Microbiologically Influenced Corrosion (MIC) by Sulfate-Reducing Bacteria (SRB) has been the subject of extensive studies for the past five decades and several theories have been proposed to explain the mechanisms governing MIC. Recently, a corrosion mechanism for the action of SRB on iron was proposed, which seeks to explain the initiation of this kind of corrosion in order to improve prevention and mitigation techniques. This mechanism was supported on the fact that there should be a local pH decrease at the metal/solution interface. So, the principal aim of this work was to prove this theory by evaluating an iron interface exposed to a culture medium rich in nutrients and SRB. The H2S concentration produced was also evaluated to establish its relation with the pH at the interface. To measure these parameters during 24 hours, pH and H2S microelectrodes with tip diameters of 10 µm were used. A SRB pure culture of Desulfovibrio desulfuricans subs. desulfuricans was used, grown as batch culture in modified ATCC 1249 medium with and without ferrous ions, in order to determine their influence on the pH and H2S behavior. Attack morphology and biofilm structure in both mediums were also determined by scanning electron microscopy (SEM), placing iron sheets at vertical and horizontal position. The results showed that pH into the biofilm decreased depending on the ferrous ions presence. SEM photomicrographs showed that MIC damage was localized, characterized as isolated and linked holes, but the more sever condition for metal surfaces occurred when they were placed at horizontal position and ferrous ions were present. In this case, linked holes within areas of general corrosion were observed. Also, It was seen that bacteria and corrosion products (ferrous sulfides in this case) were mixed and formed a complex biofilm structure that covered the iron surfaces, being in some cases protective depending on the ferrous ions presence. Finally, the statement that pH decreases at vertical position in mediums with ferrous ions could be possible, because the corrosion severity in this kind of medium was higher than the one observed in the medium without ferrous ions but lower than at horizontal position. Also, it was confirmed the influence of ferrous ions and ferrous sulfides aggressiveness in the corrosion process.
Microbiologically Influenced Corrosion (MIC) is a phenomenon where microorganisms produce or accelerate a corrosion process. It provokes serious damages for several industries and all the systems which involve the use of natural waters (oil, shipping, electrical industry, among others)1. One of the main types of bacteria associated with MIC is Sulfate-Reducing Bacteria (SRB)2, 3. They are capable of using sulfate as electron acceptor in their anaerobic respiration process, producing or excreting hydrogen sulfide (H2S) to the environment. These microorganisms typically coexist in naturally occurring biofilms, forming complex consortia on corroded metal surfaces. The interaction of the biofilm and the substratum originates a new physical and chemical environment at the metal/solution interface.
