JP2551274B2 - Surface treatment method for aluminum materials - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method for suppressing a corrosion reaction of aluminum or an aluminum alloy (hereinafter referred to as an aluminum material).

[0002]

2. Description of the Related Art Various alloying elements are added to aluminum in order to impart necessary properties such as mechanical properties and chemical properties to aluminum-based materials. In addition, even if the material is treated as pure aluminum, Fe, S, which are associated with it from the manufacturing stage,
It contains a small amount of impurities such as i and Cu.

For example, when Cu is contained in aluminum, the matrix is solid-solution strengthened and the mechanical properties of the aluminum-based material are improved. However, Cu, which does not form a solid solution in the matrix, precipitates at the grain boundaries as a CuAl ₂ -based intermetallic compound. Then, a potential difference is generated between the precipitated CuAl ₂ -based intermetallic compound and the matrix, which causes corrosion of the aluminum-based material.

The corrosion reaction is limited by the reaction between the CuAl ₂ -based intermetallic compound exposed on the surface in the very initial stage and the matrix surface. The CuAl ₂ -based intermetallic compound that is buried inside the aluminum-based material and is not exposed on the surface does not participate in the corrosion reaction in the initial stage.
Such a corrosion reaction is not limited to the CuAl ₂ -based intermetallic compound, and the same applies to other intermetallic compounds. Therefore, it is expected that, when a treatment for preventing the intermetallic compound from being exposed on the surface of the aluminum-based material, the corrosion resistance is improved without affecting the mechanical properties of the aluminum-based material.

For example, when the surface of an aluminum-based material is coated with a high-purity aluminum layer, the corrosion resistance of the aluminum-based material is improved. Further, in Japanese Patent Application Laid-Open No. 2-97700, an A-based material on the surface is formed by alternately repeating anode polarization processing and cathode polarization processing on an aluminum-based material.
A method has been proposed in which 1-Fe-based crystallized substances are preferentially dissolved to highly purify and homogenize the surface of an aluminum-based material. Since the aluminum-based material treated by this method has a homogeneous high-purity aluminum surface, it is possible to form a uniform chemical conversion treatment film having excellent corrosion resistance.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
When the aluminum-based material is treated by the method described in the publication, the Al-Fe-based crystallized substance on the surface is preferentially dissolved. However, aluminum-based materials contain various alloy elements or impurity elements such as Cu in addition to Fe. Similarly, these elements also become crystallized substances or intermetallic compounds and are precipitated from the matrix, and are likely to be the starting point of corrosion.

Among them, CuAl ₂ type intermetallic compounds are
It was exposed on the surface of the aluminum-based material in almost the same state as before treatment. From this, the method of repeating the anode polarization treatment and the cathode polarization treatment is effective only for an aluminum-based material containing Fe as an alloying element or an impurity, and is not versatile.

The present invention has been devised to solve such problems, and by controlling the electrolysis conditions, it is possible to form crystallized substances such as CuAl ₂ intermetallic compounds without dissolving the matrix. The purpose is to preferentially remove from the surface of the aluminum-based material to form a uniform high-purity aluminum layer on the surface of the aluminum-based material.

[0009]

In order to achieve the object, the surface treatment method of the present invention comprises immersing an aluminum-based material in an electrolyte solution for cathode polarization, and then for polarization on the anode side in the electrolyte solution. The potential of the aluminum-based material is periodically changed between the natural corrosion potential and the pitting corrosion potential.

As the aluminum-based material, for example, a material containing Cu as an alloy element or an impurity element is used. As the electrolyte solution, various acid solutions such as nitric acid, which have been conventionally used for electrolytic treatment of aluminum-based materials, are used.

[0011]

[Operation] It was found that crystallized substances such as CuAl ₂ -based intermetallic compounds are preferentially removed from the surface of an aluminum-based material by the electrolytic operation described below. The apparatus shown in FIG. 1 was used for the experiment. That is, the treatment liquid 11 is put in the electrolytic bath 10, and the test piece 12 is opposed to the counter electrode 13 such as platinum. The treatment liquid 11 is kept at a uniform temperature by being stirred by the impeller 14 and being heated by the heater 15.

From the test piece 12 and the counter electrode 13, lead wires 21 and 22 connected to the potentiostat 20 are led out. The potentiostat 20 is further connected to the reference electrode chamber 30 by a lead wire 23.

The reference electrode chamber 30 has a saturated K inside.
A reference electrode 32 such as a silver chloride electrode is immersed in a Cl aqueous solution 31. A salt bridge 33 connects the saturated KCl aqueous solution 31 and the test piece 12. A saturated KCl aqueous solution 31
Solid KCl 34 exists therein, and the saturated concentration of the saturated KCl aqueous solution 31 is maintained.

An aluminum alloy containing 1.3% by weight of Cu was used as the test piece 12. As the processing liquid 11, E
A 60% concentration HNO ₃ aqueous solution containing 10 g / l of DTA was prepared. When the pitting potential E _pit of the test piece 12 in the treatment liquid 11 was measured in advance, it was +1.15 V (A
g / AgCl).

When the test piece 12 was immersed in the treatment liquid 11 and the natural corrosion potential E _corr was measured, it was +0.26 V (Ag /
AgCl). At this time, the test piece 12 and the counter electrode 13
There is no current flowing between and. This state is shown as a point P ₁ in the voltage-current curve of FIG.

Then, the test piece 1 was run at a speed of -0.1 V / sec.
The potential of 2 was adjusted to -1.0 V (Ag / AgCl), and the cathode polarization was performed. At this time, the test piece 12 and the counter electrode 13
A current −i ′ of 0.33 A / 4.5 cm ² flowed in the direction of the dotted line between and ( ₂ ) (P _{2 in} FIG. ₂ ). At this time, the test piece 12
The surface area of one side of the sample was 4.5 cm ² .

When the test piece 12 was held at the cathode polarization potential E _ca for 5 minutes, the current flowing from the counter electrode 13 to the test piece 12 decreased to 0.23 A / 4.5 cm ² (P _{3 in} FIG. 2). Due to this cathode polarization, generation of hydrogen gas was observed from the surface of the test piece 12. Then, the surface of the test piece 12 was cleaned.

Next, the potential of the test piece 12 is set to +0.1 V /
Seconds a rate of no more than the pitting potential E _pit + 1.0V (A
anodic polarization up to g / AgCl). Due to this potential change, as shown by the solid line in FIG.
The electric current i flowed toward 3. The current at this time is 3.5
It was mA / 4.5 cm ² (P _{4 in} FIG. 2). After this, the potential was promptly changed to the spontaneous corrosion potential E at a rate of -0.1 V / sec.
_It returned to _corr (P _{5 in} Figure 2). When the natural corrosion potential E _corr was reached, the current i stopped flowing between the test piece 12 and the counter electrode 13.

Further, when the potential was moved to +1.0 V (Ag / AgCl) in the positive direction at a rate of +0.1 V / sec, the current i started to flow again from the test piece 12 to the counter electrode 13 (P in FIG. 2). ₆ ). However, the current i at this time was smaller than the current of 3.5 mA / 4.5 cm ² flowing at the point P ₄ .

After that, when the potential was repeatedly changed between the natural corrosion potential E _corr and the anode polarization potential E _an ,
As shown in FIG. 2, the current i flowing from the test piece 12 to the counter electrode 13 gradually decreased at the anodic polarization potential E _an and converged to a substantially constant value (about 1.2 mA / 4.5 cm ² ) (P in FIG. 2). _n ). In this state, CuAl _{2 on} the surface of the test piece 12
It shows that crystallized substances such as intermetallic compounds were completely eluted.

However, the crystallized substances on the surface can also be removed by subjecting the test piece 12 to anodic polarization while the potential is kept constant. But in this case
The reaction for eluting the crystallized substance was slow, and it took 5 times as long as when the potential was repeatedly changed between the natural corrosion potential E _corr and the anode polarization potential E _an . Further, as the anodic polarization treatment is prolonged, the matrix partially dissolves, and the test piece 12 after the treatment has rough skin.

As the treatment liquid 11, it is preferable to use, for example, an aqueous nitric acid solution having a concentration of 10% or more, since the matrix is not dissolved and only the crystallized substance is efficiently eluted. The upper limit of the concentration of the nitric acid aqueous solution is not particularly specified. Further, in order to prevent the Cu ions eluted in the treatment liquid from re-precipitating on the aluminum surface,
Substances such as EDTA that forms a complex compound with ions, sodium salt of EDTA, and oxalic acid that chemically bonds with Cu ions to form an insoluble salt can be added, preferably in the range of 1 to 10%.

A large amount of hydrogen gas is generated by the cathode polarization, and the surface of the aluminum-based material is cleaned and activated. This cathodic polarization is performed by a constant potential method or a constant current method. The electrolysis time is preferably 1 to 10 minutes in order to obtain the required cathode polarization effect.

When performing cathodic polarization by the potentiostatic method, the difference Δ between the natural corrosion potential E _corr and the set potential E _{ca of} cathodic polarization is Δ.
It is preferable to adjust the set potential E _ca so that E (= E _corr −E _ca ) is in the range of 0.1 to 2.0V. When the potential difference ΔE is less than 0.1 V, the cathode reaction is not sufficiently performed and the surface activation effect is insufficient. Conversely, the potential difference Δ
When E exceeds 2.0 V, the amount of hydrogen gas generated sharply increases, making it difficult to hold the aluminum-based material at a constant potential.

When the cathode polarization is performed by the constant current method,
It is preferable to flow a cathode current I _ca of 01 to 1 A / 4.5 cm ² . Cathode current I _ca is 0.01A / 4.5
If it is less than cm ² , the cathode reaction is insufficient and the surface of the aluminum-based material cannot be activated as expected. On the other hand, when the cathode current I _ca exceeds 1 A / 4.5 cm ² , the hydrogen gas generation becomes abrupt and the current changes unstablely, making it difficult to maintain a constant processing state.

When the cathode polarization potential E _ca is shifted to the anode polarization potential E _an , the speed of shifting the potential to the plus side, that is, the potential sweep speed is maintained in the range of 0.05 to 5 V / sec. When the potential sweep rate is less than 0.05 V / sec, the reaction proceeds slowly and the preferential dissolution reaction of the crystallized substance is unlikely to occur, and the time required for one operation becomes long, which lowers the workability. On the other hand, at a potential sweep speed exceeding 5 V / sec, a rapid reaction occurs and the current changes drastically,
The electrolytic reaction under certain conditions becomes difficult. The potential sweep rate at the time of transition from the anodic polarization potential E _an to the natural corrosion potential E _corr is also maintained in the range of 0.05 to 5 V / sec.

Anodic polarization is performed by periodically changing the potential of the aluminum-based material between the natural corrosion potential E _corr and the pitting corrosion potential E _pit, and repeating polarization-depolarization.
At this time, when the potential of the aluminum-based material exceeds the pitting potential E _pit and becomes noble, dissolution of the matrix itself in addition to crystallized substances such as CuAl ₂ -based intermetallic compounds starts, and pitting corrosion occurs on the surface of the aluminum-based material. Occurs. On the contrary, when the aluminum-based material has a base potential lower than the natural corrosion potential E _corr , Cu ions eluted in the treatment liquid are deposited on the surface of the aluminum-based material.
The effect of the anodic polarization process is lost.

When anodic polarization is performed after cathodic polarization, a large anodic current is initially observed as the polarization increases. This is because CuAl ₂
It shows that crystallized substances such as intermetallic compounds are preferentially and actively eluted. When the potential of the aluminum-based material is depolarized to the natural corrosion potential E _{corr and} then anodic polarized again, the anode current gradually decreases and the current flowing at the anodic polarization potential E _an converges to a constant value.

By anodic polarization, crystallized substances such as CuAl ₂ -based intermetallic compounds on the surface of the aluminum-based material are preferentially removed. The polarization-depolarization repetition period is about 2 to 20 cycles, and the current flowing at the anode polarization potential E _an has a constant value (about 1.2 m for Al-1.3 wt% Cu alloy).
When converged to A), the electrolysis operation is terminated. Usually 5
At ~ 20 cycles, current convergence is seen.

_Pitting potential E _pit and spontaneous corrosion potential E
_{The corr} varies depending on the type of aluminum-based material to be treated, the type, concentration, temperature, etc. of the electrolytic treatment liquid. Therefore, it is preferable to measure the pitting corrosion potential E _pit and the spontaneous corrosion potential E _corr in advance prior to the electrolytic treatment, and to determine the cathode polarization potential E _ca and the anode polarization potential E _an corresponding thereto.

The surface of the aluminum-based material is activated by cathodic polarization. However, on the surface of aluminum material,
Natural oxide film and dirt are attached. Therefore, in order to remove the natural oxide film, stains and the like to smoothly activate the surface, the surface of the aluminum-based material may be cleaned by ordinary alkali etching using an aqueous solution of sodium hydroxide or the like.

[0032]

EXAMPLE 1.3% by weight of Cu was added to high-purity aluminum, which was melted, cast and rolled to obtain an aluminum plate having a plate thickness of 1.0 mm. This aluminum plate is kept at a temperature of 70 ° C for 1
After dipping in a 0% NaOH aqueous solution for 1 minute and washing with water, it was further dipped in a 30% nitric acid aqueous solution for 1 minute, washed again with water, and dried. By this pretreatment, foreign matters such as rolling oil adhering to the surface of the aluminum plate were completely removed.

The pretreated aluminum plate was used as a test piece and subjected to electrolytic treatment under the conditions shown in Table 1. In addition, EDTA was added to the treatment liquid in an amount of 10 g / l in order to suppress reprecipitation of Cu ions eluted by the anode treatment. In addition, the spontaneous corrosion potential in Table 1 is expressed on the basis of a silver chloride reference electrode. In Table 1, untreated test pieces before electrolytic treatment are listed as Comparative Example 2.

The surface of each test piece after electrolytic treatment and the untreated test piece (Comparative Example 2) were observed with a scanning electron microscope.
As shown in FIG. 7, white and granular crystallized substances a were observed on the surface of the untreated test piece. Further, the granular crystallized substance a is
It was distributed along the grain boundaries. When this crystallized substance a was analyzed by X-ray diffraction, it was found to be a crystallized substance mainly composed of a CuAl ₂ -based intermetallic compound.

On the other hand, in the test piece electrolytically treated in accordance with the present invention, as shown in FIGS. 3 to 5, the white particles found on the surface of the untreated test piece (FIG. 7) were eliminated,
Instead, blackish holes b were observed. This indicates that crystallized substances were removed from the surface of the test piece by anodic polarization. Further, no trace of erosion could be detected in the matrix c.

On the other hand, in the test piece of Comparative Example 1, the crystallized substance was not completely removed as shown in FIG. 6, and residual crystallized substance d was observed on the surface of the treated test piece. It is speculated that this is because the nitric acid concentration of the treatment liquid used in Comparative Example 1 was insufficient and the dissolution reaction of the crystallized substance during anode polarization did not sufficiently occur.

[0037]

[Table 1]

When comparing Comparative Examples 1 and 2 with Examples 1 to 3, the crystallized substances were completely removed from the surface of the test piece electrolytically treated according to the present invention, and the matrix surface was not corroded at all. It is clear that they have not received any. Therefore, when the anodizing treatment was performed on the test pieces of Examples 1 to 3, a uniform colored film could be formed.
In addition, the occurrence of corrosion starting from crystallized substances was not observed.

[0039]

As described above, according to the present invention, the crystallized substances such as CuAl ₂ -based intermetallic compounds exposed on the surface of the aluminum-based material can be completely removed without damaging the matrix surface. Can be removed. A high-purity aluminum layer is formed on the surface layer of the treated aluminum-based material, resulting in excellent corrosion resistance. Further, when the treated aluminum-based material is subjected to chemical conversion treatment, anodic oxidation treatment, etc., a uniform film is formed without being adversely affected by crystallized substances. Moreover, Cu that is solid-dissolved in the matrix and crystal precipitates that are not exposed on the surface and do not participate in the corrosion reaction are not affected by the cathode polarization and the anode polarization. Therefore, the surface layer can be modified without adversely affecting the mechanical properties expected of the aluminum-based material.

[Brief description of drawings]

1 shows the experimental setup used to perform cathodic and anodic polarization.

FIG. 2 is a graph showing changes over time in the potential applied to the test piece.

FIG. 3 is a sketch of the surface condition of a test piece electrolytically treated in Example 1 of the present invention, observed with a scanning electron microscope at a magnification of 5000.

FIG. 4 is a sketch of the surface condition of a test piece electrolytically treated in Example 2 of the present invention, observed with a scanning electron microscope at a magnification of 5000.

FIG. 5 is a sketch of the surface condition of a test piece electrolytically treated in Example 3 of the present invention, observed with a scanning electron microscope at a magnification of 5000.

6 is a sketch of the surface condition of the electrolytically treated test piece of Comparative Example 1 observed with a scanning electron microscope at a magnification of 5000. FIG.

FIG. 7 is a sketch of the surface condition of an untreated test piece observed with a scanning electron microscope at a magnification of 5000.

[Explanation of symbols]

E _corr spontaneous corrosion potential E _pit pitting potential E _ca cathodic polarization setting potential E _an anodic polarization setting potential

Claims (1)

(57) [Claims] 1. An aluminum-based material is immersed in an electrolyte solution for cathode polarization, and then polarized on the anode side, and the potential of the aluminum-based material is adjusted between the spontaneous corrosion potential and the pitting potential in the electrolyte solution. A surface treatment method for an aluminum-based material, which is characterized by varying periodically.