Use of percolation theory to predict the probability of failure of sensitized, austenitic stainless steels by intergranular stress corrosion cracking

The underlying premise is that, for a component to fail by IGSCC, a continuous pathway of susceptible (sensitized) grain boundary facets must exist through the grain structure. Calculation of the percentage of sensitized grain boundaries necessary to form a continuous pathway provides a criterion for predicting IGSCC. The theoretical results, obtained using a computer model, are that material with less than 23% of sensitized grain boundaries will not fail by IGSCC, material with between 23 and 89% of sensitized boundaries will show mixed ductile and brittle failure in a slow strain rate test, and material with more than 89% sensitized boundaries will show only brittle intergranular failures. A range of sensitized structures has been examined experimentally using the electrochemical potentiokinetic reactivation method (EPR). Slow strain rate IGSCC experiments in which the cracking was induced by very dilute solutions of sodium thiosulphate were used to test the predictions of the percolation model. The critical percentages of sensitized grain boundaries were obtained with sensitization heat treatments at 650°C of about 4 h (for 23%) and 12 h (for 89%).