A Study of Caustic Corrosion of Carbon Steel Waste Tanks

Solution chemistry of wastes from US atomic weapons production is controlled to inhibit corrosion of carbon steel tanks used in containment and storage. The pH, nitrate ion and nitrite ion concentrations of fresh solutions are maintained within specified limits for this purpose. In the start up process for a new waste evaporator (RHLWE), non-radioactive solutions of similar chemistry will be circulated through carbon steel piping between a steel tank and the evaporator. The evaporator is fabricated from a corrosion resistant nickel base alloy. The equipment will be exposed continuously to the hot corrosive caustic solutions. Published corrosion rates for steel in pure caustic at the elevated temperatures indicate losses >1.3 mm/y. Because the total test period for start up is relatively short, penetration will not occur. However, concern exists because the rust particles will probably circulate and precipitate throughout the system. A laboratory study was performed in order to determine corrosion to be expected for the specific waste solutions being used in start up testing of the new equipment. The test solutions used in the study were (A) 40% NaOH + 16% NaNO3 by weight, representative of concentrate produced in the evaporator; (B) 40% NaOH, a control solution; and (C) 5% NaOH + 27% NaNO3, representative of fresh waste. Corrosion rates for both simulant solutions at boiling are an order of magnitude less than for the control solution, indicating that nitrate ion significantly inhibits the effect of the hydroxide. Moreover, the corrosion rate for solution (A) is 1.5-2X that for solution (C), further emphasizing the importance of nitrate to hydroxyl ion ratio. Following determination of the corrosion rates, calculations of hydrogen gas generated as a result of the corrosion process together with analysis of the system showed that the explosive possibilities are remote. In summary, the test results indicate that the corrosion rates for steel are acceptable for the short term simulant tests for the new evaporator. The amount of particulate is probably not a concern, though filtration is recommended as an option. The hydrogen formation is also not a concern because it is in a steam environment, and is continuously swept from the storage vessel.