Use of chrome (VI)-free zinc flake systems is increasing all the time - in the automotive industry and elsewhere. These days it is impossible to imagine the motor industry without chrome (VI)-free zinc flake systems. However, other branches of industry have yet to discover the many and varied benefits of this technology. And highly varied they are, as these multifunctional systems provide significantly more than just effective anti-corrosion protection. They are able to deploy their full range of strengths, for example, when used on the bolted joints of wind turbines that are subjected to high levels of corrosive, climatic and dynamic stress.

Zinc flake systems have - since the introduction of the EU Vehicle End of Life Directive - become firmly established in the motor industry as a means of coating small components.  The capabilities of such systems have been demonstrated in impressive fashion. These systems are not just chrome (VI) free, but with coat thickness levels of between 6 and 15µm they provide not only the extremely high corrosion resistance that is required, but also fulfil a whole host of other demands: these include chemical resistance, temperature stability and above all process reliability in the assembly of complex combinations of different surfaces and shapes.

While zinc flake systems have now been certified by all carmakers, other branches of industry have yet to switch to chrome (VI)-free systems. In the wind power industry, for example, operators are looking for long-lasting and powerful anti-corrosion protection systems that can significantly prolong the intervals between very labour-intensive maintenance jobs, while on the other hand this "green industry" is also interested in surface protection systems that are environmentally friendly, efficient and harmless to human health. Chrome (VI)-free zinc flake systems thus have a very promising future in the wind power sector, too. The tried and tested Delta®-Tone and Delta®-Seal coatings have indeed already been successfully used in wind turbines for some 15 years and Dörken can thus look back on extensive experience in this area. However, it is only through the use of the more recently developed Delta-Protekt® products, some designed specifically for large components, that whole new fields are opening up, such as off-shore applications.

A Booming Sector

The demand of the wind power industry for highly effective anti-corrosion protection systems is enormous, for the sector is booming. In 2008 alone, 20,000 new wind turbines were installed in 46 countries with a potential output of around 28,000 megawatts. Up until recently, around 50 percent of such newly installed output was recorded in Europe. Now, however, the USA is No. 1 in terms of new installations and under the new American administration this trend seems certain to continue. Trade fairs, such as Wind Power 2009, which took place from 4th to 7th May in Chicago, suggest that this development will be very fast and sudden. Experts currently estimate that total wind power output will rise globally over the next five years to more than 220,000 megawatts, thus increasing by around 80 percent.

The signs for growth are therefore set fair for zinc flake systems too. They have to date been used primarily on bolts and studs in the size range M20 to M48, six tons of which are used on average in building a two-megawatt turbine. Such bolted connections are used in particular in the rotor blade assembly. They are also used on shaft pedestal bearings, coupling flanges and rotating assemblies of large-diameter anti-friction slewing rings. All of these threaded connections are to some extent exposed to extreme climatic corrosive conditions, as well as dynamic stress from wind loading and the turbine’s own operation. On top of this come structural forces from screw pretensioning and the mass of the connected elements. A comparison with cars impressively shows how great the strains are on wind turbines just through their operation alone: 20 years at a rotation speed of 20rpm corresponds to a car being driven for a total of around 350,000 kilometres. And this work has to be done day and night, often under extreme conditions, such as great fluctuations in temperature.

Avoiding Brittle Fracture Failure

Due to the high and diverse stresses involved, the threaded components used in wind turbines are high strength grades. These have a susceptibility to brittle fractures. Such brittle fracture failure is usually the consequence of hydrogen penetration during the surface treatment process. This is something that needs to be taken into account when selecting the anti-corrosion protection to be used. The protective coating also has to have specific friction coefficients in order to enable connections to be easily screwed together on site even when working against the clock under difficult conditions. Not least, however, the coating must provide highly effective and long-lasting anti-corrosion protection especially in offshore locations.

Zinc flake systems are able to fulfil these requirements as a package. The basecoat provides active cathodic protection and, with a layer thickness of just 10µm (regardless of layer build-up, shape of the parts or type of application) achieves corrosion endurance times as per DIN EN ISO 9227 of over 1,000 hours. The coating can be applied using all standard methods of application. Which method to choose depends on the size and shape of the parts. For large steel components, like those used in wind turbines, the methods used are primarily spraying or what is known as spin coating, a dip and centrifuge process for rack-mounted parts. As no hydrogen is introduced in the coating process, it is possible to rule out any danger of the high strength components suffering hydrogen embrittlement caused by the coating procedure. After application, the protective coat is cured for c. 30 minutes at an object temperature of on average 240°C. The low curing temperatures prevent the high strength steel components from being damaged by high temperatures.

A subsequent organic topcoat enables the anti-corrosion protection to be increased significantly. In contrast to the basecoat, this topcoat is not electrically conductive. The additional protection from corrosion is provided here by isolating the basecoat from corrosive attack. Depending on the choice of topcoat, further requirements can also be fulfilled, such as colouring, chemical, UV ray and extreme temperature resistance and defined properties of slide and friction.

Global Presence

Nearly all major wind turbine manufacturers have now approved the use of zinc flake systems for coating high strength connecting elements. As these companies – just like the carmakers – are global players, it is key to the success of this technology that the manufacturers of the zinc flake systems are also represented globally and are able to provide qualified technical advice in the respective local markets. Highly efficient, multifunctional anti-corrosion protection systems, a global presence and variable methods of application, ensure high quality coating results can be achieved all over the world: what has been "practiced" by the carmakers for 25 years, can now be transferred relatively easily to the wind power industry. And the wind for doing so is set fair.

For Information:
Dörken MKS-Systeme GmbH & Co.KG
Wetterstr. 58, 58313 Herdecke, Germany
Tel. +49.23.3063.243
Fax +49.23.3063.354
E-mail: jselent@doerken.de
www.doerken-mks.de