Is Localized Corrosion a Necessary Precursor to Alkaline Nitrate SCC of Carbon Steels?

Abstract Stress corrosion cracking (IGSCC) and localized corrosion are the two primary documented threats to the integrity of underground carbon steel radioactive waste storage tanks at the Hanford Site. Numerous studies of IGSCC in alkaline nitrate-rich waste simulants provide anecdotal evidence of corrosion features at IGSCC initiation sites, suggesting a possible link between localized corrosion and IGSCC. However, no clear correlation has been established between critical potentials for localized corrosion and the critical cracking potential. Electrochemical tests in a range of alkaline nitrate environments were performed on carbon steel. The results indicate evidence of localized corrosion in the form of pitting, intergranular corrosion, and crevice corrosion. Detailed SEM characterization of the localized corrosion sites revealed preferential dissolution of ferrite in the pearlite structure. Cross sectional metallographic analysis confirmed the presence of significant intergranular corrosion. The observations support the hypothesis that localized corrosion in the form of pits or crevices can transform into intergranular corrosion. Intergranular corrosion sites can then act as initiation sites for IGSCC, providing a framework in which to link localized corrosion and IGSCC in alkaline nitrate/nitrite environments. It is hypothesized that cementite in pearlite can serve as sites for nitrate reduction thereby altering the local chemistry in the pit. The change in local chemistry might promote the change in corrosion morphology to intergranular corrosion.