Fig. 2. Ecological accomplishments have taken a high priority at the company for many years. Regular internal and external audits are, for example, run at every production site in order to the review the company’s environmental protection measures.
Photo courtesy MAN
duplicated production line would mean higher logistical and manufacturing costs. Furthermore, since the EU End-of-Life Vehicle Directive came into force coatings with chrome (VI) content are becoming less and less available and therefore also more expensive. In search of alternatives for the previously used systems
consisting of coatings with chrome (VI) content and post- lubrication, the company quickly became aware of Dörken MKS-Systeme GmbH & Co. KG from Herdecke. Dörken’s zinc flake systems have not only been chrome (VI)-free from the outset, but through the company’s close contacts with many representatives of the value-added chain, it was also possible to link up quickly with an international fastener manufacturing company, which was then incorporated into the early discussions. The aim here was to specify the parts provided for extensive test cycles and to define all key additional factors, such
as fastening parameters and bearing surfaces. The parts tested were high-strength steel bolts and nuts ranging in size from M12 to M16. In addition to resistance to base metal corrosion of at least 720 hours, the lubricant effect demanded by MAN was also particularly important in order to also ensure smooth fitting of such parts in practice. Various zinc flake based Cr(VI)- free coating systems that had been tested had not been able to satisfactorily fulfil the coefficient of friction requirements. Against the background of MAN’s specifications, coating
experiments were run using two selected systems in the pilot plant at the Herdecke site. The coating patterns were tested both at Dörken and at the fastener manufacturer’s premises, using the tests that had been previously jointly defined. In the subsequent assessments a solution already began to crystallise based on a zinc flake coating and a customised form of post- lubrication. In further tests it was possible to confirm the new coating’s functionality. The basecoat selected, Delta-Protekt KL 100, provides
Fig. 3. Commercial vehicle manufacturer MAN will from now on be using a chrome (VI)-free coating from Dörken for the surface protection of high-strength steel fasteners.
Photo courtesy Dörken MKS-Systeme GmbH & Co KG. 34
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active cathodic protection and with a coating thickness of just 10µm achieves – dependent upon layer build-up, shape of the parts and form of application – corrosion resistance times as defined by DIN EN ISO 9227 of up to 1000 hours. Another factor of importance for use on high-strength steel fasteners is that as no hydrogen is introduced in the coating process, any danger of hydrogen embrittlement caused by the process can be confidently ruled out. The coating can be applied in all standard application methods. Which method to choose depends on the size and shape of the parts. In MAN’s case parts of smaller dimensions are being coated using the dip spin process, while for bolts >M16 spin coating has proved the most cost-effective method. After coating and preliminary drying, the protective coat gets cured for an average of 30 minutes with the part at temperatures of 200°C to 250°C.
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