The Effect of Rhodium Addition on Degradation Behavior of Ir-10 mass％Rh Alloy during Oxidation at Elevated Temperatures

This paper focuses on effect of rhodium addition on degradation behavior of Ir-10 mass％Rh alloy for a spark plug during oxidation at elevated temperatures. Oxidation tests were conducted at the temperature range from 1173 K to 1473 K in Ar-21％O2 gas mixture to clarify the effect of rhodium addition on degradation behavior of the alloy. As a result, there are three effects of rhodium addition to prevent the alloy from the degradation: (1) dissolution of rhodium into iridium oxide scale at below 1273 K, which decreases activity of iridium oxide in the oxide scale, (2) dissolution of rhodium into the alloy at above 1273 K, which decreases activity of iridium in the alloy and (3) formation of rhodium oxide, which decreases surface area of the alloy. All effects contribute to decrease vapor pressure of volatile iridium oxide, resulting in suppression of the degradation.