Parameters determining the breakaway oxidation behaviour of ferritic martensitic 9%Cr steels in environments containing H₂O

Abstract

Minor alloying elements can significantly influence the oxidation behaviour of ferritic martenstic steels, and above all the time to the beginning of breakaway oxidation. This has been found to be particularly true when oxidation occurs in the presence of water vapour. The oxidation of 9%Cr steels has been investigated for times of up to y10 000 h in dry air and in air with both 4% and 10% water vapour. The steels were commercial and quasicommercial versions of P91, E911 and Nf616. The content of the alloying elements, primarily Si, but also W, Mn, and Mo, was varied for the purpose of investigating their effect on breakaway oxidation. This study not only involved weight gain measurements but also characterising the Cr depletion in the substrates. Additionally, the role that intrinsic growth stresses play with regard to the incubation time to breakaway was investigated by high temperature in situ X-ray diffraction and in situ acoustic emission analysis. The results show that the occurrence of breakaway is the result of cracking processes in the oxide scale in combination with a severe chromium depletion of the subsurface zone.CEST/2115Keywords: breakaway oxidation, 9%Cr steels, alloy composition, Cr-subsurface depletion, water vapour M. Schütze (schuetze@dechema.de), D. Renusch and M. Schorr are in the Karl-Winnacker-Institut der DECHEMA e.V., D-60061 Frankfurt am Main, Germany. Manuscript

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