EP0298007A1 - Multistage anodising process of aluminium - Google Patents

Abstract

Multiple anodising of an aluminium element is carried out by (a) surface polishing degreasing or pickling and water rinsing, (b) anodising in an aq. electrolyte contg. 180-200 g/l sulphuric acid and 5-10 g/l aluminium sulphate at 12-15 V d.c., followed by cleaning by immersion in water; (c) colouring in a demineralised water bath contg. (in)-organic pigments (for chemical colouring) or metal salts (for electrolytic colouring), followed by cleaning by immersion in water; (d) sealing in boiling demineralised water; (e) selectively removing the anodised layer and rinsing by immersion in water; (f) re-anodising under the same conditions as in step (b) with corresponding colouring of the new anodised layer; and (g) sealing in boiling demineralised water.

Description

The present invention relates, as its title indicates, to a process for carrying out several anodizations on an aluminum element by means of the treatment of the latter by the corresponding electrolytic processes in an acidic or anodic oxidation medium.

Until now, the aluminum elements were subjected to a single anodizing process since, if anodizing on an already anodized surface, the second treatment deteriorates or partially destroys the first. The decoration of an aluminum element in two or more colors was only possible while using successive surface lacquers. With the process recommended here, the same element can be subjected to several consecutive anodic oxidation processes without damaging the first layer deposited on it. To this end, the element must be subjected to a mechanical treatment of rectification or polishing eliminating any kind of irregularities present on its surface in order to subsequently obtain a uniform anodization thereof; to follow, it is necessary to carry out the cleaning of the entire surface in an acidic, alkaline medium or with degreasers and then to subject this element to a washing with water to remove any remainder of the degreaser.

Once the operations included in this basic treatment have been carried out, we proceed to a second phase of the process which comprises a chemical treatment of anodization in an electrolyte composed of sulfuric acid and water in a proportion of 150 200 grams of sulfuric acid per liter and aluminum or aluminum sulphate dissolved in an approximate proportion of 5 to 10 grams per liter and through which a continuous current is passed through aluminum and the acid medium from 10 to 15 volts; this produces nascent oxygen which causes the oxidation of the aluminum and the covering of the element with an oxide or alumina film, amorphous in the center and crystalline on the periphery, on which it is created under the action of the solvent of the electrolyte a porosity in the center of the alumina cells giving rise to the film which will have a variable thickness depending on the duration of the exposure time in the anodization bath, of approximately 15 microns, to allow the posterior coloring of the elements.

Before coloring these aluminum elements, they must be immersed in water to remove any remaining acid and, subsequently, in a coloring bath containing organic or inorganic pigments in the event of coloring chemical and metallic salts in the event of electrolytic coloring, which are introduced into the pores of the anodic layer and the aluminum elements are then washed to introduce them subsequently in a boiling demineralized water bath which causes the closure of the gates and the absorption by each of the molecules of the anode layer of a water molecule, the anode layer transforming into alumina monohydrate.

To subject the aluminum element to a second anodization, it is necessary to rectify the zones which it is desired to anodize and to eliminate the layer formed during the first process, leaving, in these zones, the aluminum to sight, but it is not possible to degrease the surface with alkaline products as these would damage the layer obtained by anodic oxidation.

Once the element has been rectified in the zones which it is desired to anodize, the chemical process is repeated under conditions analogous to those used in the previous treatment and an anodic film is obtained there; it is also necessary to carry out a coloring bath and subsequently introduce the element in boiling demineralized water in order to close the pores of the anode layer created during the second process.

If we want to obtain a greater combination of colors on the profile, we can machine the areas where we want to introduce a new color and subject the profile to a new anodization and a new coloring of this one by correlatively following the phases applied during the previous process.

It is also possible to obtain a combination of lacquering and several anodizations. For this purpose, it is necessary to deposit first of all on the element a layer of lacquer or paint, to machine the zones where one wishes to carry out anodization and to follow the stages quoted above to obtain one or several anodizations in combination with the lacquering performed in the first place.

The phases of the process having been sufficiently described, it is manifested for all appropriate purposes that it is not limited to exact details of the exhibition but that, on the contrary, we will introduce the modifications considered appropriate provided that these do not alter the essential characteristics which are claimed below:

Claims (1)

1. Method for carrying out several anodizations on an aluminum element by means of the treatment thereof by an anodic oxidation process characterized by the fact that it comprises a mechanical basic treatment process consisting in polishing its surface and cleaning comprising degreasing or pickling thereof by means of an acidic, alkaline medium or by means of degreasers and subsequent washing with water to remove any remains of degreaser, an anodization process proper where the element to be treated is introduced into an electrolyte composed of water with sulfuric acid in a proportion of 180 to 200 grams of sulfuric acid per liter and of alumina or aluminum sulfate in a proportion of 5 to 10 grams per liter, through which a direct current of 12 to 15 volts is passed, which causes the creation of an alumina coating on the element, of a variable thickness depending on the exposure time , after s which is carried out cleaning by immersion of the element in water, a coloring process where it is introduced into a bath of demineralized water with organic or inorganic pigments in case of chemical coloring and metallic salts in electrolytic staining, a cleaning phase by immersion in water, a process of sealing the pores of the anodized layer where the element is introduced into boiling demineralized water, whereby the molecules of the anodic layer increases their volume and absorbs water molecules, the anodic layer transforming into aluminum monohydrate and the increase in volume causing the closing of the pores, a mechanical phase where the layer deposited during the first is destroyed by rectification anodization process, a washing phase by immersion in water, a second anodization process under the same conditions as the first with the corresponding coloring anodic layer and, finally, a phase of closing the pores by immersion in boiling demineralized water; this process can be repeated while maintaining the above-mentioned steps to obtain anodized surfaces on the same element in consecutive processes and, consequently, in different colors; as an alternative, it can be made on a previously lacquered profile, the areas that one wishes to decorate must be machined before performing anodization.