High Temperature Corrosion of Metallic Materials in Composed Oxidizing Environments

Abstract

AbstractBasing on actual theoretical approach and experimental results, the mechanism of sulphide formation beneath the oxide scale grown on metals in SO2-O2 atmospheres has been described. It has been shown that in spite of much lower sulphur partial pressure in the oxidizing atmosphere than the dissociation pressure of the sulphide to be formed, the sulphidation process takes place beneath the oxide scale. This, at the first sight, unexpected behavior results from the fact that sulphur is diffusing inwards through the primary oxide scale in the molecular form, i.e. SO2 molecules. Reaching thus metal-scale interface, where the oxygen partial pressure is very low, virtually equal to the dissociation pressure of the oxide forming the scale, SO2 ⇔ O2 + ½S2 equilibrium is shifted to the right, as a result of which the partial pressure of sulphur vapor dramatically increases, reaching the value several orders of magnitude higher than that needed for sulphide formation.Analogous situation is observed during oxidation of chromium steels in CO2-O2 atmospheres. In this case, namely, carburisation process is observed beneath the oxide scale, in spite of the fact that carbon activity in this environment is several order of magnitude lower than that required for chromium carbide formation. This again unexpected situation becomes understandable if one assumes – like in the case of metal oxidation in SO2 containing atmosphere – that carbon is transported through the oxide scale in the form of CO2 molecules.The final conclusion is, that the explanation of the mechanism of sulphide formation beneath the oxide scale on metals and of carburization beneath the oxide scales on steels constitutes the important step forward, leading to the better understanding of high temperature corrosion mechanisms of metallic materials, observed in multicomponent agresive gases.