JPH05112892A - Aluminum material excellent in filiform rust resistance - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to an aluminum material having excellent thread rust resistance. [Constitution] Fe, C on the surface of aluminum or its alloy
At least one of o, Cu, Cr, and Mn is discontinuously coated so that the aluminum surface is partially exposed. [Effect] By doing so, chemical conversion treatment such as phosphoric acid treatment is improved, and it is possible to prevent generation of thread rust after coating.

Description

Detailed Description of the Invention

[0001]

[Industrial field of use] Aluminum and / or its alloys are often used by coating for automobiles, transportation materials, building materials, household electrical equipment materials, etc. The present invention is used for coating, especially pretreatment for coating. The present invention relates to the surface treatment of aluminum and its alloys, which can suppress the occurrence of filiform corrosion in the field where it is used after being subjected to a phosphate treatment.

[0002]

2. Description of the Related Art Aluminum or its alloy is one of the features that it has excellent corrosion resistance in a neutral general use environment. However, when it is used by painting, it causes filiform corrosion under the coating film. It has the drawback of being easy and spoiling the paint appearance. It is known that the pretreatment of coating greatly contributes to the inhibition of filiform corrosion, and a method of forming a chromate chromate or chromate phosphate film has been carried out with relatively good performance. However, when assembling aluminum-based materials and steel materials to perform pre-painting treatment, such as in a car body painting line, chromate-based treatment cannot be applied to steel materials, and only phosphate treatment can be used. This is a pretreatment method capable of forming a film on a material. However, there has been a problem that the phosphate-treated aluminum or its alloy has inferior thread rust resistance as compared with steel materials because sufficient performance cannot be obtained.

It is difficult to completely coat a phosphate coating on aluminum and its alloys. It is known that when Al is etched in a phosphating bath, AlPO ₄ precipitates on the surface and inhibits the formation of a zinc phosphate crystal film.
As a method for improving this, JP-A-61-157693.
Japanese Patent Laid-Open Publication No. 2003-242242 discloses coating the surface of aluminum with a Zn or Zn-based plating layer in an amount of 1 to 5 g / m ² . This method is a technique for uniformly forming a phosphate coating on the surface of an aluminum material by coating an aluminum surface on which a phosphate coating is difficult to form with a Zn-based plating layer that can easily form a phosphate coating. .. However, even if a phosphate film can be formed, there is a problem that the yarn rust resistance is not so improved.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to solve such a problem, and in coating applications where a phosphate treatment is applied, in order to form a phosphate film having high corrosion resistance, yarn rust resistance is required. The object is to provide aluminum and / or an alloy thereof having excellent properties.

[0005]

In order to achieve the above-mentioned object, the present invention provides a surface of aluminum or its alloy with F
A yarn resistant to aluminum, which is obtained by discontinuously coating an aluminum surface with at least one of e, Co, Ni, Cu, Cr and Mn, or an alloy of at least one of these metals and another metal. The main point is an aluminum material having excellent rusting properties.

[0006]

The aluminum material to which the present invention is applied includes
For example, those specified in JISZ3263 wrought aluminum alloy, or alloys having chemical components similar to them are used.

Filiform corrosion is caused by the tip of the thread rust under the coating film being the anode,
Corrosion progresses due to the action of the oxygen concentration cell which becomes the cathode later, and it is considered that the growth of thread rust is governed by the diffusion rate of O ₂ through the porous rust layer at the tail of the thread rust. A
In the case of 1, since the rust layer is not as porous as in the case of steel, part of the cathode is taken up by the part that does not yet rust around the thread rust tip, and the area around the thread rust tip is OH ions generated by the oxygen reduction reaction of the cathode. Has been observed to become alkaline. It is considered that the alkali dissolves the Al matrix and the phosphate film, breaks the chemical bond of the coating film, deteriorates the adhesion of the coating film, and promotes the growth of thread rust. The present inventors considered that the properties of the phosphate film are the most important for suppressing the growth of thread rust. That is, a phosphate film having good properties can suppress the oxygen reduction reaction and have a high resistance to alkali.

It is understood that in the phosphate film forming reaction on Al, nucleation is slow even in a fluoride ion-added treatment bath, and coarse crystals and unreacted portions are liable to occur, and thus the thread rust resistance is inferior. .. However, there are granular and strict crystals in the produced zinc phosphate crystals, and this crystal form is Z
It is clearly different from the needle- or leaf-shaped zinc phosphate crystals formed on n. The latter has many voids between crystals and cannot be a barrier capable of suppressing the oxygen reduction reaction. The inventors of the present invention have considered that a strict coating film with few inter-crystal voids can be obtained by forming a phosphate coating film having a uniform grain size of granular crystals on Al, and arrived at the present invention. ..

If Fe, Co, Ni, Cu, Cr, Mn, etc. are partially or discontinuously deposited on the surface of the aluminum material, these metals become the cathode in the phosphate reaction and the metal from the anode is removed. Promotes the dissolution of Al. Fluorine ions in the treatment bath suppress the desorption of H ions from hydrated Al ions and act as an auxiliary in raising the pH of the base interface. Thus, the pH of the interface sharply rises, and zinc phosphate crystals are deposited past the AlPO ₄ production region. It is presumed that the zinc phosphate crystals deposited on the anode become granular crystals due to the epitaxy relationship with the etched Al surface. The crystal nuclei generated on the anode grow and cover the cathode portion, and a strict crystal having a uniform grain size of about 5 μm and few inter-crystal voids can be obtained.

In the absence of effective cathodes such as Fe, Co, Ni, Cu, Cr, and Mn, the dissolution rate of Al in the phosphating bath is not sufficient, and the pH increase rate at the base interface is slow. AlPO ₄ is formed in the aluminum oxide, and the precipitation of zinc phosphate crystals is hindered. Since the crystal growth stage is started with few nucleation sites, coarse crystallization occurs, and acicular zinc phosphate secondary crystals are formed at the cathode part.

It is not only that the corrosion potential is noble with respect to Al that Fe, Co, Ni, Cu, Cr, Mn, etc. are effective cathodes for the phosphate reaction of Al.
It is presumed that the effect of the catalytic action on the H ₂ generation reaction of the cathode contributes. Fe, Co, Ni, Cu,
Cr, Mn, etc. may be precipitated alone on Al, or may be precipitated with two or more alloys. Further, it may be precipitated as an alloy with a metal other than these. Zn, B, P, C, etc. are used as an alloy.

Since the present invention is basically based on the promotion of dissolution of the Al base material during the phosphating treatment, the cathode metal must be deposited so that the Al surface is sufficiently exposed. Further, it is desirable that the particles are dispersed and deposited as uniformly as possible. The coverage of the cathode metals is preferably 10 to 90%. The adhesion amount is preferably about 0.1 to 1 g / m ^{2 on} average.

As a mode of precipitation of Fe, Co, Ni, Cu, Cr, Mn, etc., aluminum or its alloy is subjected to alkali cleaning and / or acid cleaning, and then immersed in an acidic bath or alkaline bath containing the above metal ions, Precipitate by chemical substitution reaction. Alternatively, it may be deposited by performing cathodic electrolysis in the above bath.

[0014]

【Example】

[0015]

[Table 1]

Note 1: The above aluminum plate was replaced with 5% NaOH.
Washing with an aqueous solution and then with a 5% sulfuric acid aqueous solution, immersing in sulfuric acid of pH = 1 containing the above metal ions + 3 g / l ammonium fluoride bath, or performing cathodic electrolysis in a sulfuric acid bath of pH = 1 to perform aluminum Metal was deposited on the material surface.

Note 2: Phosphate film Chemical conversion treatment was carried out in a fluoride ion-added phosphate treatment bath.

The coating state is ◯: full surface coating, ×: there is a non-adhered portion Note 3: Thread rust resistance Cationic electrodeposition coating 20 μm, intermediate coating, top coating,
An aluminum plate with a knife scratch reaching the base plate was applied to the total coating film thickness of 80 μm, and salt spray was applied for 85 days a day.
%, 40 ° C. wet for 5 days, and allowed to stand indoors for 1 day. The maximum thread rust length was measured after a test for 8 weeks under a cycle environment condition.

[0019]

EFFECTS OF THE INVENTION According to the present invention, as is apparent, an aluminum material having excellent resistance to thread rust can be obtained, and the applications of the aluminum material can be expanded, resulting in great industrial effects.

Claims (2)

[Claims] 1. A surface of aluminum or its alloy,
An aluminum material excellent in yarn rust resistance, characterized in that the aluminum surface is discontinuously coated with at least one of Fe, Co, Ni, Cu, Cr, and Mn. 2. A thread rust-resistant rust-proof according to claim 1, characterized in that an alloy of at least one of Fe, Co, Ni, Cu, Cr and Mn and another metal is discontinuously destroyed. Aluminum material with excellent properties.