The present invention relates to a process for the formation of colored conversion coatings on aluminum substrates using an acidic aqueous composition as defined in claim 1 on file. The invention further relates to an acidic aqueous composition as defined in claim 1 on file that can be used in such a process. The formation of chromium-free conversion layers on metal surfaces is covered by extensive prior art literature as cited, for example, in WO 94/28193. Such conversion layers are colorless and transparent so that the surface is bright in appearance. However, it is more desirable that the conversion coating as the result of the conversion treatment is immediately visually recognizable to the human eye. This allows to easily determine whether the process has been successfully accomplished resulting in a homogenous product. Due to the lack of color, surface analysis, for example X-ray fluorescence, is required to analyze the result of the conversion treatment process. This laborious and more time-consuming approach is the reason that hexavalent chromium based products, which are known to be carcinogenic, are still used in the architectural aluminum market.

GB 2 097 024 A discloses a process for passivation of metal surfaces, particularly for treating zinc, cadmium and aluminium surfaces, for improving the corrosion resistance properties and to further enhance the appearance of such surfaces by imparting a yellow or a blue-bright coating thereto. Accordingly, there is a need for a process for the formation of colored conversion coatings on aluminum substrates using hexavalent chromium-free substances in order to avoid the usage and presence of carcinogenic substances in the process and the obtained product. The coatings thus produced should be readily and easily recognizable without the need for more laborious technical procedures.
The present invention provides such a coating process and is based on the inventors' finding that a colored conversion coating can be formed on aluminum substrates by bringing an aluminum substrate into contact with an acidic aqueous as defined in claim 1 on file comprising chromium(III), the elements Mo and/or W, a source of fluoride, and at least one oxidizing agent having a standard reduction potential in a range from +1,0 to +1,8 V (SHE).
The advantages of the thus produced coatings are that they allow to judge homogeneity of the product by the human eye. In addition, it has been found that the processes described herein provide for a corrosion resistant conversion with improved adhesion properties of organic layers which may be applied afterwards, for example in form of a paint, an adhesive layer or a protective layer, and the like. Additionally, compared to prior art compositions no expensive and time consuming disposal of hexavalent chromium substances is needed.

The present invention is thus directed to a process for the formation of colored conversion coatings on aluminum substrates wherein an aluminum substrate is brought into contact with an acidic aqueous composition, comprising

1. a) 0.1 to 2 g/kg calculated with respect to the element Cr of at least one water-soluble compound as a source of chromium(III),
2. b) 0.01 to 0.2 g/kg calculated with respect to the elements Mo and/or W of at least one water-soluble compound of the elements Mo and/or W,
3. c) at least one water-soluble compound as a source of fluoride,
4. d) 0.01 to 1.0 g/kg calculated on a H₂O₂ equivalent basis of at least one oxidizing agent different from the components a) to c) having a standard reduction potential in a range from +1.0 to +1.8 V (SHE),

"Water-soluble", as used herein, refers to a solubility of at least 1 g of the respective compound in 1 kg of deionised water (κ<1 µScm^-1) at 20°C.
The compositions used in the process of this invention are substantially free of hexavalent chromium. "Substantially free", as used in this connection, means that the hexavalent chromium content is below 5 mol%, preferably below 1 mol%, of the total chromium content.

The acidic aqueous composition in a process according to the invention has a pH value between 0 and 7, preferably between 1 and 6, more preferably of 2,5 and 4, even more preferably of about 3. The pH value may be adjusted by an acid, for example, an aqueous acid and/or by a buffer system well known to the skilled person. The acids can be, without limited to, HCI, HNO₃, H₂SO₄, and/or H₃PO₄, preferably H₂SO₄ or HNO₃. Furthermore, HNO₃ can be used as component d). A buffer system may be formed, for example, with a conjugated base of H₃PO₄ and/or ammonia. The pH in the context of this invention relates to the negative logarithm to base 10 of the activity of hydronium ions at a temperature of 25 °C.

In the described process according to the invention, the aluminum substrate is brought into contact with an acidic aqueous composition as defined in claim 1 on file with the contacting being achieved by any suitable method known in the art. Those can, for example, include spray-coating, dip-coating, spin-coating, printing and the like. The contacting step can be conducted manually or automatically. The aluminum substrate can be brought once or several times into contact with the acidic aqueous composition according to the present invention, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, and more times.

In a preferred embodiment of the present invention the aluminum substrate is brought into contact with the acidic aqueous composition in an immersion or spraying process, preferably in a spraying process. In a preferred embodiment the contact time is at least 10 seconds, but preferably not more than 600 seconds, more preferably not more than 200 seconds.
In a further preferred embodiment the temperature of the acidic aqueous composition in a process of this invention is at least 15 °C, but preferably not higher than 80 °C, more preferably not more than 70 °C.

The acidic aqueous composition according to the invention comprises the components described above being characterized herein further.
The at least one water-soluble compound as a source of chromium(III) may be any compound that is soluble in water according to the above definition. The preferred source for Cr(III) are salts of Cr(III), including, but not limited to, chromium(III)trifluoride (CrF₃), chromium(III)nitrate, chromium(III)acetate, chromium(III)gluconate and/or chromium(III)sulfate, preferably the source is chromium(III)trifluoride. Aside from those mentioned, other suitable salts may also be used, all of which are known to those skilled in the art. Examples for such salts are chromium(III)chloride hexahydrate, chromium(III)hydroxide, etc.

In the present invention, component a) of the acidic aqueous composition amounts to at least 100 ppm, more preferably more than 500 ppm, but preferably not more than 2 g/kg, more preferably not more than 1,5g/kg calculated with respect to the element Cr and relative to the aqueous acidic composition. The term "ppm" in the context of this invention relates to "parts per million of weight", so that 1 ppm equals to 0,0001 wt.-%.
The at least one water-soluble compound of the element Mo and/or W according to component b) of the acidic aqueous composition may be any compound that is soluble in water according to the above definition. Examples of suitable compounds are known to those in the art and include, for example and without limitation, molybdates, phosphomolybdic acid, molybdenum chloride, tungstates, such as sodium tungstate, and the like. In a preferred embodiment, the water-soluble compound of the element Mo is a molybdate. Molybdates can be discrete or polymeric and have any suitable counterion.

In the present invention, component b) of the acidic aqueous composition amounts to at least 10 ppm, preferably to at least 20 ppm, but preferably to not more than 200 ppm, more preferably to not more than 100 ppm calculated with respect to the elements Mo and/or W and relative to the aqueous acidic composition.
The water-soluble compound as a source of fluoride according to component c) of the acidic aqueous composition may be selected from hydrofluoric acid, simple fluorides, such as sodium fluoride or chromium(III)fluoride, and also complex fluoro acids, such as fluorotitanic acid or fluorozirconic acid as well as their water-soluble salts.
The at least one oxidizing agent, which is different from the components a) to c), has a standard reduction potential in a range from +1,0 to +1,8 V (SHE). This standard reduction potential refers to the commonly known standard hydrogen electrode (SHE), which is a redox electrode and the hydrogen's standard electrode potential is declared to be zero at all temperature to form a basis for comparison with all other electrode reactions. An "oxidizing agent" according to compound d) of this invention preferably does not encompass dissolved oxygen or water-soluble compounds of metal elements. Accordingly, "different from the components a) to c)" means that none of the compounds a) to c) can simultaneously be considered the oxidizing agent in the sense of the present invention, but that both have to be different species. In other words, this does also mean that the oxidizing agent is not a compound that falls within the definition of components a) to c), i.e. is no source for chromium(III), molybdenum, tungsten, or fluoride. The at least one oxidizing agent different from the components a) to c) may be, for example, HNO₃ or H₂O₂.

In the present invention, the amount of component d) of the acidic aqueous composition is in the range of 0.01 to 1.0 g/kg, more preferably of 0.05 to 0.5 g/kg calculated on a H₂O₂ equivalent basis.
In a further preferred embodiment component d) is selected from water-soluble peroxides and/or oxyacids of the elements nitrogen, sulfur or chlorine as well as their water-soluble salts, preferably from hydrogen peroxide.

In yet another preferred embodiment of invention, the amount of free fluoride within the acidic aqueous composition is in the range of 10 to 200 ppm and relative to the aqueous acidic composition. The free fluoride content in the acidic aqueous composition in a process of this invention can be determined directly in an acidic aqueous composition of this invention by making use of a calibrated fluoride-sensitive electrode at a temperature of 25 °C.

In a further preferred embodiment of the invention, the acidic aqueous composition comprises in total less than 50 ppm, more preferably less than 10 ppm of water-soluble compounds of the elements Zr and/or Ti relative to the total composition. These compounds tend to interfere with the formation of a colored conversion coating based on chromium(III) so that their presence is less preferred.
After contacting with the acidic aqueous composition described herein, the metal surfaces can be rinsed with water, for example with deionized water. Depending on the nature of the subsequent coating with organic polymers, the metal surfaces are optionally dried after rinsing with water. If the coating with organic polymers is carried out, for example, by immersing the metal surfaces in a water-based paint dispersion, there is no need for drying after rinsing. However, if the coating based on organic polymers is an adhesive or powder coating, for example, the metal surfaces are preferably dried before this step.
The aluminum substrate may be provided in any shape, for example, as aluminum stripes, aluminum plates, or aluminum parts. The aluminum parts treated in accordance with the invention may be joined to other metal parts through the adhesive layer.
The aluminum substrate in a process of this invention may be provided in any shape, for example, as aluminum stripes, aluminum plates, or aluminum parts.

The aluminum substrate that is brought into contact with the acidic aqueous composition according to the first aspect of this invention, can be used without any pre-treatment or can be pre-treated, for example, with an alkaline cleaning solution or acidic descaling solution suitable for the inventive process. All of the pre-treatment methods and agents are well known to the skilled person. The pre-treating can be conducted manually or automatically. For example, the aluminum substrate can be briefly pickled in cold concentrated nitric acid or the surfaces may be alternatively rubbed down with a squeegee. Optionally, the substrate can be simply rinsed with water, for example with deionized water, and may additionally be descaled with an acidic solution. These treatments may be used separately or in combination. The solutions used for any pre-treatment may have a temperature suitable for the pre-treatment, preferably in a range from 10 to 70 °C, from 20 to 55 °C, or about 25 °C, depending on the solution and the desired effect of the solution. Furthermore, the aluminum substrates can be pre-treated for 0.5 to 20 minutes, preferably for 1 to 10 minutes, more preferably for about 3 minutes.

According to the invention an acidic aqueous composition is encompassed that comprises

1. a) 0.1 to 2 g/kg calculated with respect to the element Cr of at least one water-soluble compound as a source of chromium(III),
2. b) 0.01 to 0.2 g/kg calculated with respect to the elements Mo and/or W of at least one water-soluble compound of the elements Mo and/or W,
3. c) at least one water-soluble compound as a source of fluoride,
4. d) 0.01 to 1.0 g/kg calculated on a H₂O₂ equivalent basis of at least one oxidizing agent different from the components a) to c) having a standard reduction potential in a range from +1.0 to +1.8 V (SHE),

The total fluoride content in acidic aqueous compositions according to the second aspect of this invention can be determined as described in DIN 38 405-D-4-1 in a buffered sample volume (TISAB: "Total Ionic Strength Adjustment Buffer") taken from the acidic aqueous composition by making use of a calibrated fluoride-sensitive electrode at 25 °C sample volume temperature.
All embodiments described herein with relation to the process of this invention are equally applicable to the compositions of this invention and vice versa. This especially means that all preferred embodiments disclosed herein in relation to the compositions used in the described processes apply similarly to the described compositions.

Within a process sequence as listed below aluminum panels (AA6060) have been treated in order to yield colored coatings. The coatings have been tested with respect to adhesion properties towards a polyester-based organic resin.

1. 1. Alkaline Cleaning (Ridoline G 34 A (3,5 %), 55 °C, 3 min)
2. 2. Rinse
3. 3. Acidic Etching (Grametal DX 255 A (3 %), 25 °C, 3 min)
4. 4. Rinse
5. 5. DI rinse
6. 6. Conversion Coating Treatment (35 °C, 45 sec, Spraying Pressure: 1 atm)
7. 7. DI rinse

According to the above-mentioned process sequence the aluminum panels had been treated while the conversion treatment was performed through making use of the following different conversion coating compositions.

34 ppm Ti from H₂TiF₆ as source compound 56 ppm Maleic Acid - Methylvinylether Copolymer 35 ppm Polyvinylalcohol

The pH value was 3.0 ± 0.1.

Conversion Coating Composition of Comparative Example CE2 comprising additionally 25 ppm Molybdenum from ammonium heptamolybdate as the source compound.

1,06 g/kg Cr(III) from CrF₃ as source compound 66 ppm Zr from H₂ZrF₆ as source compound 12 ppm Mo from (NH₄)₆Mo₇O₂₄ · 4H₂O 120 ppm NH₄HF₂ 50 ppm H₂O₂

An amount of HNO₃ to adjust the pH value to 3.0.

Conversion Coating Composition of Example E1 comprising 25 ppm Molybdenum from ammonium heptamolybdate as the source compound.

Conversion Coating Composition of Example E1 comprising 50 ppm Molybdenum from ammonium heptamolybdate as the source compound.

Conversion Coating Composition of Example E1 comprising 100 ppm Molybdenum from ammonium heptamolybdate as the source compound.

All treated parts have been painted with a polyester powder coating (Corro-Coat PE-F series 2403, Jotun A/S, Norway) and cured for 20 min at 180-190°C.

Results in the Wet Adhesion Test are summarized in Table 1 and reveal that the examples according to the invention (E1-E4) adhere sufficiently to the aluminum panel and are superior to chromium-free compositions independent of the additional presence of molybdenum (CE1, CE2). Table 1 Example Wet Adhesion Test* CE1 0-0 CE2 3-4 E1 0-0 E2 0-1 E3 0-0 E4 2-1 * 2 hours exposure in deionized boiling water and 1 hour at 25°C at 40% air humidity; assessment of delamination at cross hatch cut according to DIN ISO 2409

The visibility of Cr(III)-Mo conversion coating (E1-E4) starts with 12 mg/kg Mo and increases fast with further additions. 25-50 mg/kg Mo gives a very good color that appears to be ideal for real life application. Color intensity is similar to Ti-Mo technology (CE2).