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

Manganese 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 regeneration of current capacity demonstrated by natural biofilms could only be matched by the MnO2 coatings under oxygen-saturated conditions. This indicated that both chemical and biological reoxidation probably take place in natural biofilms. Results were consistent with the idea that MnO2 recycling (perhaps augmented by iron and nitrogen cycling) within natural biofilms is sufficient to account for observed long-term galvanic and crevice corrosion rates.