Effect of precorrosion by galvanostatic anodic polarization and temperature on the formation rate of iron (ii) carbonate film

Master's thesis in Environmental technologyCorrosion on the inside walls of steel pipelines and process equipment is mostly caused by CO2 which can cause property damage on the production and the transport of oil and natural gas. In order to control corrosion of steel in CO2 environment, it is important to consider the formation of surface film and their influence on the corrosion rate. Iron (II) carbonate (FeCO3) is an insoluble corrosion product which forms a film that potentially can be act as protective layer on the corroding surface. The presence of Fe3C structure seems to be important in order to make the protective film which reduces the corrosion rate. In order to form the protective FeCO3 formation film, a forced precorrosion is stimulated anodic current in order to enrich the amount of exposed Fe3C (carbide) which facilitates the FeCO3 formation film. The specimens which were used on these experiments are X65, St52 and St33 in base solution 1 g/kg NaCl and 50% wt MEG under precorrosion. Furthermore, 100 mmol/kg NaHCO3 was added in the solution after precorrosion. The three steel qualities have different chemical composition especially the carbon (C), chromium (Cr), manganese (Mn) and silicon (Si) fraction which may show the different corrosion rate trend. The precorrosion times used in these experiments are 24 and 48 hours with temperatures of 40⁰C and 80⁰C, followed by free CO2 corrosion for 216 hours. Electrochemical impedance spectroscopy, potentiodynamic cathodic sweeps, open circuit potential and Rp/Ec trend are methods used to follow the corrosion of these steels. In addition, SEM picture with EDS analysis is conducted to describe the electrode surface