THE MECHANISM OF SRB ACTION IN MIC BASED ON SULFIDE CORROSION AND IRON SULFIDE CORROSION PRODUCTS

摘要

Different studies have been made using conventional electrochemical polarization techniques to establish mechanisms for the action of sulfate reducing bacteria (SRB) in microbiologically influenced corrosion (MIC). Nevertheless, there has been practically no detailed follow-up correlating the corrosive process with time and open circuit potential, corrosion products, sessile bacterial growth and attack morphology, to establish the mechanisms of this action. This research project designed an experimental structure to make these correlations utilizing the hydrogen permeation technique as its principal electrochemical tool to follow the cathodic reaction of hydrogen evolution, coupled with any other electrochemical, microbiological or chemical technique that would permit following up on the anodic reaction. The study began by reviewing the classic cathodic depolarization theory using an SRB strain identified as Desulfovibrio desulfuricans subespecie desulfuricans ATCC 7775 and a palladium sheet as a substrate with and without cathodic polarization. Later, sheets of carbon steel and iron were used to study the ferrous ions influence in the progression of the corrosive attack and corrosion products, correlated with open circuit potential, sessile growth and hydrogen permeation. Results obtained permitted establishing a useful correlation for proposing the mechanism for this bacterial action in three stages. The first was controlled by the adsorption of bacterial cells and iron sulfide products, principally mackinawite and pyrite, over the metallic surface, activating it through the formation of micro galvanic corrosion cells which generated a hydrogen permeation peak. The second stage showed bacterial and inorganic equilibrium, in which the metal was slightly ennobled by the formation of a more compact iron sulfide film mixed with polymers generated planktonically by the bacteria. The third stage was controlled by a severe, localized corrosive process configured into groups of deep, rounded holes, produced mainly by local reduction of pyrite to mackinawite, due to the acidity generated by bacterial corrosion, and its subsequent detachment, leaving the base metal active facing a very large cathode made up of different iron sulfide products adhering to the metal: mackinawite, pyrite, esmitite, marcasite, greigite, troilite and pyrrotite.