The oxidation of high-purity iron-chromium-aluminum alloys at 800°C

Abstract

A systematic study is presented of the oxidation of pure iron-chromium-aluminum alloys at 800°C, in pure oxygen, at a pressure of 200 Torr. Oxidation characteristics are described with reference to kinetic measurements, scale topographies and morphologies, and also possible growth mechanisms. An oxide map is used to show that alloys may be classified into four categories depending on the external scale that forms: Fe₂O₃, Cr₂O₃, Al₂O₃, or Al₂O₃ with iron-oxide nodules. Alloys containing less than 2–2.5 wt. % aluminum formed either Fe₂O₃ or Cr₂O₃ as an external scale, depending on the chromium content, and internal, rod-like protrusions of Al₂O₃. At higher aluminum concentrations, Al₂O₃ was always present as an external scale, although this was interspersed by iron-oxide nodules at chromium concentrations of less than 5 wt. %. A model based on Wagner's secondary getter concept is proposed for eliminating nodule nucleation. Evidence is also present that indicates that at 800°C, alumina scale decohesion occurs prior to void formation and that voids are the result of thermal etching beneath lifted scales.