Factors Influencing Passivity Breakdown on UNS N08800 in Neutral Chloride and Thiosulfate Solutions

In this work, the passivity degradation of UNS N08800 in solutions containing Cl − and S 2 O 3 2− is studied by using polarization curve, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Experimental results reveal that the passivity breakdown heavily depends on anodic potentials, concentration ratios of Cl − to S 2 O 3 2− , and the composition of materials. A combined effect between Cl − and S 2 O 3 2− on pitting corrosion is observed at high anodic potentials at which the passive film is broken down; in this situation, S 2 O 3 2− ions can enter into the pits by electromigration, and they are reduced to S ads and S 2− , stabilizing the metastable pits and accelerate the pit growth rate. However, there is no such combined effect at low anodic potentials when the passive film is intact, on the contrary, S 2 O 3 2− is beneficial to passivity in chloride solutions. At low anodic potentials, the interaction of S 2 O 3 2− with an intact passive layer is weak, and the adsorption of S 2 O 3 2− would mitigate the detrimental effect of Cl − ions.