Galvanic effects induced by siderite and cementite surface layers on carbon steel in aqueous CO2 environments

Siderite (FeCO3) and cementite (Fe3C) layers develop naturally on carbon steel surfaces in aqueous carbon di-oxide (CO2) environments. This study evaluates galvanic corrosion induced by such layers when coupled to bare carbon steel. In CO2-saturated, 50 degrees C, pH 5 conditions, the Fe3C-filmed carbon steel acted as the net cathode, significantly enhancing bare steel corrosion rates. Galvanic currents induced by the FeCO3-filmed steel were much lower, with FeCO3 removed from the surface as Fe3C was revealed concomitantly on bare steel. It is proposed that the presence of Fe3C amongst the FeCO3 layer is responsible for galvanic interaction, rather than FeCO3 itself.