Chemical Thermodynamics of High Temperature Reactions In Metal Dusting Corrosion

An experience of high temperature metal deterioration in atmospheres containing carbon monoxide and hydrogen has been further studied in an effort to determine the mechanism of attack. In addition to metallurgical studies of severely pitted samples of stainless steel, a thermodynamic study of gas mixture equilibria and the reactions of these mixtures with metals and metal oxides is presented. The results of these studies indicate the dominant role of chromium and its affinity for carbon which can lead to severe carburization and disintegration of the surface layers of chromium-containing alloys. The following explanation of metal dusting, supported by thermodynamic evaluation is postulated.

In the reducing and carburizing atmosphere, the normally protective surface oxide layer on the alloy steel may be attacked generally or locally, thus exposing metal to carburization. It appears that reduced nickel metal could play a role in this action and in the production of carbon. Because the carburizing potential is strong and the diffusion rate of carbon is appreciable, the surface layers can become rich in carbon and precipitation of Cr₂₃C₆ can occur in the grain boundaries. Direct reaction of chromium and components of the gas phase is possible. Ultimately, excessive formation of M₂₃C₆ and M₇C₃ leads to matrix consumption, metal disintegration and dusting.