DE2420704B2 - Process and for the continuous anodizing of an aluminum strip and device for carrying out this process - Google Patents

Description

JO

The invention relates to a method for the continuous anodizing of an aluminum strip, the strip being guided successively through a number of electrolytic cells and being flowed through in the strip running direction first by a direct current running in the strip running direction and then by a second direct current running in the opposite direction to the strip running direction, as well as a device for Carrying out the process with anodizing and contact cells. The term "aluminum" is intended to include alloys based on aluminum which, like pure aluminum, can be electrolytically oxidized. More particularly, the invention relates to an apparatus for continuous anodizing of aluminum strip, tapes, u -wire, rods, profiles. The like., Λ * which are generally referred to as the aluminum strip.

Aluminum and aluminum-based alloys in strip, wire or tape form are continuously anodized for several years by a variety of methods ^r> o. Such anodized products are used for electrical and decorative applications, for the production of household appliances, automotive trim, building materials, agricultural equipment, furniture, sporting goods, cans, ^r> 5 container closures, lithographic plates, transformers and on many other economic and industrial fields.

Two basic w> Additional methods applied, namely one such using a contact roller or -bar and on the other hand an electrochemical process using a cathodic Contact cell. μ

The first-mentioned method suffers from numerous shortcomings. For example, tici Aluminum strips to be dry to electrolysis / 1:

avoid which otherwise the contact roller or rail anodically and in its surface Would cause dimples. Another difficulty is arcing between the two surfaces with a separation between them, soft by the presence of edge ridges or produced by aluminum chips on the surface of the strip itself. The arcing causes pitting in the aluminum and pitting and oxidation of the aluminum Contact member itself

When applying the method using the cathodic contact cell, one of the Limiting factors for the current that can be introduced into the strip are that all of the current flows into one Cross-sectional area of the continuous strip must be introduced. This causes a surge in electricity caused by a non-anodized strip that is not protected by an oxide coating. This can lead to Burn throughs lead to the formation of inadequate oxide coatings. So far have been the problem of arcing when using a solid contact member and the problem the burnout due to a rush current in the contact cell method as necessary restrictions Accepted in the continuous anodizing of aluminum because all the electricity is used for the anodizing process must be removed from the continuous strip in a single pass. For this, the The fact that the anodic oxide coating formed during anodizing is responsible electrical insulator is

According to the invention, however, the introduction of the anodizing in more than one cross section of the Allows continuous aluminum strip, creating a device for preventing burn-through of the continuous strip as a result of an action on it as it enters the anodizing cell Electricity surge is created.

Various devices are known for the electrochemical anodizing of aluminum. In the CH-PS 2 05 183, which corresponds to DE-PS 6 83 169, a device with two cells is shown in Fig. 1, in which the The contact cell is at the front in the direction of passage of the aluminum strip and the anodizing cell is at the back, if not AC power is used. The disadvantage here is that the oxidation is only carried out in the last cell, see above that the first cell remains ineffective for the oxidation.

Fig. 2 of said patent shows a series of a total of six cells, wherein an alternating current between the first and second, the third and fourth and the fifth and sixth is applied, the voltage of which increases in the direction of passage of the object to be anodized. The rising The voltage is obviously supposed to cope with the increasing resistance caused by the build-up of the oxidation layer Take into account. However, a current flow remains between the second and third as well as fourth and fifth cell out. In this embodiment, alternating current is used in which one polarity is not is detectable.

In the embodiment according to FIG. 3, direct current is used as an alternative to alternating current supply of the circuits, but the first, third and fifth cells are just like the second, fourth and sixth Cell connected in parallel. According to the layer thicknesses that occur, a current flow is therefore at Use of direct or alternating current practical

can only be done from the first to the second cell. The current flow between the third and fourth cells is already greatly reduced and practically no longer present between the fifth and sixth. It there is also the fact that the first cell is positive and therefore works as a contact cell, which is disadvantageous actual coating only applied in the second cell will.

From DE-PS 7 18 975 a similar device is known in which additional washing baths are provided are. These rolls prevent contamination of the successive baths. Regarding the actual However, they are irrelevant to the anodizing process.

The US-PS 10 68 410 shows in Fig. 3 three in a row switched anodizing cells, which are fed by a common power source. Since the Resistance is lowest in the first cell, the available current is practically exclusively from the first to the second cell will flow and no two-stage oxidation will occur.

All previously known devices for anodizing have the disadvantage that either the order output is unsatisfactory or the aluminum strip only moves through the baths at low speed can be moved.

It is the object of the invention to avoid these disadvantages of the prior art and to proceed and to provide an apparatus for continuously anodizing aluminum which is opposite to the after Devices known from the prior art have an increased application rate or with the same application enables a significantly increased speed of the aluminum strip. A burning through of the continuous strip as a result of an act on it as it enters the anodizing cell Electric surge can be safely prevented.

The solution to the problem posed in the continuous anodizing of an aluminum strip, whereby the strip is successively passed through a number of electrolytic cells and thereby in the direction of belt travel first of a direct current running in the direction of travel of the tape and then of a second of the Direct current running in the opposite direction is flowed through, consists in the fact that the Direct currents flow in two independent circuits.

The direction of current flow is defined and the direction of movement of the electrons, i. H. from minus after plus.

In contrast to the prior art, the method according to the invention results for the first time in the advantageous possibility of specifically setting defined voltages between the two cells, for example between the first and second cells a lower voltage than between the second and third cell. Contrary to known methods, this has the consequence that between In the first and second lines, a maximum voltage can be set at which good oxidation occurs takes place, but a flaking of the oxide film is still reliably avoided.

A higher voltage can then be set between the second and third cells enables the already existing oxide layer to be penetrated and a maximum further one here as well Apply oxide layer.

The device according to the invention for carrying out the method is characterized in that in Direction of travel one after the other an anodizing cell with one with the negative pole of a first direct current source connected electrode, a contact cell with one connected to the positive pole of the direct current source Electrode and a further electrode connected to the positive pole of a second direct current source and a second anodizing cell is arranged with an electrode connected to the negative pole of the direct current source

ίο are.

The essence of the invention is that an anodizing cell is followed by a contact cell which is provided with two electrodes for closing a direct current circuit and which makes it possible to connect a further anodizing cell to apply a further coating layer. Characterized in that two separate DC power sources are present, it is ensured here that also between the contact cell and the second Eloxierzelle the full load current flows If the three cells from the same power source supplied, the total current would practically in view of the low resistance in the first Eloxierzelle only flow through this part of the circuit, and the application in the downstream anodizing cell would be omitted or would only be very slight. The definition of the current flows can be done in such a way that in the first circuit the current flows from the first direct current source in the running direction of the strip and in the second direct current circuit the current is in the opposite direction

jo the tape running direction is set The result of this Placement is not just an expected increase in order output or, for the same order, one up to Double possible speed increase of the aluminum belt, but for the specialist

J5 is surprising due to the arrangement according to the invention achieved an increase by a factor of 4-5, d. H. instead of the usual 7 m per minute, a surprising improvement can be achieved with the same application and continuing the eluting quality can be run at a speed of 33 mm per minute.

In the following, an embodiment of the invention is explained in more detail with reference to a drawing. the The figure shows a flow diagram of a device according to the invention for the continuous anodizing of aluminum.

According to the figure, the device for the continuous electrolytic anodizing of an aluminum strip 12 a first anodizing cell 10, a contact cell 20 and a second anodizing cell 10 '.

Each cell has a suitable container 16 for holding an electrolyte 14. In the anodizing cell 10, a cathode 18 connected to a first direct current source 24 is arranged. In the contact cell 20 an anode 22 is arranged, which is connected to the same direct current source 24. the Second anodizing cell 10 'has a cathode 18' which is connected to a second direct current source 24 '. the Contact cell 20 has a second anode 22 'which is connected to the same direct current source 24'.

W) The aluminum strip 12 is made with the help of conventional, arranged in the manner shown in the figure guide rollers continuously one after the other through the first Anodizing cell 10, the contact cell 20 and the second anodizing cell 10 'passed through.

(ν · C ^ r from the two separate direct current sources 24 and 24 'introduced into the contact cell 20 anodizing current flows in both directions, so that the Current carrying capacity of the running aluminum strip

current

(A)

25 25 25

κι 25 25 20

fens 12 is effectively doubled. In other words: table

The one in the contact cell 20 from the DC power source

Anodizing current applied to the strip 12 flows in the opposite direction to the direction of travel of the strip 12 in the upstream anodizing cell 10, in which part of the intended, porous oxide coating is formed. The anodizing is carried out in the anodizing cell 10 'by the power source 24' The anodizing current supplied by the strip 12 and by the contact cell 20 is completed and transferred along the strip to the anodizing cell 10 '. When the strip 12 from the anodizing cell 10 ' emerges, a porous oxide coating of the desired thickness has been formed on it.

The aluminum strip 12 can be known per se Submit before reaching the Eioxierabschniiis Using conventional techniques it can be cleaned, degreased, or otherwise pretreated while he is after leaving the anodizing section, sealed, colored or otherwise using known methods can be post-treated.

The invention is explained in more detail below in examples. In the examples, about 100 200 mm large panels or sheets were used, and in all examples both sides of the aluminum sheets were used anodized, the electrolyte concentration being 230 g / l aqueous sulfuric acid.

Time Time Breakdown Anodic Cathodic tension (S) (S) (V DC) 60 - 380 60 60 380 60 30th 380 30th - 290 30th 30th 290 30th 15th 290 20th 20th 220

B e i s ρ i e 1 1

Aluminum samples were in the sulfuric acid electrolyte at constant electrolyte concentration and constant temperature anodized. The DC breakdown values were determined and are summarized in Table 1 below

The results of this example show that a reversal of polarity or a siromania via an oxide coating has no adverse or harmful influence on the anodized oxide coating This is shown by the breakdown values of the DC voltage, which in the case of the samples, which are cathodic and are identical for the samples that were not cathodically treated.

Example 2

Triplicate sets of aluminum samples were prepared, all during the same period of time and anodized with the same current density. Set No. 1 was just anodized. Of the Set # 2 was anodized and left in the electrolyte for a while while the polarity was reversed was (cathodic samples). Set No. 3 was anodized and also left in the electrolyte without a current was applied. In all three sets, the anodizing time was the time it took to reverse polarity and the period of time during which the samples were left in the electrolyte, in each case the same.

Table 2

Current Actual Voltage "reverse" temp, dense current impulse voltage

ke

(A / dm) (A)

(ν)

(V)

Weight d. Weight d. Weight d. Weight set set set loss ("/.)
No. 1 No. 2 No. 3 between

Set no.
(mg / cm ² ) (mg / cm ² ) (mg / cm ² ) 1 and 2

150 4.3

150 4.3

150 4.3

15.5
15.5
15.5

16 2 30th 0.86 0.84 0.81 2 13th 2 40 0.79 0.72 0.72 9 10 2 50 0.71 0.46 0.48 33

All samples were sealed or densified in boiling water. The weight of the oxide coating formed was determined by first weighing the sample and then removing the oxide coating by soaking it in a hot chromic acid-phosphoric acid solution and then reweighing the samples. The weight difference divided by the total area of the sample gave the weight in mg per cm ² of oxide coating on the original sample. The results are summarized in Table 2.

This example shows that there is only a slight difference in weight the oxide coating between the polarity reversed samples and the case consists in which the anodized samples were soaked in the anodizing electrolyte. This shows that the Loss of oxide weight primarily due to the solvency of the electrolyte and not to that The current flows through the cover sheet when the polarity is reversed when the samples are cathodic When evaluating the data in Table 2, it should be noted that the The dissolving effect of the electrolyte increases with increasing temperature, which is responsible for the greater oxide weight loss is responsible for the tests carried out at higher temperatures

Example 3

Aluminum samples were anodized using the following three steps:

1. The sample was ^{anodized at 40 °} C. and a current density of 5.38 A / dm ² for a predetermined period of time

2. With the current density remaining at the specified value, the polarity was reversed, whereby the sample became cathodic with changes in temperature and time were made.

3. The aluminum sample was again at 4O ⁰ C and a current of 5.38 A / dm ² for a predetermined period anodized

Amperage of 5.38 A / dm ² anodized for a period equal to the total time of steps 1 and 3. Process step 2 has been omitted.

All samples of this example were sealed or densified in hot water, and the oxide coating weights were determined in the manner described in Example 2. The dates are in the following Tables 3 and 4 summarized:

Comparative samples 3 Chip
tion
Step 1 became at 40 ° C and one: <> Chip
tion
step 3 Time
step
1 + 2 Oxide over
pull weight Oxide over-
Zugsgewichl
the ver
same sample Weight
loss of
sample Tabel . (V) (V) (s) (mg / cm ² ) (mg / cm ² ) (%) Line
Step 1 13.5 Time
step 2 Chip
tion
step 2 Tempe
rature
step 2 Time
step 3 13th 120 0.6956 0.7666 9.26 (S) 13.0 (S) (V) (C) (S) 13th 120 0.6969 0.7446 6.41 54 13.0 48 2 60 66 13th 120 0.7544 0.7770 2.90 54 13.0 48 2 50 66 13th 120 0.7421 0.7556 1.78 54 13.5 48 2 40 66 13th 240 1.1205 1.4305 21.67 54 13.5 48 2 40 66 13th 240 1.2677 1.4460 12.32 108 13.0 96 4th 60 132 13th 240 1.3265 0.3552 7.77 108 96 4th 50 132 108 96 2 40 132

Table 4

current Tempe Oxide over Oxide over Weight density rature pull weight pull weight loss of X nioxating step 2 the sample the ver sample MMt equally probc (A min) (C-) (mg / cm ² ) (mg / cm ² ) (%) 100 60 0.6956 0.7666 9.26 100 50 0.6969 0.7446 6.41 100 40 0.7421 0.7556 1.78 100 40 0.7544 0.7770 2.90 200 60 1.1205 1.4305 21.67 200 50 1.2677 1.4460 12.32 200 40 1.3265 1.4382 7.77 320 60 1.2871 2.1610 40.43 320 50 1.6630 2.1739 23.50 320 40 1.9866 2.1675 8.34 Annotation: Current density = 5.38 A / drrr. Anodized pen = = 40 C.

It is known that an anodic oxide requires a forming voltage in the range of 12-13 V (see Finishing of Aluminum, Wernick and Pinner). The above example shows that if the polarity is reversed and the anodized aluminum samples are made the negative or cathodic pole of the cell, the anodized samples show an unusual appearance, while at the same current density used for the anodizing, the voltage is 1-2 V drops. Only a minimal amount of heat is generated by the influence of resistance and, as shown, there is practically no loss of weight.

4 (i This unique property enables continuation the application of the current to the aluminum strip where the anodic oxide coating By using this process it is now possible to make aluminum strips to anodize continuously with direct current without affecting the contact roller process or the common contact cell processes, in which the entire current in a single operation via a applied across a single cross-section of the strip, the restrictions imposed occur. About that In addition, the invention enables the problem of burnout to be largely eliminated by the anodizing direct current can be introduced directly into the strip 12 by means of the contact cell, After the oxide coating has been formed, there is also a substantial increase in application rate achieved or with the same order, the speed of the aluminum strip can be increased by the factor 4-5 can be increased.

1 sheet of drawings

Claims (2)

Patent claims: 1. A process for the continuous anodizing of an aluminum strip, the strips one after the other is passed through a number of electrolytic cells, first in the direction of belt travel from a direct current running in the direction of tape travel and then from a second direct current in the direction of travel of the tape oppositely running direct current is flowed through, characterized in that, that the direct currents flow in two independent circuits. 2. Device for performing the method according to claim 1 with anodizing and contact cells, characterized in that one anodizing cell (10) with one with one after the other in the direction of belt travel , the negative pole of a first direct current source (24) connected electrode (18), a contact cell (20) with an electrode (22) connected to the positive pole of the direct current source (24) and another with the positive pole of a second direct current source (24 ') connected electrode (22'), and a second Anodizing cell (10 ') with an electrode (18') connected to the negative pole of the direct current source (24 ') are arranged.