Models describing the degradation of FeAl and NiAl alloys induced by ZnCl2–KCl melt at 400–450 °C

In this study, a mechanism to account for the corrosion of Fe–Al and NiAl alloys beneath molten ZnCl2–KCl at 400–450 °C in air is described. All the examined materials experienced enhanced corrosion, with the formation of porous surface scales or, in some cases, in combination with local attack of the matrix. Comparatively, the corrosion resistance of Fe–Al and NiAl alloys is markedly improved with increased Al contents, whereas different microstructural evolutions are observed for the two systems. For Fe–Al alloys, the outermost layer of corrosion product is a mixed region composed of a large amount of Fe2O3, KCl and metallic zinc particles, overlying a mixture of aluminium oxide and KCl that acts as the intermediate layer. Moreover, a multi-layered Fe2O3 scale is produced on the surface of the matrix. For Ni–Al alloys, however, the outermost layer is mainly a mixture of KCl, metallic Zn, and some alumina particles in local regions, but no nickel oxide is detected. Beneath this outer layer is an Al-rich oxide layer combined with KCl impurity, and then a metallic nickel bulk containing aluminium oxide precipitation, in contact with the matrix.