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Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes

Abstract X20CrMoV121 is a 12%Cr martensitic steel which has been used in power plants in Europe for many decades. Superheater tubes exposed for various durations up to 135,000 h in power plants in Denmark at steam temperatures varying from 450 to 575°C were investigated. Light optical and scanning electron microscopy was used to investigate steamside oxide morphologies. At all temperatures there is a double layered oxide, however, the inner:outer oxide thickness is not always equal. At the lower steam temperature range of <500°C, there is an internal oxidation zone at the oxidation front indicating that chromium is less mobile at these temperatures. At a higher steam temperature range of 540 – 575°C the inner oxide consists of chromium rich and chromium poor oxide running parallel to the oxidation front indicating that the chromium is more mobile within the steel. Both types of morphology have been observed in the laboratory, however, the internal oxidation is observed up to 600°C and the chromium rich oxide striation are observed at 700°C.