Intergranular Corrosion of Austenitic Stainless Steel

The well‐known susceptibility of austenitic stainless steels to intergranular corrosion after heat‐treatment in the temperature range of 550°–800°C ( i.e. , "sensitization") has long been attributed to depletion of Cr from regions of the alloy matrix adjacent to grain boundaries in which had precipitated. Those regions of the steel in which the local Cr composition falls below about 12% have a diminished ability to form a passive film and hence corrode preferentially. We have made thermodynamic calculations of the equilibria in and near grain boundaries in order to determine the chromium content in the vicinity of carbide particles. The calculations show that the equilibrium chromium content is a strong function of the temperature and of the carbon and alloy content of the steel. It is inferred from the analysis that variations in the susceptibility to intergranular attack are determined primarily by changes in the equilibrium chromium content near the carbides and not by changes in the number and distribution of particles. For instance, above 800°C the chromium content near the carbides is high enough to produce passivity, and the alloy is immune not because of the absence of carbides but because chromium depletion is not severe. Experimental studies of rates of grain boundary attack as functions of temperature and composition have confirmed the predictions of the analysis. A further factor of less importance is the distribution of carbides in the grain boundary. Calculations were made to determine the Cr concentration gradient within the boundary between carbide particles as a function of temperature and composition as well as the Cr gradient normal to the boundary. These calculations predict a strong interdependence between susceptibility to intergranular corrosion, carbide particle spacing, and sensitizing temperature. Experimental results on thinned samples of sensitized material corroded in a Strauss solution and examined by electron microscopy are in good agreement with predicted effects.