Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C26/00—Coating not provided for in groups C23C2/00 - C23C24/00
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/42—Pretreatment of metallic surfaces to be electroplated of light metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/42—Pretreatment of metallic surfaces to be electroplated of light metals
  • C25D5/44—Aluminium

Definitions

• the invention relates to a method for producing coated workpieces made of light metal or a light metal alloy and the workpieces produced in this way.
• Magnesium has a density of 1.7 g / cm 3 and is therefore considerably lighter than other light metals already used in construction, such as aluminum with a density of 2.7 g / cm 3 and titanium with a density of 4.5 g / cm 3 .
• magnesium is already being used increasingly in the field of drive units.
• a major disadvantage of light metals is that they are very non-precious and thus very susceptible to corrosion.
• most light metals form a passivation layer of the corresponding metal oxide in the ambient air by reaction with oxygen. Although this passivation layer protects against corrosion, but may be detrimental to the further processing of the light metal, because this is the surface texture changed.
• the passivation layer of oxides is therefore often undesirable and other systems for corrosion protection must be used.
• the light metals are provided with metallic coatings, where usually metals are used, which are less noble than the base material.
• these coatings can act as sacrificial anode at injury sites and provide cathodic protection to the base material.
• this possibility is very limited in magnesium, since Mg is very base and therefore no common metal is more available as a sacrificial anode. Because of its base properties, Mg is often used as a sacrificial anode in steel ships.
• magnesium is a very base metal, which is considerably less noble even compared to aluminum and zinc.
• the coating of magnesium base material with aluminum is therefore particularly pay attention to the electrochemical compatibility, otherwise there is a risk that a bimetallic corrosion between the base material and the coating material takes place, which leads to corrosion of the base material.
• This is especially critical for magnesium because it has an extremely negative voltage potential.
• Magnesium is therefore attacked by bimetallic corrosion in contact with almost all common metallic engineering and coating materials to a much greater extent than aluminum or zinc. For example, magnesium corrodes alkaline corrosion products with a pH of greater than 11. These corrosion products attack aluminum, which goes into solution from a pH of about 9.4.
• Another problem of corrosion of magnesium is the strong alkalinity of the corrosion products.
• the alkaline corrosion products of magnesium can damage coatings but also other amphoteric contact metals such as aluminum or zinc. For this reason, the paint adhesion to magnesium components is difficult to control and the lifetime of pure aluminum coatings on magnesium materials is extremely low.
• DE 38 04 303 A1 proposes a method for improving the adhesion of galvanic aluminum layers to metal workpieces by applying an adhesion-promoting layer.
• a nonaqueous electrolyte is used to apply the primer layer of iron, iron and nickel, nickel, cobalt, copper and alloys of the above-mentioned metals or tin-nickel alloys.
• a galvano-aluminum layer is applied to the intermediate layer.
• the application of the intermediate layer of a non-aqueous electrolyte is essential, otherwise embrittlement of the metal workpiece occurs when using an aqueous electrolyte by the hydrogen formed during the electrolysis. This adversely affects the often used low alloy high strength steels.
• a nonaqueous electrolyte to apply the metallic intermediate layer, the embrittlement of the workpieces is avoided.
• EP 1 141 447 B1 discloses electrolytes for coating workpieces with layers of an aluminum / magnesium alloy. Such a coating is particularly necessary when compounds with magnesium parts are to be produced because the corrosion products of the magnesium are alkaline and attack the aluminum surface coatings. By using aluminum / magnesium alloys contact corrosion is avoided here and causes a long-term stability of the coating. It is proposed steel fasteners for contact with magnesium components, especially in the automotive industry with aluminum / magnesium alloys. In EP 1 141 447 B1, no metallic interlayers interposed between the workpiece and the corrosion reducing layer of an aluminum / magnesium alloy are disclosed.
• the technical object of the invention is therefore to provide a corrosion protection system for light metals as base materials, in particular magnesium available, which has an extremely low bimetallic corrosion sensitivity, is adherent and provides permanent protection of the base material over a long time and with appropriate mechanical stress.
• intermetallic phases which are an ideal corrosion protection system for light metals.
• the subsequent heat treatment produces various intermetallic phases which become more zinc-rich or aluminum-enriched towards the surface of the coated base material and more magnesium-rich in the direction of the base material.
• the bimetallic corrosion is considerably reduced, since several layers of different alloys are formed. This results in lower gradations of the voltage potentials and thus a lower susceptibility to corrosion compared to the base material than in the exclusive coating without subsequent heat treatment.
• the determined thicknesses of the corrosion protection layer are considerably larger, because a part of the base material is included in the formation of the corrosion protection layer. As a result, the better adhesion is also achieved because the applied corrosion protection layer is better anchored by the heat treatment in the base material.
• the substrate is made of magnesium or a magnesium alloy.
• Alloy compositions with aluminum may be mentioned in particular as magnesium alloys, wherein the aluminum content may be greater than zero to 10% by weight of aluminum and lower amounts of zinc, manganese, silicon, rare earths and the rare earth-related elements scandium and yttrium may be contained.
• the layer of step a) is applied to the substrate material of a non-aqueous electrolyte or of an aqueous electrolyte.
• the method according to the invention relates to light metals and light metal alloys.
• light metals is meant a collective term for metallic elements of low density.
• the light metals include metals such as aluminum, scandium, yttrium and titanium.
• technically important light metals which are also preferred in the context of the invention, are aluminum, magnesium, zinc, titanium or alloys thereof.
• various layer systems comprising substrate and corrosion protection layer are included. These include first the system in which the substrate is made of magnesium or a magnesium alloy and the layer of aluminum, zinc or alloys thereof. Another system encompassed by the present invention is the system wherein the substrate is aluminum or an aluminum alloy and the layer is magnesium, zinc or alloys thereof.
• systems which consist of three or more layers and thus contain one or more intermediate layers are also detected.
• the substrate is magnesium or a magnesium alloy and the first layer (intermediate layer) is aluminum, zinc or alloys thereof and the second layer (surface layer) is aluminum, magnesium, zinc or alloys thereof.
• the first and second layers are not the same metal.
• the substrate is made of aluminum or an aluminum alloy and the first layer (intermediate layer) is magnesium, zinc or alloys thereof and the second layer (surface layer) is aluminum, magnesium, zinc or alloys thereof.
• the metals of the first and second layers are not the same.
• the heat treatment takes place both with respect to the temperature and its duration such that an alloy comprising metal of the surface layer of the substrate and metal or metal alloy of the applied layer is formed at least in the boundary region between the substrate and the applied layer.
• the temperature of the heat treatment is between 250 ° C and 700 ° C, preferably between 300 ° C and 650 ° C, and more preferably between 250 ° C and 600 ° C.
• the duration of the heat treatment is preferably between 1 second and 5 hours, preferably between 30 seconds and 2 hours and most preferably between 1 minute and 1 hour.
• the layer is subjected to a further treatment.
• this may be an anodic oxidation, which is preferably anodization of the layer.
• Each applied layer preferably has a layer thickness of 0.1-100 ⁇ m.
• the layer thickness is 0.5 ⁇ m-70 ⁇ m, more preferably 1 ⁇ m-50 ⁇ m, preferably 2 ⁇ m-40 ⁇ m, more preferably 3 ⁇ m-30 ⁇ m, more preferably 4 ⁇ m-28 ⁇ m and most preferably 5 ⁇ m - 25 ⁇ m.
• solutions of the aforementioned metals can be used as possible electrolytes.
• the metals may be present as halides, sulfates, sulfonates or fluoroborates.
• the electrolytes may contain other additives, such as complexing substances.
• a layer deposited from an aqueous electrolyte or an arbitrary portion of such a layer can also be deposited without external current.
• step a) When the layer or one of the layers of step a) is electrodeposited from nonaqueous electrolytes, it is possible to use all nonaqueous electrolytes known to those skilled in the art.
• the electrolyte preferably contains organoaluminum compounds of the general formula (I) and (II): M [(R 1 ) 3 Al- (H-Al (R 2 ) 2 ) n -R 3 ] (I) Al (R 4 ) 3 (II) wherein n is 0 or 1, M is sodium or potassium and R 1 , R 2 , R 3 , R 4 may be the same or different, wherein R 1 , R 2 , R 3 , R 4 is a C 1 -C 4 Alkyl group and a halogen-free, aprotic solvent is used as the solvent for the electrolyte.
• the electrolyte used may be a mixture of the complexes K [AlEt 4 ], Na [AlEt 4 ] and AlEt 3 .
• the molar ratio of the complexes to AlEt 3 is preferably 1: 0.5 to 1: 3 and more preferably 1: 2.
• the electrolytic deposition of the layer can be carried out using a soluble anode containing the metals intended for deposition.
• This anode may either contain the metals intended for deposition as a metal alloy, or several soluble anodes of the respective pure metals may be used. If a layer containing an aluminum / magnesium alloy is to be deposited, it is possible to use a soluble aluminum and a likewise soluble magnesium anode or an anode made of an aluminum / magnesium alloy. Electrodeposition can also be accomplished using an insoluble anode.
• the non-aqueous electrolyte electrolytic coating is preferably carried out at a temperature of 80 to 105 ° C. A temperature of the plating bath of 91-100 ° C. is preferred.
• an electrically conductive layer is applied to the substrate before the layer is applied by electroplating in step a).
• the electrical current conducting layer can be applied to the substrate by any method known to those skilled in the art.
• the layer conducting the electric current is applied to the substrate by metallization.
• step b) of the method according to the invention the temperature and / or duration of the heat treatment is selected so that at least in the boundary region between substrate and applied layer of step a) an alloy containing metal of the surface layer of the substrate and metal and / or metal alloys the deposited layer is formed.
• the temperature and / or the duration of the heat treatment should be chosen so that they are matched to the properties of the substrate and the specific coating applied.
• an intermetallic phase generally forms on the surface of the coated workpiece, in which the layer applied in step a) is converted either partially or continuously into the intermetallic phase.
• the coated substrate below / along the liquidus line of the resulting material mixture.
• the liquidus line is the melting temperature of the material mixture formed as a function of the specific composition.
• the content of aluminum in the surface layer is 100%.
• a magnesium-aluminum alloy will be formed which has a specific melting point. Now, if the temperature during the heat treatment is chosen so that just reaches the melting point of the alloy formed, or just below, so this heat treatment is to be understood as a heat treatment below / along the liquidus line of the resulting material mixture.
• the heat treatment of the coated substrate is carried out so that a liquid phase is formed on the surface of the coated substrate. This is achieved by treating at a temperature which is higher than the melting temperature of the resulting surface layer.
• the heat treatment can be carried out under a protective gas atmosphere.
• a protective gas is used which does not react with the coated material.
• the protective gas is a noble gas, such as argon.
• the heat treatment takes place in a protective gas atmosphere.
• the heat treatment can also be carried out in air.
• the layer can be subjected to a further treatment.
• a further treatment all treatment methods which are familiar to the person skilled in the art can be used.
• the treatment may be an anodic oxidation, which is preferably anodization of the layer.
• Such a treatment is useful if in step a) a layer containing aluminum was applied.
• the coated workpiece used in the method of the present invention is preferably a rack product, a bulk product, an endless product or a molded part.
• the coated workpiece is a wire, a tape, a screw, a nut, a concrete anchor, a machine component, an engine, an engine part or a turbine blade.
• the workpieces produced by the process according to the invention have an adhesive corrosion protection layer, which have a very low tendency to bimetallic corrosion and are durable. There is thus found a corrosion protection for corrosion-sensitive light metals, in particular aluminum alloy and magnesium materials, with a largely adapted to the base material resting potential.
• magnesium contact corrosion is avoided by providing the magnesium with a coating which is not an active anode to the substrate material, but at the same time is compatible with and does not attack zinc, aluminum and zinc / aluminum and aluminum / magnesium alloys attacked by these.
• the improvement of the paintability of magnesium materials is achieved by these with a substrate compatibleratibe rak which can be phosphated in the low zinc process including the bi- and trication. If so coated substrates are provided with attachment means, unexpectedly, no degradation of the coated substrates occurs. In particular, when using fasteners that are coated with aluminum / magnesium alloys and thus very hard, brittle and less ductile, the coating is not damaged during assembly and adheres firmly to the coated material. Since therefore the corresponding corrosion protection layer is not damaged, the coated material is permanently protected even after assembly against corrosion and especially against contact corrosion.
• Samples were produced at various temperatures, the material being magnesium and the coating aluminum or zinc.
• the obtained layers were characterized by solid electron microscopy and quantitatively analyzed by an electron beam microprobe.
• the heat treatment of the samples was carried out in normal atmosphere chamber furnaces under argon atmosphere.
• the samples were aluminized with aprotic solutions.
• 20% of the zinc coating was electroless and 80% electroplated
• Example 1 System magnesium electrolytically aluminized
• Example 1 A Versatile at 460 ° C
• Example 1A In the same manner as in Example 1A, the samples were this time heated to 465 ° C for a period of 9 minutes. Examination of the cooled samples showed that diffusion occurred between the magnesium material and the aluminum layer. A layer growth of more than 50 ⁇ m was determined with an aluminum content of about 35 atomic%, which indicates an intermetallic phase Al 2 Mg 3 .
• the base material magnesium and a coating of zinc were carried out with the base material magnesium and a coating of zinc.
• the basic material used was a magnesium alloy AZ91, to which a zinc layer was usually applied at 20% without external current and at 80% by electroplating.
• the layer thickness on the sample was 12 ⁇ m.
• the originally deposited pure zinc coating is characterized. Their composition can after the diffusion treatment until to 14 atomic% of magnesium. It can thus vary in composition between almost pure zinc over the two-phase region zinc / Mg 2 Zn 11 up to the pure Mg 2 Zn 11 .
• the layers which have approximately the composition MgZn 2 are regarded as layer B.
• the characterization as layer C applies to layers with a zinc content of 60 atom% zinc, which corresponds approximately to a composition Mg 2 Zn 3 .
• the layers For a depletion of the layers to values between 28 atom% and 52 atom% zinc, the layers should be declared as C '. This corresponds to a composition range which may include the intermetallic phases MgZn and Mg 7 Zn 3 or the magnesium rich eutectic.
• Layer D is used to characterize a system with less than 8 atomic% zinc that has been formed from the substrate material by zinc deposition, without any complete formation of intermetallic phases. It can therefore consist of its mixed crystal (MKr), which can contain a maximum of 2.4 atom% of zinc and in addition consist of a certain proportion of low-zinc intermetallic phases.
• MKr mixed crystal
• Table 1 Time in minutes Layer A Layer C Layer C ' Layer D 30 90 at.% Zn 60 at.% Zn 43 at.% Zn MKR 8 ⁇ m 2 ⁇ m 3 ⁇ m 15 ⁇ m 90 89 at.% Zn 60 at.% Zn 52 at.% Zn MKR 5 ⁇ m 2 ⁇ m 10 ⁇ m 15 ⁇ m 180 84 at.% Zn Layer C completely in layer C 'passed 43 at.% Zn MKR 5 .mu.m 15 ⁇ m 45 ⁇ m
• the table shows that during the heat treatment an immigration of the zinc into the magnesium takes place, in particular along the grain boundaries.
• the outer layer A receives by immigration of magnesium from the base material has a lower content of pure zinc.
• the layers B and C are formed by diffusion of the surface zinc present in the substrate or by magnesium diffusion in the opposite direction to the outside. The mixing results in a layer growth of about 300%.
• Example 2A The same series of experiments as in Example 2A was carried out at temperatures of 380 ° C. This heating times were used between 15 minutes and 90 minutes.
• Table 2 shows the structure of the layers obtained. ⁇ b> Table 2 ⁇ / b> Time in minutes Layer A Layer B Layer C ' Layer D 15 78 - 90 at.% Zn about 25 at.% Zn 1-3 at.% Zn 6 ⁇ m porous Mg 2 Zn 11 4 ⁇ m 8 ⁇ m 15 87 at.% Zn 26 + 32 at.% Zn 1-3 at.% Zn 6 ⁇ m porous Mg 2 Zn 11 4 ⁇ m 8 ⁇ m 30 8 ⁇ m porous 35 at.% Zn ⁇ 2 at.% Zn 6 ⁇ m 8 ⁇ m 30 8 ⁇ m porous 35-37 at.% Zn 2 at.% Zn 7 .mu.m 10 ⁇ m 60 Conversion of layer A into layer B 72 at.% Zn 35-37 at.% Zn 2 at.% Zn 8
• layer C also depleted of zinc after a primary enrichment phase over time. After reaching a maximum of the layer thickness, this part is thinner again. He continues to spread mainly along the grain boundaries.
• Layer D was always found with a zinc concentration of less than or equal to 3 atomic%, and would thus be homogeneous. This layer also shows a smooth transition to the zinc content of the substrate and can expect a maximum in the layer thickness as a function of time.
• alloy layers by heat treatment of zinc coatings on magnesium substrates is well possible.
• the process involves intermediate liquid phases which, at the time of completion of the heat treatment, leave the resulting alloy layers solid or at least partially solid.
• the layer composition is very complex due to the large number of possible intermetallic phases. Furthermore, it was found that the alloy layers undergo a significant increase in volume and thickness compared to the pure zinc coatings at the outset by magnesium incorporation and the associated density decrease.

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• 2004
  • 2004-08-04 EP EP04103745A patent/EP1624093A1/fr not_active Withdrawn
• 2005
  • 2005-07-27 WO PCT/EP2005/053676 patent/WO2006013184A1/fr unknown

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