Local supersaturation and the growth of protective scales during CO 2 corrosion of steel: Effect of pH and solution flow

Abstract

By correlating in-situ synchrotron X-ray diffraction measurements with electrochemical measurements using a rotating disc electrode, we demonstrate the critical dependence on the local supersaturation of the kinetics of formation of a protective crystalline scale on the surface of carbon steel during CO2 corrosion in brine at elevated temperature. We show that the total current is the sum of a current due to dissolution of iron and a current due to growth of a crystalline layer. We show that the dissolution current and the surface supersaturation are controlled by the thickness of an initially-formed amorphous layer. As in earlier work at room temperature, we infer that the amorphous layer dissolves as a carbonato-iron complex with surface concentration of the dissolving species determined by the electrode potential, and speculate on the importance of the chemistry of this dissolution reaction in determining the corrosion result. We construct a simple transport-reaction model, which shows that the supersaturation is determined by the precipitation rate constant of colloidal FeCO3 and by the product of the current for Fe dissolution and the diffusion boundary layer thickness. Using this model, we show crystal growth rate varying quadratically with supersaturation at pH 6.8 and linearly at pH 7.3. The effects of electrode potential, surface roughness, microstructure and flow are simply to change supersaturation by changing the current density per unit projected area flowing through the amorphous initially formed layer. Variation of brine concentration has no effect. We illustrate the sensitivity to solution flow of the crystallinity of the final scale. We show that siderite is the first crystalline product and that chukanovite follows, with a delay time that decreases with increasing pH. The ratio of chukanovite to siderite is low at sufficiently high pH and increases with decreasing pH, possibly through a maximum. From the results, we advance ideas concerning the importance of local microenvironments and local fluctuations in mass-transport rate.