Hydrogen in chromium:: Influence on the high-temperature oxidation kinetics in H2O, oxide-growth mechanisms, and scale adherence

Oxidation of 1-mm thick chromium samples with 7 to < I wt, ppm hydrogen has been studied at 900 degrees C in a closed reaction chamber. The gases used were O-2, H2O, and gas mixtures of Oz; H2O at a total pressure of about 20 mbar. By means of oxygen labeling in two-stage oxidations, H2O followed by (H2O)-O-18, the position of oxide growth within the oxide scale was determined with secondary-ion mass spectrometry (SIMS). The results are consistent with visual observations and scanning electron microscopy (SEM), which shows flat oxides with good adherence when formed in H2O. When replacing O-2 out H2O in the atmosphere, the oxidation rate increases by a factor of two with an increase of oxide growth at both the inner and outer part of the oxide. An increased metal (cation) transport is observed when 5 wt. ppm hydrogen is present in the chromium metal prior to oxidation in both Oz and H2O. This is detrimental for the adherence of the oxide scale. The uptake of hydrogen in H2O exposure for 1600 min was measured to 1.5 wt. ppm. In exposures to O-2 + H2O mixtures, no H-2 is formed and no net water is consumed as long as O-2 is present. A plausible reaction scheme based on an experiment with (H2O)-O-18 is presented to explain this observation.