Effect of phase transitions in Fe-Cr-Al alloys on their corrosion resistance in gases and liquid metals

Abstract

We investigate the process of oxidation of Fe-Cr-Al alloys in argon and liquid sodium at a temperature of 650°C for 1000 h, analyze distinctive features of the process of formation of surface oxide and sub-oxide layers, and demonstrate the effect of phase transitions in alloys on the corrosion losses in these media. The process of oxidation of ferrochrome alloys in the region of homogeneity of α-solid solutions in both media results in the formation of oxide layers based on Al2O3 on the surface of aluminum-containing alloys. In an atmosphere of argon, the intense growth of the oxide layer promotes the formation of residual stresses followed by its destruction and exfoliation, which leads to an increase in corrosion losses. In liquid sodium, aluminum improves the corrosion resistance of ferrochrome alloys, which is explained both by the suppression of formation of unstable compounds (sodium chromites and ferrates) and the appearance of an interlayer of σ-phase inclusions between the Al2O3 film and the matrix. This interlayer inhibits the growth of the protective oxide layer and enhances its adhesion to the matrix. The σ-phase is formed in homogeneous ferrochrome α-alloys as a result of saturation of their surface layers with carbon present in sodium. If the composition of Fe-Cr alloys is close to equiatomic, their corrosion resistance catastrophically decreases as a result of the bulk α → σ transformation which is typical of both media.