Modified Magnetite Surface Layers on Carbon Steel in Aqueous CO₂ Environments

Through implementation of electrochemical and surface analytical techniques, we investigate the behavior of magnetite (Fe₃O₄) and mixed metal (M) magnetite (M_xFe_3-xO₄) layers on carbon steel surfaces in CO₂ environments. Initially, Fe₃O₄ layers formed naturally at 250°C are evaluated, to fully characterise the behaviour of Fe₃O₄ layers and their role in carbon steel corrosion across a range of CO₂-saturated conditions. Gravimetric measurements were performed to determine corrosion rates, complemented by the implementation of X-ray diffraction and scanning electron microscopy to shed further light on the influence of Fe₃O₄ layers on carbon steel corrosion. To simplify the evaluation of metal dopants, Fe₃O₄ and M_xFe_3-xO₄ layers were formed on carbon steel by electrodeposition (in sodium hydroxide, metal sulphate solutions at 80°C for 30 min) with similar characteristics to natural layers. The layered coupons were then immersed in a CO₂-saturated, pH 5, 50°C solution at atmospheric pressure (aggressive conditions for Fe₃O₄ survival). EIS measurements were implemented to evaluate the corrosion behaviour of the underlying steel, whilst galvanic corrosion was measured using zero resistance ammetry by coupling the layered coupon to a bare carbon steel coupon. The addition of magnesium and zinc to the Fe₃O₄ layer enhanced galvanic interaction but showed similar corrosion protection to Fe₃O₄ layers. The addition of manganese reduced galvanic currents marginally but dramatically reduced the corrosion protection provided by the layer.

Surface layers often form on carbon steel surfaces in carbon dioxide (CO₂) saturated environments and under certain conditions can offer corrosion protection to the underlying steel.^1-9 One such layer, magnetite (Fe₃O₄) is a semiconductor, having a reported electrical resistivity of the order of 10⁻² to 10⁻¹ Ω·cm and band gap of 0.1 eV.^{10, 11} The conductive properties of Fe₃O₄ are of significant importance when understanding the corrosion behaviour of carbon steel, as Fe₃O₄ can readily establish a galvanic couple with the steel surface upon which it has formed. This can lead to aggressive corrosion rates of the bare carbon steel (i.e. regions not covered by Fe₃O₄) when only partial coverage of Fe₃O₄ is achieved.^12-14 Of particular interest in this study is the role of Fe₃O₄ surface layers on carbon steel in CO₂ corrosion environments. In these conditions, Fe₃O₄ is typically formed in neutral-alkaline pH conditions and temperatures >100°C, and has been suggested to reduce uniform corrosion rates on carbon steel when formed.^{1, 15} Fe₃O₄ is also sometimes observed at temperatures <100°C on carbon steel, forming alongside iron carbonate (FeCO₃), and has been suggested to play a role in a process known as ‘pseudo-passivation’, resulting in a rise in the open circuit potential (OCP) and high levels of corrosion protection.^{3, 16, 17} However, as the conductivity of the layer can initiate galvanic and localized corrosion, uncertainty surrounds the true protective properties of Fe₃O₄ layers.