Propagation of Localised Corrosion: FEM Approach M. Stroe *,1 , R. Oltra 1 , B. Vuillemin 1 , G. Girardin 2 1. Institut Carnot de Bourgogne (ICB), 2. AREVA NP *ICB, UMR 5209 CNRS-Université de Bourgogne,9 Av. A. Savary, BP 47870 F-21078 DIJON Cedex, France, Mioara.Stroe@u-bourgogne.fr Abstract: Crevice corrosion of stainless steel in chloride media was investigated based on a 11 species model. The role of parameters like pH inside the crevice, external potential, chloride concentration of the bulk solution and crevice geometry was studied. A thermodynamic criterion for arrest / propagation of the crevice corrosion was used in order to identify the conditions for which the localised corrosion can propagate. This study shows that there is a combination of crevice geometry and potential for which the crevice is not blocked by precipitates and thus the corrosion can proceed. The time – dependent resolution showed the evolution of the conditions inside the crevice from the initiation of the corrosion until the steady state was reached. Keywords: crevice corrosion, stainless steel, chloride. 1. Introduction Crevice corrosion is a phenomenon of localised corrosion that develops in confined regions of a metallic material in contact with a corrosive medium, with the majority of the surface being in contact with the same medium and in a state of relative immunity. The most frequent cases relate to the passive materials (stainless steels, aluminium and sometimes titanium alloys) and occur mainly in aerated media containing halides (generally chlorides). The mechanism of the crevice corrosion involves three stages: incubation, initation and propagation. During incubation the occluded solution is depleted in oxygen due to limited diffusion from the external environment. The absence of oxygen leads to the separation between the anodic site (inside the crevice) and the cathodic site (the external surface). The anodic process, that continues inside the crevice, increases the concentration of metal cations. Due to hydrolysis of metal cations, the acidity inside the cavity increases. The excess of positif charges inside the crevice determines a diffusion flux of anions (chloride) from bulk solution toward the crevice. Thus, during incubation period the occluded solution becomes exempted of oxygen, more acidic and more concentrated in chloride, whithout any apparent damage of the metallic surface. This period is characterised by the “time of incubation” which can vary from a few hours to years. The inititation stage is intermediate between the incubation and the active propagation of the corrosion. Two theories were proposed for the initiation mechanism: critical crevice solution (CCS) theory and ohmic drop (IR- drop) theory. According to the first mechanism [1], the metallic surface inside the crevice is activated due to increase of the agressivness of the solution (increased acidity and chloride content). The process is autocatalytic: the acidification and chloride enrichement induce an enhancement of the corrosion rate. This theory allows an accurate estimation of incubation time and can explain the dependency on the composition of the sensitivity to localised corrosion. The CCS theory neglects the effects of potential. The second theory proposed, IR – drop theory [2], considers only the effect of the potential, neglecting the role of changes of solution chemistry inside the crevice. According to this theory, the separation of the cathodic and the anodic sites occurs due to depletion of the oxydant within the crevice. A potential drop between the external surface and the interior of the cavity is developed. For the metals with Tafel behaviour, the potential drop will reduce the corrosion, and thus it has beneficial consequences. For metals that exhibit active/ passive transition, the ohmic drop can bring the potential inside the crevice in the active zone. In this case, the potential drop will have a negative effect, enhancing the corrosion. This theory explains the location of the attack but its main weakeness is that it can be applied only for the materials that exhibit active – passive transition. The next stage is the propagation of the crevice corrosion. During this stage, the anodic current can increase, and thus enhances the rate of localised corrosion. Other processes like new Excerpt from the Proceedings of the COMSOL Users Conference 2007 Grenoble