Scale Formation of Alloy 602 CA During Isothermal Oxidation at 800–1100 °C in Different Types of Water Vapor Containing Atmospheres

Abstract

The oxidation behavior of the nickel base Alloy 602 CA in atmospheres, relevant to gas separation units in fossil fired power plants, was investigated in the temperature range 800 to 1100 °C. Isothermal oxidation kinetics were determined in Ar–7 %H2O, Ar–4 %H2–7 %H2O, Ar–20 %O2–7 %H2O and, for comparative purposes, in Ar–20 %O2. The alloy formed an external alumina scale during oxidation at 800 °C, regardless of the atmosphere. Increasing the temperature results in gradual replacement of the alumina scale by an external chromia layer and internal alumina precipitates. The chromia growth rate is affected by the gas composition, due to titanium incorporation in the scale and outwardly protruding metallic nodules. The external alumina scale formation was strongly affected by surface cold work during specimen grinding. This effect, which promoted external alumina scale formation, was retained during exposure at 800 °C but was rapidly lost at higher temperatures resulting in external chromia scale formation and internal oxidation of aluminum.