Effects of Neutron Irradiation on Transpassive Corrosion Behavior of Austenitic Stainless Steels

Irradiated and unirradiated austenitic stainless steels (SS) were exposed to a transpassive potential (1,050 mVSCE) in a sulfuric acid (H2SO4) solution, and the corrosion responses were studied. Applicability of the transpassive technique to evaluation of radiation-induced segregation (RIS) of impurities was considered. The charge density during the potentiostatic transpassive test and the corrosion morphology after the test were functions of both the material chemistry and irradiation. The charge density of the unirradiated materials was correlated successfully with the bulk impurity level, as defined in terms of the parameter (wt% silicon + wt% phosphorus x 10). The charge density increased steeply above an impurity level parameter of 0.5, which corresponded to the region where intergranular attack (IGA) became visible. The change in charge density after neutron irradiation also was a function of the impurity level. Irradiation resulted in a considerable increase in charge density and grain-boundary attack (GBA) for commercial-purity (CP) type 304 SS (UNS S30400). Furthermore, small multiple sites of preferential dissolution were observed clearly in the matrix. It was postulated that the small dissolution sites in the matrix may have been faulted dislocation loops that were segregated with silicon or phosphorus. Charge density in the potentiostatic transpassive test seemed to be an indicator of the degree of RIS of impurities.