Sodium hexabromoiridate(III) for the electroplating of Ir–Ni and Ir–Re–Ni alloy coatings

Abstract

Ir–based alloy materials are of great interest for high temperature, extreme environment and special applications. In this work, Ir–Ni and Ir–Re–Ni alloy coatings were electroplated from aqueous solutions. As the choice of iridium (Ir) compound is crucial to obtain good Ir–based alloys by electrodeposition, sodium hexabromoiridate(III) was added as Ir salt in the electrolyte, in order to obtain high-quality Ir–based alloys with high Ir-content. The bath chemistry consisted of sodium hexabromoiridate(III), sodium perrhenate and nickel(II) sulfate heptahydrate as electroactive species, and citric acid as complexing agent. The effects of bath composition and plating time on the chemical composition, surface morphology, Faraday efficiency (FE), and crystallographic phases of Ir–Re–Ni alloy coatings were studied. The surface morphology, elemental composition percentage and crystallographic phases of the prepared coatings were analyzed with a scanning electron microscopy, X–ray energy dispersive spectroscopy and X–ray diffraction techniques, respectively. Ir–Re–Ni alloys with Ir-content as high as 31 at%, Re-content as high as 63 at%, and a FE as high as 100 % were obtained. The Ir-content in Ir–Ni and Ir–Re–Ni alloys from the bromo electrolyte system was higher than that from the chloride electrolyte system, although the (Re+Ir)-content in the ternary alloys was almost kept the same for two systems. The electrochemical deposition mechanisms for Ir–Ni and Ir–Re–Ni alloys deposited from the bromo electrolyte system were investigated.