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The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 °C using dilute thiosulfate solutions containing excess sulfate or (for Alloy 600) chloride. A potentiostatic scratch method was used. In sulfate solutions, all alloys pitted at low potentials, reflecting their lack of protective Mo. The alloys demonstrated the most severe pitting at a sulfate : thiosulfate concentration ratio of ∼40. Alloy 600 pitted worst at a chloride : thiosulfate ratio of ∼2000. The results are interpreted through the mutual electromigration of differently charged anions into a pit nucleus, and differences in the major alloy component.

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A reaction-transport model in a one-dimensional pit geometry is constructed to study the effect of both cation complexation and concentration-dependent diffusivity on the IR potential drop at steady state and with an anodic limiting current density. When there is no cation complexation, the B value from Galvele's pitting potential equation remains at 59 mV, even when the ionic diffusivities are functions of chloride concentration. However, the B value is found to depend on the anionic charge and the formation of cation complexes. When doubly charged anions are assumed, the B value decreases to 30 mV as predicted from both numerical and analytical results. For the pit dissolution of Fe-17Cr alloy in chloride solution, the B value increases up to 100 mV, in agreement with experiments, assuming dissolved chromium can make cationic chloro-complexes. Such results can be rationalized via the distribution of potential and ionic fluxes at the pit mouth which not only maintain the critical chemistry for pit stability but also sustain the complexation reaction inside the pit.