PIT Stability Criteria of Nickel Base Alloys In Extremely Dilute Sulfate-Thiosulfate Solutions

Pitting of stainless steel in chloride-free solutions containing thiosulfate was discovered independently in the 1980s by Andrew Garner and by Newman (1). Sulfate plays the role of chloride, allowing strong acidification of the pit solution, while thiosulfate is reduced to an activating sulfur species.

Certain localized corrosion phenomena in nuclear steam generators (SGs) are hypothesized to occur at low temperature during shutdowns. Significantly, all SG alloys (600..800..690) are subject to sulfate-thiosulfate pitting.

The lower concentration limits for pitting in this kind of environment are being probed. A complication is that very low thiosulfate concentrations are easily destroyed by air oxidation – when an experiment was attempted in a micromolar-level thiosulfate solution without sulfate (expecting no pitting), pitting occurred easily.

A possible factor is that the “B” value in  Epit = A – B log (Cl^-) is lower than its normal value of 60-100 mV. Indeed, the derivation of this equation, wherein B log (Cl^-)  is related to the IR drop in the pit, predicts that B should be 30 mV for a doubly charged anion like sulfate.  1.      R.C. Newman, Pitting of stainless alloys in sulfate solutions containing thiosulfate ions. Corrosion, 41, 450‑453 (1985).