JPH10212586A - Chromating treatment for hot-dip coated steel plate excellent in black change resistance and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for chromating treatment by which contamination by hexahydric chromium is hardly generated and further a change to black hardly take place, too, during the storing of a zinc-aluminium system alloy hot-dip coated steel plate even when the amount of adhered chromium is increased, in the chromating treatment of the lead-aluminium system alloy hot-dip coated steel plate. SOLUTION: The chromating process is executed in such a way that a chromating treatment liquid, which is formed by reducing hexahydric chromium of a water-soluble chromium compd. by adding an oxy carboxylic acid compd. so as to attain the ratio of CR<6+> /total Cr to be <=0.1, is applied on the surface of the zinc-aluminium system alloy hot-dip coated steel plate, which is then dried without being washed with water. A coating film of metallic oxide of metallic salt may be formed by atomizing an aqueous soln. contg. at least one kind among Co salt, Fe salt and Ni Salt on the surface of a plated layer and pyrolyzing the metallic salt with heat of a steel plate during the time when the plated layer immediately after hot dipping is cooled from the molten state to 170 deg.C before this chromating treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for chromate treatment of a hot-dip zinc-aluminum alloy steel sheet having excellent blackening resistance and corrosion resistance.

[0002]

2. Description of the Related Art Zinc-aluminum alloy hot-dip galvanized steel sheets are used more frequently in building materials because they have better corrosion resistance than hot-dip galvanized steel sheets. Therefore, the surface is subjected to a chromate treatment at the time of manufacture. In this chromate treatment, a chromate treatment solution in which a water-soluble chromium compound such as chromic anhydride, chromate, and dichromate is dissolved is applied to a steel sheet.
The coating type which is dried without washing with water is mainly used, but hexavalent chromium is easily eluted from the chromate film, and in order to make the film hygroscopic, the treatment liquid is usually hexavalent chromium. The corrosion resistance and moisture resistance of the chromate film are enhanced by reducing a part of the chromate film to insoluble trivalent chromium with a reducing agent.

[0003] This reduction of hexavalent chromium is carried out by reducing starch,
Although organic compounds such as polyvinyl alcohol and formalin and inorganic compounds such as hydrogen peroxide have been used, the reduction ratio of hexavalent chromium to trivalent chromium is reduced by 50% with these reducing agents.
%, The trivalent chromium changes to chromium hydroxide or the like, causing gelation or sedimentation of the treatment liquid, making application to a steel sheet difficult. For this reason, the reduction ratio of hexavalent chromium is set to 50% or less. However, at such a reduction rate, if the amount of chromium adhering to the chromate film is increased to increase the corrosion resistance, there is a problem that yellow coloring due to hexavalent chromium occurs.

[0004] Further, when a zinc-aluminum alloy hot-dip coated steel sheet is subjected to a coating type chromate treatment having a high hexavalent chromium concentration, its surface becomes gray-black (hereinafter referred to as "a few months") in a relatively short period of time even in a normal storage state. (Referred to as black discoloration), and the appearance was impaired. It is believed that the blackening is caused by the lack of oxygen due to the shielding effect of the chromate film as follows. That is, in the case of the coating type chromate treatment, hexavalent chromium is reduced to trivalent chromium after the formation of the chromate film, and zinc on the surface of the plating layer is oxidized. Further, when a compound such as chromic anhydride that lowers the pH of the processing solution is used as the water-soluble chromium compound, an oxide film on the surface of the plating layer is removed when the processing solution is applied, and active zinc is exposed. After the formation of the chromate film, the exposed zinc is oxidized. However, if the plating layer surface is covered with a chromate film, the oxygen supply during oxidation is insufficient, so that zinc becomes an oxygen-deficient oxide,
This is presumed to be black. This blackening is also observed in a zinc-aluminum alloy hot-dip coated steel sheet in which Mg, Si, Pb, misch metal or the like is added to the plating layer to reduce non-plating and improve corrosion resistance.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to a method for producing a zinc-aluminum alloy hot-dip coated steel sheet which does not cause coloring due to hexavalent chromium even if the amount of chromium is increased, and has excellent blackening resistance and corrosion resistance. -To provide a method of processing

[0006]

According to a first aspect of the present invention, a water-soluble chromium compound, hexavalent chromium, is reduced by adding an oxycarboxylic acid compound so that the ratio of Cr ⁶⁺ / total Cr becomes 0.1 or less. The obtained chromate treatment liquid is applied to the surface of a hot-dip galvanized steel sheet of a zinc-aluminum alloy and dried without washing with water.

In a second aspect of the present invention, there is provided the chromate treatment method according to the first aspect, wherein phosphoric acid or a phosphoric acid compound is contained in the chromate treatment solution in a ratio of P / total Cr = 0.1 to 4.0. It is characterized by applying the added one.

[0008] Furthermore, the third invention provides a method for manufacturing a hot-dip galvanized zinc-aluminum alloy steel sheet immediately after hot-dip galvanizing until the coating layer is cooled from a molten state to 170 ° C.
An aqueous solution containing at least one of e-salt and Ni-salt is sprayed on the surface, and the metal salt is thermally decomposed by the heat of a steel sheet to form a metal oxide film of the metal salt. Chromium is converted to C by adding an oxycarboxylic acid compound.
A chromate treatment solution reduced so that the ratio of r ⁶⁺ / total Cr is 0.1 or less is applied to the surface of the zinc-aluminum alloy hot-dip coated steel sheet and dried without washing with water. Is what you do.

A fourth aspect of the present invention is the chromate treatment method according to the third aspect of the present invention, wherein phosphoric acid or a phosphoric acid compound is added in a ratio of P / total Cr = 0.1 to 4.0 as a chromate treatment solution. The method is characterized in that the applied material is applied.

[0010]

The present inventors have conducted various studies on a reducing agent that does not cause the chromate treatment solution to gel even when the reduction ratio of hexavalent chromium is 50% or more. Even when the total amount of chromium was reduced to trivalent chromium, it was found that the treatment liquid was stable. And
According to this, since the content of hexavalent chromium can be reduced, even if the amount of chromium attached during the chromate treatment is increased, it is possible to prevent coloring by hexavalent chromium, and it is reduced to trivalent chromium after coating. Since the amount of chromium (VI) is small and the oxidation of the zinc-aluminum alloy plating layer is reduced, the occurrence of blackening can be suppressed, and since the treatment solution does not gel, the treatment solution pH adjusts the oxide film on the plating layer surface. It has been found that the generation of black discoloration can be suppressed because the pH can be increased to a neutral state of pH 6 to 8 which is not dissolved and removed.

The reason why the treatment solution does not gel when the oxycarboxylic acid compound is used is not clear, but the hydroxyl group of the oxycarboxylic acid compound reduces hexavalent chromium to trivalent chromium and the carboxyl group changes to trivalent chromium. It is thought to coordinate with the chromium compound to prevent gelation. Examples of oxycarboxylic acid compounds include tartaric acid, malonic acid, citric acid,
Lactic acid, glycolic acid, glyceric acid, tropic acid, benzylic acid, hydroxyvaleric acid and the like can be mentioned, and these reducing agents may be used alone or in combination. Since the reducibility differs depending on the compound, the addition amount is determined while grasping the reduction to trivalent chromium.

The reduction of hexavalent chromium to trivalent chromium is carried out using Cr
The ratio of ^{6 +} / total Cr is set to 0.1 or less. If it is larger than 0.1, the content of hexavalent chromium is large and the elution of the hexavalent chromium from the chromate film is increased, so that zinc is oxidized and blackened easily. The concentration of the water-soluble chromium compound is desirably 1 to 40 g / L for all Cr. 1g
If it is less than / g, the corrosion resistance is insufficient, and if it is more than 40 g / L, it tends to gel.

The reduction by the oxycarboxylic acid compound can be promoted by adding phosphoric acid or a phosphoric acid compound to the treatment solution with a small addition amount as shown in FIG.
Further, the chromate film can be made of a hardly soluble chromium phosphate film. As the phosphoric acid compound, a water-soluble compound such as ammonium dihydrogen phosphate is used, and the amount of addition is adjusted so that P / total Cr = 0.1 to 4.0. 0.
When it is less than 1, the improvement in corrosion resistance due to insolubilization of the film is small, and when it is more than 4.0, the stability of the treatment liquid is reduced.

The treatment liquid is applied to the material to be treated in the same manner as in the case of the conventional coating type chromate treatment.
It is carried out by a coating method, an air curtain method, an electrostatic atomization method, a squeeze roll coating method, etc., and a method of drying without washing with water. The temperature of the steel sheet during the treatment may be room temperature, but the temperature of the treatment liquid is preferably heated to 40 to 50 ° C. in order to promote the evaporation of water after coating. Drying after application is 60-2.
Preferably it is 50 ° C.

In some cases, a water-soluble or water-dispersible organic resin is added to the chromate treatment liquid. In such a case, the addition amount is preferably 20 to 500 g / L.
If it is less than 20 g / L, it is difficult to form a uniform film,
If it is more than 00 g / L, the viscosity becomes high, and the application becomes difficult. Further, it has been conventionally possible to add a polymer resin powder having a melting point of 100 ° C. or more as a lubricant to the organic resin-containing chromate treatment liquid to enhance the lubricity of the chromate film at the time of press working or the like. It is done, but this chrome
A resin powder having a melting point of 100 ° C. or more, such as polyethylene, polypropylene, and fluororesin, may be added to the treating solution.

The chromate treatment method according to the present invention is characterized in that a water-soluble Co salt or a Fe salt is used until the plating layer of the zinc-aluminum alloy hot-dip steel sheet immediately after hot-dip coating is cooled from the molten state to 170 ° C. , Spraying an aqueous solution containing at least one Ni salt, and thermally decomposing the metal salt by heating the steel sheet to form a metal oxide film of the metal salt, the metal oxide film covers the plating layer, Since the contact with the chromate film is cut off, the occurrence of blackening can be further suppressed.

The metal salt contained in the aqueous solution is a Co salt, Fe
The reason for using the salt and the Ni salt is that metal salts other than these metal salts have little effect or exhibit an adverse effect. As salts, nitrates and chlorides are preferred.
This is because the metal oxide film is formed by other salts such as acetate, sulfate, fluoride, etc.
This is because they are chemically unstable and are easily eluted, and the remaining unreacted substances and decomposed substances act as corrosion factors to inhibit blackening resistance.

The salt concentration and temperature of the aqueous solution are not particularly limited, but the concentration is preferably adjusted to be 0.1 to 20 g / L in terms of metal ion concentration. This is because when the metal ion concentration is less than 0.1 g / L, the amount of metal oxide formed on the surface of the plating layer is small, and the blackening prevention effect is small.
In addition, when the spangle is minimized by spraying the unsolidified plating layer, the spangle cannot be minimized uniformly, and 20 g / L.
If the ratio exceeds 1, the blackening prevention effect is improved, but a color peculiar to the dissolved salts appears, and the entire surface is colored. The pH of the aqueous solution is preferably adjusted to 6 to 8. This is because if the pH is too low or too high, the plating layer is at a high temperature and reacts instantaneously with the plating layer, causing etching and micro-corrosion, and becomes a starting point of corrosion as well as discoloration. .

The spraying of the aqueous solution is performed while the temperature of the plating layer is 170 ° C. or more while the plating layer is in a molten state. The temperature of the plating layer at the time of spraying is set to 170 ° C. or higher because the thermal decomposition of the metal salt is not efficiently performed in a short time at a temperature lower than this temperature. When the metal salt is a nitrate, it is thermally decomposed to an oxide even at about 20 ° C., but when the plating layer temperature is such a low temperature, the thermal decomposition efficiency is reduced.

The oxides generated by thermal decomposition vary depending on the type of salt and the thermal decomposition temperature. In the case of Co salt, Co ₃ O ₄ , Co
A single film of O or spinel type CoAl ₂ O ₄ or a composite film of two or more thereof is formed. In the case of the Fe salt, a single or composite film of FeO, Fe ₂ O ₃ , Fe ₃ O ₄ or the like is formed, and in the case of the Ni salt, a film of NiO, Ni ₂ O ₃ or the like is formed.

The spray amount of the metal salt aqueous solution is such that the amount of the metal salt attached is 1 to 100 mg / m ² . If the amount is less than 1 mg / m ² , the effect of preventing blackening is insufficient, and if it exceeds 100 mg / m ² , coloring occurs.

[0022]

【Example】

Example 1 A chromate treatment solution shown in Table 1 was prepared by adding an oxycarboxylic acid compound and phosphoric acid to an aqueous solution of ammonium chromate and reducing a part of hexavalent chromium to trivalent chromium. Thereafter, this treatment liquid was stored at a liquid temperature of 60 ° C. for one month, and the case where no gelation or precipitation of the treatment liquid was observed was evaluated with a symbol “○”, and the case where it was observed was evaluated with a symbol “X”. At the same time, the treatment liquid was changed to Zn-4% Al-0.5% Mg.
It was applied to a hot-dip-alloyed steel sheet by a roll coater and dried at an ultimate sheet temperature of 120 ° C. Table 1 shows the composition of the chromate treatment solution.
It shows the stability and the amount of Cr adhering to the chromate film.

[0023]

[Table 1]

Next, the following appearance and performance tests were carried out on the chromate-treated steel sheet treated with one having good treatment solution stability as shown in Table 1. Table 2 shows the results. (1) Color Tone The degree of yellowness of the treated steel sheet was measured in accordance with JIS Z 8730 by La.
The measurement was performed using the b value according to the b method, and those having a b value of less than 3.0 were evaluated with the symbol ○ and those having the b value of 3.0 or more were evaluated with the symbol x.

(2) Blackening Resistance Test The treated steel sheet was left in an atmosphere at a temperature of 50 ° C. and a relative humidity of 98% or more for 100 hours, and the brightness of the treated steel sheet was measured according to JIS Z 873.
The L value measured by the Lab method according to
The above items were evaluated with the symbol ◎, those with less than 75 and 70 or more were evaluated with the symbol 、, those with less than 70 and 60 or more were evaluated with the symbol 、, and those with less than 60 were evaluated with the symbol ×.

(3) Corrosion resistance test A salt spray test (JIS Z 2371) was carried out for 120 hours.
, Those with 5% or more and less than 10% were evaluated with the symbol で, those with 10% or more and less than 50% were evaluated with the symbol 、, and those with 50% or more were evaluated with the symbol x.

[0027]

[Table 2]

Example 2 After adjusting the coating weight of a Zn-4% Al-0.5% Mg alloy hot-dip coated steel sheet immediately after hot-dip coating by a gas drawing method, when the coating layer temperature was 170 ° C. or higher, a Co salt was used. , F
The aqueous solution of e-salt or Ni-salt was sprayed by atomization with compressed air. Thereafter, a chromate treatment solution was applied to this steel plate by a roll coater, and dried at an ultimate plate temperature of 120 ° C. As a chromate treatment solution, tartaric acid is added to an aqueous solution of ammonium chromate (Cr concentration 10 g / L),
^{6 +} / All Cr reduced to 0.05 was used. Table 3 shows the metal salt aqueous solution used, the amount of metal adhered, the amount of phosphoric acid added to the chromate treatment solution, and the amount of Cr adhering to the chromate film. Table 4 shows the results of the same performance test as in Example 1.

[0029]

[Table 3] (Note) In the chromate treatment of Comparative Example No. 11, an aqueous solution of chromic anhydride was applied.

[0030]

[Table 4]

[0031]

As described above, the chromate treatment of the aluminum-based alloy hot-dip coated steel sheet of the present invention is performed by using an oxycarboxylic acid compound as a hexavalent chromium reducing agent to convert hexavalent chromium to trivalent chromium. Even if the reduction rate is 90% or more, since a processing solution that does not cause gelation or precipitation of the processing solution is used,
Even if the amount of chromium adhering to the chromate film is increased to increase the corrosion resistance, yellow coloring by hexavalent chromium does not occur.

Further, since the treatment solution has a low hexavalent chromium concentration and a neutral hydrogen ion concentration, blackening hardly occurs on the surface of the zinc-aluminum alloy hot-dip coated steel sheet during storage. Further, this blackening is caused when the plating layer temperature immediately after hot-dip plating is 170 ° C. or higher, such as Co salt, Fe salt, Ni salt.
When an aqueous solution containing at least one kind of salt is sprayed and a metal oxide film of a metal salt is formed before the chromate treatment by thermal decomposition using heat from a steel sheet, the generation of the metal oxide film becomes more difficult.

[Brief description of the drawings]

FIG. 1 shows the relationship between the rate of reduction to trivalent chromium and the amount of tartaric acid when hexavalent chromium in the chromatized solution was reduced by adding tartaric acid and when further reduced by adding phosphoric acid. It is a graph.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Shinichi Kamoshida 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd.

Claims (6)

[Claims] 1. A zinc-aluminum alloy comprising a chromate treatment solution obtained by reducing hexavalent chromium of a water-soluble chromium compound by adding an oxycarboxylic acid compound so that the ratio of Cr ⁶⁺ / total Cr becomes 0.1 or less. A method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance, which is applied to the surface of the hot-dip coated steel sheet and dried without washing with water. 2. The method according to claim 1, wherein a phosphoric acid or a phosphoric acid compound is added at a ratio of P / total Cr = 0.1 to 4.0 as a chromate treatment solution. A method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance. 3. The method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance according to claim 1, wherein the pH of the chromate treatment liquid is 6 to 8. 4. A hot-dip galvanized steel sheet of a zinc-aluminum alloy immediately after hot-dip coating
An aqueous solution containing at least one of a Co salt, an Fe salt, and a Ni salt is sprayed on the surface until the metal salt is cooled down to 0 ° C., and the metal salt is thermally decomposed by the heat of a steel sheet to form a metal oxide film of the metal salt. After the formation of chromium, a chromate treatment solution obtained by reducing hexavalent chromium of a water-soluble chromium compound by adding an oxycarboxylic acid compound so that the ratio of Cr ⁶⁺ / total Cr becomes 0.1 or less is used as the zinc-aluminum-based solution. A method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance, wherein the method is applied to the surface of a hot-dip coated steel sheet and dried without washing with water. 5. The chromate treatment method according to claim 4, wherein a phosphoric acid or a phosphoric acid compound added in a ratio of P / total Cr = 0.1 to 4.0 is applied as a chromate treatment solution. A method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance. 6. The method for chromate treatment of a hot-dip coated steel sheet having excellent blackening resistance and corrosion resistance according to claim 4, wherein the pH of the chromate treatment liquid is 6 to 8.