Modeling and simulation of passive film formation and breakdown in chloride ion containing electrolytes – A Point Defect Model extension

Chloride ion-induced passive film breakdown is an important aspect for corrosion engineering and research. Most of the underlying mechanisms are not easy to investigate due to the challenges in obtaining precise experimental data. In this context, physical models can be highly beneficial. This contribution presents an extension to the well-established Point Defect Model, developed to simulate, and investigate passive film dynamics. A reaction mechanism involving both chloride ion-induced film dissolution and chloride ion penetration is introduced. Additionally, a coupling approach from the passive state to transpassive metal dissolution was realized, and pH-regulating electrochemical reactions, such as oxygen reduction, were incorporated at the film/solution interface. The extended model provides transient insights into passive film forming and degrading mechanisms, and accurately captures easily measurable material responses, such as open circuit potential and the critical breakdown potential, for varying chloride ion contents. Model calibration and verification were achieved through both literature review and own electrochemical investigations with AISI 316 L stainless steel. Simulation results reveal a decrease in passive film thickness and an increase in electric field strength within the passive film for increasing chloride ion contents. Furthermore, simulation results indicate the absence of chloride ions within the passive film of AISI 316 L. Additionally, the hypothesis of chloride ion-induced vacancy condensate formation at the metal/film interface was investigated, concluding with a successful condensate simulation, followed by a film dissolution due to film growth impediment.