Modelling of crevice corrosion

Abstract

This thesis describes advances in the mathematical modelling of the initiation period of crevice corrosion. Mass transport was treated rigorously and based on infinitely dilute solution theory. Chemical reactions were decoupled from transport processes by the assumption of equilibrium. The first simulation describes the processes that occur during initiation of crevice corrosion on a titanium crevice, immersed in an oxygenated, aqueous sodium chloride environment. For mass transport purposes, the crevice was assumed to be unidimensional and the bulk solution composition was invariant. The model predicted a rapid decrease in pH in the crevice due to the hydrolysis of the dissolution product. The crevice mouth did not deoxygenate which caused a slight increase in the pH. A significant amount of chloride ion was predicted to migrate into the crevice, with the highest concentration occurring at the crevice tip. A study on the effect of the size of the crevice gap, demonstrated that narrower crevices initially have a greater ohmic potential drop, which decreases as the crevice solution becomes more concentrated. In a second model, the computational domain was extended further into the bulk solution. Mass transport and chemical reaction were simulated in both the crevice and bulk solutions, and charge transfer processes were determined for the crevice walls and bold surface. The crevice and the bold surface were electrostatically coupled using Mixed Potential theory. A simulation was performed for a titanium crevice immersed in deaerated sodium chloride solution. As expected, the predictions showed very little change in pH and chloride ion concentration. The model was initially unstable but the fluctuations in concentration quickly subsided, as the conductivity of the solution increased. The extended model was then used to simulate the crevice corrosion on a titanium crevice in a neutral oxygenated chloride solution. The extended model predicted a rapid decrease in pH, once the crevice was deoxygenated, to a value lower than that predicted by the one dimensional model. The predicted levels of chloride ion in the crevice with the extended model were also lower. The concentrations of the bulk solution species were determined and it was found that there were small changes in the levels of all species. The value of the bulk solution pH at the bold surface initially increased slightly and this increase proceeded to spread through the solution. The most significant change in concentration in the bulk solution occurred with oxygen. The levels of oxygen decreased at the bold surface in sufficient quantities to deoxygenate the crevice mouth. This resulted in a pH decrease at that location. It was concluded from this study that the constant bulk solution boundary condition, that is implemented in all other crevice corrosion models, is adequate except for the oxygen concentration. (Abstract shortened by UMI.)