EP1624091A1 - Workpieces coated with an aluminium/magnesium alloy or with aluminium having a zinc interlayer - Google Patents

Abstract

An aluminium workpiece has a first coating of zinc or a zinc alloy under a second outer coating of aluminium or aluminium/magnesium alloy. An independent claim is also included for the method of manufacturing the workpiece, where the metallic intermediate layer is galvanically separated from an aqueous electrolyte.

Description

The present invention is directed to aluminum / magnesium alloy or aluminum coated workpieces, as well as to a method of making the same.

The deposition of aluminum, magnesium or aluminum / magnesium alloys on workpieces made of base metals is a tried and tested means of protecting these workpieces from corrosion. You will be provided at the same time with a decorative coating. The protective metal layer is predominantly deposited galvanically on the workpiece. The deposited metal layer significantly improves the corrosion resistance of the workpiece. It turns out, however, that the corrosion resistance of the workpiece depends on the adhesion of the applied protective layer on the workpiece. If there is insufficient adhesion of the protective layer to the workpiece, the protective layer is easily removed, e.g. Example, when screwing a provided with a surface layer of aluminum, magnesium or an aluminum / magnesium alloy screw as a workpiece in a second workpiece. As a result, corrosion, in particular contact corrosion, occurs at these points. This corrosion inevitably leads to the destruction of the workpiece. A long-term prevention of corrosion is therefore not guaranteed.

There have been various attempts in the art to solve this problem.

In DE 31 12 919 A1 it is proposed to provide metal-coated iron workpieces with a bonding intermediate layer of cobalt, cobalt alloys or nickel-containing cobalt and then to apply an aluminum layer by electroplating. The serving as a bonding agent interlayer is electroplated from an aqueous medium. Optionally, after the galvano-aluminum layer has been applied to the adhesion-promoting layer, the galvano-aluminum layer can be chromated. As a result, the corrosion resistance is further improved.

DE 38 04 303 proposes a method for improving the adhesion of galvanic aluminum layers to metal workpieces by applying an adhesion-promoting layer. A non-aqueous electrolyte is used to apply the primer layer of iron, iron and nickel, nickel, cobalt, copper and alloys of the above-mentioned metals or tin-nickel alloys. After application of the intermediate layer as an adhesion-promoting layer on a metal workpiece, a galvano-aluminum layer is applied to the intermediate layer. In this case, the application of the intermediate layer of a non-aqueous electrolyte is essential, since otherwise embrittlement of the metal workpiece occurs when using an aqueous electrolyte by the hydrogen formed during the electrolysis. This adversely affects the often used low alloy high strength steels. By using a non-aqueous electrolyte to apply the metallic intermediate layer, the embrittlement of the workpieces is avoided.

Both in DE 31 21 919 A1 and in DE 38 04 303 A1, pure galvano-aluminum layers are applied to the workpieces provided with an intermediate layer. Both documents do not describe the application of aluminum / magnesium alloys on workpieces.

EP 1 141 447 B1 discloses electrolytes for coating workpieces with layers of an aluminum / magnesium alloy. Such a coating is particularly necessary when compounds with magnesium parts are to be produced because the corrosion products of the magnesium metal are alkaline and attack the aluminum surface coatings. By using aluminum / magnesium alloys, the contact corrosion is here avoided and causes a long-term stability of the coating. It is proposed to coat steel fasteners for contact with magnesium components, especially in the automotive industry with aluminum / magnesium alloys. In EP 1 141 447 B1, no metallic interlayers interposed between the workpiece and the corrosion reducing layer of an aluminum / magnesium alloy are disclosed.

DE 102 57 737 B3 describes a process for electrolytic magnesium deposition on galvanized strip. Here, a substrate made of sheet metal, preferably a steel sheet is provided with a zinc or zinc alloy coating. Subsequently, an electrolytic deposition of magnesium takes place on the zinc layer. The coated substrate is then subjected to a heat treatment to form a Mg-Zn alloy phase. Ribbons with improved corrosion resistance are obtained with constant surface quality and processability.

However, the application of pure magnesium layers has the disadvantage that these layers are relatively soft and often when used in fasteners, the coating is damaged or removed solely by the mechanical stress during bonding. This leads to damage of the corrosion protection layer, so that in these parts early corrosion is observed. Another disadvantage is that magnesium has no corrosion resistance in the acidic range. In contrast, Al / Mg alloys have a higher corrosion resistance, which extends into the acidic range.

The aluminum / magnesium layers of the prior art applied to a workpiece are very hard and brittle in contrast to pure magnesium layers, so that the problems described above do not occur here. Furthermore, the magnesium content of the Al / Mg alloy can be customized can be adjusted so that the hardness of the layer can be adapted to the particular application requirements.

Be fasteners that are provided with an aluminum / magnesium layer, such. As screws, used for attachment of components, but there is a risk that the screws roughen the components by the surface applied to the fastener aluminum / magnesium layer surface and destroy in the worst case. This danger exists in particular if the components consist of relatively soft or brittle materials, such. B. of magnesium. Due to the superficial destruction of the component this in turn can be exposed to increased corrosion, which can lead to the destruction of the component.

Basically, the above-described corrosion occurs increasingly at high pH values, so that in both the aforementioned cases of destruction of the surface layer of the fastener or the component, the corrosion rate increases at high pH values.

The technical object of the present invention is to provide coated workpieces which have an abrasion-resistant coating and improved corrosion resistance, especially in the alkaline range and in combination with other materials show reduced corrosion, especially when the coated workpieces as fasteners for fastening components be used.

The technical object of the present invention is achieved by a coated workpiece comprising a substrate, at least one metallic intermediate layer of zinc or a zinc alloy applied to the substrate and a layer applied to the intermediate layer comprising an aluminum / magnesium alloy or aluminum.

Preferably, the substrate contains a metal and / or a metal alloy. Alternatively, the substrate may be a metallized substrate, wherein the substrate may be metallized over the whole area or part of the area. Preferred substrates contain plastics.

The substrate may further contain ingredients selected from the group consisting of iron, steel, iron alloy, non-ferrous metals, die-cast zinc, die-cast aluminum, titanium, titanium as alloy, magnesium, magnesium die cast or mixtures thereof, the aforementioned metals preferably present as an alloying ingredient in the substrate.

The metallic intermediate layer preferably has a layer thickness of 0.1 μm to 30 μm. In a further preferred embodiment, the layer thickness of the metallic intermediate layer is 0.5 μm to 20 μm, more preferably 1 μm to 10 μm and most preferably 1.5 μm to 8 μm. In the case of zinc, the intermediate layer is particularly preferably 4 to 8 microns.

The layer applied to the metallic intermediate layer containing an aluminum / magnesium alloy preferably contains 0.5 to 70% by weight of magnesium. In a further preferred embodiment, 1 to 50% by weight, more preferably 2 to 40% by weight, in another preferred embodiment 3 to 30% by weight, preferably 4 to 25% by weight and most preferably 5 to 20% by weight. % Magnesium in the aluminum / magnesium alloy.

Preferably, the layer containing an aluminum / magnesium alloy has a layer thickness of 0.1 .mu.m to 100 .mu.m. In a further preferred embodiment, the layer thickness is 0.5 μm to 70 μm, more preferably 1 μm to 50 μm, preferably 2 μm to 40 μm, more preferably 2 μm to 30 μm, further preferably 2 μm to 28 μm most preferably 2 μm to 25 μm.

The layer containing the aluminum / magnesium alloy is preferably the surface layer of the coated workpiece. Alternatively, at least one layer may be applied to the layer containing the aluminum / magnesium alloy, which is preferably a passivation.

The coated workpiece is preferably a rack fabric, a bulk material or an endless product, wherein the coated workpiece is preferably a wire, a tape, a screw, a nut, a concrete anchor, a fastener or a machine component. Preferably, the coated workpiece is used in the automotive industry in the transmission, engine and body area. It can be an oil pan or a transmission oil pan.

• a) Aufbringen mindestens einer metallischen Zwischenschicht aus Zink oder einer Zinklegierung auf ein Substrat und
• b) Aufbringen einer Schicht enthaltend eine Aluminium/Magnesium-Legierung oder Aluminium auf die metallische Zwischenschicht.

A further subject of the present invention is the provision of a method for producing a coated workpiece, comprising the steps:

• a) applying at least one metallic intermediate layer of zinc or a zinc alloy to a substrate and
• b) applying a layer containing an aluminum / magnesium alloy or aluminum on the metallic intermediate layer.

Preferably, in step a) the metallic intermediate layer is deposited from an aqueous solution or a non-aqueous solution.

In a preferred embodiment, the metallic intermediate layer is chemically deposited.

Alternatively, in step a), the metallic intermediate layer can be electrodeposited from an aqueous electrolyte. Possible electrolytes are solutions of the metal salts of zinc. These may be present as halides, sulfates or sulfonates. The electrolytes may contain other additives such. B. complexing substances.

Alternatively, it is also possible in step a) to electrodeposit the metallic intermediate layer from a non-aqueous electrolyte. Possible electrolytes include compounds of molybdenum or vanadium or any other of the aforementioned metals which may contain the intermediate layer. The metals are preferably in the form of halides, which may be complexed with ether, in particular diethyl ether and / or acetylacetonate (acac).

$$\mathrm{AI}{\left({\mathrm{R}}^4\right)}_3$$

wobei n gleich 0 oder 1 ist, M gleich Natrium oder Kalium ist und R¹, R², R³, R⁴ gleich oder verschieden sein können, wobei R¹, R², R³, R⁴ eine C₁ bis C₄ Alkylgruppe sind und als Lösungsmittel für den Elektrolyt ein halogenfreies, aprotisches Lösungsmittel eingesetzt wird.Preferably, in step b) the layer containing an aluminum / magnesium alloy or aluminum is deposited from an anhydrous electrolyte. In a further preferred embodiment, in step b) the layer containing an aluminum / magnesium alloy is electrodeposited from an anhydrous electrolyte. The electrolyte used may be any electrolyte which is familiar to the person skilled in the art. In particular, the electrolyte contains organoaluminum compounds of the general formulas (I) and (II) $$\mathrm{M}\left[({\mathrm{R}}^1{)}_3\mathrm{AI}-(\mathrm{H}-\mathrm{AI}({\mathrm{R}}^2{)}_2{)}_{\mathrm{n}}-{\mathrm{R}}^3\right]$$

$$\mathrm{AI}{\left({\mathrm{R}}^4\right)}_3$$

wherein n is 0 or 1, M is sodium or potassium and R ¹ , R ² , R ³ , R ^{4 may be the} same or different, wherein R ¹ , R ² , R ³ , R ^{4 is} a C ₁ to C ₄ Alkyl group and a halogen-free, aprotic solvent is used as a solvent for the electrolyte.

The electrolyte can also be a mixture of the complexes K [AlEt ₄ ], Na [AlEt ₄ ] and AlEt ₃ . The molar ratio of the complexes to AlEt ₃ is preferably 1: 0.5 to 1: 3 and more preferably 1: 2.

The electrolytic deposition of the layer containing an aluminum / magnesium alloy on the workpiece is carried out using a soluble aluminum and a likewise soluble magnesium anode or using an aluminum / magnesium alloy anode.

The electrolytic coating is preferably carried out at a temperature of 80 to 105 ° C. A temperature of the plating bath of 91 to 100 ° C is preferred

In a preferred embodiment, an electrically conductive layer is applied to the substrate before the metallic intermediate layer is applied in step a).

The electrical current conducting layer can be applied to the substrate by any method known to those skilled in the art. Preferably, the layer conducting the electric current is applied to the substrate by metallization.

When the coated workpiece of the present invention is used as a fastener, no deterioration of the coated workpiece occurs. Although a surface layer of an aluminum / magnesium alloy is very hard, brittle and poorly ductile, it still adheres very firmly to the coated workpiece after and during use as a fastener. Furthermore, the coating used consisting of the intermediate layer and the surface layer is so elastic that it has not changed adversely after use as a fastener. For example, when the coated workpiece is screwed into a component as a screw, the surface treatment of the workpiece yields. This then leads to a further reduction of the stress on the coated workpiece. Since no destruction of the metallic intermediate layer and the surface layer containing an aluminum / magnesium alloy occurs, the coated Workpiece also after and during its use safe from corrosion, in particular against contact corrosion, protected.

These advantages mentioned have not provided with the aluminum / magnesium coating provided workpieces of the prior art. Either the surface layer consisting of an aluminum / magnesium layer is destroyed so that then corrosion of the workpiece occurs or else the very hard and brittle layer of aluminum / magnesium alloy of the workpieces of the prior art destroys the surface of the components to be fastened in such a way, that these are then exposed to increased corrosion.

The present invention will be illustrated by, but not limited to, the following examples:

Examples: example 1

A sheet of the size 100 x 25 x 1 mm of steel St37 is provided with a zinc intermediate layer with a thickness of about 4 microns. The zinc layer is electrodeposited from an aqueous zinc electrolyte. Subsequently, a layer of an aluminum / magnesium alloy with a magnesium content of 20 wt .-% and a layer thickness of 12 microns is deposited by electrodeposition of a non-aqueous electrolyte on the zinc layer. The coated sheet is bent lengthwise at an angle of 180 °, leaving the deposited metal layers intact in the area of the bending edge.

Comparative Example 1

A sheet of the size 100 × 25 × 1 mm steel St37 is provided with a layer of an aluminum / magnesium alloy with a magnesium content of 20 wt .-% and a layer thickness of 12 microns by electrodeposition of a non-aqueous electrolyte. The sheet coated in this way is bent lengthwise by an angle of 180 °, parts of the coating tearing open at the bending edge and partly peeling off in the form of very fine needles.

Example 2

Five screws of size M6 x 55 are provided with an aluminum / magnesium alloy with a magnesium content of 15 wt .-% and a layer thickness of 16 microns by electroplating of a non-aqueous electrolyte.

Five additional screws of size M6 x 55 are provided with a zinc layer with a layer thickness of approx. 4 μm. The zinc layer is electrodeposited from an aqueous zinc electrolyte. Subsequently, an aluminum / magnesium alloy with a magnesium content of 15 wt .-% and a layer thickness of 16 microns by electroplating of a non-aqueous electrolyte is applied to the zinc layer.

All screws are screwed in half into a nut of appropriate size and then screwed out again. The treated screws are then hung in a salt spray chamber and their corrosion behavior is examined. It turns out that the screws provided with a zinc interlayer, a longer time elapses until a first corrosion of the screws is recognizable.

Example 3

Screws of size M5 x 5 are electroplated in a drum with an aluminum / magnesium alloy with a magnesium content of 10 wt .-% and an average layer thickness of 14 microns from a non-aqueous electrolyte.

Additional screws are provided with a zinc intermediate layer with a layer thickness of approx. 4-6 μm zinc. The zinc layer is applied galvanically from an aqueous zinc electrolyte. Subsequently, on the zinc layer in a drum, a layer of an aluminum / magnesium alloy with a magnesium content of 10 wt .-% and an average layer thickness of 15 microns from a non-aqueous electrolyte is applied by electroplating.

Three equally coated screws are screwed all the way into a housing made of an aluminum / magnesium alloy with a bag nut suitable for the screw diameter. The corrosion behavior in a salt spray chamber is examined below. It turns out that the housings into which the screws were screwed with an intermediate layer of zinc only corrode significantly later. The housing with the screws without zinc interlayer show at a previous time first signs of corrosion.

In summary, it can be seen from the above examples that the metallic intermediate layer shows a significant improvement in the corrosion resistance and a considerable Verlungsverbesseung the applied aluminum / magnesium alloy. Coated workpieces comprising a substrate, a metallic intermediate layer applied to the substrate and a layer applied to the intermediate layer comprising an aluminum / magnesium alloy exhibit a superior profile of properties which the workpieces of the prior art do not have.

Claims (15)

1. A coated workpiece comprising a substrate, at least one metallic intermediate layer of zinc or a zinc alloy applied to the substrate and a layer applied to the intermediate layer containing aluminum or an aluminum / magnesium alloy. 2. The coated workpiece according to claim 1, characterized in that the surface of the substrate is electrically conductive. 3. The coated workpiece according to claim 1 or 2, characterized in that the substrate contains a metal and / or a metal alloy and / or is a metallized substrate. 4. The coated workpiece according to one or more of the preceding claims, characterized in that the substrate ingredients selected from the group iron, steel, iron alloy, non-ferrous metals, zinc die casting, die-cast aluminum, titanium, titanium as an alloy, Magnesium, magnesium casting or mixtures thereof, wherein the aforementioned metals are preferably present as an alloying component in the substrate. 5. The coated workpiece according to one or more of the preceding claims, characterized in that the intermediate layer has a layer thickness of 0.1 .mu.m to 30 .mu.m. 6. The coated workpiece according to one or more of the preceding claims, characterized in that the layer applied to the intermediate layer containing an aluminum / magnesium alloy contains 0.5 to 70 weight percent magnesium. 7. The coated workpiece according to one or more of the preceding claims, characterized in that the layer applied to the intermediate layer has a layer thickness of 0.1 .mu.m to 100 .mu.m. 8. The coated workpiece according to one or more of the preceding claims, characterized in that the coated workpiece is a rack commodity, a bulk material or an endless product, wherein the coated workpiece is preferably a wire, a tape, a screw, a nut, a concrete Anchoring or a machine component is. 9. A process for producing a coated workpiece comprising the steps of: a) applying at least one metallic intermediate layer of zinc or a zinc alloy to a substrate and b) applying a layer containing an aluminum / magnesium alloy or aluminum on the metallic intermediate layer. 10. A method for producing a coated workpiece according to claim 10, characterized in that in step a) the metallic intermediate layer is deposited from an aqueous solution or a non-aqueous solution. 11. A method for producing a coated workpiece according to claim 10 or 11, characterized in that in step a) the metallic intermediate layer is electrodeposited from an aqueous electrolyte. 12. A method for producing a coated workpiece according to claim 10, characterized in that in step b) the layer containing an aluminum / magnesium alloy or aluminum is deposited from an anhydrous electrolyte. 13. A method for producing a coated workpiece according to claim 13, characterized in that in step b) the layer containing an aluminum / magnesium alloy or aluminum is electrodeposited from the anhydrous electrolyte. 14. A method for producing a coated workpiece according to one or more of claims 10 to 14, characterized in that on the substrate, before the metallic intermediate layer is applied in step a), an electric current conducting layer is applied. 16. A method for producing a coated workpiece according to claim 15, characterized in that the electrically conductive layer is applied by metallization of the substrate.