The high temperature oxidation of 11% chromium steel: Part I - Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O2 and O2 + 10 and 40% H2O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI-XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α-(Cr,Fe)2O3 oxide in dry O2 at 600°C. In mixtures of oxygen and 10 or 40% H2O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO2(OH)2(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron-rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O2/H2O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H2O. The thick oxide formed in O2/H2O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.