Chrome-free pretreatment of high-class aluminium profiles

Koenigsdorf Oberflächentechnik changed its pretreatment plant from a chrome(VI)-containing process to the eco-friendly Oxsilan technology offered by Chemetall. Thus, the company already meets future REACH requirements and supplies high-quality, chromium-free aluminium profiles to renowned facade construction companies.

The core activity of Koenigsdorf Oberflächentechnik GmbH & Co. KG in Wolfhagen, Germany, is the refining of metal surfaces. With anodizing technologies and powder coatings, the company has been actively ensuring since the 1960s that building facades can withstand the most diverse weather conditions for many decades. Besides a long-lasting, decorative surface, the refined aluminium profiles and sheets produced by this company also afford excellent corrosion protection. With a workforce of around 130 employees, the GSB- and Qualanod- member Koenigsdorf Oberflächentechnik coats around 10,000 square meters per day, mainly aluminium surfaces, on a total of three production lines. The horizontal line – where a yellow chromating process ran – was successfully changed to the silane-based Oxsilan technology in April 2013.

Changeover to chromium-free technologies – a necessary evil?

Chromium, as a hazardous substance, is on the ECHA candidate list and chrome(VI) processes are scheduled to come under very stringent regulation probably in August 2018. Architects are now increasingly asking for chrome-free materials. Often if the suppliers are not able to produce them, they will simply not be considered in the bidding process. With these new requirements approaching, many companies are now searching for viable, non-chrome alternatives. Just like Koenigsdorf Oberflächentechnik. The first contacts between this company and Chemetall, a global leader in metal pretreatment technologies, were established at the Aluminium 2012 trade fair in Düsseldorf, Germany. In the same year, the planning process started to change the plant to the chrome-free Oxsilan technology.

“Thanks to the changeover to Oxsilan we can now process high-class aluminium profiles and galvanized material with one of the most eco-friendly coating technologies available”, says Dieter May, Managing Director of Koenigsdorf Oberflächentechnik. “With the chrome-free Oxsilan technology, we are excellently positioned for the future. The new process offers many benefits in terms of health and environmental protection, production flexibility and also potential process costs savings”.

Minor plant adaption

Only minor conversions were required for the changeover from the former yellow chromating plant to the chrome-free technology. With regard to the process sequence there is little difference between chromium-free and chromium-containing technologies. The metal substrates have to be pre-cleaned and rinsed before a conversion coating can be applied. Even the component line hangers made from steel could still be used. As opposed to most other chrome-free processes, the multi-metal Oxsilan technology can cope with the iron dissolution in the bath. And Koenigsdorf Oberflächentechnik did not need to invest in expensive stainless steel hangers.

For the changeover, only the cascaded rinse was adapted and the rinsing technology optimized. The improved nozzle technology in the four-chamber spray plant ensures better rinsing of the cavities. A glass bowl filter was installed in the Oxsilan bath to remove any particles sized between 10 and 25 µm.

During the changeover, Koenigsdorf Oberflächentechnik opted for the installation of a new reverse osmosis plant instead of refilling its existing deionized (DI) water unit. Thus, it was possible to reduce the DI water retention capacity from 6,000 l/h to 1,500 l/h. Further benefits of the fully-automatic reverse osmosis unit are its low maintenance and low consumption of regeneration chemicals. In addition, the plant is also relatively inexpensive to acquire; a refill of exchanger resins for the existing DI unit would have cost nearly the same amount. The DI unit is now re-used in the anodizing plant.

Plant cleaning

Plant cleaning was a matter of just a couple of days. While the first three process tanks, the pickling tank and two rinsing tanks, were drained and mechanically cleaned, the chromating zone and the downstream rinsing bath were cleaned using chemicals. In this process, first a cleaner bath containing an alkaline cleaner (Gardoclean R1700F) was prepared in the rinsing tank downstream of the chromating bath. This spray rinsing tank was operated for two hours before pumping the cleaner solution into the chromating zone. The rinsing tank was then filled with tap water and neutralized with sulfuric acid (Gardacid P). The cleaner solution in the chromating zone was left to react overnight at a temperature of 30 °C. The next day, the spray pumps were switched on for two to three hours before draining the tank, filling it with tap water and neutralizing it with Gardacid P until reaching a pH in the range of five to six. After that, the bath was made up using DI water and the single-component Oxsilan Al 0510 product. “For plant cleaning, our estimates regarding the effluent volume roughly follow the formula: number of bath tanks x tank volume x four”, says Dipl. Ing. Mike Schirdewahn, Technical Field Service, Chemetall, who was responsible for the changeover preparation phase .

Robust: ten times higher conductivity

In terms of rinsing water consumption, significantly less DI water is required downstream of the Oxsilan treatment line than in the case of the chromating process. Given the robustness of the technology, it was possible to raise the conductivity value of the dripping water in the DI rinse upstream of the Oxsilan bath to 200 – 300 µS/cm. This corresponds to a conductivity value which is roughly ten times higher than that of other no-rinse passivating processes. As the new reverse osmosis plant does not allow for a circulation of the water, part of the DI water is discarded directly downstream of the Oxsilan tank. Consequently, the DI water consumption remains on nearly the same level as before.

Analyses confirm superior quality

To assess the quality of the pretreatment, a visual check of the components is frequently performed after their leaving the adhesive water dryer. This was certainly the biggest change for the employees of Koenigsdorf Oberflächentechnik who had so far been able to judge the quality of the pretreatment by a uniform, yellow iridescent, wipe-proof coating. The assessment of the nearly colorless Oxsilan coating was facilitated by introducing a so-called droplet test that evidences the coating by way of a color reaction.

“Our employees feel comfortable working with the bath analysis”, says Dipl. Ing. Neufeld, Plant Manager at Koenigsdorf Oberflächentechnik. “Thanks to the automatic metering of the single-component Oxsilan and degreasing technologies, we can maintain a stable operation of our baths”. Time expenditure for the bath analyses basically does not differ from that of the former process: degreasing is measured once or twice, the concentration of the Oxsilan bath three times per day. In addition to measuring pH and conductivity, the zirconium content and the coating weight are also determined by means of photometry.

Long-term quality confirmed by rapid analysis

A new analysis method provides quick information about the quality and corrosion protection offered by the conversion coating. Whereas results confirming the long-term quality could only be obtained by means of a 500 – 1,000 hour salt spray test so far, the new measuring method now allows assessing the quality within a period of only three to seven hours. “With the new resting potential analysis, we receive confirmation of the quality of our coated components on the same day. That is what I call a truly innovative process control and quality test and it tells me that I have found the right partner”, says Dieter May.

Benefits of the new technology

A good planning, minor plant modifications and a thorough cleaning of the plant are the prerequisites for a successful changeover to chrome-free technologies. The Oxsilan process has been running smoothly at Koenigsdorf Oberflächentechnik since April this year. In addition to a robust process, simplified effluent treatment as well as improved workers’ safety, also the same-day analysis confirming the excellent long-term quality of the coated components rank among the great benefits of the Oxsilan technology. Precise information regarding the costs cannot be provided as yet given the short operating time. Presently, it is estimated that the changeover is cost-neutral. Further optimization activities, in particular with regard to water consumption, will enable substantial process cost savings and would thus leverage the full potential of the eco-friendly Oxsilan technology.

This article has been published in IPCM Sept/Oct 2013 issue:

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About Chemetall:

Chemetall, a division of Rockwood Holdings, Inc. (NYSE: ROC), is a leading global supplier of special chemicals with a focus on processes for the surface treatment of metals and plastics. Chemetall is headquartered in Frankfurt am Main, Germany, and comprises about 40 companies and 22 production sites worldwide. With 2,000 employees, the Group achieved sales of about 551 million Euro (2012). More at




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