Page 56 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 3. Metal corrosion protection potential of methanogenic communities Introduction
Metal constructions are widely used in waterways, such as wharf structures, pile foundations, and metal sheet piles. Their corrosion poses significant safety risks and has significant economic impact, with total costs estimated at 2.2 trillion dollars per year globally (Beech et al. 2014). Microbially influenced corrosion contributes to at least 10 to 20% of total corrosion damage, which emphasizes the relevance of this process (Flemming 1995; Dinh et al. 2004). Microorganisms influence metal corrosion both under oxic and anoxic conditions by changing the chemical environment through metabolic processes and biofilm formation on metal surfaces (Beech and Gaylarde 1991; Beech et al. 2014). Under oxic conditions, both abiotic and microbial processes lead to the formation of hematite (Fe2O3), and ferric hydroxide [Fe(OH)3], and active acidification of the environment, which increases corrosion rates (Rossum 1983).
Under anoxic conditions, the iron corrosion is largely influenced by microbial activity (Enning et al. 2012). In marine environments, sulfate-reducing bacteria induce iron corrosion under electroconductive conditions, which is a major issue in pipeline steel (Enning et al. 2012; AlAbbas et al. 2013). In anoxic freshwater environments, sulfate reducing Desulfovibrio species can cause corrosion of metal surfaces when sulfate is abundant (12.5-35.2 mM), as shown under laboratory conditions (Rao et al. 2000, 2005; Ilhan-Sungur, Cansever and Cotuk 2007). Under low-sulfate conditions, as for most freshwater ecosystems, iron oxidation mainly proceeds via the formation of ferrous hydroxide [Fe(OH)2], which is transformed into magnetite (Fe3O4), with the formation of H2 as main electron sink (Saheb et al. 2013).
In addition to their role in corrosion, microorganisms have also been found to potentially be involved in corrosion protection through the development of corrosion-protective layers (CPLs). A Dutch study indicated that such natural CPLs were associated with corrosion inhibition in organic-rich freshwater environments in five freshwater field sites (Kip et al. 2017). CPLs are formed by multispecies biofilms, which likely exert several protective mechanisms, including the removal of corrosive substances, e.g., the removal of oxygen through aerobic respiration (Herrera and Videla 2009); growth inhibition of corrosion causing microbes, e.g., through antimicrobial production by non-corrosive microorganisms (Videla and Herrera 2009); and the formation of a corrosion protective layer, e.g., through overproduction of extracellular polymeric substances and mineral precipitation (Zuo 2007; Kip et al. 2017).
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