Mechanism of magnetite formation in high temperature naphthenic acid corrosion by crude oil fractions

•Protective oxygen-containing layer was found in corrosion by crude oil fractions.•Oxygen-containing layer was formed underneath iron sulfide layer.•Oxygen-containing layer was formed due decomposition of iron naphthenates.•Low content of chromium enhanced layer protectiveness. Naphthenic acids and sulfur compounds present in crude oils may cause corrosion in oil refineries. In some cases, corrosion product scale formed in these oils may provide corrosion protection, but in other cases this may not be true. Current research investigates the nature of protective scales formed on steel corroded by crude oil fractions using microscopic and analytical techniques. This research identifies a thin oxygen-containing layer in-between the steel and a thicker iron sulfide outer layer contributing to the protection against naphthenic acid corrosion. Formation of this oxygen-containing layer is likely due to naphthenic acids in the crude oil.