Pitting of steam-generator tubing alloys in solutions containing thiosulfate and sulfate or chloride

The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 degrees 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 similar to 40. Alloy 600 pitted worst at a chloride : thiosulfate ratio of similar to 2000. The results are interpreted through the mutual electromigration of differently charged anions into a pit nucleus, and differences in the major alloy component.