Electrochemical Polarization Behavior of 304L Stainless Steel in a Thin Layer of Chloride Solution

Abstract

Spent nuclear fuels are stored in stainless steel canisters. The containers are cooled naturally by a convective airflow through the annulus between the overpack and the canister. This air flow draws dust into the overpacks, and the surfaces of containers are covered with dust. Salts carried by the dust will deliquesce as heat generated by radioactive decay declines over time. The deliquescence of salt deposit could induce various forms of corrosion attack such as pitting and chloride-induced stress corrosion cracking (SCC). In this study, the corrosion behavior of 304L SS - a typical dry storage container material, is evaluated by simulating the deliquescence of salt deposit using a thin electrolyte test condition. Electrochemical corrosion test was performed on 304L Stainless Steel in 3.5 wt% NaCl solution at 20 OC. The tests were carried out in a newly fabricated electrochemical cell with exposed surface area of test specimen being 1 cm2. The cell has a movable PTFE wall which houses a Pt foil as a counter electrode. The adjustable wall enables the thickness of electrolyte to be varied between the SS specimen and the counter electrode. The thickness of the electrolyte was maintained at 1 mm in all the tests. This resulted in the volume of the electrolyte to be 10 ml. Open circuit potential, cyclic polarization and potentiostatic tests at different passivation potential were carried out. The potentiostatic passivation tests were followed by electrochemical impedance spectroscopy and Mott-Schotky analysis in order to understand the initiation of localized corrosion. For comparison, the electrochemical tests are being performed by using bulk electrolytes. The electrochemical polarization behavior in the thin film electrolyte was more or less similar to that in the bulk electrolyte. However, the pitting protection was shifted in the negative direction by about 80 mV in the thin electrolyte as compared to that of bulk electrolyte. Potentiostatic tests conducted at potentials below the pitting protection potential did not show pit initiation. Relatively a quick pit initiation event was observed when the applied potential was 60 mV anodic to the pitting protection potential. Bulk electrolytes showed no passivity breakdown at this potential. The results will be analyzed based on the EIS data, semiconducting properties of the passive film, and UV-Vis absorption of the test electrolytes.