Oxidation of Mo-Si-B Alloys and Coatings in a Water Vapor Environment

Abstract

For high temperature applications associated with power generation, materials are often subjected to a combustion environment containing water vapor. Under this condition, alloys that develop protective oxide surface layer in a dry air environment exhibit a significant reduction in effectiveness. Oxide scales such as Cr2O3, SiO2, B2O3 and Al2O3 react with water vapor at high temperature to generate volatile species that result in an increase in oxidation attack. Upon oxidation of Mo-Si-B alloys the resulting borosilica scale provides some protection that depends on the B/Si ratio, but the scale suffers accelerated attack in a water vapor environment. After a pack cementation co-deposition of (B\u2009+\u2009Si) an enhanced resistance to water vapor attack is observed that is related to the incorporation of alumina in the borosilica during pack cementation. An analysis of the water vapor exposure at 1300 °C reveals an estimated coating lifetime of about 82,000 h.