Protection of Fe-Cr-Al alloys in sulfidizing environments by means of an α-Al₂O₃ scale

Abstract

The sulfur corrosion behavior of ferritic Fe-22.2Cr-5.5Al and Fe-10.2Cr-5.1Al (wt.%) alloys was studied in sulfur vapor and in a 10%H ₂ S -H ₂ (vol. %) atmosphere at 900°C after preoxidation of the alloys at 1000°C in oxygen to form an α-Al ₂ O ₃ scale. The immunity time before onset of sulfidation attack to form a layered scale containing chromium and iron sulfides was dependent upon the α-Al ₂ O ₃ scale thickness and the nature of the sulfidizing atmosphere. In pure sulfur at low vapor pressure,$P_{S_2 }$ =8.1×10 ⁻⁵ atm, the sulfide scale initially developed by the diffusion of metal cations through the α-Al ₂ O ₃ barrier. On the other hand, the sulfide was nucleated and grew beneath the Al ₂ O ₃ barrier when the alloys were exposed in the H ₂ S -H ₂ atmosphere at $P_{S_2 }$ =1.5×10 ⁻⁵ atm. It was possible to demonstate by calculations from a gas-oxide-metal model for sulfur adsorption and diffusion in the solid phases that the types of initial sulfidation attack in these atmospheres were determined by sulfur threshold concentrations at the alloy-oxide and oxide-gas interfaces.