Behaviour and Mechanisms of Alkali Metal Sulphate-Induced Cyclic Hot Corrosion in Relation to Gradients and Preoxidised MCrAlY-Type Coatings

In this study, two modified MCrAlY-type coatings were prepared by the same arc ion plating process. ‘Gradient annealing’ and ‘gradient annealing plus preoxidation’ were adopted as post-treatments for the two coatings. A two-layer scale of a mixed oxide layer and a pure alumina layer with the alumina next to the substrate was formed on the latter coating. A cyclic hot corrosion test was carried out on them with a conventional MCrAlY-type coating as a reference. The kinetics and evolution of the microstructure showed them to have greater hot corrosion resistance at 900 °C. The preformed inner alumina scale by preoxidation retarded the occurrence of internal oxidation–sulphidation to some extent. Additionally, the internal sulphidation–oxidation model is elucidated and extended.