Platinum Metals Rev., 1968, 12, (1), 20
Platinum and Palladium Electrodeposits on Refractory Metals
Aqueous Electrolytes to Yield Thick Coatings
The platinum metals are well known for their resistance to oxidation and chemical corrosion over a wide range of temperatures, these properties being exhibited to a higher degree than for any other metal. However, economic considerations prohibit the total construction in these metals of large parts for arduous duty at high temperatures and it has been the practice to utilise the mechanical properties of the refractory metals and to protect the surfaces against oxidation by cladding with platinum or its alloys. It is difficult to construct complex parts in this manner, and thus there is a need for electro-deposition systems capable of yielding substantial thicknesses.
Platinum group metal plating baths have been in use for many years in the jewellery industry and more recently in the production of contact surfaces in electronic and communication equipment. Here, however, the coatings are thin (for jewellery less than 50 micro-inches and for contacts from 50 to 300 micro-inches) and the solutions in use generally give cracked deposits at thicknesses in excess of 500 micro-inches.
A recent study at the U.S. Bureau of Mines by S. D. Cramer, C. B. Kenahan, R. L. Andrews and D. Schlain (R.I. 7016, September 1967) has examined the use of the previously available proprietary electrolytes for deposition on refractory metals such as niobium, molybdenum, tantalum, titanium, tungsten, vanadium and zirconium at thicknesses in excess of 2000 micro-inches. In almost every case they were found to be inadequate in the sense of producing either non-adherent or heavily cracked plates. Three platinum and two palladium formulations were, however, found to be more successful, and these solutions are set out in the table. Deposits of 0.010 to 0.020 inch were produced, but the platinum electrolyte No. 2 was chemically unstable and was not finally recommended.
The authors point out that the pre-treatment of the refractory metal is of the utmost importance, total failures occurring when an inadequate procedure was employed. Indeed a considerable part of the report is concerned with this aspect of the problem, and a table of procedures found to be satisfactory is included. Thick, adherent deposits of platinum and palladium could be formed on some metals after a cathodic pre-treatment in an alkali cyanide fused salt mixture containing platinum ions.
Stress measurements of the deposits were taken in a number of instances, and electron probe studies on the effect of high temperature anneals on interdiffusion are reported; little diffusion of the deposit into the substrate took place but diffusion of substrate into deposit was extensive.
This investigation adds a great deal to our knowledge of heavy platinum metal deposition and is to be commended to those working in this field.