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01256 RESPONSE OF PASSIVE ALLOYS WITH N- AND P-TYPE PASSIVE FILMS TO MANGANESE IN BIOFILMS

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Product Number: 51300-01256-SG
ISBN: 01256 2001 CP
Author: S.C. Dexter, S. Maruthamuthu
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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. MnO2 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 MnO2 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 MnO2. Since p-type films on NO6455 are deficient in free electrons, the MnO2 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. Key Words: Passive film, Manganese Dioxide, Ennoblement, n-type, p-type
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. MnO2 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 MnO2 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 MnO2. Since p-type films on NO6455 are deficient in free electrons, the MnO2 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. Key Words: Passive film, Manganese Dioxide, Ennoblement, n-type, p-type
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