The initial stages of high-temperature corrosion of Fe-Cr-Ni and Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

Abstract

The initial corrosion reactions on a number of Fe-Cr-Ni and Cr-Ni alloys when exposed in a H₂-CH₄ gas mixture ofa _c =0.8 and$p_{O_2 }$ ≈ 10⁻³⁰ bar at1098 K have been investigated. All alloys were studied with surfaces in boththe cold-worked and electropolished conditions. The experiments, in whichexposure times up to 400 rain were used, were carried out in a conventionalcorrosion rig and also in a small reaction chamber contained within an X-rayphotoelectron spectrometer. The use of XPS, SEM, and TEM showed thattransient α-Cr₂O₃ layers formed on all alloy surfaces during the heat-up period.On the electropolished surfaces, the oxide layers developed nonuniformly withrespect to microstructure and thickness, whereas homogeneous, compact, andfine-grained α-Cr₂O₃ layers formed on the cold-worked specimens. Followingan incubation period, M₇C₃ carbides nucleated on the surfaces; they grew byconsuming the less stable α-Cr₂O₃ and also through diffusion of metal fromthe alloy substrate. The mode of nucleation of the M₇C₃ and its rate of growthwere strongly dependent on the character of the transient α-Cr₂O₃ layers. Afterremoval of α-Cr₂O₃, carbon ingressed into the substrate where M₂₃C₆ wasformed internally. These results are broadly in line with the thermodynamicstability predictions and are discussed in terms of simple models.