Various pre-treatment methods prior to lacquering (Topic: 14057)

The usual pre-treatment steps prior to lacquering (powder coating) may be replaced by the following methods or steps when it is necessary:

Blast treatment

Blast treatment or blasting may be used as the first step in a pre-treatment process for instance to remove impurities or/and to get a rougher surface.

Chemical conversion coatings

Chromate and phosphochromate conversion coatings are the most used types of chemical conversion coatings, but because of pollution problems and health reasons other types of conversion coatings are necessary.

Non-chromium processes are conversion coating processes without chromium. Chromium compounds are not wanted in contact with foodstuffs. Environmental and health problems have meant a search for suitable alternatives. So far none of these processes is equivalent to the chromate-phosphate or chromate processes in terms of corrosion performance, but alternative processes are starting to be used industrially, where they give good adhesion of base lacquers and protective interior coatings. The appearance of these coatings is clear, with a coating weight normally less than 0,1 g/m2. A non-chromium conversion coating may have zirconium, aluminium, oxygen and fluorine as the main constituents of the surface film.

Phosphate processes are conversion coating processes based on phosphate, usually zinc phosphate. The phosphating solution may be applied either by immersion or spray. Typical production time for a sprayed coating is 1ñ2 minutes at 50ñ55ƒC. Excess coating chemicals being removed in a water rinse. A chromic acid rinse may be applied before lacquering. A typical coating weight varies between 1,5ñ3,0 g/m2, depending largely on the pre-treatment and the aluminium alloy being treated. Typical production time for an immersed coating is 3ñ5 minutes at 50ñ55ƒC, with a coating weight between 1,5ñ3,5 g/m2.

The main use of phosphating processes on aluminium is in situations where mixtures of metal, such as aluminium, iron, steel and zinc, are being processed together, for example in a car body assembly. Phosphate coatings on aluminium do not provide such good corrosion resistance as the chromate coatings, and in marine environments an increased depth of pitting has been observed when phosphates are present.

Anodising

Anodising is also used as a pre-treatment prior to lacquering. Due to the pores in the anodic oxide coating, organic coatings will obtain good adhesion. Whether the oxide coating should be sealed or not before lacquering has been discussed a lot. If the profiles have to be stored prior to coating, sealing is recommended, because impurities can give an inferior surface quality. The reason not to seal is due to the theory that the lacquer constituents should have the ability to penetrate into the pores, but the solid particles are too big. But an unsealed coating may give a minor increase in adhesion properties compared with a sealed coating.

The anodising may be carried out in a sulphuric acid electrolyte under conditions described in "Process steps for normal architectural anodising", but phosphoric acid and chromic acid electrolytes are also very often used. (See also "Anodising in various electrolytes"). The oxide thickness is normally lower than required for normal anodising. Film thicknesses between 3ñ8 µm are usual.