Anions such as sulphate inhibit pitting of stainless steels by raising the pitting potential. One might, therefore, expect sulphate to raise the critical pitting temperature (CPT), below which no stable pitting occurs at any potential. Surprisingly, this expectation turns out to be false. Addition of OSM Na$O, to 1M NaCl lowers the CPT of type 904L steel from 49 to 38°C. Since metastable pitting of this alloy occurs even at room temperature, the effect of sulphate concerns the ability of metastable pits to become stable. Microscopic examination reveals that the stabilization is associated with the lacy metal cover that develops over the pit. When the inhibitor is present, the enhanced passivation reduces the porosity of the pit cover and helps the pit to retain its local environment.

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