Localized Corrosion of Stainless Steel in a Nuclear Waste Cooling Water System— Part 7: Direct Radiation Experiments

To investigate the efficacy of sodium nitrate (NaNO3) as an inhibitor for large corrosion pits in stainless steels subject to direct radiation, tests were carried out using a design of artificial pit specimen that had been developed specially for this purpose. The artificial pits also incorporated tensile stress to detect any susceptibility to stress corrosion cracking during the inhibition phase when the pit chemistry would be in transition. Specimens were made from three austenitic stainless steels: 18Cr-13Ni-1Nb, UNS S30403, and UNS S34780 weld. NaNO3 dosed to a nitrate concentration of 50 g/L effected a major reduction in the current between the stainless steel pit specimens and spatially separated cathodes made from stainless steel mesh. In most cases, however, the residual currents had turned negative toward the end of the test. This has been attributed tentatively to a radiolysis effect on the pit chemistry to produce nitrous acid (HNO2). Stress was also incorporated into the specimens to detect an...