Carbon steel is thermodynamically unstable in water with dissolved CO2 and the only reason that carbon steel is so attractive and can be so widely used in oil and gas production is that the steel surface becomes covered by a protective layer of corrosion products oil mineral scale or inhibitors. Based on recent findings and understanding the paper will discusses the CO2 corrosion mechanism with particularly focus on how CO2 affects the water chemistry the electrochemical reactions on the bare steel surface and the initiation and growth of protective corrosion product films. As many sweet systems contain organic acids that affect the solution chemistry and the formation and stability of the FeCO3 corrosion product films organic acids need also to be considered when the effect of CO2 is discussed.