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    High-per­for­mance sur­face pro­tec­tion for con­crete screws

    Concrete screws are employed wherever specific components need to be secured in a concrete base. Outdoors in particular, they are exposed to a wide range of stresses and environmental influences. In the case of especially high corrosion protection requirements, the coating of the concrete screws with a tailored zinc flake system is a wise choice.

    High stresses on concrete screws

    Whether it is fixing crash barrier holders to asphalt on the autobahn, securing façade substructures to the frame of a building or as anchoring for fences, balcony or stair balustrades: Concrete screws – also known as screw anchors – are used in a wide range of areas and are required to withstand numerous stresses. In addition to sometimes extreme environmental conditions such as temperature fluctuations, rain, sun rays or salty sea air, there are also often mechanical and chemical impacts – for example from de-icing salt. A further problem of fastening: concrete is a highly alkaline material, making it “toxic” for many surface coatings. If screws are coated with an unsuitable “protective film”, this often leads to premature failure or safety defects with expensive repairs. In view of this, depending on the area of use, the effective protection of the screws against corrosive attack is urgently required.

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    Concrete screws are used for tasks such as fixing crash barrier holders to asphalt on the autobahn and need to withstand numerous stresses.

    Solutions currently commonly used for concrete screws

    In practice, concrete screws made from carbon steel or stainless steel are typically used today. To create corrosion protection, the screws made from carbon steel are either electroplated and passivated or alternatively coated via hot-dip galvanisation. However, these coatings often come up against their limitations, particularly where the corrosion protection requirements are very high and further functional requirements also need to be fulfilled. The protective performance achievable with a galvanic coating is comparatively low and unsuitable for use with concrete screws under tough environmental conditions. In many cases, the problem with hot-dip galvanisation is that the high process temperatures during application place great stress on the parts. In recent years, stainless steel concrete screws have established themselves as a common solution for projects with high corrosion protection requirements. These do not require additional surface protection, but are comparatively very expensive.

    Zinc flake system as advantageous alternative

    Zinc flake coatings are a good choice whenever a reliable and capable surface protection is required for carbon steel concrete screws. The micro-layer corrosion protection system delivers very high protective performance at low coat thicknesses of just 8–20 μm. The base coat consists of flake-like particles embedded in a binder matrix. These cross-link on the component to generate corrosion protection. An additional top cat provides the coating with multifunctional characteristics such as defined coefficients of friction or chemical resistance, for example against cleaning media or - very important - the highly alkaline concrete. The individual coats are dried at low process temperatures of max. 240°C. Zinc flake systems can therefore also fulfil the requirements of ISO 12944 C5 - very high.

    Application of the coating is typically using the classic dip-spin process. Particularly important here is also that no hydrogen is generated in the coating process. This means that there is no danger of hydrogen-induced stress corrosion cracking. For this reason, zinc flake coatings are especially suited for high-tensile steel (> 1000 MPa). Finally, the wafer-thin protective film is also advantageous from an ecological and economic viewpoint thanks to the low use of resources.

    Die Applikation der Beschichtung erfolgt in der Regel im klassischen Tauch-Schleuder-Verfahren. Besonders wichtig dabei ist auch: Beim Beschichtungsprozess wird kein Wasserstoff erzeugt. Somit ist die Gefahr einer applikationsbedingten wasserstoffinduzierten Spannungsrisskorrosion nicht vorhanden. Aus diesem Grund eignet sich die Zinklamellenbeschichtung besonders gut für hochfeste Stähle (> 1000 MPa). Nicht zuletzt erweist sich der hauchdünne Schutzfilm wegen des geringen Ressourceneinsatzes auch aus ökologischer und ökonomischer Sicht als vorteilhaft.

    Zinc flake coatings proven in practice

    Following comprehensive corrosion tests conducted together with DÖRKEN Coatings, companies such as the internationally-leading fastener specialist Rawlplug have been using the zinc flake systems from Herdecke to coat concrete screws for some time now – whenever very high corrosion protection requirements need to be met. The Rawlplug concrete screws also satisfy the ETA (European Technical Assessment) approval  required for use in safety-relevant areas.

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    Zinc flake systems offer active cathodic protection of more than 2688 hours against base metal corrosion acc. to ISO 12944-6 in test program 2 (cyclical aging testing).

    Conclusion

    The zinc flake coating is advisable for use with carbon steel concrete screws wherever very high requirements of corrosion protection need to be met as well as functional characteristics such as media resistance fulfilled. From an economic viewpoint also, these represent an advantageous alternative to stainless steel concrete screws.