Intergranular corrosion of stainless steel

A quantitative description of the degree of sensitization in austenitic stainless steels is developed and is used to predict the likelihood of failure by IGSCC. The 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. The predictions obtained using a computer model based on this premise were that material with less than 23{percent} of sensitized grain boundaries would not fail by IGSCC, that material with between 23 and 89{percent} of sensitized boundaries would show mixed ductile and brittle failure in a slow strain rate test, and that material with more than 89{percent} sensitized boundaries would show only brittle intergranular failures. These predictions were confirmed by experiments on room temperature, thiosulphate induced cracking. An accurate estimate of the proportion of sensitized grain boundaries was derived using quantitative image analysis on micrographs of specimens etched using the EPR method: this method reliably differentiated susceptible from non-susceptible boundaries as far as the room temperature thiosulphate induced cracking was concerned; as noted in Volume 1 of this report, however, a different etching method is needed reliably to assess susceptibility under BWR conditions. 20 refs., 12 figs.