JP2000273656A - Hot dip aluminized steel sheet excellent in corrosion resistance and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip aluminized steel sheet which restrains white rust, discoloring, etc., and keeping a good surface state over a long period. SOLUTION: This hot dip aluminized steel sheet has a hot dip aluminized layer formed on the surface of a substrate steel and the hot dip aluminized layer is covered with another oxidized film containing a zirconium compound. The anodized film containing the zirconium compound is formed by dipping the hot dip aluminized steel sheet into an electrolytic solution prepared by adding 0.005-0.1 mol/l k2ZrF6 to <20 wt.% sulfuric acid solution to execute the anodization treatment. Since a dense anodic oxidized film is formed, sufficient corrosion resistance is obtd. even the film thickness is small and workability is secured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Al-coated steel sheet having improved corrosion resistance such as white rust resistance and discoloration resistance, and a method for producing the same.

[0002]

2. Description of the Related Art A hot-dip Al-plated layer formed on the surface of a hot-dip Al-plated steel sheet reacts with an oxidizing component in the air or water to form a dense oxide film or hydroxide film on the surface. Since the hot-dip Al plating layer is covered with an oxide film or a hydroxide film, it exhibits excellent corrosion resistance in a normal environment as compared with a galvanized steel sheet. Taking advantage of this advantage, hot-dip Al-plated steel sheets are often used simply by chromate treatment or clear coating. Molten Al exhibiting good corrosion resistance
Even in the case of a plated steel sheet, the surface of the plating layer may be discolored or the plating layer may be corroded in applications such as a structural material that is exposed to a severe corrosive environment for a long time in a coastal zone, an industrial zone, or the like. Particularly in recent years when environmental pollution is serious, there is an environment where the hot-dip Al plating layer is easily discolored and corroded. When discoloration or corrosion occurs, the appearance of the structure is significantly impaired. The hot-dip Al-plated steel sheet used as a structural material is significantly improved in corrosion resistance in salt spray environment and wet environment by chromate treatment. However, it can be used in acid baths such as sulfuric acid and chromic acid and alkali baths such as sodium hydroxide. Improving the corrosion resistance of hot-dip Al-coated steel sheets by anodic oxidation, in which a hot-dip aluminum-coated steel sheet is immersed and electrolytically treated, has also been partially implemented (JP-A-63-5)
No. 7795).

[0003]

In a hot-dip Al-plated steel sheet, the thickness of the hot-dip Al-plated layer is limited, and it is difficult to form a thick anodic oxide film. That is, the anodic oxide film formed on the surface of the hot-dip Al plating layer is thinner than the anodic oxide film applied to a general aluminum material, and does not have a sufficient barrier effect of blocking the environment. In addition, pinhole defects and the like in the generated anodic oxide film are likely to be the starting point of pitting corrosion, so that sealing treatment after anodic oxidation treatment is essential. For this reason, it is difficult to improve the corrosion resistance by ordinary anodic oxidation treatment, and the molten Al
The beautiful surface skin of the plating layer is not maintained for a long time.

[0004]

SUMMARY OF THE INVENTION The present invention has been devised to solve such a problem, and a thin anodic oxide film containing a zirconium compound is formed on the surface of a hot-dip Al plating layer. Even with the hot-dip Al-plated layer, the same anodizing effect as that of a normal aluminum material was exhibited, and white rust and discoloration were suppressed even when used as a structural material exposed to a severe corrosive environment for a long time. Molten Al
It is intended to provide a plated steel sheet. The hot-dip Al-plated steel sheet of the present invention is characterized in that a hot-dip Al-plated layer is formed on the surface of the base steel, and the hot-dip Al-plated layer is covered with an anodized film containing a zirconium compound in order to achieve the object. I do. This hot-dip Al-plated steel sheet is prepared by adding K ₂ ZrF ₆ to a sulfuric acid solution having a concentration of 20% by weight or less by 0.005%.
It is manufactured by immersing a hot-dip Al-plated steel sheet in an electrolytic solution having a concentration of 0.1 mol / l and anodizing it to form an anodized film containing a zirconium compound on the surface of the hot-dip Al plated layer.

[0005]

The present inventors have conducted various investigations on the effects of the anodic oxide film formed on the surface of the hot-dip Al plating layer on the improvement of corrosion resistance. As a result, when anodizing the hot-dip Al-plated steel sheet in a sulfuric acid bath to which a Zr-based anionic species such as K ₂ ZrF ₆ is added, an anodic oxide film containing a zirconium compound is formed. We found that it was greatly improved. The reason why the zirconium compound works to improve the corrosion resistance is not clear, but it is presumed that the reason is that the anodized film becomes dense and the film structure changes. That is, by adding K ₂ ZrF ₆ , molten Al
It has been found that the etching action on the plating layer is strengthened and the anodic oxide film is densified, and as a result, it is considered that the corrosion resistance is improved. In addition, the corrosion resistance is improved by Zr incorporated in the film forming a barrier layer having a large barrier property, and a zirconium compound eluted from the film in a corrosive environment stabilizing Al-based corrosion products. It is inferred.

[0006] Even if the obtained anodic oxide film has the same thickness as the anodic oxide film formed in the electrolytic bath of sulfuric acid alone, it has remarkably excellent corrosion resistance and has an effect on discoloration of the plating layer surface and corrosion of the plating layer. Restrained. Since sufficient corrosion resistance is obtained even with a thin anodic oxide film, workability of the hot-dip Al-plated steel sheet is also ensured. The effect of improving corrosion resistance becomes significant when the zirconium compound contained in the anodic oxide film is 0.02% by weight or more in terms of Zr. Normally, when bending, drawing, or the like is performed on a hot-dip Al-plated steel sheet on which a thick anodic oxide film is formed, cracks are likely to occur in the surface layer of the hot-dip Al-plated layer that has been hardened by the anodizing process. The crack acts as a starting point of corrosion and causes red rust to be generated in a processed portion having reduced corrosion resistance. On the other hand, the anodic oxide film containing a zirconium compound has sufficient corrosion resistance even with a thin film.
l The degree of hardening of the surface layer of the plating layer is low.
The generation of cracks during drawing or the like is suppressed.

[0007] The zirconium compound is added to the electrolyte.
T_Two ZrF₆ , (NH_Four )_Two [ZrF₆ ], Zr (C
H_Three COO)_Four From an anionic species such as ZrF
_Four , ZrO_Two Etc. are taken into the anodic oxide film. Z
Even if the r-type anion species is added to the electrolyte, the current-voltage curve
The sulfuric acid concentration has a greater effect on
It does not adversely affect the film formation conditions. The following
In the description of K_Two ZrF ₆ Represents Zr-based anion species
Let K_Two ZrF₆ Is K in the electrolyte⁺ And ZrF₆ ^2- To
Dissociate. Dissociated ZrF₆ ^2-Is the anion species of the electrolyte
Is taken into the film in the same manner as. In this regard, zirconium conversion
Simply sealing the anodic oxide film with the compound-containing solution
The zirconium compound was not incorporated into the anodic oxide film,
No satisfactory corrosion resistance improvement effect is exhibited. On the other hand, zirconium
Anodized film containing compound has a dense film structure
Therefore, it is possible to omit the sealing treatment after the anodizing treatment
It is.

As the electrolyte, a sulfuric acid solution having a concentration of 20% by weight or less is used. Sulfuric acid is preferred because it is inexpensive and relatively easy to handle. When the sulfuric acid concentration exceeds 20% by weight, it becomes difficult to handle the electrolytic solution. The amount of K ₂ ZrF ₆ added to the electrolytic solution is determined in the range of 0.005 to 0.1 mol / l. The amount of K ₂ ZrF ₆ added is 0.
If the amount is less than 005 mol / l, the corrosion resistance is not sufficiently improved. Conversely, if the amount exceeds 0.1 mol / l, the etching action on the hot-dip Al plating layer becomes too large, and the corrosion resistance is rather reduced. The electrolysis conditions include a current density of 5 to 150
A / dm ² and an electrolysis time of 1 to 10 seconds are employed. 5A /
At a current density of less than dm ^2, a film having a sufficient film thickness is not formed because the processing time is short. On the other hand, when the current density exceeds 150 A / dm ² , the coating is burned even with a short-time treatment, and the appearance is impaired. When continuously anodizing a hot-dip Al-coated steel strip, it is necessary to efficiently form an anodic oxide film in a short time in consideration of productivity. Although the processing time required for the anodic oxidation treatment is shortened by setting the current density high, the processing conditions do not become unstable even when the current density is increased because the sulfuric acid bath is used as the electrolytic solution. In addition, the anodic oxide film required for improving the corrosion resistance can be made as thin as 0.05 to 0.2 μm, so that the efficiency of the anodic oxidation treatment can be further improved.

[0009]

EXAMPLE A 200 mm × 100 mm test piece was cut out from a hot-dip Al-plated steel sheet having a basis weight of 40 g / m ² . The test piece was immersed in an electrolytic solution prepared by adding a zirconium salt K ₂ ZrF ₆ to a sulfuric acid solution, and the bath temperature was 30 ° C. and the current density was 10%.
Anodic electrolysis was performed at A / dm ² for 10 seconds. Observation of the surface of the hot-dip Al-plated steel sheet after anodic electrolysis revealed that the film thickness was about 0.5 mm.
A 2 μm anodic oxide film was formed. The thickness of the anodic oxide film was almost constant without being affected by the added amount of K ₂ ZrF ₆ . Each anodized test specimen was subjected to a salt spray test to investigate corrosion resistance. In the salt spray test, 1
The periphery of the test piece cut out to a size of 50 mm × 70 mm is masked, and is cooled to 35 ° C. in accordance with JIS Z2371.
5% NaCl aqueous solution was sprayed on the test piece. After the salt spray was continued for 24 hours, the surface of the test piece was observed to check for the occurrence of white rust and the presence of discoloration, and the area ratio of the white rust and the discolored portion to the entire surface of the test piece was determined.

As can be seen from the investigation results in Table 1, it is understood that the corrosion resistance of the anodic oxide film is improved by adding K ₂ ZrF ₆ to the electrolytic solution. Effective K ₂ ZrF ₆
Varies depending on the sulfuric acid concentration of the electrolytic solution.
The effect of improving corrosion resistance was observed in the range of 005 to 0.1 mol / l. However, K ₂ ZrF is added to a sulfuric acid solution having a concentration of 10% by weight.
_In Test No. 7, in which an electrolyte solution containing 0.03 mol / l of ₆ was added, the entire surface of the test piece was discolored after the salt spray test for 24 hours. After anodizing the test piece surface of test number 3, E
SCA analysis showed that ZrF ₆ , ZrO ₂
Was detected, and it was found that Zr was incorporated in the anodized film. The film form also changed from a porous state to a dense structure as the amount of K ₂ ZrF ₆ added increased. It is considered that the dense anodic oxide film has a strong barrier effect of shielding the hot-dip Al plating layer from the environment, and as shown in Table 1, excellent corrosion resistance was imparted to the hot-dip Al-plated steel sheet.

[0011]

[0012]

As described above, according to the present invention, the melting A
Since the 1-plated steel sheet has an anodized film containing a zirconium compound formed on the surface of the hot-dip Al plating layer, the anodized film is dense and the barrier effect of shielding the hot-dip Al plated layer from the environment is enhanced. In addition, sufficient corrosion resistance can be obtained even with a thin anodic oxide film.
There is no need to form a thick anodic oxide film that easily causes cracks during drawing or the like, and workability is ensured even after anodizing. Therefore, it is used as a structural material that maintains a good surface state without white rust or discoloration even when exposed to severe corrosive environments such as coastal areas and industrial environments for a long period of time.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Minoru Saito, Inventor Minoru, 5th Ishizu Nishimachi, Sakai City, Osaka Prefecture F-term in Nisshin Steel Co., Ltd. Technical Research Laboratory 4K027 AA05 AA22 AB05 AB48 AC82 4K044 AA02 AB02 BA10 BA20 BB03 BB11 BC02 CA11 CA16

Claims (2)

[Claims] 1. A hot-dip Al plating layer is formed on the surface of a base steel, and a molten A
1 Hot-dip Al-coated steel sheet with excellent corrosion resistance covered with a plated layer. 2. K ₂ Z is added to a sulfuric acid solution having a concentration of 20% by weight or less.
A dipped Al-plated steel sheet is immersed in an electrolytic solution containing 0.005 to 0.1 mol / l of rF ₆ and subjected to anodizing treatment to obtain molten A
(1) A method for manufacturing a hot-dip Al-plated steel sheet having excellent corrosion resistance in which an anodized film containing a zirconium compound is formed on the surface of a plating layer.