An investigation into the CO2 corrosion of mild steel using conventional electrochemical techniques and high resolution microscopy

In this study the corrosion mechanism of mild steel in CO2 saturated brines was investigated. Electrochemical polarisation (LPR and Tafel polarisation) experiments were used to study the corrosion rate of the steel specimens exposed to CO2 saturated brines, aerated and de-aerated brines. The presence of the different dissolved was shown to affected the cathodic reaction rather than the anodic. The highest corrosion rates were obtained for the steels in the CO2 saturated brines. Changes in the anodic and cathodic reactions were observed as several environmental parameters were investigated, such as, temperature, CO2 pressure and the effect of various inhibitors. The results revealed that the corrosion rates increased linearly with temperature for de-aerated and aerated steel specimens between a 25°C and 80°C, whereas the corrosion rates for specimens exposed to the CO2 saturated brines only followed a linearly dependence at temperature less than 55°C. Increasing the CO2 partial pressure resulted in a higher corrosion rates.The inhibition effect of several different kinds of inhibitors on the CO2 corrosion of mild steel was also investigated. Furthermore, a detailed study of the corrosion morphology of mild steel was carried out using Scanning Electron Microscopy. Chemical information about the corroded steel surfaces was provided by Energy Dispersive X-ray Analysis. Throughout the analysis it was demonstrated that temperature, exposure time and vessel size had significant effects on the surface topography. The initial stage of corrosion results in the segregation of grain boundaries and Fe3C platelets. Moreover, the formation of a passive FeCO3 film was clearly observed on CO2 corroded steel surfaces, dependant on the environment parameters. In addition, the Atomic Force Microscopy was also used to study the surface morphology of CO2 corroded specimens. The application of this technique was quite limited, in that the only samples we were able to study were those that had been corroded for 24 hours. Even so the AFM proved a useful tool due to its ability to provide a three dimensional aspect of the corroded surface. The results of this study have demonstrated the feasibility of using the AFM to study localised corrosion sites caused by CO2 corrosion. Several key observations indicated that the observed surface morphology using the AFM directly correlate with those identified by the SEM.