JPH0335378B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a surface treatment method for aluminum or aluminum alloy, in which aluminum or aluminum alloy is selected from chlorite, bromite, iodate, chlorate, bromate, and iodate. Aluminum or aluminum alloy whose corrosion resistance, hydrophilicity, and paint film adhesion are significantly improved by treatment with a basic aqueous solution containing at least one type of compound to form a hydrated oxide film on the surface. The purpose of this invention is to provide a method for surface treatment. Conventionally, the chromate method, boehmite method, MBV method, or EW has been used as a chemical conversion treatment method to generate an oxide film on the surface of aluminum or aluminum alloy (hereinafter simply referred to as aluminum alloy).
Various measures such as laws have been proposed. Among these methods, the chromate method generates an oxide film with good corrosion resistance on the aluminum alloy surface in a short time at low temperatures, and although this oxide film has good paint film adhesion, it is also water repellent. ,
A fatal drawback is that chemical conversion treatment using the chromate method cannot be used in fields where hydrophilicity is required, such as for heat exchange media (fin materials) or PS plates. In addition, the chromate method has the disadvantage that the wastewater after chromate treatment contains harmful ions such as chromium and cyanide, so that the wastewater treatment is burdensome. In addition, the boehmite method uses aluminum alloy at 80°C.
Since the treatment is carried out in demineralized water heated to a temperature higher than The drawback is that it is difficult to obtain a homogeneous film. Therefore, due to these drawbacks, this method is not suitable at all as a treatment for precoating materials when aluminum alloys are used as heat exchange medium materials. Therefore, in order to compensate for these drawbacks, it has been proposed to add ammonia or triethanolamine, etc., to accelerate the dissolution reaction and enable processing to be completed in a short time. It is difficult to balance the formation, and the corrosion resistance is not improved much even though the oxide film increases, and is inferior to, for example, a film formed by a chromate treatment method. In addition, for MBV method, EW method, etc., the processing temperature is usually
The temperature is as high as 90 to 100°C, and the processing time is usually long, 3 minutes or more. Therefore, like the boehmite method described above, it is unsuitable for processing precoated materials. In addition, a chemical conversion treatment method (Special Publications Showa) in which an aluminum alloy is treated with an aqueous solution of sodium hypochlorite or potassium hypochlorite to form an oxide film on the surface.
53-32772) has also been proposed, but the corrosion resistance of the oxide film produced by this chemical conversion treatment method depends on the material quality and tempering, the oxidizing agent concentration in the treatment bath, the amount of basic substance added, and the pH based on dissolved Al. In other words, when generating a hydrated oxide film at a treatment temperature of 70°C or higher in a basic bath where the effect of adding an oxidizing agent is recognized, if the pH of the treatment bath is too high, for example, the PH
When it is 11.5 or higher, dissolution is promoted more than oxidation,
Especially Al-Fe compounds, Al-Fe-Si compounds, Al-
Mn compound, Al-Fe-Mn compound, Al-Fe-Mn
−Al around Si compounds and intermetallic compounds
Local dissolution of the substrate is promoted, intermetallic compounds fall off, and bits with a diameter of approximately 2 to 10 μm and a depth of approximately 1 μm or more calculated from the difference in surface roughness before and after treatment are frequently generated. There were many defective parts where no oxide film was formed, and the corrosion resistance was significantly deteriorated. As a result of extensive research into a chemical conversion treatment method that produces a film with good corrosion resistance, hydrophilicity, and paint film adhesion without causing pollution problems such as drainage treatment, the inventor discovered that aluminum alloys can be treated with chlorite. , bromite, iodate, chlorate, bromate,
When treated with a basic aqueous solution containing at least one compound selected from iodates (for example, by dipping or spraying), the hydrated oxide film obtained by this surface treatment exhibits corrosion resistance, hydrophilicity, and coating adhesion. They have discovered that they are excellent in all respects, and have accomplished the present invention. Next, a method for surface treatment of aluminum or aluminum alloy according to the present invention, which produces a hydrated oxide film having excellent corrosion resistance, hydrophilicity, and coating adhesion, will be described. The concentration of the oxidizing agent such as chlorite or chlorate used in the present invention is preferably about 0.0001 to 3.5 g/, more preferably about 0.05 to 3.5 g/
The amount is 3.5 g/, particularly preferably 0.5 to 1 g/. In other words, if the concentration is too low, the oxidizing power is weak,
Also, if the concentration is too high, it will be difficult to obtain a uniform film,
Also, more oxidizing agents are taken out during processing,
It becomes uneconomical. To make the treatment liquid basic, caustic soda is usually used, but caustic potash, calcium hydroxide, sodium carbonate, boric acid, and basic amines (triethanolamine, ammonia, etc.) may also be used. However, it is important to keep the pH of the basic bath at about 11.5 or less, preferably about 10-10.
It is best to leave it at 11. In other words, if the pH becomes too high, exceeding 11.5, the base aluminum around the intermetallic compound will dissolve, resulting in the formation of many pits and a decrease in corrosion resistance.
If the PH is too small (less than 10), the amount of aluminum dissolved decreases and the rate of film formation slows, that is, the amount of film formed is small, so corrosion resistance tends to decrease. Therefore, the pH of the processing solution should be adjusted to 10 to 11.
If this is done, a balance between aluminum dissolution and oxidation will be obtained, the number of pits due to shedding of intermetallic compounds will be greatly reduced, and a dense hydrated oxide film will be produced. Regarding the processing time, longer is not better; for example, it is about 60 to 180 seconds. In other words, there is an intermetallic compound on the surface layer of the aluminum alloy, and there are also intermetallic compounds below this intermetallic compound, so even if the hydrated oxide film becomes thicker during treatment, the intermetallic compound will This is because they fall off and cause film defects in the hydrated oxide film. Therefore, if the treatment time is set to about 60 to 180 seconds and the amount of hydrated oxide film generated is about 3 to 6 mg/ ^dm2 , not only will there be fewer film defects and excellent corrosion resistance, but also productivity will be improved. will also improve. When treated in this manner, the surface condition of the hydrated oxide film is approximately 2 to 10 μm in diameter due to the shedding of intermetallic compounds, and approximately 1 μm in depth calculated from the difference in surface roughness profile before and after treatment. The number of pits generated is approximately 120/mm ² or less, and the corrosion resistance is excellent. Furthermore, as the purity of aluminum increases, the number of intermetallic compounds that exist decreases.
Furthermore, since the size is reduced, the corrosion resistance is further improved. Further, after treatment with a basic aqueous solution containing chlorite or the like, the hydrophilicity can be further improved by performing a post-treatment such as sodium silicate treatment. Next, specific examples of the present invention will be described. Examples 1 to 10 JIS1200O material (75mm x 150mm x 0.12mm) aluminum alloy was treated with sodium chlorite (Example 1),
Sodium bromate (Example 2), sodium bromite (Example 3), ammonium iodate (Example 4), ammonium iodate (Example 5), sodium baseite and sodium chloride (equal weight, Example 6), sodium chlorate (Example 7), sodium chlorite (Example 8), calcium chlorite (Example 9), magnesium iodate (Example
10) is immersed in a basic aqueous solution containing each of the above to form a hydrated oxide film on the surface. Comparative Examples 1 to 5 The same material as in Example 1 was subjected to conventional boehmite treatment (desalinated water, Comparative Example 1), chromate treatment (Alodine #1200, Comparative Example 2), and basic aqueous solution treatment without oxidizing agent ( Comparative Examples 3 and 4) and sodium hypochlorite aqueous solution (Comparative Example 5). The properties of the films obtained in Examples 1 to 10 and Comparative Examples 1 to 5 were measured as shown in the table. The condition of the formed film was determined by visual observation, the amount of film formed was determined by the film weight measurement method according to JIS H8680, and the corrosion resistance was determined by JIS H8680.
It is shown by the rating number display method after 336 hours of salt spray test with Z2371 (a number of 10 is the best, and the smaller the number, the worse it is). It shows the contact angle with water droplets (the smaller the contact angle, the better the hydrophilicity).Also, the adhesion of the paint film is determined by spray painting with polyurethane paint and then applying JIS H8681.
This shows the paint film adhesion of a sample subjected to a 48-hour CASS test (100/100 is the best, and the paint film adhesion worsens as the molecules get smaller). The number of pits generated due to the falling off of intermediate compounds is approximately 2 to 10 μm in diameter.
The number of pits with a depth of about 1 μm or more is calculated by averaging over 10 sheets of material and expressed as the number per mm ² .

【table】

[Table] As described above, the method for surface treatment of aluminum or aluminum alloy according to the present invention is to treat aluminum or aluminum alloy with chlorite, bromite, iodate, bromate, or iodate. By treating with a basic aqueous solution containing at least one compound selected from among these, a hydrated oxide film is generated on the surface of aluminum or aluminum alloy, resulting in excellent corrosion resistance, hydrophilicity, and coating adhesion. In addition, it has the advantage of being highly productive as it can be processed in a short time and can be done at low cost.

Claims (1)

[Claims] 1 Aluminum or aluminum alloy is treated with chlorite, bromite, iodate, chlorate,
treated with a basic aqueous solution containing at least one compound selected from bromates and iodates,
A method for surface treatment of aluminum or aluminum alloy, characterized by forming a hydrated oxide film on the surface of aluminum or aluminum alloy.