Method for treating the surface of a metallic substrate

Abstract

An advantageous method for treating the surface of a metallic substrate made of aluminum or an aluminum alloy, comprising the following steps: providing a water-based mixture with a sol, comprising alkoxy silanes of general chemical formula Si(OR).sub.4 and organoalkoxy silanes of general chemical formula R″Si(OR′).sub.3, in which R and R′ are linear or branched, short-chained hydrocarbon groups with at least one hydroxyl group and R″ is an organic group with a glycidoxy-, merkapto-, amino-, methacryl-, allyl- and/or vinyl-group, applying the mixture to the surface of the metallic substrate and at least in sections, hardening the mixture with a formation of a sol-gel coating connected to the metallic substrate.

Claims

1. A method for treating a surface of a metallic substrate of aluminum or an aluminum alloy, comprising: furnishing a water-based mixture having a sol, having alkoxysilanes with a general chemical formula Si(OR).sub.4 and organoalkoxysilanes having the general chemical formula R″Si(OR′).sub.3, in which R and R′ are linear or branched short-chain hydrocarbon radicals with at least one hydroxyl group and R″ is an organic group having a glycidoxy, mercapto, amino, methacryl, allyl, and/or vinyl group, applying the mixture to the surface of the metallic substrate, and at least regionally hardening the mixture, forming a sol-gel coating bonded to the metallic substrate.

2. The method of claim 1, wherein R is a 2-hydroxyethyl radical.

3. The method of claim 1, wherein R′ is a 2-hydroxyethyl radical.

4. The method of claim 1, wherein R″ is a 3-glycidoxypropyl radical.

5. The method of claim 1, wherein the sol comprises: 5 to 10 weight % tetrakis(2-hydroxyethoxy)silane; and 0.5 to 3.5 weight % 3-glycidoxypropytris(2-hydroxyethoxy)silane.

6. The method of claim 1, wherein the mixture is acid-free.

7. The method of claim 1, wherein the mixture further comprises metal oxides of zirconium, aluminum and/or titanium, having the general chemical formula M(OR″′).sub.x, in which R″′ is a linear or branched short-chain hydrocarbon radical selected from the group consisting of an n-propyl, isopropyl and n-butyl radical.

8. The method of claim 7, wherein R″′ has at least one hydroxyl group.

9. The method of claim 7, wherein the mixture further comprises a complexing agent selected from the group consisting of an organic acid and a β-diketone.

10. The method of claim 7, wherein the mixture comprises: 0.3 to 4.5 weight % metal oxides selected from the group consisting of tetra-(n-propyl) zirconate and tetra(isopropyl) titanate; and 0.3 to 9 weight % of a complexing agent comprising acetic acid.

11. The method of claim 1, wherein a surface of the sol-gel coating is bonded to an organic layer comprising an epoxy and/or polyurethane adhesive layer or a lacquer layer.

12. The method of claim 1, wherein surfaces of two metallic substrates, each having a sol-gel coating, are bonded to one another via an adhesive layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, the subject of the invention is shown in further detail in terms of an exemplary embodiment. In the drawings:

(2) FIG. 1 is a schematic representation of the structural formulas of the precursors used and of the complexing agent;

(3) FIG. 2 is a plan view on two surfaces, treated with a mixture having organosilanes, of metallic substrates; and

(4) FIGS. 3a and 3b are micrographs of a plan view on surfaces of metallic substrates treated with organosilanes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) For coating according to the invention of a surface of an aluminum alloy, this surface is as a rule first degreased and/or pickled. Depending on the composition of the alloy of the metallic substrate, various means and mixtures of alkaline and acidic systems are available, for instance based on potassium hydroxide solution or sulfuric acid in combination with fluorides. In the exemplary embodiment, for alloys of the AA 7075 series, this is done by alkaline degreasing using potassium hydroxide solution and surfactants. Next, rinsing is done with deionized water, whereupon pickling is performed using a mixture of sulfuric and hydrofluoric acid. The degreasing or pickling can be done for instance by means of an immersion or spraying method (spray degreasing method or spray pickling method). After a further rinsing with deionized water, the metal sheet is dried. Hot air can suit the purpose, or other drying methods may be employed, for instance using nonwoven-covered rollers.

(6) After that, coating of the surface is done, using a mixture according to the invention (M_erf) made from 6.2 weight % tetrakis(2-hydroxyethoxy)silane—marked 1 in FIG. 1; 1.3 weight % of 3-glycidoxypropytris(2-hydroxyethoxy)silane—marked 2 in FIG. 1; 2.5 weight % tetra-(n-propyl) zirconate—marked 3 in FIG. 1; 3.2 weight % acetic acid—marked 4 in FIG. 1; and
the remainder being water.

(7) In this kind of exemplary mixture, and this mixture itself can be considered a sol, a reaction equilibrium ensues in which approximately 12.5 weight % of volatile organic compounds, predominantly ethylene glycol, are present.

(8) As needed, various additives can be added to the mixture of the invention (M_erf), in order to adapt the processing properties of the sol to the requirements at the time. These additives may for instance be wetting agents, flow and leveling agent, foam-damping agents, dispersion aids, or UV stabilizers. Still other inorganic or organic inhibitors can also be added to further increase the corrosion resistance. For example, mercaptobenzylthiazoles, mercaptobenzimidazoles, Mg(NO.sub.3).sub.2, Ce(NO.sub.3).sub.2, Cr(NO.sub.3).sub.3 and comparable substances would be suitable, for example.

(9) The application of the mixture is done by immersion coating, but as already noted, this can also be enabled in other ways—that is, the viscosity of this mixture is advantageous, so that an application can easily be done with the aid of rollers, which is advantageous especially if large-area coatings are needed.

(10) A further essential improvement is that the pot life of this mixture—depending on the conditions under which the method takes place—can be as long as several hours. In contrast to known methods, this is a considerably longer length of time, which clearly makes the method of the invention easier.

(11) The gelification that ensues takes place with the aid of the method as described, also in an advantageous way. The gel that develops is decisively improved over the prior art in terms of its makeup, cross-linking, stability, and especially in its bonding with the metal surface. This must be due not least to the fact that in the course of the hydrolysis, ethylene glycol is formed, which as already described does not volatilize until the application to the substrate, has emulsifying properties and leads to a better course of the coating method. In this connection, the advantage that until drying, practically no toxic and/or explosive vapors escape should be mentioned again.

(12) The drying can take place at a substrate temperature of 100-200° Celsius, preferably 130° Celsius; in the process it is demonstrated that even with this comparatively rapid performance, complete coverage of the surface by the hardened coating is preserved. However, it is also conceivable to perform drying at low temperatures, for instance by choosing a comparatively high rate of air convection. Typically, the temperature can be 60° Celsius—with infrared heat sources, even lower temperatures are entirely conceivable, which can be advantageous for sparing adjacent other materials, such as seals.

(13) This can be seen from FIG. 2, which shows two coated metal sheets 5, 6 comprising the aluminum alloy AA 7075. The sheet 5 was treated with a mixture of the prior art (M_SdT) of 6 weight % tetraethoxy silane and 1.9 weight % 3-glycidoxypropyltrimethoxy silane, 1.7 weight % tetra-(n-propyl) zirconate and 1.6 weight % acetic acid, the remainder being water—while the sheet 6 was being treated with the mixture of the invention in accordance with this exemplary embodiment. Thus on the surface of the sheet 5, even with the naked eye, cracks 7 and areas 8 of increased thickness of the sol-gel coating can be seen, while in contrast, in the case of the sheet 6 treated according to the invention, a comparatively uniform, homogeneous coating takes place. Disadvantageously, the coating of the sheet 5, that is, the surface of the sheet 5, has areas 8 and cracks 7 that not only represent reduced protection of the sheet 5 but also impair an ensuing treatment/processing of the sheet 5. It is thus also demonstrated that the method of the invention makes it possible, with the aid of a comparatively thin coating, to ensure that the surface of the sheet 6 is fully covered and protected, which not least produces the aforementioned advantages upon possible ensuing lacquering or upon the connection of two metallic substrates having a surface with a sol-gel coating that are connected to one another via an adhesive layer. This is also significant because the sol-gel coating, with increasing thickness, is less resistant to mechanical stresses.

(14) In micrographs as well, the especially advantageous embodiment of the coating can be seen. This is shown in FIG. 3a, in which fissuring extending over the coating can be seen, but the fissures are distributed comparatively uniformly and in terms of their width are in the low single-digit micrometer range. Suitably advantageous coatings can also be achieved with a metal sheet 9 comprising a 6XXX aluminum alloy—as can be seen from FIG. 3b.

(15) The aforementioned advantages, in the results of testing the tensile strength, are as shown in Table 1. For that purpose, the surfaces of metal sheets of an AA7075 alloy were treated with the above-recited mixture of the prior art (M_SdT), and these treated surfaces were glued together with the aid of an epoxy adhesive. In the same way, this was done with metal sheets using the mixture of the invention (M_erf). The tensile strength of these bonded lap joints was determined by exerting a tensile force that acts parallel to the adhesive surface and to the primary axis of the metal sheets.

(16) TABLE-US-00001 TABLE 1 Coatings τ.sub.max [N/mm.sup.2] M_SdT 28.2 M_erf 32.7 τ.sub.max represents the maximum force per unit of surface area that resists overlapping adhesion in the tensile shear test.

(17) Thus it can be demonstrated that the mixture of the invention not only offers advantages in terms of processing capability and environmental and health protection, etc., but also, a metallic substrate with an especially advantageous sol-gel coating is obtained, whose morphology is clearly and decisively superior to the prior art. It is thus demonstrated that and for what reason the characteristics of the coating produced according to the invention can offer improvements for instance with regard to the strength of adhesive bonds there are also the advantages of two treated surfaces.