JP7352322B1 - Surface treatment method for aluminum or aluminum alloy - Google Patents

Abstract

An object of the present invention is to provide an anodizing treatment for aluminum or an aluminum alloy and a method for sealing the anodic oxide layer, which can reduce the porosity of the anodic oxide layer of aluminum or an aluminum alloy and seal the pores to a high degree. [Solution] Anodizing is performed at a current density of 0.8 to 3.0 A/dm2 using an electrolytic sulfuric acid bath obtained by electrolyzing sulfuric acid, and then a mixed bath containing a metal salt and a fluoride or a metal fluoride is applied. Room-temperature sealing treatment is performed using a bath containing a compound, and then pressurized steam sealing treatment is performed. [Selection diagram] Figure 1

Description

The present invention relates to a method for surface treatment of aluminum or aluminum alloy by anodizing film treatment and pore sealing treatment.

Aluminum or aluminum alloys are widely used around us. However, although a dense and stable natural oxide film is formed in the air, the film thickness is very thin, about 2 nm, and can easily corrode depending on the usage environment. Therefore, artificial oxidation treatment is performed to obtain a sufficient oxide film.
Numerous pores are formed on the surface of the oxide film formed after anodizing. The pores present in this anodic oxide film are chemically active and therefore easily react with oxygen and other chemicals, so it is common to perform a sealing treatment to improve corrosion resistance. As this sealing treatment method, boiling water sealing, pressurized steam sealing, room temperature nickel sealing, high temperature nickel sealing, etc. are known.

Conventionally, in surface treatment methods that involve anodizing aluminum or aluminum alloys and then sealing them, in order to improve corrosion resistance, efforts have been made to review the sealing treatment rather than considering the anodizing treatment. .

JP-A-50-117648 (Patent Document 1) discloses an invention related to room-temperature nickel sealing that improves corrosion resistance by containing a polar solvent and metal fluoride, and JP-A-56-062991 (Patent Document 2) ), after forming an anodized film, one or more of the commonly used sealants such as metal salts, ammonium derivatives, amine compounds, alkali hydroxides, or boron compounds are added as the first sealing treatment. A two-stage sealing treatment that improves corrosion resistance by immersing in an aqueous solution at 5 to 80°C containing a concentration ranging from .1 g/L to saturation concentration, and then immersing in water heated to 60 to 100°C as a second sealing treatment. provides law.

Furthermore, in the domestic standards for anodic oxide coatings on aluminum alloys, JIS H8601-1968 specifies hydration sealing as a sealing method, and JIS H9500-1971 and JIS H9501-1971 specify hydration sealing as a sealing method. Processing conditions are specified, and in the case of boiling water sealing, the addition of sealing aids is permitted.

In recent years, inventions have been made that combine multiple sealing treatments to improve corrosion resistance. Specifically, Japanese Patent No. 5,686,608 (Patent Document 3) combines low-temperature Ni sealing and pressurized steam sealing. The temperature of the pressurized steam sealing at that time is 140°C or higher.

In pressurized steam sealing, the higher the treatment temperature (steam temperature) (the saturated steam pressure is determined by the treatment temperature) and the longer the treatment time, the better the corrosion resistance. Furthermore, the higher the processing temperature, the shorter the processing time. (Non-patent document 1)
Therefore, at actual sites, in consideration of productivity, processing is carried out at a high temperature of 140° C. or higher and in a short period of time.

Furthermore, in Japanese Patent No. 6,667,191 (Patent Document 4), a process of low-temperature Ni sealing + pressurized steam sealing (100 to 160°C) + silica gel coating is performed. Since pressurized steam sealing is not sufficient for sealing, silica gel coating is performed.
Furthermore, in Japanese Patent No. 5878133 (Patent Document 5), pressurized steam sealing is performed after immersion in a fluorine-based polymer solution, and sealing with a polymer is added.

Japanese Unexamined Patent Publication No. 117648/1983 Japanese Unexamined Patent Publication No. 56-062991 Patent No. 5686608 Patent No. 6667191 Patent No. 5878133

Yoshiji Kamiyama et al., Metal Surface Technology Field Pamphlet, Vol. 15, No. 3, p. p. 2-8 (1968) Sachiko Ono et al., Surface Technology, Vol. 66, No. 8, p. p. 364-371 (2015)

As described above, in order to improve the corrosion resistance of automotive or outdoor aluminum alloy products, the above-mentioned conventional anodizing treatment method and anodic oxide film sealing treatment method have been used, but there are the following problems.
When current aluminum alloy products are used outdoors, the surface becomes white after about a year due to the effects of acid rain, etc., and customers have complained about parts that are easily visible to the human eye, resulting in free replacements. There are also cases where this has been done. Therefore, there is a need for a treatment method that does not cause whitening of aluminum products.

The present invention has been made in view of the above-mentioned conventional problems, and is a surface treatment for aluminum or aluminum alloy that can reduce the pore diameter of the anodic oxide film of aluminum or aluminum alloy and seal the pores to a high degree. The purpose is to provide a method.

The inventors did not seek improvement only in the conventional sealing process, but also investigated both anodizing treatment and sealing process. Specifically, by using electrolytic sulfuric acid in the anodizing bath, the porosity is reduced and the intrusion of corrosive liquid is suppressed, and the pores are sealed using pressurized steam at a low treatment temperature to shorten the length of cracks on the surface. prevent the invasion of

That is, the first embodiment of the surface treatment method for aluminum or aluminum alloy of the present invention is obtained by electrolyzing sulfuric acid having a sulfuric acid concentration of 5 to 30% by mass , and has an oxidation-reduction potential of 1,100 mV or more. Using an electrolytic sulfuric acid bath having a voltage of less than 500 mV , aluminum or an aluminum alloy is anodized at a current density of 0.8 to 3.0 A/dm ² , and then the aluminum or aluminum alloy contains a metal salt and a fluoride. It is characterized in that room temperature sealing treatment is performed using a mixed bath or a bath containing a metal fluoride compound, and then pressurized steam sealing treatment is performed at a treatment temperature of 105 to 125°C .

In another aspect of the surface treatment method for aluminum or aluminum alloy, in the above aspect of the invention, the oxidation-reduction potential of the electrolytic sulfuric acid bath used for anodic oxidation is 1,100 mV or more and less than 1,500 mV.

In another aspect of the surface treatment method for aluminum or aluminum alloy, in the above aspect of the invention, the electrolytic sulfuric acid bath has an oxidizing agent concentration of 1 to 10 g/L.

In another aspect of the method for surface treatment of aluminum or aluminum alloy, in the above aspect of the invention, the temperature of the electrolytic sulfuric acid bath during the anodizing treatment is 5 to 30°C.

In another aspect of the surface treatment method for aluminum or aluminum alloy, in the above aspect of the invention, the applied voltage during the anodic oxidation treatment is 10 to 25 V.

According to the present invention, good corrosion resistance can be obtained by reducing the surface porosity in anodizing treatment, followed by room temperature nickel sealing treatment and subsequent pressurized steam sealing.

FIG. 1 is a schematic cross-sectional view of an apparatus used in an embodiment of the present invention. FIG. 3 is a diagram comparing the corrosion resistance of various sealing methods through a hydrochloric acid resistance test. FIG. 3 is a diagram showing the relationship between oxidant concentration and redox potential. It is a figure which shows the example of the process temperature in pressurized steam sealing, and the crack length which generate|occur|produces on the surface. It is a pull bay diagram of aluminum. It is a Pourbay diagram of nickel. This is an example of redox potential. FIG. 3 is a diagram showing the relationship between porosity and hydrochloric acid resistance test.

Below, a method for surface treatment of aluminum or aluminum alloy, which is one embodiment of the present invention, will be described in detail.
The surface treatment method for aluminum or aluminum alloy of this embodiment includes an anodizing process in which pores are formed on the surface by treatment in an anodizing solution, and a sealing process on the pore surface after this anodizing process. and a sealing step.
Note that the type, composition, etc. of aluminum or aluminum alloy to be treated are not particularly limited, and any material to be subjected to anodizing treatment can be selected.

[Anodizing process]
<Mechanism of action>
Aluminum or aluminum alloy combines with oxygen in the air to form a natural oxide film as thin as about 2 nm. This form of oxide film is called a barrier-type oxide film, but since the oxide film itself does not conduct electricity, during anodizing treatment, a voltage higher than the dielectric breakdown voltage is applied, causing pores to grow in the depth direction and thickening the oxide film. do. This oxide film form is called a porous oxide film. As the voltage is lower, the number of dielectric breakdown points is smaller and the current density is also lower, so less aluminum ions are eluted and the pore diameter becomes smaller (Non-Patent Document 2).
When a voltage higher than dielectric breakdown is applied and current begins to flow, Al → Al ³⁺ + 3e ^-・・・・・・・・・・・・(1)
Al dissolves according to the reaction. The eluted Al ³⁺ reacts with oxygen generated by the water splitting reaction represented by another reaction formula (2) of the anode reaction, and becomes Al ₂ O ₃ .
3H ₂ O → 3(O) + 6H ⁺ + 6e ^-・・・・・・・・・(2)
The overall reaction of formulas (1) and (2) is expressed by the following formula (3).
2Al + 3H ₂ O → Al ₂ O ₃ + 6H ⁺ + 12e ^- (3)
As can be seen from equation (1), the anodizing process is performed with a balance between the dissolution rate of Al determined by the flowing current and the rate at which eluted Al ³⁺ is oxidized according to equation (3), that is, the oxidation rate, and the oxidation rate is The faster the pore size, the smaller the pore diameter.
The higher the redox potential of the anodic oxidation bath, the faster the oxidation rate. In the present invention, an electrolytic sulfuric acid bath obtained by electrolyzing sulfuric acid is used.

When sulfuric acid is electrolyzed, hydrogen sulfate ions (HSO ₄ ⁻ ) in the sulfuric acid solution undergo the reaction of formula (4) to form peroxodisulfuric acid (S ₂ O _{8 )} , which has a high redox potential as shown in FIG. ^2- ) is generated. If the anodic oxidation treatment is carried out in this electrolytic sulfuric acid bath, the oxidation rate of Al ³⁺ will be increased, and pores with a small pore diameter can be obtained.
2HSO ₄ ^- → 2H ⁺ + S ₂ O ₈ ^2- + 2e ^- (4)

<Processing conditions in electrolytic sulfuric acid bath>
The temperature of the anodizing treatment (processing temperature) is preferably 5 to 30°C. If the treatment temperature is too low, the treatment time will be long; if the treatment temperature is too high, the dissolution rate of Al will increase and the pore size will increase. The number of pores per unit area also decreases if the treatment temperature is too low, and increases if the treatment temperature is too high. Further, the current density is preferably 0.8 to 3.0 A/dm ² . If the current density is too low, the pores will not open, and if the current density is too high, the rate of dissolution of Al will increase and the diameter of the pores will increase. The number of pores per unit area also decreases when the current density is too low, and increases when the current density is too high. Since anodic oxidation is performed with a constant current, the voltage will vary, but will be in the range of 10 to 25V.

Further, the sulfuric acid concentration is preferably 5 to 30% by mass. If the sulfuric acid concentration is too low, pores will not be formed, and if it is too high, the pore diameter will become large. If the number of pores per unit area is too high, it will increase.

<Generation of oxidizing agent>
Peroxodisulfuric acid is obtained by electrolyzing sulfuric acid, but since peroxodisulfuric acid is unstable, it self-decomposes and becomes peroxodisulfuric acid. Since this peroxomonosulfuric acid also has a high redox potential as seen in FIG. 7, it is desirable to electrolyze the two oxidants so that the combined concentration is 1 to 10 g/L. When the oxidizing agent concentration is less than 1 g/L, the redox potential becomes less than 1,100 mV as shown in Figure 3, which is almost equivalent to a sulfuric acid bath, the oxidation rate of Al ³⁺ is the same as that of a sulfuric acid bath, and the pore diameter becomes larger, and at the same time the number of pores per unit area also increases. Although a high oxidizing agent concentration does not have any adverse effect on the anodic oxidation process, the oxidation rate of Al ³⁺ is sufficiently fast, which is disadvantageous in terms of costs such as electric power required to generate more oxidizing agent.

Further, there are no particular limitations on the method of electrolysis.
In this embodiment, the surface can be made to have fine pores by anodizing using electrolytic sulfuric acid, and for example, the porosity can be reduced to 10% or less, preferably 9% or less. can.
Figure 8 shows the relationship between porosity and hydrochloric acid resistance test. When the porosity is 9% or less, corrosion resistance that greatly exceeds the acceptance standard of 200 minutes can be obtained.

[Sealing process]
In this embodiment, the room temperature nickel sealing method and the pressurized steam sealing method are used as the sealing treatment. FIG. 5 shows a Pourbay diagram for aluminum, and FIG. 6 shows a Pourbay diagram for nickel. Aluminum oxide trihydrate (Al ₂ O _{3.3H 2} O) is stable without corrosion at pH 4.0 to 8.5, and nickel hydroxide (Ni(OH) ₂ ) is stable without corrosion at pH 9 to 12. It is. In order to provide corrosion resistance in both acidic and alkaline environments, nickel hydroxide, which is resistant to alkaline atmospheres, is precipitated in the lower layer using nickel sealing at room temperature, and then aluminum oxide hydrate, which is resistant to acidic atmospheres, is sealed in the upper layer using pressurized steam sealing. A two-stage sealing process is performed in which precipitation occurs in the pores.

<Processing conditions for room temperature nickel sealing>
As for the normal temperature nickel sealing liquid, a commercially available sealing liquid can be used. Examples of the processing conditions are shown below for reference.
・Drug concentration in sealing liquid: 3 to 7 g/L
・Processing temperature: 20-35℃
・Processing time: 0.5-2 minutes/μm
・pH: 5.2-6.5
Note that, in the present invention, the processing conditions for room temperature nickel sealing are not limited to the above conditions.

Pressurized steam sealing is the most reliable way to generate aluminum oxide hydrate, as shown in Figure 2, but the difference in linear expansion coefficient between the aluminum alloy material and the oxide film (Al alloy: 23 x 10 ^{- 6} /K, Al ₂ O ₃ :3×10 ⁻⁶ /K), cracks occur inside and on the surface of the film. Although it is almost closed at room temperature, when used outdoors, corrosive liquid can enter through this crack and cause accelerated whitening. Therefore, we invented a treatment method that achieves sufficient corrosion resistance by optimizing the sealing temperature.

<Processing conditions for pressurized steam sealing>
The treatment temperature is preferably 105 to 125°C and the treatment time is preferably 60 to 90 minutes. When the processing temperature is higher than 125° C., larger cracks are more likely to occur due to the above-mentioned difference in coefficient of linear expansion. On the other hand, if the treatment temperature is low, the rate of hydration reaction between the aluminum alloy and water and the sealing rate will be slow. If the treatment time is too short, sufficient sealing cannot be performed, and if the treatment time is too long, there is no adverse effect per se, but productivity will decrease.

In this embodiment, it is possible to obtain aluminum or aluminum alloy with suppressed whitening and excellent corrosion resistance.

Below, a processing device 1 that implements this embodiment will be described based on FIG. 1.
The processing device 1 has a processing tank 2 containing sulfuric acid, and the processing tank 2 is provided with an Al plate 8 and a cathode 9 to be subjected to anodizing treatment. A DC power supply 3 is connected between the two. Further, in the processing tank 2, a porous body 4B is arranged at the tip of an air supply line 4A, and an air pump 4 is interposed in the air supply line 4A.
Further, in the treatment tank 2, the ends of a sulfuric acid liquid supply line 5A and an electrolyte solution return line 5B, each of which has a circulation pump 5 interposed therein, are disposed. An electrolytic cell 6 is received in between. Inside the electrolytic cell 6, an anode 6A, a cathode 6B, and a bipolar electrode 6C are arranged, and the sulfuric acid to be sent passes between each electrode. A power source (not shown) is connected between the cathode 6A and the cathode 6B.

Next, a processing method using the processing device 1 will be explained.
Sulfuric acid L with a sulfuric acid concentration of 5 to 30% by mass is stored in the processing tank 2, and the circulation pump 5 is operated to circulate the sulfuric acid between the processing tank 2 and the electrolytic cell 6. During circulation, in the electrolysis cell 6, a voltage is applied between the anodes 6A and 6B to electrolyze the sulfuric acid together with the bipolar electrode 6C.
Electrolysis of sulfuric acid forms peroxodisulfuric acid, some of which self-decomposes to produce peroxomonosulfuric acid.
At this time, air is sent by the air pump 4 through the air supply line 4A, and the air is supplied from the porous body 4B to the sulfuric acid in the processing tank 2. Air is supplied to make the temperature within the processing tank uniform and to diffuse chemical species such as ions near the electrodes.
The total oxidizing agent concentration of peroxodisulfuric acid, peroxomonosulfuric acid, and hydrogen peroxide produced by self-decomposition of peroxomonosulfuric acid produced by electrolysis is set to 1 to 10 g/L.
The electrolysis of sulfuric acid may be stopped after a predetermined oxidizing agent concentration is obtained, or may be continued so as to maintain the oxidizing agent concentration.

In the treatment tank 2, an Al plate 8 and a cathode 9 are installed so as to be immersed in sulfuric acid. The installation may be performed before accommodating the sulfuric acid, or after the electrolysis has started.
Once electrolyzed sulfuric acid is prepared, anodization treatment is performed. At this time, the temperature of the sulfuric acid is preferably 5 to 30°C. The temperature of sulfuric acid can be adjusted using an appropriate heater or cooler.
In the anodizing process, a constant current voltage is applied between the Al plate 8 and the cathode 9 by the DC power supply 3. The current density is set to 0.8 to 3.0 A/dm ² . It is desirable that the oxidation-reduction potential at this time is 1,100 mV or more and less than 1,500 mV. Further, the voltage applied between the electrodes is 10 to 25V.
In the anodic oxide film treatment, the surface has fine pores, and the porosity can be reduced to 10% or less.

After the treatment of the anodized film, room-temperature sealing treatment is performed using a mixed bath containing a metal salt and a fluoride or a bath containing a fluorinated metal compound. In this case, a conventional device can be used.

After room temperature sealing treatment, pressurized steam sealing treatment is performed. In pressurized steam sealing, it is desirable that the treatment temperature be 105 to 125°C. Pressurized steam sealing can be carried out in 60 to 90 minutes, for example, and can be carried out using conventional equipment.

EXAMPLES The present invention will be explained in more detail by showing Examples and Comparative Examples below. However, the present invention is not limited in any way by these descriptions.

(Example 1)
Using the apparatus shown in Figure 1, an aluminum-magnesium-silicon alloy A6063 plate was anodized using an electrolytic sulfuric acid bath obtained by electrolyzing sulfuric acid, followed by room temperature nickel sealing and pressurized steam sealing. I did it. Table 1 shows the anodizing treatment conditions, and Table 2 shows the corrosion resistance test results.

<Generation of oxidizing agent>
The processing conditions for electrolyzing sulfuric acid are shown below.
・Volume of electrolytic cell 6: 0.5L
・Material of anode 6A and cathode 6B: Diamond electrode (diameter 150mm)
・Material of bipolar electrode 6C: Diamond electrode (diameter 150mm)
・Current density: 2A/dm ²
・Solution circulation flow rate: 3L/min

<Anodizing treatment>
The specifications and processing conditions of the processing tank 2 are as follows.
・Volume of processing tank 2: 25L
・Dimensions of A6063 plate (Al plate 8): 100mm x 50mm x thickness 3mm
- Dimensions of cathode 9: 100mm x 50mm x thickness 3mm
・Material of cathode 9: A1050 (industrial pure aluminum)
・Distance between Al plate and cathode: 20mm
・Current density: 1.5A/dm ²
・Anodizing bath Acid concentration: 10% by mass
Oxidizing agent concentration: 5g/L
Bath temperature: 20℃
・Processing time: 20 minutes

The Al plate 8 on which this anodic oxide film was formed was first subjected to room temperature nickel sealing treatment, and then pressurized steam sealing was performed.

<Room temperature nickel sealing conditions>
・Nickel concentration in sealing liquid: 5g/L
・Processing temperature: 20℃
・Processing time: 20 minutes ・pH: 5.7

<Pressure steam sealing conditions>
・Processing temperature: 115℃
・Processing time: 75 minutes

Subsequently, an alkali resistance test and a hydrochloric acid resistance test were conducted as corrosion resistance tests. The results are shown in Table 2.

<Alkali resistance test>
For the alkali resistance test, the "alkali dropping test" of the alkali resistance tests specified in JIS H8681-1 was adopted. A solution prepared by dissolving sodium hydroxide in pure water to a concentration of 100 g/L was dropped under the following test conditions, and the number of drops until bubbles were generated was compared. 30 times or more was considered a pass.
・Test atmosphere temperature: 35±1℃
・Test area: Approximately 28mm ² (diameter 6mm)
・Amount of test liquid dropped: Approximately 16mg
・Test liquid dropping interval time: 5 seconds

<Hydrochloric acid resistance test>
Hydrochloric acid resistance test The sample was immersed in hydrochloric acid with a solution temperature of 35°C and a hydrochloric acid concentration of 10N, and the time until hydrogen was generated was measured. A time of 200 minutes or more was considered a pass.

(Example 2, Example 3, Example 4)
The sealing treatment conditions were the same as in Example 1, with the treatment temperature on pressurized water being 105°C, 115°C or 125°C, and the anodic oxidation treatment conditions being variously changed as shown in Table 1, and a corrosion resistance test was conducted. The results of the corrosion resistance test are also shown in Table 2.

[Comparative example 1]
For comparison, the sealing treatment conditions were the same as in Example 1, and the anodizing treatment was performed using a sulfuric acid bath instead of an electrolytic sulfuric acid bath. Table 1 shows the anodizing treatment conditions, and Table 2 shows the results of the corrosion resistance test.
[Comparative Examples 2 to 4]
For comparison, the anodizing treatment conditions were the same as those of the present invention, and the sealing treatment was variously changed as shown in Table 1. In Comparative Example 2, the treatment temperature in pressurized steam sealing was 145°C, in Comparative Example 3, only room temperature nickel sealing treatment was used, and in Comparative Example 4, only pressurized steam sealing treatment was performed. The combinations are shown in Table 1, and the results of the corrosion resistance test are shown in Table 2.

As is clear from Table 2, when the anodizing treatment method for aluminum or aluminum alloy and the sealing treatment method for the anodic oxide film of Examples 1 to 4 were carried out, it was found that the alkali resistance test and the hydrochloric acid resistance test were satisfied. The passing standard for the hydrochloric acid resistance test is 200 minutes, but 300 minutes or more is desirable.

1 Processing device 2 Processing tank 3 DC power supply 4 Air pump 4A Air supply line 4B Porous body 5 Circulation pump 5A Sulfuric acid feed line 5B Electrolyte return line 6 Electrolytic cell 6A Anode 6B Cathode 6C Bipolar electrode 8 Al plate 9 Cathode L Sulfuric acid

Claims (4)

Using an electrolytic sulfuric acid bath obtained by electrolyzing sulfuric acid with a sulfuric acid concentration of 5 to 30% by mass and having an oxidation-reduction potential of 1,100 mV or more and less than 1,500 mV , aluminum or aluminum alloy is heated at a current density of 0.8. The aluminum or aluminum alloy is anodized at ~3.0 A/dm ² , and then sealed at room temperature using a mixed bath containing a metal salt and a fluoride or a bath containing a metal fluoride compound. A method for surface treatment of aluminum or aluminum alloy, characterized in that a pressurized steam sealing treatment is performed at a treatment temperature of 105 to 125°C . The method for surface treatment of aluminum or aluminum alloy according to claim 1 , wherein the electrolytic sulfuric acid bath has an oxidizing agent concentration of 1 to 10 g/L. The method for surface treatment of aluminum or aluminum alloy according to claim 1 or 2, wherein the temperature of the electrolytic sulfuric acid bath during the anodizing treatment is 5 to 30°C. The method for surface treatment of aluminum or aluminum alloy according to claim 3, wherein the applied voltage during the anodizing treatment is 10 to 25V.

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