JPH0753911B2 - Chromate treatment method for galvanized steel sheet - Google Patents

Abstract

Zinciferous surfaces, particularly those of galvanized steel, may be effectively protected against corrosion, either as treated or after painting, while remaining easy to weld, by contact with an aqueous solution containing chromium in both +6 and +3 valence states, phosphate, a tertiary alcohol or similar oxidation stable, water soluble organic compound as a wetting agent, and a silane coupling agent, followed by drying with a film from the solution still in place on the surface. A thin coating containing 10 - 200 mg/m<2> of chromium is formed on the surface to provide a protective layer that is also an excellent base for paint.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to an electrogalvanized steel sheet, an electrogalvanized steel sheet, and an alloyed galvanized steel sheet (these steel sheets are generically referred to as a zinc-based plated steel sheet). ), A chromate treatment method capable of forming a chromate film excellent in uniform coating property and excellent in corrosion resistance, alkali resistance, welding resistance and coatability (coating adhesion and corrosion resistance after coating).

[Prior Art] Chromate treatment solutions have long been composed of chromic acid or dichromic acid aqueous solutions, but various methods have been proposed for forming a poorly soluble coating when treated with acid or alkali after forming the chromate coating. Has been done. Hereinafter, these conventional techniques and the advantages and disadvantages thereof will be described.

As a method for forming a poorly soluble chromate film on the surface of a zinc-based plated steel sheet, the invention of JP-A-50-158535 discloses a chromate solution of chromic anhydride-phosphoric acid-water-soluble or water-dispersible polymer compound. As for Cr ⁶⁺ in the treatment solution, use one that has been reduced to Cr ³⁺ by 70% or more with a reducing agent such as ethylene glycol. However, since the chromate film formed by the practice of this invention contains a polymer,
It is excellent in poor solubility, corrosion resistance, and paintability, but has weaknesses such as poor weldability.

Next, the chromate solution disclosed in Japanese Examined Patent Publication No. Sho 61-58522 specifies the components of chromic acid-chromic acid reduction product-silica sol system. However, when the surface-treated steel sheet on which the chromate film is formed by the method of the present invention is processed and coated, mainly hexavalent chromium in the chromate film is easily eluted by the alkali cleaning before coating, that is, the alkali resistance is poor. There are problems that the corrosion resistance of the coating is lowered and that the coating contains silica, so that welding, for example, spot weldability is poor.

Next, JP-A-58-22383 and JP-A-62-83478 disclose the use of a silane coupling agent as a hexavalent chromium reducing agent in a chromate treatment liquid. Although the films formed by the methods of these inventions have excellent coating film adhesion, the chromate film formed by the former method does not contain silica or organic polymer and thus has poor alkali resistance. Since the latter invention contains colloidal silica, it has a problem of insufficient spot weldability.
When the performance of each component of the prior art in the above chromate treatment method is examined, the organic polymer and silica sol tend to enhance the corrosion resistance but deteriorate the weldability. Further, since the silane coupling agent tends to reduce Cr ⁶⁺ , it tends to make the corrosion resistance of the chromate film unstable.

[Problems to be Solved by the Invention] Accordingly, the present invention solves various problems of the prior art in the chromate treatment method of zinc-based plated steel sheet, and has excellent uniform coating property, corrosion resistance, alkali resistance, weld resistance, and workability. Another object of the present invention is to provide a method for forming a chromate film having excellent coatability.

[Means for Solving the Problems] In the present invention, in order to solve the problems of the prior art in the chromate treatment method for zinc-based plated steel sheet,
First, the following measures were taken for the aqueous chromate solution to be applied. That is, a. In order to effectively exert the action of the silane coupling agent, the content of hexavalent chromium and the content of trivalent chromium are specified, and the chromium ratio (Cr ³⁺ / Cr ⁶⁺ weight ratio) is also specified.

b. an organic solvent selected from tertiary alcohols and acetonitrile C ₄ -C ₈ be specific amount in the chromate solution in order to improve the uniform application of the chromate bath. This solvent can remain relatively stable in the chromate solution at temperatures up to 35 ° C., so that it maintains the above-mentioned effects and does not adversely affect the quality of the chromate film.

c. Containing a specific amount of phosphate ion in the chromate solution to contribute to the improvement of alkali resistance and corrosion resistance of the coating. Therefore, the value of the ratio of PO ₄ ^3- / Cr ⁶⁺ + Cr ³⁺ was also specified.

d. Before applying the chromate solution, a silane coupling agent is added to the chromate solution at a specific molar ratio with respect to the Cr ⁶⁺ concentration in the solution, mixed, and then the mixture solution is applied to the surface of the zinc-based plated steel sheet. What I did. By this means, Cr ⁶⁺ in the chromate solution applied to the plating surface is subjected to the reducing action of the silane coupling agent and the solvent in the process of drying the chromate coating solution, and the network bond proceeds due to the binding action of the silanol group. Be able to form a chromate film with excellent corrosion resistance and alkali resistance.

e. The coating should not contain silica or organic polymer, which increases the interlayer insulation resistance, in the aqueous chromate solution so that the formed coating does not impair the weldability.

The inventor of the present invention has completed the present invention by making an ingenuity as described above for an aqueous chromate solution. That is, the present invention includes a hexavalent chromium 3.0~50g /, 3 chromium 2.0～40G / a, phosphoric acid ions (PO ₄ ^3-) 1.0~100g /
And 3 to at least one organic solvent selected from tertiary alcohols having 4 to 8 carbon atoms and acetonitrile.
Consists of 50g / and chromium ratio (Cr ³⁺ / Cr ⁶⁺ weight ratio)
Is 0.25 to 1.5, and the weight ratio of phosphate ions to total chromium (PO ₄ ^3- / Cr ⁶⁺ + Cr ³⁺ ) is 0.1 to 1.2. A silane coupling agent is added to the aqueous chromate solution in the chromate solution. After adding and mixing in a molar ratio of 0.05 to 0.30 with respect to the hexavalent chromium concentration, the chromate solution is applied to the surface of the zinc-plated steel sheet and then dried to give a chromium adhesion amount of 10 to 200 mg / m ^{2 on the surface.} The present invention relates to a chromate treatment method for a zinc-based plated steel sheet, which comprises forming the chromate film.

First, the constitution of the aqueous chromate solution used in the method of the present invention will be described. This chromate solution uses water as a solvent and contains 3.0 to 50 g / Cr ⁶⁺ and 2.0 to 40 g / Cr ³⁺ as basic components. Cr ⁶⁺ less than 3.0 g / and 2.0 g /
If the amount of Cr ³⁺ is less than, it will be difficult to form a chromate film that exhibits satisfactory corrosion resistance. Conversely, if the amount of Cr ^{6+ is} greater than 50 g / and the amount of Cr ^{3+ is} greater than 40 g /, the viscosity of the chromate solution will be high and chromate The stability of the liquid becomes poor and it becomes difficult to control the chromium deposition amount. Also important in the amount of chromium is Cr ⁶⁺ and Cr
The content ratio of ³⁺ and the chromium ratio (Cr ³⁺ / Cr ⁶⁺ ) is 0.25.
Must be in the range of ~ 1.5. The chrome ratio is
If it is less than 0.25, the Cr ⁶⁺ concentration in the chromate solution becomes so high that when the silane coupling agent is added to the solution, the reduction reaction of Cr ^{6+ in} the chromate solution by the silane coupling agent easily occurs. As a result, the chromate solution is heated, and as a result, Cr ^{6+ is} generated by the solvent in the chromate solution.
The progress of the reduction reaction and the volatilization of the solvent are likely to occur, leading to deterioration of the quality of the chromate solution. On the other hand, when the chromium ratio exceeds 1.5, the chromate solution is likely to gel and the corrosion resistance of the formed chromate film is deteriorated. The chromium ratio is controlled by adding a reducing agent such as ethanol, methanol, oxalic acid, starch or sucrose, if necessary.

As another component, the chromate solution of the present invention is 1.0 to 100 g /
Containing PO ₄ ^3- . PO ₄ ^3- is preferably added in the form of orthophosphoric acid (H ₃ PO _4). PO ₄ ^3- amount decreases corrosion resistance, alkali resistance of the chromate film is less than 1.0 g /, because 100 g / super In the contrary to progress rapidly reduced with a silane coupling agent of Cr ⁶⁺ in the chromate bath chromate bath Reduce the quality of. Of the PO ₄ ³⁻ amount, what is particularly important is the ratio to the amount of total chromium (Cr ⁶⁺ + Cr ³⁺ ) in the chromate solution. The ratio of PO ₄ ^3- / total Cr is preferably in the range of 0.1 to 1.2. If this ratio is less than 0.1, the alkali resistance and corrosion resistance of the chromate film tend to decrease, and conversely 1.2
If it exceeds the limit, the reduction action of Cr ^{6+ in} the chromate solution by the silane coupling agent is extremely likely to proceed, and the Cr ⁶⁺ in the chromate solution is reduced to a large amount or almost Cr ³⁺ before coating. The quality of the liquid deteriorates, and it becomes difficult to form a film suitable for the purpose of the present invention.

To improve the coating property of the chromate solution as another component, at least one solvent selected from a C _{4 to} C ₈ tertiary alcohol and acetonitrile as a wettability improver, and in some cases, a plurality of the above solvents may be used. The mixture is added to the chromate solution at 3 to 50 g / formula. All of the above solvents are approximately
It has a relatively high stability against Cr ⁶⁺ in the chromate solution within 35 ° C, and it has the effect of improving the wettability of the chromate solution on the plating surface without any adverse effect on the quality of the chromate film. . Therefore, the amount corresponding to the improvement of the wettability contributes to the improvement of the uniform coating property of the chromate solution during the high speed operation. If the amount of solvent is less than 3 g /, it will be difficult to confirm the effect, and if it exceeds 50 g / g, the effect cannot be expected to be improved by adding more than that, and it is not preferable economically and in working environment. Generally, the above-mentioned solvent is added in proportion to the total chromium concentration in the chromate solution or the coating speed. The tertiary alcohol of C ₄ -C ₈ specifically t- butanol, and the like t- amyl alcohol.

If necessary, 0.2-10 g / zinc ion can be added to the aqueous chromate solution to further improve the alkali resistance. If the amount of zinc ion is less than 0.2 g /, it is difficult to confirm the improvement effect, and conversely, if it exceeds 10 g /, it becomes a treatment liquid.
Cr ^{3+ tends} to precipitate, which is not preferable. Zn ions are added to the chromate solution in the form of zinc white, zinc carbonate, zinc phosphate or zinc hydroxide. Furthermore, if necessary, complex fluoride can be added to the chromate solution alone or together with zinc. The addition amount is preferably in the range of 0.2 to 8 g / converted to F. A preferred specific example of the complex fluoride is fluorozirconic acid, fluorotitanic acid, fluorosilicic acid or fluoroboric acid. By adding these amounts to the chromate solution, etching of the zinc-based plating surface by the chromate solution proceeds. As a result, complexed fluorides complex the eluted metal ions, or the zinc ions added to the chromate solution with the metal ions, and the metal or zinc complex compound becomes a constituent of the chromate film, resulting in corrosion resistance and coating properties of the film. It contributes to improvement. F
If the converted amount is less than 0.2 g /, it is difficult to confirm the effect of adding the complex fluoride, while if it exceeds 8 g, the corrosion resistance of the chromate film is reduced.

It is desirable that the aqueous chromate solution as described above is maintained at a temperature of about 35 ° C. or lower, preferably about 25 ° C. or lower after blending with the silane coupling agent, and is used as soon as possible after the adjustment. Before coating, apply the silane coupling agent to the chromate solution in a molar ratio (silane coupling agent / Cr ⁶⁺ molar ratio) to the molar concentration of Cr ^{6+ in} the chromate solution.
Add within the range of 0.05 to 0.30 and mix. The silane coupling agent is not particularly specified, but preferable ones include those represented by the following [I] and [II].

[I] YRSiX [II] YSiX R ... Alkyl group X ... Methoxy group or ethoxy group Y ... Vinyl group, mercapto group, glycidoxy group or methacryloxy group Specifically, as a silane coupling agent, vinyl is used. Trimethoxysilane, vinyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxy Examples thereof include propylmethyldimethoxysilane and the like. still,
As described above, the present invention does not specify the type of the silane coupling agent, but the one represented by the above general formula has a relatively high solubility in an aqueous chromate solution and is relatively large in improving the corrosion resistance of the chromate film. It is particularly preferable because it contributes.

If the addition molar ratio of the silane coupling agent is less than 0.05, it becomes difficult to confirm the effect of improving the alkali resistance of the chromate film,
On the other hand, if it exceeds 0.3, the stability of the chromate solution tends to gradually decrease, that is, Cr ³⁺ increases in the chromate solution and gelation tends to occur easily. More preferably, they are mixed in a molar ratio within the range of 0.1 to 0.2.

The chromate solution mixed with the silane coupling agent as described above is applied to the surface of the zinc-based plated steel sheet by, for example, a roll coater, and then dried. In the present invention, the drying conditions are not specified, but preferably the plate temperature of the plated steel sheet is 60 to 150 ° C. and dried for 5 to 10 seconds to obtain a chromium adhesion amount of 1
A chromate film of 0 to 200 mg / m ² is formed. However,
After the silane coupling agent is added to the aqueous chromate solution, the temperature of the mixed solution is maintained at about 35 ° C, preferably about 25 ° C or lower. When the chromium concentration is relatively low, it is sufficiently stable for about 1 month. On the contrary, when the chromium concentration is high, it is necessary to apply within one week after the above formulation.

The amount of chromium deposited is not particularly limited, but if it is less than 10 mg / m ² , the corrosion resistance of the coating and the corrosion resistance after coating are insufficient, and 200 mg / m ²
If it exceeds m ^2, it will be difficult to control the amount of chromium deposited on the chromate film, and the effect of improving corrosion resistance will saturate, and it will be difficult to expect any further effect, and part of the chromate film will be easily removed by external force. It causes a decrease in sex. The pH of the aqueous chromate solution specified in the present invention is not specified, but is preferably 1.0 to 3.0.

[Function] In the process of blending a silane coupling agent in an aqueous chromate solution and applying it to the surface of a zinc-plated steel sheet and drying, Cr ⁶⁺ , Cr ³⁺ and Cr ³⁺ in the aqueous chromate solution are mainly caused by thermal energy during drying. When each component of PO ₄ ^3- reacts with the plating surface, the following colorless coatings (a) and (b), a green coating (c) and (d) and (e) The represented golden color film is a forming component of the chromate film.

(A) Zn (OH) ₂ (b) Cr (OH) ₂ (c) CrPO ₄・ 4H ₂ O (d) ZnO ・ 3Zn (OH) ₂・ CrO ₃ , 3Zn (OH) ₂・ CrO ₃ , 2Zn ( OH) ₂ · CrO _{3 and} other zinc chromate compounds (e) Cr (OH) ₃ · Cr (OH) · CrO ₄ (chromic chromate) Next, the silane coupling agent will be described.

Here, taking a silane coupling agent having a trimethoxy group as an example and showing it as YRSi (OCH ₃ ) ₃ , this causes hydrolysis as shown in the following formula (1), that is, (1) YRSi (OCH 3 ₃ ) ₃ + 3H ₂ O → (f) YRSi (OH) ₃ + 3CH ₃ OH The heating energy after application of the chromate solution causes, for example, the chromic chromate of (d) above to cause a condensation reaction as illustrated in FIG. Together, the chromic chromate crosslinks with the hydrolyzed component (f) of the silane coupling agent or methanol reduces hexavalent chromium in the chromic chromate, as further illustrated in FIG. Thus, it is presumed that complicated cross-linking is carried out with the hydrolysis product of the chromic chromate and the silane coupling agent to form a network-like polymer structure pair.
Therefore, the above-mentioned (a), (b), (c), within the structure of the network polymer chromium compound shown in FIG. 1 and FIG.
It is presumed that each component of (d) exists in a blocked state or in a state of being bound to the high molecular weight chromium compound.

The chromate film having a network-like molecular structure thus formed has a strong alkali resistance due to the addition of a silanol group binding effect, that is, a film in which chromium in the film is difficult to elute by alkali cleaning. Moreover, it is presumed that the network-like molecular structure also contributes to the improvement of corrosion resistance and coatability.

Furthermore, since the present coating does not contain silica and organic polymer compounds, the interlayer insulation resistance value is relatively low, and therefore the welding resistance is also excellent.

Hereinafter, examples will be given in order to more specifically explain the effects of the present invention, and will be shown together with comparative examples.

[Examples] (1) Method for preparing chromate coating solution Chromate coating solution No. A shown in Table 1 was prepared by first dissolving 200 g of chromic anhydride in 500 g of water and adding phosphoric acid (75
% Aqueous solution) and methanol (18 g) at 80-90 ° C
After heating for a period of time to reduce the Cr ³⁺ / Cr ⁶⁺ weight ratio to 1.0 and cooling, 26 g of t-butanol and water were added to bring the total amount to 1 kg. Hereinafter, this solution is referred to as an aqueous chromate solution.

Next, this water-based chromate solution was added to the total chromium content (Cr ⁶⁺ + C
r ³⁺ ) is 40 g / and the amount of t-butanol is 1
It was diluted with water to 0 g /, and a silane coupling agent (manufactured by Toshiba Silicone, γ-glycidoxypropyltrimethoxysilane) was added and stirred at 9 g to prepare a chromate coating solution No. A.

Below, chromate coating liquids No.B to K are chromate coating liquids N
It was prepared to have the composition shown in Table 1 by the same procedure as oA.

(2) Chromate treatment method The chromate coating solution thus prepared was applied to the surface of an electrogalvanized steel sheet and an electrogalvanized nickel alloy-plated steel sheet by the process shown below, and dried to obtain one obtained in Table 2. It was shown to.

Steel plate to be treated (* 1) → Alkali degreasing (* 2) → Washing → Roll squeezing → Drying (air drying) → Chromate application → Roll squeezing → Drying (* 3) → Evaluation test (* 1) The steel sheet to be treated is double-sided Galvanized steel sheet (galvanized amount 20g / m ² / 20g / m ² ) and double-sided electric zinc nickel alloy plated steel sheet (plated amount 20g / m ² / 20g / m ² , nickel 11w
(T% content), and the size used was an oiling material with a size of 200 × 300 mm and a plate thickness of 0.8 mm.

(* 2) Alkaline degreasing is a 2% aqueous solution of weak alkaline degreasing agent (Nippon Parkerizing Palclean 342).
Spraying was performed for 30 seconds at a temperature of 60 ° C.

(* 3) For drying, the plate temperature was 100 ° C. and the drying time was 7 seconds.

(3) Painted plate manufacturing method Chromate treated steel plate is as it is, or (4) below
-After alkali cleaning described in (a), apply baking type melamine alkyd paint (Dainippon Paint Co., Ltd. Dericon 700 white), bake and dry at 140 ° C for 20 minutes, and paint plate (coating thickness 25μ
m) was prepared.

(4) Performance evaluation test (a) Alkali resistance test Chromate-treated steel sheets were washed with alkali under the following conditions, and the amount of chromium deposited before and after that was measured by fluorescent X-ray (mg / m ² ).
The alkali resistance was expressed by the following formula. That is, the smaller the value of% is, the more excellent the alkali resistance is, and the value of 0 is that the alkali is not affected in this test.

Alkali cleaning is an alkaline degreasing agent whose main component is sodium silicate (Nippon Parkerizing PALCLEAN N364S)
Was spray-treated for 2 minutes at a temperature of 60 ° C.

(B) Corrosion resistance Electrogalvanized steel sheet Test piece before and after alkaline cleaning (size 70 x 150 mm)
The salt spray test prescribed in JIS-Z-2371 was carried out for 150 hours, and the corrosion resistance was evaluated from the state of white rust generation on the entire area of the test piece.

◎: White rust occurrence area ratio 0% ○: Same as above 10% △: Same as above 10% or more and less than 30% ×: Same as above 30% or more Electrogalvanized nickel alloy plated steel sheet A test piece before and after alkaline cleaning, salt spray for 4 hours,
A 50-cycle test is conducted by the compound corrosion test method, which consists of dry (60 ° C) 2 hours and wet (50 ° C, RH 95% or more) 2 hours as one cycle, and corrosion resistance is evaluated from the state of red rust on the entire area of the test piece. did.

◎: Red rust occurrence area ratio 0% ○: Same as above 10% △: Same as above 10% or more and less than 30% ×: Same as above 30% or more (c) Corrosion resistance of coated plate A scratch that reaches the base metal in the coating film is inserted with a cutter, and a salt water spray test is performed. The electrogalvanized steel sheet was carried out for 200 hours, and the electrogalvanized nickel alloy plated steel sheet was carried out for 300 hours. After execution, peel off the adhesive tape (cellophane tape) and judge the maximum peeling width on one side from the scratch (unit: m
m).

(D) Adhesion of coating film Gobang eye test 1m for a test piece coated without alkali cleaning
A square of m square was cut with a cutter so as to reach the base metal, an adhesive tape (cellophane tape) was attached to one surface of the test piece and rapidly peeled off, and the degree of peeling of the coating film was observed.

Erichsen Extrusion Test 6 mm was extruded by an Erichsen extruder with respect to a test piece coated without alkali cleaning, and a cellophane tape was adhered to rapidly exfoliate, and the degree of exfoliation of the coating film was observed.

The coating film adhesion of the above items was evaluated according to the following four stages according to the degree of peeling of the coating film.

◎: Coating peeling 0% ○: Same as above 10% △: Same as above 10% or more and less than 30% ×: Same as above 30% or more (e) Welding resistance For electric zinc nickel alloy plated steel sheet, spot welding is continuously performed under the following conditions. If it is carried out, the welding terminal gradually deteriorates and the weldability deteriorates. Therefore, the weldability can be judged by the deterioration degree. That is, for each 100 hit points, another test piece of 30 × 100 mm is welded, and the tensile shear strength of the test piece is 400 kg.
I recorded the number of hits until I could maintain it.

Welding surface: Coated surface-Non-coated surface Pressure: 200kg Current: 8.5kA Energization time: 10 cycles Electrode: R40 (radius type), material-Chromium-copper (f) Chromate coating solution uniform coating test A water-soluble organic solvent was added to the indicated chromate coating liquid No. G to evaluate the uniform coatability of the chromate film in roll coater coating. In addition, Table 3 shows the surface tension of the chromate coating solution.

The uniform coatability of the chromate film is shown in the following three stages.

○: Uniform △: Slight repelling ×: Large repelling In the chromate film containing the silane coupling agent, the organic functional group of the silane coupling agent, that is, the vinyl group, the mercapto group, the glycidoxy group or the methacryloxy group reacts with the functional group of the overcoat resin to improve coating adhesion.

Therefore, Examples No. 1 to 12 within the scope of the present invention are excellent in coating adhesion and Comparative Example N containing no silane coupling agent.
o.3, 5, 8 and Comparative Examples Nos. 1, 4, 6 having addition amounts or contents outside the claimed range have poor coating adhesion.

[Advantages of the Invention] As described above, according to the present invention, the surface of a zinc-based plated steel sheet is excellent in uniform coating property, and also has alkali resistance, corrosion resistance, coatability (coating adhesion and corrosion resistance after coating), and corrosion resistance. It is possible to form a chromate film having excellent weldability.

[Brief description of drawings]

FIG. 1 is a diagram showing a condensation reaction of chromic chromate, and FIG. 2 is a diagram showing a crosslinking reaction of chromic chromate.

Claims (8)

[Claims] 1. Hexavalent chromium 3.0 to 50 g / and trivalent chromium 2.0
~ 40g /, phosphate ion (PO ₄ ^3- ) 1.0-100g /,
3 to 50 g / at least one organic solvent selected from tertiary alcohols having 4 to 8 carbon atoms and acetonitrile
And has a chromium ratio (Cr ³⁺ / Cr ⁶⁺ weight ratio) of 0.25 to
A silane coupling agent was added to an aqueous chromate solution having a weight ratio of phosphate ions to total chromium (PO ₄ ^3- / Cr ⁶ + Cr ³⁺ ) of 0.1 to 1.2 and hexavalent chromium in the chromate solution. After adding and mixing in a molar ratio of 0.05 to 0.30 in terms of concentration, the aqueous chromate solution is applied to the surface of a zinc-plated steel sheet and then dried to deposit chromium on the surface of a chromium coating of 10 to 200 mg / m ² . A method for chromating a zinc-based plated steel sheet, which comprises forming 2. The number of carbon atoms contained in the aqueous chromate solution is 4-8.
The method for chromate treatment of galvanized steel sheet according to claim 1, wherein the tertiary alcohol is selected from t-butanol and t-amyl alcohol. 3. The method for chromate treatment of zinc-plated steel sheet according to claim 1, wherein the silane coupling agent contained in the aqueous chromate solution is selected from those represented by the following [I] and [II]. [I] YRSiX [II] YSiX R ... Alkyl group X ... Methoxy group or ethoxy group Y ... Vinyl group, mercapto group, glycidoxy group or methacryloxy group 4. A zinc-based plated steel sheet is coated with a water-based chromate solution and dried at a plate temperature of 60 to 150.degree.
The method for chromate treatment of a zinc-plated steel sheet according to any one of claims 1 to 3, which is carried out for 10 seconds. 5. The water-based chromate solution further contains zinc ions in an amount of 0.
The chromate treatment method of the zinc-based plated steel sheet according to any one of claims 1 to 3, containing 2 to 10 g /. 6. The zinc ion is supplied from zinc white, zinc carbonate, zinc phosphate or zinc hydroxide.
5. The chromate treatment method for zinc-plated steel sheet according to any one of 1 to 5. 7. The chromate treatment method for zinc-plated steel sheet according to claim 1, wherein the aqueous chromate solution further contains complex fluoride in an amount of 0.2 to 8 g / F. 8. The chromate treatment method for zinc-plated steel sheet according to claim 1, wherein the complex fluoride is supplied from fluorozirconic acid, fluorotitanic acid, fluorosilicic acid or fluoroboric acid.