Protected Against Wind and Weather

Building structures are everyday subjected to extreme situations: fluctuating temperatures in summer and winter, precipitation and varying ambient air on the coast, in industrial surroundings or on land. Fasteners such as construction screws are particularly affected by these corrosive stresses.

Commonly known as “timber screws”, they have the function of both tool and fastener and come in a range of different types. Adjusting screws for wood-on-wood connections, spacer bolts, cladding screws or dry wall screws. Characteristic here is that the screws cut their own thread when fastened to the timber. The advantage of this is that the screws can be employed without pre-drilling and therefore faster and more economically. Work stages such as thread cutting or forming as well as cleaning after shaving removal are not required.

Construction screws are typically made from steel or stainless steel. However, the fact that they are often bolted to components made from other materials (e.g. brackets) means that there is a risk of contact corrosion. The screws are used primarily for roof constructions, building carports or for play equipment and safety considerations mean they need to be well protected against corrosion. In addition to the effects of precipitation and fluctuating temperatures, however, the contact with chemical substances such as impregnating agents stress the timber screws and can lead to corrosion and therefore destruction of the components.

If the construction screws were not made directly from stainless steel, but from steel, they were previously protected against corrosion with the aid of galvanic systems. There are various forms of passivation that can be used here: Yellow passivation, which contains chromium(VI), or alternative passivation forms on a chromium(III) basis.

Zinc flake systems represent an ideal alternative to prevent the corrosion of timber screws. The zinc flake system from Dörken Coatings consists of flake-like particles embedded in a binder matrix. The zinc flake basecoat is responsible for active cathodic protection.

This means that in the event of damage to the coating the ignoble zinc in the basecoat sacrifices itself on contact with water and oxygen in favor of the more noble steel base material. In conclusion a topcoat is added, which serves to extend corrosion protection whilst also ensuring additional functionalities such as defined coefficient of friction, chemical resistance and color. A colorless lubricant can subsequently be applied to timber screws, influencing and favoring the fastening torque. This ensures a fastening that is not only secure, but also simple and stable.

The zinc flake coating systems are especially suitable for timber screws, as they can be applied in very thin coats (8–20 μm) and are chemical resistant in particular. A further advantage: the zinc flake systems from Dörken MKS are also suitable for high-tensile steel, as no hydrogen is generated in the coating process and there is therefore no risk of hydrogen-induced stress corrosion cracking. The low cross-linking temperature of up to 240°C also avoids damaging the steel through excessive heat. The zinc flake systems have also been granted general technical approval from the Deutsches Institut für Bautechnik (DIBt) – the German institute for building technology – and fulfil the requirements of corrosion resistance class C5.