CO2 Corrosion Inhibition and Oil Wetting of Carbon Steel With Ferric Corrosion Products

The wettability of steel and corrosion product layers may influence the risk of corrosion in pipelines carrying oil and gas. While FeCO₃ is the main corrosion product of carbon dioxide (CO₂) corrosion, ferric corrosion products may be present. Wettability of some ferric corrosion products on carbon steel (rust (FeO(OH)), iron carbonate (FeCO₃) exposed to aerated solution) was investigated through dispersion, contact angle and inhibitor performance tests. Inhibitor performance was studied in CO₂ corrosion tests (3 Wt% NaCl, 60 ºC, 20 vol-% oil) under 1 bar CO₂ or aerated conditions. A refined, low aromatic, oil was used. Contact angles were measured on steel coupons with surface deposits, and dispersion tests were carried out with particles of the respective compounds.
Addition of the corrosion inhibitors changed the surfaces from preferential water-wetting situation to intermediate wetting. At high concentrations (>100 ppm), one of the inhibitors made the rust particles fully oil-wet. Electrochemical impedance measurements suggested that one of the inhibitors modified the wettability of the oxidized FeCO₃ , enabling the porous layer  to retain an oil film after exposure to oil. The retained oil caused a significant drop in corrosion rate. A similar effect was also seen on a rusted carbon steel surface.
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