The role of corrosion inhibitor is explained by the function of surfactant, i.e., partition from oil to water, dispersion in water and adsorption on metal surface. This study selected stearic acid as a surfactant and paid attention to the effect of oil on corrosion inhibition. Stearic acid was treated as a single component corrosion inhibitor. The corrosion inhibitor efficiency of stearic acid was observed in a metal-oil-water system. An acyclic saturated hydrocarbon, i.e., an alkane was tested as oil because it has a simple chemical structure. Stearic acid was dissolved in alkanes and injected into a synthetic brine. The roles of alkane and water were evaluated by measuring corrosion rates of carbon steel. As a result, the inhibitor efficiency increased, although stearic acid has low solubility in water. This can be explained by hypothesizing that stearic acid forms mixed micelle with alkane and to disperse into the brine. The effect was enhanced with the shorter carbon chain length of alkane having higher solubility in water. The inhibitor efficiency was affected by pH, suggesting that the adsorption of stearic acid depends on its dissociation in water.
Corrosion inhibitors are used for carbon steel pipelines in the oil and gas industry. Based on current understanding,1 the inhibitor molecules mitigate corrosion through adsorption to the internal surface of the pipeline, forming a barrier film that impedes electrochemical corrosion reactions at the metal surface. Micellization is a key factor of the surfactant distributions. Most corrosion inhibitors consist of active ingredients, commonly surfactants, and other chemical agents in a solvent. The surfactants are usually amphiphilic organic compounds, meaning the molecules contain both hydrophilic groups (called "polar head") and hydrophobic groups (called "nonpolar tail"). The hydrophobic tail groups are normally straight chain hydrocarbon. The chain length, i.e., the number of carbon atoms, affects the formation and hydrophobicity of the film adsorbing on the metal surface. Based on empirical data, the desirable length is more than C12.2
