EP0399169B1 - Chemical process to avoid a rainbow effect, provoked by an oxide layer that is caused during the polishing of pieces of aluminium or pieces of aluminium alloys - Google Patents

Description

The invention relates to a chemical process for avoiding a rainbow effect, which is caused by the oxide layer formed when parts made of aluminum or aluminum alloys are shiny, in which the parts are degreased or cleaned in a first process step and electrolytically alkaline polished in a second process step will.

Aluminum parts, such as aluminum pressed or rolled profiles, are among others in the building hardware and lighting industry or in particular in the automotive industry with e.g. Window border systems or as a decorative frame, target lists and. Like. Used. It is also known to use parts made of aluminum or aluminum alloys that have a colored anodized surface.

Such parts made of aluminum or aluminum alloys are first ground, then polished, degreased, regardless of whether they are to be left natural or colored and polished, the parts being treated in a glossy manner in an alkaline electrolyte operated by direct current. It has been shown that the oxide layer that forms when the parts shine produces an iridescent rainbow effect that is extremely undesirable. While the above-mentioned effect is usually only slightly visible with colorless (natural-colored) anodized layers, the rainbow effect, especially with colored anodized layers, is often so strong that it is no longer acceptable. A treatment of the parts in chromium-phosphoric acid solutions has already been proposed, although it has been shown that such a treatment leads to a reduction in gloss and to the etching of the previously alkaline-polished aluminum parts, so that such a treatment of the parts in forbids practice.

The present invention is based on the object of demonstrating a chemical process for avoiding a rainbow effect which is caused by the oxide layer which forms when parts made of aluminum or aluminum alloys are shiny.

This object is achieved on the basis of a method of the type mentioned at the outset, in a simple, advantageous and progressive manner according to the invention, in that the parts are subjected to a further treatment after the luster, in which the parts are first rinsed in water, then in a chromic acid solution exposed to about 50 g / l CrO₃ at about 98 ° C for about 3 minutes and finally rinsed again in a sodium bisulfite solution. The invention therefore provides that the parts made of aluminum or aluminum alloys are rinsed in water and then exposed in a chromic acid solution, the oxide layer or the oxide film (Schmand) being removed. The subsequent rinsing of the parts in a sodium hydrogen sulfite solution has the effect that the 6-valent chromium is reduced to the 3-valent chromium.

Experiments carried out here have shown that the teaching shown leads to the avoidance of the rainbow effect, without adversely affecting the previously polished aluminum.

The parts treated in this way can be left natural or colored. For dyeing, the parts are exposed in a subsequent process step in an electrolyte containing metal salts using alternating current and then chemically adsorbed in a further process step in a dye bath containing azo dye.

Claims (1)

1. A chemical process to obviate a rainbow effect which results from the formation of an oxide layer when parts of aluminium or aluminium alloys are burnished by a method in a first stage of which the parts are degreased or cleaned and in a second stage electrolytically burnished under alkaline conditions, characterised in that the parts, after burnishing, are further treated by being rinsed in water, immersed in a chromic acid solution containing approximately 50 g/l CrO₃ at approximately 98 °C for approximately 3 minutes and finally being rinsed again in a sodium bisulphite solution.