Corrosion of ferritic–martensitic steel HT9 in supercritical water

The oxidation of a 12% Cr ferritic–martensitic steel, HT9 has been examined after exposure to supercritical water (SCW) at 500 °C with two different dissolved oxygen contents, 25 wppb and 2 wppm, for exposure periods of up to 505 h. Samples exposed to the higher oxygen content SCW showed Cr-rich blades and generally a much finer oxide grain structure than samples exposed to the lower oxygen content SCW which have a porous surface oxide structure. A double-layer oxide structure developed on all samples exposed in 25 wppb oxygen content SCW, consisting specifically of an Fe-rich outer magnetite layer and a Cr-rich inner oxide layer composed of spinel and ferrite. An outer hematite layer was also found for samples exposed to 2 wppm oxygen content SCW. Samples whose surface was pre-implanted with oxygen and then exposed to SCW with 25 wppb oxygen content for 505 h showed a slightly lower weight gain compared to the untreated samples. This difference in weight gain appears to be associated with a preferred orientation in the outer oxide layer.