Carbon steel corrosion under anaerobic–aerobic cycling conditions in near-neutral pH saline solutions. Part 2: Corrosion mechanism

Abstract A corrosion mechanism has been developed to describe tubercle formation along pipeline steels during successive anaerobic–aerobic cycles. Small concentrations of O2 under nominally anaerobic conditions can lead to the separation of anodes and cathodes. Under subsequent aerobic conditions localized corrosion is then promoted by O2 reduction on the general magnetite-covered surface. Subsequently, the conversion of magnetite to maghemite passivates the general surface, and focuses corrosion within one major tubercle-covered pit. On switching from aerobic to anaerobic conditions, corrosion is temporarily supported by the galvanic coupling of lepidocrocite (γ-FeOOH) reduction (to γ-Fe-OH·OH) to steel dissolution primarily within the tubercle-covered pit.