Microstructural investigation of the effect of water vapour on the oxidation of alloy 353 MA in oxygen at 700 and 900°C

Abstract

The objective of this work was to study the impact of water vapour on the corrosion behaviour of the austenitic stainless steel 353MA at 700 and 900°C through a detailed microstructural characterisation of the oxide scales formed, after 168 hours, in O2 and O2 with 40% H2O. The oxidized samples were investigated by scanning electron microscope/energy dispersive X-ray, focused ion beam and transmission electron microscope/energy dispersive X-ray. At 700°C 353MA forms a Cr-rich protective (Fe,Cr)2O3 oxide scale, with some silica at the oxide/metal interface. Breakaway oxidation occurs in H2O/O2 mixtures because the oxide scale is depleted in Cr due to the formation of CrO2(OH)2(g). However, the microstructural investigation indicated that a healing Cr-rich oxide layer formed beneath the Fe-rich oxide after some time. This could be a result of the high Cr/Fe ratio of 353MA. The behaviour at 900°C was different. In spite of the loss of Cr from the oxide scale, breakaway oxidation did not occur, i.e. the oxide scale remained protective. The microstructural investigation showed a thick, almost continuous silica layer at the oxide/metal interface, which may act as a diffusion barrier at the higher temperature.