In the search for chromate free pre-treatment processes for aluminum, the most obvious attempt was to replace the element chromium in the chromating process by an atom that might have the same properties, would not be carcinogenic and largely available. The choice fell on the elements Titanium and Zirconium whereby the substance hexafluoro titanium acid and hexafluoro zirconic acid would fit very well in the formulation of a chromate free process based on the mechanism of a chromating reaction.
Tests have now shown that the processes based on zirconium give good conversion coatings on aluminum and are more stable than processes based on polymers or silanes which can only be applied as no-rinse processes.
The conversion coatings formed are much thinner than a chromate layer, but will always give excellent adhesion to any coating and often give similar results in the artificial corrosion tests. Meanwhile, more or less by chance, it has been established that these layers not only form a good conversion coating on the aluminum, but also on steel and galvanized steel.
FIRST APPLICATION
Several years ago the first processes were used in practice to replace zinc phosphating on steel by a cheaper and more environmentally friendly process. The processes were then characterized as "Nanotechnology" due to the fact that the formed layers were a few hundred nanometers thick, but because of the fact that the word Nanotechnology has quite a different meaning in science, the processes by C&T Technics bv collectively are referred to as Thin Layer Technology. Because a zinc phosphating process always goes together with a separate degreasing step, the first Thin Layer processes were set up as follows:
- Degreasing
- Rinsing
- Thin Layer Process
- Rinsing
- Rinsing with deionized water
STEP FORWARD
When C&T technics bv in 2006 ended the cooperation with Chemetall a partnership was formed with the British company H3P Technologies. This company developed a new generation Thin Layer processes, where the Thin Layer is formed during the degreasing in one step. In this way the process, that originally could only be used in zinc phosphating plants, was suited for any iron phosphate plant with the following process flow.
- Degreasing / Thin Layer Formation
- Rinsing
- Rinsing with deionized water
The first test in practice in the Netherlands was carried out in early 2007 in an iron phosphating plant that was converted into a combined Degreasing/Thin Layer process. The original process was as follows:
- Degreasing / phosphating (iron phosphate)
- Rinsing
- Rinsing
- Post rinse (Passivation)
The first results were satisfactory, where the advantage was achieved in the treatment of aluminum and galvanized steel because, without additional passivation, the corrosion resistance clearly was better than before. But in the treatment of steel the desire remained to improve the degreasing and during line stops, the steel products were susceptible to flash rusting. In October 2008, the process was further improved by a small alteration that improved the degreasing and minimalised flash rusting during line stops.
COST COMPARISON
The Thin Layer Process consists essentially of one component for bath make-up and replenishing. However, a pH adjustment component is needed when making up the bath, but the usage is so low that it can be neglected in the cost comparison. Because the process uses a much lower concentration than a liquid iron phosphate process and consumption during the process is very low, the production costs are lower in spite a higher price per kg. Furthermore, the sludge production during the process is approximately 5 times lower than in an iron phosphate, so the lifetime is longer and disposal costs are lower. An important advantage is the fact that the heat exchanger of the bath remains perfectly clean when applying the new process.
CORROSION RESISTANCE
Besides the usual mechanical testing of powder-coated products by the customer in the initial phase the system was regularly tested on corrosion resistance in the salt spray and filiform tests at C&T technics. From the beginning the results were very good on all substrates. After the modification of the process in October 2008 further corrosion tests were carried out to determine whether the alteration had affected the corrosion resistance. The results of these tests were identical, so without further changes the process is used in practice.
SUMMARY The introduction of this thin layer technology opens the possibility for reducing the costs of iron phosphating and improves corrosion resistance at the same time. It is also possible to treat different substrates in one process where the final quality of the coated aluminum and galvanized steel is significantly better than with an iron phosphating process. In some cases it is possible to replace the zinc phosphating process by a Thin layer process. In the U.K. the first zinc phosphate line with cathodic electropaint has been converted into this Thin Layer Technology and the results are excellent. For more information or to make an appointment call +31-413-331083 or e-mail us at
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