EP1060291A2

EP1060291A2 - Method for producing a corrosion protective coating and a coating system for substrates made of light metal

Info

Publication number: EP1060291A2
Application number: EP99911717A
Authority: EP
Inventors: Andreas Dietz; Volker Von Der Heide
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 1998-02-26
Filing date: 1999-02-26
Publication date: 2000-12-20
Anticipated expiration: 2019-02-26
Also published as: EP1060291B1; US6703135B1; WO1999043869A3; JP2002505379A; DE59900592D1; DE19807823A1; WO1999043869A2; JP4567187B2

Abstract

The invention relates to a method for producing a corrosion protective coating for a substrate made of light metal or of a light metal alloy. According to the invention, a non-electrically conductive first layer (10) is first deposited on the substrate (5), whereby the non-electrically conductive first layer (10) is produced by anodic oxidation of the substrate (5). Later, the non-electrically conductive first layer produced by anodic oxidation is subjected to a redensification. Afterwards, a metallic layer (20) is deposited on the non-electrically conductive first layer (10) without using current. Later, a third layer (30) is deposited on the metallic second layer (20). By using this method, wheel rims and wheel trim, for example, for motor vehicles can be chromed.

Description

Process for the production of a corrosion-protective coating and layer system for substrates made of light metal

The invention relates to a method for producing a corrosion-protective coating for a substrate made of a light metal or a light metal alloy.

Light metals, especially aluminum, are used in more and more technologies due to their low specific weight. The disadvantage here is that they are very susceptible to corrosion due to their electrochemically base character. They are therefore provided with a wide variety of corrosion protection layers. A known method of this type is to deposit metal layers on the light metal electrolessly or galvanically. This is particularly interesting if there are high decorative demands on the surface.

A practical example of this are light metal rims or actually light metal decorative rims. Due to the high decorative demands, especially for this product, for example, it would be desirable if it could be chrome-plated. Known chrome-plated rims of this type initially meet the high decorative requirements, but are very susceptible to damage and then quickly lose their attractiveness.

In DE 196 21 881 A1 it is therefore proposed to first apply a primer layer of powder or wet lacquer in a method for chrome-plating car rims made of an aluminum alloy, then drying the rim, applying a plastic wet lacquer layer, drying it again and finally performing galvanic chrome plating. These different process steps are quite complex and require multiple rearrangements of the intermediate products in other devices. Added to this is the time required for drying. A method known from DE 195 39 645 A1 works in a similar manner. A light metal rim for motor vehicle wheels is also coated there, and this rim is initially provided with a full paint job. An intermediate coating is formed over it as a powder coating or as a bright nickel plating in order to level the surface structure. This creates a smooth base for a decorative gloss metallization that can finally be deposited by electroplating.

For the electroplating of aluminum, the handbook of electroplating, published by Heinz W. Dettner and Johannes Elze (1964), volume 1, part 2, chapter 15, page 1034, suggests firstly applying an intermediate metal layer with good adhesive strength, alternatively a porous one Oxide film mentioned. The subsequent metallic layers are each applied galvanically. This is possible because both the metallic intermediate layer and the porous oxide layer are conductive. In the same article it is already stated that this makes the substrate easily vulnerable. Coatings produced in this way are therefore not corrosion-protecting, on the contrary.

Specifically and in practice, the methods in the prior art for producing decorative layers with good corrosion protection, for example on aluminum, look something like this: A thin zinc layer with a zincate stain is applied to an aluminum substrate without current. This is followed by direct galvanic copper plating and then a galvanic application of a duplex or tri-nickel layer with the aim of leveling and corrosion protection. A thin layer of shiny chrome is then electroplated over it.

This layer system provides sufficient corrosion protection for the aluminum substrate as long as there is no mechanical damage in the layer that affects the metal substrate. If there is such deep damage to the layer or the layer system, a so-called galvanic element is created in which the outer layer acts as a cathode and the substrate as an anode, which is oxidized.

Although chromium is a chemically very base metal, the formation of a thin oxide layer on the surface (called passivation) gives it a very positive potential. Oxygen is then reduced on this surface, which is very large compared to the aluminum exposed by the damage. The oxidation process is the conversion of metallic aluminum to Al ³⁺ . Due to the very large cathode surface of the chromium oxide, the corrosion of the aluminum at this damaged point is dramatic. One speaks here of a catastrophic failure of the corrosion protection layer.

The object of the present invention is to propose an alternative method for producing a corrosion-protective coating for a substrate made of a light metal and a corresponding layer system which is less sensitive to such damage.

This object is achieved by a method for producing a corrosion-protective coating for a substrate made of a light metal or a light metal alloy, in which an electrically non-conductive first layer is first applied to the substrate, the non-conductive first layer being produced by anodic oxidation of the substrate , later the non-conductive first layer formed by anodic oxidation is subsequently densified, then a metallized layer is applied without current to the non-conductive first layer and later a third layer is applied to the metallic second layer.

This object is further achieved by a layer system, comprising a substrate made of light metal or a light metal alloy, but not one conductive first layer, which consists of an oxide of the substrate material, an electrolessly applied second layer of one or more metals, a third layer.

For smoothing and leveling, a smoothing metallic intermediate layer is preferably applied between the electrolessly applied second layer and the third layer. The third layer is particularly preferably a decorative third layer.

The problems can be solved with such a method and such a layer system. The invention makes use of the knowledge that the prerequisite for the catastrophic failure of the known layer systems is the electrical conductivity between the anode and cathode in the oxidation process after the damage.

According to the invention, the electrical conductivity is now reliably prevented at precisely this point. The electrode flow is provided by an electrically non-conductive layer between the substrate on the one hand and the outer layers on the other. These outer layers can both continue to maintain the previous corrosion protection properties which have led to the good functionality in undamaged layer systems, and on the other hand they can have the decorative effects such as a bright chrome layer and / or be wear-resistant and / or low-friction.

The electrically non-conductive layer can be produced by physical processes, for example PVD (physical vapor deposition) or plasma CVD (chemical vapor deposition), by simple polymer layers, that is to say lacquers, or by electrochemical processes.

It is particularly preferred if the non-conductive layer is produced by anodic oxidation of the substrate. If the substrate is preferably aluminum, an anodizing process is used to produce the non-conductive layer. The metal substrate is simply switched as an anode and the surface is oxidized by applying a voltage. This oxide layer is chemically relatively inert and forms an ideal electrical barrier, especially after appropriate post-treatments.

A metal layer, preferably made of nickel, copper or another metal, which can be deposited without current, is then applied to this non-conductive first layer, here preferably the oxide layer.

Copper can then be applied to this metal layer by means of galvanic processes in order to level the still rough surface, to compensate for mechanical stresses or to give it a shine, and nickel for similar purposes and in particular for additional corrosion protection is also conceivable.

The decorative layer, in particular chromium, which is also external in the prior art, can then be applied to this layer.

In the prior art there was previously the conviction that it is possible to metallize plastics without current by roughening the non-conductive plastic surface and activating it accordingly. In the case of ceramics, this was previously thought to be possible only to a limited extent. Oxide layers are ceramics and the activation and adhesive metallization encountered problems. In order to achieve the very good adhesive strength of electrolessly applied metal layers, particularly on plastics, mechanical clinging of the two surfaces to one another, the so-called push-button effect, has previously been considered. This is only possible to a limited extent on oxidic surfaces. According to the invention, a very successful and advantageous method is preferably used here. This is because the layer created by anodic oxidation, that is to say the anodized layer in the case of aluminum, is compressed again. This is because the process of oxidation creates pores that should be reduced in size to prevent the non-conductive layer from being destroyed and possibly creating conductive bridges. This post-compression can be done using various methods, one of which is the so-called hot water compression. By immersing it in boiling water, the resulting anhydrous AI ₂ O _{3 is converted} into a boehmite type AI ₂ O ₃ x H ₂ O. This leads to an increase in the volume of the material around the pores, so that the pores shrink themselves.

This effect is now also used to activate the surface simultaneously with this compaction step. Activation in the sense of metallizing non-conductive surfaces is possible in this case by applying conductive crystallization nuclei. Preference is given to using a noble metal seed as the crystallization seed, in particular conductive palladium seeds.

These palladium nuclei or other crystallization nuclei are applied to the surface during compaction and thus also penetrate into the pores during their size reduction.

This now enables these crystallization nuclei to not only cause metallization in the next process step as the application of a further layer to the non-conductive first layer, but also for this metallization to take place in the now reduced pores. As a result, the conductive, metallic second layer protrudes into the pores, undercuts also occurring and ensuring very good interlocking and adhesion of the non-conductive first layer with the conductive second layer. After the second layer has been applied, it is possible without problems to subsequently galvanically deposit further desired layers, and finally also a decorative, in particular chrome, layer. Further preferred features are characterized in the subclaims.

The invention is described below using an exemplary embodiment with reference to the drawings. Show it:

1 shows a schematic section through a layer system according to the invention; and

FIG. 2 shows an enlarged schematic sectional illustration through an enlarged detail from FIG. 1.

FIG. 1 shows a layer system in a very schematic form. A substrate 5 consists of a light metal or a light metal alloy, in particular aluminum. For example, it can be a light alloy rim for a motor vehicle.

On the substrate 5, an electrically non-conductive first layer 10 can be seen, which in particular can be an oxide made of the material of the substrate 5, for example aluminum oxide. The aluminum oxide can have been produced by an anodizing process.

There is a conductive, metallic second layer 20 on the non-conductive first layer 10. This is deposited on the layer 10 without current.

The second layer 20 is followed by a further metallic intermediate layer 25, which is used in particular to flatten the usually relatively rough surface of electrolessly deposited layers 20.

The intermediate layer 25 is in particular electrodeposited. This leads to slight material differences between the second layer 20 and the intermediate layer 25, even if both should consist of nickel, for example, since chemically "electrolessly" deposited nickel contains, among other things, phosphorus or boron constituents, but electrodeposited nickel does not. However, this is irrelevant to the functionality of the layer system. Finally, the intermediate layer 25 is followed by a layer 30, for example a bright chrome layer, which on the outside represents the decorative and / or low-friction and / or wear-resistant properties of the finished product. If the decorative properties of the finished product are particularly important, for example in the case of motor vehicle rims, a bright chrome layer is applied. If the layer is to be particularly wear-resistant, hard chrome would be considered, for low-friction outer decorative layers nickel-teflon or lead.

In FIG. 2, the boundary area between the layers 10 and 20 is greatly enlarged during the manufacturing process, but is again shown schematically.

The non-conductive first layer 10, which is produced in particular by anodic oxidation of the substrate 5, has a surface 12. However, this surface 12 is not completely flat, but rather has a large number of pores 13; this is, in particular, a result of the manufacture of an anodizing process.

These pores 13 are now reduced in size or the layer 10 is “compressed” by immersing the substrate 5 with the surface 12 in boiling water in a so-called hot water compression. Together with the boiling water, the activation of the surface 12 is also preferably effected by the application of conductive crystallization nuclei 18, in particular palladium nuclei. These crystallization nuclei 18 get into them due to the initially large pores 13 and remain there even after the compression process when the pores 13 have become smaller.

After the boiling water has been removed, the palladium or crystallization nuclei 18 remain on the surface 12 and in particular in the pores 13. Now the metallization takes place precisely through the crystallization nuclei 18 through the materials which are now applied without current second layer 20 instead, in particular of copper and / or nickel. As a result, these materials extend into the pores or form particularly intensive contacts with the material of the layer 10 there. This leads to firm adhesion of the layer 20 not yet shown in FIG. 2 to the layer 10 by forming undercuts.

Reference list

Substrate non-conductive first layer surface of the non-conductive layer pores nuclei metallic second layer intermediate layer decorative third layer

Claims

Expectations

1 . Method for producing a corrosion-protective coating for a substrate made of a light metal or a light metal alloy, in which

- An electrically non-conductive first layer (10) is first applied to the substrate (5), the non-conductive first layer

(10) is produced by anodic oxidation of the substrate (5), later the non-conductive first layer (10) formed by anodic oxidation is subsequently compacted,

a metallized layer (20) is then applied without current to the non-conductive first layer (10),

- And later a third layer (30) is applied to the metallic second layer (20).

2. The method according to claim 1, characterized in that aluminum or magnesium or an alloy is used as substrate (5) using at least one of these two light metals.

3. The method according to claim 2, characterized in that an aluminum process is used for the substrate (5) and for producing the non-conductive first layer (10).

4. The method according to any one of the preceding claims, characterized in that at the same time with the densification also an activation by applying conductive nuclei (18) on the surface (12) or in the pores (13) formed by the anodic oxidation in the surface (12) of the first layer (10).

5. The method according to claim 4, characterized in that noble metal nuclei, in particular palladium nuclei, are used as conductive crystallization nuclei (18).

6. The method according to any one of the preceding claims, characterized in that the second metallic layer (20) is produced by electroless metallization using copper and / or nickel.

7. The method according to any one of the preceding claims, characterized in that between the metallic second layer (20) and the third layer (30) still a smoothing metallic intermediate layer (25) is applied.

8. The method according to any one of the preceding claims, characterized in that a decorative third layer is applied as the third layer (30).

9. The method according to claim 8, characterized in that for the decorative third layer (30) chromium is in particular electrodeposited.

10.The method according to any one of the preceding claims, characterized in that a material with low-friction and / or wear-resistant properties is used for the third layer (30).

1 1. layer system, produced by a method according to any one of the preceding claims.

12. Layer system, having

- a substrate (5) made of light metal or a light metal alloy,

- a non-conductive first layer (10) thereon, which consists of an oxide of the substrate material, - a second layer (20) of one or more metals applied without current, thereon

- A third layer (30).

13. Layer system according to claim 12, additionally between the second layer (20) and the third layer (30) a metallic smoothing intermediate layer

(25).

14. Layer system according to claim 12 or 13, characterized in that the substrate (5) consists of aluminum or magnesium or an aluminum or magnesium alloy.

15. Layer system according to one of claims 12 to 14, characterized in that the layer thickness of the non-conductive first layer (10) is between 1 μm and 200 μm.

16. Layer system according to one of claims 12 to 15, characterized in that the layer thickness of the electrolessly applied second layer (20) is between 0.5 μm and 20 μm.

17. Layer system according to one of claims 12 to 16, characterized in that the layers of the metallic intermediate layer (25) between 2 microns and

20 μm.

8. rim or decorative rim made of light metal, in particular for motor vehicles, coated by one of the methods according to claims 1 to 10 or provided with a layer system according to one of claims 11 to 17.