DE3244217A1 - METHOD FOR ANODICALLY OXYDATING ALUMINUM 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

PATENT ADVOCATE DR. HANS-GÜNTHER EGGERT, DIPLOMA CHEMIST

5 COLOGNE 41, Räderscheidtstrasse

■ 3-

Cologne, November 29, 1982

Electro Chemical Engineering GmbH., Poststrasse 9, Zug / Switzerland

"Process for anodic oxidation of aluminum alloys"

PATENT ADVOCATE DR. HANS-GUNTHER EGGERT ₁ DIPLOMA CHEMIST

5 COLOGNE 51, OBERLÄNDER UFER 90

■ "■■■ 'j" "*"

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 in a moving aqueous electrolyte be arranged together with one or more cathodes and a voltage to generate substantially periodically short-term current pulses with a high flow of current are applied to the workpieces and the cathode (s).

It has long been known objects made of aluminum by anodic oxidation in an aqueous electrolyte with a thick (e.g. 5o μπι), hard and abrasion-resistant oxide layer Mistake. The aluminum workpieces oxidized in this way 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 produce thick, to form 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 found application.

These electrolytes have the ability to dissolve back the oxide layer formed. In order to reduce this resolving power and the desired thick, hard and abrasion-resistant oxide

BATH ORIGINAL

To reach layers, the electrolyte is cooled.

During anodic oxidation, a porous oxide layer forms on the aluminum, the thickness of which increases with the oxidation time. Due to this growth of the oxide layer the electrical contact resistance increases between Electrolyte and workpiece. A voltage drops across this contact resistance as a function of the anodization current from .. This electrical power absorbed 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 generated by the electrolyte, which is moved for better heat dissipation and dissipated through the workpiece. Especially with thin workpieces and at edges with a small radius of curvature, the heat dissipation through the workpiece itself is negligible small amount. The main part of the Joule "see heat is therefore." dissipated by the electrolyte. It should be noted, however, that the heat flow increases as the layer thickness increases is increasingly hindered from the oxidized metal side to the electrolyte. With 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 only a slight electrical Performance degradation occurs. The resulting Joule heat can be easily removed by the electrolyte flowing past at the aluminum oxide / electrolyte interface, so that there can be no heat build-up in the oxide layer. With greater layer thicknesses, there is also a decrease in performance larger, so that increasing heating occurs. The dissolving power of the electrolyte decreases

with "heating up too much, so that the formation of thick, hard Layers is hindered. No more layer growth takes place when the rate of dissolution of the oxide by the electrolyte is equal to the rate of formation of the oxide. For workpieces with pointed, sharp-edged Parts or different wall thicknesses occur due to the high rate of dissolution, local burns because the heat generated cannot be evenly distributed and dissipated through the metal. The redemption can lead to the destruction of the workpiece.

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

In the process known from US Pat. No. 2,920,018 the type mentioned at the beginning, the anodization is carried out with pulsating direct current, the frequency of which is the mains

BATH ORIGINAL

frequency corresponds. By means of a phase control the positive (or negative) half-waves become voltage pulses cut out. The current flow time, i.e. the duration of a single pulse, corresponds approximately to one Third of the subsequent switch-off time. During this switch-off time there is practically no Joule heat more is generated / the switch-off time is used to reduce the voltage generated during the previous voltage pulse Joule heat dissipate.

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

Furthermore, US Pat. No. 3,857,766 discloses a process for anodic oxidation, especially of aluminum alloys containing copper known, in which a base direct current of low voltage is a pulsating direct current, which is at least 6 voltage pulses per second is superimposed. The oxidation takes place with a constant current. As an electrolyte a mixture of sulfuric acid and oxalic acid is used. Also the hard anodized layers produced by this process are not always satisfactory, especially in the case of special aluminum alloys The layer formation is poor or incomplete in critical areas.

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■ 2

The object of the invention is to avoid the disadvantages of the known methods and a method of the type mentioned at the beginning Art to create with which hard oxide layers even on thin-walled and pointed, sharp-edged workpieces can be produced that have sufficient mechanical properties, especially with regard to abrasion resistance and thickness.

This object is achieved in that the voltage remains switched on as long as a noticeable build-up of the Oxide layer takes place and is then switched off until the Joule heat generated essentially is discharged.

The duration of the voltage pulses is advantageously greater than 1/10 of a second, for example between o, 1 and 1.5 seconds and is therefore relatively long.

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 the anodizing voltage is specially adapted to the chemical and physical processes involved in building up an oxide layer is. This has a decisive influence on the layer formation mechanism. The initial process of layer formation is known to take place in the following stages:

1. Formation of a supersaturated with dissolved aluminum Zone on the aluminum surface caused by the anodic dissolution.

2. Formation of supercritical germs in active places.

3. Growth of the germs with the simultaneous formation of new germs.

4. Development of a primary structure, i.e. complete coverage the metal surface with germs.

OHlGINAL

5. The germs grow together to form a homogeneous layer.

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

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

3 SO ₄ ² "+ 2 Al - * ■ 3 SO ₃ ² " + Al ₂ O ₃ 3 SO ₃ ² "+ H ₂ O - * ■ 3 H ₂ SO ₄

From the reaction equations it becomes clear that the electrical

2_ lyt at the electrolyte / metal interface with SO. -Ions

2-

depleted and locally _{enriched in SO 3} ions.

After switching off the current, the moving electrolyte will equalize the concentration. Of the The concentration gradient can be reduced by the decay of the concentration polarization by means of an elec-

Detect electron beam oscillographs. It was found that the anodizing voltage after switching off the The current does not immediately drop to a value close to 0, but that the reduction in the potential of about 3 to 5 volts takes a time of 0.1 to 0.5 seconds.

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

The specified time period during which the voltage pulses are applied is long compared to the prior art extremely favorable for building up an oxide layer. The interruption of the voltage between two voltage pulses should last 0.1 to 2 seconds; this time is advantageously between 0.1 and 1 seconds. The relationship from the duration of a voltage pulse to the duration of a switch-off time should be 0.5 to 5.

With these ratios, anodization with a very high current density is optimal for all aluminum alloys, however achieved particularly advantageously in alloys with high electrochemically more noble alloy additions, surprising Good oxidation results were obtained when several (at least five to a maximum of one hundred) samples were connected in parallel achieved with the method according to the invention.

The temperature of the electrolyte or 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 new voltage pulse created. Depending on the need, the switch-off times at the beginning or at the end of the oxidation are longer or shorter one.

BATH ORIGINAL

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

Further features of the invention emerge from the remaining claims.

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

FIG. 1 shows a longitudinal section through an electrolyte tank for carrying out the method according to the invention, FIG. 2 shows a time diagram of the voltage applied between the cathode and anode in a device according to FIG. 1, FIG. 3 shows a graphic representation of the time course of the current flowing through the electrolyte. The time scale is right corresponds to the time scale in FIG. FIG. 4 shows a graphic representation of the course over time the layer thickness, the time scale corresponds to FIGS. 2 and 3, and

FIG. 5 shows a graphic representation of the time profile of the temperature change in the electrolyte on the same time scale as FIGS. 2 to 4.

The device used to carry out the method according to the invention is shown in FIG. One takes

Electrolyte tank 1 an electrolyte bath 2 with 180 g / l sulfuric acid and 15 g / l oxalic acid. In this electrolyte bath 2, an anode 3 and a cathode 4 are immersed and above Supply lines 5 and 6 are connected to a voltage supply device 7. The anode 3 is composed of an anode holder 8 and the workpieces to be treated 9.

Via a pipe 1o, which leads to a pump 11, is electrolyte liquid is constantly sucked out of the electrolyte bath 2 and into the electrolyte bath via an electrolyte guide tube 12 2, the returned current being directed to the workpiece 9.

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

The voltage supply device 7 supplies a rectified output voltage over a time tu-tjr as shown in FIG. 2 is graphically represented. This voltage can, as can be seen from the figure, be rectangular or a Have harmonics of any technically possible voltage form. This voltage U lies across the supply lines and 6 to the cathode 4 and the anode 3 and causes a current to flow through the electrolyte. The timing 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; it also remains at the same value for the entire anodizing time. The power supply device has a current limitation which _{limits the current during a time t ^ to t 4} , so that the current pulses are approximately square-wave. If the current limit is not used, the current curve begins to decrease immediately after time t ₄ .

ORIGINAL'

ό IkI * LM

A3

Conveniently, a DC voltage produced within the power supply unit 7 between 20 and 60 volts is so switched by a switch on and off, that the voltage profile shown in Figure 2, ergibt- The switch-on times t ₁ to t ₂ are According to the invention between 0.1 and 3 seconds , the pause times t ~ to t_ are between 0.1 and 2 seconds. The ratio of operating times to idle 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., As already explained and due to the automatic current limitation. Due to the structure of the oxide layer, as can be seen in FIG. 4, and the associated increase in the volume resistance of this oxide layer, the current i decreases continuously starting with the time t, since the voltage U is constant according to FIG. The layer thickness (FIG. 4) increases accordingly during the time t ₁ to t. almost constant to. As the current i decreases, the increase in layer thickness will decrease.

With the method according to the invention, as shown in FIG. 3, also, for example, in time t. are you. about the total anodization time with constant current of the current pulses or with constant voltage starting with time t until t [- be anodized.

In the figure 5 is the temperature increase compared to the initial state shown. The temperature increase in the layer rises and fluctuates during the time t 1 to t 1 then by a constant value.

The curves shown in FIGS. 2 to 5 are intended purely schematically to illustrate the pulse current technology according to the invention clarify. The shape of the voltage and the current, especially the rectangular shape, does not always correspond to this technically achievable. Often the tension and

- 3 ■ - / If-

Current increases are not linear from 0 to the nominal value, also the drops at the end of the pulse are not always so sharp, as shown. In many experiments it was found that the rise time and the fall time are about 1/10 of a second lies. However, the shape of the curve has no influence on the anodization result.

The inventive method makes it possible, despite very high

2 pulse current densities of a maximum of 8o A / dm very thin-walled workpieces such as those with sharp edges and corners without To anodize signs of burns. The inventive method is characterized in that the anodization starting directly with a high current density. However, the current can optionally be limited as shown in FIG will.

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 have been carried out. The aim of these experiments was to take constant anodization conditions (such as the type and composition of the electrolyte, bath dimensions, electrolyte temperature, Bath movement etc) to investigate the influence of the anodizing current on the layer formation and layer quality.

Various technical materials were used as anodizing material Aluminum alloys, especially aluminum alloys with a higher copper content.

BATH ORIGINAL

JZ44Z I /

-AS '

example 1

For this example, different workpieces were made

different substances such as

1. AlCuMg 2: Cu 4.3% Mg 1.6%

2. AlCuMgPb: Cu 4.4% Mg 1.4% Pb 1.5%

3. AlCu4Ni2Mg: Cu 4.1% Ni 2, ο% Mg 1.5%

4. AlCuSNi1.5: Cu 5.3% Ni 1.6%

5. AlSi 17CuMg: Si 18.8% Cu 1, * 4% Mg 1.3%

6. AlMgSiI: Si 1.1% Mg 0.8% examined.

The following four methods were used:

1. Direct current method. The experiments were made with pure Direct current with a residual ripple of about 3% under constant current, through automatic readjustment of the Anodisa ion tension carried out.

2. Polarized pulsating direct current. The current pulse duration was about 50% at a frequency of 50 Hz. The current constancy was achieved by manual regulation of the voltage adhered to. This second process is similar to that known from US Pat. No. 2,920,018.

3. Alternating current superimposed direct current. The trials were carried out with constant current. At a frequency of 50 Hz, the residual ripple was around 3o%. This the third process corresponds essentially to the process known from US Pat. No. 3,857,766.

4. Pulse current method according to the invention. These tests were carried out with a constant voltage, which was each set manually to the desired target value. The ratio of the switch-on to the switch-off times was 2 for all experiments ^! '0.' The effective current was measured.

Typical measurement results are summarized in the following three tables.

Table no. 1 workpiece A

procedure

2
2

3
3

Current density
A / dm ²

8th
16

12th
22-5

- 8th

Voltage (V) start end time
(min,)

Layer thickness
(Around)

18th

19th

! 23

19th

22nd

35

45

hardness
kp / mm ^

Surface roughness
(Rt) in around

Remarks

j Very uneven layer formation, experiment interrupted j

- 52 28o

Workpiece edge burned

- 48

27o

Workpiece edge burned

- 52
- 48

31o

24o

24

18th

28

31

j layer equal- ι '. «<<

i moderate j

Layer evenly

(Shift same-

I moderate;

jshift same-;

j moderate I.

τ r> Vi— I

Table no.

Workpiece B

procedure

Current density
A / dm ²

35-6

Voltage (V) start end_

17th

16

16

4o

32

31

42

Time (min.)

Layer thickness

in the)

hardness

kp / W

Surface roughness

(Rt) i n um

Remarks

Burns, no stratification

ο - 3o

ο - 34

- 54

The experiment was interrupted because there was no uniform layer formation,

Interrupt the experiment as there is no uniform layer formation. Strong redemption

35o

1 8 uniform layer formation

Table No. 3

Workpiece C

Voltage (V)
Anfa ng En de

Layer thickness hardness

_kp / mni ^/

- 52

- 54 ο - 21

- 5o ο - 31

- 53 - 54

43o 4oo

Surface roughness in., Jam.

6 13

Comments |

I interrupt the attempt because there is a burn

! 4oo i

: j

; j

j Attempt interrupted due to combustion

45o 38o

! 22nd

X.

i'lkk'lM

The test results summarized in Tables 1 and 2 clearly show that under the chosen test conditions anodization of aluminum alloys with high Containing electrochemically noble metals is not possible if after the first process, i.e. with direct current, is being worked on.

That according to the invention proves to be particularly advantageous Procedure. It is the only process that enables all workpieces to be produced without any signs of burns at a high current density and to oxidize with short anodization times. Furthermore, the tables show that with the method according to the invention, the Layer qualities can be improved, including those Al alloys with low contents of electrochemically more noble metals, as shown in Table 3.

Example 2

In addition to the described method of discontinuous anodization In baths, the process according to the invention proves to be particularly advantageous for continuous anodization. Continuous anodization is to be understood as the anodization of strips or workpieces that is continuous be drawn through the anodizing bath and possibly through rinsing or recompaction baths.

In the case of discontinuous anodization, as described, the current density is controlled using the direct current method the voltage is kept constant, this is not possible with continuous anodization. With the continuous Anodizing only allows the voltage to be kept constant while the current density occurs when it occurs of the strip or workpiece is very high and decreases sharply with the dwell time due to the build-up of an oxide layer, see above that the current density is very low shortly before it leaves the bath. The applied voltage is therefore severely limited

on the current density at the beginning of the entry of the strip or workpiece into the bath, in order to avoid burns that by restricting the removal of the Joule 'see heat by means of the moving electrolyte is given.

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

When the strip or workpiece enters the bath, the current density, due to the high voltage applied, is

2 is very high and can have values of up to 80 A / dm. In the The following power break, however, will see the Joula heat transported away by the moving electrolyte and also, as described, the polarization voltages reduced.

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

There is also the possibility of thick layers of over 3o μΐη to achieve as they are desired in most cases for hard anodization.

In addition, aluminum alloys with high proportions can be particularly advantageously used with the method according to the invention continuously coat on electrochemically noble alloy elements.

BAD ORIG / _NAL

oik kin

An aluminum foil made of AlMgI (dimensions: Thickness 1.0 mm, width 300 mm, length 500 mm) in a moving electrolyte with a lowering speed of o, 2 m / min, introduced and after the complete lowering of the Foil in the electrolyte, the anodization continued for 2 minutes. The applied voltage was 35 volts and the power on time was 0.4 seconds, the switch-off time was 0.2 seconds.

The sample sheet showed current density despite the high initial current density

from about 9o A / dm no burns. The layer thickness was 40 to 55 μm, with the part of the film being included, of course the longest anodization time had the greatest layer thickness.

Claims (1)

Patent claims: . Process for anodic oxidation of workpieces i made of an aluminum alloy, especially with a high ■ 5 content of copper and / or more noble metals, with the ; Workpieces in a moving aqueous electrolyte ! arranged together with one or more cathodes ι. and a voltage is essentially periodic ι to generate short-term current pulses with ι 10 high current flow to the workpieces and the cathode (s) j is created, characterized in that the j The voltage remains switched on as long as ! a noticeable build-up of the oxide layer takes place and j is then switched off until the The Joule generated j 15 see heat essentially dissipated j is. j 2. The method according to claim 1, characterized in that i that a voltage pulse of 10 to 80 V, especially i 20 special 20 to 60 V is applied. '3. A method according to claim 1 or 2, characterized marked j records that the duration of the voltage pulses i is 0.1 to 3 s, in particular 0.1 to 1.5 s. j 4. The method according to any one of claims 1 to 3, characterized ! marked that the duration of the switch-off time between two voltage pulses 0.1 to 2 s, in particular Is 0.1 to 1 s. 5. The method according to any one of claims 1 to 4, characterized in that the ratio of the duration of a
Voltage pulse for the duration of a switch-off time 0.5 to 5. 6. The method according to any one of claims 1 to 5, characterized in that that the current density is kept constant during a voltage pulse. 7. The method according to any one of claims 1 to 6, characterized in that that the current density is kept constant during the duration of the anodic oxidation. 8. The method according to any one of claims 1 to 5, characterized in, that the voltage during a voltage pulse and during the duration of the anodic oxidation is kept constant. 9. The method according to any one of claims 1 to 8, characterized in that that first the current density and then the voltage during the duration of the anodic oxidation is kept constant. 10. The method according to claim 8, characterized in that
the workpieces, in particular strip-shaped material, are allowed to run through the electrolyte bath. 11. The method according to any one of claims 1 to 10, characterized in, that the voltage remains switched off after a voltage pulse until the preferably The temperature measured on the workpiece has fallen below a threshold value. BATH