2001 Volume 50 Issue 7 Pages 330-333
It has been known that the open-circuit potentials of stainless steels exposed to natural seawater are ennobled and that there is a possibility of ennoblement caused by hydrogen peroxide in biofilms formed on the surfaces of stainless steels. In this study, hydrogen peroxide detected in biofilms formed on SUS 316L specimens exposed to natural seawater for a month was resolved by means of enzyme (Catalase or Peroxidase), and the open-circuit potentials of the specimens were simultaneously measured for the purpose of explaining the relation between hydrogen peroxide and the ennoblement.
The activity of the enzyme depends on the temperature and was controlled with adjusting the water temperature. At room temperature (23°C) the enzyme was inactive, and the open-circuit potentials of the specimens covered with biofilms formed by microorganisms in natural seawater reached approximately 0.6V vs. SHE. At the elevated water temperature (33°C) the enzyme was still more active than at room temperature, and the open-circuit potentials of the specimens in enzyme-dosed seawater fell to values equal to those without biofilms (0.2V vs. SHE), while those with no enzyme remained nobler than 0.5V vs. SHE. These results revealed that the enzyme resolved hydrogen peroxide in biofilms at 33°C and that the displacement of hydrogen peroxide reduced the open-circuit potentials to values as negative as those with no biofilm.
The open-circuit potentials were ennobled again according to the drop in water temperature. This result indicated that experimental procedures mentioned above had influence on neither the activity of microorganisms nor the structure of biofilms, and that the microorganisms in the biofilms continued to produce hydrogen peroxide.
Therefore, it is concluded that a major cause of the ennoblement induced by marine microorganisms is the presence of hydrogen peroxide in biofilms.