Pitting corrosion localized acidification

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

It is worth noting that, in whatever way a pit nucleus was bom, its further development to form a pit embryo (growing metastable pit) depends on the electrolyte concentration which locally sets up in the electrolyte when the metal dissolves. The development of the pit embryo implies the local stabilization of an acidic corrosive medium, differing from the surrounding one. This local acidification induces a local dissolution of the metal in the active state, which is compensated only by the diffusion of the corrosion products. This dissolution in turn provokes an acidification due to the hydrolysis reactions and the process is self-sustained, provided that fire diffusion or the electromigration is slow enough in the electrolyte. Formally [3h,5d], if Jis the anodic current andXrepresents the concentration in corrosion products, one has dX/dt = KJ V, X) - DX, where the metal-electrolyte potential difference F is a control parameter (J increases with V) and parameters K and D represent respectively the production in corrosion products by the anodic dissolution and their dilution into the electrolyte by a diffusion process. The steady state conditions write dX/dt = 0. The system is locally stable when... 

Corrosion develops as pitting at the fuel-water interface. Like any other type of corrosion, it follows an electrochemical mechanism. Oxidation of kerosene by bacteria releases organic acids that modify the pH of the medium. Microbial deposits form anodic sites by local acidification. The oxidation reaction consumes the oxygen dissolved in kerosene and in water. 

A marked increase in aluminum Ep values towards anodic potentials was observed in the presence of marcescens, even in the sterile medium, suggesting the metal surface may experience some protective action by these bacteria. Local acidification enhanced by adhesion processes taking place at the metal/mycelia interface accounts for some of the specific effects of resinae in the corrosion process of aluminum alloys in fuel/water systems (Salvarezza et al., 1979). The acidification also has been reported in the literature as a differential effect between two Pseudomonas spp. in relation to aluminium corrosion. Acidity can prevent repasivation and may hinder the formation of a protective oxide film. Therefore, under acidic conditions, pitting of the metal by chloride anions occurs at more cathodic potentials than in neutral solution (Salvarezza et al., 1983). 

---