Corrosion of Mild Steel in Cultures of Ferric Iron Reducing Bacterium Isolated from Crude Oil II. Mechanism of Anodic Depolarization

A protective ferric covering was formed on mild steel coupons by reaction with the oxidative inhibitor, nitrite. In cultures of a ferric iron reducing bacterium, there was a loss of passivity of coupons and polarization studies revealed intense depolarization of the anode, while in the absence of the bacterium, the metal remained passive. The anodic depolarization was accompanied by conversion of ferric to ferrous compounds and was marked by the change in color of the medium from dark brown to a greenish hue. Electron micrographs revealed that in the absence of the organism, a dense, crystalline surface deposit covered the metal, but was extensively removed in the presence of the bacterium. The bacterium caused anodic depolarization of mild steel by removing or preventing the formation of a protective ferric covering.