Effect of marine microorganisms and biofilms on cathodic current

In order to explain the mechanism that works in microbiologically influenced enhancement of corrosion rates, cathodic currents were measured on type 329J4L stainless steel exposed to seawater, and a biological analysis on biofilms was conducted. The cathodic current densities measured at 0.1 V vs. SHE were ranging from 0.3 μA/cm² to 3 μA/cm², after the specimen had been held at 0.1 V vs. SHE in natural seawater. On the other hand, the cathodic current densities at 0.1 V vs. SHE after the exposure to natural seawater under an open circuit condition, were higher than 1 μA/cm ² in the early stage of the measurement. However, they fell to values lower than 0.1 μA/cm² in 12 h. The current densities measured at 0.2 V vs. SHE were approximately 2 μA/cm² in the initial stages and the final stages of the measurement, after the beforehand exposure to natural seawater at 0.2 V vs. SHE. The current densities measured at 0.2 V vs. SHE after the beforehand exposure to natural seawater under an open circuit condition, were higher than 1 μA/cm² in the initial stages of the measurement and lower than 0.1 μA/cm² in the final stages. In the case of measurements at 0.3 V vs. SHE, the average values of cathodic current densities were approximately 0.2 μA/cm² after the beforehand exposure to natural seawater at 0.3 V vs. SHE, and lower than 0.01 μA/cm ² after the beforehand exposure to natural seawater in an open circuit state. The cathodic current densities after the exposure to synthetic seawater, were lower than 0.01 μA/cm² in the cases of measurement at 0.1 V vs. SHE, 0.2 V vs. SHE and 0.3 V vs. SHE. The data on DNA base arrangement detected by the denaturing gradient gel electrophoresis test in a biofilm formed at 0.2 V vs. SHE in natural seawater, was different from the data detected in a biofilm formed under an open circuit condition. It is, therefore, concluded that some specific types of microorganisms selectively attach to steel surfaces under cathodic conditions, and that such microorganisms are the cause of large cathodic currents resulting in high corrosion rates.