EP0333048A1 - Method for producing corrosion and wear resistant protective coatings on magnesium and magnesium alloys - Google Patents

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

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Publication number
EP0333048A1
EP0333048A1 EP89104236A EP89104236A EP0333048A1 EP 0333048 A1 EP0333048 A1 EP 0333048A1 EP 89104236 A EP89104236 A EP 89104236A EP 89104236 A EP89104236 A EP 89104236A EP 0333048 A1 EP0333048 A1 EP 0333048A1
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Prior art keywords
magnesium
protective layer
bath
direct current
phosphate
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EP89104236A
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German (de)
French (fr)
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EP0333048B1 (en
Inventor
Edith Luise Schmeling
Benno Prof.Dr.-Ing. Röschenbleck
Michael Hans Weidemann
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Electro Chemical Engineering GmbH
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Electro Chemical Engineering GmbH
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/30—Anodisation of magnesium or alloys based thereon

Definitions

  • Various methods are known for increasing the corrosion resistance and wear resistance of the surface of magnesium and magnesium alloys. These processes include chemical and electrochemical processes such as chromating and anodizing.
  • the degreased magnesium parts connected as anode are immersed in an electrolyte bath. If a current flows in this electrolyte, the negatively charged anions migrate to the anode and are discharged there. This creates atomic oxygen, which leads to the formation of magnesium oxide. This anodic coating is firmly anchored to the magnesium surface.
  • the oxidizing agents or peroxy compounds used in the known processes for the anodic oxidation of magnesium or magnesium alloys contain transition metals such as e.g. Chromium, vanadium or manganese. This has proven to be disadvantageous because some of these transition metal compounds are built into the protective layer produced on the magnesium surface, which can be seen from the color. The installation of these transition metal compounds leads to a reduction in the corrosion and wear resistance of the protective layer.
  • the object of the present invention is therefore to provide a method for producing a protective layer on magnesium or magnesium alloys by anodic oxidation, in which a protective layer with increased corrosion resistance and wear resistance is produced.
  • a particularly corrosion-resistant and wear-resistant protective layer can be produced by anodic oxidation on magnesium or magnesium alloys if the conditions specified in the main claim are met at the same time.
  • borate or sulfate anions are used according to the invention which form peroxides, which decompose easily, but which easily replicate due to the high current density in the pores of the protective layer formed. Borate and sulfate anions have proven to be particularly suitable here, since they only reach the cathode to a small extent as a result of the transfer and are reduced thereon.
  • the electrolyte must contain such anions that form poorly soluble compounds with the magnesium to be oxidized.
  • phosphate ions come here in combination nation with fluoride or chloride ions in question. If, according to the invention, a magnesium-aluminum alloy is anodically oxidized, the aluminum illuminations formed result which, with magnesium ions, result in a poorly soluble magnesium aluminate.
  • the protective layer that forms must also have pores or conductive points so that a sufficient current flow is ensured. This is achieved by the fluoride or chloride ions added to the electrolyte bath according to the invention.
  • the bath is therefore adjusted to a pH of 5 to 12, preferably between 8 and 9, in particular by adding buffering substances.
  • motor-generator units with adjustable speed, in which a change in the speed leads to a proportional change in frequency.
  • the AC voltage is adjusted to the desired percentage of the DC voltage by means of a regulating transformer in accordance with the DC voltage.
  • the frequency with which the alternating current is available from the network is preferably selected, for example in the Federal Republic of Germany at 50 Hz or in the USA at 60 Hz.
  • the anodic oxidation according to the invention can also be carried out with rectified alternating current, the frequency of which is 50 Hz or 60 Hz, with a ripple of 15 to 35%.
  • the rectification can be done by one-way circuit M1, preferably by Mit telepoint circuit M2 (according to DIN draft 41 761).
  • the current generated in this way is smoothed by suitable inductors, which reduce the ripple to 15-35% (literature, for example: R. Jäger, Power Electronics Fundamentals and Applications, Berlin 1977) page 75).
  • a direct current pulsed at 30 to 70 Hz the switch-off time between two voltage pulses being equal to or twice as long as the duration of the voltage pulses.
  • the pulsing of the direct current can take place both by electronic and mechanical switches which are controlled by a frequency generator. Suitable electronic switches are e.g. Switching thyristors.
  • a similar current profile can also be generated by one-way rectification M1 (according to DIN Draft 41 761) of an alternating current from 30 to 70 Hz with leading edge.
  • the length of the voltage pulses can be controlled by changing the phase gating angle (literature e.g.: O. Limann, Electronics without Ballast, Kunststoff 1973, page 347).
  • Amines which react weakly alkaline and generally have dissociation constants between 10 ⁇ 2 and 10 ⁇ 7 are particularly suitable for buffering the electrolyte bath.
  • Cyclic amines such as pyridine, ⁇ -picoline, piperidine and piperazine are particularly suitable as such amines.
  • these amines are readily water-soluble.
  • Other readily water-soluble amines that can be used are, for example, sodium sulfanilate, dimethylamine, ethylamine, diethylamine or triethylamine. Hexamethylenetetramine is used in a particularly preferred manner.
  • the voltage is preferably increased to 400 volts.
  • the current density is in particular 1 to 2 A / dm2.
  • a low-alkali aqueous electrolyte bath according to the invention is preferably to be understood as one which contains less than 100 mg / l alkali ions.
  • the ions to be avoided are those of the alkali metals lithium, sodium, potassium etc.
  • the ammonium ion is not considered an alkali ion here.
  • the content of the borate or sulfate ions in the aqueous electrolyte bath 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 the fluoride or chloride ions to be used in combination with the phosphate ions is calculated as HF or HCl 5 to 35 g / l.
  • the workpieces made of magnesium or magnesium alloys are subjected to the usual chemical pretreatments for degreasing, in particular an alkaline cleaning with a strongly alkaline bath. This is usually followed by acid pickling, for example with dilute aqueous solutions of phosphoric acid and sulfuric acid and, if necessary, also activation with hydrofluoric acid.
  • the protective layers produced according to the invention on the surface of the magnesium alloys or the pure magnesium are preferably still painted or subjected to an aftertreatment.
  • the protective layers produced according to the invention form a very good primer for lacquers, as are common for workpieces made of magnesium, aluminum or zinc.
  • These include Two-component paints based on polyurethane, acrylic resin, epoxy resin and phenolic resin paints.
  • Products 3, 4, 5 and 6 showed a clearly recognizable increase in the corrosion resistance of the layers.
  • the layer treated in product 6 also resulted in a significant reduction in the coefficient of friction.
  • an aftertreatment can also be carried out with a solid lubricant which can anchor itself in the existing pores.
  • lubricants are, for example, fluorinated and / or chlorinated aliphatic and aromatic hydrocarbon compounds as well as molybdenum disulfide and graphite.
  • a preferred aftertreatment of the protective layers according to the invention is carried out with the aqueous solution of an alkali silicate.
  • the MgOH 2 present in the protective layer particularly in the pores, reacts with the alkali silicate to form sparingly soluble magnesium silicate and alkali hydroxide.
  • the workpiece with the protective layer removed from the alkali silicate bath is preferably exposed to an atmosphere rich in carbon dioxide.
  • the remaining "water glass" from the silicate treatment with the CO2 of the atmosphere forms SiO2 and alkali carbonate, since the stronger carbonic acid displaces the weaker silica from its compound.
  • the pores of the protective layer are closed by the SiO2, this process being accelerated by the gassing with CO2.
  • SiO2 Since with the use of stronger acids in the outer region of the pores, SiO2 precipitates rapidly, the alkali silicate located in the interior of the pores can then no longer react. The continuous precipitation of SiO2 in the pores by the weak carbonic acid, however, results in a much better protection against corrosion.
  • the present invention further relates to magnesium alloys coated with a protective layer containing magnesium phosphate and magnesium fluoride with a thickness of 15 to 30 ⁇ m and a wear resistance, measured with the Taber abraser (CS 10, 10 N), of less than 20 mg mass loss after 10,000 revolutions are.
  • a protective layer containing magnesium phosphate and magnesium fluoride with a thickness of 15 to 30 ⁇ m and a wear resistance, measured with the Taber abraser (CS 10, 10 N), of less than 20 mg mass loss after 10,000 revolutions are.
  • a protective layer which meets these conditions can be applied, for example, using the method according to the invention described above.
  • the corrosion resistance of the magnesium alloys according to the invention is preferably less than 10 corrosion points / dm 2 after a sample of the alloy has been exposed to an exposure time of 240 h in the salt spray test in accordance with DIN 50021 SS.
  • the magnesium casting alloys of the ASTM designations AS41, AM 60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61, are particularly suitable for the process according to the invention for producing corrosion-resistant and wear-resistant protective layers.
  • the protective layer preferably additionally contains hydroxide, borate, aluminate, phenolate or silicate ions.
  • the protective layer preferably contains, in particular in the pores, silicon dioxide, which can be obtained by the after-treatment of the protective layer described above with an aqueous solution of an alkali silicate.
  • the color of the protective layer applied to the magnesium alloys according to the invention is preferably white to whitish-gray or beige.
  • the surfaces of magnesium or magnesium alloys to be treated were first pretreated in an alkaline cleaning bath.
  • This cleaning bath had the following composition: Sodium hydroxide 50 g / l Trisodium phosphate 10 g / l Wetting agent / synthetic soap 1 g / l
  • the pickling was carried out at a temperature of 20 ° C., the treatment time being about 30 seconds. After pickling, the surface sample was activated in hydrofluoric acid.
  • the anodic oxidation was carried out with a direct current superimposed with an alternating current of 50 Hz. A voltage rising to 240 V was used. The duration of the anodic oxidation was approximately 15 minutes. The layer thickness of the protective layer produced on the treated surfaces was approximately 20 ⁇ m.
  • the magnesium alloy AZ 91 was anodized in an electrolyte of the following composition and under the specified conditions: Hydrofluoric acid (H2F2) (40%) 28 g / l Phosphoric acid (H3PO4) (98%) 58 g / l Boric acid (H3BO3) 35 g / l Hexamethylenetetramine 360 g / l pH value: 7.0 - 7.3 adjusted with NH4OH (25%) Current density: 1.4 A / dm2 (rectified alternating current, ripple approx. 28%) Final voltage: 325 V Electrolyte temperature: 15 ° C Exposure time: 15 minutes
  • the layer thickness obtained was 21 ⁇ m.
  • the wear resistance in the Taber Abraser test was 30 mg mass loss after 104 revolutions.
  • the corrosion and wear resistance of the layer obtained was analogous to that described in Example 2.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Preventing Corrosion Or Incrustation Of Metals (AREA)
  • Powder Metallurgy (AREA)

Abstract

To produce corrosion- and wear-resistant protective coatings on magnesium or magnesium alloys by anodic oxidation, use is made of a low-alkali aqueous electrolytic bath which contains a) borate or sulphate anions, and b) phosphate and fluoride or chloride ions and which is adjusted to a pH of 5 to 11, preferably 8 to 9. The applied direct current is interrupted for a short time or partially reversed in order to permit the formation of magnesium phosphate and magnesium fluoride or chloride and, possibly, magnesium aluminate. This produces coatings without or with only very slight inherent coloration which can readily be coloured and which yield a good adhesive base for lacquer coatings or post-treatments.

Description

Magnesium gewinnt als metallischer Leichtbauwerkstoff (Dichte 1,74 g/cm³) in vielen Industriezweigen, wie z.B. im Flugzeugbau, in der Raumfahrttechnik, im Feingerätebau, in der optischen Industrie und im Automobilbau zunehmend an Bedeutung. Magnesium hat jedoch als Konstruktionswerkstoff den Nachteil, daß seine Korrosionsbeständigkeit ohne vorhergehende Oberflächenbehandlung gering ist. Es sind verschiedene Methoden bekannt, um die Korrosions­beständigkeit und Verschleißfestigkeit der Oberfläche von Magnesium und Magnesiumlegierungen zu erhöhen. Zu diesen Verfahren zählen chemische und elektrochemische Verfahren wie z.B. das Chromatieren und die anodische Oxidation.Magnesium wins as a metallic lightweight material (density 1.74 g / cm³) in many branches of industry, e.g. in aircraft construction, in aerospace technology, in the construction of fine devices, in the optical industry and in automobile construction. Magnesium, however, has the disadvantage as a construction material that its corrosion resistance is low without previous surface treatment. Various methods are known for increasing the corrosion resistance and wear resistance of the surface of magnesium and magnesium alloys. These processes include chemical and electrochemical processes such as chromating and anodizing.

Bei der anodischen Oxidation tauchen die als Anode geschalteten entfetteten Magnesiumteile in ein Elektrolytbad. Wenn in diesem Elektrolyt ein Strom fließt, wandern die negativ geladenen Anionen zur Anode und werden dort entladen. Hierbei entsteht atomarer Sauerstoff, der zur Bildung von Magnesium­oxid führt. Dieser anodische Überzug ist festhaftend auf der Magnesiumoberfläche verankert.In the case of anodic oxidation, the degreased magnesium parts connected as anode are immersed in an electrolyte bath. If a current flows in this electrolyte, the negatively charged anions migrate to the anode and are discharged there. This creates atomic oxygen, which leads to the formation of magnesium oxide. This anodic coating is firmly anchored to the magnesium surface.

Die bekannten elektrochemischen Verfahren zur Beschich­tung von Magnesium durch anodische Oxidation arbeiten entweder mit starken Oxidationsmitteln oder aber mit Peroxiden oder Substanzen, die bei anodischer Pola­risation in Peroxyverbindungen überführt werden (s. z.B. canadische Patentschrift Nr. 568 653). Es kann davon ausgegangen werden, daß der für die Oxidation verantwortliche atomare Sauerstoff durch Zerfall der Peroxyverbindungen gebildet wird, die dann bei hoher Stromdichte in den Poren der auf dem Magnesium be­findlichen Isolierschicht wieder neu gebildet werden. Bei Verwendung starker Oxidationsmittel wie Chromat, Vanadat, Permanganat erfolgt die Bildung des atomaren Sauerstoffs durch Reduktion des jeweiligen in dem Oxidationsmittel in seiner höchsten Oxidationsstufe vorhandenen Elements, anschließend erfolgt die Rückoxidation.The known electrochemical processes for coating magnesium by anodic oxidation either work with strong oxidizing agents or with peroxides or substances which are converted into peroxy compounds by anodic polarization (see, for example, Canadian Patent No. 568 653). It can be assumed that the atomic oxygen responsible for the oxidation by decay of the Peroxy compounds is formed, which are then formed again at high current density in the pores of the insulating layer on the magnesium. If strong oxidizing agents such as chromate, vanadate or permanganate are used, the atomic oxygen is formed by reducing the respective element present in the oxidizing agent in its highest oxidation state, followed by reoxidation.

Die bei den bekannten Verfahren zur anodischen Oxidation von Magnesium oder Magnesiumlegierungen verwendeten Oxidationsmittel bzw. Peroxyverbindungen enthalten Übergangsmetalle wie z.B. Chrom, Vanadium oder Mangan. Dies hat sich deshalb als nachteilig erwiesen, weil ein Teil dieser Übergangsmetallverbindungen in die auf der Magnesiumoberfläche erzeugte Schutzschicht eingebaut wird, was sich an der Färbung erkennen läßt. Der Einbau dieser Übergangsmetallverbindungen führt zu einer Verringerung der Korrosions- und Verschleiß­beständigkeit der Schutzschicht.The oxidizing agents or peroxy compounds used in the known processes for the anodic oxidation of magnesium or magnesium alloys contain transition metals such as e.g. Chromium, vanadium or manganese. This has proven to be disadvantageous because some of these transition metal compounds are built into the protective layer produced on the magnesium surface, which can be seen from the color. The installation of these transition metal compounds leads to a reduction in the corrosion and wear resistance of the protective layer.

Die Aufgabe der vorliegenden Erfindung besteht deshalb darin, ein Verfahren zur Erzeugung einer Schutzschicht auf Magnesium oder Magnesiumlegierungen durch anodische Oxidation zur Verfügung zu stellen, bei dem eine Schutz­schicht mit erhöhter Korrosionsbeständigkeit und Ver­schleißfestigkeit erzeugt wird.The object of the present invention is therefore to provide a method for producing a protective layer on magnesium or magnesium alloys by anodic oxidation, in which a protective layer with increased corrosion resistance and wear resistance is produced.

Weiter ist es Aufgabe der vorliegenden Erfindung, Schichten ohne bzw. mit nur sehr geringer Eigenfärbung zu erzeugen, die gut färbbar sind und einen guten Haftgrund für Lackierungen oder Nachbehandlungen abgeben.It is a further object of the present invention to produce layers without or with only very little intrinsic coloration, which are easy to dye and give a good primer for lacquering or post-treatment.

Zur Lösung dieser Aufgabe dient ein Verfahren der anodischen Oxidation, bei dem man ein alkaliarmes wässriges Bad verwendet, das

  • a) Borat- oder Sulfatanionen, und
  • b) Phosphat- und Fluorid- oder Chloridionen enthält,
und auf einen pH-Wert von 5 bis 11, vorzugsweise 8 bis 9 eingestellt ist.
Die Stromzufuhr erfolgt in der Weise, daß man den zugeführten Gleichstrom kurzzeitig unterbricht oder partiell gegenpolt um die Ausbildung von Magnesium­phosphat und Magnesiumfluorid oder -chlorid und gegebenenfalls Magnesiumaluminat zu ermöglichen.To solve this problem, an anodic oxidation process is used, in which an alkali-poor aqueous bath is used
  • a) borate or sulfate anions, and
  • b) contains phosphate and fluoride or chloride ions,
and is adjusted to a pH of 5 to 11, preferably 8 to 9.
The current is supplied in such a way that the direct current supplied is briefly interrupted or partially reversed in order to enable the formation of magnesium phosphate and magnesium fluoride or chloride and, if appropriate, magnesium aluminate.

Es hat sich überraschenderweise gezeigt, daß sich durch anodische Oxidation auf Magnesium oder Magnesiumlegie­rungen eine besonders korrosionsfeste und verschleiß­beständige Schutzschicht erzeugen läßt, wenn gleich­zeitig die im Hauptanspruch genannten Bedingungen einge­halten werden. Um den für die Oxidation des Magnesiums erforderlichen atomaren Sauerstoff anzubieten, verwendet man erfindungsgemäß Borat- oder Sulfatanionen, die Peroxyde bilden, die zwar leicht zerfallen, sich aber infolge der hohen Stromdichte in den Poren der gebil­deten Schutzschicht leicht nachbilden. Borat- und Sulfatanionen haben sich hierbei als besonders geeignet erwiesen, da sie infolge der Überführung nur in geringem Maße zur Kathode gelangen und an dieser reduziert werden.It has surprisingly been found that a particularly corrosion-resistant and wear-resistant protective layer can be produced by anodic oxidation on magnesium or magnesium alloys if the conditions specified in the main claim are met at the same time. In order to offer the atomic oxygen required for the oxidation of magnesium, borate or sulfate anions are used according to the invention which form peroxides, which decompose easily, but which easily replicate due to the high current density in the pores of the protective layer formed. Borate and sulfate anions have proven to be particularly suitable here, since they only reach the cathode to a small extent as a result of the transfer and are reduced thereon.

Weiterhin wurde gefunden, daß der Elektrolyt solche Anionen enthalten muß, die mit dem zu oxidierenden Magnesium schwerlösliche Verbindungen bilden. Erfindungsgemäß kommen hier Phosphationen in Kombi­ nation mit Fluorid- oder Chloridionen in Frage. Wenn erfindungsgemäß eine Magnesium-Aluminium-Legierung anodisch oxidiert wird, bilden sich aus den vorhan­denen Aluminium Aluminationen, die mit Magnesiumionen ein schwerlösliches Magnesiumaluminat ergeben.Furthermore, it was found that the electrolyte must contain such anions that form poorly soluble compounds with the magnesium to be oxidized. According to the invention, phosphate ions come here in combination nation with fluoride or chloride ions in question. If, according to the invention, a magnesium-aluminum alloy is anodically oxidized, the aluminum illuminations formed result which, with magnesium ions, result in a poorly soluble magnesium aluminate.

Die sich bildende Schutzschicht muß außerdem noch Poren oder leitfähige Stellen aufweisen, damit ein ausreichender Stromfluß gewährleistet ist. Dies wird durch die erfindungsgemäß dem Elektrolytbad zuge­setzten Fluorid- oder Chloridionen erreicht.The protective layer that forms must also have pores or conductive points so that a sufficient current flow is ensured. This is achieved by the fluoride or chloride ions added to the electrolyte bath according to the invention.

Weiterhin hat sich gezeigt, daß es wichtig ist, daß nahe der zu beschichtenden Magnesiumoberfläche das richtige Verhältnis von Anionen zu Kationen vorliegt, da nur so eine hinreichend stabile dichte Schutz­schicht erzeugt wird. Bei Verwendung eines konstanten Gleichstroms würde es in Nähe der Anode zu einer Anreicherung der Anionen kommen. Inbesondere würden sich dort die eine hohe Beweglichkeit aufweisenden OH⊖-Ionen stark anreichern, was zur Ausbildung einer Schicht von Mg(OH)₂ auf der Magnesiumoberfläche führen würde. Die Bildung einer Mg(OH)₂-Schicht ist jedoch wegen der geringeren Stabilität dieser Schicht uner­wünscht. Außerdem würde durch eine Zunahme der Konzentration an OH⊖-Ionen in unerwünschter Weise die Bildung von molekularem Sauerstoff O₂ begünstigt. Erfindungsgemäß wird deshalb das Bad, insbesondere durch Zugabe von puffernden Substanzen auf einen pH-Wert von 5 bis 12, vorzugsweise zwischen 8 und 9 eingestellt.Furthermore, it has been shown that it is important that the correct ratio of anions to cations is present near the magnesium surface to be coated, since this is the only way to produce a sufficiently stable, dense protective layer. If a constant direct current were used, the anions would accumulate in the vicinity of the anode. In particular, the high mobility OH eine ions would accumulate there, which would lead to the formation of a layer of Mg (OH) ₂ on the magnesium surface. The formation of a Mg (OH) ₂ layer is undesirable because of the lower stability of this layer. In addition, an increase in the concentration of OH ⊖ ions would undesirably favor the formation of molecular oxygen O₂. According to the invention, the bath is therefore adjusted to a pH of 5 to 12, preferably between 8 and 9, in particular by adding buffering substances.

Man kann die in Nähe der zu beschichtenden Oberfläche gewünschte Konzentration an Anionen, die in die Schutzschicht eingebaut werden sollen, dadurch er­zielen, daß man anstelle eines konstanten Gleichstroms einen kurzzeitig unterbrochenen Gleichstrom zuführt oder aber partiell gegenpolt, um so die Ausbildung von Magnesiumphosphat und Magnesiumfluorid oder -chlorid und - falls eine Aluminium enthaltende Magnesiumle­gierung oxidiert wird - die Ausbildung von Magnesium­aluminat zu ermöglichen.
Vorzugsweise arbeitet man mit einem konstanten Gleich­strom mit überlagertem Wechselstrom einer Frequenz von zweichen 10 und 100 Hz. Die Überlagerung erfolgt durch Reihenschaltung von Gleichstromquelle und Sinusstrom­quelle, dessen Wechselspannungsanteil 15 - 30 % des Gleichspannungsanteils beträgt. Die Erzeugung von Wech­selstrom einstellbarer Frequenz zur Überlagung des Gleichstromes kann mit Hilfe von Frequenzumformern erfolgen. Dies sind z.B. Motor-Generatoreinheiten mit regelbarer Drehzahl, bei denen eine Änderung der Dreh­zahl zu einer proportionalen Frequenzänderung führt. Hierbei wird die Wechselspannung durch einen Regel­transformator entsprechend der Gleichspannung auf den gewünschten %-Anteil der Gleichspannung eingestellt. Vorzugsweise wird die Frequenz gewählt, mit der der Wechselstrom aus dem Netz zur Verfügung steht, also z.B. in der Bundesrepublik Deutschland mit 50 Hz oder in den USA mit 60 Hz.You can achieve the desired concentration of anions to be built into the protective layer in the vicinity of the surface to be coated by supplying a briefly interrupted direct current instead of a constant direct current or partially reversing the polarity, so as to form magnesium phosphate and magnesium fluoride or -chloride and - if an aluminum-containing magnesium alloy is oxidized - to enable the formation of magnesium aluminate.
It is preferable to work with a constant direct current with superimposed alternating current with a frequency of two 10 and 100 Hz. The superimposition is carried out by connecting the direct current source and the sine current source in series, the alternating voltage component of which is 15-30% of the direct voltage component. Frequency adjustable frequency can be generated to superimpose the direct current with the help of frequency converters. These are, for example, motor-generator units with adjustable speed, in which a change in the speed leads to a proportional change in frequency. Here, the AC voltage is adjusted to the desired percentage of the DC voltage by means of a regulating transformer in accordance with the DC voltage. The frequency with which the alternating current is available from the network is preferably selected, for example in the Federal Republic of Germany at 50 Hz or in the USA at 60 Hz.

Um den Aufwand für das geeignete Stromprofil zu mindern, kann erfindungsgemäß die anodische Oxidation auch mit gleichgerichtetem Wechselstrom, dessen Frequenz 50 Hz bzw. 60 Hz beträgt, mit einer Welligkeit von 15 bis 35 % durchgeführt werden. Die Gleichrichtung kann so wohl durch Einwegschaltung M1, vorzugsweise durch Mit­ telpunktschaltung M2 (nach DIN Entwurf 41 761) er­folgen. Die Glättung des so erzeugten Stromes erfolgt durch passende Induktivitäten, die die Welligkeit auf 15 - 35 % herabsetzen (Literatur z.B.: R. Jäger, Leistungselektronik Grundlagen und Anwendungen, Berlin 1977) Seite 75).In order to reduce the effort for the suitable current profile, the anodic oxidation according to the invention can also be carried out with rectified alternating current, the frequency of which is 50 Hz or 60 Hz, with a ripple of 15 to 35%. The rectification can be done by one-way circuit M1, preferably by Mit telepoint circuit M2 (according to DIN draft 41 761). The current generated in this way is smoothed by suitable inductors, which reduce the ripple to 15-35% (literature, for example: R. Jäger, Power Electronics Fundamentals and Applications, Berlin 1977) page 75).

Alternativ hierzu ist auch das Arbeiten mit einem mit 30 bis 70 Hz gepulsten Gleichstrom möglich, wobei die Ausschaltzeit zwischen zwei Spannungsimpulsen gleich bis doppelt so lang ist, wie die Dauer der Spannungs­impulse. Die Pulsung des Gleichstromes kann sowohl durch elektronische wie mechanische Schalter erfolgen, die mit einem Frequenzgenerator angesteuert werden. Als elektronische Schalter eignen sich z.B. Schalt­tyristoren. Ein ähnliches Stromprofil kann auch durch Einweggleichrichtung M1 (nach DIN Entwurf 41 761) eines Wechselstromes von 30 bis 70 Hz mit Phasenanschnitt er­zeugt werden. Durch Änderung des Phasenanschnittwinkels läßt sich die Länge der Spannungsimpulse steuern (Li­teratur z.B.: O. Limann, Elektronik ohne Ballast, Mün­chen 1973, Seite 347).Alternatively, it is also possible to work with a direct current pulsed at 30 to 70 Hz, the switch-off time between two voltage pulses being equal to or twice as long as the duration of the voltage pulses. The pulsing of the direct current can take place both by electronic and mechanical switches which are controlled by a frequency generator. Suitable electronic switches are e.g. Switching thyristors. A similar current profile can also be generated by one-way rectification M1 (according to DIN Draft 41 761) of an alternating current from 30 to 70 Hz with leading edge. The length of the voltage pulses can be controlled by changing the phase gating angle (literature e.g.: O. Limann, Electronics without Ballast, Munich 1973, page 347).

Zum Abpuffern des Elektrolytbads eignen sich insbeson­dere Amine, die schwach alkalisch reagieren und in der Regel Dissoziationskonstanten zwischen 10⁻² und 10⁻⁷ aufweisen. Als solche Amine kommen insbesondere zyklische Amine wie z.B. Pyridin, ß-Picolin, Piperi­din und Piperazin in Frage. Diese Amine sind in der Regel gut wasserlöslich. Andere gut wasserlösliche Amine, die verwendet werden können, sind z.B. Natriumsulfa­nilat, Dimethylamin, Ethylamin, Diethylamin oder Tri­ethylamin. In besonders bevorzugter Weise wird Hexa­methylentetramin verwendet.Amines which react weakly alkaline and generally have dissociation constants between 10⁻² and 10⁻⁷ are particularly suitable for buffering the electrolyte bath. Cyclic amines such as pyridine, β-picoline, piperidine and piperazine are particularly suitable as such amines. As a rule, these amines are readily water-soluble. Other readily water-soluble amines that can be used are, for example, sodium sulfanilate, dimethylamine, ethylamine, diethylamine or triethylamine. Hexamethylenetetramine is used in a particularly preferred manner.

Erfindungsgemäß wird vorzugsweise bei einer bis 400 Volt ansteigenden Spannung gearbeitet. Die Stromdichte beträgt insbesondere 1 bis 2 A/dm².According to the invention, the voltage is preferably increased to 400 volts. The current density is in particular 1 to 2 A / dm².

Unter einem erfindungsgemäßen alkaliarmen wässrigen Elektrolytbad ist vorzugsweise ein solches zu ver­stehen, das weniger als 100 mg/l Alkaliionen enthält. Die zu vermeidenden Ionen sind die der Alkalimetalle Lithium, Natrium, Kalium etc. Das Ammoniumion wird hier nicht als Alkaliion angesehen.A low-alkali aqueous electrolyte bath according to the invention is preferably to be understood as one which contains less than 100 mg / l alkali ions. The ions to be avoided are those of the alkali metals lithium, sodium, potassium etc. The ammonium ion is not considered an alkali ion here.

Der Gehalt der Borat- oder Sulfationen in dem wäss­rigen Elektrolytbad beträgt vorzugsweise 10 bis 80 g/l. Der Gehalt an Phosphationen berechnet als H₃PO₄ liegt vorzugsweise zwischen 10 und 70 g/l. Die Menge der in Kombination mit den Phosphationen zu verwendenden Fluorid- oder Chloridionen be­trägt berechnet als HF bzw. HCl 5 bis 35 g/l.The content of the borate or sulfate ions in the aqueous electrolyte bath 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 the fluoride or chloride ions to be used in combination with the phosphate ions is calculated as HF or HCl 5 to 35 g / l.

Vor der anodischen Oxidation unter den erfindungs­gemäßen Bedingungen werden die Werkstücke aus Mag­nesium- oder Magnesiumlegierungen den üblichen chemischen Vorbehandlungen zum Entfetten, insbesondere einer alkalischen Reinigung mit einem stark alka­lischen Bad unterworfen.
Anschließend folgt üblicherweise ein Säurebeizen z.B. mit verdünnten wässrigen Lösungen von Phosphorsäure und Schwefelsäure und erforderlichenfalls auch noch eine Aktivierung mit Flußsäure.Before the anodic oxidation under the conditions according to the invention, the workpieces made of magnesium or magnesium alloys are subjected to the usual chemical pretreatments for degreasing, in particular an alkaline cleaning with a strongly alkaline bath.
This is usually followed by acid pickling, for example with dilute aqueous solutions of phosphoric acid and sulfuric acid and, if necessary, also activation with hydrofluoric acid.

Die erfindungsgemäß erzeugten Schutz schichten auf der Oberfläche der Magnesiumlegierungen oder des Reinmag­nesiums werden vorzugsweise noch lackiert oder einer Nachbehandlung unterzogen.The protective layers produced according to the invention on the surface of the magnesium alloys or the pure magnesium are preferably still painted or subjected to an aftertreatment.

Die erfindungsgemäß erzeugten Schutzschichten bilden einen sehr guten Haftgrund für Lacke, wie sie für Werkstücke aus Magnesium, Aluminium oder Zink üblich sind. Dazu gehören u.a. Zweikomponenten-Lacke auf Polyurethanbasis, Acrylharz-, Epoxydharz- und Phenol­harzlacke.The protective layers produced according to the invention form a very good primer for lacquers, as are common for workpieces made of magnesium, aluminum or zinc. These include Two-component paints based on polyurethane, acrylic resin, epoxy resin and phenolic resin paints.

Erprobt wurden unter vielen anderen die folgenden handelsüblichen Produkte:

  • 1.) Aqualac 8,
  • 2.) VP 5140 (Degussa) Methacrylsäureester,
  • 3.) VKS 20 (Phenolharz),
  • 4.) Araldit 985 E,
  • 5.) Wasserglas + CO₂
  • 6.) PTFE-Dispersion
The following commercially available products were tested among many others:
  • 1.) Aqualac 8,
  • 2.) VP 5140 (Degussa) methacrylic acid ester,
  • 3.) VKS 20 (phenolic resin),
  • 4.) Araldite 985 E,
  • 5.) water glass + CO₂
  • 6.) PTFE dispersion

Die Produkte 3, 4, 5 und 6 ergaben eine deutlich er­kennbare Steigerung der Korrosionsbeständigkeit der Schichten. Die in Produkt 6 behandelte Schicht ergab zusätzlich eine erhebliche Verminderung des Reibungs­koeffizienten.Products 3, 4, 5 and 6 showed a clearly recognizable increase in the corrosion resistance of the layers. The layer treated in product 6 also resulted in a significant reduction in the coefficient of friction.

Zur Verbesserung der tribologischen Eigenschaften (Gleitfähigkeit, Trockenschmiereigenschaften) einer derartig beschichteten Oberfläche kann auch eine Nach­behandlung mit einem Festschmierstoff erfolgen, der sich in den vorhandenen Poren verankern kann. Als solche Schmierstoffe eignen sich z.B. fluorierte und/­oder chlorierte aliphatische und aromatische Kohlen­wasserstoffverbindungen sowie Molybdändisulfid und Graphit.To improve the tribological properties (lubricity, dry lubrication properties) of a surface coated in this way, an aftertreatment can also be carried out with a solid lubricant which can anchor itself in the existing pores. Such lubricants are, for example, fluorinated and / or chlorinated aliphatic and aromatic hydrocarbon compounds as well as molybdenum disulfide and graphite.

Eine bevorzugte Nachbehandlung der erfindungsgemäßen Schutzschichten erfolgt mit der wässrigen Lösung eines Alkalisilikats. Durch diese Nachbehandlung reagiert das in der Schutzschicht besonders in den Poren vor­handene MgOH₂ mit dem Alkalisilikat zu schwerlöslichem Magnesiumsilikat und Alkalihydroxid. Vorzugsweise wird das dem Alkalisilikatbad entnommene Werkstück mit der Schutzschicht in einem zweiten Schritt einer kohlen­dioxidreichen Atmosphäre ausgesetzt. Dabei bildet das restliche "Wasserglas" aus der Silikatbehandlung mit dem CO₂ der Atmosphäre SiO₂ und Alkalicarbonat, da die stärkere Kohlensäure die schwächere Kieselsäure aus ihrer Verbindung verdrängt. Durch das SiO₂ werden die Poren der Schutzschicht geschlossen, wobei dieser Prozeß durch die Begasung mit CO₂ noch beschleunigt wird. Da bei Verwendung von stärkeren Säuren im äuße­ren Bereich der Poren eine rasche Fällung von SiO₂ erfolgt, kann das im Inneren der Poren befindliche Alkalisilikat dann nicht mehr reagieren. Die durch­gehende Fällung von SiO₂ in den Poren durch die schwache Kohlensäure ergibt dagegen einen wesent­lich besseren Korrosionsschutz.A preferred aftertreatment of the protective layers according to the invention is carried out with the aqueous solution of an alkali silicate. As a result of this aftertreatment, the MgOH 2 present in the protective layer, particularly in the pores, reacts with the alkali silicate to form sparingly soluble magnesium silicate and alkali hydroxide. In a second step, the workpiece with the protective layer removed from the alkali silicate bath is preferably exposed to an atmosphere rich in carbon dioxide. The remaining "water glass" from the silicate treatment with the CO₂ of the atmosphere forms SiO₂ and alkali carbonate, since the stronger carbonic acid displaces the weaker silica from its compound. The pores of the protective layer are closed by the SiO₂, this process being accelerated by the gassing with CO₂. Since with the use of stronger acids in the outer region of the pores, SiO₂ precipitates rapidly, the alkali silicate located in the interior of the pores can then no longer react. The continuous precipitation of SiO₂ in the pores by the weak carbonic acid, however, results in a much better protection against corrosion.

Die vorliegende Erfindung betrifft weiter Magnesium­legierungen, die mit einer Magnesiumphosphat und Magnesiumfluorid enthaltenden Schutzschicht mit einer Dicke von 15 bis 30 µm und einer Verschleißbeständig­keit gemessen mit dem Taber-Abraser (CS 10, 10 N) von weniger als 20 mg Massenverlust nach 10 000 Um­drehungen überzogen sind.The present invention further relates to magnesium alloys coated with a protective layer containing magnesium phosphate and magnesium fluoride with a thickness of 15 to 30 μm and a wear resistance, measured with the Taber abraser (CS 10, 10 N), of less than 20 mg mass loss after 10,000 revolutions are.

Die Aufbringung einer Schutzschicht, die diesen Bedingungen genügt, kann z.B. mit Hilfe des oben geschilderten erfindungsgemäßen Verfahrens erfolgen.A protective layer which meets these conditions can be applied, for example, using the method according to the invention described above.

Die Korrosionsbeständigkeit der erfindungsgemäßen Magnesiumlegierungen beträgt nach Aufbringen der Schutzschicht vorzugsweise weniger als 10 Korro­sionspunkte/dm² nachdem eine Probe der Legierung einer Expositionszeit von 240 h im Salzsprühtest gemäß DIN 50021 SS ausgesetzt wurde.After the protective layer has been applied, the corrosion resistance of the magnesium alloys according to the invention is preferably less than 10 corrosion points / dm 2 after a sample of the alloy has been exposed to an exposure time of 240 h in the salt spray test in accordance with DIN 50021 SS.

Für das erfindungsgemäße Verfahren zur Erzeugung von korrosions- und verschleißbeständigen Schutz­schichten eignen sich außer Reinmagnesium insbe­sondere die Magnesiumgußlegierungen der ASTM-­Bezeichnungen AS41, AM 60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61, EZ33, HZ32 sowie die Knetlegierungen AZ31, AZ61, AZ80, M1, ZK60, ZK40.In addition to pure magnesium, the magnesium casting alloys of the ASTM designations AS41, AM 60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61, are particularly suitable for the process according to the invention for producing corrosion-resistant and wear-resistant protective layers. EZ33, HZ32 and wrought alloys AZ31, AZ61, AZ80, M1, ZK60, ZK40.

Vorzugsweise enthält bei den erfindungsgemäßen Magnesiumlegierungen die Schutzschicht zusätzlich Hydroxid-, Borat-, Aluminat-, Phenolat- oder Silikat­ionen. Die Schutzschicht enthält vorzugsweise ins­besondere in den Poren Siliciumdioxid, das durch die oben beschriebene Nachbehandlung der Schutzschicht mit einer wässrigen Lösung eines Alkalisilikats erhalten werden kann. Die Farbe der auf die erfindungsgemäßen Magnesiumlegierungen aufgebrachten Schutzschicht ist vorzugsweise weiß bis weißlich-grau oder beige.In the magnesium alloys according to the invention, the protective layer preferably additionally contains hydroxide, borate, aluminate, phenolate or silicate ions. The protective layer preferably contains, in particular in the pores, silicon dioxide, which can be obtained by the after-treatment of the protective layer described above with an aqueous solution of an alkali silicate. The color of the protective layer applied to the magnesium alloys according to the invention is preferably white to whitish-gray or beige.

Im folgenden wird das erfindungsgemäße Verfahren an­hand der Beispiele näher erläutert.The method according to the invention is explained in more detail below with the aid of the examples.

Beispiel 1example 1

Die zu behandelnden Oberflächen von Magnesium bzw. Magnesiumlegierungen wurden zunächst in einem al­kalischen Reinigungsbad vorbehandelt. Dieses Reini­gungsbad hatte die nachfolgende Zusammensetzung: Natriumhydroxid 50 g/l Trinatriumphosphat 10 g/l Netzmittel/synthetische Seife 1 g/l The surfaces of magnesium or magnesium alloys to be treated were first pretreated in an alkaline cleaning bath. This cleaning bath had the following composition: Sodium hydroxide 50 g / l Trisodium phosphate 10 g / l Wetting agent / synthetic soap 1 g / l

Auf diese Behandlung im alkalischen Reinigungsbad folgte eine Beizung in einem Bad der nachfolgenden Zusammensetzung: Phosphorsäure (85 %ig) 380 ml/l Schwefelsäure (98 %ig) 16 ml/l Wasser 604 ml/l This treatment in the alkaline cleaning bath was followed by pickling in a bath of the following composition: Phosphoric acid (85%) 380 ml / l Sulfuric acid (98%) 16 ml / l water 604 ml / l

Die Beizung wurde bei einer Temperatur von 20°C durch­geführt, wobei die Behandlungsdauer ca. 30 Sekunden betrug. Nach der Beizung wurde die Oberflächenprobe in Flußsäure aktiviert.The pickling was carried out at a temperature of 20 ° C., the treatment time being about 30 seconds. After pickling, the surface sample was activated in hydrofluoric acid.

Anschließend erfolgte die anodische Oxidation zur Er­zeugung der erfindungsgemäßen Schutzschicht. Dabei wurde ein Elektrolytbad mit der nachfolgenden Zusammen­setzung verwendet: HF 30 g/l H₃PO₄ 60 g/l H₃BO₃ 70 g/l pH-Wert 8,9 (eingestellt mit Ammoniak).The anodic oxidation then took place to produce the protective layer according to the invention. An electrolyte bath with the following composition was used: HF 30 g / l H₃PO₄ 60 g / l H₃BO₃ 70 g / l pH 8.9 (adjusted with ammonia).

Die anodische Oxidation erfolgte mit einem mit Wechsel­strom von 50 Hz überlagerten Gleichstrom. Es wurde eine bis 240 V steigende Spannung verwendet. Die Dauer der anodischen Oxidation betrug ca. 15 Minuten. Die Schicht­dicke der auf den behandelten Oberflächen erzeugten Schutzschicht betrug ca. 20 µm.The anodic oxidation was carried out with a direct current superimposed with an alternating current of 50 Hz. A voltage rising to 240 V was used. The duration of the anodic oxidation was approximately 15 minutes. The layer thickness of the protective layer produced on the treated surfaces was approximately 20 μm.

Beispiel 2Example 2

Nach einer Vorbehandlung wie bei Beispiel 1 wurde die Magnesiumlegierung AZ 91 in einem Elektrolyten nachfol­gender Zusammensetzung und den angegebenen Bedingungen anodisiert: Flußsäure (H₂F₂) (40 %ig) 28 g/l Phosphorsäure (H₃PO₄) (98 %ig) 58 g/l Borsäure (H₃BO₃) 35 g/l Hexamethylentetramin 360 g/l pH-Wert: 7,0 - 7,3 eingestellt mit NH₄OH ( 25 %ig) Stromdichte: 1,4 A/dm² (gleichgerichteter Wechselstrom, Welligkeit ca. 28 %) Endspannung: 325 V Elektrolyt-Temperatur: 15°C Expositionszeit: 15 Minuten After pretreatment as in Example 1, the magnesium alloy AZ 91 was anodized in an electrolyte of the following composition and under the specified conditions: Hydrofluoric acid (H₂F₂) (40%) 28 g / l Phosphoric acid (H₃PO₄) (98%) 58 g / l Boric acid (H₃BO₃) 35 g / l Hexamethylenetetramine 360 g / l pH value: 7.0 - 7.3 adjusted with NH₄OH (25%) Current density: 1.4 A / dm² (rectified alternating current, ripple approx. 28%) Final voltage: 325 V Electrolyte temperature: 15 ° C Exposure time: 15 minutes

Nach der Anodisation wurde die erhaltene Schicht nach Patentanspruch 10 und 11 behandelt.After the anodization, the layer obtained was treated according to claims 10 and 11.

Die erhaltene Schichtdicke betrug 21 µm.The layer thickness obtained was 21 μm.

In einem Korrosionstest nach DIN 50 021 SS zeigte die so erhaltene Schicht nach 500 Stunden 2 Korrosions­punkte / dm².In a corrosion test according to DIN 50 021 SS, the layer obtained in this way showed 2 corrosion points / dm 2 after 500 hours.

Die Verschleißbeständigkeit im Taber-Abraser-Test betrug 30 mg Massenverlust nach 10⁴ Umdrehungen.The wear resistance in the Taber Abraser test was 30 mg mass loss after 10⁴ revolutions.

Beispiel 3Example 3

Nach Vorbehandlung wie bei Beispiel 1 wurde die Mag­nesiumlegierung Mg Al 6 Zn in einem Elektrolyten nachfolgender Zusammensetzung und den angegebenen Parametern anodisiert: Flußsäure (H₂F₂): (40 %ig) 30 g/l Phosphorsäure (H₃PO₄) (98 %ig) 60 g/l Borsäure (H₃BO₃): 70 g/l Dimethylamin: (40 %ig) 260 g/l pH-Wert: 8,4 eingestellt mit NH₄OH (25 %ig) Stromdichte: 1,4 A/dm² (gepulster Gleichstrom 40 Hz, Einschalt : Ausschaltzeit = 1:2) Elektrolyt-Temperatur: 15°C Endspannung 320 V am Ende der Behandlungszeit kurzfristig 400 V Expositionszeit: 25 Minuten Nachbehandlung: wie im Beispiel 2 After pretreatment as in Example 1, the magnesium alloy Mg Al 6 Zn was anodized in an electrolyte with the following composition and the specified parameters: Hydrofluoric acid (H₂F₂): (40%) 30 g / l Phosphoric acid (H₃PO₄) (98%) 60 g / l Boric acid (H₃BO₃): 70 g / l Dimethylamine: (40%) 260 g / l pH: 8.4 adjusted with NH₄OH (25%) Current density: 1.4 A / dm² (pulsed direct current 40 Hz, switch-on: switch-off time = 1: 2) Electrolyte temperature: 15 ° C Final voltage 320 V at the end of the treatment period 400 V for a short time Exposure time: 25 minutes Post-treatment: as in example 2

Die Korrosions- und Verschleißbeständigkeit der erhal­tenen Schicht war analog der im Beispiel 2 beschriebenen.The corrosion and wear resistance of the layer obtained was analogous to that described in Example 2.

Claims (17)

1. Verfahren zur Erzeugung von korrosions- und verschleißbeständigen Schutzschichten auf Magne­sium oder Magnesiumlegierungen durch anodische Oxidation, dadurch gekennzeichnet, daß man ein alkaliarmes wässriges Elektrolytbad verwendet, das
a) Borat- oder Sulfatanionen, und
b) Phosphat- und Fluorid- oder Chloridionen
enthält, und auf einen pH-Wert von 5 bis 11, vorzugweise 8 bis 9 eingestellt ist, und daß man den zugeführten Gleichstrom kurzzeitig unterbricht oder partiell gegenpolt, um die Ausbildung von Magnesiumphosphat und Magnesiumfluorid oder -chlorid und gegebenen­falls Magnesiumaluminat zu ermöglichen.1. A process for the production of corrosion and wear-resistant protective layers on magnesium or magnesium alloys by anodic oxidation, characterized in that one uses a low-alkali aqueous electrolyte bath, the
a) borate or sulfate anions, and
b) phosphate and fluoride or chloride ions
contains, and is adjusted to a pH of 5 to 11, preferably 8 to 9, and that one briefly interrupts or partially reverses the supplied direct current in order to enable the formation of magnesium phosphate and magnesium fluoride or chloride and optionally magnesium aluminate. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß mit einem konstanten Gleichstrom mit überlager­tem Wechselstrom von 10 bis 100 Kz, dessen Strom­dichte 15 bis 35 % des Gleichstroms beträgt, ge­arbeitet wird.2. The method according to claim 1, characterized in that a constant direct current with superimposed alternating current of 10 to 100 Kz, the current density of which is 15 to 35% of the direct current, is used. 3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß mit gleichgerichtetem Wechselstrom mit einer Welligkeit von 15 bis 35 % gearbeitet wird.3. The method according to claim 1, characterized in that one works with rectified alternating current with a ripple of 15 to 35%. 4. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß mit einem mit 30 bis 70 Hz gepulsten Gleich­strom gearbeitet wird, wobei die Ausschaltzeit zwischen zwei Spannungsimpulsen gleich bis doppelt so lang ist, wie die Dauer der Spannungsimpulse.4. The method according to claim 1, characterized in that one works with a pulsed with 30 to 70 Hz direct current, the switch-off time between two voltage pulses being equal to or twice as long as the duration of the voltage pulses. 5. Verfahren nach einem der Ansprüche 1 bis 4, da­durch gekennzeichnet, daß das Bad mit einem Amin abgepuffert ist.5. The method according to any one of claims 1 to 4, characterized in that the bath is buffered with an amine. 6. Verfahren nach einem der Ansprüche 1 bis 5, da­durch gekennzeichnet, daß das Bad durch Hexame­thylentetramin abgepuffert ist.6. The method according to any one of claims 1 to 5, characterized in that the bath is buffered by hexamethylenetetramine. 7. Verfahren nach einem der Ansprüche 1 bis 6, da­durch gekennzeichnet, daß bei einer Stromdichte von 1 bis 2 A/dm² gearbeitet wird.7. The method according to any one of claims 1 to 6, characterized in that one works at a current density of 1 to 2 A / dm². 8. Verfahren nach einem der Ansprüche 1 bis 7, dadurch gekennzeichnet, daß bei steigender Spannung bis 400 Volt gearbeitet wird.8. The method according to any one of claims 1 to 7, characterized in that the voltage is increased to 400 volts. 9. Verfahren nach einem der Ansprüche 1 bis 8, dadurch gekennzeichnet, daß das Bad weniger als 100 mg/l Alkaliionen enthält.9. The method according to any one of claims 1 to 8, characterized in that the bath contains less than 100 mg / l alkali ions. 10. Verfahren nach einem der Ansprüche 1 bis 9, dadurch gekennzeichnet, daß die Schicht mit der wässrigen Lösung eines Alkalisilikats nachbehandelt wird.10. The method according to any one of claims 1 to 9, characterized in that the layer is post-treated with the aqueous solution of an alkali silicate. 11. Verfahren nach Anspruch 10, dadurch gekenn­zeichnet, daß das dem Alkalisilikatbad ent­nommene Werkstück mit der Schutzschicht einer kohlendioxidreichen Atmosphäre ausgesetzt wird.11. The method according to claim 10, characterized in that the workpiece removed from the alkali silicate bath with the protective layer is exposed to a carbon dioxide-rich atmosphere. 12. Verfahren nach einem der Ansprüche 1 bis 11, dadurch gekennzeichnet, daß die Schutzschicht lackiert wird.12. The method according to any one of claims 1 to 11, characterized in that the protective layer is painted. 13. Magnesiumlegierung gekennzeichnet durch eine Magnesiumphosphat und Magnesiumfluorid enthaltende Schutzschicht einer Dicke von 15 bis 30 µm und einer Verschleißbeständigkeit, gemessen mit dem Taber-Abraser (CS 10, 10 N) von weniger als 20 mg Massenverlust nach 10 000 Umdrehungen.13. Magnesium alloy characterized by a protective layer containing magnesium phosphate and magnesium fluoride with a thickness of 15 to 30 µm and a wear resistance, measured with the Taber abraser (CS 10, 10 N) of less than 20 mg mass loss after 10,000 revolutions. 14. Magnesiumlegierung nach Anspruch 13, gekenn­zeichnet durch eine Korrosionsbeständigkeit von weniger als 10 Korrosionspunkten/dm² nach einer Expositionszeit von 240 h im Salzsprüh­test nach DIN 50 021 SS.14. Magnesium alloy according to claim 13, characterized by a corrosion resistance of less than 10 corrosion points / dm² after an exposure time of 240 h in the salt spray test according to DIN 50 021 SS. 15. Magnesiumlegierung nach Anspruch 13 oder 14, dadurch gekennzeichnet, daß die Schutzschicht zusätzlich Magnesiumhydroxid-, -borat, -aluminat, -phenolat oder -silikat enthält.15. Magnesium alloy according to claim 13 or 14, characterized in that the protective layer additionally contains magnesium hydroxide, borate, aluminate, phenolate or silicate. 16. Magnesiumlegierung nach einem der Ansprüche 13 bis 15, dadurch gekennzeichnet, daß die Schutz­schicht, insbesondere in den Poren, Siliziumdioxid enthält.16. Magnesium alloy according to one of claims 13 to 15, characterized in that the protective layer, in particular in the pores, contains silicon dioxide. 17. Magnesiumlegierung nach einem der Ansprüche 13 bis 16, dadurch gekennzeichnet, daß die erzeugte Schutzschicht weiß bis weißlich-grau oder beige ist.17. Magnesium alloy according to one of claims 13 to 16, characterized in that the protective layer produced is white to off-white or gray or beige.
EP89104236A 1988-03-15 1989-03-10 Method for producing corrosion and wear resistant protective coatings on magnesium and magnesium alloys Expired - Lifetime EP0333048B1 (en)

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US7025111B2 (en) 2000-05-26 2006-04-11 Daimlerchrysler Ag Method for coating a metallic component
WO2003056055A1 (en) * 2001-12-24 2003-07-10 Universität Hannover Magnesium workpiece and method for generation of an anti-corrosion coating on a magnesium workpiece
EP2003218A1 (en) 2007-06-12 2008-12-17 Yamaha Hatsudoki Kabushiki Kaisha Anodised magnesium alloy member, method for producing the same, and transporter comprising the same
WO2013070669A1 (en) * 2011-11-07 2013-05-16 Synthes Usa, Llc Lean electrolyte for biocompatible plasmaelectrolytic coatings on magnesium implant material
KR20140091579A (en) * 2011-11-07 2014-07-21 신세스 게엠바하 Lean electrolyte for biocompatible plasmaelectrolytic coatings on magnesium implant material
US9066999B2 (en) 2011-11-07 2015-06-30 DePuy Synthes Products, Inc. Lean electrolyte for biocompatible plasmaelectrolytic coatings on magnesium implant material
US9682176B2 (en) 2011-11-07 2017-06-20 DePuy Synthes Products, Inc. Lean electrolyte for biocompatible plasmaelectrolytic coatings on magnesium implant material
KR102122707B1 (en) 2011-11-07 2020-06-29 신세스 게엠바하 Lean electrolyte for biocompatible plasmaelectrolytic coatings on magnesium implant material

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JPH01301888A (en) 1989-12-06
DE3808609A1 (en) 1989-09-28
US4978432A (en) 1990-12-18

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