THE INITIAL POTENTIAL AND CURRENT DISTRIBUTIONS OF THE CREVICE CORROSION PROCESS

A computational model has been developed to calculate the potential and current distributions in the electrolyte phase and on the electrode surface for a system in which a part of the anode is passivated while the rest remains in the active dissolution state. The computation employs the finite element method allied with a boundary variation and a trial and error technique. From the obtained distributions, the location of the boundary between the active and passive regions on the anode can be predicted. In the case of a crevice, this means that a critical distance into the crevice exists beyond which active corrosion (crevice corrosion) takes place. In addition to the active/passive behavior of the material, solution conductivity, applied potential at the sample's outer surface, crevice gap and depth dimensions, and passive current density influence this critical distance to different degrees. The developed software package may also be used for (i) IR-induced crevice corrosion under open-circuit conditions where the electrode potential at the outer surface is established in the passive region by an oxidant, (ii) the design of anodic protection systems, and (iii) the design of some corrosion resistant coatings and alloys. Applications of the model are given for a crevice whose polarization curve is the same as that of the bulk solution, a situation that is approached in buffered solutions, complexing solutions (e.g., Fe in an ammoniacal solution) or in solutions of pH adjusted to the equilibrium pH of the hydrolysis reaction. Alternatively, the polarization curve of the crevice electrolyte composition (either measured or estimated) can be used. The values of the critical distance into a crevice for pure iron in buffered ammoniacal (pH 9.7) and acetic acid (pH 4.6) solutions are calculated and shown to be in good agreement with available (pH 4.6) literature values.