Assured corrosion protection for wind energy units

High-performance corrosion protection for screwed fasteners in high-tensile steel enables wind energy units to be largely maintenance free. Zinc flake systems have proven their reliability in the area of rotor blade fastenings for over 30 years.

The rotor blades are fundamental, key components of a wind energy unit. They are used to extract energy from flows of air and move this to the generator. The blade lengths of up to 85 metres and a weight of up to 25 tonnes mean that the fastening of the rotor blades to the rotor hub is particularly significant. This is achieved using fasteners, consisting of so-called double ends, barrel-nuts and hexagonal nuts, typically in the M30 to M36 dimension range.

Diverse stresses and requirements

The rotor blades and thereby also their screwed fasteners are subjected to extreme climatic as well as dynamic stress from wind load and the operation of the unit itself. This is joined by static forces from prestressing of the bolts and the mass of the connected elements. As a consequence of the high level of stress for the screwed fasteners, the choice of corrosion protection is especially important. The protective coating needs to meet specified coefficients of friction in order to guarantee problem-free fastening on the construction site, typically with considerable time pressure and under difficult conditions. In addition, the coating must offer highly-effective and long-lasting corrosion protection to ensure the long-term, reliable operation of the unit.

Rotor blade fastener comprising double end, barrel nuts, hexagonal nuts and washers.

Corrosion protection with multiple benefits

The extremely thin zinc flake coatings from Dörken MKS fulfil all of these requirements. They consist of a basecoat and an organic or inorganic topcoat tailored to the respective components. The zinc-containing basecoat determines the corrosion properties and is responsible for the cathodic corrosion protection. In addition to adding further protection, the tailored topcoat enables the observance of defined coefficients of friction and therefore safe and fast assembly. A further advantage of the zinc flake systems: coated screws and bolts do not display setting behaviour, meaning that cost-intensive maintenance work is reduced and service life increased. Even the risk of application-related, hydrogen-induced stress corrosion cracking can be avoided, due to the fact that no hydrogen is involved in the coating process. In addition, the low cross-linking temperature of around 200°C avoids changes in the properties of the high-tensile components. The highly-effective and long-lasting corrosion protection offered by zinc flake systems is displayed in comprehensive outdoor weathering tests conducted at the Deutsches Institut für Bautechnik (DIBt) and the Federal Institute for Materials Research and Testing (BAM). They tested the proven system of DELTA®-TONE 9000 and DELTA®-SEAL, certifying it with the corrosiveness class “C5-M mittel” – permitting use in coastal and offshore areas with high salt exposure. The further developed zinc flake coatings DELTA-PROTEKT® KL100 and DELTA-PROTEKT® KL120 display corrosion protection characteristics beyond these and are continuously specified for new requirements.

Efficient coating with the racked dip-spin procedure

The size of the rotor blade fasteners in particular means that the proven racked dip-spin procedure is advisable for coating the components. This application technology enables even, high-performing protective coatings to be achieved.

The racked dip-spin process has proved itself in the coating of the components.

Following extensive pretreatment, at the beginning of this process the bolts or barrel nuts are fixed to a rack or multiple small supporting points, or alternatively placed individually in a basket with grille. After this, the unit is dipped in the coating medium in the scope of a precisely-defined process. This ensures that the smallest hollow spaces are reliably coated. After the rack has been removed from the bath, the centrifuge is utilised. The intensity and duration of this spinning process depends on the geometry and desired coat thickness of the parts. All excess material in the area of the interior spaces is effectively spun off. In the last step the entire unit including the rack is subjected to the drying and cross-linking process. In this the coated parts are hardened and cross-linked in a special oven with a defined temperature and duration. This results in an even and adhesive coating that enables problem-free and secure fastening to the rotor blades of the wind energy unit in subsequent use.

Conclusion: High-performance coating solutions help to extend maintenance intervals and working life of wind energy units, whilst also increasing levels of efficiency.