Cathodic protection (CP) is one of the corrosion control techniques mostly used with quite satisfactory results; however, where the phenomenon is microbiologically influenced corrosion (MIC), the efficiency of such control systems decreases significantly, due to a number of factors, among which the following could be mentioned: MIC can increase the kinetics of the corrosion reactions, increasing the CP current necessary to achieve a given level of polarization; the microorganism can attack pipeline coating, increasing exposed metal surface area and further increasing the CP current required to achieve the desired polarization,1 and the extracellular polymeric membrane (EPS) produced by microorganisms can generate an effect of ohmic drop2. This paper presents a review of the experiences reported worldwide related to the problems of corrosion in presence of sulfate reducing bacteria (SRB) with cathodic polarization. It also gives a review of the best criteria of polarized potential to be maintained when the problem is generated by MIC.
The corrosion of exposed metallic structures in a corrosive electrolyte is one of the most important problems found in the industry. If the corrosion is not controlled it can result in higher costs to repair or substitute equipment and pipes, even higher when accidents unexpectedly happen and they can cause human losses. A good corrosion control system is thus important. In the case of external corrosion a lot of time is spent in the selection, design and installation of a protection system to control or prevent this problem, guarantying the good operation of a plant. Cathodic protection, considered at the beginning as a scientific curiosity, acquired the status of an advanced technology in the decade of the 60´s. Its application field extends from the protection of buried metallic structure and marine facilities to prevention of stress corrosion cracking, fatigue corrosion, and dezincification, among others3, with certain effectiveness under determined circumstances. In the case of buried and submerged structures, cathodic protection has been applied with enough success and big benefits at -0.850 V vs Cu/CuSO4 reference electrode (CSE) as protective potential criterion to extend their useful life, specifically in high corrosiveness media; however, when they contain microorganisms like sulfate reducing bacteria (SRB), this potential criterion changes to -0.950 V vs CSE to polarize the metallic structure adequately. This statement was proposed by Butlin and Vernon4 in 1949 and by Horvath and Novak5 in 1964, based on thermodynamic data of Fe-S-H2O system, and was afterwards experimentally verified by Fisher6 in 1981 who studied the cathodic protection of pipelines in North Sea sediments and muds containing SRB. Then, in 1984 Barlo and Berry7 determined that more negative potentials than -0.95 V vs CSE are required to prevent corrosion when SRB are present. External microbiologically influenced corrosion is a serious dilemma in the industry. In the past and even today, it has been recognized that -0.95 V vs CSE is not enough to protect the pipelines in a wet clay soil or submerged in sea or brackish water.
