JPH0445599B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for pretreating aluminum or an aluminum alloy. (Prior art and its problems) Aluminum easily oxidizes in the air, forming an oxide film on its surface. This natural oxide film inherently has corrosion resistance. However, natural oxide films are usually extremely thin and non-uniform, resulting in insufficient corrosion resistance as a whole. Therefore, aluminum or aluminum alloys are subjected to various pretreatment methods in order to improve their corrosion resistance and adhesion during painting. Pretreatment methods for aluminum or aluminum alloys include anodizing (alumite) and boehmite (α-Al ₂ O ₃ H ₂ O), which is a chemical conversion treatment for aluminum or aluminum alloys in water.
Bayerite (β-Al ₂ O ₃ 3H ₂ O) treatment and acid chromate treatment are generally known.
Widely applied. In the case of anodizing, not only is it difficult to form a uniform film depending on the aluminum material, but AC4C materials used in aluminum castings and automobile wheels are particularly susceptible to alkaline etching, sulfuric acid, and phosphoric acid baths. Anodic oxidation causes problems such as the loss of the luster of the material and the occurrence of gray or black coloration that remains even after clear coating. Additionally, there are problems in terms of the size of the facility used for anodizing and the economical efficiency. In the case of chemical conversion treatments, boehmite and bayerite treatments have problems such as poor corrosion resistance or low-temperature adhesion, and are particularly prone to thread rust. Although acid chromate treatment has excellent rust prevention properties, when used on articles with sharp edges such as automobile wheels, thread rust occurs from the sharp edges. Therefore, in order to prevent the occurrence of thread rust, various pretreatment agents, paints, and conditions for their use have been investigated, but no satisfactory comprehensive solution has been found. (Object of the Invention) As a result of research to solve these problems, it was discovered that very good results can be obtained when aluminum is treated with a small amount of electricity in an aqueous phosphate solution. (Structure of the Invention) That is, the present invention provides for applying aluminum or an aluminum alloy to a voltage of 100 to 250 V in a phosphate aqueous solution with a pH of 6 to 12.
The present invention relates to a pretreatment method for aluminum or aluminum alloy, which is characterized by carrying out anodic electrolytic treatment at a voltage of . The phosphates used in the present invention include ammonium phosphate, potassium phosphate, sodium ammonium hydrogen phosphate, diammonium hydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, dihydrogen phosphate, Ammonium hydrogen, potassium dihydrogen phosphate, calcium dihydrogen phosphate, sodium dihydrogen phosphate, manganese dihydrogen phosphate, lithium dihydrogen phosphate, and as diphosphates, potassium pyrophosphate, sodium pyrophosphate, pyrroline. Examples of the acid sodium dihydrogen and triphosphate include sodium tripophosphate. A treatment liquid is obtained by dissolving the above phosphate in water, preferably pure water. The concentration of phosphate in the treatment solution is 0.1 to 15% by weight, preferably 1 to 15% by weight.
It is 5% by weight. If it is less than 0.1% by weight, the amount of current will be small and the processing will take time. If the amount exceeds 15% by weight, the washing water will be contaminated and it will be extremely uneconomical. The temperature of the treatment liquid is suitably 5 to 40°C, preferably 15 to 30°C. Due to the treatment of the present invention, some interference colors may appear, but these will disappear when clear coating is applied. A titanium compound, a tin compound, a zirconium compound, etc. may be added to the treatment liquid in advance in order to prevent uneven treatment and to increase the effect of suppressing thread rust. The titanium compound is a water-soluble compound such as titanium sulfate, the tin compound is such as tin sulfate, and the zirconium compound is zirconium sulfate. In addition, a chromate compound may be mixed into the treatment liquid. Chromate compounds work to improve corrosion resistance and are very effective against thread rust.
The amount of chromate compound added is preferably 50 ppm ~
It is 1000ppm. Below 50 ppm, the addition of chromate compounds has no effect. In addition, if it exceeds 10,000 ppm, the object to be treated will turn yellow and the color will not disappear even after painting.
Examples of chromate compounds include chromium sulfate and chromic anhydride, which are soluble in water and generate chromate ions. According to the present invention, a phosphoric acid film is uniformly formed on the aluminum surface by using aluminum or an aluminum alloy as an anode and applying a direct current between it and a counter electrode. In this case, aluminum does not lose its luster at all, but rather, depending on the energization conditions, a film with beautiful interference fringes can be obtained. This film is considered to be a non-conductor, and the current value decreases over time when current is applied at a constant voltage. Furthermore, it can be seen that a film is formed because the voltage increases when a constant current is applied.
However, since it does not form a perfect insulator,
After the treatment of the present invention, a coating film can be further formed by electrodeposition coating. As the treatment bath, since the pH of the treatment liquid is 6 to 12, an iron container can be used in this case. In addition, the electricity is applied at 100 to 250V for 5 to 10 minutes. In the case of constant current energization, the current density is 0.2 to 5 A/100 ^cm2 for 5 seconds to 10 minutes, preferably 0.5 to 2 A/
Electricity is applied for 30 seconds to 2 minutes at 100 cm ² , that is, an amount of electricity of 1.0 to 3000 coulombs/cm ² , preferably 15 to 240 coulombs/cm ² . (Effects of the Invention) According to the present invention, rust in aluminum or aluminum alloy, particularly thread rust, can be suppressed by a simple treatment technique of applying a small amount of electricity to aluminum or aluminum alloy in a phosphate aqueous solution. In particular, since only a small amount of current is required, equipment can be downsized and costs can be reduced. (Example) Examples will be described below. Example 1 1960 g of pure water was taken in a stainless steel beaker, and 40 g of sodium pyrophosphate was dissolved in this pure water to prepare a treatment liquid. Next, an aluminum alloy plate (AC4C) was immersed in the treatment liquid to serve as an anode, and a stainless steel beaker was used as a cathode, and a DC voltage of 200 V was applied between these electrodes for 7 minutes. During this time, the temperature of the treated solution was maintained at 25° C. and stirred with a magnetic stirrer. After energizing,
Pull up the aluminum alloy plate from the treatment solution for 80~
It was dried at 120°C for 10 minutes. Next, a thermosetting acrylic resin clear was applied to the treated board, and after baking and drying, a thread rust test was conducted. The results are shown in Table-1. Example 2 The same operations and tests as in Example 1 were carried out, except that 1960 g of pure water was taken in a stainless steel beaker, and 40 g of sodium tripolyphosphate was dissolved in this pure water to prepare a treatment solution (pH 9.8). The results are shown in Table-1. Example 3 The same procedure as in Example 1 was carried out, except that 1960 g of pure water was placed in a stainless steel beaker, and 40 g of a mixture of 5 parts of titanium sulfate and 95 parts of sodium pyrophosphate was dissolved in the pure water to obtain a treatment solution (pH approximately 7.5). and conducted a test. The results are shown in Table-1. Example 4 The same procedure as in Example 1 was carried out except that 1960 g of pure water was taken in a stainless steel beaker, and 40 g of a mixture of 5 parts of tin sulfuric acid and 95 parts of sodium pyrophosphate was dissolved in this pure water to obtain a treatment solution (pH 7.0). Performed operations and tests. The results are shown in Table-1. Example 5 1997 g of pure water was placed in a stainless steel beaker, and 3 g of a mixture of 5 parts of zirconium sulfate and 95 parts of sodium pyrophosphate was dissolved in the pure water to obtain a treatment solution (pH 7.2). The procedure was the same as in Example 1. and conducted a test. The results are shown in Table-1. Example 6 The same procedure as in Example 1 was carried out, except that 1680 g of pure water was taken in a stainless steel beaker, 300 g of sodium tripolyphosphate was dissolved in this pure water, and then 20 g of chromic anhydride was dissolved to obtain a treatment solution (pH 6.0). and conducted a test. The results are shown in Table-1. Comparative Example 1 An aluminum alloy (AC4C) was immersed in an electrolytic solution according to JISH9500.7.2 to serve as an anode, and current was applied between it and a counter electrode at a current density of 2 A/100 cm ² for 5 minutes (alumite film thickness 1 to 2 μm). Thereafter, it was thoroughly washed with water, dried at 100 to 120°C for 10 minutes, and then a thermosetting acrylic resin clear was applied, and after baking and drying, a thread rust test was conducted. The results are shown in Table-1. Comparative Example 2 Aluminum alloy (AC4C) was subjected to a standard treatment with known chromic anhydride (5000 ppm) without energization, then a thermosetting acrylic resin clear was applied, and a yarn rust test was conducted after baking and drying. The results are shown in Table-1. Comparative Example 3 Aluminum alloy (AC4C) was degreased and immediately painted in the same manner as in Comparative Example 1, and after baking and drying, a thread rust test was conducted. The results are shown in Table-1. Comparative Example 4 The same process as in Example 1 was carried out, except that phosphoric acid was used instead of sodium pyrophosphate, the pH was adjusted to 3, and a DC voltage of 30 V was applied for 2 minutes, and the thread rust test was carried out. was carried out. The results are shown in Table-1. Thread rust test method: Place a cross-shaped cut (reaching to the substrate) on the painted board and perform salt water spray test for 48 hours, then relative humidity 80-90.
%, and leave in a sealed container adjusted to a temperature of 40°C for 480 hours.

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Claims (1)

[Scope of Claims] 1. A pretreatment method for aluminum or aluminum alloy, which comprises subjecting aluminum or aluminum alloy to anodic electrolysis treatment at a voltage of 100 to 250 V in a phosphate aqueous solution having a pH of 6 to 12. 2. The method according to item 1, wherein the phosphate is a polyphosphate. 3. The method according to item 1, wherein the aqueous solution contains at least one of a titanium compound, a tin compound, a zirconium compound, and a chromate compound. 4. The method according to item 1, wherein the concentration of phosphate in the aqueous solution is 0.1 to 15%. 5 The concentrations of titanium compounds, tin compounds, zirconium compounds, and chromate compounds in the aqueous solution are
The method according to item 3, wherein the amount is 50 ppm to 10,000 ppm. 6 The amount of electricity for electrolytic treatment is 1.0 to 3000 coulombs/100
The method according to paragraph 1, wherein cm ² . 7. The method according to item 1, wherein the amount of electricity in the electrolytic treatment is 15 to 240 coulombs/ ^cm2 .