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

Abstract

High-temperature oxidation behavior and the mechanism of hyper duplex stainless steel S32707 were investigated at 900, 1000, 1100 and 1200 °C in air. Some interesting oxidation characteristics were observed, and the corresponding oxidation mechanisms were proposed. At 900 °C, oxidation follows a linear rate law. The combined actions of MoO3 volatilization, molten molybdate dissolution reaction and block-like σ phase and Cr2N precipitation cause catastrophic oxidation. When the temperature is raised to 1000 and 1100 °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 °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.