JPS5858427B2 - Method of forming boehmite film on aluminum surface - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a chemical oxide film, particularly a boehmite film, on an aluminum surface.

In this specification, the term "aluminum" means
It shall cover pure aluminum, commercially available aluminum with small amounts of impurities, and aluminum alloys in which aluminum accounts for the majority.

When aluminum is heat-treated using deionized water or distilled water, boehmite (A1203/I) is formed on the aluminum surface.
It has been well known for a long time that a n2O) film is formed.

Recently, a method of forming the above-mentioned boehmite film by adding a small amount of basic compounds such as ammonia or amines to deionized water or distilled water to prepare a slightly basic treatment liquid has become known. It is being

This method is mainly used as a corrosion-resistant treatment for aluminum heat exchangers used in bathtubs and radiators.Furthermore, the resulting film has good adhesion to the synthetic resin painted surface, and unlike the chromate method, it has problems with pollution. Since there is no risk of causing any damage, it has been used as a base treatment for painting in place of the chromate method.

However, this method had several drawbacks as follows.

In other words, (a) the boehmite film formed by this method has sufficient corrosion resistance against ordinary water, hot water, and hot water, but is not resistant to highly corrosive liquids such as seawater. However, it could hardly be said that it had good corrosion resistance.

(b) This method requires boiling treatment followed by pressurized steam treatment as the heat treatment for aluminum.
The work took a long time and was inefficient.

(c) The preparation water for the treatment liquid used in this method must be deionized water or distilled water.

If water containing various ions, such as industrial water, tap water, or underground water, is used, not only will no boehmite film be formed, but the aluminum surface will turn brown or black.

Therefore, when using deionized water made with degraded ion exchange resin as bathing water, or when using bathing water mixed with washing water used in pretreatment, the above-mentioned problems may occur on the aluminum surface. There is a risk of causing discoloration.

Therefore, sufficient consideration was always required for the management of bath water.

That is the point.

The present invention was made in view of the above-mentioned drawbacks, and an object of the present invention is to provide a method for forming a boehmite film that overcomes all of these drawbacks.

This invention contains a lithium salt, a basic organic compound, an alkaline earth metal salt or an aluminate, and has a pH of 6.
The gist is a method of forming a boehmite film on an aluminum surface by immersing boiling aluminum at a boiling temperature of 60° C. to 13 in a treatment solution having a temperature of 60° C. to 13.

The lithium salt in this invention may be any water-soluble salt; examples thereof include lithium chloride, chloride such as bromide; lithium nitrate, sulfate, and acetate. , inorganic salts such as carbonates; organic salts such as oxalates and lactates of lithium.

These lithium salts may be used alone or in combination of two or more.

The structure and formation rate of the formed film vary depending on the type of lithium salt used.

For example, when lithium chloride is used as the lithium salt, a thick film with plate-like crystals is produced, and when an inorganic salt of lithium such as nitrate or sulfate is used, a thick film with granular crystals is produced. However, when organic salts of lithium, such as oxalates and lactates, are used, a translucent, dense, and thinner film is produced than when using inorganic salts.

The concentration of the lithium salt in the treatment liquid is preferably 10-3 to 5 mol/l, particularly preferably 10-2 to 0.5 mol/l.

If the concentration is less than 10-3 mol/l, the formation of the boehmite film will be uneven; on the other hand, if the concentration is less than 5 mol/l
If it exceeds 0, lithium hydroxide will precipitate in the treatment solution, which is undesirable in either case.The basic organic compound is used to make the boehmite film formed on the aluminum surface uniform in thickness. It has the function of adjusting pH and preventing precipitation of lithium hydroxide.

Examples of this include hydrazine, monoethanolamine,
Examples include amines such as jetanolamine, triethanolamine, monoethylamine, diethylamine, and triethylamine.

The concentration of the basic organic compound is preferably 101 to 1 mol/l, particularly preferably 10-3 to 0.1 mol/l.

If the concentration is less than 10-5, the formation of the boehmite film will not proceed sufficiently and precipitation of lithium hydroxide in the treatment solution will not be prevented; on the other hand, if the concentration exceeds 1 mol/l, Both cases are unfavorable because the pH of the treatment liquid becomes too high.

Examples of alkaline earth metal salts used include hydroxides, carbonates, bicarbonates, acetates, phosphates, borates, silicates, and aluminates of magnesium, calcium, and strontium; As acid salts, alkali metal salts and alkaline earth metal salts are mainly used.

The pH of the treatment liquid is within the range of 6-13.

If the pH is less than 6, the formation of a boehmite film on the aluminum surface will be insufficient, and if the pH exceeds 13, the etching of the aluminum surface by the alkali will proceed too much, so either case is unfavorable.

A particularly preferred pH range is 8-12.

As the pH adjuster, the above-mentioned basic organic compounds such as aqueous ammonia are used.

The immersion treatment of aluminum is carried out at a temperature of the treatment liquid of 60° C. to boiling temperature.

If the temperature of the treatment liquid is less than 60°C, it is not preferable because it takes a long time to form a boehmite film.

The higher the temperature is within the above range, the thicker the film can be obtained.

The longer the immersion treatment time, the thicker the film can be formed, but usually a sufficiently thick film can be obtained in 5 to 60 minutes.

The bath water used for preparing the treatment liquid may be water containing various ions such as deionized water, distilled water, tap water, or ground water.

Although the reason for this is not clear, it is thought to be as follows.

That is, the lithium salt in the present invention has the effect of preventing ions such as iron ions, which cause the aluminum surface to discolor from brown to black, from adhering to the aluminum surface in the treatment solution.

This immersion treatment leaves the aluminum surface with a thickness of approximately 0.
A boehmite film of 1 to 5 μm is formed.

This film has no fear of forming pores and has excellent corrosion resistance even against seawater.

In addition, when the treatment liquid in this invention further contains an alkali metal salt or alkaline earth metal salt other than lithium salt, the treatment liquid is thicker and has better corrosion resistance than when it does not contain it. A boehmite film is formed.

Further, the boehmite film formed according to the present invention has approximately 1
The corrosion resistance of the coating can be further improved by performing pressurized steam treatment at a pressure of ~5 kg/crtr.

As described above, this invention uses a lithium salt, a basic organic compound, and an alkaline earth metal salt or an aluminate treatment solution, so it is possible to form a thick, dense, and robust boehmite film. Therefore, aluminum can be completely protected from seawater and the like.

Moreover, in addition to deionized water and distilled water, industrial water, tap water, underground water, etc. can be used as bath water, and the processing time can be shortened to improve work efficiency.

Examples of this invention will be shown below.

However, the present invention is not limited to these Examples.

In each example, a boehmite film was formed on the surface under the conditions shown in Table 1 using an aluminum material of JIS Al10O-H24.

Furthermore, performance tests were conducted on the obtained films as shown in Table 2.

(The deionized water has a resistivity of 500,000Ω or more, and the tap water is tap water from Sakai City, Osaka Prefecture.

) [The measurement of the film weight is based on JIS H8680 "
The test was carried out according to the film weight test method in "Anodized film thickness test method".

The alkaline resistance value is determined by the time from dropping NaOH solution until bubbles are generated from the aluminum surface under the same conditions as the alkali dropping test method in JISH8681 "Corrosion resistance test force and method of anodized film". This is an evaluation of corrosion resistance against.

Corrosion tests using a pitting corrosion accelerating solution were conducted using NaC11 (0,0
05% by weight) and Cu 804 ・5H20 (0,OO5
The test was carried out by immersing the sample in a solution containing (% by weight).

Claims (1)

[Claims] 1. Aluminum surface treated by immersing a boiling point of 60°C to a boiling point in a treatment solution containing a lithium salt, a basic organic compound, and an alkaline earth metal salt or an aluminate and having a pH of 6 to 13. A method of forming a boehmite film on.