Abstract

The steel structures of offshore wind turbine foundations have so far only been protected cathodically (galvanic anodes/impressed current systems) in the underwater area. More recent German regulations recommend an additional coating in order to reduce the required anode masses (1). In most cases, organic top coatings are used for this purpose, which significantly reduces the necessary protective current requirement. However, organic top coatings lose their barrier protection effect due to damage, defects and ageing, which increases the protective current requirement again. Mechanical loads already during transport and installation ("pile driving"), as well as (hydro-)abrasion by sand near the seabed during the period of use, make it useful to consider the tribological properties.

Introduction

Metallic coatings as a protective coating are characterized by excellent corrosion protection behavior and show extreme resistance to mechanical loads as well. Pure metallic coatings or duplex systems are already being used successfully in other areas of offshore structures. For example, areas in the tidal water zone, such as boat landings, usually receive a duplex system consisting of thermal spayed coating and a fitting topcoat. Add-on parts are often protected exclusively by a metallic zinc coating. A thermal spray coating in the submerged zone thus represents a logical alternative to the organic topcoat system.

Thermal Spraying Zinc-Aluminum in Submerged Zone

Thermal spraying has also been used in the offshore wind energy sector for a long time. Components such as the boot landings (Fig 1) on the offshore towers and other components in the area of the water exchange zone are specified as duplex coating, consisting of a zinc-aluminum spray coating (TSZA) and a subsequent multi-layer paint coating. This design is a proven system. In field tests carried out over many years by the MPA Stuttgart (Federal Materials Testing Institute of the State of Baden Württemberg) (3) together with the Grillo company, the corrosive resistance of thermally sprayed zinc coatings and in particular of zinc alloy coatings with up to 22 wt.% Al without subsequent color coating could be proven (Fig 2). Up to these alloy contents of aluminum in the zinc alloy, the cathodic corrosion protection remains completely intact.

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