Microbial iron corrosion and possible corrosion protection measures in monopiles of offshore wind power systems

Microbially influenced corrosion of iron (MIC) is a widespread problem in the maritime industry, especially for support structures of offshore wind turbines, including monopiles. The steel-made, cylindrical hollow piles are driven into the seabed, trapping seawater, and thus exhibit only a limited exchange with the environment. Microbial activity and biogeochemical processes inside the monopile, which influence the MIC, are highly impacted by these special conditions. So far, in-depth studies of MIC in monopiles are lacking and commonly used corrosion protection measures only address electrochemical corrosion.

In an interdisciplinary project we aim at developing effective, environmentally friendly and economically feasible protection measures against MIC in offshore monopiles. For this purpose, a solid understanding of the environmental conditions, microbial communities and mechanisms involved in MIC inside the monopiles is necessary. Thus, we carried out in situ incubations inside a monopile in the North Sea, incubating commonly used mild steel at 6 and 21 m water depth, accompanied by measurements of biogeochemical parameters. First results show a strong seasonal stratification of the water column inside the monopile with the absence of oxygen and accumulation of biogenic methane and hydrogen sulfide (up to millimolar concentrations) in bottom waters at certain times of the year. Despite storm-related ventilation events of the water column, RNA-based taxonomic profiles of biofilms from steel samples incubated at 21 m exhibited an enrichment in sulfate reducing Bacteria and methanogenic Archaea, hypothesized to be relevant players in MIC under anaerobic conditions. Steel-associated communities at 6 m water depth, hallmarked by oxic conditions, were enriched in Fe-oxidizing Bacteria. Current and future work involves ex situ and further in situ incubations of suitable coated steel and steel alternatives as well as the development of local inhibition measures to identify the most efficient measures for MIC inhibition inside monopiles.