Thin layer technology for metal pretreatment
Chromium trioxide and related chromium salts are environmentally persistent carcinogenic chemicals which are now under phase out in Europe. These salts have been used in various formulations for the treatment of Aluminium surfaces to prevent corrosion in demanding environments and as pre-treatment prior to painting. In the search for chromate free pre-treatment processes, the most obvious solution is to replace the chromium salts by others that might have the same properties, but will not be carcinogenic and be safer for the environment and most importantly operators. The choices are restricted but products based on titanium and zirconium salts produce processes using the same reaction as chromating.
The conversion coatings formed with both elements are much thinner and more flexible than a chromate layer, but give excellent adhesion to any paint or powder coating and give similar results in artificial corrosion tests. The coatings are formed on aluminium as well as mild and galvanised steel.
Tests showed that the processes based on zirconium give good conversion coatings on aluminium which are more stable than processes based on polymers or silanes that can only be applied as no-rinse additional processes.
These innovative formulations were developed into practical processes to replace zinc phosphating on steel in a cost effective and environmentally friendly process. The new coatings were initially described as "Nanotechnology" due to the fact that the layers formed were a few hundred nanometers thick, but because this word has quite a different meaning in science, the processes collectively are now referred to as Thin Layer Technology.
Because the zinc phosphating process always required a separate degreasing step, the first Thin Layer Technology applications were used in traditional process flow:
During development these formulations evolved to produce a new generation of Thin Layer processes, where the Thin Layer is produced during the degreasing stage in a single step. The process, that originally could only be used in zinc phosphating plants, was now usable in any iron phosphate installation with savings in space, energy and water usage using a simple process flow needing only three tanks:
Degreasing / Thin Layer Formation - Rinsing -
The results were satisfactory with aluminium and galvanised steel where the improvement was evident because, without additional passivation, the corrosion resistance was clearly better than before. In the treatment of mild steel, however, during line stops flash rusting continued to be problem which was finally overcome by improvements in formulation to improve degreasing and reduce flash rusting. These are now available as the Aquus Thin Layer Process.
The Aquus Thin Layer Process consists of one component for both bath make-up and replenishing. A pH adjustment component is needed when commissioning 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 zinc or iron phosphate process and consumption during the process is very low, the production costs are lower in spite of a higher price for the Aquus product range.
As well as the usual mechanical testing of powder-coated or painted products by the customer in the initial phase the new system was regularly tested for corrosion resistance in salt spray and filiform tests. From the beginning the results were excellent on all substrates and after simplification of the process the results of further corrosion tests were identical.
The introduction of Aquus Thin Layer Technology reduces the costs of metal pre-treatment and improves corrosion resistance at the same time. Different substrates are treated in one process where the final quality of coated aluminium and galvanized steel is significantly better than with a conventional phosphating processes.
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