Effect of the crevice gap on the initiation of crevice corrosion in passive metals

Past research into the mechanism governing the time to active crevice corrosion-the incubation period-of a passive metal crevice has produced theoretical models coupled with the B-dot model, the Debye-Huckel limiting law, and other activity models to correct for nonideal behavior at moderately high concentrations. In this research, the transport model of Watson and Postlethwaite is coupled with the ionic interaction model of Pitzer to predict the effect of the crevice gap on the iR drop and chemical activity of the crevice solution. Two cathodic reactions, crevice external oxygen reduction and crevice internal hydrogen ion reduction, are assumed to balance metal dissolution. To validate the model, the experimental Type 304 (UNS S30400) stainless steel crevice of Alavi and Cottis is simulated. Model predictions improve upon predictions of past models and match observations of this experimental work within experimental uncertainty. The effect of crevice gap on a titanium crevice immersed in 0.5 M aqueous sodium chloride (NaCl) solution at 25 degreesC also is predicted. The iR drop, electrical conductivity, and chemical activity of the solution increases as the crevice gap decreases. The relationship between iR drop and deviation from charge electroneutrality of the solution is investigated.