Biofilms are able to change the electrochemical characteristics of passivable metals, creating anodic and cathodic areas and inducing electroactive effects that can directly influence the corrosion rate of the material in a specific environment. This kind of process is referred to as Microbial Influenced Corrosion (MIC), and, even if its mechanisms have been widely studied in literature, it remains a problem largely underestimated and difficult to prevent. A huge number of bacteria are generally involved in MIC process and one of the difficulties of carrying out laboratory tests is to reproduce this great variety. The technique employed in this study aims at overcoming these issues using a Microbial Fuel Cell (MFC) as environment to test the microbial corrosion behavior of metallic samples. In a MFC, fuel is typically composed of the organic substance dissolved in the solution that fills the anodic half-cell. Bacterial populations, inoculated in the anodic solution, build up an electroactive biofilm on the anode that catalyzes the fuel oxidation. In traditional fuel cells the anodic half-cell is separated from the cathodic one by an electrolytic membrane. In more simple MFC air-breathing systems, the biofilm growing on the cathode both consumes the oxygen diffusing through the pores, thereby guaranteeing anaerobic conditions to the anode, and catalyzes the oxygen reduction.

The redox mechanisms of electroactive bacterial occurring on the anode and cathode in a MFC are similar to those involved in microbial corrosion. Therefore laboratory studies on electrophilic bacteria acting on microbial fuel cell electrodes, offer the possibility to study in well-defined anodic and cathodic regions, the mechanisms responsible for microbiological corrosion.

This study, in particular, has been carried out in order to characterize the electrochemical behavior of Ag-doped hybrid coatings deposited on stainless steel. The hybrid coatings are based on epoxy resin formulations where the silver nanoparticles are produced in-situ with a bottom-up approach: the silver precursor undergoes a photoreduction reaction during the UV-curing process and leads to a microstructure with nanoparticles dispersed inside the polymeric matrix. The sample under study is inserted in the chamber of the fuel cell and connected to the two electrodes in order to be able to monitor and analyze the variation in time of the current flowing between each one of the three system nodes affected by the same bacteria pool. Both coated and uncoated stainless steel samples have been immersed in the MFC anodic chamber containing wastewater inoculated with swine manure and acetate (as fuel). The performance and growth of bacterial biofilm on the coated samples, in anaerobic conditions, have been investigated by means of anodic and cathodic polarization and electrochemical impedance spectroscopy (EIS). Eventually, the coatings and the biofilms have been morphologically characterized by means of field emission scanning electron microscopy.