Perfect adhesion for powder coatings

Two pre-odised and powder coated profile surfaces after the wet adhesion test. Left: pre-anodization only, right: pre-anodization plus new pre-treatment process.
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Pre-anodising is still considered the top class of chemical pre-treatment processes for aluminium surfaces and offers particularly high resistance, especially with regard to extreme surface requirements such as maritime climate or swimming pool atmosphere. In simple terms, pre-anodization has the effect of a thick galvanized layer on steel. The thick layer forms a resistant barrier against damaging influences from outside and the porous (uncompressed) structure enables good anchoring of the subsequently applied powder coating. Despite the high protective effect, pre-anodization also has disadvantages: The pre-anodization process is considerably more expensive due to the consumption of electricity and chemicals and, depending on the thickness of the layer, longer treatment times are necessary. In addition, the process also requires a very good rinsing technique to wash out remaining sulphuric acid residues and aluminium salts from the pores. If this part of the pretreatment is neglected, poor paint adhesion and possibly reduced corrosion protection on the later finished part can be the consequences. Qualicoat requires the so-called wet adhesion test for pre-odised and powder-coated components. In this test, the finished component is first boiled in fully demineralised water, then provided with a cross-cut and subsequently checked by tearing off the adhesive tape. During this test, no delamination may occur, but especially during the wet adhesion test, it is often observed in practice that inexplicable loss of adhesion occurs. This usually involves intensive troubleshooting: Was it an inadequate flushing technique? A poorly formed oxide layer? Or an error in the paint application? The actual cause is often difficult to prove afterwards.

Optimized pore rinsing and better paint adhesion

To avoid such adhesion problems and to improve process safety, Alufinish has developed the Envirox Preano pretreatment process. Due to its special chemical composition, the pore rinsing is simultaneously optimised, corrosion protection is supported by titanium-containing components and paint adhesion is improved by suitable polymers. Envirox Preano is a chromium-free process that is applied after pre-anodization and replaces the otherwise usual final rinse with deionized water. The parts are then dried and powder-coated in the usual manner. The method is used as a so-called "no-rinse" process, where further rinsing is not absolutely necessary. Alternatively, it is also possible to rinse again with fully demineralised water afterwards. The application can thus be flexibly designed, depending on the available plant zones.

 The complete article was published in the March issue of JOT.

Author(s): Thomas Sondermann, Alufinish

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