In situ synchrotron X-ray diffraction study of scale formation during CO2 corrosion of carbon steel in sodium and magnesium chloride solutions

In situ synchrotron X-ray diffraction was used to follow the formation of corrosion products on carbon steel in CO₂ saturated NaCl solution and mixed NaCl/magnesium chloride (MgCl₂) at 80 °C. Siderite (FeCO₃) was the only phase formed in NaCl solution, while Fe(OH)₂CO₃ was also detected when MgCl₂ was present. The proposed model is that siderite precipitation, occurring once the critical supersaturation was exceeded within a defined boundary layer, caused local acidification which accelerated the anodic dissolution of iron. The current fell once a complete surface scale was formed. It is suggested that MgCl₂ addition decreased the required critical supersaturation for precipitation.