A mechanistic study on semiconductivity conversion of passive films under varying sulfate to chloride concentration ratios

Abstract

Alloy 800 is an important steamgenerator material used in nuclear power plants, and so there is significant interest in the properties of passive films of this alloy under service conditions. In this work, the semiconductivity of Alloy 800 in sulfate and chloride solutions was investigated using Mott-Schottky analysis, electrochemistry impedance spectroscopy (EIS), scanning electron microscopy (SEM), and scanning electrochemical microscopy (SECM). The Mott-Schottky results show that the semiconductivity is affected by the sulfate to chloride concentration ratio; p-type semiconductivity is exhibited at high concentration ratios but transitions to n- type when the concentration ratio is low. EIS, SEM, and SECM results indicate that the degradation formof the passive filmchanges fromtranspassive dissolution to pitting as the concentration ratio decreases while the film's surface reactivity increases, an effect that is related to the semiconductivity conversion. The observed variation in semiconductivity results fromthe competitive adsorption of sulfate and chloride, a process that modifies the potential drop at the film/solution interface, changes the vacancy types and ultimately determines the semiconductivity. © Editorial office of Acta Physico-Chimica Sinica.