Steam oxidation resistance and thermal stability of chromium aluminide/chromium hybrid coating on alloy steels formed at low temperatures

This study investigated the feasibility of forming a hybrid coating with a structure consisting of a top Cr-aluminide layer and an inner Cr layer on alloy steels by using a two step process: electro-Cr plating and then pack aluminising at low temperatures. The oxidation resistance of the coating so formed was tested in ultrasupercritical steam of 650 °C and 30 MPa. The factors affecting the oxidation kinetics of the coating were studied by comparing its oxidation behaviour with that of the pack Fe-aluminide coating tested in the same ultrasupercritical steam. The thermal stability of the coating at 650 °C was investigated by a series of isothermal annealing experiments in argon atmosphere. It was demonstrated that the outer Cr-aluminide layer of the coating can improve the steam oxidation resistance of the steel substrate. The inner Cr layer can function as an effective barrier preventing the outward diffusion of Fe from the steel substrate; it can also act as a buffer zone, substantially reducing the rate of the inward Al diffusion process.