Method for producing corrosion and wear resistant protective coatings on magnesium and magnesium alloys

p0001Magnesium wins as a metallic lightweight material (density 1.74 g / cm³) in many industries, such as in aircraft construction, in space technology, the precision instrument, the optical industry and in the automotive industry is becoming increasingly important. However, magnesium has as a construction material the drawback that its corrosion resistance is poor without preliminary surface treatment. Various methods are known to increase the corrosion resistance and wear resistance of the surface of magnesium and magnesium alloys. These methods include chemical and electrochemical processes, such as the chromate treatment and the anodic oxidation.

p0002In the anodic oxidation, as the anode degreased magnesium parts dip into an electrolyte bath. In this electrolyte, a current flows, the negatively charged anions migrate to the anode and are discharged there. This results in atomic oxygen, which leads to the formation of magnesium oxide. This anodic coating is firmly anchored on the magnesium surface.

p0003The known electrochemical methods for coating magnesium work by anodic oxidation either with strong oxidizing agents or with peroxides or substances, which are converted at anodic polarization in peroxy (see, for example canadische Pat. No. 568,653). It can be assumed that the charge of the oxidation atomic oxygen formed by decomposition of peroxy compounds, which are then re-formed at high current density in the pores of Existing magnesium insulating again. When using strong oxidizing agents such as chromate, vanadate, permanganate, the formation of atomic oxygen by reduction of the respective in the oxidizing agent present in its highest oxidation state element takes place, then the re-oxidation takes place.

p0004The oxidizing agents or peroxy compounds used in the known methods for the anodic oxidation of magnesium or magnesium alloys containing transition metals such as chromium, vanadium or manganese. This has therefore been found to be disadvantageous because a part of these transition metal compounds is incorporated into the formed on the magnesium surface protective layer, which can be seen on the coloring. The incorporation of these transition metal compounds results in a reduction of the corrosion and wear resistance of the protective layer.

p0005The same applies to an anodizing solution for anodic oxidation of magnesium or its alloys, which is prepared by dissolving silicate, carboxylate and alkali metal hydroxide in water and can additionally contain borate, fluoride and phosphate (DE-A-37 15 663). This anodizing solution is alkali-rich. It has been shown that the incorporation of alkali metal ions in the layer produced in particular greatly reduces the corrosion resistance.

p0006The object of the present invention is to provide a method for generating a protective layer on magnesium or magnesium alloys by anodic oxidation are available, in which a protective layer is produced with enhanced corrosion resistance and wear resistance.

p0007It is another object of the present invention to produce layers with no or very low intrinsic color, are well-dyeable and make a good primer for painting or finishing.

p0008To solve this problem is a method of anodic oxidation, in which one uses a low alkali aqueous electrolytic bath<ul><li>a) 10 to 80 g / l borate or sulphate ions,</li><li>b) 10 to 70 g / l phosphate ions and</li><li>c) from 5 to 35 g / l fluoride ions</li></ul> and less than 100 mg / l of alkali metal ions and that is 7 to 9, adjusted to a pH value of from 5 to 11, preferably. The current supply takes place in such a way that with a rising voltage to DC power source to 400 Volts and the direct current interrupting or gegenpolt to allow the formation of magnesium phosphate and magnesium fluoride, and optionally magnesium briefly.

p0009It has surprisingly been found that can produce a particularly corrosion-resistant and wear-resistant protective layer by anodic oxidation on magnesium or magnesium alloys, when the same conditions as set out in the main claim are observed. In order to offer the necessary for the oxidation of the magnesium atomic oxygen, is used in the invention borate or sulfate anions which form peroxides which, although readily decompose, but is easy to reproduce because of the high current density in the pores of the protective layer formed. Borate and sulfate have been found to be particularly useful because they come as a result of the transfer only to a small extent to the cathode and be reduced to this.

p0010Furthermore, it was found that the electrolyte must contain those anions which form with the sparingly soluble to be oxidized magnesium compounds. According to the invention come here phosphate ions in combination with fluoride ions in question. If the invention is a magnesium-aluminum alloy is anodized, formed from the existing aluminum aluminate arising with magnesium ions a sparingly soluble magnesium.

p0011The forming protective layer must also have more pores or conductive bodies, so that a sufficient current flow is ensured. This is achieved by the invention as the electrolyte added fluoride ions.

p0012Furthermore, it has been found that it is important that close to the surface to be coated magnesium the correct ratio of anions to cations present, as only then an adequately stable dense protective layer is produced. When using a constant DC current would occur in the vicinity of the anode in an accumulation of anions. In particular, would be there, the high mobility having OH^⊖accumulate ions strongly, which would lead to the formation of a layer of Mg (OH) ₂ on the magnesium surface. However, the formation of a Mg (OH) ₂-layer is undesirable due to the lower stability of this layer. Moreover, it would an increase in the concentration of OH^⊖Ions undesirably formation of molecular oxygen O₂ favors. According to the invention is therefore the bath, in particular by the addition of buffering substances to a pH value of from 5 to 12, preferably set 8 to 9

p0013It can be desired in the vicinity of the surface to be coated concentration of anions which are to be incorporated into the protective layer, thereby achieving that is fed instead of a constant direct current a short time interrupted direct current or gegenpolt briefly so as to the formation of magnesium phosphate and magnesium fluoride, and - if an aluminum-containing magnesium alloy is oxidized - to enable the formation of magnesium aluminate. The current density is preferably 1 to 2 A / dm². Preferably, one works with a constant direct current with superposed alternating current of a frequency of biplane 10 and 100 Hz The superimposition is carried out by series connection of DC power supply and sinusoidal current source, the AC voltage component 15 -. 30% for the DC component. The generation of alternating current to the adjustable frequency Überlagung of the direct current can be effected with the aid of frequency converters. These are, for example, motor-generator units with variable speed, in which a change in the rotational speed lead to a proportional change in frequency. Here, the AC voltage is adjusted by a variable transformer according to the DC voltage to the desired% share of the DC voltage. Preferably, the frequency is selected at which the AC power is made available to the grid, eg in the Federal Republic of Germany with 50 Hz or 60 Hz in the United States.

p0014To reduce the effort for the appropriate power profile, can according to the invention, the anodic oxidation with rectified alternating current, whose frequency is 50 Hz or 60 Hz, to be carried out with a ripple of 15 to 35%. The rectification can both disposable circuit M1, preferably a center tap M2 (according to DIN draft 41 761), take place. The smoothing of the current thus produced is done by matching inductors, the ripple on 15 - decrease 35% (literature including: R. Hunter, Power Electronics Fundamentals and Applications, Berlin 1977), page 75).

p0015Alternatively, also working with a pulsed with 30 to 70 Hz DC current is possible, wherein the off time between two voltage pulses is equal to twice as long as the duration of the voltage pulses. The pulsing of the direct current may be carried out either by electronic, such as mechanical switches which are controlled by a frequency generator. When electronic switches are, for example Schalttyristoren. A similar power profile can an alternating current of 30 to 70 Hz are generated with phase by half-wave rectification M1 (according to DIN draft 41 761). By changing the current flow angle, the length of the voltage pulses can be controlled (eg literature: O. Aeschlimann, electronics without ballast, Munich 1973, page 347).

p0016For buffering of the electrolytic bath, in particular, amines which react weakly alkaline and generally have dissociation constants between 10⁻² and 10⁻⁷ suitable. As such amines especially cyclic amines such as pyridine, β-picoline, piperidine and piperazine are suitable. These amines are readily soluble in water generally. Other readily water-soluble amines that can be used are, for example, sodium sulfanilate, dimethylamine, ethylamine, diethylamine or triethylamine. Most preferably, hexamethylenetetramine is used.

p0017According to the invention is carried out at a rising voltage to 400 volts.

p0018Under a low alkali aqueous electrolyte according to the invention is preferably one to understand that contains less than 100 mg / l alkali ions. The unavoidable ions are those of the alkali metals lithium, sodium, potassium, etc. The ammonium ion is not considered here as alkali ion.

p0019The content of the borate or sulfate ions in the aqueous electrolyte is preferably 10 to 80 g / l. The content of phosphate ions, calculated as H₃PO₄ is preferably between 10 and 70 g / l. The amount of phosphate ions in combination with the fluoride ions to be used is calculated as HF from 5 to 35 g / l.

p0020Before the anodic oxidation under the conditions of the invention, the workpieces of magnesium or magnesium alloys are subjected to the usual chemical pretreatments for degreasing, in particular an alkaline cleaning with a strongly alkaline bath. Then usually follows an acid pickling eg with dilute aqueous solutions of phosphoric and sulfuric acids, and if necessary also an activation with hydrofluoric acid.

p0021The protective layers according to the invention formed on the surface of magnesium alloys or of pure magnesium are preferably still painted or subjected to post-treatment.

p0022The protective layers according to the invention produced form a very good base for paints, as are customary for workpieces made of magnesium, aluminum or zinc. These include two-component coatings based on polyurethane, acrylic resin, epoxy resin and phenolic resin coatings.

p0023Tried were among many others, the following commercially available products:<ul><li>1.) Aqualac 8,</li><li>2.) VP 5140 (Degussa) methacrylates,</li><li>3.) VKS 20 (phenolic resin)</li><li>4.) Araldite 985 E,</li><li>5.) water glass + CO₂</li><li>6.) PTFE dispersion</li></ul>

p0024The products 3, 4, 5 and 6 resulted in a clearly visible improvement in the corrosion resistance of the layers. The treated product in 6 layer additionally showed a significant reduction in the coefficient of friction.

p0025In order to improve the tribological properties (lubricity, dry lubricating properties) of such a coated surface can also be a post-treatment with a solid lubricant occur, which can be anchored in the existing pores. As such lubricants are suitable for example, fluorinated and / or chlorinated aliphatic and aromatic hydrocarbon compounds and molybdenum disulfide and graphite.

p0026A preferred treatment of the protective layers of the invention is carried out with the aqueous solution of an alkali silicate. By this treatment which reacts in the protective layer, especially in the pores existing Mg (OH) ₂ with the alkali silicate and alkali hydroxide to sparingly soluble magnesium silicate. Preferably, the workpiece is removed Alkalisilikatbad exposed with the protective layer in a second step, a carbon dioxide-rich atmosphere. Here, the remaining forms "water glass" from the silicate treatment with CO₂ atmosphere SiO₂ and alkali, as the stronger carbon dioxide displaces the weaker silicic acid from its connection. By SiO₂ the pores of the protective layer to be closed, this process being accelerated by gassing with CO₂. Since with use of stronger acids in the outer area of ​​the pores rapid precipitation of SiO₂ is performed, the alkali silicate is present in the interior of the pores can then no longer react. The continuous precipitation of SiO₂ in the pores by the weak carbonic contrast gives a much better corrosion protection.

p0027The present invention further relates to magnesium alloys, the protective layer containing a magnesium phosphate and magnesium fluoride with a thickness of 15 to 30 microns and an abrasion resistance measured by the Taber Abraser (CS 10, 10N) of less than 20 mg weight loss after 10 000 cycles coated are.

p0028The application of a protective layer which satisfies these conditions, for example can be done with the aid of the above-described inventive method.

p0029The corrosion resistance of the magnesium alloys according to the invention is by application of the protective layer is preferably less than 10 corrosion points / dm² after a sample of the alloy an exposure time of 240 hrs in the salt spray test according to DIN 50021 SS was suspended.

p0030For the novel process for producing corrosion and wear-resistant protective layers are suitable except pure magnesium particular Magnesiumgußlegierungen the ASTM designations AS41, AM 60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61, EZ33, HZ32 and wrought alloys AZ31, AZ61, AZ80, M1, ZK60, ZK40.

p0031Preferably, in the magnesium alloys according to the invention contains the protective layer additionally hydroxide, borate, aluminate, silicate or phenolate ions. The protective layer preferably contains, in particular in the pores of silica, which can be obtained by the above described after-treatment of the protective layer with an aqueous solution of an alkali silicate. The color applied to the magnesium alloys inventive protective layer is preferably white to whitish-gray or beige.

p0032The method of the invention with reference to examples in more detail.

example 1

p0033The surfaces of magnesium or magnesium alloys were first pretreated in an alkaline cleaning bath. This cleaning bath had the following composition:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">sodium hydroxide</entry><entry namest="col2" nameend="col2" align="right">50 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">trisodium</entry><entry namest="col2" nameend="col2" align="right">10 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Wetting agents / synthetic soap</entry><entry namest="col2" nameend="col2" align="right">1 g / l</entry></row></tbody></tgroup></table></tables>

p0034In this treatment in alkaline cleaning a pickling followed in a bath of the following composition: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Phosphoric acid (85%)</entry><entry namest="col2" nameend="col2" align="right">380 ml / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Sulfuric acid (98%)</entry><entry namest="col2" nameend="col2" align="right">16 ml / l</entry></row><row><entry namest="col1" nameend="col1" align="left">water</entry><entry namest="col2" nameend="col2" align="right">604 ml / l</entry></row></tbody></tgroup></table></tables>

p0035The pickling was carried out at a temperature of 20 ° C, the treatment time was about 30 seconds. After dressing the surface sample was activated in hydrofluoric acid.

p0036This was followed by the anodic oxidation for forming the protective layer of the invention. In this case, an electrolyte bath was used with the following composition:<tables id="tabl0003" num="0003"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">HF</entry><entry namest="col2" nameend="col2" align="right">30 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">H₃PO₄</entry><entry namest="col2" nameend="col2" align="right">60 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">H₃BO₃</entry><entry namest="col2" nameend="col2" align="right">70 g / l</entry></row></tbody></tgroup></table></tables>

p0037pH 8.9 (adjusted with ammonia).

p0038The anodic oxidation was carried out with a superimposed with alternating current of 50 Hz DC. It was used a 240 V voltage rising. The duration of the anodic oxidation was about 15 minutes. The layer thickness of the protective layer produced on the treated surfaces was about 20 microns.

example 2

p0039After a pre-treatment as in Example 1, the magnesium alloy AZ 91 was anodized in an electrolyte of the following composition and the conditions indicated: <tables id="tabl0004" num="0004"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Hydrofluoric acid (H₂F₂) (40%)</entry><entry namest="col2" nameend="col2" align="right">28 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Phosphoric acid (H₃PO₄) (98%)</entry><entry namest="col2" nameend="col2" align="right">58 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Boric acid (H₃BO₃)</entry><entry namest="col2" nameend="col2" align="right">35 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">hexamethylenetetramine</entry><entry namest="col2" nameend="col2" align="right">360 g / l</entry></row><row><entry namest="col1" nameend="col2" align="justify">pH: 7.0 to 7.3 adjusted with NH₄OH (25%)</entry></row><row><entry namest="col1" nameend="col2" align="justify">Current density: 1.4 A / dm (rectified AC ripple about 28%)</entry></row><row><entry namest="col1" nameend="col2" align="justify">End voltage: 325V</entry></row><row><entry namest="col1" nameend="col2" align="justify">Electrolyte temperature: 15 ° C</entry></row><row><entry namest="col1" nameend="col2" align="justify">Exposure time: 15 minutes</entry></row></tbody></tgroup></table></tables>

p0040After the anodization, the resulting layer was treated according to claim 10 and 11. FIG.

p0041The layer thickness was 21 microns.

p0042In a corrosion test according to DIN 50 021 SS, the layer thus obtained was after 500 hours 2 corrosion points / dm.

p0043The wear resistance in the Taber Abraser test was 30 mg weight loss after 10⁴ revolutions.

example 3

p0044After pre-treatment as in Example 1, the magnesium alloy Mg Al 6 was Zn in an electrolyte of the following composition and the specified parameters anodized: <tables id="tabl0005" num="0005"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Hydrofluoric acid (H₂F₂): (40%)</entry><entry namest="col2" nameend="col2" align="right">30 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Phosphoric acid (H₃PO₄) (98%)</entry><entry namest="col2" nameend="col2" align="right">60 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Boric acid (H₃BO₃):</entry><entry namest="col2" nameend="col2" align="right">70 g / l</entry></row><row><entry namest="col1" nameend="col1" align="left">Dimethylamine: (40%)</entry><entry namest="col2" nameend="col2" align="right">260 g / l</entry></row><row><entry namest="col1" nameend="col2" align="justify">pH 8.4 adjusted with NH₄OH (25%)</entry></row><row><entry namest="col1" nameend="col2" align="justify">Current density: 1.4 A / dm² (40 Hz pulsed DC, power on: off time = 1: 2)</entry></row><row><entry namest="col1" nameend="col2" align="justify">Electrolyte temperature: 15 ° C</entry></row><row><entry namest="col1" nameend="col2" align="justify">End voltage 320 V at the end of the treatment period short 400V</entry></row><row><entry namest="col1" nameend="col2" align="justify">Exposure time: 25 minutes</entry></row><row><entry namest="col1" nameend="col2" align="justify">Aftertreatment: as in Example 2</entry></row></tbody></tgroup></table></tables>

p0045The corrosion and wear resistance of the obtained film was similar to the described in Example 2. FIG.