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Rhodium — Electrodeposition and applications
Surface Technology, 12 (1981) 351 - 360
351
RHODIUM -ELECTRODEPOSITION
AND APPLICATIONS
MALATHY PUSHPAVANAM, VIDYALAKSHMI RAMAN and B. A. SHENOI Central Electrochemical Research Institute, Karaikudi-6 (India)
(Received May 1, 1980)
Summary Rhodium coatings are widely applied in electronics, decoration and many engineering fields. The thickness of the rhodium coating depends upon the n a t u r e of the application. In this paper we describe different methods of rhodium plating using various baths, the bath operating conditions, pretreatments etc. Finally, some applications of rhodium coatings are reviewed.
1. I n t r o d u c t i o n Rhodium is one of the six metals of the platinum group, all of which are characterized by high melting points, high stability and good resistance to corrosion. Rhodium is more resistant to chemical at t ack t h a n platinum and, a l th o u g h it is superficially oxidized on heating to very high temperatures in air, at normal and moderately elevated temperatures it is completely free from oxidation or tarnish. Rhodium coatings are widely used as permanent protection from atmospheric or marine corrosion in electrical or electronic engineering, as an a t t r a c t i v e and lasting finish on scientific and surgical instruments and as the ideal co n tact material in r.f. circuits. The high reflectivity and heat resistance of rhodium are made use of in instrument mirrors, cinema projectors, IR reflectors and similar apparatus. Rhodium-plated coatings are much harder, more abrasion resistant and less porous t h a n platinum deposits. Rhodium also has a much higher electrical conductivity t ha n platinum and therefore can be used in certain types of sliding or rubbing contacts and electronic equipment. The ability of rhodium to m ai nt ai n a low and stable cont act resistance owing to its complete freedom from tarnish is of exceptional value where there is no appreciable voltage and where absolute reliability is essential. Thus rhodium coatings are the only solution in r.f. and a.f. Circuits where 0376-4883/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
352 variation in the contact resistance causes spurious signals and in certain cases where tarnish or oxide films would produce unwanted components due to partial rectification. Rhodium reflecting surfaces are widely used in optical projection systems where high efficiency, heat-resisting properties and tarnish-free characteristics are essential. Typical applications are galvanometer mirrors, spotlight and projector reflectors, searchlights etc. Rhodium is usually electrodeposited for three main purposes: (1) as a decorative tarnish-resistant film for silverware, jewellery etc. up to thicknesses of 5 x 1 0 - 6 - 1 x 10 -5 in; (2) in reflectors and searchlights [1] up to thicknesses of 5 × 10- 5 . 8 x 10- s in; (3) in electrical contacts [2] where the thickness required varies up to 0.005 in. Rhodium is usually electrodeposited from sulphate, phosphate or sulphate-phosphate baths [3], although the use of sulphamate [4], perchlorate [5], fluoroborate and nitrate [6] baths has been reported. Complex baths containing citric acid, tartaric acid [7], lactic acid, boric acid, alkaline phosphate [8] and aminonitrate [9] have also been investigated. Smith et al. [10] obtained adherent coherent deposits of rhodium from a molten sodium cyanide bath containing dissolved rhodium metal and operated at 600 °C in an argon atmosphere with rotating cathodes. They claimed t h a t good coatings could be obtained on molybdenum, tungsten, nickel, Inconel, carbon steels and stainless steels. However, the bath was not as stable as the aqueous baths. Rhodium plating solutions are very difficult to prepare since pure rhodium cannot be converted to water-soluble salts unless it is in finely divided form (rhodium black) [7]. The baths are available as concentrates which can be diluted as required. Very dilute solutions are suitable only for decorative plating. More concentrated solutions are required to plate the heavy rhodium coatings [11] t h a t are used in a variety of engineering applications usually concerned with the coating of electrical contacts of the sliding or wiping type [12].
2. P r e t r e a t m e n t s
2.1. Substrate metals andundercoats Because of the high cost of rhodium it is customary to use the thinnest possible coating when tarnish resistance of the surface is the only requirement. Since rhodium coatings, like thin deposits of almost any metal, show significant porosity in this thickness range, readily corrodible metals like steel and zinc-base alloys must be provided with an undercoat deposit, usually of silver or nickel, which is sufficiently thick to provide a fairly high level of protection to the substrate [13]. These undercoats are also provided when high surface conductivity is required, when the surface is to be exposed to high temperatures and for other specialized requirements [14]. Metals like steel, aluminium etc. which are readily attacked by the strongly
353 acid plating bath are also given a dense non-porous coating of silver or nickel. Articles on which Pb-Sn solders have been used should also be preplated to avoid black staining of the solder and around the joints. The choice of undercoat is mainly governed by the application. Silver is used for h.f. circuits [15]. As the frequency increases, current flow becomes more confined to the surface layer. In extreme cases the thickness of the rhodium may have to be restricted and a silver-to-rhodium ratio established which is a compromise between conductivity and service life. Silver is preferred as the undercoat for thick rhodium deposits as it appears to minimize the risk of cracking of the highly stressed rhodium deposits. For applications involving exposure at elevated temperatures, e.g. in reflectors, or where high wear resistance is required nickel is the natural choice of undercoat. Nickel undercoats are used on copper printed circuits where the thickness of the rhodium is limited by plating solution attack on the copperlaminate adhesive and by the lifting effect of initial stresses in the rhodium deposit [16]. A silver undercoat is preferred when the coating is required to resist exposure to marine or other chloride-containing atmospheres as the potential difference between silver and rhodium in seawater at 25 °C is only 0.05 V. Nickel undercoats are superior to silver undercoats in sulphide atmospheres [13]. The thickness of the undercoat also depends on the nature of the application. 2.2. Pretreatments The very highly stressed nature of the rhodium film means that cracking and exfoliation may occur owing to poor adhesion to the substrate, particularly in the case of thick coatings. In rhodium plating the surface finish of the metal is of more t h a n usual importance in view of its high hardness, and the need to avoid loss of costly metal by polishing. Since rhodium baths have no levelling properties, surfaces should be finely finished before plating. Surfaces which have been buffed or otherwise brought to a high polish by mechanical means are not altogether suitable for the direct reception of a thick highly stressed rhodium deposit, and it is preferable to remove the disturbed surface layer of the substrate and in some cases to destroy any positive film by a light controlled etching treatment before rhodium plating is commenced. The following methods are used for preparing a substrate for rhodium plating [17]. 2.2.1. Silver
Degrease cathodically; rinse; etch anodically in 1% KCN solution at 0.2 A dm-2 for 1 min at room temperature or chemically etch in a solution containing 20 g l-1 FeC13 and 60 ml l-1 HC1 for 20 - 30 s; rinse; dip in 5% HESO4; rhodium plate. 2.2.2. N i c k e l
Degrease cathodically; rinse; etch in a solution containing 20 g 1-1 FeC13 and 60 ml 1-1 HC1; rinse; rhodium plate.
354
2.2.3. Nickel silver Degrease cathodically; rinse; etch in 1.5% H3PO 4 at 0.7 A dm-2 for 40 s; rinse; rhodium plate.
2.2.4. Beryllium copper Degrease cathodically; rinse; etch anodically; copper plate in a solution containing 125 g l - 1 CuSO 4 and 38 ml 1-1 H2SO 4 at 2 A dm - 2 for 1 min at room temperature; rinse; rhodium plate. 2.2.5. Phosphor bronze Degrease cathodically; rinse; etch in 107/o - 15% FeCl3; dip in 10% HC1; rinse; dip in 5% H2SO4; copper plate; rinse; rhodium plate. 2.2.6. Copper and its other alloys Degrease cathodically; rinse; etch in a solution containing 1000 ml H2SO 4, 50 ml HNO a, 2 - 5 ml HC1 and 125 ml water for 10- 20 s; rinse; rhodium plate. On aluminium bronze the etch produces a greyish surface with some tendency for smutting but no adhesion difficulties were encountered [18].
3. Plating baths A number of bath formulations have been used by different workers [19]. Both sulphate and phos pha t e - s ul pha t e baths have been used for the deposition of thin (up to 5 × 10- 5 in) coatings as non-tarnishing finishes for decorative purposes [20]. A typical bath contains 2 g 1-1 of rhodium as sulphate or phosphate c o n c e n t r a t e a n d 20 - 30 ml 1-1 H2SO 4 and runs at 1 - 4 A dm -2 at a temperature of 35 - 40 °C. A solution for heavier industrial deposits would have an increased rhodium c o n c e n t r a t i o n of up to 10 g 1-1 with 50 ml 1-1 H2SO4. The sulphate bath gives very smooth light grey deposits 5 x 10 -4 1 x 10-3 in thick at room temperature and 3 A dm-2. Cracking, which is minimal at room temperature, becomes extremely severe as the deposition t e m p e r a t u r e is increased to 70 °C and the deposits have a bright frosted appearance. The cracking of deposits at elevated temperatures can be reduced by increasing the H2SO 4 content of the bath to 100 ml l-1. The cathode c u r r e n t efficiency is of the order of 75% at room temperature and at 70 °C, corresponding to a deposition rate of 0.001 in of rhodium in about 1 h. Deposits are fairly smooth at c u r r e n t densities up to 10 A dm-2 but become r a t h e r matt and rough at densities above this value. As the c u r r e n t density is increased the c u r r e n t efficiency falls from 75% at 3 A dm-2 to 56% at 10 A d m - 2. As the rhodium c o n c e n t r a t i o n of the bath decreases from 50 to 20 g 1-1 during operation the c ur r e nt efficiency decreases from 75% to 30% at room temp er atu r e but remains at about 70% at 70 °C. At low rhodium concentrations the deposits resemble those obtained from the fresh bath
355 with respect to appearance and cracking incidence. At high temperatures there is no tendency to frosting of the deposit and cracking is very slight but the deposit is rough. The throwing power of this type of bath is good [21, 22]. In sulphate-phosphate baths the cathode current efficiency at room temperature is low. Deposits in the thickness range 5 x 1 0 - 4 . 1 × 10-3 in show much more variation in appearance than those obtained from sulphate baths, the finish ranging from bright to light grey frosted. The incidence of cracking is extremely high in all cases. On aging, the deposits turn milky bright with a high incidence of cracking, although the current efficiency is slightly improved. At 70 °C the current efficiency is of the order of 70% and there is a marked decrease in cracking and a loss of brightness. The deposits from this type of bath are softer than those from the sulphate baths. The internal stresses vary over a wide range and are higher than in coatings produced from sulphate baths. The phosphate bath gives deposits similar to those from the Sulphate bath although the incidence of cracking is higher. Severe cracking and a consequent frosted appearance have been observed at higher temperatures but the deposits obtained at room temperature are rough. The cathode current efficiency at room temperature is relatively low in highly concentrated baths whereas it shows a marked increase in dilute baths at room temperature. Fluoroborate baths yield frosted deposits at all rhodium concentrations at high temperatures. The current efficiency is comparable with that of the sulphate bath at room temperature but falls at higher temperatures for more concentrated solutions. Sulphamate baths give smooth deposits with less cracking. The current efficiency is relatively low but increases with the deposition temperature. For thick plating the sulphate bath appears to have advantages over the sulphate-phosphate bath as it has a higher current efficiency and produces films with lower internal stresses and higher hardness. The sulphate solution is easy to maintain. The build-up of H2SO 4 associated with the replenishment of rhodium sulphate does not have any serious detrimental effects. Leister and Benham [23] have calculated that for replenishment rhodium should be passed through the solution at a rate of 0.6 g A - l h - 1 f o r a 4 g l - 1 rhodium bath and at 0 . 4 5 g A - l h - 1 f o r a 2 g l - 1 rhodium bath. Sulphate-phosphate baths are normally used to provide a decorative jewellery finish on items which contain large quantities of soft solder or are made of steel, tin alloys or zinc-base alloys [24]. The all-phosphate bath is sometimes preferred for deposition on tin alloys [25]. The use of alternating currents for rhodium deposition has been found to give smooth and compact deposits i n sulphate and sulphate-phosphate baths [26]. A well-known disadvantage of rhodium coatings deposited from the simple acid sulphate bath (which is still widely used) is the presence of high internal stresses. These lead to the development of cracks in coatings with
356 thicknesses greater than a limiting value which varies with the electrolyte composition and the operating parameters. Reid [27] has reported t h a t a selenic acid type of solution [28] has advantages over aluminium- or magnesium-containing solutions [29] with respect to the brightness of the deposits even at substantial thicknesses. The addition of aluminium and magnesium eliminates cracks in deposits of thicknesses up to 5 × 10-4 in but no corresponding reduction in tensile stress has been found [29, 30]. The low stress bath based on the addition of selenic acid [31, 32] actually produces an increase in stress in the initial stages which decreases steadily to a very low value. Relatively thick deposits (over 10 ~m) have a much greater protective value t h a n normal coatings [33]. The addition of copper salt (0.01 - 0.5 g 1-1) has been reported to reduce stress in sulphamate baths [34]. Other additives are generally not necessary to improve the brightness of the deposit since the pure solution itself gives attractive white deposits. Sodium lauryl sulphate has been used as a wetting agent to remove pitting [24]. The presence of aluminium chloride and chromic chloride helps in the deposition of bright deposits [35] and the presence of chloride ions gives fine crystalline deposits [36].
4. Effect o f impurities The structure and colour of the deposit should be of very high quality in rhodium plating. However, rhodium plating is very sensitive to the presence of impurities which change the colour of the coating. A detailed study has been carried out by Parker [24]. Alkali metal, alkaline earth and ammonium salts have an unfavourable effect as in larg e concentrations they may impart a yellow colour to the deposit [37]. In contrast, traces of calcium ions improve the colour of the deposit. The presence of ammonium ions decreases the cathode efficiency at room temperature. Nickel, iron, copper and beryllium are not deleterious even at concentrations as high as 1 g 1-1. However, Parker [24] has reported t h a t copper concentrations above 0.005 g l- 1 give milky matt deposits from 4 g 1-1 solutions. Excessive concentrations produce copper-coloured deposits with severe cracking [38]. Very small concentrations (0.01 g 1-1) of bismuth and silver have a beneficial effect on the brightness of the deposit, but at higher concentrations dark, streaky and eventually spongy deposits are produced. Similar effects are obtained with lead and zinc. Zinc reduces the reflectivity at concentrations of 0.1 g 1-1 [39], while mercury produces dark matt deposits at concentrations above 0.1 g 1-1 [40]. The presence of mercury leads to flaking of the deposits and a black colouration [41]. Reports on the effect of tin vary from author to author. Reid has found a drop in efficiency at concentrations above 0.1 g 1-1. Parker has reported" that deposits are speckled at concentrations above 0.004 g 1-1 and become rough above 0.02 g 1-1. Leister and Benham [23] have found no effects at concentrations above 0.1 g l- 1. The addition of lead has been reported to improve the brightness of the deposit
357 and to have a beneficial effect on the limiting thickness [42]. Chloride ions have a marked effect on the character of the deposits from the 10 g l-~ sulphate solution. They become increasingly matt and rough at concentrations from 0.1 to 1.0 g 1-1 and the internal stress is reduced [38]. The presence of ferrocyanide ions produces a fall in cathode efficiency. The general effect of organic materials [41] in dilute baths is to cause darkening of the deposits and to reduce the efficiency of deposition. Small amounts of phenol and related compounds can lead to an increase in the brightness of the deposits. Organic contamination produces small circular pits in the deposits and at higher concentrations the coatings crack and develop a scaly appearance [14]. Therefore care should be taken in the selection of tank linings [43]. Under some conditions coloured deposits similar to cuprous oxide and lead peroxide are obtained which may be due to the deposition of oxides. Rhodium is also oxidized to some extent at the anode to the quadrivalent or hexavalent state (present as rhodate) whose presence appears to assist in the formation of smooth deposits. An increase in the rhodate may render the bath unfit for further use, and this is always accompanied by a change in bath colour from red brown to green black. Rhodium baths which have become inoperative owing to the presence of metal impurities can be purified by the use of ethylenediaminetetraacetic acid (EDTA). At an EDTA concentration of 0.25- 0.5 g 1-1 there is an improvement in the brightness of solutions containing tin, lead, silver, cadmium and nickel. However, there is a general lowering of cathode efficiency and hence this procedure cannot be adopted for thick rhodium plating. Brenner and Olson [39] have proposed a method for the removal of zinc, cadmium, mercury, lead, iron, cobalt and nickel by precipitation with ferrocyanide. The excess ferrocyanide is removed after filtration by the addition of ferric sulphate. This procedure requires the utmost care in view of the possible adverse effects of residual ferrocyanide in the solution. Chloride can be removed by titration with silver sulphate and silver by precipitation as chloride. Treatment with activated charcoal or alumina is recommended for organic impurities.
5. Plating techniques Barrel plating formulations have been suggested for plating small components. Parker [24] has suggested the composition given in Table 1 for barrel plating with sulphate baths. The current should be on whe~ an immersed barrel enters the solution and a higher strike current is used to improve the coverage. Immersion treatment can also be used for rhodium plating; a solution containing 5 g l- 1 rhodium and 250 ml l- I concentrated HC1 is used. The cleaned parts are immersed in the solution without agitation at 25 °C. A coating 30 pm thick is deposited in 10 min. The plates should be sealed after plating by gold plating, immersion in concentrated NH4OH for 5 min or by
358 immersion in boiling water for 15 min. Application of the technique of brush plating (the Dalik process or tampon peeling) to the plating of rhodium has been mentioned in the literature [44]. TABLE 1
Rhodium (g 1-l) H2SO4 (ml 1-1) Current density (Aft -2, ,~mA cm-2) Temperature (°C)
For deposits up to 5 x 1 0 -5 in
For deposits thicker than 5 x 1 0 -5 in
1 25 10 - 30 45
2 25 10 - 20 45
6. Applications of rhodium plating Rhodium plating is used [45] for electrical contacts, especially sliding contacts which are required to operate reliably after periods of idleness. Numerous types of slip rings and switches used in radio communication [46] and radar control gear are rhodium plated. Tests have been reported on rhodium-plated slip rings for the transmission of electroencephalograph signals [47]. High speed computer switching is another field of application. In many cases this involves printed copper switch patterns on Formica, Bakelite or Melamine laminates [48]. Rhodium plating is used solely for wear resistance in small magnetic clutches [49]. Rhodium has also been used as a diffusion barrier layer between gold and copper [50] and between silver plate and Ni-Fe alloy [51]. Gold-plated connectors are rhodium plated to provide better wear resistance [52]. Copper anodes for thermionic valves, which are gold plated to protect them against oxidation during processing and sealing into their envelopes, are plated with a thin layer of rhodium or nickel to prevent diffusion of gold into the copper during the baking out process. A u - P t or Pt-Rh spinnerets for sealed reed switches are coated with rhodium to improve their surface hardness [53]. Rhodium plating is used on titanium to reduce the contact resistance between connecting rods and graphite anodes [54], as a conducting track on tin or antimony oxide film resistors on glass [55] and to improve the life of iridium crucibles used for growing single crystals of calcium tungstate etc. by reducing the loss of iridium oxide at the high operating temperatures [56]. The use of electrodeposited rhodium in coaxial r.f. circuits has been discussed by Walter [57]. Rhodium plating of Au-Pd-plated phosphor bronze plugs has been reported to prevent bimetallic corrosion [58]. Heavy radio interference caused by a rotary converter has been completely eliminated by rhodium plating [59, 60].
359
References 1 A . H . S t u a r t , Electroplat., India, 1 (3) (1948) 88 - 90. 2 R. P i n n e r , Electroplat., India, 6 ( A u g u s t 1953) 276 - 281. 3 L. Serota, Met. Finish., 64 (9) (1966) 84- 88; 64 (10) (1966) 82- 84. 4 R. P i o n t e l l i et al., Chim Ind. (Milan), 21 (1939) 478 - 479; 22 (1940) 61 - 62. 5 F . M . L e v e r a n d E. G. Towndrow, Br. Patent 856,867, 1960. 6 E t a b l i s s e m e n t s M a r r e t , B o n n i n , Lebel a n d Geieu, French Patent 803,273, 1936. 7 C . G . F i n k a n d C. G. Lambross, Trans. Electrochem. Soc., 63 (1933) 181. 8 B a k e r a n d Co. Inc., U.S. Patent 2,057,475, 1936. 9 B a k e r a n d Co. Inc., Br. Patent 413,342, 1934; U.S. Patent 1,981,820, 1935; Br. Patent 438,365, 1935. 10 G . R . Smith, C. B. K e n a h a n , R. L. A n d r e w s a n d D. S c h l a i n , Plating, 56 (1969) 805 808. 11 A . M . Weisberg, Mater. Methods, 53 ( M a r c h 1953) 85 - 87. 12 H . C . Angus, Trans. Inst. Met. Finish., 39 (1962) 20. 13 L . L . Shreir, Corrosion, Vol. 2, George Newness, London, pp. 100 - 101. 14 R . R . B e n h a m , Platinum Met. Rev., 5 (1) (1961) 13. 15 A . J . Foster, Electroplat. Met. Finish., 28 (1) (1975) 8. 16 G . L . Vivian, Prod. Finish. (London), 17 (11) (1964) 97- 106. 17 F . H . Reid, Trans. Inst. Met. Finish., 33 (1955 - 1956) 105. 18 H . J . Weisner, Proc. Am. Electroplat. Soc., 39 (1952) 79. 19 F . H . Reid, Metall. Rev., 8 (1963) 167. 20 D . O . Walter, Electroplat. Met. Finish., 10 (2) (1957) 44. 21 K. S c h u m p e l t , Trans. Electrochem. Soc., 80 (1941) 495. 22 D . G . F o u l k e a n d D. C. J o h n s o n , Met. Finish., 61 (6) (1963) 72 - 82. 23 F . H . Leister a n d R. R. B e n h a m , Trans. Inst. Met. Finish., 29 (1951) 52- 53. 24 E . A . P a r k e r , Plating, 42 (1955) 882. 25 W . T . Lee, Met. Finish. J., 7 (1961) 63. 26 L. K a d a n e r a n d D. Dik, J. Appl. Chem. U.S.S.R., 35 (1) (1962) 196- 197. 27 F . H . Reid, Metalloberfl~tche, 15 (2) (1961) 33- 37. 28 F . H . Reid, Trans. Inst. Met. Finish., 36 (1958 - 1959) 74 - 80. 29 K. S c h u m p e l t , French Patent 1,189,477, 1958. 30 T. A. G o r t a l o v a , S. I. B r e z i n a a n d G. S. Vozdvizhenskii, Prot. Met. (U.S.S.R.), 8 (1972) 203 - 205. 31 Met. Ind. (London), 99 (7) (1961) 128. 32 F . H . Reid, Swiss Patent 362,581, 1962. 33 F . H . Reid, Trans. Inst. Met. Finish., 48 (1970) 117. 34 S i m o n i a n . A t k i n Y (Sel Rex Corp.), French Patent 1, 577,593, 1969. 35 D e u t s c h e Gold- u n d S i l b e r - S c h e i d e a n s t a l , German Patent 2,329,578, 1973. 36 U . A . Boiko, Russian Patent 382,762, 1971. 37 L. C i n a m o n , Mon. Rev. Am. Electroplat. Soc., 24 (1937) 347. 38 H . J . W i e s n e r a n d H. A. Meers, Plating, 43 (1956) 838- 848. 39 A. B r e n n e r a n d W. A. Olson, Proc. Am. Electroplat. Soc., 33 (1946) 29; U.S. Patent 2,401,331, 1946. 40 J. F i s c h e r a n d D. Weimer, Precious Metal Plating, R o b e r t Drapper, T e d d i n g t o n , 1964, p. 183. 41 S. Hill, Plating, 52 (5) (1965) 417- 419. 42 Br. Patent 626,430, 1949. 43 Electroplat. Met. Finish., 17 (5) (1964) 157. 44 Electroplat. Met. Finish., 11 (3) (1958) 91. 45 L . B . H u n t , Met. Ind. (London), 71 (17) (1947) 339 - 342. 46 N. M a r c h a n d , Trans. Inst. Met. Finish., 38 (2) (1961) 59 - 65. 47 B . M . H o r t o n , Rev. Sci. Instrum., 20 (1949) 930. 48 A. K l e i n l e a n d O. Loebich, Galvanotechnik, 53 (2) (1962) 62 - 74.
360
49 50 51 52 53 54 55 56 57 58 59 60
F . H . Reid, Met. Finish. J., 9 (1963) 431. Br. Patent 698,675, 1953. Compagnie Franqaise Thomson-Houston, Br. Patent 901,292, 1962. D . G . Folke, Plating, 51 (1963) 685 - 688. H . W . Robinson, Engelhard Industries, U.S. Patent 3,044,155, 1962. Br. Patent 881,625, 1961. Br. Patent 927,215, 1963. R . S . H o w a r t h and C. Whiteley, J. Electrochem. Soc., 110 (1963) 467. D . O . Walter, Platinum Met. Rev., 1 (1) (1957) 14 - 19. D . O . Walter, Electroplat. Met. Finish., 10 (2) (1957) 44 - 46. Electroplat. Met. Finish., 8 (7) (1955) 257. E . H . Leister, Met. Ind. (London), 19 (1954) 428.
351
RHODIUM -ELECTRODEPOSITION
AND APPLICATIONS
MALATHY PUSHPAVANAM, VIDYALAKSHMI RAMAN and B. A. SHENOI Central Electrochemical Research Institute, Karaikudi-6 (India)
(Received May 1, 1980)
Summary Rhodium coatings are widely applied in electronics, decoration and many engineering fields. The thickness of the rhodium coating depends upon the n a t u r e of the application. In this paper we describe different methods of rhodium plating using various baths, the bath operating conditions, pretreatments etc. Finally, some applications of rhodium coatings are reviewed.
1. I n t r o d u c t i o n Rhodium is one of the six metals of the platinum group, all of which are characterized by high melting points, high stability and good resistance to corrosion. Rhodium is more resistant to chemical at t ack t h a n platinum and, a l th o u g h it is superficially oxidized on heating to very high temperatures in air, at normal and moderately elevated temperatures it is completely free from oxidation or tarnish. Rhodium coatings are widely used as permanent protection from atmospheric or marine corrosion in electrical or electronic engineering, as an a t t r a c t i v e and lasting finish on scientific and surgical instruments and as the ideal co n tact material in r.f. circuits. The high reflectivity and heat resistance of rhodium are made use of in instrument mirrors, cinema projectors, IR reflectors and similar apparatus. Rhodium-plated coatings are much harder, more abrasion resistant and less porous t h a n platinum deposits. Rhodium also has a much higher electrical conductivity t ha n platinum and therefore can be used in certain types of sliding or rubbing contacts and electronic equipment. The ability of rhodium to m ai nt ai n a low and stable cont act resistance owing to its complete freedom from tarnish is of exceptional value where there is no appreciable voltage and where absolute reliability is essential. Thus rhodium coatings are the only solution in r.f. and a.f. Circuits where 0376-4883/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
352 variation in the contact resistance causes spurious signals and in certain cases where tarnish or oxide films would produce unwanted components due to partial rectification. Rhodium reflecting surfaces are widely used in optical projection systems where high efficiency, heat-resisting properties and tarnish-free characteristics are essential. Typical applications are galvanometer mirrors, spotlight and projector reflectors, searchlights etc. Rhodium is usually electrodeposited for three main purposes: (1) as a decorative tarnish-resistant film for silverware, jewellery etc. up to thicknesses of 5 x 1 0 - 6 - 1 x 10 -5 in; (2) in reflectors and searchlights [1] up to thicknesses of 5 × 10- 5 . 8 x 10- s in; (3) in electrical contacts [2] where the thickness required varies up to 0.005 in. Rhodium is usually electrodeposited from sulphate, phosphate or sulphate-phosphate baths [3], although the use of sulphamate [4], perchlorate [5], fluoroborate and nitrate [6] baths has been reported. Complex baths containing citric acid, tartaric acid [7], lactic acid, boric acid, alkaline phosphate [8] and aminonitrate [9] have also been investigated. Smith et al. [10] obtained adherent coherent deposits of rhodium from a molten sodium cyanide bath containing dissolved rhodium metal and operated at 600 °C in an argon atmosphere with rotating cathodes. They claimed t h a t good coatings could be obtained on molybdenum, tungsten, nickel, Inconel, carbon steels and stainless steels. However, the bath was not as stable as the aqueous baths. Rhodium plating solutions are very difficult to prepare since pure rhodium cannot be converted to water-soluble salts unless it is in finely divided form (rhodium black) [7]. The baths are available as concentrates which can be diluted as required. Very dilute solutions are suitable only for decorative plating. More concentrated solutions are required to plate the heavy rhodium coatings [11] t h a t are used in a variety of engineering applications usually concerned with the coating of electrical contacts of the sliding or wiping type [12].
2. P r e t r e a t m e n t s
2.1. Substrate metals andundercoats Because of the high cost of rhodium it is customary to use the thinnest possible coating when tarnish resistance of the surface is the only requirement. Since rhodium coatings, like thin deposits of almost any metal, show significant porosity in this thickness range, readily corrodible metals like steel and zinc-base alloys must be provided with an undercoat deposit, usually of silver or nickel, which is sufficiently thick to provide a fairly high level of protection to the substrate [13]. These undercoats are also provided when high surface conductivity is required, when the surface is to be exposed to high temperatures and for other specialized requirements [14]. Metals like steel, aluminium etc. which are readily attacked by the strongly
353 acid plating bath are also given a dense non-porous coating of silver or nickel. Articles on which Pb-Sn solders have been used should also be preplated to avoid black staining of the solder and around the joints. The choice of undercoat is mainly governed by the application. Silver is used for h.f. circuits [15]. As the frequency increases, current flow becomes more confined to the surface layer. In extreme cases the thickness of the rhodium may have to be restricted and a silver-to-rhodium ratio established which is a compromise between conductivity and service life. Silver is preferred as the undercoat for thick rhodium deposits as it appears to minimize the risk of cracking of the highly stressed rhodium deposits. For applications involving exposure at elevated temperatures, e.g. in reflectors, or where high wear resistance is required nickel is the natural choice of undercoat. Nickel undercoats are used on copper printed circuits where the thickness of the rhodium is limited by plating solution attack on the copperlaminate adhesive and by the lifting effect of initial stresses in the rhodium deposit [16]. A silver undercoat is preferred when the coating is required to resist exposure to marine or other chloride-containing atmospheres as the potential difference between silver and rhodium in seawater at 25 °C is only 0.05 V. Nickel undercoats are superior to silver undercoats in sulphide atmospheres [13]. The thickness of the undercoat also depends on the nature of the application. 2.2. Pretreatments The very highly stressed nature of the rhodium film means that cracking and exfoliation may occur owing to poor adhesion to the substrate, particularly in the case of thick coatings. In rhodium plating the surface finish of the metal is of more t h a n usual importance in view of its high hardness, and the need to avoid loss of costly metal by polishing. Since rhodium baths have no levelling properties, surfaces should be finely finished before plating. Surfaces which have been buffed or otherwise brought to a high polish by mechanical means are not altogether suitable for the direct reception of a thick highly stressed rhodium deposit, and it is preferable to remove the disturbed surface layer of the substrate and in some cases to destroy any positive film by a light controlled etching treatment before rhodium plating is commenced. The following methods are used for preparing a substrate for rhodium plating [17]. 2.2.1. Silver
Degrease cathodically; rinse; etch anodically in 1% KCN solution at 0.2 A dm-2 for 1 min at room temperature or chemically etch in a solution containing 20 g l-1 FeC13 and 60 ml l-1 HC1 for 20 - 30 s; rinse; dip in 5% HESO4; rhodium plate. 2.2.2. N i c k e l
Degrease cathodically; rinse; etch in a solution containing 20 g 1-1 FeC13 and 60 ml 1-1 HC1; rinse; rhodium plate.
354
2.2.3. Nickel silver Degrease cathodically; rinse; etch in 1.5% H3PO 4 at 0.7 A dm-2 for 40 s; rinse; rhodium plate.
2.2.4. Beryllium copper Degrease cathodically; rinse; etch anodically; copper plate in a solution containing 125 g l - 1 CuSO 4 and 38 ml 1-1 H2SO 4 at 2 A dm - 2 for 1 min at room temperature; rinse; rhodium plate. 2.2.5. Phosphor bronze Degrease cathodically; rinse; etch in 107/o - 15% FeCl3; dip in 10% HC1; rinse; dip in 5% H2SO4; copper plate; rinse; rhodium plate. 2.2.6. Copper and its other alloys Degrease cathodically; rinse; etch in a solution containing 1000 ml H2SO 4, 50 ml HNO a, 2 - 5 ml HC1 and 125 ml water for 10- 20 s; rinse; rhodium plate. On aluminium bronze the etch produces a greyish surface with some tendency for smutting but no adhesion difficulties were encountered [18].
3. Plating baths A number of bath formulations have been used by different workers [19]. Both sulphate and phos pha t e - s ul pha t e baths have been used for the deposition of thin (up to 5 × 10- 5 in) coatings as non-tarnishing finishes for decorative purposes [20]. A typical bath contains 2 g 1-1 of rhodium as sulphate or phosphate c o n c e n t r a t e a n d 20 - 30 ml 1-1 H2SO 4 and runs at 1 - 4 A dm -2 at a temperature of 35 - 40 °C. A solution for heavier industrial deposits would have an increased rhodium c o n c e n t r a t i o n of up to 10 g 1-1 with 50 ml 1-1 H2SO4. The sulphate bath gives very smooth light grey deposits 5 x 10 -4 1 x 10-3 in thick at room temperature and 3 A dm-2. Cracking, which is minimal at room temperature, becomes extremely severe as the deposition t e m p e r a t u r e is increased to 70 °C and the deposits have a bright frosted appearance. The cracking of deposits at elevated temperatures can be reduced by increasing the H2SO 4 content of the bath to 100 ml l-1. The cathode c u r r e n t efficiency is of the order of 75% at room temperature and at 70 °C, corresponding to a deposition rate of 0.001 in of rhodium in about 1 h. Deposits are fairly smooth at c u r r e n t densities up to 10 A dm-2 but become r a t h e r matt and rough at densities above this value. As the c u r r e n t density is increased the c u r r e n t efficiency falls from 75% at 3 A dm-2 to 56% at 10 A d m - 2. As the rhodium c o n c e n t r a t i o n of the bath decreases from 50 to 20 g 1-1 during operation the c ur r e nt efficiency decreases from 75% to 30% at room temp er atu r e but remains at about 70% at 70 °C. At low rhodium concentrations the deposits resemble those obtained from the fresh bath
355 with respect to appearance and cracking incidence. At high temperatures there is no tendency to frosting of the deposit and cracking is very slight but the deposit is rough. The throwing power of this type of bath is good [21, 22]. In sulphate-phosphate baths the cathode current efficiency at room temperature is low. Deposits in the thickness range 5 x 1 0 - 4 . 1 × 10-3 in show much more variation in appearance than those obtained from sulphate baths, the finish ranging from bright to light grey frosted. The incidence of cracking is extremely high in all cases. On aging, the deposits turn milky bright with a high incidence of cracking, although the current efficiency is slightly improved. At 70 °C the current efficiency is of the order of 70% and there is a marked decrease in cracking and a loss of brightness. The deposits from this type of bath are softer than those from the sulphate baths. The internal stresses vary over a wide range and are higher than in coatings produced from sulphate baths. The phosphate bath gives deposits similar to those from the Sulphate bath although the incidence of cracking is higher. Severe cracking and a consequent frosted appearance have been observed at higher temperatures but the deposits obtained at room temperature are rough. The cathode current efficiency at room temperature is relatively low in highly concentrated baths whereas it shows a marked increase in dilute baths at room temperature. Fluoroborate baths yield frosted deposits at all rhodium concentrations at high temperatures. The current efficiency is comparable with that of the sulphate bath at room temperature but falls at higher temperatures for more concentrated solutions. Sulphamate baths give smooth deposits with less cracking. The current efficiency is relatively low but increases with the deposition temperature. For thick plating the sulphate bath appears to have advantages over the sulphate-phosphate bath as it has a higher current efficiency and produces films with lower internal stresses and higher hardness. The sulphate solution is easy to maintain. The build-up of H2SO 4 associated with the replenishment of rhodium sulphate does not have any serious detrimental effects. Leister and Benham [23] have calculated that for replenishment rhodium should be passed through the solution at a rate of 0.6 g A - l h - 1 f o r a 4 g l - 1 rhodium bath and at 0 . 4 5 g A - l h - 1 f o r a 2 g l - 1 rhodium bath. Sulphate-phosphate baths are normally used to provide a decorative jewellery finish on items which contain large quantities of soft solder or are made of steel, tin alloys or zinc-base alloys [24]. The all-phosphate bath is sometimes preferred for deposition on tin alloys [25]. The use of alternating currents for rhodium deposition has been found to give smooth and compact deposits i n sulphate and sulphate-phosphate baths [26]. A well-known disadvantage of rhodium coatings deposited from the simple acid sulphate bath (which is still widely used) is the presence of high internal stresses. These lead to the development of cracks in coatings with
356 thicknesses greater than a limiting value which varies with the electrolyte composition and the operating parameters. Reid [27] has reported t h a t a selenic acid type of solution [28] has advantages over aluminium- or magnesium-containing solutions [29] with respect to the brightness of the deposits even at substantial thicknesses. The addition of aluminium and magnesium eliminates cracks in deposits of thicknesses up to 5 × 10-4 in but no corresponding reduction in tensile stress has been found [29, 30]. The low stress bath based on the addition of selenic acid [31, 32] actually produces an increase in stress in the initial stages which decreases steadily to a very low value. Relatively thick deposits (over 10 ~m) have a much greater protective value t h a n normal coatings [33]. The addition of copper salt (0.01 - 0.5 g 1-1) has been reported to reduce stress in sulphamate baths [34]. Other additives are generally not necessary to improve the brightness of the deposit since the pure solution itself gives attractive white deposits. Sodium lauryl sulphate has been used as a wetting agent to remove pitting [24]. The presence of aluminium chloride and chromic chloride helps in the deposition of bright deposits [35] and the presence of chloride ions gives fine crystalline deposits [36].
4. Effect o f impurities The structure and colour of the deposit should be of very high quality in rhodium plating. However, rhodium plating is very sensitive to the presence of impurities which change the colour of the coating. A detailed study has been carried out by Parker [24]. Alkali metal, alkaline earth and ammonium salts have an unfavourable effect as in larg e concentrations they may impart a yellow colour to the deposit [37]. In contrast, traces of calcium ions improve the colour of the deposit. The presence of ammonium ions decreases the cathode efficiency at room temperature. Nickel, iron, copper and beryllium are not deleterious even at concentrations as high as 1 g 1-1. However, Parker [24] has reported t h a t copper concentrations above 0.005 g l- 1 give milky matt deposits from 4 g 1-1 solutions. Excessive concentrations produce copper-coloured deposits with severe cracking [38]. Very small concentrations (0.01 g 1-1) of bismuth and silver have a beneficial effect on the brightness of the deposit, but at higher concentrations dark, streaky and eventually spongy deposits are produced. Similar effects are obtained with lead and zinc. Zinc reduces the reflectivity at concentrations of 0.1 g 1-1 [39], while mercury produces dark matt deposits at concentrations above 0.1 g 1-1 [40]. The presence of mercury leads to flaking of the deposits and a black colouration [41]. Reports on the effect of tin vary from author to author. Reid has found a drop in efficiency at concentrations above 0.1 g 1-1. Parker has reported" that deposits are speckled at concentrations above 0.004 g 1-1 and become rough above 0.02 g 1-1. Leister and Benham [23] have found no effects at concentrations above 0.1 g l- 1. The addition of lead has been reported to improve the brightness of the deposit
357 and to have a beneficial effect on the limiting thickness [42]. Chloride ions have a marked effect on the character of the deposits from the 10 g l-~ sulphate solution. They become increasingly matt and rough at concentrations from 0.1 to 1.0 g 1-1 and the internal stress is reduced [38]. The presence of ferrocyanide ions produces a fall in cathode efficiency. The general effect of organic materials [41] in dilute baths is to cause darkening of the deposits and to reduce the efficiency of deposition. Small amounts of phenol and related compounds can lead to an increase in the brightness of the deposits. Organic contamination produces small circular pits in the deposits and at higher concentrations the coatings crack and develop a scaly appearance [14]. Therefore care should be taken in the selection of tank linings [43]. Under some conditions coloured deposits similar to cuprous oxide and lead peroxide are obtained which may be due to the deposition of oxides. Rhodium is also oxidized to some extent at the anode to the quadrivalent or hexavalent state (present as rhodate) whose presence appears to assist in the formation of smooth deposits. An increase in the rhodate may render the bath unfit for further use, and this is always accompanied by a change in bath colour from red brown to green black. Rhodium baths which have become inoperative owing to the presence of metal impurities can be purified by the use of ethylenediaminetetraacetic acid (EDTA). At an EDTA concentration of 0.25- 0.5 g 1-1 there is an improvement in the brightness of solutions containing tin, lead, silver, cadmium and nickel. However, there is a general lowering of cathode efficiency and hence this procedure cannot be adopted for thick rhodium plating. Brenner and Olson [39] have proposed a method for the removal of zinc, cadmium, mercury, lead, iron, cobalt and nickel by precipitation with ferrocyanide. The excess ferrocyanide is removed after filtration by the addition of ferric sulphate. This procedure requires the utmost care in view of the possible adverse effects of residual ferrocyanide in the solution. Chloride can be removed by titration with silver sulphate and silver by precipitation as chloride. Treatment with activated charcoal or alumina is recommended for organic impurities.
5. Plating techniques Barrel plating formulations have been suggested for plating small components. Parker [24] has suggested the composition given in Table 1 for barrel plating with sulphate baths. The current should be on whe~ an immersed barrel enters the solution and a higher strike current is used to improve the coverage. Immersion treatment can also be used for rhodium plating; a solution containing 5 g l- 1 rhodium and 250 ml l- I concentrated HC1 is used. The cleaned parts are immersed in the solution without agitation at 25 °C. A coating 30 pm thick is deposited in 10 min. The plates should be sealed after plating by gold plating, immersion in concentrated NH4OH for 5 min or by
358 immersion in boiling water for 15 min. Application of the technique of brush plating (the Dalik process or tampon peeling) to the plating of rhodium has been mentioned in the literature [44]. TABLE 1
Rhodium (g 1-l) H2SO4 (ml 1-1) Current density (Aft -2, ,~mA cm-2) Temperature (°C)
For deposits up to 5 x 1 0 -5 in
For deposits thicker than 5 x 1 0 -5 in
1 25 10 - 30 45
2 25 10 - 20 45
6. Applications of rhodium plating Rhodium plating is used [45] for electrical contacts, especially sliding contacts which are required to operate reliably after periods of idleness. Numerous types of slip rings and switches used in radio communication [46] and radar control gear are rhodium plated. Tests have been reported on rhodium-plated slip rings for the transmission of electroencephalograph signals [47]. High speed computer switching is another field of application. In many cases this involves printed copper switch patterns on Formica, Bakelite or Melamine laminates [48]. Rhodium plating is used solely for wear resistance in small magnetic clutches [49]. Rhodium has also been used as a diffusion barrier layer between gold and copper [50] and between silver plate and Ni-Fe alloy [51]. Gold-plated connectors are rhodium plated to provide better wear resistance [52]. Copper anodes for thermionic valves, which are gold plated to protect them against oxidation during processing and sealing into their envelopes, are plated with a thin layer of rhodium or nickel to prevent diffusion of gold into the copper during the baking out process. A u - P t or Pt-Rh spinnerets for sealed reed switches are coated with rhodium to improve their surface hardness [53]. Rhodium plating is used on titanium to reduce the contact resistance between connecting rods and graphite anodes [54], as a conducting track on tin or antimony oxide film resistors on glass [55] and to improve the life of iridium crucibles used for growing single crystals of calcium tungstate etc. by reducing the loss of iridium oxide at the high operating temperatures [56]. The use of electrodeposited rhodium in coaxial r.f. circuits has been discussed by Walter [57]. Rhodium plating of Au-Pd-plated phosphor bronze plugs has been reported to prevent bimetallic corrosion [58]. Heavy radio interference caused by a rotary converter has been completely eliminated by rhodium plating [59, 60].
359
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