US20150354078A1

US20150354078A1 - Method for forming film on aluminum or aluminum alloy, pretreatment liquid therefor, and product thereof

Info

Publication number: US20150354078A1
Application number: US14/722,586
Authority: US
Inventors: Masahiro Fujita
Original assignee: Suzuki Motor Corp
Current assignee: Suzuki Motor Corp
Priority date: 2014-06-04
Filing date: 2015-05-27
Publication date: 2015-12-10
Also published as: JP6295843B2; JP2015229786A

Abstract

In a method for forming a film on a surface of aluminum or an alloy thereof, first, an anodic oxide film is formed on a surface of aluminum, a pretreatment is performed by immersing the aluminum or the alloy thereof on which the anodic oxide film is formed in a pretreatment liquid, and a sealing treatment using a sealing treatment liquid containing lithium ions is performed on the anodic oxide film on the surface of the aluminum or the alloy thereof subjected to the pretreatment. The pretreatment liquid contains phosphate ions and a reaction-controlling agent and has a pH in a neutral to acidic range. The reaction-controlling agent contains a compound having a carboxy group or a salt thereof or contains a compound capable of forming a hydroxide ion in an aqueous solution. This method makes it possible to uniformly form a sealing product in pores of the film.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority from Japanese Patent Application No. 2014-115918 filed Jun. 4, 2014, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method for forming a film on aluminum or an aluminum alloy, a pretreatment liquid therefor, and a product thereof. More specifically, the present invention relates to a sealing treatment for closing pores present in an anodic oxide film formed on a surface of aluminum or an aluminum alloy.
2. Description of the Related Art
The anodic oxidation treatment is employed as a method for improving the corrosion resistance of aluminum or an aluminum alloy. By the anodic oxidation treatment, an anodic oxide film is formed on a surface of the aluminum or the aluminum alloy. The anodic oxide film has a porous layer in which a large number of fine pores are present, and the porous layer is one of the causes of decrease in corrosion resistance. For this reason, to further improve the corrosion resistance, a sealing treatment for closing the fine pores is carried out after the anodic oxidation treatment.
A method employed for the sealing treatment is a high-temperature hydration type sealing treatment method in which an aluminum workpiece having an anodic oxide film formed thereon is immersed in boiling water. However, this method has problems such as the treatment time must be 10 minutes or more and a large amount of energy is necessary to maintain the boiling state of the boiling water. To solve these problems, a method using an aqueous solution containing lithium ions as a sealing treatment liquid has been developed, as disclosed in Japanese Patent Application Publication No. 2010-077532. In this sealing treatment method, the sealing treatment can be carried out even with the temperature of the treatment liquid being 25° C. and the treatment time being 1 minute. Hence, this sealing treatment method is excellent in terms of the production efficiency and energy consumption.
In addition, Japanese Patent Application Publication No. 2013-1957 discloses a method for performing a sealing treatment on a surface of an anodic oxide film formed on a surface of an aluminum workpiece. A sealing treatment liquid used in this method contains lithium ions and two reaction accelerators, that is, a reaction accelerator A selected from polyoxyethylene alkyl ethers and the like and a reaction accelerator B selected from polyvinyl alcohol and the like. In addition, the following reaction conditions are described: the sealing treatment liquid is made weakly alkaline with a pH of 8.0 to 10.0; and the immersion time in the sealing treatment liquid is 30 minutes.

SUMMARY OF THE INVENTION

The sealing treatment method using lithium ions disclosed in Japanese Patent Application Publication No. 2010-077532 is a method in which the sealing is carried out rapidly by causing the anodic oxide film to partially dissolve and form a compound with lithium ions. For this reason, the amount of the sealing product formed in the pores varies between the pore bottom portion and the surface portion, and a larger amount of the sealing product is formed near the surface of the anodic oxide film. As a result, the color tone of the anodic oxide film turns cloudy white after the sealing. This results in a problem in that the appearance quality deteriorates in the case in which a colorless paint or no paint is applied onto the anodic oxide film.
The sealing treatment method disclosed in Japanese Patent Application Publication No. 2013-1957 makes it possible to prevent the color tone of the anodic oxide film from turning cloudy white; however, this sealing treatment method has the following problem. Specifically, even when the workpiece is washed with water after the anodic oxidation treatment, the anodic oxidation treatment liquid remains on the workpiece, because the sealing treatment liquid is weakly alkaline with a pH of 8.0 to 10.0. For this reason, the contamination of the sealing treatment liquid with the anodic treatment liquid, which is strongly acidic or strongly basic, is unavoidable in the next step. Accordingly, the pH of the sealing treatment liquid is changed greatly, and it is difficult to maintain the pH within a predetermined pH range. It is described that when the pH of the sealing treatment liquid is 12.0, the dissolution of the film occurs at a surface portion of the anodic oxide film, and the finished surface has a whitish and uneven color.
In this respect, an object of the present invention is to provide a method for forming a film on aluminum or an aluminum alloy which makes it possible to prevent change in color tone of a surface of an anodic oxide film after a sealing treatment, reduce the time taken for the sealing treatment, and moreover, easily control the pH of the sealing treatment liquid, and also to provide a pretreatment liquid therefor and a product thereof.
To achieve the above-described object, an aspect of the present invention is a method for forming a film on a surface of aluminum or an aluminum alloy, the method including the steps of: forming an anodic oxide film on a surface of aluminum or an aluminum alloy; performing a pretreatment by immersing, in a pretreatment liquid, the aluminum or the aluminum alloy on which the anodic oxide film is formed; and performing a sealing treatment using a sealing treatment liquid containing lithium ions on the anodic oxide film present on the surface of the aluminum or the aluminum alloy subjected to the pretreatment, wherein the pretreatment liquid contains phosphate ions and a reaction-controlling agent and has a pH in a neutral to acidic range, and the reaction-controlling agent is a compound having a carboxy group or a salt thereof or is a compound capable of forming a hydroxide ion in an aqueous solution.
A concentration of the phosphate ions is preferably 0.1 to 4.0 mol/L. A concentration of the reaction-controlling agent is preferably at least 0.05 mol/L. A temperature of the pretreatment liquid is preferably 10 to 50° C. An immersion time in the pretreatment liquid is preferably at least 1.5 minutes. The compound having a carboxy group is preferably a carboxylic acid or a salt thereof. The compound capable of forming a hydroxide ion in an aqueous solution is preferably a hydroxide. The anodic oxide film may have a thickness of 3 to 40 μm. In the sealing treatment step, a sealing product is formed in pores present in a surface of the anodic oxide film, and the sealing product may be a lithium-aluminum hydrate containing at least LiH(AlO₂)₂.5H₂O. The pretreatment liquid is preferably a buffer solution.
Another aspect of the present invention is a pretreatment liquid used for a pretreatment in a sealing treatment on an anodic oxide film by using a sealing treatment liquid containing lithium ions, the pretreatment liquid including: phosphate ions; and a reaction-controlling agent, wherein the pretreatment liquid has a pH in a neutral to acidic range, and the reaction-controlling agent contains a compound having a carboxy group or a salt thereof or contains a compound capable of forming a hydroxide ion in an aqueous solution.
A still another aspect of the present invention is an aluminum or aluminum alloy product including: aluminum or an aluminum alloy; and an anodic oxide film formed on a surface of the aluminum or the aluminum alloy, wherein pores in a surface of the anodic oxide film formed on the aluminum or the aluminum alloy are sealed with a sealing product, the sealing product contains a lithium-aluminum hydrate, and the lithium-aluminum hydrate is uniformly formed from the surface of the anodic oxide film to pore bottom portions in the pores.
As described above, according to the present invention, the pretreatment step is performed before the sealing treatment step, and phosphate ions and a reaction-controlling agent are added to the pretreatment liquid used in this pretreatment step. In addition, the pH of the pretreatment liquid is adjusted to the neutral to acidic range. Moreover, a compound having a carboxy group or a salt thereof or a compound capable of forming a hydroxide ion in an aqueous solution is used as a reaction-controlling agent. Hence, phosphate ions are moderately captured by the anodic oxide film in the pretreatment step. The phosphate ions have an effect of inhibiting the formation of sealing products such as a lithium-aluminum hydrate formed in the sealing treatment step. Hence, it is possible to form the sealing product uniformly from the film surface to the pore bottom portions in multiple fine pores present in the surface of the anodic oxide film. This makes it possible to prevent the change in color tone of the anodic oxide film. In addition, each of the pretreatment step and the sealing treatment step is completed in a short period of about several minutes, and the pH of the pretreatment liquid is in the neutral to acidic range. Hence, the pH of the sealing treatment liquid can be controlled easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an anodic oxide film subjected to a sealing treatment and obtained by a film formation method according to the present invention.

FIG. 2 is a cross-sectional view schematically illustrating an anodic oxide film subjected to a sealing treatment and obtained by a reference film formation method.

FIG. 3 shows photographs of the appearance of surfaces of anodic oxide films subjected to sealing treatments in the Examples and Reference Examples.

FIG. 4 is an electron micrograph of a surface of an anodic oxide film subjected to a sealing treatment in the Examples.

FIG. 5 shows photographs of the appearance of surfaces of anodic oxide films subjected to a sealing treatment in the Examples and Comparative Examples after a corrosion resistance test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of a film formation method according to the present invention is described. The film formation method of this embodiment includes the steps of: forming an anodic oxide film on a surface of aluminum or an aluminum alloy; performing a pretreatment by immersing, in a pretreatment liquid, the aluminum or the aluminum alloy on which the anodic oxide film is formed; and performing a sealing treatment using a sealing treatment liquid on the anodic oxide film present on the surface of the aluminum or the aluminum alloy subjected to the pretreatment.
The material on which the film is to be formed may be aluminum or may also be an aluminum alloy. The aluminum alloy can contain a wide variety of components such as silicon and copper, without any particular limitation. A method for producing the aluminum or the aluminum alloy is not particularly limited, and this film formation method can be applied also to wrought materials, cast materials, and die-cast materials.
In the step of forming an anodic oxide film, an anodic treatment liquid is electrolyzed by using the aluminum or the aluminum alloy as a working electrode in the anodic treatment. Thus, an anodic oxide film mainly containing aluminum oxide can be formed on a surface of the aluminum or the aluminum alloy. The anodic treatment liquid is not particularly limited, and, for example, either an acidic bath of sulfuric acid, oxalic acid, phosphoric acid, chromic acid, or the like, or an alkaline bath of sodium hydroxide, sodium phosphate, sodium fluoride, or the like may be used.
A method for the electrolysis is not particularly limited, and any one of direct-current electrolysis, alternating-current electrolysis, AC-DC superimposition electrolysis, Duty electrolysis, and the like may be used. In addition, it is preferable to perform washing with water at least once after the electrolytic treatment. By performing the washing with water, the anodic treatment liquid attached to the aluminum or the aluminum alloy can be removed to some degree, or the concentration of the treatment liquid which cannot be removed can be reduced. Thus, the amount of the treatment liquid carried over to the next step can be reduced. Water used for the washing is preferably water with a lower level of impurities, such as ion-exchanged water or purified water.
The film thickness of the anodic oxide film is not particularly limited, and is preferably 3 to 40 μm. Since a sealing treatment using lithium ions is performed on the anodic oxide film, the anodic oxide film has to have a certain thickness. When the anodic oxide film has a thickness of 3 μm or more, the anodic oxide film is not lost even when subjected to the sealing treatment using lithium ions. Moreover, when the anodic oxide film has a thickness of at least 40 μm, the anodic oxide film can exhibit functions sufficient for a film of aluminum or an aluminum alloy. In addition, it is possible to improve productivity because the time necessary for the film formation can be reduced.
In the pretreatment step, the aluminum or the aluminum alloy on which the anodic oxide film is formed is immersed in a pretreatment liquid containing phosphate ions and a reaction-controlling agent and having a pH in an acidic to neutral range, before a sealing treatment on the anodic oxide film.
By immersing the anodic oxide film in such a pretreatment liquid, phosphate ions are captured on the surface and in pores of the anodic oxide film. Since phosphate ions have a function of inhibiting a hydration reaction, the formation reaction of a lithium-aluminum hydrate, which is a sealing product requiring a large number of water molecules, proceeds gently, so that the variation between the amounts of the sealing product formed in pore bottom portions in the pores and formed in a surface portion can be reduced. For this reason, it is possible to reduce change in color tone of the anodic oxide film due to reflection or refraction of light falling on the film.
As a phosphate ion source, phosphoric acids and phosphoric acid salts, which are water-soluble and contain phosphate ions, are preferable. Examples of the phosphate ion source include orthophosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, trisodium phosphate, potassium phosphate, calcium dihydrogen phosphate, lithium phosphate, ammonium phosphate, and the like. From the viewpoints of low cost and ease of handling, phosphoric acid sodium salts are preferable, and sodium dihydrogen phosphate is more preferable.
The concentration of phosphate ions in the pretreatment liquid is preferably 0.1 to 4.0 mol/L. By setting the concentration of phosphate ions to 0.1 mol/L or more, a necessary amount of phosphate ions are captured by the anodic oxide film, so that the effect of inhibiting a hydration reaction can be exhibited. In addition, setting the concentration of phosphate ions to 4.0 mol/L or less makes it possible to prevent phosphate ions from being excessively captured by the anodic oxide film and allows the reaction for the sealing treatment in the next step to proceed appropriately.
Since phosphate ions have the effect of inhibiting a hydration reaction, as described above, excess phosphate ions captured by the anodic oxide film may lead to longer sealing treatment time or may stop the sealing reaction. However, it is difficult to completely control the amount of phosphate ions captured by the anodic oxide film by controlling the concentration of phosphate ions alone. In this respect, the reaction-controlling agent is added to the pretreatment liquid to prevent phosphate ions from being excessively captured by the anodic oxide film in the present invention.
The reaction-controlling agent is a compound having a carboxy group or a salt thereof, or is a compound capable of forming a hydroxide ion in an aqueous solution. The compound having a carboxy group or the compound capable of forming a hydroxide ion in an aqueous solution can prevent phosphate ions from being excessively captured by the anodic oxide film, because the carboxy group or the hydroxide ion itself consumes the phosphate ions moderately at a solid-liquid interface between the anodic oxide film and the pretreatment liquid.
A carboxylic acid is preferable as the compound having a carboxy group, because it is acidic and easy to handle. It is possible to use, as the carboxylic acid, a wide variety of carboxylic acids, for example, saturated carboxylic acids such as formic acid, acetic acid, propionic acid, and butyric acid, dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, maleic acid, and fumaric acid, aromatic carboxylic acids such as benzoic acid, salicylic acid, phthalic acid, and trimesic acid, hydroxy acids such as citric acid, lactic acid, glycolic acid, malic acid, and tartaric acid, and other carboxylic acids such as pyruvic acid, acetoacetic acid, and aconitic acid. In addition, a compound having a relatively low water-solubility such as benzoic acid or salicylic acid may be used in the form of a carboxylic acid salt such as sodium benzoate or sodium salicylate, instead of the acid form.
As the compound capable of forming a hydroxide ion in an aqueous solution, it is possible to use a wide variety of compounds including ammonia, carbonates such as sodium hydrogen carbonate, metal salts capable of forming a hydroxide ion, and the like. Among the compounds capable of forming a hydroxide ion in an aqueous solution, hydroxides are preferable, because they are water-soluble and easy to handle. Examples of the hydroxides include sodium hydroxide, calcium hydroxide, magnesium hydroxide, potassium hydroxide, and the like. In particular, sodium hydroxide is preferable, because it is readily available and inexpensive.
The reaction-controlling agent may be a compound having both a carboxy group and a hydroxy group. Such compounds include the above-described hydroxy acids such as citric acid, lactic acid, glycolic acid, malic acid, and tartaric acid, and the above-described aromatic hydroxy acids such as salicylic acid. Of these compounds, citric acid is preferable, because it is readily available and easy to handle.
The concentration of the reaction-controlling agent in the pretreatment liquid is preferably at least 0.05 mol/L. By setting the concentration to 0.05 mol/L or more, it is possible to prevent phosphate ions from being excessively captured by the anodic oxide film. An upper limit of the concentration of the reaction-controlling agent is not particularly limited; however, if the concentration of the reaction-controlling agent is excessively high, the viscosity of the pretreatment liquid increases, so that the pretreatment liquid becomes difficult to remove by washing with water. Hence, the upper limit of the concentration of the reaction-controlling agent is preferably 10 mol/L or less, more preferably 7 mol/L or less, and further preferably 4 mol/L or less.
The ratio of the concentration of phosphate ions and the concentration of the reaction-controlling agent is not particularly limited, and, for example, the ratio of phosphate ion:reaction-controlling agent may be in a range from 1:0.25 to 1:4.
The pH of the pretreatment liquid is in an acidic to neutral range, and, for example, is 1 to 7. This is because, when the pH of the pretreatment liquid is alkaline, the sealing treatment in the next step is adversely affected by the pH, so that the closing of the pores present in an anodic oxide film may be inadequate, or the color tone of the film may change. In addition, when the pretreatment liquid is in the acidic to neutral range, the following effect is obtained. Specifically, for example, when a carboxylic acid salt is added to the pretreatment liquid, the carboxylic acid salt dissociates into a carboxylate anion and a metal ion in the pretreatment liquid. The equilibrium of the carboxylate anion in the dissociated state moves toward the carboxy group and exerts an effect of preventing phosphate ions from being excessively captured by the anodic oxide film.
In addition, the pH of the pretreatment liquid is preferably in a weakly acidic to neutral range, for example, from 5 to 7. When the pretreatment liquid is weakly acidic to neutral as described above, the change in pH of the sealing treatment liquid used in the next step can be reduced, even when the sealing treatment liquid is contaminated with the pretreatment liquid.
For adjusting the pH of the pretreatment liquid to the acidic to neutral range, the kinds, the concentrations, and the like of the phosphate ions and the reaction-controlling agent may be adjusted, or an acid such as sulfuric acid, nitric acid, hydrochloric acid, or hydrofluoric acid, or a base such as sodium hydrogen carbonate, sodium carbonate, or ammonia may be used. Note that a silicic acid compound such as sodium silicate cannot be used as the acid or base for adjusting the pH. If used, the silicic acid compound may inhibit the sealing treatment.
In addition, since the pretreatment liquid contains phosphate ions which act as a conjugate base, a buffer solution may be prepared by using the phosphate ions in combination with a weak acid or a weak base. By using a phosphate buffer having a wide-ranging buffer capacity over the weakly acidic to neutral range with the pH of from 5 to 7, the change in pH of the pretreatment liquid can be reduced even when the pretreatment liquid is contaminated with a strongly acidic or strongly alkaline anodic treatment liquid, and the pH of the pretreatment liquid can be maintained in a predetermined range.
The temperature of the pretreatment liquid is not particularly limited, and it is preferably 10 to 50° C. By setting the temperature to 10° C. or above, the action of the phosphate ions in the pretreatment liquid is activated, so that the phosphate ions can be appropriately captured by the anodic oxide film. Meanwhile, by setting the temperature to 50° C. or below, it is possible to prevent a large amount of phosphate ions from being captured by the anodic oxide film, and prevent the change in color tone of the film due to the dissolution of the anodic oxide film in the pretreatment liquid and resultant formation of fine projections and recesses on the surface. The temperature of the pretreatment liquid is more preferably 15 to 40° C. In this temperature range, the temperature can be maintained more easily, and the energy necessary for maintaining the temperature can be reduced, because the temperature is closer to normal temperature.
The immersion time in the pretreatment liquid is preferably at least 1.5 minutes. By setting the immersion time to 1.5 minutes or more, phosphate ions in the pretreatment liquid can be sufficiently captured by the anodic oxide film. The upper limit of the immersion time is not particularly limited, and it is preferably 20 minutes or less, considering production efficiency.
After the immersion in the pretreatment liquid, it is preferable to perform washing with water at least once before the sealing treatment in the next step, as in the case of the step of forming an anodic oxide film.
In the sealing treatment step, a sealing treatment using a sealing treatment liquid containing lithium ions is performed on the anodic oxide film formed on the surface of the aluminum or the aluminum alloy. By this sealing treatment, fine pores present in the surface of the anodic oxide film can be closed. As a source of the lithium ions, lithium sulfate, lithium chloride, lithium nitrate, lithium carbonate, lithium phosphate, lithium hydroxide, a hydrate thereof, or the like can be used. Of these lithium ion sources, lithium hydroxide and lithium carbonate are preferable, because their aqueous solutions are alkaline and nontoxic.
The concentration of lithium ions in the sealing treatment liquid is preferably 0.02 to 20 g/L, and more preferably 0.08 to 10 g/L. The pH of the sealing treatment liquid is preferably at least 10.5, more preferably at least 11, and further preferably at least 12. An upper limit of the pH is not particularly limited, and it is preferably at most 14. Since the pH varies depending on the lithium ion source, the pH of the sealing treatment liquid can be adjusted by using an acid such as sulfuric acid, oxalic acid, or chromic acid or a base such as sodium hydroxide, sodium carbonate, or sodium fluoride.
Regarding a method for the sealing treatment, the sealing treatment liquid may be attached to the anodic oxide film by immersing the aluminum or the aluminum alloy on which the anodic oxide film is formed in the sealing treatment liquid, spraying the sealing treatment liquid onto the surface of the anodic oxide film, or applying the sealing treatment liquid onto the surface of the anodic oxide film with an ink brush or the like. The temperature of the sealing treatment liquid is preferably 10 to 65° C., and more preferably 25 to 50° C. The time for the sealing treatment is preferably 0.5 to 5 minutes. After the sealing treatment liquid is attached, it is preferable to wash the anodic oxide film with cold water or hot water and to dry the anodic oxide film by blowing air, using a dryer, or the like.
By such a sealing treatment, a sealing product 26 can be formed uniformly from the film surface to the pore bottom portions in multiple fine pores 24 a present in the surface of an anodic oxide film 22 formed on aluminum or an aluminum alloy 10 as shown in FIG. 1. The sealing product 26 may be a lithium-aluminum hydrate 26 a and a boehmite 26 b. The lithium-aluminum hydrate 26 a is, for example, LiH(AlO₂)₂.5H₂O. The boehmite 26 b is, for example, AlO.OH.
LiH(AlO₂)₂.5H₂O is a pentahydrate, and the formation of this pentahydrate requires 5 water molecules to 1 lithium atom. The phosphate ions captured by the anodic oxide film in the pretreatment step have an effect of inhibiting a hydration reaction and exert a great influence on the formation of the lithium-aluminum hydrate, which requires a large number of water molecules. Hence, the phosphate ions can make the formation reaction of the lithium-aluminum hydrate gentle. Consequently, the conditions (necessary amounts of phosphoric acid and the reaction inhibitor, the immersion time, and the solution temperature) for reducing the change in color tone of the anodic oxide film can be reduced, shortened, and lowered.
In addition, as described above, the sealing product 26 contains the boehmite 26 b in addition to the lithium-aluminum hydrate 26 a. In a reference method, a large amount of the lithium-aluminum hydrate 26 a is formed near the surface of the anodic oxide film 22 as shown in FIG. 2, and hence exerts effects on the refraction of light and the like, so that the film looks white. On the other hand, in the present invention, since the lithium-aluminum hydrate 26 a is uniformly formed in the pores 24 of the anodic oxide film as shown in FIG. 1, the change in color tone of the anodic oxide film due to the sealing treatment can be consequently reduced. In this manner, the present invention makes it possible to prevent the change in color tone of the anodic oxide film, and to produce an aluminum or aluminum alloy product having a high corrosion resistance.

EXAMPLES

Test Example 1

Regarding Influence of Reaction-Controlling Agent in Pretreatment Liquid

Test pieces made of an aluminum alloy A1100 material were used. This was because of the following reason. Specifically, when a large amount of an alloy component is contained, an anodic oxide film is of black or yellow color because of the influence of the alloy component. Hence, by using the A1100 material which contained almost no alloy component and which allowed formation of a transparent film, the visual evaluation of the effect (the reduction of change in color tone) of the present invention was facilitated.
In Test Example 1, each test piece of the A1100 material was immersed as an anode in a sulfuric acid bath of a concentration of 200 g/L, and a direct current with a current density of 1.5 A/dm²was applied for 10 minutes. Thus, an anodic oxide film with a thickness of 10 μm was formed on a surface of the test piece. Next, for a pretreatment, pretreatment liquids (20° C.) which were aqueous solutions each having a concentration of phosphate ions of 0.5 mol/L, a concentration of a reaction-controlling agent of 0.25 or 0.5 mol/L, and a pH of 1 to 5 were prepared by using phosphate ion sources (reagent 1) and reaction-controlling agents (reagent 2) shown in TABLE 1. Note that when the aqueous solution was alkaline, the pH was adjusted to the acidic range by adding sulfuric acid. After the test piece was immersed in each of the pretreatment liquids for 5 minutes, the pretreated test piece was immersed in a sealing treatment liquid (20° C.) which was an aqueous solution having a concentration of lithium ions of 0.8 g/L and a pH of 13 for 1 minute (Examples 1 to 9).
Note that, for comparison, the series of treatments were conducted on test pieces in the same manner as in the above-described Example, except that pretreatment liquids were prepared by using compounds which did not fall within the reaction-controlling agent of the present invention (Comparative Examples 1 and 2).
Each of the test pieces of Examples and Comparative Examples was evaluated to determine whether the change in color tone of the anodic oxide film was reduced, and also whether the anodic oxide film was sealed. For the evaluation of the reduction of change in color tone of the film, a comparison of color tone with those of test pieces of Reference Examples 1 and 2 described below were made visually.
The test piece of Reference Example 1 was a test piece subjected to only the anodic treatment. Specifically, the test piece of Reference Example 1 was a test piece on which an anodic oxide film of 10 μm was formed by immersing a test piece of the A1100 material as an anode in a sulfuric acid bath of a concentration of 200 g/L and applying a direct current with a current density of 1.5 A/dm²for 10 minutes.
The test piece of Reference Example 2 was a test piece on which the sealing treatment was performed after the anodic treatment without performing any pretreatment. Specifically, the test piece of Reference Example 2 was a test piece on which an anodic oxide film of 10 μm was formed by immersing a test piece of the A1100 material as an anode in a sulfuric acid bath of a concentration of 200 g/L, and applying a direct current with a current density of 1.5 A/dm²for 10 minutes, and then on which a sealing treatment was performed by immersing the test piece in a sealing treatment liquid having a concentration of lithium ions of 0.8 g/L and a pH of 13 at a temperature of 20° C. for 1 minute.
Criteria for evaluating whether the change in color tone of the film was reduced were as follows. Specifically, in a case in which the color of the test piece of the Example or Comparative Example was close to that of the test piece of Reference Example 1 under visual observation, the pretreatment was evaluated to be effective (“good” in the Table), whereas in a case in which the color was close to that of Reference Example 2, the pretreatment was evaluated to be ineffective (“inferior” in the Table). FIG. 3 shows the appearance of each of the test pieces of Reference Examples 1 and 2, and, for reference, the appearance of the test piece (Example 40, described later) for which the pretreatment was evaluated to be effective.
In addition, for the sealing state, the surface of each anodic oxide film was observed under an electron microscope to determine whether the anodic oxide film was sealed. Note that when the sealing product was formed by the sealing treatment in an amount sufficient to close multiple fine pores present in the surface of the anodic oxide film, a thin plate-shaped substance was observed on the surface of the anodic oxide film as shown in FIG. 4. Accordingly, the sealing state was evaluated on the basis of the presence or absence of the thin plate-shaped substance in this test. TABLE 1 shows the results of the evaluations described above.

TABLE 1

	Concentration of			pH of	Reduction of
	phosphate ions		Concentration	solution after	change in color	Sealing	Overall
Reagent 1	[mol/L]	Reagent 2	[mol/L]	adjustment	tone of film	state	evaluation

Example 1	phosphoric acid	0.5	citric acid	0.25	1	good	good	good
Example 2	disodium hydrogen	0.5	citric acid	0.25	3	good	good	good
	phosphate
Example 3	sodium dihydrogen	0.5	citric acid	0.25	3	good	good	good
	phosphate
Example 4	trisodium	0.5	citric acid	0.25	3	good	good	good
	phosphate
Example 5	sodium dihydrogen	0.5	oxalic acid	0.25	3	good	good	good
	phosphate
Example 6	sodium dihydrogen	0.5	tartaric acid	0.5	3	good	good	good
	phosphate
Example 7	sodium dihydrogen	0.5	malic acid	0.5	3	good	good	good
	phosphate
Example 8	sodium dihydrogen	0.5	lactic acid	0.5	3	good	good	good
	phosphate
Example 9	sodium dihydrogen	0.5	potassium	0.25	2	good	good	good
	phosphate		hydroxide
Comp. Ex. 1	sodium dihydrogen	0.5	sulfuric acid	0.5	1	good	inferior	inferior
	phosphate
Comp. Ex. 2	sodium dihydrogen	0.5	boric acid	0.5	4	good	inferior	inferior
	phosphate

As shown in TABLE 1, in each of Examples 1 to 9 in which the pretreatment liquids were prepared by using various phosphate ion sources and carboxylic acids or various compounds capable of forming a hydroxide ion in an aqueous solution, the sealing was successfully performed, while reducing the change in color tone of the anodic oxide film. On the other hand, in each of Comparative Examples 1 and 2 in which the pretreatment liquids were prepared by using the compounds having no carboxy group and being incapable of forming a hydroxide ion in an aqueous solution by itself such as sulfuric acid or boric acid, it was not possible to control the amount of phosphate ions captured by the film, and a large amount of phosphate ions were captured, so that the sealing was performed insufficiently.

Test Example 2

Regarding Influence of pH of Pretreatment Liquid

The series of treatments were conducted on test pieces in the same manner as in Test Example 1 described above, except that an aqueous solution (20° C.) was prepared by using phosphoric acid and citric acid with the concentration of phosphate ions being 0.5 mol/L, and the concentration of the carboxylic acid being 0.25 mol/L, and that the pH of the aqueous solution was varied in a range from 1 to 13 by adding sodium hydroxide to the aqueous solution in a range from 0 to 3.5 mol/L, to prepare pretreatment liquids (Example 1, Examples 10 to 14, and Comparative Examples 3 to 6). Then, the test pieces of these Examples and Comparative Examples were evaluated for the reduction of change in color tone of the film and the sealing state in the same manner as in Test Example 1. TABLE 2 shows the results.

TABLE 2

	Concentration of	Concentration of
	phosphate ions	carboxylic acid	Concentration		Reduction of
	[mol/L]	[mol/L]	of hydroxide	Solution	change in color
	phosphoric acid	citric acid	[mol/L]	[pH]	tone of film	Sealing state	Overall evaluation

Example 1	0.5	0.25	pH was adjusted	1	good	good	good
Example 10	0.5	0.25	by addition at 0	2	good	good	good
Example 11	0.5	0.25	to 3.5 mol/L	3	good	good	good
Example 12	0.5	0.25		4	good	good	good
Example 13	0.5	0.25		6	good	good	good
Example 14	0.5	0.25		7	good	good	good
Comp. Ex. 3	0.5	0.25		8	inferior	inferior	inferior
Comp. Ex. 4	0.5	0.25		9	inferior	inferior	inferior
Comp. Ex. 5	0.5	0.25		11	inferior	inferior	inferior
Comp. Ex. 6	0.5	0.25		13	inferior	inferior	inferior

As shown in TABLE 2, in each of Examples 1 and 10 to 14 in which the pH of the pretreatment liquid was in the acidic to neutral range of from 1 to 7, the sealing was successfully performed, while reducing the change in color tone of the anodic oxide film Especially when the pH was 2 to 4, the color change reduction effect was high, and more preferred results were obtained. In each of Comparative Examples 4 to 7 in which the pH of the solution was 8 or more, the film surface was dissolved in the pretreatment liquid, and a lot of fine projections and recesses were formed, so that the color tone of the film was changed. Hence, Comparative Examples 4 to 7 were not preferable. Moreover, in each of Comparative Examples 3 to 6 in which the pH was 8 or more, even the sealing did not occur. This was presumably because the anodic oxide film was also made alkaline by the immersion in the alkaline pretreatment liquid, and consequently the pH gradient was absent between the film and the sealing treatment liquid which was also alkaline, so that it became difficult for the sealing treatment liquid to penetrate into the film.

Test Example 3

Regarding Influence of Concentration of Pretreatment Liquid

The series of treatments were conducted on test pieces in the same manner as in Test Example 1 described above, except that pretreatment liquids were prepared by using sodium dihydrogen phosphate or phosphoric acid with the concentration of phosphate ions being 0 to 5 mol/L or by using citric acid (a carboxylic acid) or sodium hydroxide (a hydroxide) with the concentration thereof being 0 to 3 mol/L (Example 3, Examples 15 to 33, and Comparative Examples 7 to 13). Note that the pHs of the pretreatment liquids were 3 to 4. Then, the test pieces of these Examples and Comparative Examples were evaluated for the reduction of change in color tone of the film and for the sealing state in the same manner as in Test Example 1. TABLE 3 shows the results.

TABLE 3

Concentration of
carboxylic acid	Concentration of		Reduction of
[mol/L]	hydroxide [mol/L]	Solution	change in color		Overall
citric acid	sodium hydroxide	[pH]	tone of film	Sealing state	evaluation

	Concentration of
	phosphate ions [mol/L]
	sodium dihydrogen
	phosphate
Example 15	0.1	0.05	0	3	good	good	good
Example 16	0.1	0.25	0	3	good	good	good
Example 17	0.25	0.25	0	3	good	good	good
Example 18	0.25	0.5	0	3	good	good	good
Example 19	0.5	0.05	0	3	good	good	good
Example 3	0.5	0.25	0	3	good	good	good
Example 20	0.5	0.5	0	3	good	good	good
Example 21	0.5	1	0	3	good	good	good
Example 22	1	0.25	0	3	good	good	good
Example 23	1	0.5	0	3	good	good	good
Example 24	1	1	0	3	good	good	good
Example 25	2	0.5	0	3	good	good	good
Example 26	2	1	0	3	good	good	good
Example 27	2	2	0	3	good	good	good
Example 28	3	1	0	3	good	good	good
Example 29	3	2	0	3	good	good	good
Example 30	3	3	0	3	good	good	good
Example 31	4	3	1	3	good	good	good
Comp. Ex. 7	0.25	0	0	4	good	inferior	inferior
Comp. Ex. 8	0.5	0	0	4	good	inferior	inferior
Comp. Ex. 9	0.5	0.03	0	4	good	inferior	inferior
Comp.	0	0.25	0	3	inferior	good	inferior
Ex. 10
Comp.	0.05	0.25	0	3	inferior	good	inferior
Ex. 11
Comp.	0.05	2	0	3	inferior	good	inferior
Ex. 12
Comp.	5	2.5	0	3	good	inferior	inferior
Ex. 13	phosphoric acid
Example 32	0.5	0	0.5	3	good	good	good
Example 33	1	0	1	3	good	good	good

As shown in TABLE 3, in each of Examples 3 and 15 to 33 in which the concentration of phosphate ions was 0.1 to 4 mol/L, and the concentration of the carboxylic acid or the hydroxide was 0.05 to 4 mol/L in total, the sealing was successfully performed, while there was reduction in the change in color tone of the anodic oxide film. On the other hand, in each of Comparative Examples 10 to 12 in which the concentration of phosphate ions was less than 0.1 mol/L, the amount of phosphate ions captured by the film was so small that the sealing reaction proceeded rapidly, and consequently, the color of the film turned white. In addition, in each of Comparative Example 13 in which the concentration of phosphate ions exceeded 4 mol/L and Comparative Examples 7 to 9 in which the concentration of the carboxylic acid or the hydroxide was less than 0.05 mol/L, the sealing did not occur, because phosphate ions were excessively captured by the film.

Test Example 4

Regarding Influence of Temperature of Pretreatment Liquid

The series of treatments were conducted on test pieces in the same manner as in Test Example 1 described above, except that a pretreatment liquid was prepared by using sodium dihydrogen phosphate and citric acid with the concentration of phosphate ions being 0.5 mol/L, the concentration of the carboxylic acid being 0.25 mol/L, and the pH being 3, and that the test pieces were immersed in the pretreatment liquid at various temperatures in the range from 5 to 60° C. (Example 3, Examples 34 to 37, and Comparative Examples 14 and 15). Then, the test pieces of these Examples and Comparative Examples were evaluated for the reduction of change in color tone of the film and for the sealing state in the same manner as in Test Example 1. TABLE 4 shows the results.

TABLE 4

Concentration of
phosphate ions
[mol/L]	Concentration of	Temperature of
sodium dihydrogen	carboxylic acid [mol/L]	aqueous solution	Reduction of change		Overall
phosphate	citric acid	[° C.]	in color tone of film	Sealing state	evaluation

Example 34	0.5	0.25	10	good	good	good
Example 3	0.5	0.25	20	good	good	good
Example 35	0.5	0.25	30	good	good	good
Example 36	0.5	0.25	40	good	good	good
Example 37	0.5	0.25	50	good	good	good
Comp. Ex. 14	0.5	0.25	5	inferior	good	inferior
Comp. Ex. 15	0.5	0.25	60	inferior	inferior	inferior

As shown in TABLE 4, in each of Examples 3 and 34 to 37 in which the temperature of the pretreatment liquid was 10 to 50° C., the sealing was successfully performed, while there was reduction in the change in color tone of the anodic oxide film. On the other hand, in Comparative Example 14 in which the temperature was less than 10° C., the action of phosphate ions in the pretreatment liquid was so sluggish that the phosphate ions were not readily captured by the film. For this reason, it was not possible to inhibit the rapid sealing reaction, and the color tone of the film changed. Meanwhile, in Comparative Example 15 in which the temperature exceeded 50° C., the action of phosphate ions was excessively activated, and a large amount of phosphate ions were captured by the film. Hence, the sealing did not occur. In addition, the anodic oxide film was dissolved in the pretreatment liquid, and a large number of fine projections and recesses were formed on the surface, so that the color tone of the film changed.

Test Example 5

Regarding Influence of Immersion Time in Pretreatment Liquid

The series of treatments were conducted on test pieces in the same manner as in Test Example 1 described above, except that a pretreatment liquid was prepared by using sodium dihydrogen phosphate and citric acid with the concentration of phosphate ions being 0.5 mol/L, the concentration of the carboxylic acid being 0.25 mol/L, and the pH being 3, and that the time for which the test piece was immersed in the pretreatment liquid was varied in the range from 1 to 20 minutes (Example 3, Examples 38 to 43, and Comparative Example 16). Then, the test pieces of these Examples and Comparative Example were evaluated for the reduction of change in color tone of the film and for the sealing state in the same manner as in Test Example 1. TABLE 5 shows the results.

TABLE 5

Concentration of
phosphate ions	Concentration of
[mol/L]	carboxylic acid		Reduction of
sodium dihydrogen	[mol/L]	Immersion time	change in color
phosphate	citric acid	[min]	tone of film	Sealing state	Overall evaluation

Example 38	0.5	0.25	1.5	good	good	good
Example 39	0.5	0.25	2	good	good	good
Example 40	0.5	0.25	3	good	good	good
Example 3	0.5	0.25	5	good	good	good
Example 41	0.5	0.25	7	good	good	good
Example 42	0.5	0.25	10	good	good	good
Example 43	0.5	0.25	20	good	good	good
Comp. Ex. 16	0.5	0.25	1	inferior	good	inferior

As shown in TABLE 5, in each of Examples 3 and 28 to 43 in which the immersion time in the pretreatment liquid was 1.5 minutes or more, the sealing was successfully performed, while there was reduction in the change in color tone of the anodic oxide film. On the other hand, in Comparative Example 16 in which the immersion time was less than 1.5 minutes, the amount of phosphate ions captured by the anodic oxide film was so small that it was not possible to inhibit the sealing reaction, and hence the color tone of the film changed.

Test Example 6

Corrosion Resistance Test

Since the aluminum alloy A1100 material, which was used in Test Examples 1 to 5, is a corrosion resistant alloy, test pieces of the ADC12 material, which is one of the general-purpose aluminum alloys, were used in Test Example 6 for evaluating corrosion resistance. In Test Example 6, each of the test pieces of the ADC12 material was immersed as an anode in a sulfuric acid bath of a concentration of 200 g/L, and a direct current with a current density of 1.5 A/dm²was applied for 10 minutes. Thus, an anodic oxide film with a thickness of 4 μm was formed on the surface of the test piece. Next, for the pretreatment, a pretreatment liquid (20° C.) which was an aqueous solution having a concentration of phosphate ions of 0.5 mol/L and a pH of 4 was prepared by using only sodium dihydrogen phosphate, and a pretreatment liquid (20° C.) which was an aqueous solution having a concentration of phosphate ions of 0.5 mol/L, a concentration of citric acid of 0.25 mol/L, and a pH of 3 was prepared by using sodium dihydrogen phosphate and citric acid. After the test pieces were immersed in these pretreatment liquids for 5 minutes, the pretreated test pieces were each immersed in a sealing treatment liquid (20° C.) which was an aqueous solution having a concentration of lithium ions of 0.8 g/L and a pH of 13 for 1 minute.
Then, the test pieces subjected the sealing treatments were subjected to a salt spray test according to JIS Z 2371 for 120 hours and were evaluated for corrosion resistance on the basis of the presence or absence of white oxidation. FIG. 5 shows the appearance of the surfaces of the test pieces at this point. In addition, whether the anodic oxide film was sealed was also evaluated under an electron microscope.
On the test piece in the case in which the pretreatment liquid contained only phosphate ions, white oxidation was formed after the corrosion resistance test as shown in FIG. 5( a). This was because phosphate ions were excessively captured by the film, and the sealing reaction did not occur. On the other hand, regarding the test piece in the case in which the pretreatment liquid contained both phosphate ions and citric acid, the sealing reaction occurred, and the corrosion resistance was improved, so that no white oxidation formed, as shown in FIG. 5( b). Note that since a blackish film was formed on the ADC12 material, it was not possible to evaluate the change in color tone. In addition, when the film surface was observed under an electron microscope, the pores were not sealed in the case in which the pretreatment liquid contained only phosphate ions. In the case in which the pretreatment liquid contained both phosphate ions and citric acid, the pores were confirmed to be sealed. These results were in agreement with the results of the actual corrosion resistance test.

Claims

What is claimed is:

1. A method for forming a film on a surface of aluminum or an aluminum alloy, comprising the steps of:

forming an anodic oxide film on a surface of aluminum or an aluminum alloy;

performing a pretreatment by immersing, in a pretreatment liquid, the aluminum or the aluminum alloy on which the anodic oxide film is formed; and

performing a sealing treatment using a sealing treatment liquid containing lithium ions on the anodic oxide film present on the surface of the aluminum or the aluminum alloy subjected to the pretreatment,

wherein the pretreatment liquid contains phosphate ions and a reaction-controlling agent and has a pH in a neutral to acidic range, and the reaction-controlling agent contains a compound having a carboxy group or a salt thereof or contains a compound capable of forming a hydroxide ion in an aqueous solution.

2. The method according to claim 1, wherein a concentration of the phosphate ions is 0.1 to 4.0 mol/L.

3. The method according to claim 1, wherein a concentration of the reaction-controlling agent is at least 0.05 mol/L.

4. The method according to claim 1, wherein a temperature of the pretreatment liquid is 10 to 50° C.

5. The method according to claim 1, wherein an immersion time in the pretreatment liquid is at least 1.5 minutes.

6. The method according to claim 1, wherein the compound having a carboxy group is a carboxylic acid or a salt thereof.

7. The method according to claim 1, wherein the compound capable of forming a hydroxide ion in an aqueous solution is a hydroxide.

8. The method according to claim 1, wherein the anodic oxide film has a thickness of 3 to 40 μm.

9. The method according to claim 1, wherein in the sealing treatment step, a sealing product is formed in pores present in a surface of the anodic oxide film, and the sealing product is a lithium-aluminum hydrate containing at least LiH(AlO₂)₂.5H₂O.

10. The method according to claim 1, wherein the pretreatment liquid is a buffer solution.

11. A pretreatment liquid used for a pretreatment in a sealing treatment on an anodic oxide film by using a sealing treatment liquid containing lithium ions, the pretreatment liquid comprising:

phosphate ions; and

a reaction-controlling agent,

wherein the pretreatment liquid has a pH in a neutral to acidic range, and the reaction-controlling agent contains a compound having a carboxy group or a salt thereof or contains a compound capable of forming a hydroxide ion in an aqueous solution.

12. An aluminum or aluminum alloy product comprising:

aluminum or an aluminum alloy; and

an anodic oxide film formed on a surface of the aluminum or the aluminum alloy,

wherein pores in a surface of the anodic oxide film formed on the aluminum or the aluminum alloy are sealed with a sealing product, the sealing product contains a lithium-aluminum hydrate, and the lithium-aluminum hydrate is uniformly formed from the surface of the anodic oxide film to pore bottom portions in the pores.