Passivation Behavior of Friction Stir Welded 304L SS in Thin Layer Electrolyte

Friction stir welding (FSW) is being considered as a potential repair welding technique for nuclear spent fuel dry storage canisters. The heat affected zones (HAZ) of the fusion welded regions of the canisters become prone to stress corrosion cracking (SCC) due to chlorides in the environment. Pitting corrosion is usually observed in the welded regions which is the precursor for SCC. The goal of the present work is to understand the effect of chloride on the atmospheric corrosion behavior of FSW 304L stainless steel. A simulated crack was introduced into hot rolled and annealed 304L plates by electric discharge machining, and FSW was performed at two different temperatures, 725 and 825 °C in order to repair the simulated crack. Figure 1(a)-(c) shows the optical images of the 825 °C FSWed 304L alloy. Electrochemical corrosion tests are being performed on FSWed 304L alloy in 3.5 wt% NaCl aqueous solution at 20 OC as a thin-layer electrolyte. A comparison will be made between the corrosion behaviors of the base metal, HAZ region and the weld nugget region for both FSWed specimen types. The electrochemical tests are carried out in an in-house designed electrochemical cell with less than 10 ml of electrolyte and variable electrolyte thicknesses in the range of 0.5 – 2 mm. Open circuit potential, cyclic polarization and potentiostatic tests at different passivation potentials are being conducted to understand the pit initiation and growth events as a function of FSW microstructures. Electrochemical impedance spectroscopy and Mott-Schottky analysis are carried out to understand the semiconducting properties of the passive film and their correlation with the passive film breakdown. This presentation will discuss the effect of FSW microstructures on the passivity breakdown in thin electrolytes containing chloride ions, and the results will be compared with that of tests conducted in the conventional bulk electrolyte arrangement. Figure 1