The unusual scaling behaviour of iron-22·9% chromium in air at 1100°C

Abstract

The oxidation of an iron-22·9% chromium alloy in air at 1100 and 1400°C has been studied using thermogravimetric measurements, metallography and electron-probe microanalysis. The scales formed at 1100°C are of four types: (i) chromium oxide, containing very small quantities of iron oxide, which grows at a declining rate in a manner characteristic of scales on alloys of higher chromium content, (ii) chromium oxide forming a “roof” over a stratified inner layer containing chromium-rich oxides beneath oxides rich in iron, (iii) multi-layer scale consisting of alternate regions of chromium-rich and iron-rich oxides and (iv) stratified scale, similar to that formed on alloys of lower chromium content, showing virtually pure outer iron oxides and a complex inner layer of chromium and iron oxides. Type (iv) scale is also readily formed at 1400°C. Scales of types (ii) and (iii) have not been reported previously in the literature. The importance of partial or complete lifting, cracking or removal of the initial Cr 2O 3 layer in determining the mode and extent of scaling is emphasized. The nature of the scales and the proposed mechanism of growth are largely in agreement with the kinetic measurements.