Modeling unidirectional corrosion damage evolution in a SS 316 pencil-electrode experiment

Highlights: • A unidirectional pit growth model under diffusion and activation control until repassivation is developed. • The model is solved to yield the potential drop, concentrations of sodium, chlorine and metal ions. • The model solution yields the pit depth as a function of time. • Analytical expressions for the pit stability product relationship during diffusion control are developed. A three-stage, unidirectional pit growth model is developed to simulate corrosion damage evolution in a pencil electrode experimental setup. Stage I models initial pit growth, in the absence of a salt film, under activation control and a constant current density. Stage I is terminated when the ionic metal concentration reaches its saturation limit. Stage II models stable pit growth under diffusion control where the metal ion flux is the same as the dissolution rate of the metal at the bottom of the pit. During Stage II, the applied bulk potential is decreased at a specified scan rate. When the bulk potential reaches the transition potential, Stage III begins. In this stage the pit growth is under activation control that is defined by a prescribed polarization curve. Stage III continues until the metal repassivates as the potential is decreased. The governing system of equations for each stage is solved analytically, where possible, or numerically to determine the potential drop and the concentrations of sodium, chloride, and metal ions within the pit. The pit depth as a function of time is determined from Faraday's Law in Stages I and III, and from a mass balance at the electrolyte/metal interface in Stage II. The cumulative pit depth is compared with experimental pit depths in the literature for stainless steel in chloride solution.