Role of Asphaltenes in Inhibiting Corrosion and Altering the Wettability of the Steel Surface

Asphaltenes (heptane insolubles) from a variety of crude oils have been identified previously as contributors to inhibition of internal corrosion of mild steel pipelines. However, the mechanism of inhibition is unknown. To explore the mechanism, carbon dioxide (CO₂) corrosion rates and wettability (oil/water contact angles) have been measured using Arab Heavy crude oil and its asphaltenes. Inhibition of CO₂ corrosion rates for carbon steel was measured using electrochemical methods in a glass cell; wettability was assessed using contact angle measurements in a multiphase goniometer. The phase behavior of asphaltenes in corrosion and wetting was evaluated in the crude, toluene (C₇H₈), or heptol (70:30 mixture of heptane [C₇H₁₆] and toluene). Inhibition on steel exposed to a hydrocarbon phase increased with the concentration of asphaltenes in toluene. Inhibition by asphaltenes dissolved in toluene appears to be more effective than in the whole crude, at equivalent concentrations of asphaltenes. At 5 wt% in toluene, asphaltenes form a strong protective layer on the carbon steel surface, which reduces the corrosion rate and makes the surface hydrophobic. When the solubility of the oil is altered to the point where asphaltenes start to flocculate, it enhances the corrosion inhibition greatly. However, the inhibition is not as persistent as for the fully dissolved asphaltenes, and the surface needs to be periodically wetted with the oil phase to maintain the protection.