Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H2/H2O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.

► High chromium steels were tested in respect to oxidation resistance in Ar–H2–H2O.
► Between 550 and 700 °C a bell-shape temperature dependence of oxidation rates was found.
► Iron base oxide scales tended to form at T < 650 °C.
► Chromia base scales were formed above 700 °C if the chromium content was higher than 16%.
► The results are interpreted on the basis of the competing diffusion processes in alloy and scale.