Abstract: The high temperature corrosion resistance of Ni-25.9Cr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4Cr-16.0Al-0.5Y-0.2Hf alloys was assessed in sulfidation/oxidation environments. In the environment with a sulfur partial pressure of 1Pa. and an oxygen partial pressure of 10-19Pa, both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃. In the initial stage, selective sulfidation of Cr suppressed the formation of the other metal sulfides, resulting in lower weight gains. In the transient stage, breakdown and cracking of Cr sulfides and insuffcient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight. In the steady-state stage, corrosion was controlled by the diffusion of anions and/or cations, which led to a parabolic rate law.

Key words: sulfidation and oxidation, corrosion resistance, M-Cr-Al-Y type alloy