Use of Microanalysis to Better Understand the High-Temperature Corrosion Behavior of Chromium Exposed to Multi-Oxidant Environments

Abstract

The corrosion behavior of metals and alloys at high temperatures in complex multi-oxidant environments is of a great interest for achieving extended service performances and improved operation efficiencies. In this basic study, the scaling reactions of pure chromium in several multi-oxidant gas mixtures were assessed. The environments studied are similar to those that exist in low-NOx burner and coal gasification atmospheres, which are very reducing and favor sulfidation and carburization, together with possible formation of Cr2O3. The effect of sulfur on chromia-scale growth kinetics was also considered. Isothermal exposures were done for up to 100 h at 871 °C (1600 °F), and comparison was made to similar exposures to air. Exposed samples were characterized in detail using some combination of X-ray diffraction and electron beam scattering and spectroscopic techniques. It was found that chromia scales formed in mixed gases containing water vapor grew much faster and had a finer grain structure than those formed in dry air. Both inward growth and outward growth of the chromia scale were inferred for the mixed-gas conditions. The effect of a high carbon potential in the gas on the scaling behavior is also discussed.