The corrosion mechanism of X65 carbon steel was evaluated in water environment by increasing CO2 partial pressure from 1 bar at 24 °C to 110 bar at 40 °C, reaching conditions that include water saturated with supercritical CO2. To perform this kind of investigation, the experimental tests were conducted only with CO2, eliminating any other influence of anions present in the solution. Electrochemical impedance measurements and weight loss tests were performed at pH 4, monitored within the whole experiment time. In addition, optical and confocal microscopy analyses were carried out to assess the carbon steel specimens after the immersion tests. The gravimetric results indicated that the corrosive process was not accelerated by increasing the pressure in H2O-CO2 systems. Concerning CO2 participation in the iron dissolution mechanism, electrochemical impedance results did not evidence a direct reaction of CO2 on the free iron surface, even when exposed to water saturated with supercritical CO2. If CO2 has any influence in the anodic mechanism, it must be detect by further experiments, however it certainly will not act directly on the free iron surface.
Key words: CO2 corrosion, supercritical CO2, H2O-CO2 systems, electrochemical impedance.