Improvement of transpassive Lntergranular corrosion resistance of 304 austenitic stainless steel by thermomechanical processing for twin-induced grain boundary engineering

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent passive intergranular corrosion such as sensitization of austenitic stainless steels, but the effect of GBE on transpassive corrosion has not been clarified. In the present study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into type 304 austenitic stainless steels containing different phosphorus concentrations. The resulting steels showed much higher resistance to transpassive intergranular corrosion during the Coriou test, in comparison with the as-received ones. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular deterioration during GBE.