The Effects of pH and Temperature on Electrodeposition of Re-Ir-Ni Coatings from Aqueous Solutions

The Re-Ir system is of great interest for high-temperature applications and extreme environments. Here, Re-Ir-Ni alloy coatings were electroplated from aqueous solutions. The effects of pH and temperature on the faradaic efficiency (FE), chemical composition, surface morphology and crystallographic structure of the coatings were studied. The results show that the pH and temperature of electrolytes have a significant effect on the electrodeposition of Re-Ir-Ni. As the pH was increased from 2.0 to 8.0, the FE, partial current densities for deposition of the three metals, and deposition rate increased. The highest Re-content (82 at%) was obtained at pH = 2.5. At pH = 2.0, the coating consisted of an amorphous phase, and no cracking was observed. When raising the pH to 4.0 and above, crystalline phases (including hydrides) formed, and both columnar crystals and micro-cracks were observed. The bath temperature affected the surface crack density and the size of the grains that form the mesoscopic colonies. At the two lowest temperatures studied herein (50 and 60°C), no codeposition of Ir was observed. The phase composition of the coating changed from amorphous Re-Ni at 50°C; to amorphous Re-Ni, hcp-Ni and hexagonal Ni2H at 60°C; to hcp-Ir0.4Re0.6, hcp-Ni and hexagonal Ni2H at higher temperatures.