinum lyfor their influence on internal stress. The first experiments on the electrolytic Recent work has shown that it is possi- deposition of the platinum group metals were ble to deposit platinum-cobalt alloys made over 150 years ago by Elkington, who which have excellent magnetic and obtained a patent in 1837 (4), and later by mechanical properties, and these alloy Bijttger (5). The electrolytes contained simple deposits look very promising for data metal salts and were rather unstable, while the storage applications and for small per- deposits could not be used for any application. manent magnet layers. A review in 1910 stated simply that “electrolytic platinum is hard” despite the fact that it listed no fewer than nine different processes (6). In general, present day electrolytes contain Research on the electrodeposition of the platinum as stable but quite different, com- platinum has been rather neglected recently, plexes. An advantage of these complexes is that and as a result there has been little discussion they show a smaller degree of hydrolysis in the of it in the literature. The last review paper on bath, as compared to simple salts, and therefore the subject, by F. H. Reid, appeared in I970 the electrolytes are much more stable. (I), while a more general review of the electro- These electrolytes may be divided into two deposition of the platinurn group metals, by groups: [a1 baths containing platinum in the one of the present authors, appeared in the divalent state and [bl those with it in the Gmelin Handbook (2). tetravalent state.. Within both groups a In the main, the published literature contains further differentiation can be made based on only general statements about the bath com- the typical salts used, which may not always be position; often information is lacking on the identical with the metal supplying compounds. preparation of the bath, its operation, and the In Tables I and I1 a survey of electrolytes pro- properties of the deposits obtained. However, a ducing worthwhile deposits is given. Tests to

Platinum Metals Rev., 1988, 32, (4), 188-197 188 produce deposits from aqueous cyanide solu- acid increases the electrolyte stability by com- tions proved to be unsuccessful (7). A Mond plexing the platinum ion. Electrolytes with Nickel Company patent describes the deposi- citrates, phosphates and ammonium com- tion of platinum from a complex amine compounds react in a weakly acidic, or sometimes pound which should generate good layers (8). even alkaline state, but their make-up is not Apparently deposition is possible if the always as simple as it may at first seem to be. cyanoamine platinum complexes contain at Interest in chloride-based electrolytes recurred least one, if possible co-ordinated, CN- group in 1931 with investigations by Grube and bound to the metal (8). The electrolyte, Reinhardt (9). Later Atkinson further however, did not find wide use. developed these baths by studying their deposition mechanism (10). He suggested as optimum Electrolytes with Divalent operating parameters: platinum as H2(PtC1,) Platinum Compounds 10-50 gA, hydrochloric acid 180-300 gA, 45- Electrolytes Based on Chlorides p°C, current density 2.5-3.5 A/dm2, current Today this type of bath has no great efficiency 15-20 per cent, and a soluble significance, however it was one of the first to platinum anode. Under carefully controlled achieve technical importance. The basic salts conditions, this solution apparently produces are PtCl,.5H20, H,PtC1,.6H20 and crack-free, ductile crystalline layers of up to (NH,),PtCl,, and in general the bath operates 2opm in thickness. As with most transition in the acidic range. The fundamentals of these metals in aqueous solutions, the pH must be solutions were developed between 1840 and kept within a narrow range in order to avoid 1900. According to Biittger (9, 500 grams of hydrolysis, which in this bath starts between citric acid should be dissolved in 2 litres of pH 2.0 to 2.2 (10). It has to be stated that water and carefully neutralised with sodium even today these baths generally tend to be hydroxide. During continuous stirring, 75 fairly unstable. Deposition begins at a high grams of dry ammonium hexachloroplatinate is polarisation, causing high hydrogen co- then added to the boiling solution which is deposition and a current efficiency which is finally diluted with water to 5 litres. Current mostly below 50 per cent. The lifetime of the efficiency is nearly 70 per cent and the solutions is not very long and in addition, they operating temperature is about w°C. are rather corrosive, which requires most base Deposits produced using this bath are fine metals to be given a proper protective inter- grained but irregular. The addition of citric mediate layer of, for example, gold, silver or

Table I Types of Electrolyte for the Electrodeposition of Platinum

Based on Platinum (11) Based on Platinum (IV)

Chloride bath Alkali hexahydroxyplatinates

Dinitrodiammineplatinum Phosphate bath (platinum-p-salt)

DNS-plating baths (Dinitrosulphatoplatinites)

Platinum Metals Rev., 1988, 32, (4) 189 resurgence in platinum electroplating (12, 13 The most simple type consists of Pt-p-sai 8-16.5 gA, ammonium nitrate 100 gA, sodiun nitrite 10 gA, ammonia (28 per cent) 50 gA: 90-95OC, current density 0.3-2.0 A/dm2, current efficiency 10 per cent, and an insoluble pure platinum anode. However the electrolyte reacts irregularly, due to changes in the nitrite concentration, which with increasing concentration influences the dissociation of the platinum complex. The initial current efficiency can be regained by boiling the bath with added NH,NO,. On reacting with NaNO, it Fig. 1 Using an acidic phosphoric-eulphuric forms NH,NO, which, as a free compound, acid bath, at 4OoC, pH 1.5 and a current density dissociates into nitrogen and water. of 1 Aldm’ a bright surface, free from cracks, may be produced During deposition nearly all nonmetallic con- stituents of the Pt-p-salt are removed from the palladium. During operation C1- ion concen- bath in gaseous form, so the electrolyte has a tration increases in most cases, generating dis- longer lifetime than halide systems. The bath coloured deposits. At higher hydrochloric acid can be replenished by frequent additions of Pt- concentrations and operating temperatures, p-salt. However its levelling power is fairly and at too low current densities, the platinum poor. Deposits are of a similar quality to those anodes dissolve, so no additions of platinum obtained from phosphate baths. The advantage salts will be necessary. At lower hydrochloric of these electrolytes is that they are relatively acid concentrations an insoluble yellow deposit easy to prepare. However, the quality of the of (NH,),PtCl, may form at the anode, acting electrodeposited layer depends largely on the as an insulating layer. Finally it should be ex- concentration of the metal salt in solution and plained why chloride electrolytes may be con- its purity; the higher the platinum concentra- sidered as of a “divalent” type. If the tion, the better the deposits.

electrolysis begins with a (PtCl,) j- solution A further improvement was reached by the Pt‘+ ion is cathodically reduced to Pt’+ periodically reversing the 5-6 A/dm2 current, and we have to consider the equilibrium: 5 seconds cathodic being followed by z seconds z(PtC1,)Z- f (PtCl,)2- + Pt + za- anodic current (14). In this way a platinum At the high concentrations of hydrochloric acid deposition rate of Sprn per hour was obtained. used, equilibrium is strongly shifted to the left It is assumed that this depolarisation of the side of the above equation. cathode also reduces the porosity of the platinum layer, producing such a low porosity Dinitrodiammine-Based Solutions that a 5cm plate (Pt) over nickel withstood In order to maintain the concentration of boiling hydrochloric acid (20 per cent) for five divalent platinum in solution and to avoid hours with no weight loss (IS). A different oxidation of Pt(I1) to Pt(IV), it is necessary to electrolyte of the Pt-p-salt type contains stabilise the Pt(I1) ion by complexing it with fluoroboric acid (16); its composition being: suitable amino compounds. The basis of this Pt-p-salt 20 gA, fluoroboric acid 50-100 gA, kind of solution is cis-dinitrodiammine- sodium fluoroborate 80-120 gA, 70-w°C, cur- platinum Pt(NH 3) ,(NO,) ,, which is frequent- rent density 2-5 A/dmz, current efficiency ly called Pt-p-salt ( I I). Its use for electroplating 14-18 per cent, insoluble platinum anode. applications was discovered by Keitel and Layers up to 7.5pm in thickness can be Zschiegner in 1931 and resulted in a strong deposited. According to Hiinsel defect free

Platinum Metals Rev., 1988, 32, (4) 190 copper overplate

\ayer -D

copper substrate

Fig. 2 Replacing ammonium salts with sodium acetate and sodium carbonate results in improved current efficiency, and enables smooth, bright layers free from cracks and pores to be deposited; left 2.5pm platinum layer, right lOpm platinum layers can be achieved only if the platinum is which ammonium salts are replaced by sodium present in the solution in the divalent state (12). acetate and sodium carbonate, which also im- A different Pt-p-salt solution contains 20-100 proves stability (I). Deposits from this bath are gA sulphamic acid (17) or phosphoric acid, or a smooth, bright and free of pores and cracks at mixture of phosphoric and sulphuric acids (18, thicknesses up to Iopm, as shown in Figure 2. 19). For all these electrolytes insoluble The use of the sodium instead of the am- platinum anodes are used; good electrolyte stir- monium salts improves current efficiency and ring and movement of the cathode are impor- avoids the formation of a salt layer on the tant. The layers produced from this electrolyte anode. The cathodic current efficiency of 35 to are crack free, shiny, bright and up to 2pm in 40 per cent may be further improved by move- thickness. Figure I shows the surface of a Ipm ment of the cathode or the electrolyte, as is thick layer deposited from a phosphoric and shown in Figure 3. sulphuric acid bath. The highest current effi- Since the metal concentration in the elec- ciency is obtainable from an electrolyte in trolyte is fairly low, the diffusion controlled transport of the dischargeable ionic species through the Nernst diffusion layer soon

52 becomes the dominating step in the discharge. Furthermore, this increases both the diffusion f 48 up overvoltage and the hydrogen deposition, $44 which gets even stronger at higher current den-

40 sities. This mechanism explains the increase in IL current efficiency with higher relative 36 z+ movements of cathode and electrolyte. Addi- 32 U tional agitation, and a reduction of the i? 28 thickness of the Nernst diffusion layer, is also Cathodic agitation lmlrnin provided by hydrogen evolution at the cathode. Bath agitation 0 01 llrnin Another method of improving the current LCathodic agitation 1 rnlmin efficiency of this bath is to increase the LWithout:I agitation operating temperature. The influence of Fig. 3 Agitation of the electrolyte and the electrolyte temperature on the cathodic current cathode results in an improvement in cur- efficiency is shown in Figure 4, a sudden in- rent efficiency at 7OoC, pH 10 and a current density of l Aldm’ crease in current efficiency near 6o°C reaches almost 60 per cent close to w0C. Below 5o°C

Platinum Metals Rev., 1988, 32, (4) 191 formed and the nitrite concentration increases, :60, 1 reducing the concentration of the dischargeable platinum ionic complex. ./ I The current density-potentialcurves of these electrolytes show very characteristic behaviour in the alkaline pH range, insofar as the current density increases up to a maximum limiting value of I. 5 A/dm2 at about 500 mV, and then within a further 50 mV it drops to a low value u TEMPERATURE, *C of 0.2 Aldrn’ before increasing again with Vig. 4 Current efficiency can also be in- potential due to hydrogen production. How far fluenced by the temperature of the this kind of “cathodic passivation” is con- electrolyte. At pH 10, 1 A/dm’ and agitation of both the electrolyte and the nected with the hydrogen co-discharge, and the cathode, a marked increase in current ef- adsorption of hydrogen, is not as yet clear. ficiency occurs at about 6OoC Another explanation might be a sudden change in pH value in the cathodic layer caused by hydrogen evolution. The critical current den- the value is only about 10 per cent. If this sity can be increased by an increase in the con- electrolyte is newly made up, the pH adjusts centration of the F’t-p-salt, as well as by the itself to about 10. The dependence of cathodic sodium acetate addition but not by changes in current efficiency on pH is approximately the sodium carbonate concentration. Tem- linear, see Figure 5. This is further proof that perature has a very decisive influence on the the importance of hydrogen co-deposition in- critical current density, while potentials remain creases with pH value. At the same time less nearly constant. “Passivation” is no longer metal is deposited-the current efficiency for observed at pH values below 7, since hydrogen metal deposition decreases with pH. A pro- evolution starts before platinum discharge nounced influence of metal deposition on cur- begins. Figure 7 shows typical current density- rent efficiency is shown by the platinum metal potential curves for the electrolyte and for concentration, Figure 6. A decrease of metal various pH values, at a temperature of 7oOC. concentration causes a reduction in current efficiency. On replenishing the electrolyte with Electrolytes Based on new Pt-p-salt, intermediate products may be Dinitrosulphatoplatinonoue Acid These electrolytes are free of ammonia or amines, and are based on the complex

60 H, Pt(N0,) ,SO., . With them it is possible to u 1 coat directly with platinum a wide range of materials including copper, brass, silver, nickel, lead and titanium. However, metals such as iron, zinc or cadmium have to be pre- plated with a dense layer of nickel or silver (20). The salts for making up the solutions are - potassium salts of nitro-, chloro- and sulphato- V 4 5 6 7 8 9 1011 platinous acid, such as: K,Pt(NO,),Cl, PH K, Pt(N0,) ?Cl, and K, Pt(N0,) ,SO, ,respec- Fig. 5 At 7OoC, 1 A/dmz and with elec- tively. If bright deposits are required the work trolyte and cathode agitation, the reletion- ship between current efficiency and pH has to be carried out at low current densities value is almost linear and the pH of the electrolyte must be kept below z by the addition of sulphuric acid.

Platinum Metals Rev., 1988, 32, (4) 192 Typical baths have the composition: dinitro- CURRENT DENSITY, Aldm2 sulphatoplatinous acid (DNS) 10 gA, sulphuric 2 1.5 1 0.5 acid up to pH 2, temperature 30 to 7ooC, current density 2.5 Aldm’, current efficiency - 200 10-15 per cent, insoluble pure platinum anode. The advantages of the DNS electrolyte are -400 claimed to be as follows: the deposits produced are smooth and bright, and do not have to be -600 polished; layers up to 25pm thickness can be -800 produced, however cracks form at greater W thicknesses; electrolytes are stable, producing -1000 2 constant results and do not degrade on stand- 5! ing; it is possible to coat a wide range of base Of5 metals without using an intermediate layer. 6 -200

Electrolytes with Tetravalent -400 Platinum Complexes Alkaline Solutions -600 In general alkaline electrolytes contain the sodium or potassium salts of hexahydroxy- -800 I platinic acid Na,Pt(OH), or K,Pt(OH),, -1000 respectively (21). A typical bath is: sodium hex- ahydroxyplatinate 20 gA , sodium hydroxide 10 Fig. 7 These typical current density gA, 75OC, pH 13, current density 0.8 A/dm’, versus potential curves show the effect of temperature and of pH value; above, con- current efficiency 100 per cent, insoluble nickel stant pH 10 and variable temperature: (a) or stainless steel anode. Deposits are dense and 7OoC, (b) 5OoC, (c) 35OC; below, con- bright, if the electrolyte is freshly made up. In stant temperature, 7OoC, and variable pH older solutions they become matte and spongy. An advantage of these kinds of electrolytes is their easy regeneration. If the pH of the elec- precipitated and can then be filtered off and trolyte is decreased by the addition of acetic used for the preparation of a new bath. How- acid, hexahydroxyplatinic acid salts are ever, a disadvantage is low stability, since during operation, or on standing, a yellow deposit forms by decomposition of the hydroxy salt according to the equation: 3 Na,Pt(OH), - Na,0.3Pt02.6H,0 + 4NaOH + H,O The reaction is catalysed by impurities in the make-up salts. Another problem is the take up of carbon dioxide from the air, so the carbonate concentration increases and has to be reduced regularly by precipitation with Ba(0H)2. In this connection potassium salts show a better performance than sodium salts. It is claimed Fig. 6 Current efficiency is also in- that an improvement in stability is obtained by fluenced by the platinum concentration in the electrolyte, as demonstrated by a bath adding sodium oxalate, sodium sulphate or at 7OoC, pH 10 and 1 A/&’ sodium acetate (22, 23). Other researchers

~ ~ ~~ ~~ ~ state, however, that these additions are detri-

Platinum Metals Rev., 1988, 32, (4) 193 Tablm II Baths for Platinum Electrodepoeition Type of bath Reference

Chloroplatinic acid H,IRCI,) Ammoniumhexachloro- INH412RCls olatinate-- --- Platinum-p-salt PtlNHdz(N0zh Dinitrosulphato- H2WN02)2S04 platinous acid Sodium hexahydroxy- NadPtlOH)sb2H~O platinate Hexahydroxyplatinic HdPtiOHlsi acid Potassium hexahydroxy- K2FIlDHlpl platinate Platinum chloride RC14.5H20

Ammonia (28%) NH3 Hydrochloric acid HCI Sodium citrate Na,C,H,0t.2H,0 Ammonium chloride NH&I Ammonium nitrate NH4tdOj Sodium nitrite NaN02 Fluoroboric acid HBF4 Sodium fluoroborate NaBF4 Sulphamic acid NH,SO,H Phosphoric acid H3% Sulphuric acid nzso4 Sodium acetate AlaCH3C00 Sodium carbonate Na2C03 Sodium hydroxide NaOH Sodium oxalate Na~C204 Sodium sulphate NazSOs Potassium hydroxide KOH Diammonium (NH,I,HPO, hydrogen phosphate Disodium hydrogen Na2HW4 phosphate Potassium sulphate KZS04 ~ Temperature,Temperature, OCOC 45-90 8O-iO 901% 70-90--65-100 75-100 75-100 80-90 30-70 75 65-80 75 70-90 I 70-90 CurrentCurrent density, density, Aldm2 Aldm2 2.5-3.5 0.5-1.0 0.3-2.0 2-5 0.2-2 0.5-3.0 0.5-3 0.5 2.5 0.8 0.8 0.75 0.3-1 I 0.3-1 tiirrantCurrent offirionpv efficiency. nor per cant cent 15-20 70 10 14-18 15 . 15 15 35-40 10-15 100 80 100 10-50 I 10-50 mental to the stability of the electrolyte since diammonium hydrogen phosphate they were inclined to increase the tendency for (NH,),HPO, 20 gA, disodium hydrogen precipitation of insoluble platinum compounds phosphate Na,HPO, 100 gA, ammonium (12). chloride 20-25 gA, 7o-9o0C, current density According to Davies and Powell, a typical 0.3-1.0 A/dm2, current efficiency 10 to 50 per bath composition is: sodium hexahydroxy- cent, insoluble platinum anode (4). Layers up platinate 18.5 gA, sodium hydroxide 5.1 gA, to a thickness of 0.5pm can be deposited. By in- sodium oxalate 5.1 gA, sodium sulphate 30.8 creasing the platinum concentration up to 5-10 gA, 65-8ooC, current density 0.8 A/dm2, cur- gA it is, however, possible to obtain deposits rent efficiency 80 per cent, insoluble pure which, after dissolution of the substrate metal, platinum anodes (24). With this electrolyte remain as solid foils, tubes or other hollow dense, sparkling platinum deposits can be pro- structures. Using a similar electrolyte com- duced which are comparable to rhodium elec- parable results were reported by Grube and trodeposits (25). However if the platinum Beischer (7). A great disadvantage of this elec- concentration drops below 3 gA the current effi- trolyte is that it is difficult to prepare. Before a ciency drops to only a few per cent. At higher freshly prepared solution can be used, it has to platinum concentrations (12 gA) current den- be left until a deposit which forms is completely sities of up to 2.5 A/dm2 may be achieved. Cur- dissolved. If the solution is made up without rent efficiency versus temperature data display ammonium phosphate, the deposits are rather a maximum of about 80 per cent at 65 to 7ooC, porous and sometimes spongy. Apparently the which is not further improved, even at higher ammonium phosphate improves the dissolution temperatures. Large temperature variations of of an (NH,)+-containing platinum complex in the bath during deposition may generate scaling the solution. Under certain conditions, this of the layers (25). Gold, silver, copper and their electrolyte also forms a layer of insoluble yellow alloys may be used as base materials. However salt which serves as an insulating “barrier” on the electrolyte seems to be tolerant of impuri- the anode surface; this is most likely am- ties, especially against cyanides, which mask moniumhexachloroplatinate. the platinum ions, so reducing current efficiency. Platinum must always be present in the Platinum Alloy Deposits tetravalent state of the hydroxy complex. The A general review of work on the electro- bath tolerates up to 300 gA potassium carbonate deposition of platinum alloys up to 1963 has but only 60 to 80 gA sodium carbonate, before been given by Brenner (28). The earliest alloy cathodic current efficiency drops (26). For deposition seems to have taken place in 1894 deposition onto metals which react in strong from alkaline cyanide solutions. A patent alkaline solutions potassium hydroxide may be describes the deposition of platinum with replaced by 40 gA of potassium sulphate (26). cobalt, nickel, copper, zinc, cadmium and tin (29). A later literature survey of the subject Phosphate Based Electrolytes covering the period from 1965 to I970 was As early as 1855 Roseleur and Lanaux sug- given by Krohn and Bohn (30). However their gested the use of phosphate based electrolytes remarks are very general and no information on for the deposition of platinum (27). The baths electrolyte compositions is given. Bogenschiitz contain platinum as a chloro compound, such and George mention the compositions of certain as Pt(1V) chloride, hexachloroplatinic acid or electrolytes for the deposition of platinum with its alkali salts. To improve conductivity alkali ruthenium, palladium, iridium and rhenium, as phosphates and ammonium phosphates are well as a solution for producing platinum-cobalt used; the presence of the latter is supposed to alloys. Regrettably information on deposition enhance deposition. Pfanhauser suggests as the conditions and properties is not given (31). electrolyte: platinum(1V) chloride 7.5 gA, Angus studied platinum-palladium and

Platinum Metals Rev., 1988, 32, (4) 195 used a chloride electrolyte with ammonium a 1007 cmulg hexachloroplatinate and the respective iridate, and disodium hydrogen phosphates as a buffer (pH 8.5) for the platinum-iridium alloy deposition. The best platinum-rhenium alloys were produced from a fluoroboric acid solution; however sulphamic acid caused high internal stresses in the layers. An acidic platinum-iridium alloy bath (pH 1-2) is described by Tyrell (34). The bath is based on hexabromoplatinic acid and hexabromoiridic acid with I. 5 gfl iridium and 3.5 gA platinum. The iridium concentration in Fig. 8 The hysteresis loops of the solution and the temperature strongly in- platinum-70 atomic per cent cobalt demonstrate the attractive magnetic pro- fluence the iridium content of the deposit. At 4 perties of this material and 30 per cent iridium the layers tend to crack. However, layers with 10per cent iridium can be produced up to a thickness of Iopm. The deposition of platinum-cobalt alloys and the properties of the deposits have been studied recently in the authors’ Institute (35). The basic electrolyte composition was sodium acetate, sodium carbonate, platinum-p-salt, I cobalt sulphate and triethanolamine. It was 0 10 20 30 40 50 6 shown that it is possible to deposit platinum- COBALT CONTENT, atomic pcr Cent cobalt alloys containing low to high cobalt con- Fig. 9 The hardness of electrodeposited centrations (more than 50 atomic per cent), platinum-cobalt depends largely upon the cobalt content. Here the base metal con- which combine high hardness and wear resist- centration in the electrolyte was 10 g/l, pH ance with excellent magnetic properties. For 5, and current density 2 Aldm’ example, the deposits show high coercive forces of up to 400 kA/m at a relatively small platinum-ruthenium alloys, for depositing layers containing 20 to 80 per cent platinum (32). In these baths, current density is the dominant influence on alloy composition. Electrolytes for both alloys are quite different. In the platinum-palladium solution, with increasing current density, less palladium is co- deposited, while in the platinum-ruthenium alloy solution the ruthenium content of the layers increases from 10 to 50 per cent as the current density is raised from 0.5 to 2.0 Ndm’ . Studies by HHnsel showed that electrodeposited alloys of platinum-7 per cent iridium Fig. 10 Platinum-cobalt alloys, containing and platinum-5 per cent rhenium have a much 10-12 weight per cent cobalt, can be deposited as bright shiny layers which are crackfree at higher hardness than pure platinum layers, the thicknesses up to about 6pm, and which show ex- increase being more than 200 per cent (33). He cellent adhesion to the substrate

Platinum Metals Rw., 1988, 32, (4) 196 anisotropy, making them very interesting from these electrolytes is strongly influenced by materials for applications such as data storage electrolyte temperature and pH value. If the devices or permanent magnets, see Figure 8. electrolyte is operated in the neutral or alkaline The hardness of the layers depends strongly range, no more than 10 to 12 weight per cent on their cobalt concentration, increasing cobalt cobalt can be co-deposited with platinum. resulting in an increase in hardness. In par- Depending on the pH value, the current effi- ticular, an alloy with about so atomic per cent ciencies are between 15 and 30 per cent. All cobalt shows a major increase in hardness, as deposits are shiny bright, show excellent ad- illustrated in Figure 9. hesion and are crack-free up to thicknesses of As yet it is not clear how much the effect is 6pm (Figure 10). due to the deposition of platinum-cobalt in the Acknowledgements ordered state. For comparison, metallurgically We wish to thank Impala Platinum Limited for produced platinum-so atomic per cent cobalt supporting our work on the electrodepsition of alloys exhibited hardness values around 200 platinum and platinum alloys. Part of the work is HV, while electroplated deposits of this com- based on a Diplom-Arbeit by M. Wiesner, made at Forschungsinstitut fiir Edelmetalleund Metallchemie position have a hardness of 700 HV. in collaboration with Professor Dr. H. Ropwag of the The composition of layers electrodeposited Fachhochschule, Aalen, West Germany.

References

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Platinum Metals Rev., 1988, 32, (4) 197