JPH0544090A - Chromate treatment method for galvanized steel sheet - Google Patents

Abstract

PURPOSE:To realize a plating method for providing excellent corrosion resistance and coating adhesion and high degree of whiteness by using chromating liquid at a specified composition, pH value and temp. CONSTITUTION:A chromating liquid containing 5-70g/l Cr<6+>+Cr<3+>, 2-60g/l Zn<2+>, 1-40g/l MoO4<2->, 10-100g/l SO4<2-> and 5-100g/l at least one of oxide sols of Al2O3, Sb2O5, SiO2, SnO2, TiO2 and ZrO2 as a solid material, at which Cr<3+>/total amt. of Cr in wt. ratio is <=0.5 and MoO4<2->/Zn<2+> in wt. ratio is 0.02-1 and of which pH value is 1-5 and a temp. is 30-70 deg.C, is used, and a cathodic electrolysis processing for galvanized steel sheet is carried out with 0.5-40C/dm<2> amt. of electricity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate treatment of a zinc-based plated steel sheet capable of imparting excellent corrosion resistance, paint adhesion, and a uniform color tone with high whiteness to a zinc or zinc alloy plated steel sheet. It is about the method.

[0002]

2. Description of the Related Art A zinc-plated steel sheet or a zinc alloy-plated steel sheet (hereinafter "zinc-based steel sheet") having a zinc-plated coating or a zinc alloy-plated coating (hereinafter referred to as "zinc-based plating coating") formed on at least one surface thereof. The term "plated steel sheet" is widely used for steel sheets for automobiles, steel sheets for household appliances, steel sheets for building materials, and the like.

Recently, in the automobile industry, in order to save energy, the weight of automobile bodies has been actively reduced. Therefore, automobile manufacturers have made the plate thinner and provided rust prevention measures. Development of zinc-based plated steel sheet having excellent paint adhesion and corrosion resistance is required. In the home electric appliance industry, in order to reduce the cost of the product, it is often the case that the steel sheet for home electric appliances is used unpainted.
There is a demand for the development of a zinc-based plated steel sheet which is excellent in bare corrosion resistance and has a beautiful appearance with a uniform color tone with high whiteness. Further, in order to improve long-term durability of a building material, the building material industry is required to develop a zinc-based plated steel sheet having excellent paint adhesion and corrosion resistance as a rust preventive measure.

As a zinc-based plated steel sheet satisfying the above-mentioned requirements, there is known a chromate-treated zinc-based plated steel sheet obtained by subjecting a zinc-based plated steel sheet to a chromate treatment to form a chromate coating on the zinc-based plated coating. ing.

For the chromate treatment method, a zinc-based plated steel sheet is sprayed with a chromate-treated solution, or a zinc-based plated steel sheet is immersed in the chromate-treated solution to form a zinc-based plated film and a chromate. Reaction treatment solution, reacting chromate treatment consisting of forming a chromate film on the surface of the zinc-based plating film, applying the chromate treatment liquid on the surface of the zinc-based plating film, Coating type chromate treatment consisting of drying without washing with water,
There is also an electrolytic chromate treatment which comprises electrolytically forming a chromate film on the surface of a zinc-based plating film.

The reactive chromate treatment has the following problems. That is, in general, the chromate treatment liquid easily reacts with the metal. Therefore, the metal ions eluted in the chromate treatment liquid are contained in the chromate film, and have a significant adverse effect on the quality of the chromate film. In the reactive chromate treatment, the surface of the zinc-based plating film of the zinc-based plated steel sheet to which the chromate treatment is applied is particularly susceptible to the influence. Therefore, the amount and quality of the chromate film formed on the zinc-based plating film may vary depending on the plating conditions before the chromate treatment, the pretreatment conditions such as pickling and alkali washing, and the fluctuations in the operating conditions such as the line speed. The quality of the chromate film tends to be unstable due to large changes. Further, it cannot be applied to a zinc-based plating film having poor reactivity with a chromate treatment liquid.

The coating type chromate treatment has the following problems. That is, in the case of the coating type chromate treatment, by adjusting the components of the chromate treatment liquid, the influence of the reaction between the chromate treatment liquid and the zinc-based plating film can be suppressed to some extent and the chromate film can be controlled. However, in order to control the amount of the chromate coating film, it is necessary to change the concentration of the chromate treatment liquid and to properly set the coating conditions by the coater, so it is extremely difficult to quickly respond to the operating conditions. Furthermore, since the wettability differs depending on the surface properties of the chromate treatment liquid and the zinc-based plating film, defects such as cissing and coating unevenness are likely to occur on the chromate film.

On the other hand, in the electrolytic chromate treatment, cathodic electrolysis is performed in an aqueous solution containing Cr ⁶⁺ as a main component with a zinc-based plated steel sheet as a cathode to form a chromate film on the surface of the zinc-based plated film. Consists of forming. Therefore, according to the electrolytic chromate treatment, the adhesion amount of the chromate coating is not affected by the type of zinc-based plated steel sheet or the surface texture of the zinc-based plated coating, and the amount of chromate coating is determined by the current density and the amount of electricity during cathodic electrolytic treatment. Further, there are advantages that the quality can be easily and stably controlled, and uneven adhesion is less likely to occur.

Therefore, research on a method of forming a chromate film on the surface of a zinc-based plated film by such an electrolytic chromate treatment and thereby improving the corrosion resistance and paint adhesion of a zinc-based plated steel sheet has been conventionally conducted. For example, Japanese Patent Publication No. 1-24880 discloses a method consisting of the following. And Cr ⁶⁺ and Cr ^3+, and silica and / or silicate, and, NO ₃ ^- contains the ratio of Cr ³⁺ / Cr ⁶⁺ is in the range of 1/50 1/3 Using the adjusted chromate treatment solution, a zinc-based plated steel sheet is subjected to cathodic electrolysis treatment (hereinafter referred to as prior art 1).

Japanese Patent Publication No. 1-14436 discloses a method consisting of the following: Cr ⁶⁺ , PO ₄ ^3- , and fluorine compounds 1
Seed or two, silica and / or silicate, Z
At least one of n, Ni, Co, Al, Mg, Sn, Pb, Mn ions
Using a chromate treatment liquid containing a seed, zinc-based plated steel sheet, aluminum alloy plated steel sheet, subjected to cathodic electrolytic treatment to lead or lead alloy plated steel sheet, to form a chromate film on the plating film, then, Immediately perform anodic electrolysis treatment (hereinafter referred to as prior art 2).

Japanese Unexamined Patent Publication No. 2-145797 discloses a method consisting of the following. Cr0 ₃ , SiO ₂ and H ₂ ZrF ₆ as main components,
Furthermore, in a chromate treatment liquid containing H ₂ SO ₄ and F ⁻ , a zinc-based plated steel sheet is subjected to cathodic electrolysis treatment to form a chromate coating on the surface of the zinc-based plated coating (hereinafter referred to as Prior Art 3). ).

[0012]

The methods of the prior arts 1 and 2 described above have the following problems. That is, compared with the prior art, although some improvement in corrosion resistance or paint adhesion is recognized, it is still insufficient in improving both. That is, when consideration is given to the improvement of paint adhesion, desired corrosion resistance cannot be obtained. On the other hand, when the amount of the chromate film is increased to improve the corrosion resistance, the chromate film is unevenly colored and its appearance deteriorates. As described above, according to Prior Art 1 and 2, it is difficult to form a chromate film having excellent corrosion resistance, paint adhesion and appearance.

[0013] In particular, the methods of the prior art 1, when a zinc-plated steel sheet was continuously chromate treatment, NO ₃ which is contained in the chromate processing solution ^- by elution of the plated metal is promoted, eluted The formed metal ions are accumulated in the chromate treatment liquid. As a result, silica and / or silicate in the chromate-treated solution gels and precipitates in the solution, and this precipitate adheres to the surface of the chromate film. When the zinc-based plated steel sheet is continuously chromated by the method of Prior Art 2, the chromate film formed by the cathodic electrolytic treatment is partially dissolved or peeled off during the anodically electrolytic treatment to form sludge, which causes chromate It precipitates in the treatment liquid, and this precipitate adheres to the surface of the chromate film. As described above, according to the prior arts 1 and 2, as a result of the deposit in the chromate treatment liquid adhering to the surface of the chromate film, the color tone of the formed chromate film becomes non-uniform and the appearance thereof is significantly deteriorated.

In the case of the method of Prior Art 3, H ₂ ZrF ₆ contained in the chromate-treating solution gives a chromate film having a high whiteness, but the stability of the chromate-treating solution is poor. However, it is difficult to obtain a stable and uniform color appearance.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a zinc-based plated steel sheet with excellent corrosion resistance and paint adhesion, and a high-whiteness and uniform color tone appearance. It is to provide a chromate treatment method for zinc-based plated steel sheet.

[0016]

[Means for Solving the Problems] The inventors of the present invention have conducted extensive studies to solve the above problems. As a result, a certain amount of Cr ⁶⁺ + Cr ³⁺ , Zn ²⁺ , MoO ₄ ²⁻ , SO ₄ ²⁻ , and Al ₂ O
₃ , Sb ₂ O _5, SiO ₂ , SnO _2, TiO _{2, and} ZrO ₂ are contained in at least one group, and Cr ³⁺ / total Cr amount and MoO ₄ ^2- / Zn ²⁺ are within a predetermined range. Using a chromate treatment solution with a predetermined pH value and temperature inside, by a predetermined amount of electricity, by subjecting the zinc-based plated steel sheet to cathodic electrolytic treatment, if a chromate film is formed on the zinc-based plated film It has been found that a zinc-based plated steel sheet can be provided with excellent corrosion resistance and paint adhesion, and an appearance with a uniform color tone with high whiteness.

The present invention has been made on the basis of the above-mentioned findings. The present invention relates to a zinc or zinc alloy plated steel sheet having a zinc plated coating or a zinc alloy plated coating formed on at least one surface. On the other hand, by performing a cathodic electrolysis treatment in a chromate treatment liquid, consisting of forming a chromate film on the zinc or zinc alloy plating film, in the chromate treatment method of the zinc-based plated steel sheet, as the chromate treatment liquid, Cr ⁶⁺ + Cr ³⁺ (total amount of chromium): 5~ 70g / l, Zn 2+: 2~ 60g / l, MoO 4 2-: 1~ 40g / l, SO 4 2-: 10~100g / l, And Al ₂ O ₃ , Sb ₂ O _5, SiO ₂ , SnO _2, TiO _2, and ZrO ₂ containing at least one oxide sol: 5 to 100 g / l as solid content, and Cr ³⁺ / total Cr amount weight ratio: 0.5 or less, MoO ₄ ^2- / Zn ²⁺ weight Ratio: 0.02-1, pH value is 1-5, treatment liquid with temperature of 30-70 ° C is used, and the zinc or zinc alloy-plated steel sheet is treated with the electric quantity of 0.5-40C / dm ^2. On the other hand, it is characterized by performing cathodic electrolysis treatment.

[0018]

Next, the reason why the component composition of the chromate treating liquid is limited as described above in the present invention will be explained. (1) Cr ⁶⁺ and Cr ³⁺ : As Cr ⁶⁺ , chromic anhydride,
Also, one or a mixture of two or more of chromic acid, an alkali metal salt of dichromic acid and an ammonium salt is used. Cr ³⁺ is produced by the reduction reaction of Cr ⁶⁺ in the treatment liquid during the cathodic electrolysis treatment. Incidentally, Cr ³⁺ may be contained in the treatment liquid in advance by reducing Cr ⁶⁺ to Cr ³⁺ with alcohols or tannic acid, or Cr ³⁺
The compound of 1 may be directly added to the treatment liquid. Cr ⁶⁺ and Cr ³⁺
And the total Cr content should be limited to within the range of 5 to 70 g / l. If the total chromium content is less than 5 g / l, the formation efficiency of the chromate film is poor. On the other hand, the total chromium content is 70 g / l
Even if it exceeds the above, not only is it uneconomical because the coating properties cannot be further improved, but there is also the problem that the working environment is contaminated by the chromate treatment liquid that has adhered to the steel strip and is taken out of the chromate treatment tank. Occurs. Cr ³⁺ has no special effect on the quality of the chromate film, but rather has the advantage of increasing the efficiency of film formation. However, when the Cr ³⁺ / total Cr amount exceeds 0.5 by weight, the stability of the chromate treatment solution is lowered. Therefore, Cr ³⁺ / total
The Cr content should be 0.5 or less by weight.

(2) Zn ²⁺ : As Zn ²⁺ , zinc sulfate is used. Such Zn ²⁺ can be added by dissolving it in the treatment liquid in the form of metal, oxide, carbonate, chloride or the like when adjusting the ion concentration in the chromate treatment liquid. Zn ²⁺ has a function of uniformly depositing a chromate film and forming an appearance with high whiteness. The amount of Zn ²⁺ should be limited to the range of 2 to 60 g / l. Zn ²⁺
If the amount of Zn ²⁺ is less than 2 g / l, the above-mentioned effect cannot be obtained, while if the amount of Zn ²⁺ exceeds 60 g / l, the corrosion resistance after coating is deteriorated. More preferable Zn ²⁺ content is 4 to 40 g / l
Within the range of.

(3) MoO ₄ ²⁻ : As MoO ₄ ²⁻ , an alkali metal salt of MoO ₄ ²⁻ , an alkali metal salt, an ammonium salt or the like is used. MoO ₄ ^2-A has an effect to improve corrosion resistance. The amount of MoO ₄ ^2- should be limited to the range of 1 to 40 g / l. If the amount of MoO ₄ ^2- is less than 1 g / l, the above-mentioned effects cannot be obtained, while if the amount of MoO ₄ ^2- exceeds 40 g / l, the stability of the chromate treatment liquid decreases and Precipitation of oxide sol easily occurs. MoO ₄ ^2- / Zn ²⁺ is the weight ratio
It should be limited to the range of 0.02 to 1. MoO ₄ ^2- / Zn ²⁺
However, if the weight ratio is less than 0.02, the corrosion resistance after coating is deteriorated. On the other hand, when the weight ratio of MoO ₄ ²⁻ / Zn ²⁺ exceeds 1, the whiteness of the chromate film is lowered.

(4) SO ₄ ²⁻ : Conventionally, when a small amount of SO ₄ ²⁻ is contained in the chromate treatment liquid, a non-uniform coating film is formed by the cathodic electrolysis treatment, and corrosion resistance and other properties are deteriorated. It is said that the coating film is likely to be blackened.
However, according to the study of the present inventors, SO ₄ ^2-
By containing a large amount in the range of up to 100 g / l, the stability of the chromate treatment liquid is dramatically improved without impairing quality characteristics such as corrosion resistance and paint adhesion, and it has a uniform appearance. It has been found that a chromate film can be formed. Therefore, in the present invention, SO ₄ ²⁻ is contained in the chromate treatment liquid within the range of 10 to 100 g / l. The amount of SO ₄ ^2- , as described above, is 10 to 10
It should be limited to the range of 0 g / l. The amount of SO ₄ ^2- is 10 g /
If it is less than l, the above-mentioned effect cannot be obtained. On the other hand, if the amount of SO ₄ ^2- exceeds 100 g / l, no further effect can be obtained and it is uneconomical. The more preferable content of SO ₄ ^2- is 20
~ 100 g / l.

(5) Al ₂ O ₃ , Sb ₂ O _5, SiO ₂ , SnO _2, TiO _2, and At least one oxide sol of ZrO ₂ : Al ₂ O
₃ , Sb ₂ O _5, SiO ₂ , SnO _2, TiO _2, and Most of the oxide sols of ZrO ₂ are colloidalized by adjusting the surface charges and counterions of hydroxides or oxides, and are stably dispersed in the treatment liquid. Such an oxide sol has a function of improving corrosion resistance and coating adhesion. The amount of oxide sol should be limited to the range of 5 to 100 g / l as solids. If the amount of oxide sol is less than 5 g / l, the above-mentioned effects cannot be obtained. On the other hand, if the amount of oxide sol exceeds 100 g / l, the oxide particles tend to peel off from the chromate film, resulting in coating adhesion. Sex deteriorates. The oxide sol having a smaller particle diameter tends to give excellent corrosion resistance to the chromate coating, and the oxide sol having a larger particle diameter tends to give excellent paint adhesion to the chromate coating. Therefore, it is effective to mix and use an oxide sol having a large particle diameter and an oxide sol having a small particle diameter. However, an oxide sol having a too large particle size deteriorates dispersion stability in the treatment liquid. Therefore, it is preferable to use an oxide sol having a particle diameter of 50 mmμ or less.

(6) Cl ⁻ , F ⁻ , BF ₄ ⁻ , SiF ₆ ²⁻ and NO
₃ ^- at least one ^{of: Cl -, F -, BF} 4 -, Si
F ₆ ²⁻ and NO ₃ ⁻ have the function of uniformly dissolving the surface of the plating film and forming a uniform chromate film. In particular, it effectively acts on a plating film having a thick oxide film formed by a hot dipping method or the like, or a plating film containing a large amount of alloying elements and having non-uniform active points. Therefore, in the present invention, if necessary, Cl ⁻ , F ⁻ , BF
₄ ^-, SiF ₆ ^2- and NO ₃ ^- at least one of,
It is contained in the chromate treatment liquid. Cl ^-, F ^-, B
The amount of at least one of F ₄ ⁻ , SiF ₆ ²⁻ and NO ₃ ⁻ should be limited to the range of 0.01 to 0.1 gram ion / l. If the amount of at least one of Cl ⁻ , F ⁻ , BF ₄ ⁻ , SiF ₆ ²⁻ and NO ₃ ⁻ is less than 0.01 gram ion / l, the above effect cannot be obtained. On the other hand, if it exceeds 0.1 gram ion / l, the plating surface is excessively dissolved, resulting in deterioration of corrosion resistance.

(7) Additives: In order to improve the stability of operation, the following additives may be added to the chromate treatment liquid. PH buffers such as borates, phosphates and phthalates to suppress pH fluctuations at the treated interface during electrolysis and to form stable chromate coatings, ammonium, alkali metal sulfates and A chelate compound of amines and carboxylic acids that fixes the metal ions in the chromate treatment solution and prevents the formation of sludge by fixing electrical conduction aids such as halides.

(8) Others: In the chromate treatment liquid,
Fe, Al, Mn, Co, Pb, Sn, Cu, and Ag ions, which are eluted from the zinc-plated steel sheet or electrode to be chromated, may inevitably be mixed in, but these are 0.1% in total. There is no problem if it is less than gram ion / l. In addition, as a cation, alkali metal ions such as Na and K contained in SO ₄ ² -supplied salts or additives such as electric conduction auxiliary agents and pH buffers, alkaline earth metal ions such as Mg and Ca, or, It may contain ammonium ions, but these do not adversely affect the effects of the present invention and are not particularly limited.

(9) pH value of chromate treatment liquid: The pH value of the chromate treatment liquid should be limited within the range of 1 to 5. When the pH value of the chromate treatment liquid is less than 1, the efficiency of depositing the chromate film decreases. Furthermore, at pH values below 1,
After the electrolytic treatment, the chromate treatment liquid adhering to the surface of the chromate coating partially redissolves the chromate coating, resulting in deterioration of corrosion resistance and paint adhesion, and the appearance of the chromate coating tends to be uneven. Become.
On the other hand, when the pH value of the chromate treatment liquid exceeds 5, the stability of the chromate treatment liquid is lowered and a precipitate is likely to be generated in the treatment liquid. The pH value can be adjusted by adding sulfuric acid, alkali metal hydroxide or the like to the chromate treatment liquid.

(10) Temperature of chromate treatment liquid: The temperature of the chromate treatment liquid should be limited to the range of 30 to 70 ° C. When the temperature of the chromate treatment liquid is lower than 30 ° C., undissolved substances are likely to be generated in the treatment liquid, and the electric conductivity is lowered. On the other hand, when the temperature of the chromate treatment liquid exceeds 70 ° C., the treatment liquid adhered to the surface of the chromate coating immediately dries after the electrolytic treatment, resulting in a problem that the appearance of the chromate coating remarkably deteriorates due to contact with a roll or the like. Occurs.

(11) Electricity of Cathodic Electrolysis The amount of chromate coating adhered can be controlled by changing the current density and the treatment time, that is, the amount of electricity during cathodic electrolysis for chromate treatment. The amount of electricity is 0.5
To 40 C / dm ² should be limited. Electricity is 0.
If it is less than 5 C / dm ² , the chromate film is not uniformly formed, and desired quality characteristics cannot be obtained. On the other hand, the amount of electricity is 40C /
When it exceeds dm ² , the chromate film is formed excessively thick, resulting in a decrease in whiteness and deterioration in appearance. The current density for cathodic electrolysis is preferably in the range of 0.2 to 40 A / dm ² . The current density is less than 0.2 A / dm ² or 40A / dm ² greater, problems arise which lead to a decrease in precipitation efficiency of the chromate film.

The zinc-based plated steel sheet to be chromated according to the present invention includes electroplating method, hot dipping method,
Various zinc-based plated steel sheets with a plated coating formed by vapor deposition plating, ion plating, etc., for example, Zn
Zinc alloy plated steel sheet such as plated steel sheet, Zn-Ni series, Zn-Fe series, Zn-Mn series, Zn-Al series or zinc composite plated steel sheet, or the above, Zn, Zn-Ni, Zn-Fe, Zn- It is possible to use a so-called dispersed galvanized steel sheet in which a metal oxide, a polymer, or an insoluble salt such as barium chromate is dispersed in a zinc-based plating film such as Mn or Zn-Al.

The chromate-treated zinc-based plated steel sheet on which the chromate film has been formed by the method of the present invention is used as an unpainted steel sheet or a steel sheet for coating base in the state of being washed with water and dried. However, if necessary, the steel sheet may be subjected to a commonly used chromate sealing treatment or a post-treatment with an organic composite treatment liquid. Next, the present invention will be described in more detail with reference to Examples and comparison with Comparative Examples.

[0031]

【Example】

Example 1 For an electrogalvanized steel sheet having a galvanized coating of 20 g / m ² formed on its surface by electroplating, the composition and pH within the scope of the present invention shown in Tables 1 and 2 are shown. A chromate film was formed on the galvanized film by using a chromate treatment solution having a value and a temperature and subjecting it to cathodic electrolysis under the following electrolysis conditions within the scope of the present invention. Then, this was washed with water and dried to prepare Samples Nos. 1 to 23 of the present invention shown in Tables 1 and 2. Current density: 10A / dm ² Processing time: 1.0 second Electricity: 10C / dm ²

[0032]

[Table 1]

[0033]

[Table 2]

For comparison, the above galvanized steel sheet was prepared by using a chromate treatment liquid whose composition, pH value and temperature, which are also shown in Tables 1 and 2, are outside the scope of the present invention. Cathodic electrolysis was performed under the same conditions as above to form a chromate film on the galvanized film, and comparative sample Nos. 1 to 14 shown in Tables 1 and 2 were prepared.

Example 2 An electrogalvanized steel sheet having a galvanized coating of 20 g / m ² formed on its surface by electroplating has the following composition within the scope of the present invention. Using a chromate treatment liquid, cathodic electrolysis treatment was performed under the electrolysis treatment conditions within the scope of the present invention shown in Table 3 to form a chromate coating film on the galvanized coating film. Next, this was washed with water and dried to prepare Samples Nos. 24 to 33 of the present invention shown in Table 3. Cr ⁶⁺ + Cr ³⁺ : 10g / l (Cr ³⁺ / total Cr: <0.05) Zn ²⁺ : 5g / l MoO ₄ ^2- : 5g / l SO ₄ ^2- : 30g / l SiO ₂ : solid content 15g / l

For comparison, the electrogalvanized steel sheet was subjected to cathodic electrolysis treatment using a chromate treatment solution having the same composition as above and under electrolysis conditions outside the scope of the present invention shown in Table 3 together. , A chromate film was formed on the galvanized film, and the comparative specimen No. 15 to
20 were prepared.

[0037]

[Table 3]

Example 3 Hot-dip zinc having a Zn-Al alloy plating film containing 55 wt.% Al and 1.6 wt.% Si in an amount of 75 g / m ² formed on its surface by the hot dipping method. For alloy-plated steel sheet, using a chromate treatment solution having the following component composition, pH value and temperature within the scope of the present invention, and subjecting it to cathodic electrolysis under the electrolysis conditions within the scope of the present invention, Zn A chrome coating was formed on the Al alloy plating coating. Then, this was washed with water and then dried to obtain a sample No. 3 of the present invention.
4,35 were prepared. For comparison, for the hot-dip zinc alloy-plated steel sheet, a chromate treatment liquid having a composition outside the scope of the present invention shown below was used.
o.21 was prepared.

Sample No. 34 of the present invention Chromate treatment liquid Total Cr: 20 g / l (Cr ³⁺ / Total Cr
^{<0.05) Zn 2+: 6g /} l MoO 4 2-: 3g / l SO 4 2-: 30g / l SiO 2: 10g / l SiF as solid content ₆ ² 0.03 click Bu Ramuion / l NO ₃ ^-: 0.01 Gram ion / l pH value: 3.2 Temperature: 45 ° C Cathodic electrolysis conditions Current density: 8A / dm ² Treatment time: 1 second Electricity: 8C / dm ²

Sample No. 35 of the present invention Chromate treatment liquid Total Cr: 20 g / l (Cr ³⁺ / Total Cr
^{<0.05) Zn 2+: 6g /} l MoO 4 2-: 3g / l SO 4 2-: 20g / l SiO 2: 10g as solids / l BF ₄ ^-: 0.1 click Bu Ramuion / l pH value: 3.2 Temperature: 45 ℃ Cathodic electrolysis conditions Current density: 8A / dm ² Treatment time: 1 second Electricity: 8C / dm ²

Specimen for comparison No. 21 Chromate treatment solution Total Cr: 20g / l (Cr ³⁺ / Total Cr
^{<0.05) Zn 2+: 6g /} l MoO 4 2-: 3g / l SO 4 2-: 20g / l SiO 2: 10g as solids / l BF ₄ ^-: 0.12 click Bu Ramuion / l pH value: 3.2 Temperature: 45 ℃ Cathodic electrolysis conditions Current density: 8A / dm ² Treatment time: 1 second Electricity: 8C / dm ²

Example 4 For a hot dip galvanized steel sheet on which a Zn-Fe alloy plating film containing 11 wt.% Fe in an amount of 45 g / m ² was formed on the surface by the hot dipping method, On the Zn-Fe alloy plating film, a chromate treatment solution having a component composition, a pH value and a temperature within the scope of the present invention shown in is used, and a cathodic electrolysis treatment is performed under electrolytic conditions within the scope of the present invention. A chromate film was formed on the film. Next, this was washed with water and then dried to prepare Sample No. 36 of the present invention. For comparison, a comparative sample No. 22 was prepared by using a chromate treatment liquid having the following composition outside the scope of the present invention for the hot-dip galvanized steel sheet.

Sample No. 36 of the present invention Chromate treatment liquid Total Cr: 10 g / l (Cr ³⁺ / Total Cr
^{<0.05) Zn 2+: 5g /} l MoO 4 2-: 5g / l SO 4 2-: 20g / l SiO 2: 15g as solids / l F ^-: 0.01 click Bu Ramuion / l pH value: 3.3 Temperature: 40 ℃ Cathodic electrolysis conditions Current density: 8 A / dm ² Treatment time: 0.5 seconds Electricity: 4 C / dm ²

Specimen for comparison No. 22 Chromate treatment liquid Total Cr: 10g / l (Cr ³⁺ / Total Cr
<0.05) Zn ²⁺ : 1g / l SO ₄ ^2- : 4g / l SiO ₂ : 15g / l pH value as solid content: 3.3 Temperature: 40 ° C Cathodic electrolysis condition Current density: 8A / dm ² Treatment time: 0.5 seconds Electricity: 4 C / dm ²

Example 5 An electrogalvanized steel sheet having a Zn-Ni alloy plated coating containing 12 wt.% Ni in an amount of 30 g / m ² formed on the surface by electroplating was prepared as follows. On the Zn-Ni alloy plating film, a chromate treatment solution having a component composition, a pH value and a temperature within the range of the present invention shown in is used, and a cathodic electrolysis treatment is performed under electrolytic conditions within the range of the present invention. A chromate film was formed on the film. Next, this was washed with water and then dried to prepare Samples Nos. 37 and 38 of the present invention.
For comparison, Comparative Specimen Nos. 23 and 24 were prepared by using a chromate treatment solution having the following composition outside the scope of the present invention for the electrogalvanized steel sheet.

Sample No. 37 of the present invention Chromate treatment liquid Total Cr: 10 g / l (Cr ³⁺ / Total Cr
^{<0.05) Zn 2+: 5g /} l MoO 4 2-: 2g / l SO 4 2-: 30g / l SiO 2: 10g / l Cl as solids ^- 0.01 click Bu Ramuion / l pH value: 3.5 Temperature: 40 ° C Cathodic electrolysis conditions Current density: 10 A / dm ² Treatment time: 0.5 seconds Electricity: 5 C / dm ²

Sample No. 38 of the present invention Chromate treatment liquid Total Cr: 15 g / l (Cr ³⁺ / Total Cr
0.12) Zn ²⁺ : 5g / l MoO ₄ ^2- : 2g / l SO ₄ ^2- : 20g / l SiO ₂ : 15g / l as solid content pH value: 3.3 Temperature: 45 ° C Cathodic electrolysis condition Current density: 10 A / dm ² Processing time: 0.5 seconds Electricity: 5 C / dm ²

Comparative specimen No. 23 Chromate treatment liquid Total Cr: 15g / l (Cr ³⁺ / Total Cr
0.12) Zn ²⁺ : 1g / l MoO ₄ ^2- : 1g / l SO ₄ ^2- : 2g / l SiO ₂ : 10g / l as solid content pH value: 3.3 Temperature: 45 ° C Cathodic electrolysis condition Current density: 10A / dm ² Processing time: 0.5 seconds Electricity: 5 C / dm ²

Specimen for comparison No. 24 Chromate treatment solution Total Cr: 15g / l (Cr ³⁺ / Total Cr
^{0.12) Zn 2+: 5g / l} MoO 4 2-: 2g / l SO 4 2-: 20g / l Cl -: 0.01 click Bu Ramuion / l pH value: 3.5 Temperature: 45 ° C. cathodic electrolysis conditions Current density: 10 A / dm ² Processing time: 0.5 seconds Electricity: 5 C / dm ²

The test described below was conducted on each of the specimens thus prepared. (1) Corrosion resistance test The test piece was subjected to a salt spray test specified in JIS-Z-2371, the white rust generation area after 72 hours was measured, and the result was shown in%. (2) Paint Adhesion Test A commercially available alkyd melamine-based paint was applied to the surface of the test piece by about 30 μm to form a coating film. Next, 100 checker-like notches were formed in the coating film immediately after coating at intervals of 1 mm, and then extruded by 7 mm with an Erichsen tester, and then an adhesive tape was applied and peeled off on the surface of the extruded portion. As a result, the peeling area is 50
The number of coating films less than% was evaluated as the coating residual rate (%). (3) Appearance The appearance of the test piece was visually evaluated.

The test results of each of the above-mentioned specimens are shown in Tables 4-6.
Shown in.

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

As is clear from Tables 4 to 6, Comparative Sample No. 1 in which the total amount of Cr in the chromate treatment liquid is out of the range of the present invention, is small, and as a result, the formation efficiency of the chromate film is lowered, resulting in a color tone Became uneven, and corrosion resistance and paint adhesion became unstable. Comparative sample No. 2 having a large Cr ³⁺ / total Cr ratio exceeding the range of the present invention discolored to gray as a result of increased sludge in the chromate treatment liquid, and
Corrosion resistance and paint adhesion became unstable. Comparative sample No. containing no MoO ₄ ^2- in the chromate treatment solution
Comparative specimen No. 4 containing no 3 and Zn ²⁺
In both cases, the corrosion resistance and paint adhesion were poor, and the comparative sample No. 4 turned gray.

The amount of MoO ₄ ²⁻ in the chromate treatment solution was outside the range of the present invention and was small, Comparative Specimen No. 5 and Zn ²⁺
Comparative sample No. 6 whose amount is out of the range of the present invention
Had poor corrosion resistance and paint adhesion, and Comparative Sample No. 6 turned gray. Comparative specimens No. 7, 8 having a large MoO ₄ ^2- / Zn ²⁺ ratio exceeding the range of the present invention
Had poor corrosion resistance and paint adhesion and turned gray or brown. Comparative sample No. 9 having a small MoO ₄ ²⁻ / Zn ²⁺ ratio outside the range of the present invention had poor corrosion resistance, and blisters occurred in the corrosion resistance test after coating. Comparative sample No. 10 in which the amount of SO ₄ ^2- in the chromate treatment liquid was outside the range of the present invention was small, the treatment liquid deteriorated with time, and the appearance, corrosion resistance and paint adhesion were unstable. ..

Comparative sample No. 11 in which the amount of the oxide sol in the chromate treatment solution was outside the range of the present invention and was small had poor corrosion resistance and paint adhesion. Comparative specimens No. 12 and 13 in which the amount of the oxide sol is more than the range of the present invention,
Due to the peeling of the oxide from the chromate film, the corrosion resistance and paint adhesion were unstable, and the appearance color tone was also uneven. Comparative sample No. 14 in which the oxide sol was not contained in the chromate treatment liquid had poor corrosion resistance and paint adhesion.

Comparative sample No. 15 in which the pH value of the chromate treatment solution was out of the range of the present invention, the dissolution of the plating film occurred and the efficiency of formation of the chromate film decreased, resulting in discoloration to gray and corrosion resistance. And the paint adhesion was unstable. Comparative sample No. 1 having a large pH value exceeding the range of the present invention
In No. 6, sludge increased with time in the treatment liquid, the formation efficiency of the chromate film decreased, and corrosion resistance and paint adhesion were unstable.

In Comparative Sample No. 17 in which the temperature of the chromate treatment liquid was outside the range of the present invention and was low, the formation efficiency of the chromate film and the electric conductivity of the chromate treatment liquid were lowered. Comparative sample No. 18 in which the temperature of the chromate treatment liquid was higher than the range of the present invention, the drying occurred after the chromate treatment, resulting in discoloration to brown, and the corrosion resistance and paint adhesion were unstable. .. The amount of electricity and the current density during cathodic electrolysis treatment are out of the range of the present invention and are small.
In No. 19, the formation efficiency of the chromate film was lowered, the corrosion resistance and paint adhesion were poor, and the color changed to pale yellow. The amount of electricity at the time of cathodic electrolysis treatment is large, exceeding the scope of the present invention.
Corrosion resistance and paint adhesion were poor, and it turned gray.

Comparative Specimen No. 21 having a large amount of BF ₄ ⁻ in the chromate treatment solution exceeding the range of the present invention discolored to gray as a result of dissolution of the plating film, and had corrosion resistance and paint adhesion. Was unstable. MoO _{4 in} chromate solution
² is not contained, Zn ²⁺ and SO ₄ ^2- content is specimen No.22 comparative less out of the scope of the present invention, discolored to gray, and, corrosion resistance and paint adhesion properties also poor It was Comparative sample No. 23, in which the amount of Zn ^{2+ and the} amount of SO ₄ ²⁻ in the chromate treatment solution were out of the range of the present invention, was discolored to gray, and the corrosion resistance and paint adhesion were poor. Comparative sample N containing no oxide sol in the chromate treatment liquid
In o.24, the corrosion resistance and paint adhesion were poor.

On the other hand, the sample Nos. 1 to 38 of the present invention are
It was excellent in appearance, corrosion resistance and paint adhesion.

[0062]

As described above, according to the present invention,
Excellent corrosion resistance and paint adhesion on zinc-based plated steel sheets,
In addition, an industrially useful effect capable of imparting a uniform color tone appearance with high whiteness is brought about.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Masaaki Yamashita 1-2, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Toyofumi Watanabe 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Inside the steel pipe company

Claims (2)

[Claims] 1. A zinc or zinc alloy-plated steel sheet having a zinc-plated coating or a zinc alloy-plated coating formed on at least one surface thereof is subjected to cathodic electrolysis treatment in a chromate treatment solution to obtain the zinc or A chromate treatment method for a zinc-based plated steel sheet, which comprises forming a chromate coat on a zinc alloy plated coat, wherein the chromate treatment liquid is Cr ⁶⁺ + Cr ³⁺ (total chromium amount): 5 to 70 g / l, Zn ²⁺ : 2 to 60 g / l, MoO ₄ ^2- : 1 to 40 g / l, SO ₄ ^2- : 10 to 100 g / l, and Al ₂ O ₃ , Sb ₂ O _5, SiO ₂ , SnO _2, TiO _2, and ZrO ₂ containing at least one oxide sol: 5 to 100 g / l as a solid content, and Cr ³⁺ / total Cr amount weight ratio: 0.5 or less, MoO ₄ ^2- / Zn ²⁺ weight Ratio: 0.02-1, pH value is 1-5, treatment liquid with temperature of 30-70 ° C is used, and the zinc or zinc alloy-plated steel sheet is treated with the electric quantity of 0.5-40C / dm ^2. A method for chromate treatment of zinc-based plated steel sheet, characterized in that cathodic electrolysis treatment is performed. 2. The chromate treatment liquid is Cl ⁻ , F ⁻ ,
The method according to claim 1, further comprising 0.01 to 0.1 gram ion / l of at least one of BF ₄ ⁻ , SiF ₆ ²⁻ and NO ₃ ⁻ .