Catastrophic oxidation mechanism of hyper duplex stainless steel S32707 at high temperature in air

High-temperature oxidation behavior and the mechanism of hyper duplex stainless steel S32707 were investigated at 900, 1000, 1100 and 1200 degrees C in air. Some interesting oxidation characteristics were observed, and the corresponding oxidation mechanisms were proposed. At 900 degrees C, oxidation follows a linear rate law. The combined actions of MoO3 volatilization, molten molybdate dissolution reaction and block-like sigma phase and Cr2N precipitation cause catastrophic oxidation. When the temperature is raised to 1000 and 1100 degrees C, the deleterious effects of MoO3 and molten molybdates are greatly enhanced. Moreover, undesirable air-nitriding occurs and further accelerates the oxidation. The synergistic effects of MoO3 volatilization, molten molybdate dissolution reaction and air-nitriding strongly promote catastrophic oxidation. Surprisingly, the oxidation at 1200 degrees C follows a near-parabolic rate law. The disappearances of MoO3, molten molybdates and air-nitriding lead to the formation of a rather compact, complete and protective oxide layer. The oxidation resistance of the steel has been clearly improved.