Abstract
The corrosion behavior of carbon steel in 5.0M MDEA and DGA solutions was investigated by potentiodynamic polarization and impedance measurements under 4.5MPa carbon dioxide (CO2) at 100°C. The corrosion-product layers formed on the carbon steel in these conditions were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). A tight corrosion-product layer, which consists of iron carbonate (FeCO3) crystal, was formed on the carbon steel in the 5.0M MDEA solution, but a loose corrosion-product layer, which consists of iron carbide (FeC3) and FeCO3, was formed in the 5.0M DGA solution. In the 5.0M MDEA solution, the corrosion rate of carbon steel was low (-1μAcm-2). The anodic and catholic reactions were inhibited by the tight corrosion-product layer. Particularly, the diffusion of the species responsible for the catholic reaction was strongly inhibited by the layer, and of course, that was the determining-step. In this case, the corrosion potential was about -0.6V vs. Ag/AgCl, and the main catholic reaction may be the reduction of carbonic acid (H2CO3). Contrarily, in the 5.0M DGA solution, DGA-carbamate formed a soluble complex with Fe(II), and thereby carbon steel corroded at a high corrosion rate (-2×103μAcm-2). The corrosion potential was about -0.8V vs. Ag/AgCl, and the main catholic reaction may be the reduction of DGA-ammonium ion.
