Microstructure and Isothermal Oxidation of Ir-Rh Spark Plug Electrodes

High-temperature oxidation tests were performed on pure iridium, rhodium, and the iridium alloys, IrRh10, IrRh25, and IrRh40, at 1100 °C in a stable air environment for 60 h. The results of the oxidation were analyzed by X-ray photoelectron spectroscopy (XPS). Microstructural changes of the Ir-Rh alloys were characterized by scanning electron microscopy (SEM). XPS analysis results show that the main oxide of the Ir-Rh alloy in a 1100 °C environment was Rh₂O₃, and SEM analysis shows that the surfaces of the Ir-Rh alloys after oxidation formed both linear and ellipse-shaped corrosion pits, and had the same direction with the wire-drawing process. The oxidation behavior of Ir-Rh alloys, including the mass change, the reason for the mass loss, and the role of Rh in improving oxidation resistance performance, are discussed.