Corrosion protection is always the last step during a production process. When a job is behind schedule, there is often pressure on the paint shop to make up for lost time by hastening its part of the production process. But trying to gain time by rushing the painting process can have costly consequences.

Painting offshore structures in a paint shop costs from €15 to €25 per m², depending on the setup of the workshop and the paint system. The cost of coating repair work performed on site and onshore is about 5 to 10 times more, due to the logistics of getting men and materials to the job site and limited access to structures due to weather conditions. If repair work must be done offshore, when corrosion protection fails prematurely, the cost can increase to more than €1000/m2, between 40 and 60 times the cost of doing the job in the paint shop.

Onshore exposure implies generally cyclic dew/condensation, with or without minor salinity, and moderate exposure to sunlight, resulting in moderate corrosion at holidays, weak points and damaged areas of the coating system. In contrast, offshore exposure implies long-term exposure to humidity with high salinity, intensive influence of UV light, wave action and the presence of a splash zone area, and high corrosive stress resulting in speedy corrosion at vulnerable areas of the coating system.

This difference in exposure severity is also reflected in the mass and thickness loss per unit surface of low-carbon steel and zinc in the first year of their unprotected exposure. For example, in Germany, the onshore corrosivity is evaluated as being about C3, according to ISO 12944, which corresponds to a thickness loss of 25 to 50 μm. In comparison, for splash zone areas, as much as 500 μm thickness loss has been observed in the first year of service.

Key factors for corrosion protection

A qualified coating system is not enough to guarantee a successful corrosion protection. Certificates and approvals are often overestimated. Indeed, the EN ISO 12944, part 5 (German issue) lists nine factors that are decisive for the durability of a coating system. The coating system itself is only one, and other important factors include the design of the structure (access possibilities), the design of edges and weld seams, the workmanship of the applicator, the specific condition of the steel before surface preparation, and the exposure of the paint system immediately after completing application.

In effect, failure statistics show that the paint system itself is seldom the reason for the premature failure of the corrosion protection. The paint industry is mainly supplying 'half-products': that is, two or more component-reactive resins. It is the applicator who effectively creates a chemically new substance that in the end provides corrosion protection. Failure can occur during this process if the mixing of the components, the temperature control, the time requirements, and the application of the mixture onto correctly prepared surfaces are not all carried out properly. That is why permanent quality control and supervision by qualified professionals are required.

Design considerations

The design and fabrication of steel used in wind turbine construction is critical to its corrosion protection. Before the applicator starts their job, the steel builder has to provide a structure suited to being painted successfully. It is a law of nature that corrosion will always start at edges and weld seams – which is where the responsibility of the steel builder begins.

Coatings can only protect accessible surfaces – a fact often neglected in design. EN ISO 12944, part 3, Design Considerations, recommends that appropriate distances required for tools in corrosion protection work are respected. It also recommends minimum dimensions for openings that provide access to confined areas, minimum dimensions for narrow spaces between surfaces, and the incorporation of design features that may be used to avoid deposits accumulating or water being trapped.

Construction designs that create insufficient access for painting should be avoided. During the fabrication of an offshore steel structure, the recommendations of ISO 8501, part 3 (preparation grades of welds, edges, and other areas with surface imperfections) should also be considered. This relatively new standard, which appears not to be well known at engineering offices and steel construction companies, is aimed at the steel builder, not the paint applicator. It gives recommendations about the required condition of the steel surface if paints are to be applied later for long-term corrosion protection.

For offshore structures, preparation grades P2 or P3 should be specified, depending on the relevant details of the structure. This means the steel surface should be free from welding spatters and slags, pores, undercuts and laminations. The surface also needs to be dressed (by grinding) to remove irregular and sharp-edged profiles.

 

Performance testing an offshore corrosion protection system on WindFloat, a semi-submersible floating structure for offshore wind turbines that has been deployed with a 2 MW Vestas turbine off the Portuguese coast (Hempel)

Pores are often found, particularly on welds, and inspectors are often asked if pores can be filled up with paint. Pores should never be filled with paint or filler. The steel builder has to repair them, according to the demands of EN ISO 5813, weld quality B.

Correctly designed weld seams are crucial to successful corrosion protection. Handmade weld seams on offshore structures generally need to be ground by the steel fabricator before they can be painted. Poor weld seam design will cause steel to corrode quickly.

It is often helpful to have something to show to the welder to clarify what is meant by the right design for corrosion protection at a weld seam. The NACE Standard RP 0178, for example, provides photographic examples of the front and back sides of good and bad weld seams.

Finally, the steel fabricator has to be careful about the condition of the carbon steel to be used to build an offshore structure. Unpainted steel is characterised by four rust grades according to EN ISO 8501, part 1. For offshore structures, it is strongly recommended to use steel with a rust grade not worse than B (Norsok M501), or not worse than C (EN ISO 12944). The service life of a coating system depends directly on the rust grade of the initial steel and the degree of surface preparation (Sa2 and Sa3 according to EN ISO 8501, part 1).

In summary, the steel fabricator has to be aware that he is constructing an offshore structure that needs certain access and surface conditions for successful corrosion protection.

Surface preparation and paint application

The workmanship of the applicator is very important for a coating system's long service life. Half of all premature corrosion protection failures are application-related; thus all operations must be high quality and quality control checks must be frequent. Correct surface preparation and coating thickness are definitely the most important application parameters. Planning and informing the work crew of the different procedures and, preferably, signing off on the results are necessary to guarantee the right level of quality.