A Model of Crack Electrochemistry for Steels in the Active State Based on Mass Transport by Diffusion and Ion Migration

A model of the electrochemical conditions in a static crack has been developed for steel in the active state based on the steady‐state mass transport of species by diffusion and ion migration. The main reactions considered were anodic dissolution, hydrolysis of ferrous ions, and cathodic reduction of hydrogen ions and water with the assumption that the electrode reactions were taking place both at the tip and on the walls of the crack. The reduction of oxygen in the crack was not included since it was demonstrated in a separate analysis that this reaction is insignificant in a static crack at steady state. The pH and potential drop in the crack have been evaluated as a function of external potential, crack dimensions, and other parameters assuming a bulk solution of 3.5% . The effect of dissolution and hydrolysis of alloying elements and also the buffer reactions of seawater on the pH within a crack were also assessed, but mass transport by ion migration was neglected in these cases. The theoretical predictions of the potential drop and pH were in reasonable agreement with values determined experimentally in an artificial crevice of the structural steel BS 4360 50D in 3.5% and in artificial seawater.