Application of a Reaction-Transport Model with Cation Complexation and Variable Diffusivity to Galvele’s Criteria for Pitting Corrosion

Abstract

A reaction-transport model in a one-dimensional pit geometry is constructed to study the effects of cation complexation, hydrolysis and multiple aggressive anions on the mass transport of ions, and the IR drop, within a pit at the anodic limiting current density. In particular, the study focuses on the B value in Galvele’s pit model, which states the dependence of pitting potential (Epit ) on chloride concentration ([Cl-]) Epit = A- B log[Cl-] where the B value is predicted to be 60 mV from Galvele’s model [1]. However, this value is found to vary up to around 100 mV in practice for stainless steels, indicating that the IR drop inside the developing pit is not explainable by a simple model [1-4]. The present model is constructed at steady state and with a limiting anodic current density (fixed dissolved metal concentration at the bottom of the pit), which is carried by metal-containing species. When there is no cation complexation or hydrolysis, the B value remains at 60 mV in NaCl and 30 mV in Na2SO4, even when the ionic diffusivities are functions of chloride concentration. However, the B value is found to vary between 30 mV and 60 mV at different ratios of anionic bulk concentration in the mixed solution of singly and doubly charged anions (such as chloride-thiosulfate systems). Due to the charge difference between these anions, the bulk ratio between the anionic concentrations is not preserved inside the pit, which results in an enrichment of the doubly charged anions at the bottom of the pit. For a one metal system, the effect of hydrolysis on the B value is negligible compared with that of cation complexation, which is very significant and could potentially explain a lot of practical observations. For an Fe-17Cr alloy, the B value increases up to 100 mV, in agreement with experiments, assuming dissolved chromium can make cationic chloro-complexes. The obtained results can be rationalized via the change of potential gradient and ionic fluxes near the top of the pit at different bulk NaCl concentration, to maintain the critical chemistry for pit stability and the complexation reaction inside the pit.