Mechanism of magnetite formation in high temperature corrosion by model naphthenic acids

• Magnetite scales were found in naphthenic acid (NAP) corrosion.
• Magnetite scales were formed due to thermal decomposition of iron naphthenates.
• Formation and protectiveness of magnetite scales depended on structure of NAP.
• Carboxylic acids confirm corrosion observations for commercial NAP.

Naphthenic acid (NAP) corrosion is a major concern for refineries. The complexity of NAP in crude oil and the sulfidation process hinder a fundamental knowledge of their corrosive behavior. Studies with model acids were performed to explore the corrosion mechanism and magnetite scales were found on carbon steel. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray diffraction methods detected differences in the quantity and quality of magnetite formed by model acids. These scales exhibited different resistance to higher severity NAP corrosion in a flow through apparatus. Magnetite is proposed to be formed by thermal decomposition of iron naphthenates.