Replacement inhibitors for high level waste tank cooling coils

Sodium chromate has been an effective corrosion inhibitor for the cooling coil systems in Savannah River Site (SRS) waste tanks for over 40 years. Due to age and operating history, cooling coils occasionally fail allowing chromated water to leak into the environment. The costs of reporting and cleaning up chromate spills became significant enough for SRS to consider alternate inhibitors. Confirmatory tests were performed to assess the effectiveness of three alternative corrosion inhibitor systems for the waste tank cooling water systems: (1) sodium molybdate (250 ppm as Mo)/sodium hydroxide (pH 10), (2) sodium molybdate (50 ppm as Mo)/sodium silicate (50 ppm as Si), and (3) sodium nitrite (500 ppm)/ sodium hydroxide (0.01 M). The tests were conducted under stagnant conditions to simulate a worst-case scenario. The results showed that these inhibitors were as effective as chromate at minimizing general corrosion at solution temperatures between 30--70 C. However, the initiation of localized attack in crevice regions, in solutions containing the alternative inhibitors at 70 C, was observed. Also, for the nitrite and the molybdate systems to be effective, suitable biocide is needed. On the other hand, interval coupon tests showed that the molybdate inhibitor systems prevented significant propagation of the localized attack