JPH0745720B2 - Method for forming opaque anodic oxide coating on aluminum alloy material - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming an opaque anodic oxide film on an aluminum alloy material.

Conventional Technology and Problems There are several known methods for forming an opaque anodic oxide film on an aluminum alloy material.

For example, the Emmatal method and the so-called chromic acid method in which electrolytic treatment is performed using potassium titanium oxalate as an electrolytic solution are known. However, in the former, the electrolytic solution is expensive and
The management of the electrolytic treatment bath is complicated, and the latter has a problem that the problem of pollution due to chromium may occur and the film formation rate is slow.

Also known is a method in which the surface of the underlying aluminum surface is roughened in advance by a chemical satin treatment, and then an ordinary anodic oxidation treatment is performed. However, in this method, since the matte-finishing bath is aged severely, not only must the satin-finishing bath be well controlled in order to maintain the uniformity of quality, but the coating often did not become opaque.

Therefore, the present applicant has already proposed the following method for forming an opaque anodic oxide film of aluminum in order to solve the above problems. The method relating to this proposal is disclosed in Japanese Examined Patent Publication No.
As disclosed in Japanese Patent No. 617, outline, Cu0.05-4.0
The first step of forming an anodic oxide film on an aluminum alloy material consisting of an aluminum-based alloy containing 100% by oxalic acid electrolysis bath, and by rapidly dropping the voltage from the previous voltage value to 5 to 35 V in the same electrolysis bath Although it comprises a second step of making the anodic oxide film opaque by electrolytic treatment, it is not always sufficiently satisfactory in the following points. That is, since the aluminum alloy material to which the same method can be applied is limited to one containing 0.05 to 4.0% of Cu as a main component, the range of material selection is relatively narrow. Further, in particular, when a cream-colored opaque film is formed, the first step must be performed by AC / DC superposition electrolysis, which is a disadvantage of the electrolytically treated product, and variations in film thickness and color are likely to occur. .

The present invention has been made in view of the above circumstances, and an object thereof is an aluminum-based alloy that does not contain Cu as a main component, and has a relatively wide range of main components, and an opaque oxide. A coating can be formed in a short time with simple treatment bath management, is cost effective, and does not cause pollution problems. An object of the present invention is to provide a method for forming an opaque anodic oxide film on an aluminum alloy material that does not require electrolysis.

Solution to Problem A method for forming an opaque anodic oxide coating on an aluminum alloy material according to the present invention is an anodizing treatment of an aluminum alloy material made of an aluminum-based alloy containing 0.5 to 6.0% of Mg in an oxalic acid electrolytic bath to anodize the surface. A first step of forming an oxide film,
Subsequently, the second electrolytic step is characterized by a second step in which the voltage is rapidly decreased from the previous voltage value to 5 to 35 V in the same electrolytic bath to carry out a constant voltage electrolysis treatment to make the anodized film opaque. In this specification,% is based on weight.

Here, if the Mg content of the aluminum-based alloy is less than 0.5%, satisfactory opacity cannot be obtained, and conversely, if it exceeds 6.0%, hot and cold workability deteriorates, and stress resistance increases. Corrosion cracking property also decreases, so it is limited to the range of 0.5 to 6.0%.

The first step is to use the aluminum-based alloy in an oxalic acid electrolysis bath by alternating current, direct current or alternating current / direct current
Anodizing electrolysis treatment is performed until a film having a predetermined thickness is obtained. Here, the concentration of oxalic acid is preferably 1 to 5
%, Particularly preferably 2-3%. This electrolysis may be constant current electrolysis or constant voltage electrolysis.

In the second step, the voltage in the first step is rapidly lowered to a predetermined value, and then the processed product in the first step is subjected to constant voltage electrolysis. When constant voltage electrolysis is carried out after the voltage is drastically dropped in this way, the current does not flow immediately, and after several seconds to several minutes have elapsed, it gradually starts to flow and reaches a steady state after a while. This phenomenon is called a recovery phenomenon. Due to this recovery phenomenon, an anodic oxide film is formed and the underlying aluminum alloy surface of the film is uniformly roughened. The diffuse reflection on the rough surface causes the anodic oxide film formed in the first step to appear opaque.
Here, although the voltage drop from the voltage in the first step to the voltage in the second step is performed rapidly, in actual operation, the energization in the first step is temporarily stopped, and the voltage is again reduced to the predetermined voltage at the beginning of the second step. The method of applying the voltage is preferably used.

If the voltage in the second step is less than 5V, the above-mentioned recovery effect is small, so that the roughening of the underlying surface does not proceed. On the contrary, if it exceeds 35V, an oxide film is rapidly generated and the underlying rough surface disappears. Therefore, it is limited to the range of 5 to 35V. A particularly preferred voltage range is 15 to 20V. Further, the current in this step may be any of alternating current, direct current, or superposed alternating current / direct current.

The electrolytic treatment time of the second step is not particularly limited, but if it is less than 5 minutes, the roughening of the aluminum base alloy does not proceed. On the contrary, if it exceeds 25 minutes, the underlying rough surface tends to disappear. 25 minutes, especially 10 to 20 minutes are preferred.

Examples Example A As samples, six kinds of plate-shaped aluminum alloy materials (sample numbers (1) to (6)) having the compositions shown in Table 1 were prepared,
After etching these with 5% NaOH at 50 ° C x 5 minutes, put each sample in a 3% oxalic acid electrolytic bath, use these as the anode, carbon as the cathode, and keep the bath temperature at 35 ° C.
Anodic oxidation treatment was performed by direct current electrolysis at 1.3 A / dm ² for 15 minutes (first step). Then, after stopping the energization, each sample was subjected to DC constant voltage electrolysis under the conditions shown in Table 2 to carry out the opacification treatment (second step). The results are shown in Table 2.

Example B As samples, three pieces of sample No. (1) shown in Table 1 were prepared, and these were etched under the same conditions as in Example A. In each of the two electrolytic baths, the positive and negative currents were made the same and the negative currents were made different in three in the alternating current / direct current superimposing flow. By an electrolysis, the anodizing treatment which is the first step was completed. Then, after stopping the energization, each sample was subjected to DC constant voltage electrolysis at 15V for 20 minutes in the same electrolytic bath to complete the second step of opacifying treatment. The results are shown in Table 3.

EFFECTS OF THE INVENTION Since the present invention is configured as described above, according to the method of the present invention, the main component is an aluminum-based alloy containing Mg instead of Cu, and the component range of Mg is relatively wide ( Mg content 0.5-6.0%) Aluminum alloy material,
Not only can an opaque oxide film be formed in a short time by simple treatment bath management, it is also advantageous in terms of cost and there is no risk of pollution problems.

Moreover, especially when it is desired to obtain a cream-colored opaque oxide film, the method of the present invention makes it possible to achieve the object without AC / DC superposition electrolysis.

Claims (1)

[Claims] 1. A first step of forming an anodic oxide film on the surface of an aluminum alloy material made of an aluminum-based alloy containing 0.5 to 6.0% Mg by anodic oxidation in an oxalic acid electrolytic bath, followed by the same electrolysis. A method for forming an opaque anodic oxide coating on an aluminum alloy material, which comprises a second step of making the anodic oxide coating opaque by subjecting the anodic oxide coating to a constant voltage electrolysis treatment by rapidly decreasing the voltage from the previous voltage value to 5 to 35 V in a bath.