Through implementation of electrochemical and surface analytical techniques, we investigate the behavior of magnetite (Fe3O4) and mixed metal (M) magnetite (MxFe3-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 MxFe3-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.