Cathodic currents measurements and biological analysis were conducted for type 329J4L stainless steel exposed to natural seawater for the purpose of finding a factor in microbiological promotion of crevice corrosion propagation. The cathodic current density measured at 0.2 V vs. SHE rose from values below 0.1 μA/cm 2 and became higher than 2 μA/cm 2 in summer. However, there was no obvious rise in cathodic current densities and their values were lower than 0.1 μA/cm 2 at the other seasons. 16S rRNA gene analysis revealed that bacteria belonging to Genus Algibacter or Class Gammaproteobacteria were specific to biofilms on the sample with the rise in the cathodic current density above 2 μA/cm 2 in summer. The population of all kinds of bacteria did not have positive effect on cathodic currents. It is, therefore, concluded that large cathodic currents resulting in high corrosion rate are mainly induced by the action of specific bacteria, and that there is a possibility of Algibacter or Gammaproteobacteria having action to enlarge cathodic currents.
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