The interaction in austenitic stainless steel between the stabilizing elements and carbon or nitrogen and its effect on corrosion behaviour

Abstract

Though “weld decay” in austenitic CrNi steels can be eliminated by the use of stabilizing elements to lock up the carbon and nitrogen, related effects are still observed under onerous conditions, e.g. exposure to hot nitric acid. A simple model is developed in which it is assumed that the behaviour of an alloy depends on the concentrations in solutions of two elements which form a precipitate of form MX and the amount of precipitate formed, and that the concentrations in solution are given by the dissociation constant of the precipitate. The model is extended to cover the cases where a single precipitate of form M iX j occurs, and where a third element Y also reacts with M so that two precipitates MX and MY are formed. Existing data on the corrosion behaviour of austenitic CrNi steels are considered in terms of the model. The model provides a satisfactory fit of the data and explains many of the known facts about the behaviour of these alloys after various heat treatments and the relative performance of titanium-and niobium-stabilized steels.