Abstract
Chromium specimens were annealed in hydrogen to reduce the level of sulphur impurity and thus improve the oxide adhesion. The growth mechanism of the chromia scale in 0·1 atm oxygen at 950°C was investigated by oxidising sequentially in natural oxygen and then in gas enriched in 18O; the distribution of 18O in the scale was determined using imaging secondary ion mass spectroscopy (SIMS). From preliminary results it was found that substantial oxygen diffusion occurred. This differs from the growth mechanism of chromia on vacuum annealed material which, as far as solid state transport is concerned, mainly involves cation transport. The theory that sulphur promotes cation diffusion via grain boundaries in chromia and that removal of the sulphur favours oxygen diffusion is supported by the results. From examination of the distribution of the oxygen isotopes at the scale/gas interface, it is shown that their distribution is not uniform, from which it can be inferred that the growth of oxide at that interface was not uniform during oxidation in the 18O-rich gas.MST/967
Published Version
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