Laboratory Reproduction of Potential Ennoblement of Stainless Steels in Natural Seawater

Abstract The localized corrosion susceptibility of stainless steels, such as type 316L, immersed in natural seawater is higher than those in synthetic seawater. It is widely observed that the corrosion potential of stainless steels in natural seawater becomes noble over +0.3V vs SCE. This phenomenon is caused by the influence of the microorganisms in the natural seawater, and therefore it is considered as Microbially Influenced Corrosion (MIC). In this paper, the laboratory reproduction of the corrosion potential ennoblement was examined on the basis of the metabolize of aerobic bacteria (pH, dissolved oxygen and metabolite). The electrochemical measurements in natural seawater and synthetic seawater with bacterial mediums were carried out. The corrosion potential of stainless steels became noble in only natural seawater, and cathodic current density increased around corrosion potential, these results suggest that the presence of oxidizer on the metal surface which has higher redox potential than that of oxygen. As the corrosion potential became noble during the logarithmic phase of bacterial growth in the result of the examination with isolated bacteria, it is suggested that the aerobic metabolism of bacteria effects the ennoblement of corrosion potential. The ennoblement was also reproduced in laboratory by the addition of oxidase simulated the metabolism of aerobic bacteria. The oxidase produced the peroxide by the reduction of oxygen which has higher redox potential than that of oxygen, therefore it is considered that the corrosion potential ennoblement is caused by the peroxide generated by the metabolism of aerobic bacteria included in the biofilm formed on the surface of the stainless steels in natural seawater.