Hydrogen in chromium:: Influence on the high-temperature oxidation kinetics in O2, oxide-growth mechanisms, and scale adherence

Oxidation of chromium in 20 mbar isotopically labeled molecular oxygen was studied at 900 degrees C. The samples comprised specimens with as-received contents of hydrogen and specimens where this hydrogen had been eliminated by vacuum annealing. Two-stage oxidation was performed by exposure of the samples to O-16,16(2) ana O-18,18(2). Reaction rates were studied by measuring the change in oxygen pressure in a closed reaction-chamber with a mass spectrometer during oxidation. The oxide formed was characterized by secondary-ion mass spectrometry (SIMS) and scanning electron microscopy (SEM). The adherence of the oxide scale to the underlying metal substrate is poor and the oxidation rate is initially slightly higher when the metal contains hydrogen prior to oxidation. It is shown by O-18 depth profiles that chromium diffusion within the Cr2O3 scale is increased and oxygen diffusion is decreased for metal containing hydrogen. It is believed that the poor adherence of the oxide scale, in this case, is due to low anion diffusion relative to cation diffusion, which leads to an insufficient portion of oxide growth at the metal-oxide interface.