Stratified oxide scale growth on two Cr-Ni steels oxidized in high-pressure steam at 800°C

Abstract

A commercial 18% Cr-11% Ni steel and a laboratory-made 21% Cr-11% Ni alloy have been oxidized in steam at 50 atm pressure and with approximately 0.2 ppm O₂ at 800°C for 50–1000 hr. The oxide scales have been studied by optical microscopy, replica electron microscopy, x-ray, and selected-area-electron diffraction and microprobe analysis. The diffusion processes have been studied by gold markers and autoradiography by means of the reaction¹⁸O (p, n)¹⁸F. On both materials internal oxidation in combination with external scale formation is observed. The interface between the external scale and the inner internally oxidized scale coincides very closely with the original metal surface. The oxide phases present have been identified. The scale is of fairly even thickness on the 21% Cr-11% Ni alloy and grows by a cubic rate law. The scale on the commercial steel is of very irregular thickness; areas with protective Cr-oxide alternate with areas of the above-mentioned structure, so-called nodules. The zones of internal oxidation in the nodules are frequently intersected by bands of compact oxide. The growth mechanism of the nodules is discussed in terms of the theory for internal oxidation. The reason for the beneficial effect of increased Ni content with constant Cr content is discussed briefly.