In Situ Synchrotron X-Ray Diffraction Studies of CO2 Corrosion of Carbon Steel with Scale Inhibitors ATMPA and PEI at 80°C

In situ synchrotron x-ray diffraction was used to investigate experimentally the effect of two common scale inhibitors, amino trimethylene phosphonic acid (ATMPA) and polyethyleneimine (PEI), on the corrosion rate and protective scale formation process during carbon dioxide (CO2) corrosion of carbon steel. These experiments were performed under mild anodic polarization in sodium chloride (NaCl) solutions at similar to 1 bar CO2, 80 degrees C, and pH 6.5, such that a protective scale would form readily in the absence of any organic scale inhibitors. These results were compared with longer-term open-circuit weightloss corrosion tests in similar conditions but at up to 30 bar CO2. The scale inhibitors had complex, concentration-dependent effects but in general had a very significant inhibitory effect on the nucleation and growth of siderite (FeCO3), and as a consequence resulted in an increased corrosion rate. The results of pulsed galvanostatic experiments were consistent with the scale inhibitors acting by adsorption onto siderite nuclei formed in the solution close to the surface.