High-per­for­mance pro­tec­tion for fas­ten­ers in the con­struc­tion sec­tor

Solar or photovoltaic units are growing steadily in popularity – the large sheets can already be seen on many buildings. However, it should not be forgotten that not only the panels but also the fasteners that secure them are exposed to various climatic conditions day after day. Component-specific corrosion protection is therefore all the more important.

Whilst in the construction of actual façades the fasteners of the cladding are subject to very strict specifications, such as the mounting guidelines of the IFB (international lightweight metal construction association), there are also many applications in external construction fields such as solar panels or photovoltaic units that are not covered by these stringent requirements. Specifically, this means that steel fasteners can be used in these areas, and these are not required to be manufactured from austenitic stainless steels as is the case with façade construction fasteners. Despite this, contractors and fitters are faced with numerous challenges. For example, there is the problem of contact corrosion when fastening two different materials together. If steel bolts with an unsuitable “protective film” are screwed to an aluminium profile, for example, this often results in premature corrosion and therefore to safety defects and expensive repairs. In view of this, the effective protection of screws and bolts against corrosive attack is urgently required, especially for safety-relevant components.

Corrosion protection solutions currently in common use for fasteners in the construction field

In practice, fasteners 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 fasteners under tough environmental conditions. In many cases, the problem with hot-dip galvanisation is that the high annealing temperatures during application place great stress on the parts. In addition, the thickness of the coating is relatively high – which can result in difficulties in assembly for screwed fasteners with internal drive in particular. In recent years, stainless steel screws and bolts 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 solution

Zinc flake coatings are a good alternative whenever reliable and capable surface protection is required for carbon steel fasteners. 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 topcoat lends the coating additional functionalities such as defined coefficient of friction as well as an additional barrier effect against moisture and oxygen. The individual coats are dried at low curing temperatures of max. 240°C. Zinc flake systems offer active cathodic protection of more than 1,000 hours against base metal corrosion (red rust) in salt spray testing according to DIN EN ISO 9227. In the event of damage to the coating the ignoble zinc in the base coat sacrifices itself on contact with water and oxygen in favour of the more noble steel base material.

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Vergleich: Mit Dörken beschichtete Schrauben vor (links) und nach (rechts) dem Salzsprühnebeltest DIN EN ISO 9227.

Comparison: bolts coated with Dörken before (left) and after (right) salt spray testing according to DIN EN ISO 9227.


Application of the coating is typically using the classic dip-spin process. Also particularly important here: 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.

Zinc flake coatings proven in practice

In a joint project carried out by Dörken and Reisser-Schraubentechnik the enormous capability of the zinc flake system could be demonstrated. Reisser Schraubentechnik GmbH of Ingelfingen is one of the leading manufacturers of stainless-steel screws and bolts in Europe and a licensee of DELTA-PROZINC® and zinc flake systems from Dörken. The years of co-operation between Reisser-Schraubentechnik and Dörken continue to bear fruit: together they develop new, innovative coatings and successfully undertake projects in partnership. These include fasteners for solar or photovoltaic units. When fastened to aluminium profiles, the coated steel bolts displayed a resistance to contact corrosion of over 2,000 hours in salt spray tests in accordance with DIN EN ISO 9227. As a consequence, Reisser-Schraubentechnik now uses the zinc flake systems from Herdecke for the coating of fasteners, particularly in areas with high corrosion protection requirements. A final important point: all Reisser bolts and screws with zinc flake coating fulfil (where required) the ETA approvals (European Technical Assessment) or general building approvals (AbZ) for use in safety-relevant areas.

Zinc flake systems offer active cathodic protection of more than 1,000 hours against base metal corrosion (red rust) in salt spray testing according to DIN EN ISO 9227

Zinc flake systems offer active cathodic protection of more than 1,000 hours against base metal corrosion (red rust) in salt spray testing according to DIN EN ISO 9227

Conclusion: Proven coating system for the highest requirements

The zinc flake coating is advisable for use with carbon steel fasteners wherever very high requirements of corrosion protection need to be met and where functional characteristics such as enhanced barrier effect against moisture are required. From an economic viewpoint also, these represent an advantageous alternative to stainless steel fasteners.