Properties of the Passive Film on Alloy

This study investigates the properties of the passive film formed on Alloy 22 in chloride solutions through electrochemical impedance spectroscopy. The primary objective was to analyze the behavior of the passive and transpassive ranges at a temperature of 90°C in 1 M NaCl. The methodology involved utilizing a RX-(R//CPE) circuit model to interpret the impedance data across a range of chloride concentrations and pH levels at open circuit potential. Results indicated the presence of two distinct time constants at both the onset and conclusion of the passive range, suggesting that resistances related to ion transfer occur at the film interfaces rather than within the film itself. Furthermore, it was found that the protective characteristics of the passive film enhanced over time due to the film's thickening and aging processes, with both film resistance and space charge layer thickness increasing with potential. At elevated potentials, the oxidation of Cr3+ to Cr6+ was observed, leading to transpassive dissolution of the film. The findings reveal a transition in electronic characteristics of the film from n-type to p-type semiconductivity at higher passive potentials, highlighting the complexity of the film's behavior in chloride-rich environments.