A quantitative description of the degree of sensitisation in austenitic stainless steels has been developed based on percolation theory. This is used to predict the likelihood of failure of sensitised stainless steel by intergranular stress corrosion cracking (IGSCC). The underlying premise is that, for a component to fail by IGSCC, a continuous pathway of susceptible (sensitised) grain boundaries must exist through the grain structure. Calculation of the percentage of sensitised grain boundaries necessary to form a continuous pathway provides a criterion for predicting IGSCC. The theoretical result, obtained using a computer model (which takes no account of the anisotropy induced by the applied strain) is that material with less than 23.8% of sensitised grain boundaries will not fail by IGSCC.

A range of sensitised structures has been examined experimentally using the electrochemical potentiokinetic reactivation method (EPR). An accurate estimate of the degree of sensitisation in terms of the percentage of sensitised grain boundaries was obtained using quantitative image analysis on micrographs of specimens from the EPR test.

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. These have demonstrated that the modified examination method provided a more reliable means of determining susceptibility to IGSCC than the conventional interpretation of EPR data.

Subject
Steel structures, Corrosion failures, Materials, Intergranular stress corrosion cracking, Crack initiation, Clusters, Probability, Corrosion attacks, Mechanical failure, Stainless steel, Grain boundary, Heat treatment, Grain
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1988
UKAEA
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