Abstract
CO2 corrosion is a complex process and a large number of variables are involved, such as: pH, temperature, chloride concentration, fugacity and system total pressure. This work presents electrochemical impedance spectroscopy and gravimetric results of tests carried out at pH 4, which was controlled within the whole experiment time. The impedance diagrams obtained with and without CO2 presented the same interfacial process, suggesting the same corrosion mechanism for both. Therefore, no evidence was found for a direct reaction of CO2 on Fe surface in the experimental conditions presented herein. In specific test conditions, local pH measurements demonstrated huge pH shifts whereas bulk pH remained constant. Thus, a strict controlled pH is mandatory to study corrosion mechanisms. For the same pH, comparing solutions with and without carbon dioxide, the weight loss was six times higher in environments containing CO2 and eight times higher in CO2/brine conditions. The CO2 buffering effect, previously reported in the literature, was only observed in brine condition. The results suggest a very good correlation between electrochemical and gravimetric data obtained at the same pH.
