Corrosion of the interior steel surfaces of offshore monopiles

The present paper reports on four-year field investigations aimed at ascertaining internal corrosion rates for offshore wind-power generation monopiles, despite these being designed to obviate internal corrosion through oxygen exclusion. Results for mild steel strips within the enclosed, quiescent, uPVC simulated seawater environments are presented as a function of relative to mean tide level, period of exposure and localised pollution of seawater. Detailed pit depth measurements are also reported for various exposure periods, with the magnitude of such corrosion displaying a bi-modal behaviour as a function of exposure time, consistent with much other data. No evidence was found of the classical corrosion profile for vertical steel structures (e.g. piles) or of accelerated low water corrosion usually associated with microbiologically influenced corrosion caused by elevated seawater nutrient concentrations. It is proposed that the observed corrosion behaviours primarily are the result of the shielded quiescent environment. The practical implications are briefly discussed.