Oil Wetting and Carbon Dioxide Corrosion Inhibition of Carbon Steel with Ferric Corrosion Products Deposits

Abstract

Abstract Wettability of carbon steel with corrosion product films (iron carbonate [FeCO3], FeCO3 with oxidized surface, and rust [FeO(OH)]) was investigated through contact angle and inhibitor performance tests. Two corrosion inhibitors, an oleic imidazoline compound (OI) and a phosphate ester compound (PE), were used. The inhibitor performance was studied in carbon dioxide (CO2) corrosion tests at 60°C, 1 bar CO2, 3 wt% sodium chloride (NaCl), and 20 vol% oil, where the samples were alternately exposed to oil and aqueous phase. A refined, low aromatic oil was used in the tests. Addition of both inhibitors lead to significant changes in the wettability of oxidized FeCO3 surfaces. The contact angle tests showed that a transition from a preferential water-wet state to a preferentially oil-wet state was achieved for both inhibitors. Electrochemical measurements indicated that addition of OI enabled the oxidized FeCO3 surface to retain an oil film after immersion in oil. Electrochemical impedance spectra (EIS) measurements revealed a significant increase in the measured, uncompensated resistance at high frequencies after immersion in oil. The apparent retained oil film caused a significant drop in corrosion rate in aerated solutions. The reduction in corrosion rate remained stable even after a return to a CO2-saturated solution. A similar effect was also seen on a rusting carbon steel surface. No significant lasting effect of oil exposure was seen for PE in the inhibitor performance tests.