Effect of Pre Corrosion by Galvanostatic Anodic Polarization and Temperature on the Formation Rate of Iron Carbonate Film
Carbon dioxide (CO2) corrosion is the main type of corrosion studied in oil and gas industry. The major concern with CO2 corrosion is that it causes failure of equipment in the main downhole tubing and transmission pipelines, and thus may cause disruption in the production. Carbon dioxide gas dissolved in formation or injection water forms carbonic acid (H2CO3). Carbonic acid is corrosive to carbon steel or low alloy steel. Corrosion rates in a CO2 system can reach very high levels, but it can be effectively inhibited. One way of inhibition is by favoring the formation of the protective iron carbonate (siderite) scale. Magnetite scales are also formed in CO2 systems, and corrosion product scales often consist of layers or mixtures of siderite and magnetite. Experiments were carried out in order to observe the iron carbonate scale formation on three common carbon steel materials of grades X65, St52 and St33. The materials were exposed to CO2 saturated brine at 40°C and 80°C for precorrosion periods of twenty four or forty eight hours. The corrosion of the samples were monitored using Electrochemical Impedance Spectroscopy (EIS), Potentiodynamic Cathodic Scan (PSC), Rp/Ec trend and the samples were analysed by Scanning Electron Microscopy (SEM) with Energy Dispersed X-Ray System (EDS) analysis after experiments. In conclusion, the result of the experiments showed that the galvanostatic stimulated precorrosion and temperature had significantly contributed to the protective FeCO3 formation on surface steel. Both for 40°C and 80°C, protective scales are formed on surface steel after few days to a week of exposure time.