MIXED-OXIDANT ATTACK OF HIGH-TEMPERATURE ALLOYS IN CARBON-CONTAINING AND OXYGEN-CONTAINING ENVIRONMENTS

The corrosive degradation of high-temperature alloys in environments containing more than one oxidant cannot, in general, be predicted from a knowledge of the response of the materials to the individual oxidants. In the present study, the phenomenological changes associated with the degradation of iron-nickel-chromium base alloys in carbon-oxygen environments have been investigated by examining the microstructural changes in samples exposed to such environments for extended periods of time. The results of these studies have led to the formulation of a model which proposes that the material exposed to the reaction environment experiences five stages of microstructural changes close to the surface before severe degradation sets in. The end of Stage V is the start of severe degradation, which contributes to a complete modification of the microstructure. This, in turn, leads to a rapid deterioration of the mechanical properties of the material.