Development of Charge Transport Model to Study the Diffusion-Controlled Dissolution and Multi-Pit Interaction in Pitting Corrosion

Abstract

Recently, we developed a reaction-transport model that incorporates diffusion, migration, and complexation to examine the charge transport inside a one-dimensional corrosion pit [1]. In this presentation, we will demonstrate the application of this model to study the “diffusion-controlled” dissolution of a metal in a pit from a more advanced point of view, such that the limiting anodic current density varies with the bulk NaCl concentration. An analytical result on anodic limiting current density as functions of pit depth, diffusivity, and chloride concentration was derived explicitly and coupled with artificial pit experiments. In a dilute chloride solution with low conductivity, the limiting current density is increased due to the increase of the migration flux near the pit mouth. On the other hand, the presence of Na+ at the bottom of the pit is significant for moderate to high bulk NaCl concentrations, which increases the limiting current density to some degree. The proposed full mass transport model can be used to predict limiting current densities at different ranges of bulk NaCl solutions. At above 2.5 M NaCl, the conventional diffusion-only model can give reasonably accurate results, but the effect of Na+ at the bottom of the pit on the solubility of metal ions and the limiting current density should be considered.Finally, we will discuss the extension of the current work to the two-dimensional pit model to investigate the propagation and the interaction mechanism of multiple pits. The 2D multi-pit simulation under galvanostatic conditions shows the interaction between pits of different sizes. Due to the increase of the local chemistry inside the larger pit, the dissolution rate at the surface is faster and suppresses the growth of the neighboring pits.Reference: V. A. Nguyen, A. G. Carcea, M. Ghaznavi and R. C. Newman, The Effect of Cation Complexation on the Predicted “B” Value in Galvele’s Pit Model, J. Electrochem. Soc., 166, C3297 (2019).