Role of Manganese Dioxide in Corrosion in the Presence of Natural Biofilms

• Published in: Corrosion (Houston, Tex.) Vol. 57; no. 10; pp. 863 - 873
• Main Authors: Ruppel, D.T., Dexter, S.C., Luther, G.W.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.10.2001; NACE
• Subjects: Applied sciences; Biofilms; Calomel electrode; Cathodes; Circuits; Coatings; Corrosion; Corrosion environments; Corrosion rate; Crevice corrosion; Current density; Disproportionation; Dissolved oxygen; Drawing and ironing; Electric charge; Exact sciences and technology; Galvanic corrosion; Heavy metals; Ions; Manganese; Manganese compounds; Manganese dioxide; Metals. Metallurgy; Nitrogen cycle; Oxidation; Polarization; Regeneration; Regeneration (biological); Reoxidation; Seawater; Stainless steel; Stainless steels; Surface phenomena; Open circuit; Manganese oxides; Corrosion potential; Stainless steel; Biofilm; Bacterial corrosion; Current density; Sea water corrosion
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3290313

ABSTRACTManganese dioxide (MnO2) has been suspected of being the active agent for the observed enhancement of crevice and galvanic corrosion rates involving passive metal cathodes in the presence of natural marine biofilms. To investigate the viability of this mechanism, polymeric MnO2 was precipitated onto UNS N08367 stainless steel, which was then polarized potentiostatically at ­550 mV vs saturated calomel electrode (SCE) in natural seawater under various conditions to measure the effect of MnO2 on current capacity. The same alloy was also polarized with natural biofilms on the surface for comparison. The data showed that MnO2 was capable of increasing both the open-circuit potential of the coupon and the current that can be drawn from it, even in the absence of biofilms. The observed effect of MnO2 on current density was greater than could be accounted for by reduction of all the MnO2 deposited. Comparison of the total charge passed during polarization with the amount of MnO2 deposited indicated that each Mn+2 ion was being reoxidized 2 to 5 times during the polarization process by a combination of dissolved oxygen and disproportionation of a soluble Mn+3 intermediate to MnO2 and Mn+2. The current supplied by MnO2 coatings under air-saturated conditions was similar to that provided by coupons with natural biofilms. However, the rapid regenera-Biofilms have long been known to alter the chemistry and electrochemistry at metal surfaces. Mollica and Trevis were the first to report an observed increase in the open-circuit potential (OCP), or ennoblement, of passivating metals when immersed in natural waters.1 Since then, numerous studies have confirmed this finding.2-5 In addition to changing the potential, it has been shown recently by Dexter and LaFontaine that biofilm growth on the stainless steel cathodes of galvanic couples increased the corrosion rate of carbon steel and aluminum anodes vs control tests without biofilms.6 They also found that cathodic polarization of UNS N08367(1) coupons with natural biofilms revealed a systematic increase in current density at the potential of the galvanic couples as compared to bare (control) coupons.6 The general purpose of the work reported in this paper was to investigate how natural marine biofilms are able to sustain an increased current at the cathode surface and to maintain it over the two-month period of the galvanic corrosion tests.