Influence of Sulphate-reducing Bacteria on the Corrosion Potential of Iron

Abstract

THE potential assumed by an iron electrode immersed in a culture of sulphate-reducing bacteria has been studied by Hadley¹, using a heterotrophic medium. These experiments indicated that the inoculation of the sterile medium, in which the potential of an iron electrode had been allowed to reach a steady value, was followed by a movement of the potential in the anodic (negative) direction. This, in turn, was followed by a rise to a steady value more cathodic (positive) than the initial value in a sterile medium. These changes of potential are shown diagrammatically in the accompanying graph. We feel, however, unable to agree with the explanation given for the first fall of potential, namely, that it is due to bacterial utilization of hydrogen with consequent depolarization of the cathodic areas. Such a depolarization should lead to a potential change in the cathodic (positive) direction, as may be seen from a consideration of the Evans polarization diagram for a corroding system. We also feel unable to accept the statement that the final steady potential(− 0.370 V., normal hydrogen scale) is due to the establishment of the Fe/FeS/S″ equilibrium, on the grounds that this value is too positive for such an equilibrium. It is, in fact, more positive than the standard Fe/Fe⁺⁺ potential(− 0.441 V.) and would thus imply a concentration of ferrous ions of greater than 1 M. Such a concentration seems impossible in the presence of the large amounts of sulphide produced under heterotrophic conditions.