EIS Response of MIC on Carbon Steel

Abstract Microbially influenced corrosion of carbon steel under sulphate reducing (sulphide-producing) bacterial activity (SRB) results in the formation of both ferrous sulphides as well as biofilm on the metal surface. The electrochemical characteristics of the ferrous sulphide/steel interface as compared to the biofilm/ferrous sulphide/steel interface has been studied with EIS, DC polarisations (Tafel, LPR) and a potentiostatic step technique. The electrochemical response is related to a threshold sulphide concentration above which very characteristic changes such as indications of finite diffusion and high interfacial capacitance appear. These effects are strongly enhanced in the biologically active environment as compared to the sterile solutions possibly due to an enhanced porosity in the biofilm/ferrous sulphide surface layers. The effect of these features is that EIS may be used for identifying the formation of biofilm/ferrous sulphide layers but unfortunately also that corrosion rate estimation by these electrochemical techniques is unreliable when these layers form. These considerations are also relevant for other corrosion systems where film formation might mask the electrochemical response from the metal surface.