Abnormal oxidation of stabilized ferritic stainless steels in water vapor

The influence of water vapor on the high temperature oxidation of several stabilized ferritic stainless steels with various Si and Cr contents has been studied by thermogravimetry at 900 degreesC in pure water vapor or dry oxygen, followed by the characterization of the oxide scales formed. Oxidation of Fe-Cr steels in pure water vapor was characterized by a non-protective breakaway oxidation due to the formation of iron oxide nodules on the sample surface. Decreasing the H(2)O partial pressure or increasing the Si or Cr content enhanced the oxidation resistance of the alloy by delaying the breakaway appearance. This abnormal oxidation is thought to be initiated by the arrival of oxygen- and hydrogen-containing species on the chromium depleted metal/oxide interface. The mechanism of the effect of water vapor is discussed and some assumptions proposed.