Chrome free surface coatings for bolts and other fasteners

Paul Boughton

Rudolf Knott and Dr Jochen Schneider look at the benefits of using a chrome (VI)-free coating for the surface protection of high-strength steel fasteners.

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. Its layered structure made up of a zinc flake coating with co-ordinated post-lubrication achieves the specified high level of corrosion resistance and fulfils at the same time the coefficient of friction requirements.

MAN Truck & Bus AG of Munich is the largest company in the MAN Group and one of the world's leading suppliers of commercial vehicles and transportation solutions. As a pioneer in all areas of relevant technology, the company is constantly looking for new solutions to reduce fuel consumption, exhaust emissions and noise level. The MAN TGX 18400, for example, received the 'Green Truck 2011' award, having impressed judges with the lowest level of fuel consumption and CO2 emissions on the European market.

However, 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 review the environmental protection measures in place and to continually improve the company's sparing use of water and material resources and the reduction in air pollutants. Some of these measures are concerned, for instance, with reducing use of coolants and lubricants and the associated consumption of water, while others relate to optimising fasteners' anti-corrosion coating from a 'green' perspective.[Page Break]

Responsible decision-making

Although EU Directive 2000/53/EC, which has been in force since July 2007, does not apply to vehicles over 3.5tons, MAN decided to switch surface coatings for all bolts and other fasteners to chrome (VI)-free alternatives. This decision was, of course, taken partly to reduce the risks to human being and the environment. However, commercial and logistical considerations also played a part. That's due to the fact that at every stage of the production process right through to assembly chrome (VI)-free parts have to be strictly separated from parts with a chrome (VI) content. And a 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.[Page Break]

The basecoat selected, Delta-Protekt KL 100, provides 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 30minutes with the part at temperatures of 200°C to 250°C.

Subsequently a new form of post-lubrication with multifunctional properties is applied using Delta-Lube 10. The material has been specifically designed for mass-produced parts that are assembled using automatic bolting systems. It produces a dry lubricant coat and ensures constant overall coefficients of friction with a value based by MAN on the VDA range of between 0.09 and 0.14µg. For the user that means predictable and - in relation to tightening torque and clamping force - reproducible results and shorter assembly times. The post-lubrication also gets applied by the dip spin process or spin coating and then gets dried for c. 15 to 20minutes at 50°C to 60°C. In order to reliably identify the newly coated parts, the new post-lubrication has a greenish colour.

The new coating structure has now been approved by MAN as part of the MAN 183 company norm. Key factors for this global player in the commercial vehicles industry were not just the technical features of the solution, but also Dörken MKS Systems' global presence through local subsidiaries and the application of the products by licensed coating companies, which using co-ordinated application techniques achieve the same high quality of coating results all over the world.

Enter √ at

Rudolf Knott, Central Engineering Division, MAN Truck & Bus A, and Dr Jochen Schneider, Customer Service, Dörken MKS-Systeme GmbH & Co KG, Herdecke, Germany.

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