Microbial induced corrosion of steel structures, used for transport or storage of fuels, chemical weapons or waste radionuclides, is an environmental and economic threat. In non-sulfidic environments, the exact role of methanogens in steel corrosion is poorly understood. From the non-sulfidic, methanogenic sediments of the Baltic Sea corrosive communities were enriched using exclusively Fe0 as electron donor and CO2 as electron acceptor. Methane and acetate production were persistent for three years of successive transfers. Methanosarcina and Clostridium were attached to the Fe0, and dominated metagenome libraries. Since prior reports indicated Methanosarcina were merely commensals, consuming the acetate produced by acetogens, we investigated whether these methanogens were capable of Fe0 corrosion without bacterial partners (inhibited by an antibiotic cocktail). Unassisted, methanogens corroded Fe0 to Fe2+ at similar rates to the mixed community. Surprisingly, in the absence of competitive bacteria, Baltic-Methanosarcina produced six times more methane than they did in the mixed community. This signifies that Baltic-Methanosarcina achieved better corrosion alone, exclusive of an operative bacterial partner. Our results also show that together with acetogens, Methanosarcina interact competitively to retrieve electrons from Fe0 rather than as commensals as previously assumed.