Abstract
This paper summarizes the field and laboratory studies on microbially influenced corrosion (MIC) of buried pipelines and its control. MIC occurs when bacteria in soil are in contact with bare areas of the pipe. During our work on MIC we have demonstrated that two cases are to be considered. One is bare cast iron pipes, the other a coating defect of cathodically protected steel pipelines. The bacteria responsible for severe corrosion of cast iron pipes is iron bacteria (IB) that form extensive tubercles resulting in rapid graphitic corrosion under tubercles. The evaluation of catholic protection reliability on steel pipe in soils in the presence of bacteria such as IB and sulfate-reducing bacteria (SRB) was performed with steel specimens exposed to soils containing above mentioned bacteria potentiostatically polarized in the potential range from -0.65V to -1.3V (Cu/CuSO4). A steel specimen simulates a coating defect. In the case of sandy soil containing IB, when catholic polarization potential is applied more negative than the generally accepted protection potential of -0.85V (Cu/CuSO4), catholic protection is achieved substantially by a decrease in number of IB due to the environmental changes including a decrease in Eh and an increase in pH caused by the process of cathodic reaction. On the other hand, in the case of clay containing SRB, when catholic polarization potential is applied more negative than the recommended protection criteria for active SRB clay of -0.95V (Cu/CuSO4), cathodic protection is achieved. In the potential range from -0.95V to -1.1V (Cu/CuSO4), SRB can thrive actively resulting in the formation of adherent iron sulfide layers on the steel surface, thereby cathodic protection is achieved by their protective properties without ruptures. At the more negative potentials than -1.1V (Cu/CuSO4), sufficient catholic protection is achieved due to the formation of excess hydroxyl ions generated by enhanced cathodic reaction.
