Protective and nonprotective behavior of 11% Cr steel in O-2+H2O environment at 450-700 degrees C

Artikel i vetenskaplig tidskrift, 2004

The oxidation of X20 steel (CrMoV 11 1) in dry O2 and O2+10 or 40%H2O was investigated at 450-700°C. The flow rate was 2.5 cm/s and the exposure time was 168 h. The oxidized samples were investigated gravimetrically and by a number of surface analytical techniques including environmental scanning electron microscopy/energy-dispersive X-ray, grazing incidence X-ray diffraction, and glow discharge optical emission spectroscopy. The oxidation behavior was strongly influenced by temperature and pH2O. In dry O2 the scale was protective up to 700°C, the chromium concentration in the scale increasing with temperature. In O2+H2O environment chromium was lost by vaporization of CrO2(OH)2, resulting in a deterioration of the ability of the oxide to protect the metal. This resulted in rapid oxidation in O2+40%H2O at high temperature (650 and 700°C) and at low temperature (500°C), forming a thick duplex (Fe2O3/Fe2CrO4) scale. In the intermediate temperature range (550-600°C) the rate of oxidation was not increased. The results are interpreted in terms of the ability of the material to compensate for chromium vaporization by supplying chromium to the protective oxide by diffusion.