海水中におけるステンレス鋼の微生物腐食機構

Abstract

Stainless steels, such as types 316L and 304 have high susceptibilities to pitting and crevice corrosion in natural seawater even though they have low susceptibilities in synthetic seawater. Recently, this type of corrosion has been recognized as a kind of Microbially Influenced Corrosion (MIC) in aerobic aqueous environments. The mechanism of MIC on stainless steels in natural seawater is studied by the electrochemical and biological techniques. Measurements of cathodic polarization curves of stainless steels were carried out in natural and synthetic seawater. The corrosion potential of stainless steels was measured in seawater and culture medium of the bacteria isolated from natural seawater. The corrosion potential of stainless steels became noble in natural seawater compared with synthetic seawater, and the cathodic polarization behavior indicates that some substances exist, which have higher redox potential than that of oxygen. It has become clear that the susceptibility to corrosion of stainless steels in natural seawater is effected by bacteria in the mature biofilm. As a result of the experiment using isolated bacteria, the corrosion potential became noble in the logarithmic phase of bacterial growth. These results suggest that the aerobic metabolism of the bacteria causes the ennoblement of stainless steels. Hydrogen peroxide, which could be generated due to the reduction of oxygen in the presence of enzyme (oxidase), was detected in the mature biofilm. Hydrogen peroxide has a higher redox potential than that of oxygen. Therefore, this result supports the fact that the hydrogen peroxide, as an intermediate substance in the aerobic metabolism of bacteria in the biofilm, causes the ennoblement of stainless steels in natural seawater. The potential ennoblement and the occurrence of crevice corrosion of stainless steels were successfully reproduced in the laboratory by using the synthetic seawater with the addition of oxidase and substrate.