JPH0431035B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an anodizing solution for magnesium or its alloy. More specifically, the present invention relates to an anodizing solution for forming an oxide film with excellent corrosion resistance and wear resistance on the surface of magnesium or its alloy. [Prior Art] The present invention relates to a method for improving the previously filed "Anodizing Solution for Magnesium or Its Alloy" (Japanese Patent Application No. 61-269562 (Japanese Unexamined Patent Publication No. 63-100195)). Magnesium or its alloys are among the most lightweight and have excellent mechanical properties among practical metals, but their field of use is limited because they are chemically active and easily corrode. Therefore, various surface treatment methods have been proposed and implemented in the past. Chemical treatment methods and anodic oxidation treatment methods are generally known as methods for preventing corrosion of magnesium or its alloys. The chemical treatment method is a method in which magnesium or its alloy is immersed in a treatment solution containing chromium or manganate, etc., to chemically form an anti-corrosion film, such as JIS H-8651 (1978) 1 to 4. This includes species and seven species. On the other hand, the anodizing method is a method in which magnesium or its alloy is immersed in a treatment solution containing chromate, aluminate, or fluoride as its main component, and an oxide film is electrochemically formed using an AC or DC power source. For example, the types 5 and 6 of JIS H-8651 (1978) and the MIL standard (MIL-M-45202)
The Dow17 Act or the HAE Act falls into this category. [Problems to be Solved by the Invention] Although there are various surface treatment methods as described above, all of these conventional methods have problems. for example,
Because chemical treatment methods are used for temporary corrosion protection or as a base for painting, the film formed on magnesium or its alloy cannot be expected to have a high degree of corrosion resistance, and the wear resistance of the film is also extremely low. . On the other hand, the anodizing treatment method is a treatment method that aims to form a highly corrosion-resistant and wear-resistant film on magnesium or its alloy. There are problems with abrasion resistance, and the color tone of treated products is dark green in the Dow17 method and dark brown in the HAE method, which is poor in decoration.Therefore, there is a desire from many quarters for the development of an even better surface treatment method. ing. [Means for Solving the Problems] The present invention is an anodizing solution for magnesium or an alloy thereof, which is prepared by dissolving a silicate, a carboxylate, an alkali hydroxide, and a fluoride in water. The present invention relates to a novel anodizing solution characterized by being a lithium salt or a potassium salt. The treatment liquid of the present invention forms an excellent corrosion-resistant film even when the treatment liquid is composed of only the above four components, but the treatment liquid contains one or more compounds selected from borates and chromates. When dissolved, the corrosion resistance or wear resistance of the oxide film is further improved. When magnesium or its alloy is anodized using an anodizing solution containing appropriate amounts of these ingredients, a glass-like hard oxide film containing forsterite (2MgO・SiO ₂ ) as the main component is formed on its surface. It was also found that the film thickness reached a maximum of 30 μm. The color tone of these films is white except when chromate is added to the treatment solution, and the color tone of these films is white except when chromate is added to the treatment solution, and it is different from conventionally known anodized films, such as Dow 17 method (dark green) and HAE method (dark brown). It is superior in terms of decoration compared to other materials, and also has extremely superior corrosion resistance and abrasion resistance. The metals targeted by the anodizing solution of the present invention are:
There is no particular restriction as long as it is magnesium or its alloy, and it is applicable to general structural or industrial materials with a magnesium content of 70% or more, and magnesium alloys containing other elements such as aluminum, zinc, manganese, zirconium, silicon, and rare earths. Ru. The compounds constituting the anodizing solution of the present invention are:
In terms of water solubility, alkali metal salts are preferred, and the amount used is 5 to 65 g of silicate/
(SiO ₂ standard), carboxylate 10-120g/, alkali hydroxide 25-250g/, fluoride 1-40g/
, borate 2-50g/, chromate 2-50g/
selected from the range. Examples of the silicates include lithium silicate, sodium silicate, and potassium silicate, with lithium silicate and potassium silicate being preferred. The appropriate amount to be used is 5 to 65 g/, preferably 15 to 50 g/based on SiO ₂ . The amount of silicate is 65
When the amount is more than g/g, the formed oxide film becomes rough and the corrosion resistance of the film decreases. Examples of carboxylic acid salts include monocarboxylic acids such as formic acid, acetic acid, and propionic acid, dicarboxylic acids such as oxalic acid, malonic acid, and succinic acid, and alkali metal salts such as oxycarboxylic acids such as lactic acid, aquarite acid, and citric acid. Among them, lithium salts or potassium salts of oxycarboxylic acids are preferred. Its usage amount is 10-120g/, preferably 30-80g/
g/ is appropriate. The amount of carboxylate is 120
Even if the amount is more than g/g, the effect of addition will not increase so much. The effect of the addition here is the densification of the anodic oxide film, and it is presumed that the magnesium salt of carboxylic acid, which is temporarily formed on the object to be treated during the anodizing treatment, is involved in the densification of the film. Examples of the alkali hydroxide include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Among them, potassium hydroxide is preferred. Its usage amount is 25-250g/, preferably 100g/
~200g/ is appropriate. Alkali hydroxide is 25
If the amount is less than g/, the electrolytic voltage of the product to be treated will be high, making anodizing treatment difficult. On the other hand, 250g/
If the amount is more, the growth rate of the film will be slow and a processed product with the desired film thickness cannot be obtained within a certain processing time, which is disadvantageous in practice. Examples of the fluoride include lithium fluoride, sodium fluoride, and potassium fluoride, with potassium fluoride being preferred. The appropriate amount to use is 1 to 40 g/, preferably 2 to 20 g/. If the amount of fluoride is less than 1 g/g, the electrolytic current of the product to be treated becomes unstable, making it difficult to carry out the intended anodic oxidation treatment. On the other hand, if the amount is more than 40 g, the oxide film formed becomes rough and the physical properties of the film deteriorate. The anodizing solution of the present invention is composed of the above-mentioned four components, but if higher abrasion resistance is expected for the oxide film to be formed, borate is added. Examples of the borate include lithium metaborate, sodium metaborate, potassium metaborate, and the like, with potassium metaborate being preferred. The amount used is 2-50g/, preferably 10-30g/
is appropriate. Addition of more than 50 g/borate does not significantly improve the abrasion resistance of the coating. In preparing the anodizing solution, it is acceptable to select two or more compounds from each compound group, but it is more appropriate to select one compound and mix them. When magnesium or its alloy is oxidized using the anodizing solution prepared in this way, a white hard film with excellent corrosion resistance, wear resistance, and decorative properties can be formed on the surface. When a film is expected to have the following properties, chromate is dissolved in the treatment solution. In this case, the color tone of the oxide film formed is yellow-green. Examples of the chromate include sodium dichromate and potassium dichromate. Among them, potassium dichromate is preferred, and the appropriate amount to be used is 2 to 50 g/preferably 5 to 30 g/. Even if chromate is added in an amount of more than 50 g/m, it is not expected to significantly improve the corrosion resistance of the oxide film, and is rather economically disadvantageous. When anodizing magnesium or its alloy using the anodizing solution of the present invention, the temperature of the treatment solution is adjusted to 60°C or lower, preferably 40°C or lower. If the temperature of the treatment solution is higher than 60°C, part of the oxide film formed on the surface of magnesium or its alloy will dissolve, making it difficult to form the desired film thickness within the specified time and reducing the corrosion resistance of the film. decreases. When anodizing magnesium or its alloy using the anodizing solution of the present invention, the power source used is preferably an alternating current, and the current density is 0.2
-5 A/ ^dm2 , preferably 1.5-4.0 A/ ^dm2 is suitable. If the current density is less than 0.2A/ ^dm2 , it is difficult to obtain the desired hard oxide film;
If it is larger than 5 A/dm ² , the concentration of sparks generated on the surface of magnesium or its alloy becomes intense, making it difficult to form a homogeneous oxide film. Items treated with the anodizing solution of the present invention may be used for that purpose immediately after washing and drying, but after immersing the treated item in a mixed solution of chromate and heavy fluoride for several minutes. If a conventionally known post-treatment (sealing treatment) method of rinsing, washing, and drying is employed, the oxide film is stabilized and the corrosion resistance is further improved. Next, the present invention will be further explained in detail by showing Examples and Comparative Examples, but the present invention is not limited to the Examples. [Example] Corrosion resistance in each example was determined by spraying salt water on the anodized product for a predetermined period of time in accordance with the method described in JIS-Z-2371 "Salt water spray test" and measuring the corrosion loss before and after the spray test. Wear resistance was determined by a flat abrasion test (load 400gf, number of reciprocating frictions 60DS/ minutes, abrasive paper #320, material SiC),
The number of reciprocating frictions (DS value) required to wear out a film with a thickness of 1 μm was measured. Examples 1 to 5 A test piece measuring 60 mm in length x 50 mm in width was cut from a magnesium alloy plate (AZ31, thickness 3 mm), the surface was polished with #320 abrasive paper, and after washing with alkali and acid, the composition shown in Table 1 ( Unit: g/) using anodizing solution, treatment solution temperature 25℃, current density 3.0A/dm ²
(AC, voltage 50 to 100 V) and a treatment time of 30 minutes. Example 6 The anodized product of Example 1 was immersed for 3 minutes in a post-treatment solution consisting of 50 g of sodium dichromate and 50 g of potassium difluoride, washed with water, and dried. Example 7 The anodized product of Example 4 was immersed in the post-treatment solution of Example 6 for 3 minutes, washed with water, and dried. Example 8 Using the anodizing solution of Example 1, the current density was
Anodizing treatment was carried out under the same treatment conditions as in Example 1 except that the anodic oxidation treatment was changed to 2.0 A/dm ² . Comparative Example 1 Anodizing treatment was performed according to the conventionally known Dow 17 method. i.e. ammonium hydrogen fluoride 240
g/, sodium dichromate 100g/, 85%
Using a treatment solution consisting of 90ml of phosphoric acid, the test piece (AZ31 alloy) of the example was treated at a treatment solution temperature of 80℃ and a current density of
The anodic oxidation treatment was carried out under the conditions of 3A/dm ² (alternating current) and a treatment time of 30 minutes. Comparative Example 2 Anodizing treatment was performed according to the conventionally known HAE method. That is, aluminum hydroxide 35g/, potassium hydroxide 165g/, potassium fluoride 35g/
Using a treatment solution consisting of , trisodium phosphate 35g/, and potassium permanganate 20g/, the treatment solution temperature was 25℃,
The anodizing treatment was carried out under the conditions of a current density of 2 A/dm ² (alternating current) and a treatment time of 30 minutes. After washing the treated product with water, add 20g of sodium dichromate/
After-treatment was carried out using a treatment solution consisting of 100 g of ammonium difluoride. The surface-treated products of Examples 1 to 8 and Comparative Examples 1 and 2 were subjected to film thickness measurements, corrosion resistance tests, abrasion resistance tests, and color tone observations. The results are shown in Table 2.

【table】

[Table] [Effects of the Invention] The film formed by anodizing using the anodizing solution of magnesium or its alloy according to the present invention has better corrosion resistance, wear resistance, and decorative properties, especially corrosion resistance, than conventionally known films. Are better. Since the treatment liquid of the present invention is alkaline and the treatment temperature is around room temperature, the electrolytic cell can be manufactured from iron, plastic, etc., which is economically advantageous.

Claims (1)

[Scope of Claims] 1. An anodizing solution for magnesium or its alloy, which is obtained by dissolving a silicate, a carboxylate, an alkali hydroxide, and a fluoride in water. 2. The anodizing solution according to claim 1, wherein the silicate, carboxylate, and fluoride are all lithium salts or potassium salts, and the alkali hydroxide is potassium hydroxide. 3. The anodizing solution according to claim 1 or 2, wherein at least one compound selected from borates and chromates is dissolved in the solution.