EP0112439B1 - Process for the anodic oxidation of aluminium alloys - Google Patents

Abstract

1. Process for anodic oxidation of workpieces made from an aluminium alloy, particularly with high copper and/or nobler metal content, the workpieces being arranged in a moving aqueous electrolyte together with one or more cathodes and a voltage being applied mainly periodically to produce current pulses of short duration with high current conduction to the workpieces and the cathode(s), characterized in that the voltage remains switched on each time as long as a noticeable build-up of the oxide layer results and then is switched off until the Joule effect produced is mainly eliminated.

Description

The invention relates to methods for anodic oxidation of workpieces made of an aluminum alloy, in particular with a high content of copper and / or noble metals, the workpieces being arranged in a moving aqueous electrolyte together with one or more cathodes and a voltage essentially periodically Generation of short-term current pulses with high current flow is applied to the workpieces and the cathode (s).

It has long been known to provide objects made of aluminum by anodic oxidation in an aqueous electrolyte with a thick (eg 50 ₁ 1m), hard and abrasion-resistant oxide layer. The aluminum workpieces thus oxidized can be used wherever wear-resistant surfaces are necessary.

These so-called hard oxide layers have hardness values that correspond to 5 to 10 times the hardness of the base material. A large number of electrolytes have been developed for hard anodization, with which it is possible to form thick, hard and abrasion-resistant oxide layers. The best known of these electrolytes are based on sulfuric acid, whereby on the one hand pure sulfuric acid of different concentrations and on the other hand mixed electrolytes (e.g. sulfuric acid and oxalic acid) have been used.

These electrolytes have a redissolving power of the oxide layer formed. In order to reduce this redissolving power and to achieve the desired thick, hard and abrasion-resistant oxide layers, the electrolyte is cooled.

In the case of anodic oxidation, a porous oxide layer forms on the aluminum, the thickness of which increases with the oxidation time. As a result of this growth of the oxide layer, the electrical contact resistance between the electrolyte and the workpiece increases. A voltage drops across this contact resistance depending on the anodization current. This electrical power consumed by the contact resistance is converted into Joule heat. This leads to an increase in temperature in the oxide layer.

The released Joule heat is dissipated through the electrolyte, which is moved for better heat dissipation, and through the workpiece. In particular with thin workpieces and on edges with a small radius of curvature, the heat dissipation through the workpiece itself is negligible. Most of the Joule heat is therefore dissipated by the electrolyte. It should be noted, however, that as the layer thickness increases, the heat flow from the oxidized metal side to the electrolyte is increasingly impeded. In the case of anodic oxidation with direct current, the current density is therefore limited.

In the initial stage of anodization, the layer thickness is initially small, so that there is only a slight drop in electrical power. The resulting Joule heat can easily be dissipated by the electrolyte flowing past the aluminum oxide / electrolyte interface, so that no heat build-up can occur in the oxide layer. With larger layer thicknesses, the drop in performance also increases, so that increasing heating occurs. The back-dissolving power of the electrolyte increases sharply with heating, so that the formation of thick, hard layers is hindered. No more layer growth occurs when the rate of dissolution of the oxide through the electrolyte is equal to the rate of formation of the oxide. In the case of workpieces with pointed, sharp-edged parts or different wall thicknesses, local burns occur due to the high dissolution rate, since the heat generated cannot be evenly distributed and dissipated by the metal. The redissolution can lead to the destruction of the workpiece.

In addition to these thermal problems, which also occur with pure aluminum, the anodic oxidation of aluminum alloys with a high content of electrochemically nobler alloy elements, such as of copper, the following problem must also be added: The electrochemically nobler metals or metal phases embedded in the aluminum matrix form defects in the oxidation, since these metals either have a higher dissolution rate than the aluminum (such as the intermetallic phase of copper) or are not soluble in the electrolyte , such as Lead deposits, which also have a high electron conductivity compared to the oxide formed. These impurities prevent a homogeneous formation of germs, which initiate the primary growth of the oxide layer, on the aluminum matrix. In the case of intermetallic phases, which, like copper, preferentially dissolve, the current flow at the impurities increases very strongly, so that Joule heat occurs increasingly, which in turn leads to increased redissolution.

In the method of the type mentioned at the beginning of US Pat. No. 2,920,018, the anodization is carried out with pulsating direct current, the frequency of which corresponds to the mains frequency. Voltage pulses are cut out of the positive (or negative) half-waves by means of a phase control. The current flow time, i.e. the duration of a single pulse, corresponds to approximately one third of the subsequent switch-off time. During this switch-off time, practically no more Joule heat is generated; the switch-off time is used to dissipate the Joule heat generated during the previous voltage pulse.

However, the oxide layers produced by this known method are not always satisfactory. In fact, practical tests have shown that even with relatively short-term voltage pulses and, accordingly, inevitably longer breaks between the individual voltage pulses, a uniform layer growth is not always achieved. The heat dissipation at the critical points is not sufficient. Accordingly, the layer formation at critical points and with special alloys is not satisfactory.

Furthermore, US Pat. No. 3,857,766 describes a process for the anodic oxidation, especially of copper-containing ones Aluminum alloys are known in which a pulsating direct current, which has at least 6 voltage pulses per second, is superimposed on a basic direct current of low voltage. The oxidation takes place with a constant current. A mixture of sulfuric acid and oxalic acid is used as the electrolyte. The hard anodization layers produced by this process are also not always satisfactory, especially in the case of special aluminum alloys. The layer formation is poor or incomplete at critical points.

The object of the invention is to avoid the disadvantages of the known methods and to provide a method of the type mentioned at the outset with which hard oxide layers can also be produced on thin-walled and pointed, sharp-edged workpieces which have sufficient mechanical properties, in particular with regard to abrasion resistance and thickness.

This object is achieved in that the voltage remains switched on for as long as there is a noticeable build-up of the oxide layer and is then switched off until the Joule heat generated is essentially dissipated.

The duration of the voltage pulses is advantageously greater than 1/1 second, for example between 0.1 and 1.5 seconds and is therefore relatively long.

• 1. Ausbildung einer mit gelöstem Aluminium übersättigten Zone auf der Aluminiumoberfläche, hervorgerufen durch die anodische Auflösung.
• 2. Ausbildung überkritischer Keime an aktiven Stellen.
• 3. Wachstum der Keime bei gleichzeitiger Neubildung von Keimen.
• 4. Ausbildung einer Primärstruktur, d.h. vollständige Bedeckung der Metalloberfläche mit Keimen.
• 5. Zusammenwachsen der Keime zu einer homogenen Schicht.

The method according to the invention is characterized in that the duration of the switch-on time and the duration of the switch-off time of the anodizing voltage are specially adapted to the chemical and physical processes involved in building up an oxide layer. This has a decisive influence on the layer formation mechanism. As is well known, the initial process for layer formation takes place in the following stages:

• 1. Formation of a zone oversaturated with dissolved aluminum on the aluminum surface, caused by the anodic dissolution.
• 2. Training of supercritical germs in active places.
• 3. Growth of the germs with simultaneous new formation of germs.
• 4. Formation of a primary structure, ie complete covering of the metal surface with germs.
• 5. Growing together of the germs into a homogeneous layer.

In the method according to the invention, a relatively high voltage is applied during the specified period of time. With the resulting high current flow, the anodization rate is high from the start, so that despite the described, preferred high rate of dissolution of intermetallic phases (e.g. copper), the aluminum matrix is activated to form nuclei and an even layer formation is achieved even on critical parts.

During the electrochemical process of anodic oxidation, the electrolyte at the electrolyte / metal surface interface becomes depleted of the ions necessary for the oxidation, so that a strong concentration polarization occurs. In a sulfuric acid electrolyte, the aluminum is oxidized via the suifations. The following reactions take place:

It is clear from the reaction ^equations that the electrolyte at the electrolyte / metal interface ^becomes poor at S0 ₄ ^2-- ions and locally accumulates at S0 ₃ ^2-- ions.

After the current is switched off, a concentration equalization will occur due to the moving electrolyte. The reduction in the concentration gradient can be demonstrated by the decay of the concentration polarization using an electron beam oscillograph. It was found that the anodization voltage does not immediately drop to a value of approximately 0 after the current is switched off, but that the reduction in the potential of approximately 3 to 5 volts takes a time of 0.1 to 0.5 seconds.

On the other hand, however, an approximately constant voltage does not appear until 0.2 to 0.8 seconds after the current has been switched on, as can also be verified by means of an electron beam oscillograph.

The specified, in comparison to the prior art, long period of time during which the voltage pulses are present is extremely favorable for the construction of an oxide layer. The interruption of the voltage between two voltage pulses should last 0.1 to 2 seconds, advantageously this time period is between 0.1 and 1 seconds. The ratio of the duration of a voltage pulse to the duration of a switch-off time should be 0.5 to 5.

With these conditions, optimal anodization with a very high current density is achieved with all aluminum alloys, but particularly advantageously with alloys with high electrochemically more noble alloy additives. Surprisingly good oxidation results were achieved when several (at least five to a maximum of one hundred) samples were connected in parallel using the method according to the invention.

The temperature of the electrolyte or of the workpiece, in particular at critical points, is advantageously measured. Only after the temperature measured in this way has dropped back to a predetermined value is a renewed voltage pulse applied. Depending on the need, the switch-off times are longer or shorter at the beginning or at the end of the oxidation.

The hard anodization according to the method according to the invention can be used for workpieces made of cast or wrought alloys. In addition, workpieces made of sintered aluminum can also be coated satisfactorily with high alloy proportions of electrochemically more noble elements, such as copper, using the method according to the invention. Due to the high rate of formation of the oxide layer and the reduced redissolution, it succeeds on the porous Sintered metal material to form relatively homogeneous layers.

Further features of the invention result from the remaining claims.

An embodiment of the invention is explained in more detail below and described with reference to the drawing.

• Figur 1 einen Längsschnitt durch eine Elektrolyt-Wanne zur Durchführung des erfindungsgemäßen Verfahrens,
• Figur 2 ein Zeitdiagramm der zwischen Kathode und Anode in einer Vorrichtung gemäß Figur 1 angelegten Spannung,
• Figur 3 eine graphische Darstellung des zeitlichen Verlaufs des durch den Elektrolyten fließenden Stromes. Der Zeitmaßstab stimmt mit dem Zeitmaßstab in Figur 2 überein.
• Figur 4 eine graphische Darstellung des zeitlichen Verlaufs der Schichtdicke, der Zeitmaßstab stimmt mit den Figuren 2 und 3 überein, und
• Figur 5 eine graphische Darstellung des zeitlichen Verlaufs der Temperaturänderung im Elektrolyten bei gleichem Zeitmaßstab wie die Figuren 2 bis 4.

In this show:

• FIG. 1 shows a longitudinal section through an electrolyte trough for carrying out the method according to the invention,
• FIG. 2 shows a time diagram of the voltage applied between cathode and anode in a device according to FIG. 1,
• Figure 3 is a graphical representation of the time course of the current flowing through the electrolyte. The time scale corresponds to the time scale in FIG. 2.
• FIG. 4 shows a graphical representation of the course over time of the layer thickness, the time scale corresponds to that of FIGS. 2 and 3, and
• 5 shows a graphical representation of the time course of the temperature change in the electrolyte at the same time scale as FIGS. 2 to 4.

FIG. 1 shows the device used to carry out the method according to the invention. An electrolyte pan 1 receives an electrolyte bath 2 with 180 g / l sulfuric acid and 15 g / l oxalic acid. An anode 3 and a cathode 4 are immersed in this electrolyte bath 2 and connected to a voltage supply device 7 via leads 5 and 6. The anode 3 is composed of an anode holder 8 and the workpieces 9 to be treated.

Electrolyte liquid is constantly sucked out of the electrolyte bath 2 via a pipeline 10, which leads to a pump 11, and is returned to the electrolyte bath 2 via an electrolyte guide tube 12, the returned current being directed towards the workpiece 9.

Furthermore, a cooler 13 (heat exchanger) of a cooling unit 14 projects into the electrolyte bath 2. This cooling unit 14 is controlled by a contact thermometer 15, which also extends into the electrolyte bath 2.

The voltage supply device 7 supplies a rectified output voltage over a time t _i -t ₂ , as is shown graphically in FIG. 2. As can be seen in the figure, this voltage can be rectangular or have a ripple of any technically possible voltage form. This voltage U is applied to the cathode 4 and the anode 3 via the leads 5 and 6 and causes a current to flow through the electrolyte. The time course of this current flow i is shown graphically in FIG.

As FIG. 2 shows, the voltage supplied by the voltage supply device 7 is constant during each individual voltage pulse, and it also remains at the same value for the entire anodization time. The power supply device 7 has a current limitation which limits the current during a time t to t ₄ , so that the current pulses are also approximately rectangular. If there is no current limitation, the current curve begins to decrease immediately after the time t ₄ . A DC voltage between 20 and 60 volts generated within the voltage supply device 7 is expediently switched on and off by a switch in such a way that the voltage curve shown in FIG. 2 results. According to the invention, the switch-on times t to t ₂ are between 0.1 and 3 seconds, the pause times t ₂ to t ₃ are between 0.1 and 2 seconds. The ratio of operating times to break times is approximately in the range from 0.5 to 5, in the exemplary embodiment shown this ratio is 2.

The first current pulses are constant up to time t _4, as already explained and due to the automatic current limitation. Due to the structure of the oxide layer, as can be seen from FIG. 4, and the associated increase in the volume resistance of this oxide layer, the current i decreases continuously with time t ₄ , since the voltage U is constant according to FIG. Accordingly, the layer thickness (FIG. 4) increases almost constantly during the time t until t ₄ . The layer thickness increase will decrease as the current i decreases.

With the method according to the invention, as shown in FIG. 3, anodization can also be carried out, for example, in the time t until t ₄ over the entire anodization time with constant current of the current pulses or with constant voltage starting with time t ₄ to t ₅ .

FIG. 5 shows the temperature increase compared to the initial state. The temperature increase in the layer increases during the time t until t ₄ and then fluctuates around a constant value.

The curves shown in FIGS. 2 to 5 are intended to illustrate the pulse current technology according to the invention purely schematically. The shape of the voltage and the current, especially the rectangular shape, does not always correspond to what is technically achievable. Frequently, the voltage and current increases are not linear from 0 to the nominal value, and the drops at the end of the pulse are not always as sharp as shown. Many experiments have shown that the rise and fall times are around 1/10 of a second. However, the curve shape has no influence on the anodization result.

The method according to the invention makes it possible to anodize very thin-walled workpieces such as those with sharp edges and corners in spite of very high pulse current densities of at most 80 A / dm ² without any signs of combustion. The process according to the invention is characterized in that the anodization is started directly with a high current density. However, the current can optionally be limited as shown in Figure 3.

In order to determine the differences and advantages of the method according to the invention compared to the known method, a large number of comparative studies were carried out.The aim of these experiments was to determine the influence of the under constant anodization conditions (such as electrolyte type and composition, bath dimensions, electrolyte temperature, bath movement, etc.) Examine anodizing current for layer formation and layer quality.

Technical materials of various aluminum alloys, in particular aluminum alloys with a higher copper content, served as the anodization material.

example 1

• 1. AICuMg 2: Cu 4,3 % Mg 1,6 %
• 2. AICuMgPb: Cu 4,4 % Mg 1,4 % Pb 1,5 %
• 3. AICu4Ni2Mg: Cu 4,1 % Ni 2,₀ % Mg 1.5 %
• 4. AICu5Ni1,5: Cu 5,3 % Ni 1,6 %
• 5. AISi17CuMg: Si 18,8 % Cu 1;4 % Mg 1,3 %
• 6. AlMgSi1: Si 1,1 % Mg 0,8 % untersucht.

For this example, different workpieces made of different materials such as

• 1. AICuMg 2: Cu 4.3% Mg 1.6%
• 2.AICuMgPb: Cu 4.4% Mg 1.4% Pb 1.5%
• 3.AICu4Ni2Mg: Cu 4.1% Ni 2, ₀ % Mg 1.5%
• 4.AICu5Ni1.5: Cu 5.3% Ni 1.6%
• 5. AISi17CuMg: Si 18.8% Cu 1; 4% Mg 1.3%
• 6. AlMgSi1: Si 1.1% Mg 0.8% examined.

• 1. Gleichstrom-Verfahren. Die Versuche wurden mit reinem Gleichstrom bei einer Restwelligkeit von etwa 3 % unter Stromkonstanz, durch automatische Nachregelung der Anodisationsspannung durchgeführt.
• 2. Polarisierter pulsierender Gleichstrom. Die Stromimpuls dauer betrug bei einer Frequenz von 50 Hz etwa 50 %. Die Stromkonstanz wurde durch Handregelung der Spannung eingehalten. Dieses zweite Verfahren ist dem aus der US-PS-2 920 018 bekannten Verfahren ähnlich.
• 3. Wechselstrom-überlagerter Gleichstrom. Die Versuche wurden mit Stromkonstanz durchgefuhrt. Bei einer Frequenz von 50 Hz betrug die Restwelligkeit etwa 30 %. Dieses dritte Verfahren entspricht im wesentlichen dem aus der US-PS-3 857 766 bekannten Verfahren.
• 4. Impulsstromverfahren nach der Erfindung. Diese Versuche wurden mit konstanter Spannung durchgefuhrt, die jeweils manuell auf den gewunschten Sollwert eingestellt wurde. Das Verhältnis der Einschalt- zu den Ausschaltzeiten betrug für alle Versuche 2,0. Gemessen wurde der Effektivstrom.

The following four methods were used:

• 1. DC method. The tests were carried out with pure direct current with a residual ripple of about 3% under constant current, by automatic readjustment of the anodization voltage.
• 2. Polarized pulsating direct current. The current pulse duration was about 50% at a frequency of 50 Hz. The constant current was maintained by manual control of the voltage. This second method is similar to the method known from US Pat. No. 2,920,018.
• 3. AC superimposed DC. The tests were carried out with constant current. At a frequency of 50 Hz, the ripple was about 30%. This third method corresponds essentially to the method known from US Pat. No. 3,857,766.
• 4. Pulse current method according to the invention. These tests were carried out with constant voltage, which was set manually to the desired setpoint. The ratio of the switch-on and switch-off times was 2.0 for all tests. The effective current was measured.

Die in den Tabellen 1 und 2 zusammengefaßten Versuchsergebnisse zeigen deutlich, daß unter den gewählten Versuchsbedingungen eine Anodisation von Aluminiumlegierungen mit hohen Gehalten an elektrochemisch edleren Metallen nicht möglich ist, wenn nach dem ersten Verfahren, also mit Gleichstrom, gearbeitet wird.Typical measurement results are summarized in the following three tables.

The test results summarized in Tables 1 and 2 clearly show that under the chosen test conditions an anodization of aluminum alloys with high levels of electrochemically noble metals is not possible if the first method, ie with direct current, is used.

The method according to the invention proves to be particularly advantageous. It is the only process that makes it possible to oxidize all workpieces without combustion phenomena with a high current density and short anodization times. Furthermore, the tables show that the coating qualities can be improved with the method according to the invention, even of such Al alloys with low contents of more noble electrochemical metals, as shown in Table 3.

Example 2

In addition to the described method of discontinuous anodization in baths, the method according to the invention proves to be particularly advantageous for continuous anodization. Continuous anodization is understood to mean the anodization of strips or workpieces which are drawn continuously through the anodization bath and, if appropriate, through rinsing or post-compression baths.

While in the case of discontinuous anodization, as described, the current density is kept constant by regulating the voltage using the DC method, this is not possible in the case of continuous anodization. With continuous anodization, there is only the possibility of keeping the voltage constant, while the current density when the strip or workpiece enters is very high and decreases significantly with the dwell time due to the build-up of an oxide layer, so that the current density shortly before it leaves the bath is very low. The applied voltage is thus strongly limited by the current density at the beginning of the entry of the strip or workpiece into the bath, in order to avoid burns, which is due to the restriction of the removal of the Joule heat by means of the moving electrolyte.

On the other hand, the maximum achievable layer thickness is also limited by the limitation of the applied voltage. With the method according to the invention, as described, it could be shown that very high voltages and the resulting very high current densities are possible.

When the strip or workpiece enters the bath, the current density is very high due to the high voltage applied and can have values of up to 80 A / dm ² . In the subsequent pause in the current, however, the Joule heat is removed by the moving electrolyte and, as described, the polarization voltages are also reduced.

The process of the pulse current method according to the invention thus makes it possible to reduce the anodization time considerably, so that the throughput in existing strip anodization systems can be increased considerably if the same layer thickness is to be achieved.

It is also possible to achieve thick layers of more than 30 11 m, as is required in most cases for hard anodization.

With the method according to the invention, aluminum alloys with high proportions of electrochemically more noble alloying elements can also be coated particularly advantageously continuously.

For the experiment, an aluminum foil made of AIMg1 (dimensions: thickness 1.0 mm, width 300 mm, length 500 mm) was placed in a moving electrolyte at a lowering speed of 0.2 m / min. introduced and after completely lowering the film in the electrolyte, the anodization continued for 2 minutes. The applied voltage was 35 volts and the current switch-on time was 0.4 seconds, the switch-off time 0.2 seconds.

The sample film showed no burns despite the high initial current density of approximately 90 A / dm ² . The layer thickness was 40 to 55 μm, the part of the film with the longest anodization time naturally having the greatest layer thickness.

Claims (11)

1. Process for anodic oxidation of workpieces made from an aluminium alloy, particularly with high copper and/or nobler metal content, the workpieces being arranged in a moving aqueous electrolyte together with one or more cathodes and a voltage being applied mainly periodically to produce current pulses of short duration with high current conduction to the workpieces and the cathode(s), characterised in that the voltage remains switched on each time as long as a noticeable build-up of the oxide layer results and then is switched off until the Joule effect produced is mainly eliminated.

2. Process according to Claim 1, characterised in that a voltage pulse of 10 to 80 V, particularly 20 to 60 V is applied.

3. Process according to Claim 1 or 2, characterised in that the duration of the voltage pulse is 0.1 to 3 s, particularly 0,1 to 1,5 s.

4.Process according to one of Claims 1 to 3, characterised in that the duration of the switch-off time between two voltage pulses is 0,1 to 2 s, particularly 0,1 to 1 s.

5. Process according to one of Claims 1 to 4, characterised in that the ratio of duration of a voltage pulse to the duration of a switch-off time is 0,5 to 5.

6. Process according to one of Claims 1 to 5, characterised in that the current density during a voltage pulse is kept constant.

7. Process according to one of Claims 1 to 6, characterised in that the current density in the duration of the anodic oxidation is kept constant.

8. Process according to one of Claims 1 to 5, characterised in that the voltage during a voltage pulse and in the duration of the anodic oxidation is kept constant.

9. Process according to one of Claims 1 to 8, characterised in that first the current density and then the voltage is kept constant in the duration of the anodic oxidation

10. Process according to Claim 8, characterised in that the workpieces, particularly material in strip form, are made to pass through the electrolytic bath.

11. Process according to one of Claims 1 to 10, characterised in that the voltage after a voltage pulse remains switched off until the temperature, preferably measured on the workpiece, has fallen below a threshold value.

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