Microbially influenced corrosion (MIC) behavior of stainless steels in natural Seawater, the accelerated corrosion test method and the countermeasure to MIC were studied. Electrochemical measurements and immersion tests were carried out. Furthermore, the effect of the surface. treatment of a stainless steel on the MIC resistance was studied Susceptibilities to localized corrosion of stainless steels in natural seawater are higher than those in synthetic seawater. The biofilm formed on the surface of stainless steels in natural Seawater contains the activated oxygen such as peroxide generated by the metabolism of aerobic bacteria. The corrosion potential becomes noble in the presence of the peroxide because of its higher redox potential than that of oxygen. The ennoblement of corrosion potential can be reproduced in the laboratory by the addition of oxidase to simulate the metabolism of aerobic bacteria. This method can be used to evaluate the MIC resistance of various materials in the laboratory. The surface film of stainless steels electrolyzed in the Fe (III) and Cr (VI) containing solution is the bipolar membrane which consists of the anion selective membrane in the outer layer and the cation selective membrane in the inner layer. This bipolar membrane suppresses the catholic reaction, therefore, the corrosion potential does not become noble even in the presence of the activated oxygen generated by the aerobic bacterial metabolisms, and leads to the improvement of the MIC resistance.
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.
Carbon steel was exposed for long time to an inoculation continuous culturing medium of sulfate-reducing bacteria (SRB). The feed rate of medium was 10cm3hr-1, and the concentaration of Fe2+ of medium was kept to 0.01mol kg-1. The corrosion behavior and metabolism of sulfate-reducing bacteria (SRB) were investigated by measuring weight change of the specimens and by using a phase-difference microscope, hydrogen sulfide gas detector, and measuring the amount of ferrous sulfide in the medium. The number of SRB was above 1010 cell/cm3 after the culture of 50 days. SRB evolved a significant amount of hydrogen sulfide and ferrous sulfide after the culture above 2 days. In the period of after the culture of 50days, the medium contained 2types of SRB with different shapes. One was rod like shape and the other was comma like shape. In this period, pH of medium was kept almost 7 without any pH ajusting. The weight of deposited film on the specimen increased linearly with culturing time, whereas the weight of the specimen substrate due to corrosion decreased linearly with culturing time. The corrosion rate in continuous culturing medium was larger by 7 times than that in batch culture.
There are cases of corrosion which are supposed to have been influenced by microbiological activities. It is not easy work to make the involvement of bacteria clear because of difficulties in reproducing the environmental conditions of the corrosion in question in laboratory tests. A case study introduced in this paper is a localized corrosion from cooling water environment on Ni-base alloy used in plate-type heat exchanger in chemical plants. It was made clear from observation of corrosion samples and through reproduction corrosion test in laboratory that some bacteria have contributed in causing the corrosion. With a series of laboratory tests, sterilization treatment of water was found quite effective to prevent bacteria-influenced corrosion and continuous application of biocide demonstrated a remarkable efficiency in preventing corrosion. The study results have already been adopted in actual plants and the corrosion in plate-type heat exchanger with Ni-base alloy have been largely prevented. No measurable shift of corrosion potential to noble side was observed by corrosion test. It is thought from the observation and examination above that the local environment in the plate had been severed by the influence of microbaiological activities.
This paper describes the effect of hydrogenase-positive sulfate-reducing bacteria (Desulfovibrio desulfuricans) on corrosion behavior of ductile cast iron in Postgate's medium B using electrochemical techniques such as corrosion potential, AC impedance, and polarization measurements. Uniform corrosion rate remained as low as 2.5×10-13m·s-1 presumably due to the adherent iron sulfide layer formed by the action of sulfate-reducing bacteria. Polarization behavior of ductile cast iron under actively-growing Desulfovibrio desulfuricans does not coincide with corrosion rate. Therefore, catholic depolarization is not directly related to an increrase in corrosion rate.