Carbon steel corrosion under anaerobic–aerobic cycling conditions in near-neutral pH saline solutions – Part 1: Long term corrosion behaviour

The influence of anaerobic–aerobic cycling on pipeline steel corrosion was investigated in near-neutral carbonate/sulphate/chloride solution (pH 9) over 238 days. The corrosion rate increased and decreased as exposure conditions were switched between redox conditions. Two distinct corrosion morphologies were observed. The majority of the surface corroded uniformly to produce a black magnetite/maghemite layer approximately 4.5 μm thick. The remaining surface was covered with an orange tubercle, approximately 3–4 mm in cross section. Analysis of the tubercle cross section revealed a single large pit approximately 275 μm deep. Repeated anaerobic–aerobic cycling localized the corrosion process within this tubercle-covered pit.

► Anaerobic–aerobic cycling on pipeline steel forms two distinct surface morphologies.
► Seventy-five percentage of the surface was covered by a black, compact layer ∼4.5 μm thick.
► A tubercle, ∼3 to 4 mm in cross section, covered the remaining 25% of surface.
► The tubercle cross section showed a single large pit ∼275 μm deep.