JPS6053453B2 - Aluminum chemical conversion method - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a chemical conversion method for aluminum, and aims to provide a method suitable for producing aluminum foil with low leakage current, which can be used as an anode foil for aluminum electrolytic capacitors. be.

The anode foil of aluminum electrolytic capacitors has a dense barrier-type oxide film formed on aluminum, and although the film is extremely thin at 14A per volt of applied voltage, it has high electrical insulation properties. have.

Although it is not possible to directly measure the electrical insulation properties of the oxide film, it can be indirectly determined by the leakage current value that flows when a constant voltage is applied in the electrolyte. This leakage current depends on the type of chemical electrolyte, its pH and conductivity,
Although it varies depending on temperature, current density, etc., the presence of defective parts in the oxide film has the greatest influence. The biggest factor causing these defects is impurity metals contained in aluminum. By the way, currently the only way to increase the purity of aluminum is electrolytic refining, and in fact, secondary electrolytic foil is two orders of magnitude more pure than primary electrolytic foil, so it is inevitable that The leakage current value is also extremely small. However, the electrolytic refining method consumes a lot of energy, and the price of the foil becomes extremely high. Despite this, high-purity secondary electrolytic foils are often used as anode foils for capacitors, as electric and electronic devices become more sophisticated and high quality. This can be said to be because there is a very strong demand for higher performance.The present invention realizes a chemical synthesis method that can simultaneously satisfy the two contradictory demands of higher performance and lower cost mentioned above. .

Currently, the following method is generally used to manufacture anode foil for aluminum electrolytic capacitors.

That is, first, aluminum foil is electrolytically etched in an aqueous solution such as hydrochloric acid or sodium chloride to increase its surface area. Next, this aluminum foil is chemically converted for a certain period of time in an aqueous solution containing boric acid, an organic acid, or a salt thereof as a solute. Then, for the purpose of shortening the chemical conversion time and improving the electrical properties of the chemical conversion film, the voltage application is interrupted and the aluminum foil is heated in air at a temperature within the range of 300 to 59° C. for several minutes. Usually, the above chemical conversion and heat treatment are repeated several times. A major feature of the chemical conversion method of the present invention is that a special acid treatment is performed before heat treating the foil after the first chemical conversion (hereinafter referred to as pre-chemical conversion).

Specifically, PHO. The above-mentioned preformed aluminum foil is immersed in a treatment solution prepared by dissolving magnesium chloride in an acidic aqueous solution adjusted to 1 to 6.0. The treatment conditions at this time cannot be unconditionally defined depending on the liquid temperature, PH immersion time, type of acid, amount of aluminum immersion, etc., but in general, the lower the PH and the higher the liquid temperature, the longer the immersion time. Can be shortened. Here, if the pH after treatment is lower than 0.1, it is difficult to use because the oxide film tends to dissolve. If the pH is greater than 6.0, effective treatment cannot be performed. In addition, in the treatment solution, magnesium chloride is 0.
．． If the amount is less than 1% by weight, a sufficient effect cannot be expected, so it is preferable that the amount dissolved is 0.1% by weight to a saturated aqueous solution. After completing the acid treatment step, the foil is sufficiently washed with water and then heat treated at a temperature within the range of 300 to 500C for several minutes. For foils that have undergone the steps up to this heat treatment, even if the re-forming and heat treatments are repeated thereafter, the effect of improving the properties of the film, which is the objective of the present invention, will not diminish. In other words, the oxide film formed by the chemical conversion method of the present invention is significantly improved compared to the chemical conversion film of the primary electrolytic foil described above in comparison of the leakage current value per unit capacity. It exhibits properties close to those of electrolytic foil. In addition, the treatment method itself is extremely simple and can be carried out at low cost, making it an excellent practical method. The present invention will be described below with reference to Examples, Reference Examples, and Comparative Examples.

Reference Example A commercially available secondary electrolytic aluminum foil (purity of 99.99% or more) having a thickness of 100 pm was electrolytically etched in a saline solution having a concentration of 10% according to a conventional method.

The etched foil was immersed in a 10% aluminum borate aqueous solution, used as an anode, and maintained at a temperature of 30C. After applying constant current to 100V, 100V
Constant voltage formation was continued for 10 minutes (formation performed so far is referred to as pre-formation). The foil preformed as described above was washed with water and then heat treated in a 4° C. oven for 3 minutes. Then, constant voltage formation was repeated for one more time in the same electrolytic solution at -1■■. This secondary chemically formed foil was washed with water and heat treated in a 400° C. oven for 3 minutes. Regarding the foil that has been chemically formed,
100 in an aluminum borate aqueous solution with a concentration of 10%
Leakage current value 1 minute after applying voltage of V and 120H
The capacitance at z was measured. These results are shown in the table below. Comparative Example A commercially available primary electrolytic aluminum foil (purity of 99% or more) with a thickness of 100 pm was etched in the same manner as in the above reference example,
I did the chemical conversion.

The leakage current value and capacitance are also shown in the table below. Example 1 A commercially available "primary electrolytic aluminum foil" having a thickness of 100P7rI was subjected to etching and pre-forming in the same manner as in the reference example.

Next, magnesium chloride was dissolved in an aqueous solution of hydrochloric acid with a concentration of 1% to a proportion of 1% by weight to obtain an intermediate treatment solution, and the pre-formed foil was immersed for 3 minutes while maintaining the solution temperature at 40±2°C. Processed. After the treated foil was thoroughly washed with water, it was heat-treated in a 400° C. oven for 3 minutes. The same method as in the reference example was adopted from subsequent secondary chemical formation to heat treatment. The electrical properties of the foil after completion of chemical formation are shown in the table below. Example 2 In the chemical conversion method described in Example 1, the intermediate treatment was replaced by the following method.

That is, an intermediate treatment solution was prepared by dissolving magnesium chloride in a 5% aqueous phosphoric acid solution to a concentration of 0.5% by weight, and the pre-formed foil was immersed for 3 minutes while maintaining the solution temperature at 50±2°C. . The foil after the intermediate treatment was thoroughly washed with water and heat treated at 400° C. for 3 minutes. The same methods as in Example 1 were used for other etching, pre-chemical formation, and secondary chemical formation. The electrical properties of the foil after chemical conversion are shown in the table below. Example 3 In the chemical conversion method described in Example 1, the intermediate treatment was replaced by the following method.

That is, magnesium chloride was dissolved in an oxalic acid aqueous solution with a concentration of 5% to give an intermediate treatment solution of 2% by weight,
It was immersed for 5 minutes at a liquid temperature of 70C. The treated foil was thoroughly washed with water and heat treated at 400C for 3 minutes. Other etching, pre-chemical formation, and secondary chemical formation were carried out in the same manner as in Example 1. The electrical properties of the foil after completion of chemical formation are shown in the table below. As is clear from the results in the table above, under the same etching conditions, the etching magnification of the secondary electrolytic foil is higher than that of the primary electrolytic foil.

Claims (1)

[Claims] 1. After a primary electrolytic aluminum foil as an anode foil for an aluminum electrolytic capacitor is once chemically formed, it is immersed in an acidic aqueous solution containing magnesium chloride with a pH of 0.1 to 6.0, washed with water, heat-treated, and then treated again. A chemical conversion method for aluminum that involves chemical conversion treatment.