JPS6320318B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for surface treatment of aluminum or its alloy in order to obtain aluminum products with excellent corrosion resistance. Aluminum and its alloys (hereinafter referred to as aluminum) are lightweight, and with appropriate surface treatment, corrosion resistance can be improved.
It is well known that it has excellent electrical and heat transfer performance, so it is widely used in construction materials, aircraft materials, heavy industrial products, general household goods, etc. In particular, its excellent heat transfer and light weight properties lead to reduced product weight and energy savings, so its use is likely to continue to expand in the future. Currently, there are many types of surface treatments for the purpose of preventing corrosion of aluminum products, and these are being put into practical use in each field. Specific examples include the following. (1) Chemical conversion treatment using dichromate or a chemical treatment solution containing chromate; (2) Anodization treatment in an aqueous solution of sulfuric acid, oxalic acid, or chromic acid to form a protective oxide film on the surface. (3) After the treatment in (1) and (2), apply an appropriate coating. (4) After the treatment in (2), immerse the aluminum in boiling water or expose it to steam to coat the aluminum inside the porous oxide film. Improve corrosion resistance by forming hydrates and blocking pores (referred to as sealing treatment). (5) Diffusion and penetration of zinc into the aluminum surface at a concentration of several to 10%. , it protects aluminum from corrosion through its sacrificial anode action, etc. The present invention belongs to the technical field of processing (5), and is intended to improve the following drawbacks of processing (5). That is, since the method (5) relies on the sacrificial anode action of zinc, it inevitably corrodes faster than the aluminum base material. For this reason, the surface of aluminum treated with this treatment develops white rust and corrodes more quickly than that of untreated aluminum, which has the disadvantage of seriously deteriorating the appearance. However, while zinc is present, this corrosion is limited to the surface layer and does not penetrate deeply into the interior, but once the zinc has been consumed, corrosion will eventually penetrate inside and cause major damage. A corrosion phenomenon peculiar to aluminum occurs. Therefore, in order to maintain the effects of zinc over a long period of time,
It is important to minimize consumption of zinc while taking advantage of this effect, but this method has not been developed at present. The purpose of the present invention is to develop a method that can improve the drawbacks of the above-mentioned aluminum corrosion prevention method that utilizes the sacrificial anode action of zinc. After zinc is diffused into the aluminum surface, the surface is anodized. In particular, the above objectives have been achieved. In the present invention, zinc is diffused into the surface of an aluminum base material, and the surface is anodized to form an aluminum oxide film containing zinc on the zinc diffusion layer. That is, the present invention performs zinc diffusion and penetration treatment on aluminum or its alloy, and then anodizes the surface to elute some zinc and form an oxide film containing aluminum as the main component. The present invention relates to a surface treatment method for aluminum or its alloy, which is characterized by controlling the concentration of diffused zinc. This method has the following characteristics. (1) The concentration of zinc in the cross section of an aluminum base material subjected to zinc diffusion varies depending on the diffusion conditions (temperature and time), but it can be determined by immersing aluminum in a caustic soda solution containing zinc, which is the usual method. A method in which zinc is deposited on the surface and then heated after washing with water to diffuse into the interior has the disadvantage that the amount of zinc deposited on the surface is not always constant.
Therefore, if the diffusion treatment is performed at a constant temperature and time,
Although the depth of diffusion of zinc into the aluminum base material can be controlled, the concentration of zinc in the outermost layer will be high when the amount of adhesion is large, and low when the amount of adhesion is small. This phenomenon greatly affects the anticorrosion effect of zinc. In the present invention, even in the above state,
For example, when anodizing in oxalic acid, high concentrations of zinc dissolve into the solution even during the oxidation treatment, and when the zinc concentration decreases and reaches a certain concentration, an oxide film on aluminum begins to form. The elution of zinc is stopped, resulting in the following effects. (a) The zinc concentration in the aluminum base material can be kept almost constant. (b) Since zinc is contained in the aluminum oxide film formed on the surface, this is effective in improving the corrosion resistance of the oxide film. (c) Even if the oxide film is corroded during use as an aluminum product, the zinc that has diffused and penetrated into the aluminum base material directly below it will not penetrate deeply into the interior, and the sacrificial anode effect of zinc will continue. can be used effectively. (iv) Since the oxide film can be sealed by conventional methods, the present invention can utilize the effects of both the anodic oxide film, which has excellent corrosion resistance, and the zinc diffusion treatment. (e) Since the zinc concentration on the surface can be kept constant, the quality of the anodic oxide film containing zinc can also be kept constant, making quality control easy and making it possible to always obtain a good film. (2) Since zinc does not evaporate or evaporate even when the members of the present invention are subjected to processing such as heating in a vacuum container, there are no processing restrictions on the effect (zinc diffusion-treated materials are When heated in a container, zinc, which has a high vapor pressure, volatilizes, making this type of processing impossible in the past.) As described above, the present invention (1) performs zinc diffusion infiltration treatment on aluminum or its alloy, and then performs anodization treatment on its surface to elute some zinc and oxidize aluminum as the main component. The present invention relates to a surface treatment method for aluminum or its alloy characterized by controlling the diffused zinc concentration by forming a film, and the present invention also provides (2) the anodizing treatment of (1) above. After that, hydrates of zinc and aluminum are generated in the oxide film by immersion in boiling water or exposure to steam,
Its corrosion resistance can also be improved. Furthermore, in the present invention, after carrying out the treatments (3) and (1), the zinc in the oxide film is changed into insoluble zinc chromate by immersing it in dichromic acid or a chemical solution containing chromic acid. By passivating the oxide film itself, its corrosion resistance can be improved, and after performing the treatments in (4) and (2),
After the treatment (3) with dichromic acid or chromic acid solution to improve its corrosion resistance, or after the treatment (5) (1), it is immersed in a solution containing a silicon compound to form an oxide film. By filling silicon compounds (silica sol, silica gel, etc.), a composite film containing zinc (part of the diffused zinc), aluminum (oxide film and part of the base material), and silicon is formed, and this is sealed with boiling water. Its corrosion resistance can also be improved by treatment, steam sealing, or immersion in a chemical conversion solution containing dichromate or the like. The individual treatment methods used in the present invention, such as zinc diffusion treatment and anodic oxidation treatment, are known, and commonly used conditions can be employed, but generally speaking, they are as follows. (1) Zinc replacement method After washing the surface of aluminum or its alloy with water, immerse it in alkali (caustic soda 10%) for 30 seconds or in concentrated nitric acid (60wt%) for about 2 to 5 seconds, and then wash it with water. Immerse in the following alkaline solution for about 30 seconds to 2 minutes. By this operation, a part of the aluminum or its alloy is dissolved in the alkali, but instead, zinc in the liquid is deposited on the surface of the aluminum or its alloy. Zinc oxide 70-100g/Liquid temperature 25℃ Caustic soda 400-500g/ (2) Zinc diffusion treatment After performing the treatment in (1), wash with water to remove excess alkalinity. In this case, the deposited zinc remains on the aluminum, so when it is placed in an electric furnace and heated, the zinc diffuses into the aluminum base material.
Although the temperature conditions vary depending on the purpose, it is generally carried out below the melting point of zinc (approximately 420°C). (3) Anodic oxidation treatment Using aluminum or its alloy as an anode, electricity is applied in the following electrolytic solution.

【table】

[Table] (4) Boiling water, steam treatment Boiling water: immerse deionized water at 80-100℃ for 10-60 minutes Steam treatment: Leave in steam at 120-150℃ for 20-60 minutes (5 ) Dichromate, chromate Potassium dichromate or sodium dichromate is usually used, but potassium chromate and sodium chromate are also effective. 5-20 minutes in a 2-4% solution at 80-100℃
Immersion (6) Silicon treatment compound Sodium silicate is the main agent, but methyl silicate can also be used. FIG. 1 shows an example of the distribution of the zinc diffusion treatment temperature and the zinc concentration inside the aluminum base material in the present invention. In the figure, A, B, and C are 300℃, 400℃, and 500℃, respectively.
This is the zinc concentration on the surface after zinc diffusion treatment.It varies considerably depending on the temperature. D is a zinc diffusion layer that dissolves during anodizing treatment, and E is an anodic oxide film layer, and this oxide film contains a zinc compound. A', B', and C' are the zinc concentrations when anodized after zinc diffusion at each of the above temperatures, and the differences due to temperature are small. Although the zinc concentration on the surface of this oxide film has decreased, a sufficient concentration remains to prevent corrosion of the aluminum base material. As can be seen from the figure, the zinc concentration on the aluminum surface changes depending on the diffusion temperature (more precisely, time also influences). Therefore, there is a difference in corrosion resistance in this state (high zinc concentration causes zinc to corrode quickly and cause white rust). However, when the zinc oxide layer is subsequently anodized, this high-concentration zinc layer is dissolved, and an aluminum oxide film is formed from the stage when the zinc concentration has decreased to a certain level, which prevents subsequent elution of zinc. Therefore, a treated layer consisting of the anodic oxide film and the zinc diffusion layer of the present invention is formed. Moreover, since this anodic oxide film contains zinc, its corrosion resistance is further improved. Example 1 The aluminum materials used were two types of JIS H 4000 (1982) standard 1050 and 3003, each with a width of 50 ×
Finished with dimensions of 100 mm long x 1 mm thick. The aluminum material was sequentially subjected to the following treatments. (1) Pretreatment: immersion in 10% caustic soda at 50℃ for 30 seconds → washing with water, (2) Zinc replacement treatment: immersion in an aqueous solution containing 450g of caustic soda and 60g of zinc oxide at 40℃ for 3 minutes → washing with water → drying, (3) Zinc diffusion treatment: 350℃ x 5h, (4) Anodizing treatment: 4% oxalic acid, electrolysis as anode at 25℃ for 0.5 hours, (5) Sealing treatment: 30 minutes immersion in boiling water or
(6) Passivation treatment: Sodium dichromate 5g / Potassium dichromate 5g / Sodium carbonate 4g / 90°C x 30 minutes immersion (4) Performed on items after the treatment in 5). The corrosion resistance of the aluminum material subjected to the above treatment was tested. Evaluation of corrosion resistance is based on salt spray test JIS
A test was conducted continuously for 1000 hours using the method specified in Z 2371, and the appearance of white rust was evaluated. In this test, for comparison, we used a known method of sealing the pores by anodic oxidation without any treatment, immersion in boiling water or exposure to steam, and a method of zinc diffusion infiltration treatment. Table 1 shows the results of these salt spray tests.

[Table] (Notes) ◎: No abnormality, ○: Discoloration but no white rust, △: White rust occurs, ×:
Both white rust and pitting occur.
As is clear from this result, all comparative products had white rust mainly composed of aluminum hydrate or white rust composed of zinc and aluminum hydrate (zinc diffused) occurring on the entire surface. However, pitting corrosion was observed in those that were not passivated. When zinc diffusion permeation materials were not passivated, pitting corrosion was noticeable in areas where zinc had completely disappeared. On the other hand, when the product of the present invention is not subjected to chemical conversion treatment, it discolors to grayish white, but no white rust occurs, and when it is passivated, almost no abnormalities are observed and it shows good corrosion resistance. . In addition, as a result of investigating the zinc concentration in the cross section of the present invention material using an X-ray microanalyzer, it was found that zinc was present in the pores of the anodic oxide film, and also in the aluminum base material directly under the oxide film. 0.3-0.5% zinc remained. Therefore, even if the oxide film is consumed by carrying out a corrosion test for a longer period of time, the corrosion resistance can be maintained as long as residual zinc is present. Example 2 Using aluminum 1050 material, anodic oxidation treatment was performed on aluminum 1050 materials under different zinc substitution conditions and diffusion treatment conditions, and the relationship between zinc concentration and diffusion depth was investigated. (1) Zinc replacement treatment: Caustic soda 450g/, zinc oxide 60g/, immersion temperature 25℃, 50℃ (2) Zinc diffusion treatment 300℃×5h, 24h 400℃×5h 500℃×5h (3) Anodic oxidation treatment 4 % oxalic acid as an anode for 0.5 hours at 25° C. After zinc diffusion treatment and anodization treatment, the surface zinc concentration and diffusion depth were investigated using an X-ray microanalyzer. The results are shown in Table 2.

[Table] Zinc's anticorrosion effect can be expected even at locations beyond the diffusion depth.
In the zinc substitution treatment, the longer the immersion time at the same temperature, the higher the zinc concentration on the surface of the aluminum substrate, and in the diffusion treatment, the higher the temperature, the lower the concentration. However, the diffusion depth becomes longer. Therefore, it takes a long time to obtain a deep zinc diffusion layer at low temperatures, and the working efficiency becomes extremely poor. A deep diffusion layer can be obtained by heating at a high temperature, but in this case, the diffusion of zinc mainly passes through the grain boundaries of the aluminum base material, so there is a risk that the base material will become brittle. On the other hand, when anodic oxidation is performed, some of the zinc on the surface of the aluminum base material is eluted into the oxalic acid solution and an oxide film on aluminum is formed at the same time, so the zinc concentration is not as large as the difference in the processing conditions and is uniform. There is a tendency to become In particular, when diffused at high temperatures for a short period of time, even if the crystal grain boundaries of the aluminum base material are weakened, an oxide film is formed on the surface, which can compensate for the weak points and shorten the diffusion work time. It is highly effective. In the present invention, the anodizing treatment was carried out in an oxalic acid solution, but as is clear from its nature, it is also possible in solutions of chromic acid, sulfuric acid, etc., and in particular, in chromic acid, the passivation treatment may be omitted. (See Table 3).

[Table] [Remarks] Sealing treatment conditions Passivation treatment conditions and salt spray corrosion test conditions are the same as in Table 1.
In the case of sulfuric acid, the zinc concentration on the surface decreases due to the intense elution of zinc, but the treatment itself is possible. It is clear from the results of Examples 1 and 2 that it is possible to process aluminum alloy materials that already contain 5 to 6% zinc, such as 7075 of the JIS H4000 (1982) standard. Since the mechanical properties of 1050 and 3003 are quite different, it is important to select materials from this perspective. Furthermore, as a matter of course, it is also possible to apply appropriate coating after carrying out the present invention. Example 3 Using 1050, 3003 and 5005 materials, after performing treatments (1) to (4) in Example 1, the following treatment was performed,
Its anticorrosion effect was investigated by a 1000h salt spray test. (5) Immersed in an aqueous solution containing 3% sodium silicate, then pulled up and dried. (6) Dipped in an ethyl silicate solution, pulled up and dried. (7) Implemented on items treated in (5) or (6). Example 1
(6) Passive treatment is carried out. The test results are shown in Table 4, and show much better corrosion resistance than the comparative example shown in Table 1, which was subjected to the corrosion test under the same conditions.

[Table] (Notes) ◎: No abnormality
○: Discoloration but no white rust Example 4 Using aluminum materials 1050 and 3003 as samples, zinc substitution treatment → anodization in oxalic acid was performed according to the procedure of Example 1, and then sealing treatment was not performed. The passivation treatment of Example 1 was carried out. On the other hand, we created a product without passivation treatment and conducted continuous salt spray tests.
The corrosion resistance was evaluated after 1000 hours. The results are shown in Table 5, and the passivation treatment showed no white rust at all and was in good condition. In addition, those that were not subjected to passivation treatment were discolored to gray, but white rust did not occur. As shown in Table 1 above, if zinc diffusion treatment is not performed, white rust is observed even after anodization and passivation treatment. Therefore, in this example, the effect of zinc diffusion treatment and It can be said that the effect of the subsequent passivation treatment has been recognized.

[Table] (Notes) ◎ No abnormality, ○ Discoloration but no white rust Example 5 After performing the treatments 1 to 4 of Example 1 using aluminum materials 1050 and 3003 as test materials, Example 3-5 and 6
After that, the passivation treatment of Example 1 was performed again, and the corrosion resistance was evaluated by a continuous salt spray test for 1000 hours. The results are shown in Table 6, and if the method of the present invention was applied, there were no abnormalities even after 1000 hours of testing, and the product remained healthy.

[Table] (Notes) ◎No abnormalities ○Discoloration but no white rust

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the distribution of zinc diffusion treatment temperature and zinc concentration inside an aluminum base material in the present invention.

Claims (1)

[Claims] 1. After subjecting aluminum or its alloy to zinc diffusion and penetration treatment, the surface thereof is subjected to anodic oxidation treatment to elute some zinc and form an oxide film containing aluminum as the main component. characterized by suppressing the concentration of diffused zinc,
A method for surface treatment of aluminum or its alloy. 2. After zinc diffusion and penetration treatment is applied to aluminum or its alloy, the surface is anodized to elute some zinc and form an oxide film mainly composed of aluminum, and then immersed in boiling water. A method for surface treatment of aluminum or an alloy thereof, comprising forming a hydrate of zinc and aluminum in an oxide film by immersion or exposure to water vapor. 3 After zinc diffusion and penetration treatment is applied to aluminum or its alloy, the surface is anodized to elute some zinc and form an oxide film mainly composed of aluminum, and then dichromate is applied to the surface. A surface of aluminum or its alloy, which is characterized in that zinc in the oxide film is changed into insoluble zinc chromate and the oxide film is made passivated by immersing it in a chemical solution containing salt or chromate. Processing method. 4 After zinc diffusion and penetration treatment is applied to aluminum or its alloy, the surface is anodized to elute some of the zinc and form an oxide film mainly composed of aluminum, and then exposed to boiling water. It is characterized by forming a hydrate of zinc and aluminum in the oxide film by immersing it in water or exposing it to water vapor, and then immersing it in a dichromate or a chemical solution containing chromate. A method for surface treatment of aluminum or its alloy. 5 After zinc diffusion and penetration treatment is applied to aluminum or its alloy, the surface is anodized to elute some zinc and form an oxide film mainly composed of aluminum, and then silicon compound A composite film containing zinc, aluminum and silicon is formed by immersing it in a solution containing a silicon compound to fill the oxide film with a silicon compound, and then subjecting it to boiling water sealing treatment and steam treatment. A method for surface treatment of aluminum or its alloy. 6 After zinc diffusion and penetration treatment is applied to aluminum or its alloy, the surface is anodized to elute some zinc and form an oxide film mainly composed of aluminum, and then a silicon compound is applied. A composite film containing zinc, aluminum and silicon is formed by immersing it in a solution containing silicon to fill the oxide film with a silicon compound, and then immersing it in dichromic acid or a chemical solution containing chromic acid. A method for surface treatment of aluminum or its alloy, characterized by: