INTRODUCTION
The growth of pits in carbon steel exposed to dilute (0.055 M nitrate-bearing) alkaline salt solutions that simulate radioactive waste was investigated in coupon immersion tests. Most coupons were tested in the as- received condition, with the remainder having been heat treated to produce an oxide film. Nitrite, which is an established pitting inhibitor in these solutions, was present in concentrations from 0 to 0.031 M to 0.16 M; the last concentration is known to prevent pitting initiation in the test solution at the 50°C test temperature. The depths of the deepest pits on coupons of particular exposure conditions were measure microscopically and were analyzed as simple, type 1 extreme value statistical distributions, to predict the deepest expected pit in a radioactive waste tank subject to the test conditions. While the growth rate of pits could not be established from these tests, the absolute value of the deepest pits predicted is of the order of 100 mils after 448 days of exposure. The data indicate that even nitrite concentrations insufficient to prevent pitting have a beneficial effect on limiting the growth of deepest pits.
Pitting is the principal corrosion concern for carbon steel tanks that contain dilute alkaline salt solutions. At the United States Department of Energy?s Savannah River Site, high-level radioactive liquid waste that has been prepared as the feed for the waste verification process presents carbon steel tanks with a dilute solution (nitrate concentration <1 M) capable of inducing pitting. The process for vitrifying the waste requires that the radioactive waste slurries be washed to remove soluble salts. Washing dilutes the concentration of hydroxide, which in stored waste is maintained at a concentration sufficient to prevent both pitting and stress corrosion cracking of carbon steel. The hydroxide concentration is further decreased by reaction with reaction with organic acids in certain waste streams. Through dilution and chemical reaction, the hydroxide concentration in washed waste slurries can fall to yield a pH of the order of 9.5 to 10. In this pH range, the nitrate, sulfate, and chloride anions in the waste are able to induce pitting in the carbon steel.
The initiation of pitting corrosion in SRS waste tanks that contain dilute waste slurries is prevented by maintaining specified nitrite concentrations along with a minimum pH. If2 The nitrite concentrations depend upon the waste temperature and the concentrations of the aggressive anions, according to equations that were developed from laboratory corrosion tests. A conservative safety factor of 1.5 has been incorporated into the nitrite equations to increase the confidence in the application of experimental data to the actual waste tank environment. Through regular periodic chemical analysis of the dilute waste and maintenance of the minimum safe nitrite concentration, the carbon steel waste tanks (including the cooling coils) should be protected against any pitting corrosion.
Investigations at the Savannah River Site into the pitting phenomenon in carbon steel have predominantly focused on the empirical relationship between the temperature and composition of the waste and the nitrite concentration that prevents the initiation of pitting. In the last three years work has been
