Comparison of corrosion products/scales on X65 carbon steel in a NaCI solution and a complex brine at different CO_2 partial pressures

摘要

This study investigates the contrasting characteristics of corrosion products/scales and corrosion behavior of X65 carbon steel when exposed to 4.9 wt.% NaCI solution compared to a complex brine solution at different CO_2 partial pressures (3.5 to 29.5 bar) at 200°C. The corrosion product morphology and chemistry are identified through a combination of microscopic and spectroscopic techniques. The results indicate that Ca~(2+) and Mg~(2+) ions within the complex brine significantly influence the formation of surface deposits, and have an effect on the carbon steel resistance to general corrosion, particularly at lower pressure. The surface products are mainly comprised of iron carbonate (FeCO_3) when the samples were exposed to the pure NaCI solution containing CO_2 at a partial pressure over 15.4 bar, and a mixture of FeCO_3 and magnetite (Fe_3O_4) when the partial pressure was 6.5 bar and below. However, a complex mixture of Ca, Mg and Fe-containing deposits was detected on the surface in the complex brine solution, with Fe_3O_4 only being detected at the lowest evaluated CO_2 partial pressure of 3.5 bar. The implications of the formation of such corrosion products and their relation to general corrosion are discussed, with results suggesting the formation of a thin Fe_3O_4 layer at the steel surface plays a critical role in determining the resistance of the material to general corrosion, and that the nature of the brine chemistry influences the propensity and kinetics of this layer formation.