Classifications

• • 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 produce protective layers on magnesium or magnesium alloys with anodic oxidation without or with only very little intrinsic coloration, which are easy to color and give a good primer for paintwork or aftertreatments and at the same time have increased corrosion resistance and wear resistance award.
• 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 in combination with fluoride or chloride ions are suitable here. If, according to the invention, a magnesium-aluminum alloy is anodically oxidized, the existing aluminum illuminations are formed 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 8 to 12, preferably between 10.5 and 11.5, in particular by adding buffering substances.
• 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 e.g. Motor-generator units with adjustable speed, in which a change in 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 selected, e.g. in the Federal Republic of Germany with 50 Hz or in the USA with 60 Hz.
• the anodic oxidation 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%. Rectification can take place both through one-way circuit M1, preferably through center circuit M2 (according to DIN draft 41 761). The current generated in this way is smoothed by suitable inductances, 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).
• the voltage is preferably increased to 100 volts.
• the current density is in particular 1 to 6 A / dm2.
• An alkali-rich aqueous electrolyte bath according to the invention is preferably to be understood as one which contains from 0.9 to 8.5 mol / l of alkali ions.
• Alkali ions 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 e.g. 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 paints, 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 e.g. 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” forms from the silicate treatment with the CO2 of the atmosphere 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.
• the present invention further relates to magnesium alloys with a protective layer containing magnesium phosphate, hydroxide and 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 40 mg mass loss 10,000 revs are covered.
• a protective layer containing magnesium phosphate, hydroxide and 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 40 mg mass loss 10,000 revs are covered.
• the corrosion resistance of the magnesium alloys according to the invention is preferably less than 15 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 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 to be treated of objects made of the magnesium alloy GD-MG Al 9 Zn 2 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.
• a 25 ⁇ m thick, white layer was obtained which could be dyed particularly well with commercially available dyes.
• the protective layers were treated with commercially available water glass at a concentration of 50 g / l at a temperature of 95 ° C. for 15 minutes, dried and then exposed to a CO2 atmosphere in a desiccator. Here, the water glass and the water glass present in the depth of the pores is slowly converted as SiO2. After this compaction, the layer shows 5 corrosion points after 500 hours in the corrosion test according to DIN 50 021 SS. The mass loss in the Taber Abraser test was 38 mg after 104 revolutions.

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• Chemical Kinetics & Catalysis (AREA)
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• Chemical Treatment Of Metals (AREA)
• Electroplating Methods And Accessories (AREA)

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• 1988
  • 1988-03-15 DE DE3808610A patent/DE3808610A1/de not_active Withdrawn
• 1989
  • 1989-03-09 US US07/321,253 patent/US4976830A/en not_active Expired - Fee Related
  • 1989-03-10 EP EP89104237A patent/EP0333049A1/fr not_active Withdrawn
  • 1989-03-13 JP JP1060582A patent/JPH01301889A/ja active Pending

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