Effects of Oxygen on Scale Formation in CO₂Corrosion of Steel in Hot Brine: In Situ Synchrotron X-ray Diffraction Study of Anodic Products

The effect of low concentrations of oxygen on the anodic dissolution of carbon steel in CO2-saturated aqueous NaCl at 80°C is simply to change the dissolution product from colloidal amorphous material assumed to be amorphous ferrous carbonate to a crystalline carbonate green rust, Fe6(OH)12CO3, which forms rapidly. This material is deposited from solution and does not inhibit the dissolution. Corrosion is limited by the nucleation onto the surface and growth of a crystalline scale of siderite (FeCO3), as in the absence of oxygen. The effects of oxygen and of solution flow can be understood in terms of effects on supersaturation for carbonate crystallization, and on the effect on the surface pH, caused by the precipitation of carbonate green rust. The formation of crystalline chukanovite (Fe2(OH)2CO3) is strongly affected in the presence of trace oxygen, both by flow and electrode potential, effects which are consistent with lower supersaturation and higher surface pH caused by green rust precipitation. In the presence of trace oxygen, FeOOH and Fe3O4 could be detected in small amounts, and are assumed to form as a consequence of the oxidation of chukanovite.