Production of anticorrosion coating on magnesium or alloy part, used in vehicle or aircraft construction, involves oxidation in aluminum phosphate electrolyte containing vanadium, molybdenum and/or manganese compound

The present invention relates to a method for the production of Anti-corrosion layers on surfaces of magnesium or magnesium alloys, wherein the protective layer with the components to be provided as an electrode, preferably as are connected anode and oxidized in an electrolyte, the electrolyte containing phosphate compounds and aluminum compounds.

A method of the type mentioned above is described in EP 0333049 A1. The Coating of substrates magnesium usually requires the presence of highly conductive electrolytes. In addition, it must be ensured that the pH of the Electrolyte in the neutral is possible to basic range. The electrolyte should continue have a good buffering effect to maintain the pH value of the solution over a longer to keep time constant. Therefore, the use of electrolytes which contain phosphate-containing compounds found to be suitable. The presence of Aluminum compounds designed to allow the incorporation of aluminum in the layer matrix the surface coating. This supports the layer formation mechanism and reducing the magnesium content at the surface, whereby the corrosion potential of is reduced.

The DE 199 13 242 A1 describes chemically passivated articles made of magnesium or Magnesium alloys in which a conversion layer by an electrolytic but electroless process is generated by the on passivating objects in a aqueous passivating be submerged. This includes passivating Manganites, vanadates or molybdates. The described in this document Conversion coatings are considered to be appropriate, the previously used to replace chromate. The use of chromate-free aqueous Passivating, for reasons of environmental protection and work safety sought.

It is known that the specific surface properties of magnesium and Magnesium alloys, an adaptation of the long-in aluminum alloys Application upcoming process of anodic oxidation difficult.

This is justified in the fact that the magnesium surfaces, depending on their Alloy composition, its surface treatment and their condition Ambient conditions from a mixture of hydroxides, oxides and carbonates of respective alloying elements in particular of magnesium exist.

From the viewpoint of corrosion protection is the fact that the different physical (eg. as coefficient of thermal expansion) and chemical properties (Mutual solubility) of these compounds present on the surface to microcrack to scholl shaped structures leads that under moist Ambient conditions a favorable starting position for a permanent representing corrosion attack by local cell formation. This can be accomplished by at Dominant surface not chemically and mechanically stable Mg (OH)₂ hardly meet be cast.

Thus, the aim was earlier solutions, the disadvantages mentioned above by the To work producing relatively stable protective layers.

The entry into force of appropriate environmental laws, there is a demand for Avoid Cr (VI) -containing compounds in technological processes.

To the diverse requirements of the surfaces of technical products Magnesium alloys are asked to evaluate, is a holistic Approach essential. This has both anticipated the mechanical and corrosive stress components and their interactions account.

Especially known from the automotive industry damage mechanisms of the Occurrence of corrosive load initially mostly mechanical damage to the Surface presuppose is insufficiently in the patent literature attention paid.

Magnesium alloys which future wide application fields in the body area will open up, especially need the following surface properties:

<ul><li>1. Surface hardness &lt;base material hardness</li><li>2. Corrosion properties of the surface better than those of the base material</li><li>3. adhesion strength of the surface layer on the base material &lt; Adhesion strength of the different coating layers on the surface layer</li><li>4. Higher modulus of the surface protective layer in comparison to the base material</li><li>5. High plastic deformability of the surface layer (damage tolerance)</li></ul>

The above requirements are as claims on the layer system base material - Surface layer - to understand coat.

The object underlying the invention now consists in a method of the to provide defined kind, which as corrosion protection coatings with good mechanical and chemical properties results, the disadvantages of the chemical Conversion not generated mechanically unstable and thin conversion layers exhibit. In addition, these anticorrosive coatings according to the invention are in their Properties on the one hand be compatible with the magnesium substrate surface as well as a have extremely tenacious bond to possible subsequent coats of paint.

This object provides an inventive method of the above mentioned kind with the characterizing features of the main claim.

According to the invention contains the electrolyte besides phosphate compounds and Aluminium compounds also contain at least one compound of vanadium and / or Molybdenum and / or manganese. These compounds are preferably present in the electrolyte , Contained in an amount within a range of about 10 g / l to about 150 g / l. Particularly preferred is the use of vanadate and / or molybdate and / or Manganate in an amount in the range of between about 25 g / l and about 50 g / l.

A preferred embodiment of the invention provides that the electrolyte Ethylene diamine, preferably in 50% concentration and in an amount of between about 25 ml / l and 230 ml / l, more preferably in an amount between about 30 ml / l and about 150 ml / l. By the addition of a complexing agent, the conductivity of the Electrolytes are limited, also can be in electrochemical reactions using the ligand charge hike sense and the speed of the complexes in affect electric field. It is thereby achieved that, for. Example, positively charged metal ions can migrate to the anode. The use of stable chelates with pincer-like Ligand has the advantage that the largest possible amount of electrolyte components can be complexed. As part of the development of the invention is the use of ethylenediaminetetraacetate (EDTA) waived because of his poor biological Degradability. Instead, ethylene diamine is used, determined by tests could be that this is able, less resistant to oxidation stabilize.

As part of the process of the invention is in addition to the Electrolyte composition the procedure of (anodic) oxidation process Importance. According to the invention is preferably carried out with a pulsed bipolar direct current, preferably with a frequency in the range between about 250 and about 2000 Hz or with an alternating current, preferably with a frequency of between about 10 and about 100 Hz. The current densities are preferably in the range between about 0.3 and about 5 A / dm², The ratio of the anodic to cathodic current density is preferably between about 1: 1 and about 10: 1, preferably chosen Bath voltages between about 80 V and 300 V and final voltages between about 180 and 220 V. The start of the visible spark discharge is generally between 100 and 150 V.

The coating / deposition time is preferably in the range between about 1 minute and about 40 minutes. When using alternating current are the bath voltages preferably between about 50 V and about 300 V more preferably between about 100 V and 200 V. The novel process produces one Corrosion layers preferably with layer thicknesses in the range of between about 3 microns and about 100 microns.

Before oxidation in the electrolyte, the surfaces to be coated are preferably stained with a solution, which, for example, aluminum nitrate, preferably in a concentration of between about 5 and about 25%. Further, the pickling is with a solution containing glycolic acid, preferably in a concentration of between about 10 and about 40%, more preferably in a concentration of about 35%, in The novel process is advantageous.

Another advantage is that out of a high aluminate and the presence Compounds of the elements molybdenum, manganese and / or vanadium in the used Electrolyte a good anodic oxidizability with high chemical given modifiability of the surface layer.

it is also advantageous adapted to their electrolyte composition Procedure of the anodic oxidation process. This is achieved by z. B., that parts of a preferably previously degreased and cleaned magnesium alloy are subjected to a steadily increasing bath voltage. Upon reaching the for plasma chemical reactions typical bath voltages &lt;120 V are first Spark discharges visible. This, then for a split second stable and locally limited Microplasmas provide both oxidation of directly at the Substrate surface located electrolyte components, as well as by oxidation of the the previous conversion effects already modified substrate surface. In the this stage present chemical surface composition of the parts is in essentially characterized by the presence of magnesium oxide, Magnesium hydroxide, alumina, aluminum hydroxide, as well as Al-Mg mixed oxides or -Mischhydroxiden.

To a preferred further object of the invention, the synthesis and tenacious to achieve deposition of the other oxides in the substrate surface, the bath voltage is without interruption further increased. The then present discharge phenomena cause further oxidation of the substrate surface and there is an additional Activation of the dissolved in the electrolyte present molybdates, manganites and / or Vanadates instead. These are also converted into the plasma state, thereby undergo intensive mixing with the constituents of the substrate surface and form in Fractions of a second oxides and mixed oxides. These are firmly rooted in the now finally present surface layer as a result of the sequence previous conversion and deposition processes for high adhesion having base material. This layer formation mechanism runs to the from the Plasma chemistry known from mechanisms. This means that individual for the Plasma state characteristic and visually visible spark discharges the Substrate surface scanning and then come to a halt when a Balance between the bath voltage and the dielectric strength of the layer has set. The mixed oxide of the invention can, depending on the have electrolyte composition of colors. These range from light gray over beige to black. Likewise, depending on the process parameters in reaches general coating thicknesses of between about 3 microns and about 100 microns.

A preferred embodiment of the invention has the advantage that, depending on the applied current form both cationic and anionic electrolyte components Film formation and contribute foreign oxides formed therefrom in a layer matrix be anchored, which were formed by the same oxidation process. Through this Combination of conversion and deposition effects is a high adhesion to the Underground ensured.

Another preferred aim of the invention is the selective incorporation of Aluminum compounds in the layer matrix. This not only the Film formation mechanism supported as such, but it is by so associated reduction of the Mg-content at the surface of the corrosion potential of these Alloys minimized.

The inventive method is to be able, relevant for the automotive industry generating surfaces in a single-stage process with the desired properties. These surface layers should have a high mechanical and chemical protective effect have comparable with the produced aluminum alloys anodic Oxidation layers. Furthermore, these surface layers are indeed no Cr VI include compounds containing, in terms of their anti-corrosion effect, however, having self-healing effects which are comparable to known chromate.

This achievement of self-healing effects without the use of chromate ions can thereby done that also prone to change the oxidation state of the oxides Molybdenum, vanadium and manganese from the largely chemically inert layer Mixed oxides or mixed hydroxides of aluminum and magnesium are incorporated. There the oxidizing power and the associated self-healing effect of the above Oxides is less than that of chromates, this disadvantage is inventively preferably offset by a higher quantity of foreign oxides. This is achieved by the setting of relatively large layer thicknesses of up to 100 microns possible. Another inventive advantage over purely chemical deposited conversion layers is that mechanical injury of the surface layers and the subsequent following corrosion to a protracted dissolution of Foreign oxides from the film matrix results. This results in a metered and long-lasting Self-healing effect because the layer matrix upon contact with the aqueous remote corrosion medium also relatively far from the site originally damaged Areas nor can subsequent delivery foreign oxides. This depot effect is achieved with chemical applied conversion coatings not achieved.

Another advantage of the invention is that the layer matrix by their in Range between 250 HV and 500 HV hardness lying in a position, mechanical to tolerate damage such as stone chips or cracks in a far greater extent than can be expected from conversion thin layers with thicknesses of &lt;3 microns.

Through the widespread especially in corrosion protection in automotive layer sequence z. B. Magnesium substrate surface protective layer, cathodic electrodeposition paint, filler, Basecoat, clearcoat is the coordination of the respective layer thicknesses and surface hardening recommended.

Thus, it was found that when abrasive wear applications the varnish layers initially plastically deformed tear within wider stress and spall. This is the located under the cathodic dip coating exposed surface protective layer according to the invention and also mechanically claimed. Due to the surface hardness and the modulus of elasticity, however, this layer is in able to a certain subcritical stress level by elastic cushion rebound. Exceeds the load a critical value, are in the surface layer microcracks generated the corrosive with simultaneous or subsequent Stress to the slow dissolution of the foreign oxide layer from the inside leads. This is necessary for the self-healing process condition results in a transport then the dissolving foreign oxides to the located on the crack reasons Magnesium base material. There will be a change in the oxidation state of foreign oxides and there are water-insoluble compounds, the effective temporarily to a Anticorrosion lead. The surface protective layer according to the invention is characterized through a process-related porous structure with pore sizes of eg., between 1 and 2 microns from. This layer structure has two significant advantages. The first advantage is that through the pores subsequent cathodic dip coating itself at high film thicknesses without problems is possible and the cathodic dip coating in cured state fills the pores of the surface layer of the invention and thus leads to high adhesive strength values ​​up to 50 MPa.

The second advantage of the morphology of the surface layer of the invention consists in their relatively large real surface. This ensures, in the case of a mechanical Damage to the paint layers lying thereon and the surface layer as well as in Presence of an aqueous medium corrosion that a comparatively three-dimensional flow of dissolved foreign oxides to exposed substrate surface takes place.

The present invention also relates to components made of magnesium or a Magnesium alloy whose surface with a chromate-oxide Corrosion layer is provided that a method according to the present Invention has been applied in the manner described above.

The invention further provides the use of a component made of magnesium or a magnesium alloy, which is connected by the method according to the invention a Corrosion layer was coated, as a component for the interior or exterior applications Vehicle or aircraft.

The features mentioned in the subclaims relate to preferred developments of erfündungsgemäßen process. Further advantages of the invention result from the following detailed description.

Usually the procedure is such that magnesium alloys are degreased alkaline and in a mixture of eg., 10 to 40 parts of glycolic acid, 5 to 25 parts Aluminum nitrate and water is stained. In a subsequent anodic Oxidation process using various types of current and current densities of preferably 0.3 A / dm² to 5.0 A / dm² in an aqueous electrolyte with z. B. below specified compositions, the magnesium surface with an up to 100 microns thick protective layer.

The surface layers thus produced are either using conversion effects out of the base material, as well as by targeted incorporation of Electrolyte components chemically modified.

The magnesium components are connected as electrodes, preferably as an anode. The Of current is either pulsed bipolar DC or AC current, wherein preferably the following process parameters are chosen:

<ul><li>a) A pulsed, bipolar DC:<ul><li>- Frequencies between 10 and 5000 Hz, preferably between 250 and 2000 Hz;</li><li>- Current densities between 0.3 and 5 A / dm², Preferably between 1 and 2 A / dm²</li><li>- Ratio of anodic to cathodic current density of between 1: 1 and 10: 1, preferably between 2: 1 and 4: 1;</li><li>- Bath voltages between 80 and 300 V; Beginning of the visible Spark discharge between 100 and 150 V, final voltage of between 180 and 220 V (see examples);</li><li>- Coating / deposition duration between 1 and 40 minutes, preferably 5-25 minutes.</li></ul></li><li>b) AC:<ul><li>- Frequencies between 10 and 100 Hz, preferably 50 Hz;</li><li>- Current densities between 0.3 and 5 A / dm², Preferably between 1 and 2 A / dm². more preferably 1.5 A / dm²;</li><li>- Bath voltages between 50 and 300 V, preferably between 100 and 200 V; </li><li>- Coating / deposition duration between 1 and 40 minutes, preferably 5-25 minutes.</li></ul></li></ul>

Preference is given here electrolyte solutions are used the

<ul><li>a) vanadate and / or molybdate and / or manganese compounds in the range of 10 to 150 g / l,</li><li>b included) aluminate in the range of 3 to 45 g / l (as aluminum content) and</li><li>c) phosphate compounds, in particular sodium in contents of 50 to 400 g / l ,, chelates, in particular ethylene diamine (50%) at levels of 25 to 230 m / l and ammonia (25%) included in contents of 20 to 75 ml / l.</li></ul>

The required pH values ​​are adjusted with 25% ammonia solution.

Concentrations:

<ul><li>- Phosphate compounds: 50 g / l to 400 g / l, preferably 60 g / l to 100 g / l, in particular 80 g / l;</li><li>- Aluminate: 3 g / l to 45 g / l, preferably 4-10 g / l, in particular 4.6 g / l;</li><li>- Ammonium metavanadate: 10 g / l to 150 g / l, preferably 25 to 50 g / l, in particular 30 g / l;</li><li>- Ammonium molybdate: 10 g / l to 120 gl / m, preferably 25 to 45 g / l, in particular 38 g / l;</li><li>- Ammonia (25% ig): 20 ml / l to 75 ml / l, preferably 25 to 50 ml / l, in special 35 ml / l;</li><li>- Ethylenediamine (50%): 25 ml / l to 230 ml / l, preferably 30 to 150 ml / l, in particular 50 to 75 ml / l.</li></ul>

The invention is based on the following preferred but non-limiting examples are explained.

example 1

A component made of magnesium alloy AZ31 is alkaline degreased and in a solution from 35% glycolic acid, 14% aluminum nitrate and least. Water at RT for about 3 minutes until the metallic luster stained. In an aqueous electrolyte comprising 80 g / l Potassium dihydrogen phosphate, 45 g / l sodium carbonate, 50 m / l ethylene diamine, 35 ml / l ammoniacal solution, 8 g / l of sodium hydroxide and 5 g / l aluminum hydroxide and 45 g / l hexa-4-ammonium heptamolybdate hydrate ((NH₄)₆Mo₇O₂₄ × 4 H₂O), the connected magnesium component as an anode and bipolar pulsed DC applied. The frequency amounts to 250 Hz, the current density 1 A / dm², The relationship anodic to cathodic current density is 4: 1.

From a voltage of 145 V will lead to visible spark discharges on the Surface of the component. The final voltage is 200 V and ends the process at a Layer thickness of 25 microns. There are smooth, light gray produced to dark gray layers.

example 2

A magnesium component made of alloy AZ91hp is in the same manner as in Example 1 explains pretreated.

In an aqueous electrolyte comprising 80 g / l potassium dihydrogenphosphate, 45 g / l Sodium carbonate, 50 ml / l of ethylene diamine, 35 ml / l ammoniacal solution, 8 g / l Sodium hydroxide, 5 g / l aluminum hydroxide and 30 g / l Ammoniummeta vanadate is the connected magnesium component as an anode and bipolar pulsed DC applied. The frequency of this case is 2000 Hz, the current density 1.5 ALDM² J The Ratio anodic to cathodic current density is 2: 1.

From a voltage of 130 V will lead to visible spark discharges on the Surface of the component. The final voltage is 190 V. With advancing Layer construction decreases the anodic current density and ends the process at a Film thickness of 15 microns. It produces smooth, dark brown layers.

example 3

Two components of the magnesium alloy AZ31 are degreased and alkaline in a Solution of 35% glycolic acid, 14% aluminum nitrate and least. Water at RT for about 3 minutes to stained for metallic luster.

These two components are made as an electrode in an aqueous electrolyte, comprising 40 g / l ammonium bifluoride, 30 g / l Natriumhydrogendiphosphat, 30 g / l, ammonium meta-vanadate, 50 g / l citric acid and 75 ml / l of ethylenediamine and used with AC (frequency 50 Hz) applied. The current density amounts to 1.5 A / dm², at a bath voltage of 170 V are black surface layers with a Layer thickness produces up to 65 microns.