The Electrodeposition of Platinum-Iridium Alloys

Summary

The paper describes the development of an aqueous electrolyte for the electrodeposition of platinum-iridium alloys from an acidic solution (pH 1·0 to 2·0) of platinum and iridium bromides. The initial work was concerned with establishing the conditions for depositing a platinum-30% iridium alloy on to an etched titanium substrate. The preferred solution for this process contains 5 g/1 total platinum metal in the ratio 70 Pt: 30 Ir. With this electrolyte the most convenient parameter for controlling the alloy composition is temperature; using an electrolyte of the above composition a range of platinum-iridium alloys, of controlled composition, has been produced. The deposits obtained are bright, coherent and adherent with a surface hardness in the range 200 to 250 D P N. Deposits may readily be obtained on a variety of base metals including etched titanium, nickel and gold-flashed copper. With the latter substrate deposits containing 4% and 30% iridium were cracked at thicknesses greater than 1μ, but deposits containing 10% iridium were uncracked at 5μ. Stress measurements on these latter coatings gave a value of 5 tons/in². A survey of 5 g/1 electrolytes of composition from 10 to 90% iridium showed that alloys containing up to 60% iridium could be deposited from the solutions of high iridium content. A preliminary survey of electrolytes of higher total platinum-metal content showed that when these were operated at high current densities satisfactory alloy deposits could be obtained at much higher plating rates than are normally obtained in platinum-metal plating, for example 27μ/h at 8 A/dm² from a 30 g/1 solution.