EP0459550B1 - Process for rinsing conversion coatings - Google Patents

Description

The invention relates to a method for rinsing conversion surfaces produced on surfaces made of aluminum or aluminum alloys with chromium-free solutions based on complex fluorides of titanium, zirconium and / or hafnium, and its use as a pretreatment for subsequent painting, film coating or adhesive coating.

The production of conversion layers on aluminum or aluminum alloy surfaces, in particular as pretreatment before the application of an organic coating, is carried out industrially on a large scale. Conversion layers of this type prevent or make it more difficult for corrosion to occur and, in the case of an organic coating, serve to improve the adhesion of the applied coating. The widespread chromate coatings generally meet these requirements to a sufficient extent; for reasons of workplace hygiene, environmental protection or because the treated material for special purposes, e.g. As food packaging, however, the generation of chrome-free conversion layers is increasingly preferred. The treatment solutions used for this are generally adjusted to an acidic pH and contain e.g. Titanium, fluoride, phosphate and tannin (U.S. Patent 4,017,334) or zirconium, fluoride and boron (U.S. Patent 3,964,936). A treatment solution containing hafnium and fluoride is described in FR-PS 2 417 537.

The properties of the conversion layers generated can be further improved if a passivating rinsing is subsequently carried out. For the reasons mentioned Chromium-free treatment solutions are also preferred for rinsing, which usually contain organic active ingredients. Such a rinse aid based on polyvinylphenol is described in US-A 4,376,000. However, it causes a pollution of the wastewater, in particular due to the high oxygen demand, which is necessary to break down the organic component.

The object of the invention is to provide a method for passivating rinsing of chrome-free conversion layers produced on surfaces made of aluminum or aluminum alloys with solutions based on complex fluorides of titanium, zirconium and / or hafnium, which does not have the disadvantage of rinsing solutions with organic components with regard to waste water treatment has, but improves the corrosion protection and paint adhesion in at least the same way.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the conversion layers are rinsed with aqueous solutions which contain aluminum, zirconium and fluoride with a molar ratio of Al: Zr: F of (0, 15 to 2.0): 1: (5 to 16), and whose pH is adjusted to 2 to 5.

The method according to the invention is suitable for the passivating rinsing of conversion layers which have been produced on aluminum or aluminum alloy surfaces, wherein the materials can consist of solid aluminum or solid aluminum alloy or objects plated therewith, for example, in the hot-dip method. For example, the material can be made of steel, which has been provided with an aluminum or aluminum alloy surface using the hot-dip process. Suitable alloying partners for aluminum are in particular silicon, manganese, magnesium, zinc and copper.

The conversion layers produced on such surfaces with solutions based on Ti, Zr and / or Hf are distinguished by the fact that the layer thickness is clearly below 1 μm, the layers are at least partially amorphous and that they contain no chromium. The treatment solutions by means of which the conversion layers are produced contain, in addition to titanium, zirconium and / or hafnium ions, further layer-forming and / or pickling components, such as e.g. Fluorides, phosphates, compounds of boron, and possibly passivating components such as Tannins. Suitable treatment solutions are described in the patents mentioned at the beginning.

The surface is expediently rinsed with water after the conversion layer has been produced.

The application of the rinse solution can e.g. by dipping, flooding, spraying or rolling.

A preferred embodiment of the invention consists in rinsing with a solution which contains aluminum, zirconium and fluoride in a total concentration Al + Zr + F between 0.1 and 8 g / l, preferably between 0.2 and 5 g / l. The Al: Zr: F molar ratios should advantageously be set to (0.15 to 0.67): 1: (5 to 7).

The rinse solutions used in the method according to the invention contain, inter alia, acidic aluminum fluorozirconates and, in the case of an excess of aluminum, other salts of aluminum (e.g. fluorides, tetrafluoroborates, nitrates). Their manufacture can e.g. in such a way that metallic zirconium or zirconium carbonate is first dissolved in aqueous hydrofluoric acid, complex fluorozirconic acid being formed. Then metallic aluminum or aluminum hydroxide or an aluminum salt, e.g. Nitrate, fluoride, tetrafluoroborate, formate, acetate, preferably in dissolved form, are added and, if necessary, dissolved. A possible slight clouding of the solution does not affect the effectiveness. Although the described manufacturing route is preferred, the solutions can also be prepared in other ways.

The pH of the solution is preferably adjusted with cations of volatile bases. These include in particular ammonium, ethanolammonium and di- and tri-ethanolammonium. When setting in particular higher pH values in the specified pH range and at higher concentrations in the specified range of the total concentration Al + Zr + F, the solution may become cloudy, which has no negative effect on the effectiveness of the process.

According to a further advantageous embodiment of the invention, the surfaces provided with a conversion coating are rinsed with an aqueous solution which additionally contain at least one of the anions benzoate, caprylate, ethylhexoate, salicylate in a total concentration of preferably 0.05 to 0.5 g / l. This particularly increases the protection against bright corrosion. The anions can be introduced via the corresponding acids or their salts.

The duration of the application of the rinse solution is generally between about 1 and 120 seconds, according to an expedient embodiment of the invention between 1 and 30 seconds. The application temperature can be between 20 ° C and about 80 ° C. Temperatures between 20 and 50 ° C are preferred.

Demineralized or low-salt water is preferably used for the preparation of the rinse bath. Water with a high salt content is less suitable for bath preparation.

After the passivating rinse, the surface can e.g. air-dried or oven-dried, if necessary rinsed with deionized water beforehand. An advantageous embodiment of the invention provides that the surface is accelerated after the passivative rinsing, e.g. dry with hot air or infrared radiation.

The method according to the invention is primarily used to pretreat the aluminum or aluminum alloy surfaces before painting, film coating or applying adhesives. Examples of organic coating agents used are polyesters, silicone-modified polyesters, polyvinylidene fluorides, acrylates, epoxies, epoxy-phenol resins, plastisols or organosols (e.g. PVC or acrylates).

The advantage of the method according to the invention lies in particular in the increase in the adhesion of the organic films to the metallic substrate, as is the case, for example, with correspondingly good results in the T-Bend test (ISO 1519-1973) or in the feathering test (paint adhesion when opening the can closures ) is expressed. Likewise, the durability of the organic films Corrosion stress, e.g. increased in a condensation water constant climate test (DIN 50 017 KK) or a sterilization test. In addition, the method according to the invention improves the corrosion resistance of the non-organically coated surface, as can be seen, for example, in the test for fountain blackness (no blackening when boiling in water).

The method according to the invention is explained by way of example and in more detail using the following example.

Example:

Ti

0,17 g/l

F

1,24 g/l

P₂O₅

0,09 g/l

NH₄

0,91 g/l

Tannin

0,11 g/l

Na

0,003 g/l

Biozid

0,10 g/l

Cleaned and degreased aluminum sheets were immersed in a solution for 10 seconds to produce a conversion coating, the temperature of which was 50 ° C. and had the following composition:

Ti

0.17 g / l

F

1.24 g / l

P₂O₅

0.09 g / l

NH₄

0.91 g / l

Tannin

0.11 g / l

N / A

0.003 g / l

Biocide

0.10 g / l

The sheets were then rinsed with water and, after passivation, rinsed. For this purpose, the sheets were immersed in the rinse solution for 5 seconds and then excess solution was removed by squeezing. After a drying time of 0.5 min in a convection oven at 60 ° C, the sheets were coated with a two-layer food lacquer, the first layer of which is an epoxy-phenol resin and the second layer of which is an organosol. The layer thickness of the overall paint was between 10 and 15 µm.

Then, sheet blanks with a diameter of 60 mm were punched out of the treated, approximately 0.25 mm thick sheets and wells with a diameter of 26 mm and a height of 25 mm were deep-drawn.

These cells were subjected to a sterilization test by exposure to an aqueous solution of 3% sodium chloride, 1% citric acid and 0.5% lactic acid at 121 ° C. in a pressure vessel for 40 minutes. The defects found on the cells afterwards (paint detachments, bubbles) were rated on a scale from 1 (paint in the entire mantle area of the well = useless) to 15 (no paint defects = excellent)

The rinsing solution used was diluted 1.6 g / l (rinsing solution A) or 3.2 g / l (rinsing solution B) of an aqueous concentrate with 0.855 wt .-% Al and 8.62 wt .-% Zr and 10.7 wt% F made using deionized water. The pH in both solutions had been adjusted to approximately 3.6 with ammonia.

For comparison purposes, a polyvinylphenol solution with 0.6 g / l and a pH of approximately 5 (rinsing solution C) was used for rinsing.

The composition of the rinse solutions used in the process according to the invention was:

Test results of the sterilization test

(Rating scale: 1 = unusable to 15 = excellent)

A comparison of the table values shows that the method according to the invention delivers significantly better values than the comparative example with a rinse solution based on polyvinylphenol.

Claims (12)

1. A process for rinsing chromium-free conversion coatings produced on surfaces of aluminium or aluminium alloys with solutions on the basis of complex fluorides of titanium, zirconium and/or hafnium, characterised in that the conversion coatings are rinsed with aqueous solutions which have a content of aluminium, zirconium and fluoride with a molar ratio of Al : Zr : F of (0.15 to 2.0) : 1 : (5 to 16), and the pH value of which is set to 2 to 5.
2. A process according to Claim 1, characterised in that the conversion coatings are rinsed with solutions which have a total concentration of Al + Zr + F of between 0.1 and 8.0 g/l.
3. A process according to Claims 1 and 2, characterised in that the conversion coatings are rinsed with solutions which have a total concentration of Al + Zr + F of between 0.2 and 5.0 g/l.
4. A process according to Claims 1, 2 and 3, characterised in that the conversion coatings are rinsed with solutions in which the molar ratios of Al : Zr : F are set to (0.15 to 0.67): 1 : (5 to 7).
5. A process according to one or more of Claims 1 to 4, characterised in that the conversion coatings are rinsed with solutions the pH value of which is set with cations of volatile bases.
6. A process according to Claim 5, characterised in that the conversion coatings are rinsed with solutions, the pH value of which is set with ammonium, ethanolammonium, diethanolammonium or triethanolammonium compounds.
7. A process according to one or more of Claims 1 to 6, characterised in that the conversion coatings are rinsed with solutions which additionally contain at least one of the anions benzoate, caprylate, ethyl hexoate, salicylate in a total concentration of 0.05 to 0.5 g/l.
8. A process according to one or more of Claims 1 to 7, characterised in that the conversion coatings are rinsed for 1 to 30 seconds.
9. A process according to one or more of Claims 1 to 8, characterised in that the conversion coatings are rinsed with solutions which have a temperature of 20 to 80°C.
10. A process according to Claim 9, characterised in that the conversion coatings are rinsed with solutions which have a temperature of 20 to 50°C.
11. A process according to one or more of Claims 1 to 10, characterised in that the surface is dried after the rinsing operation.
12. The application of the process according to one or more of Claims 1 to 11 as a preliminary treatment for a subsequent painting, film-coating or adhesive-coating operation.