MECHANISMS OF THE MICROBIAL CORROSION OF ALUMINUM ALLOYS
Pitting potential was used recently to assess the aggressiveness of the biological species in the microbial corrosion of aluminum alloys in hydrocarbon/water systems. This parameter is analysed here in relation to the electrolyte composition of the medium, especially with reference to chloride, nitrate, and phosphate levels. Chloride anions, usually present in hydrocarbon fluids and culture media, control the pitting process Nitrates and phosphates, used as nutrients in culture media, act as pitting inhibitors displacing the pitting potential value to high anodic regions at a low chloride/inhibitor ratio. In these conditions, microbial uptake of pitting inhibitors is the principal cause of the pitting potential decrease during microbial growth. At high chloride/inhibitor ratio or in the absence of inhibitor, metabolites production is the most important factor in the aggressiveness increase. Dodecanoate anions recently proposed as the main metabolic product leading to the passivity breakdown of aluminum alloys, show no effect on the pitting process, either in the absence or presence of chloride. The acidic metabolites produced by “Cladosporium resinae” are able to facilitate the breakdown of the passive oxide film by chloride anions decreasing the pitting potential value. The action of unidentified metabolic products or cellularlysis compounds on the corrosion process can be also envisaged.
Stephen A. Amadi, C.P. Ukpaka and Jacob B. Neeka