Abstract
For sludge washing to be conducted in existing Hanford carbon steel tanks, there must be an assurance that the tanks will be safe from failure by pitting, stress-corrosion cracking (SCC) or other failure processes when the corrosion inhibitors present in the waste are diluted during the sludge washing operation. Testing in the past has been concerned with defining the safe operating regimes in concentrated waste environments. The corrosion testing reported herein has been carried out to determine the safe chemical bounds in the dilute regime.
Six month exposure coupon tests, slow strain rate tests, and potentiodynamic scans have been completed on a statistically designed test matrix of sixteen test solutions. Uniform corrosion rates (maximum of 0.15 mpy) were too low to be an important failure process. Incipient pitting in the vapor phase was commonly found, with a few deep pits propagating at a maximum rate of 40 mpy. Slow strain rate testing revealed SCC in solutions #1 and #2, only. These two solutions are low in hydroxide, with nitrate/nitrite ratios ≥10. Crevice corrosion was also observed in these two solutions. The composition of solution #1 falls into a chemical range in which the data base (Table 1) used for SCC prevention would have predicted SCC immunity. This may be an indication this data base, which is the technical basis for preventing SCC in concentrated environments, is incomplete at the lower concentrations.
