The oxidation behavior of X20 steel in steam environments was studied isothermally between 580-640°C. Initially, the magnetite (Fe3O4) phase formed on the surface. With increasing time and temperature, the hematite (Fe2O3) phase formed. The oxide scale consisted of an inner layer divided from an outer layer by the original metal surface. A Cr-rich area was observed beneath the original metal surface. Oxide scales formed on a serviced boiler tube at 540°C for 7000h were also analyzed and found to be similar in oxide phase composition to those formed on X20 steel under laboratory conditions. However, differences existed in the microstructure and distribution of the Cr-rich area within the oxide scale. It was concluded that the oxidation mechanism under field conditions differs from that under laboratory conditions.

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