ABSTRACT

Thermal spraying of zinc and zinc alloys is a well-established process which is used for long-time corrosion protection of steel structures in maritime surrounding conditions and for offshore structures since decades.

The paper summarizes the actual arguments to use long-time cathodic corrosion protection with thermally sprayed zinc and zinc based alloys in different types of surrounding conditions based on detailed laboratory tests, results of long time field tests as well as the monitoring of metalized steel structures in different countries.

The presentation represents best technical and economic performance and a lifetime more than 30 years for thermal spraying with zinc-aluminium 15 for buildings in marine conditions.

In addition it reports about modern process technology, which allows the application of thermal spraying with zinc in factory and at site for new steel structures and for repair work with high quality and near tolerances.

INTRODUCTION

While wind turbines on land are depending on their location often accessible for repairs, choosing a suitable corrosion-protection system for offshore turbines represents a major challenge. Offshore wind turbines face greater corrosive, mechanical and biological stresses than turbines on land 1. In addition, offshore wind turbines are very difficult to access and therefore have much higher maintenance and repair costs 1. For these reasons the corrosion protection has to be very good, long-lasting (>25 years) and fault-tolerant.

The cost of applying the first coating in the plant is € 15-25/m2. A repair outside of the plant but still on land costs around 5 – 10 times more as equipment and personnel have to be transported. If repair work has to be carried out offshore, the costs rise to over € 1000/m2. In experience reports, costs are stated which are 75-100 times 1 higher than the first coating. The main reason for the high offshore repair costs are due to the fact that specially-trained personnel and equipment have to be transported by helicopter or ship to the offshore wind turbines and due to weather conditions only being favourable for short periods. Under the environmental conditions, it is extremely difficult to carry out a competent repair. Figure 5 shows an industrial climber at work on an offshore wind turbine. It is assumed that there are only 40 days a year when weather conditions allow repairs to be carried out. The weather conditions that are required are low humidity, temperatures above the dew point and little wind. A further difficulty lies in getting and keeping the surface to be repaired dry and free of chloride, as the damp, salty air repeatedly contaminates the surface and would therefore reduce the adherence of the coating. In addition, organic repair coatings are applied on multiple layers and setting times have to be heeded between the layers.

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