Timber construction screw corrosion protection thanks to zinc flake surface

Timber construction screws are exposed to severe weathering and require high-quality corrosion protection. The best possible corrosion resistance and durability for timber construction is achieved with a zinc flake coating.

Due to their often demanding operating conditions, wood construction screws place special demands on the surface coating. With their many years of experience, the specialists at the Benseler Group rely on zinc flake coating with Geomet, which meets the highest corrosion resistance as well as functional properties and friction behavior. Special processes ensure a high-quality and economical coating, even for screws up to one meter in length. In this way, Benseler supports the sustainable construction of timber buildings and contributes to a long service life with high-quality corrosion protection.

The use of wood construction screws in timber construction is diverse: it ranges from general applications such as single-cut connections in wood-wood or steel-wood to special connections with fastening systems. For example, wood screws serve as indispensable fasteners for the friction-locked connection of timber modules in timber construction and must meet a wide variety of requirements in different dimensions and shapes. Screws with diameters of 8 to 12 mm and lengths of up to 1,000 mm - often with full or partial threads and different head and tip shapes - are used in house or bridge construction in areas that are exposed to severe weather conditions. The specifications of the timber construction industry therefore require a very high corrosion resistance of over 1,500 hours until red rust occurs in accordance with DIN EN ISO 9227.

For the surface experts at Benseler, who coat all types, lengths and sizes of fasteners up to a length of approx. 1,500 mm, the zinc flake coating with Geomet for timber construction screws is the surface of choice: "The zinc flake coating is characterized by a particularly high and durable corrosion protection, which makes it predestined for demanding applications such as timber construction," says Kerstin Genz, Sales Manager at Benseler Oberflächentechnik GmbH & Co. "In contrast to conventional galvanizing processes, the specific lamella structure of the coating ensures that it reacts very slowly to corrosion, even in the event of damage." This distinguishes the zinc flake coating significantly from galvanized zinc coatings, which are often susceptible to corrosion after a short time if damaged. Furthermore, because the zinc flake is not applied electrolytically, there is no risk of hydrogen embrittlement. This coating process, e.g. using dip-spin technology, significantly reduces the risk of brittle fracture, particularly in the case of high-strength fasteners. Another advantage is the resistance of the zinc flake to many chemicals such as organic solvents.

However, the zinc flake systems offer advantages not only for corrosion protection, but also for the installation of timber construction screws: the friction coefficients can be set precisely according to customer requirements or specifications, resulting in a defined screw-in torque with a constant torsional load on the screw during installation. This also works reliably for screws with a length > 1,000 mm. As a rule, pre-drilling is not necessary and the screw can be installed using a suitable screwdriver.

Zinc flake coating is one of the most resource-efficient processes, as it offers excellent corrosion protection even with very thin layers of just 8 to 20 μm. It can be coated either by spraying or by dip-spin coating in bulk or in rack technology. The latter is suitable for all parts that cannot be spun loose due to their size or weight and is therefore used at Benseler for wood construction screws with a length of approx. 200 mm or more. The parts are inserted into the frame so that they remain in place during the dipping and drying process in the centrifuge. This prevents damage caused by snagging or knocking as well as deformation, which guarantees high process stability. Because the screws remain easily movable in the rack mount, the method enables uniform coating even at the mounting points on the rack. Screw heads with internal drives can also be coated without leaving any residue with the appropriate mounting position in the frame
. Coating with basecoat and topcoat is carried out in a large-scale production system using base frames by immersing the entire frame in the coating system, which wets the parts. Benseler removes the excess material down to the smallest droplets from the component by subsequent dripping and a defined spinning process. In the downstream oven, the parts are pre-dried on the rack and baked at a specified temperature and duration.

Benseler monitors the quality of its coatings through comprehensive tests in its own test laboratories. Coating thickness measurement, coating weight determination, adhesion testing and corrosion tests in the salt spray test in accordance with DIN EN ISO 9227 prove that the parts coated here fully meet the specification requirements. The experts also test the friction properties: Up to five complete screwing cycles (screwing in and unscrewing) allow the coating to remain intact even under intensive use. (OM-4/25)