Topics in the Mathematical Modeling of Localized Corrosion

Abstract

Localized corrosion describes dissolution processes concentrated at specific areas on the surfaces of metals. In some types of localized corrosion, enhanced dissolution rates arise from partial or complete destruction of the protection normally afforded by the passive oxide film covering the metal surface. Oxide breakdown can be due to mechanical rupture (stress corrosion cracking), the chemical action of aggressive anions such as chloride (pitting corrosion), the impaction of solid particles on the surface (erosion corrosion), or the concentration of corrosion products within small solution-filled gaps (crevice corrosion). Other localized corrosion processes are initiated at metal compositional inhomogeneities such as grain boundaries in alloys (intergranular corrosion), or interfaces between dissimilar metals (galvanic corrosion). The economic impact of all forms of localized corrosion is severe. For example, pitting and stress corrosion cracking together account for about one fourth of equipment failures in the chemical process industries. Metal dissolution rates during localized corrosion are high enough so that large concentration or potential gradients are typically found near the dissolving metal surface. Characterization of these gradients is a necessary precursor for understanding the mechanisms controlling the corrosion rate. Thus, experimental research on localized corrosion has always been closely coupled to quantitative analysis of mass transport processes by mathematical modeling. In this chapter, three examples are presented which illustrate the range of models applied to localized corrosion processes, reflecting the particular interests of the authors. Section II, written by Hebert, is a review of recent work on the modeling of pitting corrosion. The remainder of the chapter communicates results of recent work by Tribollet on galvanic corrosion (Sect. III) and on the simulation of the impedance in crevice-type geometries.