A study of corrosion of mild steel in mixtures of petroleum distillates and electrolytes

Petroleum distillates are not corrosive unless water and some other species are contained in them at particular concentrations. It was not found in the literature what water concentration and which species result in high corrosiveness of light petroleum distillates. The purpose of this study was to determine minimal ("critical") water content and corrosive species type resulting in high corrosiveness of petroleum distillates. The study was conducted using mixtures of light petroleum distillates (naphtha and gasoline) with water content < 5 vol%, and in some cases with 10 ppm sodium chloride (NaCl) solutions. The study confirmed the electrochemical origin of the corrosion mechanism and showed that the main cause of corrosion in the petroleum distillates-electrolyte mixtures was the presence of water and dissolved oxygen. "Critical" water contents were obtained for naphtha-water and gasoline-water mixtures. Salts (chlorides, sulfates, and nitrates) and organic acids (formic [HCOOH] and acetic [CH₃COOH]) were extracted from the naphtha and gasoline by the aqueous phase and were responsible for the severe corrosion of these petroleum distillates. Valuable data regarding different types of corrosion attack (pitting or uniform corrosion) on mild steel were acquired during the study.