Effect of Corrosion Inhibitors and Oil on Carbon Dioxide Corrosion and Wetting of Carbon Steel with Ferrous Carbonate Deposits Available to Purchase

The wettability of the steel surface is an important factor co-determining the risk of corrosion in multiphase pipelines for transportation of oil and gas. The present paper deals with the effect of carbon dioxide (CO₂) corrosion inhibitors on the wettability of carbon steel with ferrous carbonate (FeCO₃) deposits. The wettability was studied by contact angle measurements at 25°C, 1 bar CO₂, and 3 wt% sodium chloride (NaCl). Both oil-in-water and water-in-oil experiments were performed. Inhibitor performance was studied in CO₂ corrosion tests at 60°C, 1 bar CO₂, and 3 wt% NaCl, both in the absence of oil and in tests where the steel specimens were alternately exposed to oil and aqueous phases. Three inhibitors were investigated: two commercial, inhibitor-based chemicals (an oleic imidazoline salt [OI] and a phosphate ester [PE]), and cetyltrimethylammonium bromide (CTAB), a well-characterized quaternary ammonium compound. A refined, low-aromatic oil product was used in the tests. Both OI and PE substantially decreased the tendency of water droplets to spread on an initially oil-wet steel surface with FeCO₃, but had a negligible effect on the wettability of initially water-wet surfaces. The inhibitor performance of the OI and PE was significantly improved by the presence of oil. Corrosion rates one order of magnitude lower than in the tests without oil exposure was obtained. The addition of inhibitor had a limited effect on the corrosion rate of steel with FeCO₃ deposits in the absence of oil. The results indicated that the enhanced performance was caused by a modification of the inhibitor film. The water-in-oil contact angle measurements indicated that CTAB significantly enhanced water wettability.