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Bureau of Mines Information Circular/1984 
 
 Methods for Producing Platinum-Group 
 Metal Coatings From Molten 
 Alkali Cyanides 
 
 By Richard P. Walters and David R. Flinn 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 8974 
 
 fi 
 
 Methods for Producing Platinum-Group 
 Metal Coatings From Molten 
 Alkali Cyanides 
 
 By Richard P. Walters and David R. Flinn 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 William P. Clark, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 

 Library of Congress Cataloging in Publication Data: 
 
 Walters, R. P. (Richard P.) 
 
 Methods for producing platinum-group metal coatings from molten 
 alkali cyanides. 
 
 (Bureau of Mines information circular ; 8974) 
 
 Bibliography: p. 15-17. 
 
 Supt. of Docs, no.: I 28.27:8974. 
 
 1. Electroplating. 2. Platinum group. 3. Fused salts. 4. Cya- 
 nides. I. Flinn, David R. II. Title. III. Series: Information circu- 
 lar (United States. Bureau of Mines) ; 8974. 
 
 T&295.U4 [TS692.P56] 622s [673'. 7] 84-600022 
 
\ CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Historical perspective 2 
 
 Scope of report 3 
 
 Electrolyte preparation 3 
 
 Drying and fusing of salts 3 
 
 Platinum-group metal addition 4 
 
 Role of oxygen 5 
 
 Electrodeposition 5 
 
 Substrate surface preparation 5 
 
 Molybdenum-base alloys 5 
 
 Iron-base alloys 6 
 
 Electroplating 6 
 
 Platinum 7 
 
 Rhodium 7 
 
 Platinum-rhodium alloys 9 
 
 Palladium 10 
 
 Iridium and ruthenium 11 
 
 Substrate effects 12 
 
 Incomplete preparation 12 
 
 Microstructure 13 
 
 Summary 15 
 
 References 15 
 
 ILLUSTRATIONS 
 
 1. Cell for electrolytic addition of platinum ions to the molten cyanide 
 
 salts 4 
 
 2. Cross-sectional microstructure of a direct-current-deposited platinum 
 
 coating 8 
 
 3. Surface of a direct-current-deposited platinum coating on an Fe-lOCr 
 
 substrate 8 
 
 4. Cross-sectional microstructure of a pulse-plated platinum coating 9 
 
 5. Surface of a direct-current-deposited palladium coating on an Inconel 
 
 substrate 10 
 
 6. Surface of a direct-current-deposited iridium coating on an Fe-5Cr 
 
 substrate 11 
 
 7. Surface of a direct-current-deposited platinum coating on an incompletely 
 
 degreased molybdenum substrate 12 
 
 8. Enlargement of individual pore 13 
 
 9. Surface of a thin platinum coating on a fully annealed iron substrate 14 
 
 10. Surface of a platinum coating on a graphite-graphite composite material... 14 
 
 \6 
 

 UNIT OF MEASURE ABBREVIATIONS USED IN 
 
 THIS REPORT 
 
 °c 
 
 degree Celsius 
 
 mol pet 
 
 mol percent 
 
 g 
 
 gram 
 
 MPa 
 
 megapascal 
 
 h 
 
 hour 
 
 ms 
 
 millisecond 
 
 Hz ac 
 
 hertz alternating current 
 
 mV 
 
 millivolt 
 
 kg/mm 2 
 
 kilogram per square 
 millimeter 
 
 mV/s 
 
 millivolt per second 
 
 
 
 pet 
 
 percent 
 
 lb/in 2 
 
 pound per square inch 
 
 
 
 
 
 ppm 
 
 part per million 
 
 mA 
 
 milliampere 
 
 
 
 
 
 V 
 
 volt 
 
 mA/cm 2 
 
 milliampere per square 
 
 
 
 
 centimeter 
 
 vol pet 
 
 volume percent 
 
 min 
 
 minute 
 
 wt pet 
 
 weight percent 
 
 ym/min 
 
 micrometer per minute 
 
 
 
METHODS FOR PRODUCING PLATINUM-GROUP METAL COATINGS 
 FROM MOLTEN ALKALI CYANIDES 
 
 By Richard P. Walters 1 and David R. Flinn 2 
 
 ABSTRACT 
 
 This Bureau of Mines report reviews and assesses the technology for 
 preparing electrodeposited coatings of the platinum-group metals from 
 molten alkali metal cyanide baths. Methods are described for the prep- 
 aration and purification of the cyanide electrolyte. Vacuum drying, 
 fusion, and purity of the cyanide salts are discussed. Use of a sodium 
 borosilicate test tube as a sodium-ion-conducting membrane for the ad- 
 dition of the desired platinum-group metal is described. The optimum 
 plating parameters of potential and current density for Pt , Pd, Rh, Ru, 
 and Ir coatings that result in the best deposits are given. A method 
 for producing platinum-rhodium alloy coatings of predetermined composi- 
 tion is also described. The importance of careful substrate surface 
 preparation is illustrated. While problems still remain with respect 
 to coating porosity, careful application of the methods described per- 
 mits the preparation of adherent, coherent electrodeposited coatings 
 of the platinum-group metals. 
 
 1 Research chemist. 
 Supervisory research chemist. 
 Avondale Research Center, Bureau of Mines, Avondale, MD . 
 
INTRODUCTION 
 
 The Bureau of Mines has evaluated the 
 substitution of platinum-group metal 
 coatings for bulk platinum-group metal 
 objects as a means of reducing the con- 
 sumption of the platinum-group metals. 
 The Bureau has conducted several studies 
 of the electrodeposition of the platinum- 
 group metals from molten alkali metal 
 cyanide baths during the last two dec- 
 ades. The major incentive for this work 
 has been the need to protect materials 
 from increasingly hostile environments 
 imposed by modern technology. To with- 
 stand high-temperature environments, 
 structural materials must possess high- 
 temperature strength as well as resist- 
 ance to oxidation and corrosion. Refrac- 
 tory metals such as molybdenum, tungsten, 
 and columbium, and alloys of these metals 
 that have the required high-temperature 
 strength, are readily oxidized. Protec- 
 tion of the refractory metals could be 
 accomplished by coating them with a suit- 
 able platinum-group metal, and a compos- 
 ite material with highly desirable prop- 
 erties would result. Bureau research has 
 focused on producing platinum-group metal 
 coatings on refractory metals as well as 
 on more common materials of construction. 
 This research has shown that high-quality 
 deposits which are thick, adherent, and 
 coherent can be prepared of each of the 
 platinum-group metals, as well as select 
 binary alloys of these metals. 
 
 HISTORICAL PERSPECTIVE 
 
 In 1937, Atkinson (1)^ obtained a pa- 
 tent describing the use of a molten cya- 
 nide electrolyte for stripping and rede- 
 positing platinum-group metal coatings 
 from and onto base metal substrates. At- 
 kinson used an inert atmosphere of nitro- 
 gen over the molten bath; however, he 
 did not describe his apparatus nor the 
 results of his experiments in detail. 
 Twenty years later, Withers and Ritt pre- 
 sented a paper on the deposition of irid- 
 ium from molten cyanide electrolytes (2^) 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 and obtained a patent on this process in 
 1960 (3). The plating bath used (2-3) 
 was a 70 wt pet sodium cyanide-30 wt pet 
 potassium cyanide electrolyte at 500° C 
 under an argon atmosphere. Iridium was 
 added to the melt by passing 60 Hz ac 
 at 10 mA/cm 2 between two iridium elec- 
 trodes. In 1962, Rhoda (4^) described the 
 electrodeposits of several platinum-group 
 metals obtained from either pure sodium 
 cyanide or an eutectic mixture of 53 wt 
 pet sodium cyanide and 47 wt pet potas- 
 sium cyanide. A flow of argon gas around 
 the crucible was used to reduce the lev- 
 els of moisture and oxygen over the bath. 
 Rhoda' s study was the most extensive ear- 
 ly work on platinum-group metals electro- 
 deposition from the molten salt. Coat- 
 ings of iridium, platinum, and ruthenium 
 were obtained, but he was unsuccessful in 
 depositing palladium and rhodium. 
 
 Beginning in 1967, the Bureau conducted 
 research on the preparation of platinum- 
 group metal coatings (5-16). Initially, 
 Schlain and coworkers from 1967 to 1977 
 (5-9) investigated plating from several 
 cyanide and cyanide-cyanate or cyanide- 
 carbonate melts. They were successful in 
 designing a plating system that excluded 
 oxygen from the molten electrolytes and, 
 based upon results obtained from these 
 experiments , made recommendations as to 
 the necessity of including or excluding 
 oxygen. Schlain and coworkers were suc- 
 cessful in depositing thick coatings of 
 platinum, palladium, iridium, and ruthe- 
 nium. Platinum and iridium objects were 
 also electroformed using copper or molyb- 
 denum mandrels. A summary of the litera- 
 ture on electrodeposition from molten 
 cyanide melts up to 1977 appears in an 
 article by Harding (17) . 
 
 Starting in 1976, articles were pub- 
 lished by several workers from South 
 Africa who used the molten cyanide salt 
 system to separate platinum-group metals 
 by solvent extraction (18-25) . These 
 articles have reported on the purifica- 
 tion of the molten salts (18) , the chemi- 
 cal state of the platinum-group metals 
 in both the molten salt (19) and the 
 
quenched salt (20-21) , redox extraction 
 from the molten salt (22-23) , and the cy- 
 clic voltammetry of the platinum-group 
 metals in the molten salt (24-25) . Al- 
 though these publications do not discuss 
 deposition techniques, they are important 
 because they contain information on the 
 observed chemical states of the platinum- 
 group metals within the molten cyanide 
 system. 
 
 and McBurney reported on studies of plat- 
 inum electrodeposition from molten cya- 
 nide melts onto nickel and nickel super- 
 alloy substrates (28). 
 
 Only one company is known to presently 
 use the molten salt electrodeposition 
 process commercially. Several publica- 
 tions that describe the process have ap- 
 peared since 1975 (29-31). 
 
 Harding (26-27) reported attempts to 
 produce a platinum-iridium alloy coating, 
 but was unable to produce a sound alloy 
 deposit of predetermined composition. 
 The first successful method of producing 
 a platinum-rhodium alloy coating wherein 
 the composition could be precisely varied 
 was devised by Flinn and Manger ( 11) . 
 
 Recent Bureau studies have resolved the 
 ambiguity of the effect of oxygen through 
 the use of electrochemical techniques 
 (10, 12), determined the physical proper- 
 ties of platinum coatings (14-15) , and 
 examined the effect of current reversal 
 and pulse plating on the prorsity of 
 platinum coatings (16). In 1980 Sethi 
 
 SCOPE OF REPORT 
 
 This Bureau report describes the pro- 
 cedures involved in, and the expected 
 results of, using molten cyanide electro- 
 lytes for electrodepositing the platinum- 
 group metals. The methods used by the 
 Bureau to produce the platinum-group met- 
 al coatings have changed over the years 
 as understanding of the process has im- 
 proved. This report describes the prac- 
 tical steps Bureau researchers have found 
 necessary in both electrolyte and sub- 
 strate surface preparation to insure that 
 a high-quality deposit is obtained. Typ- 
 ical coating morphologies are also shown. 
 
 ELECTROLYTE PREPARATION 
 
 Many variations on the composition and 
 purity of molten alkali cyanides have 
 been used, from sodium cyanide to mix- 
 tures of sodium and potassium cyanide. 
 Recent research at the Bureau has been 
 standardized on a 50-50 mol pet mixture, 
 and the discussion of electrolyte prepa- 
 ration is focused on this mixture. The 
 reader should, however, be aware that no 
 evidence exists to demonstrate an advan- 
 tage of any particular combination of 
 sodium and potassium cyanide in terms of 
 deposit quality. In general, a higher 
 operating temperature for the plating 
 bath permits higher plating rates because 
 of improved mass transport, but it also 
 increases the thermal decomposition of 
 the plating bath, resulting in precipita- 
 tion of the dissolved platinum-group 
 metal(s) . 
 
 DRYING AND FUSING OF SALTS 
 
 Reagent-grade sodium and potassium cya- 
 nides are initially dried in a vacuum 
 
 furnace at 250° C for 48 h. A liquid 
 nitrogen cold trap is used to prevent 
 back diffusion of oil vapors from the 
 vacuum pump. Equimolar mixtures of the 
 dried sodium and potassium cyanides are 
 fused in air until the melt is clear and 
 free of suspended solids, usually requir- 
 ing 1 h at 515° to 600° C. The molten 
 salt is decanted into a fused-silica tray 
 to produce a thin solid sheet of salt. 
 The salt and tray are then immediately 
 placed into a helium-filled environment 
 chamber containing less than 1 ppm 2 and 
 H 2 (_10, 12). 
 
 A procedure has been reported ( 18 ) for 
 the purification of the alkali cyanide, 
 involving the use of silicon to remove 
 carbonates, that is claimed to result in 
 virtually zero electrochemically and 
 spectrochemically active impurities. Im- 
 purities such as carbonates and cyanates 
 can also be removed by electrochemical 
 oxidation (12) . However, deposits ob- 
 tained from platinum baths prepared by 
 
vacuum drying of the salts, as described, 
 were not different in quality from depos- 
 its obtained from melts that were silicon 
 treated or electrochemically oxidized. 
 We therefore infer that the small, resid- 
 ual levels of cyanate, carbonate, and 
 carbon impurities have little or no ef- 
 fect on the deposit quality. A higher 
 purity level is required to produce a 
 more stable electrolyte for palladium 
 plating, although this greater purity 
 does not entirely prevent precipitation 
 of dissolved palladium ( 12 ) . 
 
 PLATINUM-GROUP METAL ADDITION 
 
 Typically, 100 to 150 g of the equimo- 
 lar mixture is placed into a high-purity 
 AI2O3 crucible and heated to the desired 
 melt temperature, normally 570° C. All 
 electrolyte preparations are accomplished 
 in a helium-filled environmental cham- 
 ber described in RI 8656 (12) . Platinum- 
 group metals are added to the electrolyte 
 by electrolytic (anodic) dissolution of 
 the platinum-group metal of interest. 
 The example for platinum addition is 
 shown schematically at the bottom of the 
 page. 
 
 As shown in figure 1, a sodium-ion- 
 conducting membrane is used to separate 
 the cathode and anode compartments . Oxi- 
 dation products formed at the anode, 
 Pt +2 , are free to diffuse into the melt, 
 while reduction products , Na° and/or K° , 
 are isolated within the test tube used as 
 a cathode compartment. 
 
 In initial studies, the platinum-group 
 metals were added to the electrolyte un- 
 der potentiostatic control of the anode. 
 The reference electrode ( 12 ) is comprised 
 of a small, closed silica or borosili- 
 cate glass tube filled with silver chlo- 
 ride containing 5 mol pet of sodium chlo- 
 ride. A silver wire contacts the AgCl- 
 NaCl mixture. This tube is dipped into 
 the molten plating bath when coatings are 
 being electrodeposited. Although this 
 reference electrode is not useful for 
 thermodynamic measurements because of the 
 
 Graphite cathode 
 
 Platinum anode 
 
 FIGURE 1. - Cell for electrolytic addition of 
 platinum ions to the molten cyanide salts. 
 
 unknown differences in ion activities 
 across the glass separator, this refer- 
 ence is very stable and reproducible. It 
 was determined that, as long as the po- 
 tential at the anode did not become more 
 positive than approximately -1.1 V versus 
 the reference electrode, cyanogen (NCCN) 
 generation did not occur (12) . Dissolu- 
 tion currents of at least 100 mA/cm 2 were 
 readily attained for the platinum-group 
 metals studied (Ir, Pd, Pt , Ru, Rh) with- 
 out producing the undesirable cyanogen 
 product (12). 
 
 Cathode overpotentials are of no con- 
 cern during the addition of the platinum 
 metal because that process is not limited 
 by alkali metal concentration and the 
 cathodic reaction products are contained 
 in the sodium borosilicate test tube, 
 which is discarded. The practical limi- 
 tation on anodic dissolution current is 
 the amount of undercutting that occurs 
 on the platinum metal electrode at high 
 current density. Because the undercut- 
 ting results in a physical loss of the 
 platinum-group metal from the anode by 
 flaking, an anodic current density of 
 25 mA/cm 2 or less is normally used during 
 the addition of the platinum-group metal. 
 
 Pt 
 (anode) 
 
 Pt +2 in NaCN-KCN 
 
 Ion-conduct ing 
 glass 
 
 NaCN-KCN 
 Na° or K c 
 
 Graphite 
 (cathode) 
 
With no flaking, anode dissolution cur- 
 rent efficiencies were near 100 pet for 
 platinum and rhodium anodes based upon 
 the formation of Pt +2 and Rh + ' . For spe- 
 cific information on the preparation of 
 platinum, palladium, iridium, and rhodium 
 plating baths, the reader is referred to 
 RI 8656 (12). 
 
 After the current-voltage relationships 
 of the anodic dissolution process were 
 understood, the desired platinum-group 
 metal was added to the plating bath using 
 the configuration shown in figure 1. 
 Current density was controlled rather 
 than the anode potential, as had initial- 
 ly been done. Because of power supply 
 voltage limitations, initial values of 
 dissolution current were 150 to 200 mA, 
 which slowly decayed as the glass conduc- 
 tivity decreased because of replacement 
 of some of the sodium with less mobile 
 potassium in the silicate structure (12). 
 Glass test tubes, which served as the 
 sodium-ion-conducting membranes, were 
 changed after the current fell below 80 
 mA. The dissolution current limit during 
 this procedure is mostly determined by 
 the potential drop across the glass when 
 the current is applied. 
 
 ROLE OF OXYGEN 
 
 A major consideration in the use of the 
 molten cyanide system is the effect of 
 oxygen and the necessity to exclude oxy- 
 gen from the atmosphere of the plating 
 system. The cyanide ion (CN~) reacts 
 with oxygen at temperatures exceeding 
 200° C and with carbon dioxide at room 
 temperature ( 18) . This requires that the 
 molten cyanide be handled in an inert 
 
 atmosphere in order to prevent decomposi- 
 tion of the cyanide. The major impuri- 
 ties formed are cyanate (0CN~), from the 
 reaction between molten cyanide and oxy- 
 gen, and carbonate, from the reaction of 
 carbon dioxide and cyanide (18). 
 
 As well as directly affecting the com- 
 position of the molten cyanide bath, the 
 presence of an oxygen-containing atmos- 
 phere affects the ability to maintain 
 soluble platinum-group metal species in 
 the plating bath. It was originally be- 
 lieved that an oxygen atmosphere was nec- 
 essary in order to prepare a platinum or 
 palladium melt (13) . However, recent 
 work has shown that the presence of oxy- 
 gen was required only to oxidize the 
 cathode reduction products, Na° and K° , 
 in order to prevent these metals from re- 
 acting with the soluble platinum or pal- 
 ladium and precipitating it from the melt 
 (12) . Use of the test tube cathode com- 
 partment to isolate the reduction prod- 
 ucts during the electrolytic addition of 
 platinum or palladium solves this problem 
 and allows the melts to be prepared in an 
 inert atmosphere. 
 
 Iridium, rhodium, and ruthenium melts 
 cannot be prepared in the presence of 
 oxygen owing to the rapid precipitation 
 of the platinum-group metal (12-13). In 
 contrast, platinum-containing melts were 
 found to be still usable after more than 
 115 h of electrodeposition in air (13) , 
 although large amounts of insoluble car- 
 bonates had precipitated. In a helium 
 atmosphere no limit to the lifetime of 
 platinum-group-metal-containing melts has 
 been found. 
 
 ELECTRODEPOSITION 
 
 Substrate surface preparation tech- 
 niques and coating deposition parameters 
 are important factors in the preparation 
 of protective coatings. Proper substrate 
 surface preparation is essential for ob- 
 taining a coherent and adherent coating. 
 Molybdenum- and iron-base alloys have 
 been the principal substrates studied 
 because of their desirable mechanical and 
 high-temperature properties. 
 
 SUBSTRATE SURFACE PREPARATION 
 
 Molybdenum-Base Alloys 
 
 Although coatings have been produced 
 on pure molybdenum substrates, the vast 
 majority of work has focused on the alloy 
 TZM (0.41 pet Ti, 0.10 pet Zr, bal. Mo). 
 Substrates are ground initially to a 
 600-grit finish and then ultrasonically 
 
degreased with trichloroethylene. A po- 
 tassium ferricyanide solution (3 wt pet 
 K 3 Fe(CN) 6 , 0.5 wt pet NaOH, bal. H 2 0) is 
 then used to etch the surface. Approxi- 
 mately 30 min of ultrasonic etching pro- 
 duces a lightly etched surface, which is 
 rinsed with water and then 10 vol pet 
 HC1. The desired surface has a uniform 
 mat gray appearance with no discolored 
 areas. If any discoloration occurs, the 
 samples are reground and reetched. Often 
 the 10 pet HC1 does not completely remove 
 the film formed during etching. In such 
 a case the sample is dipped briefly into 
 a fresh etching solution prior to rerins- 
 ing with water and the 10 pet HC1. This 
 treatment is successful in removing the 
 film. Etching solutions are used once 
 and then discarded. Immediately prior to 
 electrodeposition the samples are again 
 ultrasonically degreased in trichloro- 
 ethylene for 10 min. The extensive sur- 
 face degreasing was found to be essential 
 for producing a coherent coating. 
 
 Iron-Base Alloys 
 
 Adherent coatings have been produced on 
 both ferritic and austenitic iron-base 
 alloys. Sample surfaces are initially 
 ground to a 600-grit finish. The samples 
 are then stress-relieved for 2 h at 
 the appropriate temperature (600° C for 
 FelOCr), quenched in water, and pickled 
 in 20 pet sulfuric acid. The final sur- 
 face finish is obtained by electropolish- 
 ing in 10 vol pet perchloric acid (70 pet 
 acid)-90 vol pet glacial acetic acid un- 
 til a mirror finish is produced. As with 
 the molybdenum samples, the iron-base 
 alloys are ultrasonically degreased in 
 trichloroethylene for 10 min immediately 
 prior to electrodeposition. It is not 
 absolutely necessary that an electropol- 
 ished surface be used; in fact, ground 
 and etched surfaces are also successfully 
 plated. If a ground and etched surface 
 is preferred in order to reduce surface 
 preparation time, the most important con- 
 siderations are that the surface is 
 (1) properly etched and (2) completely 
 degreased. 
 
 ELECTROPLATING 
 
 Electrodeposition was performed with 
 a three-electrode configuration to per- 
 mit potentiostatic control of the work- 
 ing electrode (cathode) during deposition 
 of the platinum-group metal. Potential- 
 controlled electrodeposition was neces- 
 sary to prevent codeposition of the al- 
 kali metals. Reduction potentials, the 
 reference electrode, and the equipment 
 used are described in detail elsewhere 
 (12). 
 
 For materials that do not exhibit an 
 open-circuit corrosion reaction, such as 
 molybdenum, the sample can be placed di- 
 rectly into the molten salt and the depo- 
 sition potential applied. However, mate- 
 rials such as iron-base alloys exhibit a 
 significant corrosion reaction at open 
 circuit and require special techniques to 
 prevent cementation reactions from oc- 
 curring (12). After studying the open- 
 circuit corrosion potentials of iron- 
 chromium alloys, it was determined that 
 an additional 100 mV of cathodic polari- 
 zation beyond the deposition potential 
 was required to prevent cementation reac- 
 tions. To prevent these reactions from 
 occurring when the sample is initially 
 placed into the molten salt, the poten- 
 tial is applied prior to immersing the 
 sample in the plating bath. The deposi- 
 tion potential can be adjusted to the 
 desired plating potential once the sample 
 has a coherent coating on the surface. 
 
 The high operating temperature of the 
 molten salt plating system does not ad- 
 versely affect the purity of the deposit 
 as a result of substrate diffusion into 
 the coating, with one notable exception. 
 Copper or copper alloy substrates such 
 as Monel were found to diffuse copper 
 throughout the entire coating cross sec- 
 tion during the plating process. 
 
 In the following sections, the operat- 
 ing parameters and some of the micro- 
 structures obtained are discussed. The 
 effects of various plating techniques 
 
on the porosity of platinum coatings are 
 also summarized. 
 
 Platinum 
 
 The reduction potential of Pt +2 (1 wt 
 pet in the equimolar NaCN-KCN bath) 
 to Pt° is approximately -1.7 V with re- 
 spect to the Ag-AgCl reference electrode 
 used at the Bureau ( 1_2 ) . Slow (2 mV/s) 
 cathodic polarization scans on a vitre- 
 ous carbon electrode showed that the 
 diffusion-limited current for a melt con- 
 taining 1 wt pet Pt +2 is between 12 and 
 15 mA/cm 2 at 580° C. The scans also ex- 
 hibited an oxidation wave with a magni- 
 tude that varied from melt to melt. 
 Although no study was made of the oxida- 
 tion wave, some similarities exist be- 
 tween this behavior and that observed in 
 a palladium-containing melt ( J_2 ) . This 
 oxidation wave did not have a detrimental 
 effect on deposit quality, regardless of 
 its magnitude. 
 
 Direct current deposits were porous 
 regardless of the plating current den- 
 sity. However, the least porous and best 
 quality deposit was obtained at one-half 
 the diffusion-limited current ( 16) . At 
 one-half the diffusion-limited current 
 for the l-wt-pct-Pt +2 bath, the plating 
 rate is 0.2 um/min. Without agitation, 
 deposition current efficiencies were near 
 100 pet for cathode current densities in 
 the range of one-fifth to complete diffu- 
 sion control. A typical platinum coating 
 cross-sectional microstructure for a di- 
 rect current deposit is shown in figure 
 2, where the average grain size increases 
 with coating thickness. Scanning elec- 
 tron microscope examinations of the coat- 
 ing surface reveal the highly crystalline 
 nature of the coatings (fig. 3). 
 
 Studies performed on the porosity of 
 the platinum coatings (1_6) indicate that 
 samples plated under pulse current and 
 current reversal (periodic reversal) 
 electrolysis had reduced porosity. Sam- 
 ples that were pulse-plated using a peak 
 current density of 25.4 mA/cm 2 , an "on" 
 time of 0.1 ms , and an "off" time of 0.67 
 ms produced the minimum porosity coating. 
 
 A combination of pulse plating and cur- 
 rent reversal could perhaps produce a 
 truly pore-free coating; however, this 
 was not investigated. Pulse plating also 
 produced an almost constant grain size 
 microstructure (fig. 4). The constant 
 grain size region, approximately 17 ym in 
 width, is visible between the dark bands. 
 The dark bands parallel to the substrate 
 are produced while developing the micro- 
 structure by preferential etching of lay- 
 ers deposited when the plating parameters 
 were altered. For a more detailed dis- 
 cussion of the banding effect see RI 8829 
 (16). The major disadvantage to the use 
 of pulse plating was that the average 
 current density was only 3.3 mA/cm 2 , 
 resulting in plating rates of only 0.09 
 ym/min. 
 
 Physical properties of the direct- 
 current platinum deposits have been mea- 
 sured (14) . The measured fracture or 
 adhesion strength was 331 MPa (48,000 lb/ 
 in 2 ), and the cross-sectional microhard- 
 ness, which was found to be dependent on 
 microstructure, exhibited a minimum value 
 of HK 25 = 52 kg/mm 2 . 
 
 Rhodium 
 
 Rhodium reduction occurs at -2.0 V with 
 respect to the Ag-AgCl reference elec- 
 trode (12) . Deposition should be per- 
 formed between -2.2 and -2.0 V to avoid 
 incorporation of alkali metal into the 
 coating at potentials more negative than 
 -2.2 V. Rhodium dissolves into the melt 
 electrolytically as Rh + 1 (1_2, 19-20). 
 Overpotentials on the anode should be 
 minimized by increasing the anode-to- 
 cathode surface area ratio to prevent 
 reduction in current efficiency due to 
 the formation of Rh +3 . Diffusion cur- 
 rents for a 1-wt-pct-Rh melt are approxi- 
 mately the same as those obtained for a 
 platinum-containing melt. Cathode cur- 
 rent efficiencies for deposition without 
 use of agitation are near 100 pet. Plat- 
 ing rates, surface topography, and struc- 
 ture are also equivalent to those ob- 
 tained for the platinum electrodeposits. 
 No measurements were made on the physical 
 properties of the rhodium coatings. 
 

 FIGURE 2. - Cross-sectional microstructure of a direct-current-deposited platinum coating. 
 
 FIGURE 3. - Surface of a direct-current-deposited platinum coating on an Fe-lOCr substrate. 
 
FIGURE 4. - Cross-sectional microstructure of a pulse-plated platinum coating exhibiting constant 
 grain size as a function of distance from the substrate-coating interface. The dark bands parallel to 
 the substrate are produced while developing the microstructure by preferential etching of layers de- 
 posited when the plating parameters were altered. 
 
 Platinum-Rhodium Alloys 
 
 When the plating bath contains both 
 platinum and rhodium, an alloy can be de- 
 posited that contains platinum and rho- 
 dium in a weight ratio equivalent to that 
 of the metals in the molten salt (11). 
 Deposition potentials must exceed both 
 the platinum and rhodium reduction poten- 
 tials in order to incorporate rhodium 
 into the coating. A potential of -2.2 V 
 was used in producing the alloy coat- 
 ings. Although it may be possible to 
 vary the coating composition by reducing 
 
 the plating potential and, hence, the 
 relative rate of rhodium deposition, no 
 attempt was made to perform these ex- 
 periments. All alloy coatings were pro- 
 duced by adjusting the weight percent 
 of each metal ion in the molten salt. 
 This was accomplished by alternating the 
 anode materials during addition of the 
 platinum-group metal and dissolving the 
 appropriate amount of each. Again, plat- 
 ing rates, surface topography, and struc- 
 ture were equivalent to those obtained 
 for platinum electrodeposits at the 
 diffusion-limited current. 
 
10 
 
 Palladium 
 
 The diffusion-limited current for a 
 1-wt-pct-Pd melt is approximately 20 mA/ 
 cm 2 (12) . Figure 5 gives an example of 
 a palladium coating on an Inconel sub- 
 strate. No measurements have been made 
 on the physical properties of palladium 
 deposits. 
 
 Palladium-containing melts were the 
 most difficult to produce and maintain. 
 The problem appears to be related to the 
 reduction potential of palladium, -1.35 
 V. This potential is near the redox 
 potential for the cyanamide (NCN 2- )- 
 dicyanamide (N(CN) 2 ~) couple (12, 21): 
 
 i2- = 
 
 CN" + NCN^~ = N(CN) 2 " + 2e- 
 
 (1) 
 
 The cyanamide and dicyanamide ions can 
 be produced in the molten cyanide elec- 
 trolyte either by electrolytic oxidation 
 or by contacting the melt with oxygen for 
 extended periods. Cyanamide ion has been 
 shown to be capable of reducing Pd 2+ 
 to Pd°, and the dicyanamide ion was shown 
 to cause rapid dissolution of metallic 
 palladium in the molten cyanide electro- 
 lyte (12) . Regardless of the problem of 
 maintaining the palladium in the molten 
 salt in a form that is electrochemically 
 reducible to a coating, satisfactory 
 coatings can be deposited. In preparing 
 a melt for palladium addition, extreme 
 care must be used in purifying the molten 
 salts, and no melting should be done 
 in an oxygen-containing atmosphere. 
 Melts should be preelectrolyzed prior to 
 
 FIGURE 5. - Surface of a direct-current-deposited palladium coating on an Inconel substrate. 
 
11 
 
 palladium additions. Further purifica- 
 tion by the methods of Lessing ( 18 ) is 
 also recommended. 
 
 Iridium and Ruthenium 
 
 Iridium reduction occurs at -1.64 V 
 with a diffusion-limited current of 15 
 mA/cm 2 for a 1-wt-pct melt. Current ef- 
 ficiencies were found to be dependent on 
 anode current densities, with a maximum 
 of 80 pet for the cathodic current effi- 
 ciency (12). The variability in current 
 efficiencies is due to the existence of 
 several oxidation states of iridium with- 
 in the melt (J_2, 26-27). Depending on 
 the anode current density, the iridium 
 can dissolve in the +3 or the +1 state; 
 
 also, reactions at the anode could possi- 
 bly oxidize the +3 state to higher oxida- 
 tion states. The hardness of the irid- 
 ium coatings has been reported as HK 25 
 = 800 kg/mm2 (_4) and HK, 00 = 727 kg/mm 2 
 (5). Iridium deposits are extremely fine 
 grained (fig. 6) . 
 
 Deposits of ruthenium have been ob- 
 tained from the molten cyanide electro- 
 lyte (4, 13); however, no electrochemical 
 characterizations have been performed at 
 the Bureau. Recent work in South Africa 
 ( 25 ) has shown voltammograms for ruthe- 
 nium. The hardness of ruthenium coat- 
 ings is HK 25 = 610 to 935 kg/mm 2 accord- 
 ing to Rhoda (4). 
 
 FIGURE 6. - Surface of a direct-current-deposited iridium coating on an Fe-5Cr substrate. 
 
12 
 
 SUBSTRATE EFFECTS 
 
 Coatings must be deposited in a uni- 
 form manner over the substrate in order 
 to obtain the "best" deposit in terms 
 of adherence and coherence. If the sub- 
 strate has been improperly prepared, the 
 resulting coating will have character- 
 istic deficiencies that can be identi- 
 fied. The structure of the substrate 
 also affects the type of coating ob- 
 tained. Examples of the effect of sub- 
 strate structure and improper surface 
 preparation of the coating are presented 
 below. Although these examples should 
 help reduce the time spent in isolating 
 problems of poor coating structure or 
 performance, these are by no means the 
 only types of problems that can develop 
 
 in electrodepositing 
 metals. 
 
 the platinum-group 
 
 Incomplete Preparation 
 
 In addition to the obvious need to re- 
 move dirt and thick, oxide scales from the 
 substrates, degreasing is also a very 
 important step in substrate surface prep- 
 aration. When the surface is not com- 
 pletely clean, the platinum-group metal 
 can be inhibited from depositing the con- 
 tinuous initial layer. The coating even- 
 tually will bridge the area if it is rel- 
 atively small, but a pore or hole will 
 persist if the area is large. Neither of 
 these effects is desirable since they 
 result in loss of both adherence and co- 
 herence. Figure 7 shows a sample that 
 
 FIGURE 7, - Surface of a direct-current-deposited platinum coating on an incompletely degreased molybdenum 
 substrate exhibiting porosity due to lack of uniform deposition. 
 
13 
 
 has several areas that were not complete- 
 ly degreased. A closeup view (fig. 8) of 
 one of the pores produced by the incom- 
 plete cleaning shows that no coating has 
 deposited on the substrate at the base of 
 the pore. Figure 8 also shows the colum- 
 nar structure of the deposits discussed 
 earlier. 
 
 Micros t rue ture 
 
 The type of substrate and its structure 
 can affect the coating morphology. Plat- 
 inum electrodeposits prepared on a fully 
 annealed iron substrate exhibit patterns 
 
 similar to the grain structure of the 
 substrate, as shown in figure 9. Thick 
 deposits prepared on cold-rolled or an- 
 nealed substrates have the same morphol- 
 ogy, but it is only in thin deposits that 
 the grain pattern of the substrate is 
 mirrored. Whenever the substrate reduces 
 the ability of the coating to nucleate, 
 because of incomplete cleaning or type of 
 substrate, the coating will form at the 
 least resistant site and grow. An ex- 
 treme example of this is shown in figure 
 10 for a carbon-carbon composite material 
 that was plated with platinum. 
 
 FIGURE 8. - Enlargement of individual pore where base of pore has not been completely coated 
 owing to poor surface preparation. 
 
14 
 
 FIGURE 9. - Surface of a thin platinum coating on a fully annealed iron substrate following what 
 appears to be the grain pattern of the substrate. 
 
 FIGURE 10. - Surface of a platinum coating on a graphite-graphite composite material growing 
 in clusters due to inhibition of uniform nucleation. 
 
15 
 
 SUMMARY 
 
 The molten cyanide system is a practi- 
 cal medium for preparing coatings of the 
 platinum-group metals. Coatings of high 
 quality can be produced if care is taken 
 in all steps of the process from electro- 
 lyte preparation to sample surface prepa- 
 ration to the actual electrodeposition. 
 The single problem that remains to be 
 solved is the obtainment of zero porosity 
 in the coatings. Use of novel current 
 waveforms , such as combinations of pulse 
 and current reversal plating techniques, 
 could result in coatings having zero 
 porosity. 
 
 electrolyte to prepare coatings of the 
 platinum-group metals on a commercial 
 scale and that only platinum coatings 
 are currently available. To the best of 
 our knowledge, no company is using the 
 molten cyanide system to produce electro- 
 deposits of all the platinum-group met- 
 als. In the future perhaps the molten 
 cyanide system can be used for the recla- 
 mation of the platinum-group metals from 
 scrap in much the same fashion that it is 
 currently used in South Africa to sepa- 
 rate platinum-group metals by solvent 
 extraction (22-23). 
 
 Currently, it appears that only one 
 company has used the molten cyanide 
 
 REFERENCES 
 
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 2. Withers, J. C. , and P. E. Ritt. 
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 inum Metals 
 
 Technique 
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 Electrodeposition of Plat- 
 
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16 
 
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 Electrochemical Studies of 
 
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 21. De Haas, K. S., and K. F. Fouche. 
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 23. 
 
 Redox Extractions From 
 
 Molten Alkali-Metal Cyanides. Part II, 
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 20, 1977, 2024-2029. 
 
 24. Van der Kouwe, E. Th. , and A. Von 
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 27. 
 
 The Electrodeposition of 
 
 Platinum, Iridium, and Platinum-Iridium 
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17 
 
 29. Platinum Metals Review. Heavy 31. Simon, F. Electrodeposition of 
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