Galvanic Corrosion of Carbon Steel in Carbon Dioxide Caused by Magnetite

Complicated environments in oil and gas production systems often result in the deposition of various corrosion product scales on the carbon steel surface. It is not widely recognized that many iron corrosion products, particularly magnetite, are nonmetallic semiconductors that furnish galvanic coupling to their base metal, resulting in galvanic corrosion. Laser Raman spectroscopy is a very powerful analysis technique that has been applied to identify iron oxides/hydroxides such as magnetite (Fe₃O₄), hematite (Fe₂O₃), goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) based on the unique Raman spectral band.

A galvanic couple of magnetite with mild steel was investigated under carbon dioxide conditions, and this indeed demonstrated galvanic corrosion of the steel.  The effects of carbon dioxide partial pressure, solution pH and cathode to anode surface area ratio of the magnetite/mild steel couple were studied and all of these factors were found to have a significant effect on the galvanic corrosion rate. The presence of magnetite scale may explain the high localised corrosion rates experienced in some oil production lines operating at low partial pressures of CO₂ and environments normally considered non-corrosive. Corrosion inhibitors with high inhibition efficiency in carbon dioxide corrosion of mild steel were tested, and were found not to be as effective in galvanic corrosion control. An understanding of the electrochemistry and chemistry of the corrosion products will be the key to resolve this problem.

Keywords: Galvanic corrosion, Raman spectroscopy, corrosion product scale, iron oxides, magnetite.