This paper describes the development of a new mathematical model of the incubation period of crevice corrosion. The model uses a new treatment of the transport processes, which includes both ionic migration and diffusion. An explicit finite-difference technique was applied to a uni-dimensional crevice along the crevice depth, with appropriate boundary conditions at both the crevice tip and bulk solution.
Transient concentration results demonstrate that the major reason for the pH decrease in crevice solution is the production of soluble metal hydroxides, particularly CrOH2+. No solid monomer metal hydroxides (ex. Cr(OH)3 〈s〉) were formed in any of the simulations, which suggests that metal oligomeric complexes will be formed in the crevice solution. In addition, a comparison of the simulation results and experimental literature data (for alloys 304 [UNS(1) S30400], 316L [UNS S31600], 904L [UNS N08904], and Inconel(2) 625 [UNS N06625]) are discussed.
The numerical simulation results, using Inconel 625, show that there is a distinct delineation between a metal exhibiting passivity or active crevice corrosion, for a given crevice depth or average crevice gap. If these two dimensions are not accurately determined, then accurate prediction of the incubation period of crevice corrosion is extremely difficult.
