Localized Corrosion of Stainless Steel in a Nuclear Waste Cooling Water System—Part 3: Development of a Large Artificial Pit

Abstract

To enable testing of candidate inhibitors for chloride-induced pitting of austenitic stainless steel in a radiation field, an artificial pit cell was required that was capable of sustaining corrosion at rates in the range revealed by plant inspections. Two designs of artificial pit cells are described. These allowed measurement of the internal and external cathodic contributions to the corrosion rate and changes in the composition of the artificial pit solution as time progressed. The number of electrolytic holes and the type and quantity of ferruginous sludge used were found to exert significant effects. The final design adopted on the basis of empirical testing demonstrated stable corrosion at a high rate (5 mm/y to 6 mm/y) in nominally 15 mg/L Cl− bulk solution containing 5 mg/L hydrogen peroxide (H2O2) at 60°C and stable pit solution composition for 20 days to 25 days at least. The initial strength of pit solution used was found to be unimportant, since the composition quickly gravitated toward 10 g/L to 20 g/L Cl− with a pH in the range of 1.5 to 2. For this composition, the corrosion reaction was driven almost exclusively by the external cathode, the internal hydrogen evolution ceasing to make a significant contribution.