WO2009046328A1

WO2009046328A1 - Galvanic deposition of metal layers on magnesium or magnesium alloy surfaces

Info

Publication number: WO2009046328A1
Application number: PCT/US2008/078792
Authority: WO
Inventors: Christoph Werner; Andreas MÖBIUS; Karl-Heiinz Wandner
Original assignee: Enthone Inc.
Priority date: 2007-10-05
Filing date: 2008-10-03
Publication date: 2009-04-09
Also published as: DE102007048043A1; EP2045364A2

Abstract

The present invention relates to a process for the galvanic coating of magnesium or magnesium alloy surfaces. For the deposition of different metal layers on magnesium or magnesium alloy surfaces the process according to the invention proposes to contact the surfaces to be coated exclusively with process solutions and washing solutions having a pH value ≥ pH 7 until a first metal layer has been deposited on the said surfaces. It has shown that metal layers can be deposited on a plurality of magnesium or magnesium alloy surfaces with excellent adhesive strength and with a good deposition result if contacting of the surfaces to be coated with process solutions having a pH value < 7 is avoided.

Description

GALVANIC DEPOSITION OF METAL LAYERS ON MAGNESIUM OR MAGNESIUM

ALLOY SURFACES

FIELD OF THE INVENTION

[ 0001 ] The present invention relates to a method for the galvanic deposition of metal layers on magnesium or magnesium alloy surfaces.

BACKGROUND OF THE INVENTION

[ 0002 ] Over the recent years and decades, magnesium has been increasingly used in the automotive industry, in commercial aircraft construction and in the electronics industry. Especially in the production of high-quality products magnesium or magnesium alloys are used in order to save weight. Compared to other light alloys, magnesium or magnesium alloys have the advantage that they offer excellent casting properties, so that both injection- molded parts and die-cast parts having very good production qualities can be produced from corresponding magnesium alloys or from magnesium. Particularly the production of construction parts by means of a magnesium injection molding process seems to be of high interest also in future times, especially in the field of the automotive industry. During injection molding of magnesium or magnesium alloys the same are not heated until their complete melting but only to approx 100° below the melting point. A thixotropic state of the magnesium will be achieved thereby, in which state it can be correspondingly injection- molded.

[ 0003 ] But for decorative and functional applications the substrates which have been produced by means of corresponding casting processes must be subject as a rule to a surface finishing treatment. The same may serve to corrosion protection of the magnesium or the magnesium alloy surface on one side and on the other side to the production of corresponding glossy or dull decorative surfaces by means of a deposition of metal layers on the surface.

[0004 ] In the field of galvanic and electroless coating processes, anodizing processes, plasma processes or conversion coatings such as for instance chromatizing are currently offered on the market in addition to the classical galvanic coating processes such as the deposition of copper, nickel, chromium, tin and the like.

[ 0005 ] For the time being, only galvanic coating processes are suitable for use in decorative surfaces. The above-mentioned anodizing processes and conversion coatings merely serve to corrosion protection and as a basis for a subsequent coating, e.g. by enameling.

[0006] Galvanic processes for coating magnesium or magnesium alloys have already been known for years. For instance, Modern Electroplating by F.A. Lowenheim, Wiley & Sons, London 1974, introduces processes for the galvanic or electroless coating of magnesium or magnesium alloys.

[ 0007 ] A fundamental problem during the coating of magnesium or magnesium alloys resides in the oxide film which is formed on the substrate surface and which has to be removed by a corresponding pre-treatment in prior art.

[0008 ] Accordingly, for the removal of this oxide layer pre-treatments (activations) by means of an ammonium hydrogen chloride-containing phosphoric acid solution are known from this prior art. In other processes the magnesium or magnesium alloy surfaces to be coated are activated with hydrochloric acid.

[0009 ] Magnesium alloys which are frequently employed in the industry are such of the type AZ31 to AZ91, wherein AZ symbolize aluminum and zinc that have been added as alloy elements, and the subsequent numerals identify the size of the fraction of these alloy elements in the magnesium.

[0010 ] All the processes which are known from prior art have in common that they respectively show satisfying coating results only for certain magnesium alloys. In addition, the formation of pores can be often observed during the galvanization of magnesium, which fact may result in a blistering of the metal layers that had been applied.

SUMMARY OF THE INVENTION

[0011 ] Briefly, therefore, the invention is directed to a process for the galvanic coating of a magnesium or magnesium alloy surface with a metal-based layer, the process comprising a) an alkaline cleaning process, b) an alkaline deposition process for depositing the metal-based layer after the alkaline cleaning process, and no intermediate processes between (a) and (b) which involve exposure of the surface to an acidic solution.

[0012 ] Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013 ] This application claims priority from German patent application 10 2007 048 043.3 filed October 5, 2007, the entire disclosure of which is incorporated by reference.

[0014 ] The present invention is therefore based on the object of providing a process for the galvanic coating of magnesium or magnesium alloys, which process achieves a satisfying coating result for a great number of different magnesium alloys while avoiding the formation of pores as the same is known in prior art. [0015 ] This object is solved by a process for the galvanic coating of magnesium or magnesium alloy surfaces, characterized in that during the entire coating process and until the complete coverage with a metal layer of the metal to be deposited, the surfaces to be coated are contacted exclusively with process solutions and washing solutions which have a pH value > pH7, preferably > pH 8.

[0016] Surprisingly, it has been found out that the avoidance of acidic process steps leads to clearly improved deposition results. For instance, the abandonment of acidic process steps allows a galvanic coating of a plurality of different magnesium alloys, without causing the formation of pores, blistering or delamination.

[ 0017 ] By avoiding acidic treatment solutions it is prevented that magnesium is dissolved out of the surface of the substrates to be coated. This fact also reduces the need of leveling substances existing for instance in the galvanic coating particularly of highly alloyed magnesium alloys such as AZ91.

[0018 ] The process works particularly well on the magnesium alloy AU-LITE. Here, the leveling may be almost completely omitted.

[0019 ] In one embodiment of the process according to the invention the same is subdivided into three process steps: These process steps are:

1) cleaning the substrate to be coated which consists of magnesium or a magnesium alloy;

2) etching the substrate to be coated by a zincate etchant; and

3) applying a first metal layer to the substrate.

[0020 ] The cleaning of the substrate to be coated can be performed with the aid of hot degreasing. In this case, the substrate to be cleaned is treated with a suitable degreasing agent at a temperature of about 70 - 80⁰C for a duration of 10 minutes. Here, particularly the combination of different degreasing agents is provided, the selection of the degreasing agents to be used being made in dependence of the contaminations present on the substrates or substrate surfaces to be treated. But normally the degreasing agents that are to be employed are alkaline degreasing agents.

[0021 ] To achieve a sufficient removal of the adhering contaminations from the substrate surfaces, the use of a Kavitec modulus or of eductor nozzles may be provided in accordance with the invention. Kavitec systems are high-pressure water nozzles which support the cleaning-off by cavitation effects.

[ 0022 ] For removing coarse dirt particles from the degreasing agents or degreasing solutions the use of corresponding filter systems may be provided. [ 0023 ] Subsequent to the above-described cleaning of the substrate surface and after an optional washing step in the herein described embodiment of the process according to the invention, a treatment of the substrate surface with an alkaline, cyanide-free pre-bonding etchant is performed. In one embodiment of the invention such a pre-bonding etchant is a zincate etchant, which in addition to sodium pyrophosphate (Na₄P₂O_? x 10 H₂O), zinc sulfate, sodium carbonate and sodium fluoride includes suitable wetting agents for reducing the surface tension of the zincate etchant. The zincate etchant which has been described and which is to be used in accordance with the invention has a pH value adjusted with sodium carbonate to within the range of pH 9 to 11, in preferred embodiments between 9.7 and 10.3, and in certain embodiments between 9.9 and 10.1

[0024 ] The substrate surface to be treated of the magnesium or magnesium alloy substrate to be coated is contacted with the zincate etchant at a temperature between 60 and 80⁰C, preferably between 65 and 75°C. Here, the exposing time is between 5 and 15 minutes, preferably between 9 and 11 minutes.

[ 0025 ] One example of a zincate etchant which is to be used in accordance with the invention is given in the following:

[0026] Zincate etchant: 100 - 300 g/1 sodium pyrophosphate x 10 H₂O

1. 25 - 75 g/1 zinc sulfate x 7 H₂O

2. 3 - 6 g/1 sodium carbonate

3. 2 - 6 g/1 sodium fluoride

4. 0.5 - 3 g/1 wetting agent

[0027 ] In one preferred embodiment, the composition of the zincate etchant is

1. 110 - 130 g/1 sodium pyrophosphate

2. 25 - 75 g/1 zinc sulfate x 7 H₂O

3. 4 - 5 g/1 sodium carbonate

4. 3 - 4 g/1 sodium fluoride

5. 1 - 2 g/1 wetting agent

[0028 ] Alternative to 100 - 300 g/1 sodium pyrophosphate x 10 H ₂O, 60 - 180 g/1 sodium pyrophosphate water free can be used.

[0029 ] In the previously described zincate etchant a wetting agent known as Nonpitter 62 A or a wetting agent known as EnPREP TTM WA may be preferably used as a wetting agent. Moreover, also combinations of different wetting agents may be employed.

[ 0030 ] It has been discovered that particularly good deposition results are obtained when the surface tension of the zincate etchant which is used in accordance with the invention is < 55 mN/m. Here, the stated surface tension is determined as the dynamic surface tension by means of a bubble pressure tensiometer. This determination of the bubble lifetime is based on < 500 ms. A suitable measuring instrument for the determination of the dynamic surface tension is Scienceline T60 from Sita-Messtechnik GmbH.

[ 0031 ] By the treatment with a corresponding zincate etchant, a pre-bonding zincate layer is formed on the magnesium or magnesium alloy surface to be coated. The thickness of the zincate layer is not narrowly critical and is most influenced by the nature of the substrates. In many embodiments it is a continuous layer having a thickness of on the order of less about 1 micron.

[ 0032 ] In the process according to the invention which is described herein by way of example, the above-described treatment of the surface with a zincate etchant that serves as a pre- bonding etchant and an optional intermediate water rinsing step are followed by a deposition of a first metal layer on the substrate layer to be coated. Here, different galvanically deposited metal layers such as for instance copper, nickel or chromium layers may be provided. In the following, the deposition of a copper layer on a magnesium or magnesium alloy substrate which has been pre-treated in the above-mentioned way will be described by way of example.

[ 0033 ] The substrate surface of the magnesium or the magnesium alloy substrate coated with the a zincate layer in the above-described way is contacted with a cyanidic glossy copper electrolyte, for example an electrolyte of the type CUPRALYTE 1545 from Enthone Inc., at a temperature between 40 and 55°C. For the deposition of the copper layer a current density between 0.5 and 2.0 A/dm² is set. According to the invention, a current density is to be provided here which is lower for the use of plate or billet anodes than for the use of copper pieces in corresponding anode baskets.

[0034 ] The voltage to be applied lies within a range of 2.0 to 12.0 Volts, depending on the anodes which are used and on the substrates or substrate surfaces which are to be coated. The cyanidic copper electrolyte which is used in the herein described embodiment of the process according to the invention exhibits a photometrically determined pH value within a range of pH 11.0 and pH 12.0.

[ 0035 ] The copper concentration in the copper electrolyte which is to be employed according to the invention is between 20 g/1 and 50 g/1. The free potassium cyanide concentration in the cyanidic copper electrolyte which is to be employed here is between 20 g/1 and 35 g/1. Moreover, the electrolyte includes potassium carbonate of maximally 120 g/1. The density of an electrolyte as described above is approximately 1.15 g/cm³. [0036] For the regeneration of the copper electrolyte which is employed the same may be supplemented, in accordance with the invention, with copper (I) cyanide in order to replenish the copper from the electrolyte consumption. Here, approximately 1.4 g/1 copper (I) cyanide for every 1 g/1 of copper content to be replenished are required. For adjusting the content of free potassium cyanide, potassium cyanide is added dosed in an order of approximately 2 g for every g of replenished copper.

[ 0037 ] From a copper electrolyte like one that has been described above, copper layers can be deposited at a deposition speed in the order of approximately 0.4 μm/min at a set current density of 1 A/dm² on the substrate surfaces of the magnesium or magnesium alloy substrates which have been pre-treated in the above-described way. The deposited copper layers are strongly adhering and show a uniform gloss.

[ 0038 ] According to the invention, additional metal layers can be applied to the so deposited copper layers, and this subsequent deposition of metal layers may take place both in acidic and alkaline coating electrolytes, provided that the underlying magnesium or magnesium alloy layer is completely covered with a corresponding copper layer.

[0039 ] When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0040 ] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

[0041 ] As various changes could be made in the above products and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A process for the galvanic coating of a magnesium or magnesium alloy surface with a metal-based layer, the process comprising: a) an alkaline cleaning process; b) an alkaline deposition process for depositing the metal-based layer after the alkaline cleaning process; and no intermediate processes between (a) and (b) which involve exposure of the surface to an acidic solution.

2. The process according to claim 1 further comprising applying an adherence- imparting layer to the surface by exposing the surface to an alkaline pre-bonding etchant between (a) and (b).

3. The process according to claim 2 wherein the pre-bonding etchant composition which has a dynamic surface tension < 55 mN/m at a bubble lifetime < 500 ms.

4. The process according to claim 2 wherein the pre-bonding etchant composition includes a wetting agent from the group consisting of Nonpitter 62A and EnPREP TTN WA.

5. The process of claim 2 wherein the pre-bonding etchant composition is a zincate- based etchant composition.

6. The process of claim 5 wherein the zincate-based etchant composition comprises: 100 - 300 g/1 sodium pyrophosphate x 10 H₂O

25 - 75 g/1 zinc sulfate x 7 H₂O

3 - 6 g/1 sodium carbonate

2 - 6 g/1 sodium fluoride 0.5 - 3 g/1 wetting agent

7. The process of claim 5 wherein the zincate-based etchant comprises: 110 - 130 g/1 sodium pyrophosphate

25 - 75 g/1 zinc sulfate x 7 H₂O

4 - 5 g/1 sodium carbonate

3 - 4 g/1 sodium fluoride 1 - 2 g/1 wetting agent