Experiments were conducted in six operating oil and gas production pipelines over four years to determine internal pitting corrosion rates under realistic operating conditions.
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Pitting corrosion rates were similar when the compositions of surface layers were similar.
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When a compact layer of single species formed, the surface was protected from pitting corrosion; the iron sulfide (FeS) layer was more protective than the siderite (FeCO3) layer.
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When multiple layers of several species formed, the susceptibility of the surface to pitting corrosion increased. Frequent changes in the pipeline operating conditions facilitated the formation of multiple layers.
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When no surface layer formed, the susceptibility of the surface to pitting corrosion decreased but was not eliminated. Extraneous materials (e.g., sand) on the surface facilitated pitting corrosion.
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In the absence of surface layer and extraneous materials, no pitting corrosion was observed.
