EP0607579A1 - Process for anodic oxidation - Google Patents

Process for anodic oxidation Download PDF

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Publication number
EP0607579A1
EP0607579A1 EP93120288A EP93120288A EP0607579A1 EP 0607579 A1 EP0607579 A1 EP 0607579A1 EP 93120288 A EP93120288 A EP 93120288A EP 93120288 A EP93120288 A EP 93120288A EP 0607579 A1 EP0607579 A1 EP 0607579A1
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EP
European Patent Office
Prior art keywords
approximately
electrolyte
minutes
h2so4
voltage
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EP93120288A
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German (de)
French (fr)
Inventor
Erich Kock
Volker Muss
Christoph Dr. Matz
Fekko De Wit
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Airbus Defence and Space GmbH
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Deutsche Aerospace AG
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Publication of EP0607579A1 publication Critical patent/EP0607579A1/en
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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
    • C25D11/04—Anodisation of aluminium or alloys based thereon
    • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
    • C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids

Definitions

  • the invention relates to a method for the anodic oxidation of structural elements made of aluminum or magnesium materials in a phosphorus-containing aqueous electrolyte.
  • Such a method has become known from DE 27 05 652 C2 and is preferably used in aerospace engineering for pretreating structural elements made of aluminum or high-strength aluminum alloys which are to be bonded to one another or which are combined with an organic, metallic or inorganic Coating should be provided.
  • the oxide skin created on the workpiece surface serves the purpose To improve the adhesion properties of this surface in the long term, the aim being to produce the softest possible, thin, but above all porous oxide layer, through which the effective surface is enlarged and the greatest possible number of active centers for the formation of adhesive bonds is created.
  • anodizing processes are also used in practice for pretreating workpieces made of aluminum or aluminum alloys, in which electrolytes based on chromic acid or sulfuric acid are used.
  • the oxide layers produced in these processes can be compressed and are then also suitable as sole corrosion protection.
  • Chromic acid anodizing has the advantage that almost optimal surfaces can be achieved with it in terms of morphology and the layer thickness of the oxide layer, but at the same time this method is relatively complex and time-consuming to carry out and is problematic in terms of occupational hygiene and environmental issues, since working with Chromium salts and chromic acid are associated with an increased health risk. Furthermore, this process must be carried out at an elevated temperature.
  • the oxide layers produced during phosphoric acid anodizing are generally mechanically unstable and to one Supplemented by organic coating materials. This means that the workpieces made of aluminum or aluminum alloys pretreated with these methods can only be processed and inspected without contact until the primer used for the subsequent bonding has hardened.
  • the object of the invention is to develop a method of the type mentioned in such a way that it can achieve an optimal surface finish similar to chromic acid anodizing, but at the same time offers much easier handling, shortened process duration and costs, and significantly improved environmental protection.
  • the invention solves this problem by a method having the characterizing features of claim 1.
  • the use of a mixed electrolyte of phosphoric and sulfuric acid which is provided according to the invention, creates a method with which oxide layers can be produced on workpieces made of aluminum and its alloys, which Oxide layers that can be achieved by chromic acid anodizing are not only comparable in terms of morphology and layer thickness, but also have good long-term stability.
  • the oxide layers which can be produced using the method according to the invention are notable for high adhesive strength and ductility and have good corrosion resistance.
  • Another advantage of the method according to the invention is that, by varying the process parameters, the same electrolyte can be used for the production of a compressed anodized layer, as corrosion protection and for the pretreatment in a subsequent coating or gluing. It can ideally do this in each case machining material to be adjusted.
  • the invention will be explained in more detail below on the basis of an exemplary embodiment, the figure representing the chronological course of the anodizing process in a voltage-time diagram.
  • the workpiece made of a high-strength aluminum alloy which is to be prepared for subsequent gluing or coating, for example with an organic substance, is first subjected to a cleaning and activation step using a commercially available cleaning agent and an oxidizing acid bath, for example made of nitric acid (HNO3), after each each of these steps is rinsed with water.
  • a cleaning and activation step using a commercially available cleaning agent and an oxidizing acid bath, for example made of nitric acid (HNO3), after each each of these steps is rinsed with water.
  • HNO3 nitric acid
  • the workpiece is then placed in an aqueous electrolyte which has a concentration of about 100 g / l sulfuric acid (H2SO4) and phosphoric acid (H3PO4), so that these two components are in a mixing ratio of about 1: 1.
  • the bath temperature is about 27 o C.
  • the workpiece to be oxidized is connected to the positive pole of a direct current source, while an electrode made of pure aluminum, lead or a high-alloy steel is used as the cathode.
  • the applied voltage is increased over a period of about three minutes (t 1) from zero to a value between 15 and 20 volts. This voltage value is then kept constant for about 15 minutes before the voltage is switched off. The workpiece is then removed from the bath and the residue of the The electrolyte is rinsed off in about two minutes.
  • the oxide layer produced in this way has a thickness of about 3 to 3.5 micrometers, is of high ductility and has a high pore volume and thus a large number of active centers for the formation of adhesive bonds.
  • a salt spray test with aging in accordance with DIN 50021 confirms the good long-term durability of this layer.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • ing And Chemical Polishing (AREA)

Abstract

In a process for anodic oxidation of structural elements (structural components), made of aluminium materials or magnesium materials, in an aqueous electrolyte, said electrolyte contains phosphoric acid (H3PO4) and sulphuric acid (H2SO4) in approximately equal proportions, the concentrations of the two acids in the electrolyte preferably being approximately 100 g/l, the bath temperature being approximately 27@C, and the voltage applied first being increased, over a period of approximately 3 minutes, from 0 to a value between 15 and 20 volts and then being kept constant for approximately 15 minutes. <IMAGE>

Description

Die Erfindung betrifft ein Verfahren zur anodischen Oxidation von Strukturelementen aus Aluminium- oder Magnesiumwerkstoffen in einem phosphorsäurehaltigen wäßrigen Elektrolyten.The invention relates to a method for the anodic oxidation of structural elements made of aluminum or magnesium materials in a phosphorus-containing aqueous electrolyte.

Ein derartiges Verfahren ist aus der DE 27 05 652 C2 bekannt geworden und wird vorzugsweise in der Luft- und Raumfahrttechnik zur Vorbehandlung von Strukturelementen aus Aluminium oder hochfesten Aluminiumlegierungen angewandt, die durch eine Klebung miteinander verbunden werden sollen oder die mit einer organischen, metallischen oder anorganischen Beschichtung versehen werden sollen. Die dabei auf der Werkstückoberfläche erzeugte Oxidhaut dient dazu, die Adhäsionseigenschaften dieser Oberfläche nachhaltig zu verbessern, wobei angestrebt wird, eine möglichst weiche, dünne, vor allem aber porige Oxidschicht zu erzeugen, durch die die wirksame Oberfläche vergrößert und eine möglichst große Zahl aktiver Zentren für die Ausbildung von adhäsiven Bindungen geschaffen wird.Such a method has become known from DE 27 05 652 C2 and is preferably used in aerospace engineering for pretreating structural elements made of aluminum or high-strength aluminum alloys which are to be bonded to one another or which are combined with an organic, metallic or inorganic Coating should be provided. The oxide skin created on the workpiece surface serves the purpose To improve the adhesion properties of this surface in the long term, the aim being to produce the softest possible, thin, but above all porous oxide layer, through which the effective surface is enlarged and the greatest possible number of active centers for the formation of adhesive bonds is created.

Neben dem Anodisieren in phosphorsäurehaltigen wäßrigen Elektrolyten werden in der Praxis auch Anodisierverfahren zur Vorbehandlung von Werkstücken aus Aluminium bzw. Aluminiumlegierungen eingesetzt, bei denen Elektrolyten auf der Basis von Chromsäure oder Schwefelsäure verwendet werden. Die bei diesen Verfahren erzeugten Oxidschichten können verdichtet werden und sind dann auch als alleiniger Korrosionsschutz geeignet.In addition to anodizing in aqueous electrolytes containing phosphoric acid, anodizing processes are also used in practice for pretreating workpieces made of aluminum or aluminum alloys, in which electrolytes based on chromic acid or sulfuric acid are used. The oxide layers produced in these processes can be compressed and are then also suitable as sole corrosion protection.

Für Strukturelemente, die anschließend miteinander verklebt werden sollen, werden in der Regel entweder Elektrolyten auf der Basis von Chromsäure oder von Phosphorsäure verwendet. Das Chromsäureanodisieren hat dabei zwar den Vorteil, daß sich mit ihm hinsichtlich der Morphologie und der Schichtdicke der Oxidschicht nahezu optimale Oberflächen erzielen lassen, zugleich ist dieses Verfahren aber relativ komplex und zeitaufwendig in der Durchführung und in arbeitshygienischer und ökologischer Hinsicht problematisch, da das Arbeiten mit Chromsalzen und Chromsäure mit einem erhöhten Gesundheitsrisiko verbunden ist. Ferner muß dieses Verfahren bei erhöhter Temperatur durchgeführt werden.For structural elements that are subsequently to be glued to one another, either electrolytes based on chromic acid or phosphoric acid are generally used. Chromic acid anodizing has the advantage that almost optimal surfaces can be achieved with it in terms of morphology and the layer thickness of the oxide layer, but at the same time this method is relatively complex and time-consuming to carry out and is problematic in terms of occupational hygiene and environmental issues, since working with Chromium salts and chromic acid are associated with an increased health risk. Furthermore, this process must be carried out at an elevated temperature.

Diese Problematik wird bei der Verwendung phosphorsäurehaltiger Elektrolyten zwar vermieden, jedoch sind die beim Phosphorsäureanodisieren erzeugten Oxidschichten in der Regel mechanisch instabil und auf eine Ergänzung durch organische Beschichtungsstoffe angewiesen. Dies führt dazu, daß die mit diesen Verfahren vorbehandelten Werkstücke aus Aluminium bzw. Aluminiumlegierungen bis zum Aushärten des für die nachfolgende Klebung verwendeten Primers nur berührungslos verarbeitet und inspiziert werden können.Although this problem is avoided when using electrolytes containing phosphoric acid, the oxide layers produced during phosphoric acid anodizing are generally mechanically unstable and to one Supplemented by organic coating materials. This means that the workpieces made of aluminum or aluminum alloys pretreated with these methods can only be processed and inspected without contact until the primer used for the subsequent bonding has hardened.

Aufgabe der Erfindung ist es, ein Verfahren der eingangs genannten Art so auszubilden, daß sich mit ihm eine dem Chromsäureanodisieren ähnliche optimale Oberflächenbeschaffenheit erzielen läßt, daß dabei aber zugleich eine wesentlich einfachere Handhabung, verkürzte Prozeßdauer und -kosten sowie einen wesentlich verbesserten Umweltschutz bietet.The object of the invention is to develop a method of the type mentioned in such a way that it can achieve an optimal surface finish similar to chromic acid anodizing, but at the same time offers much easier handling, shortened process duration and costs, and significantly improved environmental protection.

Die Erfindung löst diese Aufgabe durch ein Verfahren mit den kennzeichnenden Merkmalen des Patentanspruchs 1. Durch die erfindungsgemäß vorgesehene Verwendung eines Mischelektrolyten aus Phosphor- und Schwefelsäure wird dabei ein Verfahren geschaffen, mit dem sich auf Werkstücken aus Aluminium und dessen Legierungen Oxidschichten erzeugen lassen, die den mittels Chromsäureanodisieren erzielbaren Oxidschichten nicht nur hinsichtlich der Morphologie und Schichtdicke vergleichbar sind, sondern die zugleich eine gute Langzeitstabilität besitzen. Die mit dem Verfahren nach der Erfindung herstellbaren Oxidschichten zeichnen sich durch eine hohe Haftfestigkeit und Duktilität aus und weisen eine gute Korrosionsbeständigkeit auf. Ein weiterer Vorteil des erfindungsgemäßen Verfahrens besteht darin, daß sich durch eine Variation der Prozeßparameter der gleiche Elektrolyt für die Erzeugung einer verdichteten Eloxalschicht, als Korrosionsschutz und für die Vorbehandlung bei einer nachfolgenden Beschichtung oder Klebung verwenden läßt. Es kann dabei in idealer Weise dem jeweils zu bearbeitenden Werkstoff angepaßt werden. Vorteilhafte Weiterbildungen des Verfahrens nach der Erfindung im Hinblick auf eine Optimierung der Prozeßparameter sind in den weiteren Ansprüchen angegeben.The invention solves this problem by a method having the characterizing features of claim 1. The use of a mixed electrolyte of phosphoric and sulfuric acid, which is provided according to the invention, creates a method with which oxide layers can be produced on workpieces made of aluminum and its alloys, which Oxide layers that can be achieved by chromic acid anodizing are not only comparable in terms of morphology and layer thickness, but also have good long-term stability. The oxide layers which can be produced using the method according to the invention are notable for high adhesive strength and ductility and have good corrosion resistance. Another advantage of the method according to the invention is that, by varying the process parameters, the same electrolyte can be used for the production of a compressed anodized layer, as corrosion protection and for the pretreatment in a subsequent coating or gluing. It can ideally do this in each case machining material to be adjusted. Advantageous developments of the method according to the invention with a view to optimizing the process parameters are specified in the further claims.

Nachfolgend soll die Erfindung anhand eines Ausführungsbeispiels näher erläutert werden, wobei die Figur in einem Spannungs-Zeit-Diagramm den zeitlichen Ablauf des Anodisiervorgangs wiedergibt. Das für eine nachfolgende Klebung oder eine Beschichtung, beispielsweise mit einer organischen Substanz, vorzubereitende Werkstück aus einer hochfesten Aluminiumlegierung wird zunächst einem Reinigungs- und Aktivierungsschritt mit einem handelsüblichen Reinigungsmittel sowie einem oxidierenden sauren Bad, beispielsweise aus Salpetersäure (HNO₃) unterzogen, wobei es nach jedem dieser Schritte jeweils mit Wasser abgespült wird. Anschließend wird das Werkstück in einen wäßrigen Elektrolyten gebracht, der eine Konzentration von jeweils etwa 100 g/l Schwefelsäure (H₂SO₄) und Phosphorsäure (H₃PO₄) aufweist, so daß diese beiden Komponenten in einem Mischungsverhältnis von etwa 1 : 1 vorliegen. Die Badtemperatur beträgt etwa 27oC.The invention will be explained in more detail below on the basis of an exemplary embodiment, the figure representing the chronological course of the anodizing process in a voltage-time diagram. The workpiece made of a high-strength aluminum alloy, which is to be prepared for subsequent gluing or coating, for example with an organic substance, is first subjected to a cleaning and activation step using a commercially available cleaning agent and an oxidizing acid bath, for example made of nitric acid (HNO₃), after each each of these steps is rinsed with water. The workpiece is then placed in an aqueous electrolyte which has a concentration of about 100 g / l sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄), so that these two components are in a mixing ratio of about 1: 1. The bath temperature is about 27 o C.

Das zu oxidierende Werkstück wird mit dem Pluspol einer Gleichstromquelle verbunden, während als Kathode eine Elektrode aus Reinaluminium, Blei oder einem hochlegierten Stahl verwendet wird.The workpiece to be oxidized is connected to the positive pole of a direct current source, while an electrode made of pure aluminum, lead or a high-alloy steel is used as the cathode.

Wie in der Figur dargestellt, wird die angelegte Spannung über einen Zeitraum von etwa drei Minuten (t₁) von Null auf einen Wert zwischen 15 und 20 Volt erhöht. Dieser Spannungswert wird anschließend für etwa 15 Minuten konstant gehalten, bevor die Spannung abgeschaltet wird. Das Werkstück wird dann dem Bad entnommen und der auf seiner Oberfläche verbleibende Rückstand des Elektrolyten wird innerhalb von etwa zwei Minuten abgespült.
die auf diese Weise erzeugte Oxidschicht besitzt eine Dicke von etwa 3 bis 3.5 Mikrometern, ist von hoher Duktilität und weist ein hohes Porenvolumen und damit eine Vielzahl aktiver Zentren für die Ausbildung adhäsiver Bindungen auf. Ein Salzsprühtest mit einer Auslagerung nach DIN 50021 bestätigt die gute Langzeitbeständigkeit dieser Schicht.As shown in the figure, the applied voltage is increased over a period of about three minutes (t 1) from zero to a value between 15 and 20 volts. This voltage value is then kept constant for about 15 minutes before the voltage is switched off. The workpiece is then removed from the bath and the residue of the The electrolyte is rinsed off in about two minutes.
the oxide layer produced in this way has a thickness of about 3 to 3.5 micrometers, is of high ductility and has a high pore volume and thus a large number of active centers for the formation of adhesive bonds. A salt spray test with aging in accordance with DIN 50021 confirms the good long-term durability of this layer.

Im Rahmen der Erfindung ist es dabei selbstverständlich auch möglich, durch die Variation einzelner Prozeßparameter, wie Konzentration, Temperatur, Spannung oder Stromdichte eine für den jeweiligen Anwendungsfall optimale Konstellation zu erzeugen. Auch kann das in der Figur gezeigte Spannungs-Zeit-Profil durch ein entsprechendes auf den Werkstoff und die Prozeßparameter abgestimmtes Stromdichte-Zeit-Profil ersetzt werden. Neben Aluminium und dessen Legierungen kann es auch zur Vorbehandlung von Strukturelementen aus anderen Werkstoffen, beispielsweise Magnesium oder dessen Legierungen, verwendet werden.Within the scope of the invention it is of course also possible to generate an optimal constellation for the respective application by varying individual process parameters such as concentration, temperature, voltage or current density. The voltage-time profile shown in the figure can also be replaced by a corresponding current density-time profile that is matched to the material and the process parameters. In addition to aluminum and its alloys, it can also be used to pretreat structural elements made from other materials, such as magnesium or its alloys.

Claims (7)

Verfahren zur anodischen Oxidation von Strukturelementen aus Aluminium- oder Magnesiumwerkstoffen in einem phosphorsäurehaltigen wäßrigen Elektrolyten, dadurch gekennzeichnet, daß der Elektrolyt einen Zusatz an Schwefelsäure (H₂SO₄) enthält.Process for the anodic oxidation of structural elements made of aluminum or magnesium materials in a phosphorus-containing aqueous electrolyte, characterized in that the electrolyte contains an addition of sulfuric acid (H₂SO₄). Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Mischungsverhältnis von Phosphorsäure (H₃PO₄) und Schwefelsäure (H₂SO₄) etwa 1 : 1 beträgt.A method according to claim 1, characterized in that the mixing ratio of phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄) is about 1: 1. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß die Konzentration der Phosphorsäure (H₃PO₄) und Schwefelsäure (H₂SO₄) jeweils etwa 100 g/l beträgt.A method according to claim 2, characterized in that the concentration of phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄) is each about 100 g / l. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß die Temperatur des Elektrolyten bei etwa 27oC liegt.Method according to one of claims 1 to 3, characterized in that the temperature of the electrolyte is about 27 o C. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß die angelegte Spannung zwischen 15 und 20 Volt liegt.Method according to one of claims 1 to 4, characterized in that the applied voltage is between 15 and 20 volts. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß die Spannung über einen Zeitraum von etwa 15 Minuten annähernd konstant gehalten wird.A method according to claim 5, characterized in that the voltage is kept approximately constant over a period of about 15 minutes. Verfahren nach Anspruch 6, dadurch gekennzeichnet, daß die Spannung innerhalb eines Zeitraumes von etwa drei Minuten kontinuierlich von null Volt auf den konstanzen Endwert erhöht wird.A method according to claim 6, characterized in that the voltage is continuously increased from zero volts to the constant final value within a period of about three minutes.
EP93120288A 1992-12-19 1993-12-16 Process for anodic oxidation Withdrawn EP0607579A1 (en)

Applications Claiming Priority (2)

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DE19924243164 DE4243164A1 (en) 1992-12-19 1992-12-19 Anodic oxidation process
DE4243164 1992-12-19

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Cited By (2)

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US8889226B2 (en) 2011-05-23 2014-11-18 GM Global Technology Operations LLC Method of bonding a metal to a substrate
WO2017183965A1 (en) 2016-04-18 2017-10-26 Fokker Aerostructures B.V. Method of anodizing an article of aluminium or alloy thereof

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US8309237B2 (en) 2007-08-28 2012-11-13 Alcoa Inc. Corrosion resistant aluminum alloy substrates and methods of producing the same
CN110114517B (en) * 2016-08-17 2022-12-13 席勒斯材料科学有限公司 Method for producing thin functional coatings on light alloys

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DATABASE WPI Week 7824, Derwent World Patents Index; AN 78-43046A *
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8889226B2 (en) 2011-05-23 2014-11-18 GM Global Technology Operations LLC Method of bonding a metal to a substrate
WO2017183965A1 (en) 2016-04-18 2017-10-26 Fokker Aerostructures B.V. Method of anodizing an article of aluminium or alloy thereof
CN109415836A (en) * 2016-04-18 2019-03-01 福克航空结构公司 The anode oxidation method of aluminium or its alloy product
US11326269B2 (en) 2016-04-18 2022-05-10 Fokker Aerostructures B.V. Anodizing an article of aluminum or alloy thereof

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DE4243164A1 (en) 1994-06-23
JPH06299393A (en) 1994-10-25

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