Correlation of high-temperature steam oxidation with hydrogen dissolution in pure iron and ternary high-chromium ferritic steel

The high-temperature oxidation in both air and steam was examined experimentally with pure iron and a Fe-10Cr-0.08C (mass%) ternary ferritic steel. In case of pure iron, the thickness of the oxide scale formed in steam at 923 K for 360 ks was comparable to that of the scale formed in air. On the other hand, in case of the ternary ferritic steel, the oxide scale formed was much thicker in steam than in air, Thus, the oxidation rate was nearly independent of the air and the steam atmosphere for pure iron, but was dependent for the ternary ferritic steel. In the present study, this difference was investigated from a viewpoint of the hydrogen dissolution in the oxide scale during the steam oxidation. The amount of dissolved hydrogen was measured using a thermal desorption spectroscopy (TDS). It was found that the amount of the dissolved hydrogen was much larger in the ternary ferritic steel than in pure iron. Also, it was shown that the hydrogen dissolution in the ternary ferritic steel was related to the presence of (Fe,Cr)₃O₄ in the oxide scale. The defect structure in this chromium-rich oxide was modified by hydrogen dissolution, so that the ionic diffusion could be enhanced in it, resulting in the more accelerated oxidation rate in steam.