Effect of water vapour on growth and adherence of chromia scales on pure chromium (2007)

The oxidation behaviour of chromium was studied in the temperature range 950 to 1050oC. A number of atmospheres such as Ar-O2, Ar-H2-H2O, and more complex N2-O2-H2O and N2-H2-H2O were used, to allow the effects of oxygen and water vapour partial pressures to be determined. It was shown that the oxide scale formed in Ar-O2 environments was dependent on the oxygen partial pressure. Decreasing the pO2 in such gas mixtures lowered the oxidation rate and improved scale adherence. Different behaviour was found in Ar-H2-H2O. Scale adherence remained good in all low pO2 gases and decreasing the pO2 in the gas (this was done by using gases of higher H2/H2O ratio) led to an increase in the oxidation rate. The main finding of the work was that the oxide scale formed on chromium in Ar-H2O and Ar-H2-H2O gases significantly differs from that formed in Ar-O2. It appeared that oxide grains were much smaller in low pO2 gases containing water vapour than in dry high pO2 gases. It is believed that this effect of water vapour on the oxide morphology development is due to the faster adsorption and dissociation of H2O at the outer surface compared with O2, which enhance the inward transport of oxygen ions to the metal/oxide interface. It is proposed that increased oxygen grain boundary diffusion is the dominant process but molecular transport of H2 and H2O through the scale could also operate and contribute to the inner scale growth. The tracer study combined with SNMS techniques showed that in low pO2 gases a significant amount of the scale was formed at the metal/oxide interface and this supports the observations that smaller oxide grains promote inward oxygen transport. It was also found that the extent of inward growth was higher in Ar-H2-H2O than in Ar-H2O gas. This may indicate that the oxygen partial pressure of the gas or the hydrogen partial pressure were also factors determining the oxidation behaviour in low pO2 gases. Exposure in Ar-O2-H2O mixtures revealed that water vapour could markedly improve oxide scale adherence. This is believed to be a consequence of enhanced inward transport of H2O, which promotes scale growth at the metal/oxide interface. It was found that water vapour, when present in sufficient amounts in the multicomponent gas mixture could block the formation of nitrides and carbides beneath the oxide scale. The blocking effect of water vapour at the surface was especially pronounced in low pO2 gases such as N2-4%H2-4%H2O and CO-2%CO2-1%H2O. It is believed that in such atmospheres adsorption and dissociation of H2O was enhanced by the low oxygen partial pressure and therefore dissociation of water vapour was faster. The purity of chromium was shown to affect substantially both, the oxidation rate and the oxide scale adherence. This could be a key factor in the interpretation of the scatter found in the literature. Chromium specimens of different purity were investigated and it was observed that purer chromium had a lower oxidation rate and produced a less porous oxide scale. Such behaviour was particularly pronounced in Ar-O2 gases. The presence of some elements could account for the enhanced oxidation rate due to modifications in defect concentrations in the oxide. Poor scale adherence was probably affected the most by carbon, which forms the volatile oxide CO at the chromium/chromia interface at pressure of 4x10-5 atm.