JPS5858426B2 - 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 is intended to include 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.
H20) It has been well known for a long time that a film is formed.

Recently, a method has become known in which a slightly alkaline treatment solution is prepared by adding a small amount of alkaline substances such as ammonia or amines to deionized water or distilled water, and this is used to form the above-mentioned boehmite film. There is.

This method is mainly applied to aluminum heat exchangers used in bathtubs and radiators as anti-corrosion treatment.Furthermore, the resulting film has good adhesion to synthetic resin painted surfaces, and unlike the chromate method, it has problems with pollution. Because there is no risk of causing any damage, it has been used in place of the chromate method to treat the base for painting. However, this method had several drawbacks as described below.

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 high-pressure steam treatment as the heat treatment for aluminum.
The work took a long time and was inefficient ○ (c) The preparation water for the treatment solution 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 is required at all times in 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.

In this invention, a treatment liquid containing one or more types of alkaline earth metal sulfates and having a pH of 7 to 12 is added
~The gist is a method of forming a boehmite film on the aluminum surface by immersion treatment of aluminum at boiling temperature.○ The alkaline earth metal may be any metal shown in the periodic table, including magnesium, calcium, and strontium. preferable.

As the alkaline earth metal sulfate, one compound thereof may be used alone, or several kinds of compounds may be used in combination.

In the latter case, a particularly uniform and thick film can be formed.

The concentration of alkaline earth metal sulfate in the treatment solution is 0.
01 to 500 El/l is preferable, and 0.05 to 20 g/l
lj is particularly preferred.

If the concentration is less than 0.019/A', the formation of a boehmite film will not proceed sufficiently, while if the concentration exceeds 500 g/l, metal hydroxides will precipitate in the treatment solution. The case is also unfavorable.

In addition, NH4CA', (NH4) 2S is added to the treatment solution.
O.

It is also preferable to contain ammonium compounds such as.

These compounds can prevent precipitation of metal hydroxides and improve the stability of the treatment solution.

The pH of the treatment liquid is within the range of 7-12.

If the pH is less than 7, the formation of a boehmite film on the aluminum surface will be insufficient, and if the pH exceeds 12, the etching of the aluminum surface by the alkali will proceed too much, which is not preferable in either case.

A particularly preferred pH range is 8-11.

Examples of the pH adjuster include ammonia water, amines, alkali metal or alkaline earth metal hydroxides, and aluminates.

In particular, triethanolane is preferable because it increases the stability of the treatment solution and also has the effect of preventing precipitation of metal hydroxides.

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 metal chloride or sulfate in this 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.

Note that the corrosion resistance of the film can be further improved by subjecting the boehmite film formed according to the present invention to high-pressure steam treatment at a pressure of about 1 to 5 kg/crA.

As described above, according to the present invention, since a treatment liquid containing one or more types of alkaline earth metal sulfates is used, it is possible to form a boehmite film that is thick and has excellent corrosion resistance. 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 of an aluminum material of JIS Al10O-H24 under the conditions shown in Table 1.

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

[In addition, 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 alkali resistance value is evaluated by evaluating the corrosion resistance against alkalinity by measuring 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 JISH 86811 - Corrosion resistance test method for anodic oxide film. This is what I did.

Corrosion tests using a pitting corrosion accelerating solution were conducted using NaC# (0,00
The sample was immersed in a liquid containing Cu 804 .5H20 (0,005% by weight).

Claims (1)

[Claims] 1. Forming a boehmite film on the aluminum surface by immersing aluminum in a treatment solution containing one or more alkaline earth metal sulfates and having a pH of 7 to 12 at a temperature of 600 to boiling. Method.