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 S2O32- is studied by using polarizationcurve, 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 S2O32-, and the composition of materials. A combined effect between Cl- and S2O32- on pitting corrosion is observed at high anodic potentials at which the passive film is broken down; in this situation, S2O32- ions can enter into the pits by electromigration, and they are reduced to Sads and S2-, 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, S2O32- is beneficial to passivity in chloride solutions. At low anodic potentials, the interaction of S2O32- with an intact passive layer is weak, and the adsorption of S2O32- would mitigate the detrimental effect of Cl- ions. (C) The Author(s) 2017. Published by ECS.