RESPONSE OF PASSIVE ALLOYS WITH N- AND P-TYPE PASSIVE FILMS TO MANGANESE IN BIOFILMS

摘要

Marine and fresh water biofilms usually shift the open circuit potential (OCP) of stainless steels in the electropositive direction by several hundred millivolts. One of the reasons for this ennoblement is thought to be the presence of Mn in biofilms. Two stainless steels, UNS NO8367 (6XN, having an n- type passive film) and NO6455 (C4 with a p-type film) have been selected for this study. MnO_2 was coated onto these oxide films, resulting in a higher OCP (+750 mV SCE) on alloy NO8367 than on NO6455 (+360 mV). This is consistent with the interpretation that MnO_2 reduction is easier on the n- type passive film, which is rich in free electrons, than on the p-type film. Cathodic polarization curves revealed reduction peaks for Fe, Mn and Cr oxides. The Fe and Mn reduction peaks were thought to be due to the availability and flow of electrons between the passive oxide film and MnO/. Since p-type films on NO6455 are deficient in free electrons, the MnO_2 coating (which is n-type) was thought to supply electrons to the oxide film for chromium reduction. The results are consistent with the idea that bacteria, having a net negative surface charge, accumulate reduced manganese ions for re-oxidation.