A Type 430 stainless steel was exposed to Ar-20 %CO<inf>2</inf> without and with water vapour at 800 °C up to 96 h. Increasing water vapour content from 5 % to 40 % helped reduce mass gain, enhance volatilisation, and improve scale adhesion with fewer pores at scale/steel interface. Mechanisms for Cr and Mn volatilisations including the direct reactions between Cr-containing oxide and CO<inf>2</inf> were suggested with the aid of the XPS results. Mathematical relations that express the role of water vapour on concentrations of defects responsible for scale growth were proposed to help explain the reduced oxidation rate.
