Fe3C influence on the corrosion rate of mild steel in aqueous CO sub 2 systems under turbulent flow conditions

The corrosion and corrosion inhibition of mild steel in CO sub 2 saturated solutions were studied under turbulent flow conditions at different pH. Electrochemical measurements using a.c. and d.c. techniques in uninhibited solutions of pH 3.8 indicated the formation of protective surface films (FeCO sub 3 ) in short immersion times. However, as the exposure time was increased the corrosion rate always increased, an effect attributed to the increased surface area of Fe sub 3 C residue from corrosion of the steel. At pH 5.5, the corrosion rate always increased with time, behaviour also associated with the presence of FCC surface film. The huge cathodic area of Fe sub 3 C seems to have a more important impact on the electrochemical behaviour than the poorly formed FeCO sub 3 products. The effect of Fe sub 3 C on inhibition by a quaternary amine inhibitor at pH 3.8 is to increase the corrosion rate as the pre-corrosion time is increased. The FCC causes either (a) a cathodic area increase reflected in the corrosion rate increase with time or (b) a potential gradient in the pores of the Fe sub 3 C layer that prevents positively charge amine ions from reaching all anodic sites.