JPH0319303B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to surface treatment of aluminum. Aluminum surfaces herein are intended to include surfaces of aluminum and alloys formed primarily from aluminum and which can be treated in a manner similar to aluminum.

It is widely known to clean aluminum surfaces by contacting them with aqueous alkaline cleaning solutions. The pH of the cleaning solution is generally between 9 and 10 when the solution does not contain components that inhibit alkaline attack of aluminum, but the PH can be as high as 12 or 13 when the solution contains components that inhibit alkaline attack of aluminum. good. Examples of substances that inhibit alkaline attack of aluminum include sodium phosphate and sodium silicate. It is known to use complexing agents or similar substances in alkaline cleaning solutions.

For example, it is often necessary to form a coating on a clean aluminum surface that has been cleaned by treatment with one of these cleaning solutions, and conventional coating compositions for aluminum include ferricyanide or Solutions containing hexavalent chromium and fluoride with the addition of promoters such as molybdates may be mentioned. Although these solutions form coatings that provide good substrates for painting, they pose severe environmental problems in that fluoride and hexavalent chromium are both undesirable from a health and safety standpoint. For example, waste fluids containing hexavalent chromium must be treated before being discharged. These solutions are acidic and therefore the equipment applying the solutions must be resistant to both acids and fluorides.

Alkaline methods for forming coatings are also known, but these involve the application of hot solutions containing hexavalent chromium, and therefore environmental concerns exist as well.

Recently, methods have been developed for forming hexavalent chromium-free coatings. Therefore, although these methods can avoid the dangers due to the toxicity of hexavalent chromium, they are generally acidic and contain fluorides, so they also pose environmental problems. It also requires equipment that is resistant to acids and fluorides. For example, these solutions contain tannins, titanium or zirconium, and fluorides.

The object of the invention is to provide a method for forming a coating on aluminum which is a good substrate for painting and which does not have the disadvantages of using hexavalent chromium, fluorides or acidic solutions, and which allows the formation of the coating to be rapid. It's about doing.

Contains no fluorides, chromates or inhibitors
It has a pH of 9 to 12.5, has a carboxylic acid group, and exists as a free acid or a water-soluble salt.
By contacting the aluminum surface with an aqueous solution containing a compound dissolved in an amount of 0.5 to 20 g/m and an oxidizing agent consisting of a persalt or a peroxide, a paint can be formed without any chemical treatment. It has been discovered by the inventors to form a coating on the surface of aluminum that can be applied thereon.

The present invention provides a method for producing a coating, a solution by which the coating can be formed, and a method for forming the coating and optionally on the coating after washing and drying without intervening chromating and other coating steps. A method of forming a paint coating is included.

Although most aqueous alkaline cleaning solutions merely clean aluminum, the inventors found that when applied under normal conditions,
For example, it has been observed in various experiments that slightly colored coatings are formed when they contain certain types of complexing agents and are applied at higher than normal temperatures. This coating was initially considered by the inventors to be undesirable, or at least not useful. This coating does not interfere with subsequent normal coating processes such as treatment with hexavalent chromium and fluorides, so the colored film formed by the cleaning solution is likely the substrate for painting. It was discovered that during the formation of the coating acting as Now, it has further been discovered that this coating can be increased in thickness and improved to form a thicker coating, and that this coating alone can be used as a substrate for paints. As a result of the invention:
It is at least possible to form a coating on aluminum that acts as a good substrate for painting without the need to use acidic, fluoride or chromate solutions.

This coating that is formed increases in thickness favorably over any coating observed in our experimental work using cleaning solutions even under normal conditions. For example, although the weight of these coatings was too small to be measured, the weight of the coating in the present invention is generally at least 0.05 g/ ^m2 ;
In most cases it is 0.1-1.0 g/ ^m2 .

The solution must contain a compound having a carboxylic acid group and an oxidizing agent consisting of a persalt or a peroxide. This compound is in the form of a free acid or a water-soluble salt. Such compounds include, for example, acetic acid, citric acid,
Included are oxalic acid and their alkali metal salts or other water-soluble derivatives. However, it is preferred that the compound used in the invention is a metal complexing agent containing at least one carboxylic acid group. Preferred complexing agents are aminocarboxylic acids such as alanine, glycine, ethylenediamine, ethylenediaminetetraacetic acid (EDTA) and nitrile triacetic acid (NTA) and their alkali metal salts or other water-soluble derivatives thereof.

The amount of complexing agent or other dissolved compound is generally between 0.5 and 20 g/g/. If the compound is an aminocarboxylic acid, the amount measured as its sodium salt is preferably 1 to 10 g/g/. If the amount is too low, no coating will form, and if the amount is increased beyond the upper limit mentioned, no advantage will be obtained.

The pH of the solution should be between 9 and 12.5. If the pH is less than 9, a good coating will not be formed, and if it exceeds 12.5, the tendency to corrode the metal increases. Generally the PH is between 9.5 and 12, most preferably between 10 and 11.5. PH
Values are obtained by including a sufficient amount of a suitable alkaline compound in the solution. This compound must not inhibit the attack of the solution on the aluminum, and therefore many alkaline compounds such as sodium phosphate or sodium silicate, which are often used in cleaning solutions, are used to adjust the pH. It cannot be used for. Whether a compound is a deterrent can be easily determined experimentally, since if it is a deterrent it will significantly reduce the effectiveness of the solution when included in the solution. PH adjustment is preferably carried out by using sodium hydroxide as the alkaline substance, but other alkaline substances that do not produce inhibiting ions can also be used, such as sodium hydroxide or ammonium hydroxide. Generally, this alkaline substance is such a substance that in all cases introduces only the ions present in the tap water with which the solution is prepared.

As already mentioned, the solution must be free of detergents such as sodium phosphate or sodium silicate; other deterrents to avoid include surfactants such as quaternary ammonium salts. can be mentioned. The solution must also be free of fluorides or chromium compounds or any other coating components commonly used in the formation of coatings on metals, such as phosphates. In fact, the solution preferably consists only of water, a complexing agent or other dissolved compound as an alkali for pH adjustment, and optionally a non-inhibiting surfactant. The inclusion of surfactants is sometimes preferred, especially to increase the color of the coating. A preferred surfactant is betaine, but non-inhibitory nonionic or anionic surfactants may also be used. Cationic surfactants are generally deterrent and cannot be used.

In addition, the inclusion of an oxidizing agent is necessary because without the use of a suitable oxidizing agent, the formation of the coating will be extremely slow, requiring contact between the solution and the metal surface for more than 4 hours, typically for as long as 8 hours. This is especially necessary. Good coatings can be formed much more quickly using suitable oxidizing agents. Preferably, the oxidizing agent in solution is a persalt oxidizing agent, which may be sodium peroxide or hydrogen peroxide, a water-soluble persalt such as sodium persulfate, or a persalt such as perborate. Other compounds that release peroxide are introduced into the solution by adding them to the solution. Compounds such as persulfates are preferably introduced as alkali metal or ammonium compounds, such as sodium, potassium or ammonium persulfates. Other oxidizing agents such as nitriles, nitrates and chlorates are believed to be less effective than persalt oxidizing agents.

The amount of oxidizing agent will vary depending on the particular metal used. If this amount is too low, the processing time required to obtain a good coating cannot be adequately reduced to less than 4 hours; in the absence of the oxidizing agent, even more processing time would be required. Too much oxidizing agent will make the solution quite unstable. Generally, the amounts are such that a suitable coating is obtained in less than two hours. If the oxidizing agent is introduced as a persulfate, its amount should be from 1 to 50 g/, and most preferably from 3 to 20 g/, measured as sodium persulfate. Similar amounts of peroxide measured as sodium peroxide may be used. When the oxidizing agent is hydrogen peroxide, the amount is preferably a similar amount measured as "100 volumes" of hydrogen peroxide.

The temperature of the solution during contact between the solution and the metal surface is generally between room temperature and 50°C, preferably between 25°C and 45°C. Temperatures below room temperature (e.g. 20℃
below 50°C, the formation of the coating is slow.
At temperatures above , the presence of oxidizing agents increases the instability of the solution.

Contact between the solution and the metal surface is obtained by any method, such as dipping or spraying, and is carried out for the time necessary to obtain the desired coating. This time is always less than 24 hours, and generally less than 2 hours. This time is usually at least 1 minute, preferably 2 to 30 minutes.

After its formation, the coating is generally thoroughly rinsed with water and then dried. A paint coating is then applied in the usual manner without prior chromating or other coating treatments. The paint may be a lacquer.

As an example, the solution is 2 g/trisodium
NTA, 10g/sodium persulfate and PH
11 by dissolving enough sodium hydroxide in tap water. The temperature of the solution is brought to 35° C. and the cleaned panel of SIC grade aluminum is immersed in the solution for 15 minutes. A brown coating formed. The panels were then washed with water, dried, and then painted with baked-on acrylic paint. ASTM the resulting paint coating
Cross-scored according to B287 specifications and subjected to acetic acid accelerated salt spray test. After 500 hours, the panels were removed and the scores were taped. No paint sloughing occurred, indicating that this coating was a very good substrate for paint.

Similarly colored coatings are obtained by substituting potassium persulfate for sodium persulfate;
The use of ammonium persulfate also provides multicolored pigmented coatings that are blue or purple and pearlescent. By increasing the amount of the ingredients used, e.g.
Using trisodium NTA of g/g and sodium persulfate of 40 g/g gives a higher weight, less pigmented and pearlescent coating.

Claims (1)

[Scope of Claims] 1. Substantially free of inhibitors, fluorides or chromium hydrochloride, having a pH value of 9 to 12.5, and having a carboxylic acid group and existing as a free acid or a water-soluble salt of 0.5 A coating is formed on the surface of aluminum by contacting the surface of aluminum with an aqueous solution containing a dissolved compound and an oxidizing agent consisting of a persalt or a peroxide. How to treat aluminum surfaces. 2. A method according to claim 1, characterized in that the coating has a coating weight of at least 0.05 g/m ² . 3. A method of treating an aluminum surface according to claim 1 or 2, characterized in that the coating has a coating weight of 0.1 to 1.0 g/m ² . 4. The method for treating an aluminum surface according to any one of claims 1 to 3, wherein the dissolved compound is an aminocarboxylic acid or a water-soluble derivative thereof. 5. The method for treating an aluminum surface according to claim 4, wherein the dissolved compound is ethylenediaminetetraacetic acid, nitriletriacetic acid, or an alkali metal salt thereof. 6. A method of treating an aluminum surface according to any one of claims 1 to 5, characterized in that the solution contains a non-inhibitory nonionic or anionic surfactant. 7. The method for treating an aluminum surface according to claim 6, wherein the surfactant is a betaine surfactant. 8. The treatment of aluminum surface according to any one of claims 1 to 7, wherein the oxidizing agent is an alkali metal persulfate, ammonium persulfate, sodium peroxide, or hydrogen peroxide. Method. 9 The solution contains 3 to 20 g/oxidizing agent, 0.5
The aluminum surface according to any one of claims 1 to 8, characterized in that it contains from 1 to 20 g of an aminocarboxylic acid or a water-soluble derivative, and has a pH of 10 to 11.5. processing method. 10. The method for treating an aluminum surface according to any one of claims 1 to 9, characterized in that the temperature of the solution is between room temperature and 50°C. 11. The coated surface is washed with water, dried, and then coated with a paint without any prior coating treatment. How to treat aluminum surfaces. 12 Substantially free of inhibitors, fluorides or chromic acid, has a pH value of 9 to 12.5, and has carboxylic acid groups and is present as a free acid or water-soluble salt and is dissolved at 0.5 to 20 g/ 1. A solution for treating an aluminum surface, characterized in that it contains an oxidizing agent consisting of a persalt or a peroxide. 13. The aluminum surface treatment solution according to claim 12, wherein the oxidizing agent is an alkali metal persulfate, ammonium persulfate, sodium peroxide, or hydrogen peroxide. 14. The aluminum surface treatment solution according to claim 12 or 13, wherein the dissolved compound is an aminocarboxylic acid or a water-soluble derivative thereof. 15. The aluminum surface treatment solution according to claim 14, wherein the dissolved compound is ethylenediaminetetraacetic acid, nitriletriacetic acid, or an alkali metal salt thereof. 16 Contains 3 to 20 g/of oxidizing agent and 0.5 to 20 g/carboxylic acid, and has a pH of 10 to 10.
11.5, the solution for treating an aluminum surface according to any one of claims 12 to 15. 17. A treatment solution for aluminum surfaces according to any one of claims 12 to 16, characterized in that it contains a non-inhibitory nonionic or anionic surfactant. 18. The aluminum surface treatment solution according to claim 17, characterized in that it contains a betaine surfactant.