JP2770865B2 - Chromate treatment method for galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an electro-galvanized steel sheet, an electro-zinc-nickel alloy-coated steel sheet, and an alloyed galvanized steel sheet ) Can form a chromate film having excellent corrosion resistance, alkali resistance, welding resistance and coating properties (coating adhesion and corrosion resistance after coating), especially even at low temperature and / or short time drying. It relates to a chromate treatment method.

[Prior Art] Conventionally, a chromate treatment solution is a chromic acid or dichromate aqueous solution, and various methods have been proposed for forming a chromate film and then forming a film that is hardly soluble when treated with an acid or alkali. .

Hereinafter, these prior arts and their advantages and disadvantages will be described.

Japanese Patent Application Laid-Open No. 50-158535 discloses a method of forming a hardly soluble chromate film on the surface of a galvanized steel sheet, which discloses a chromate solution of chromic anhydride-phosphoric acid-water-soluble or water-dispersible polymer compound. 6 in the processing solution
Chromium ion is 70% with a reducing agent such as ethylene glycol
The above is a reduction. However, since the chromate film formed according to the embodiment of the present invention contains a polymer, it is excellent in poor solubility, corrosion resistance and paintability, but has a drawback of poor weldability.

Next, the chromate solution disclosed in JP-B-61-58552 is a chromate-chromate reduction product-silica sol system, and a surface-treated steel sheet having a chromate film formed by this method is processed. In alkaline cleaning in the pre-treatment before painting, mainly 6
There is a problem that the corrosion resistance of the film is reduced because valent chromium is easily eluted. Next, JP-A-58-22383 and JP-A-62-83478 which disclose the use of a silane coupling agent to reduce hexavalent chromium ions in a chromate treatment solution are mentioned. Although the films formed by these methods are all excellent in film adhesion, the chromate films formed by the former method are formed from a chromate treatment solution containing no phosphoric acid and have poor alkali resistance. Also, the latter method is similarly insufficient in alkali resistance.

[Problems to be Solved by the Invention] Accordingly, the present invention relates to various problems of the prior art in the method of chromate treatment of galvanized steel sheet, especially corrosion resistance, alkali resistance, welding resistance, workability and coating even at low temperature and short time drying. It is an object of the present invention to provide a method for forming a chromate film having excellent properties.

Examination of the performance of each component of the prior art in the above-described chromate treatment method reveals that organic polymers and silica tend to increase the corrosion resistance but deteriorate the alkali resistance, and the silane coupling agent contains 6% of the chromate treatment solution. Since the chromium ion tends to be reduced, the concentration of hexavalent chromium ion in the chromate treatment liquid decreases for a while during use of the film, and the corrosion resistance of the chromate film tends to be unstable.

Although various methods have been disclosed as described above, the chromate film performance of each method is strongly dependent on the drying conditions of the chromate treatment, and therefore, when the drying is at a low temperature and / or for a short time, the performance is low. Becomes insufficient.

[Means for Solving the Problems] In the present invention, in order to solve the problems of the prior art in the method of chromate treatment of a zinc-based plated steel sheet, first, the following studies are performed on an aqueous chromate solution to be applied. Was done.

That is, (1) Investigation on including phosphoric acid and hydrofluoric acid in a chromate solution for the purpose of improving alkali resistance and corrosion resistance of a film.

(2) Quantitative examination of trivalent chromium ions and hexavalent chromium ions.

(3) Quantitative examination of the sum of hydrofluoric acid and phosphoric acid and trivalent chromium ions.

As a result of the above investigations, the synergistic effect of hydrofluoric acid and phosphoric acid contained in the chromate solution applied to the plating surface creates a strong bond with the zinc-based plating surface and provides good adhesion even at low temperatures and short drying times. It has been found that a film is formed and a chromate film having excellent corrosion resistance, alkali resistance, weldability, and coatability (coating adhesion and corrosion resistance after coating) can be formed.

That is, the present invention is a hexavalent chromium ion 1 to 30 g / l,
1 to 30 g / l of trivalent chromium ion, 1 to 59 g / l of hydrofluoric acid, and 1 to 59 g / l of phosphoric acid, and the weight ratio of hexavalent / 3-valent chromium ion is 0.1 to 2.0, and phosphoric acid And the sum of hydrofluoric acid is 2
~ 60g / l, and the sum of phosphoric acid and hydrofluoric acid is (fluorine ion + phosphate ion) / 3 weight ratio of chromium ion
0.5 to 3.5, and zinc ions of 0.1 to 10 g / l and / or
Alternatively, a chromate solution that can contain 0.1 to 10 g / l of nickel ions is applied
The coating is 10-150 mg / m ^2, followed by low temperature and / or corrosion resistance in a short time drying, alkali resistance, weldability and paintability (coating adhesion and corrosion resistance after painting)
The present invention relates to a method for chromate treatment of a galvanized steel sheet, which is characterized by forming a chromate film excellent in quality.

First, the structure of the aqueous chromate solution used in the method of the present invention will be described.

This aqueous chromate solution uses water as a solvent, and
1-30 g / l of trivalent chromium ion and 1-30 g / l of trivalent chromium ion
Is contained as a basic component. A hexavalent chromium ion concentration of less than 1 g / l and a trivalent chromium ion concentration of less than 1 g / l make it difficult to form a chromate film exhibiting satisfactory corrosion resistance.
At a hexavalent chromium ion concentration of more than 30 g / l and a trivalent chromium ion concentration of more than 30 g / l, the viscosity of the chromate solution becomes high and the stability of the chromate solution deteriorates, making it difficult to control the amount of chromium adhering. What is important in the amount of chromium is the content ratio of hexavalent and trivalent chromium ions, and the hexavalent / 3valent chromium ion weight ratio (hereinafter referred to as "chromium ion weight ratio") is in the range of 0.1 to 2.0. It is necessary. If the chromium ion weight ratio is less than 0.1, the content of hexavalent chromium ions in the chromate film decreases, and the corrosion resistance decreases. vice versa,
If the chromium ion weight ratio exceeds 2.0, the hexavalent chromium ion concentration in the chromate solution will increase accordingly, and the hexavalent chromium ion in the chromate film will be easily eluted by alkali washing before painting, leading to a deterioration in the quality of the chromate film. I do.

The chromium ion weight ratio can be controlled by adding a known reducing agent such as ethanol, methanol, acid, starch, and sucrose as needed.
It can also be adjusted by dissolving trivalent chromium salts such as basic chromium carbonate and chromium hydroxide in hydrofluoric acid, phosphoric acid and chromic acid. As another component, the aqueous chromate solution of the present invention must contain a total of 2 to 60 g / l of hydrofluoric acid and phosphoric acid. If the total amount of hydrofluoric acid and phosphoric acid is less than 2 g / l, the corrosion resistance and alkali resistance of the chromate film will decrease.If the total amount exceeds 60 g / l, the chromate film will decrease in dryness, absorb moisture easily, and decrease in corrosion resistance after painting. I do. The hydrofluoric acid in these acids is 1 to 59
g / l, phosphoric acid 1-59 g / l.

Of the total amount of hydrofluoric acid and phosphoric acid, what is particularly important is the ratio of the total amount of hydrofluoric acid and phosphoric acid in the chromate solution to the trivalent chromium ion. The sum of hydrofluoric acid and phosphoric acid is (fluorine ion + phosphate ion) / 3 weight ratio of chromium ion
If it is less than 0.5, the alkali resistance and corrosion resistance of the chromate film tend to decrease, while if it exceeds 3.5, the drying property of the chromate film deteriorates, it becomes easy to absorb moisture, and the coating film adhesion after coating and the corrosion resistance decrease. The present chromate solution is an aqueous chromate solution that does not contain complex fluorides such as zirconium fluoride, borofluoric acid, and silicic hydrofluoric acid.

Incidentally, the aqueous chromate solution is 0.1 to 10 g / l as needed.
And at least one of 0.1 to 10 g / l of nickel ions can be added to further improve alkali resistance and corrosion resistance. If the content of zinc ion and nickel ion in the aqueous chromate solution is less than 0.1 g / l, it is difficult to confirm the effect of improvement. Conversely, if the content exceeds 10 g / l, trivalent chromium ions in the treatment solution will precipitate. It is not preferable because it becomes easier. Zinc ion is zinc white, zinc carbonate, zinc hydroxide, zinc phosphate or zinc fluoride, nickel ion is nickel carbonate, basic nickel carbonate, nickel hydroxide,
Alternatively, it can be added to the chromate solution in the form of nickel fluoride.

If the amount of chromium adhered is less than 10 mg / m ² , the corrosion resistance of the chromate film and the corrosion resistance after painting are insufficient. With an upper limit of the chromium coating weight is Do difficult to control the chromium coating weight of chromate film at 150 mg / m ² greater, the effect of improving the corrosion resistance chromate film is saturated, it is difficult to expect more effective, and chromate Since the coating is easily locally removed by external force, the adhesion of the coating is reduced and the weldability is reduced.

The low-temperature and / or short-time conditions for drying the chromate-treated film according to the present invention are not particularly limited, but sufficient performance of the chromate film is exhibited even when the temperature is less than 100 ° C. and the time is 5 seconds or less.

The pH of the aqueous chromate solution specified in the present invention is not specified, but is preferably 2nd to 5th.

[Action] The aqueous chromate solution described in the claims is applied to the surface of a zinc-based plated steel sheet, and then, in the process of drying, trivalent and hexavalent chromium ions in the chromate solution, zinc, mainly due to heat energy during drying. Ions, nickel ions, and each component such as hydrofluoric acid and phosphoric acid react with the zinc-based plating to form colorless compounds represented by the following (a) and (b), respectively, and (c), (d), (e) and A green compound represented by (f), and (g) and (h)
Are generated, and these are the components for forming the chromate film.

(A) ZnF _n (OH) _m where n + m = 2, (n = 0 to 2) (b) Zn ₃ (PO ₄ ) ₂ .4H ₂ O (c) NiF _n (OH) _m where n + m = 2, ( n = 0~2) (d) Ni 3 (PO 4) 2 · 8H 2 O (e) CrFx (OH) y but x + y = 3, (x = 0~3) (f) CrPO 4 · 6H 2 O ( _{g) ZnO · 3Zn (OH)} 2 · CrO 3, 3Zn (OH) 2 · CrO 3, 2Zn (OH) 2 · CrO 3 zinc chromate-based compound represented by such as _{(h) Cr (OH) 3} · Cr ( OH) .CrO ₄ (chromic chromate) As described above, the chromate solution according to the present invention can form a complex bond between the above (a) to (h) even when the drying conditions after coating are low and / or short time. It is presumed that a chromate film is present in a state in which it forms a higher-order molecular structure pair and is strongly bonded to the zinc-based plating surface. The alkali-resistance of the chromate film is enhanced by the binding effect of the chromate film having a higher molecular structure pair formed as described above. That is, the chromium in the film is hardly eluted by the alkali cleaning. Moreover, it is presumed that the higher molecular structure pairs contribute to the improvement of corrosion resistance.
Furthermore, the chromate film of the present invention does not contain an organic polymer compound, and thus has excellent alkali resistance, and has a relatively low interlayer insulation resistance value, and thus has excellent weldability.

EXAMPLES In order to more specifically explain the effects of the present invention, examples will be given below, together with comparative examples.

[Examples] (1) Preparation method of chromate coating solution Chromate coating A shown in Table 1 was prepared by first dissolving 200 g of chromic anhydride in 500 g of water, and adding 83 g of phosphoric acid (75% aqueous solution) to this aqueous solution and methanol And heated at 80-90 ° C. for 1 hour to reduce and cool the weight ratio of trivalent chromium ion / 6 chromium ion to 1.0, and then added hydrofluoric acid (55% aqueous solution) 36 g / l,
Further water was added to make the total amount 1 kg. Hereinafter, this liquid is referred to as an aqueous chromate liquid. Next, this aqueous chromate solution was diluted with water so that the total amount of chromium ions was 25 g / l, to prepare a chromate coating solution A. Hereinafter, chromate coating solutions BK are processed in the same manner as chromate coating solution A,
It was prepared to have the composition shown in Table 1.

(2) Chromate treatment method The chromate coating solution thus prepared is applied to the surface of an electrogalvanized steel sheet or an electrozinc-nickel alloy-plated steel sheet by the following process, and is dried to obtain a result of a performance test. Are shown in Table 2.

Steel plate to be treated (* 1) → Alkaline degreasing (* 2) → Washing →
Roll drawing → drying (air drying) → chromate coating → roll drawing → drying (* 3) (* 1) The steel sheet to be treated is a double-sided galvanized steel sheet (zinc plating amount 20g / m ² / 20g / m ² ) and both sides electrical zinc-nickel alloy plated steel sheet (coating weight ^{20g / m 2 / 20g / m} 2, nickel 1
An oiling material containing 1 wt% and having a size of 20 × 300 mm and a thickness of 0.8 mm was used.

(* 2) Alkaline degreasing was performed with a 2% aqueous solution of a weak alkaline degreasing agent (Nippon Parkerizing, Pulclean 342) at a temperature of 60 ° C and spraying for 30 seconds.

(* 3) The drying was performed at a plate temperature of 90 ° C. and a drying time of 4 seconds.

(3) Method of preparing painted plate The chromate-treated steel plate is used as it is, or after alkali washing as described in (4) (a) below, baking-type melamine alkyd paint (Dai Nippon Paint Co., Delicon 700 white) is applied, and 140 Baking and drying at 20 ° C for 20 minutes.
5 μm).

(4) Performance evaluation test (a) Alkali resistance test Chromate-treated steel was washed with alkali under the following conditions, and the amount of chromium adhering before and after the measurement was measured with fluorescent X-rays (mg / m ² ), and expressed as alkali resistance by did. That is, a smaller value of% indicates that the alkali resistance is better, and a value of 0 indicates that the test is not affected by the alkali at all in this test.

A = Amount of chromium before alkali washing (mg / m ² ) B = Amount of chromium after alkali washing (mg / m ² ) Alkali washing is an alkaline degreasing agent containing sodium silicate as a main component (Nippon Parkerizing Pal Clean N364
Spraying was performed with a 2% aqueous solution of S) at a temperature of 60 ° C. for 2 minutes.

(B) Corrosion resistance Electrogalvanized steel sheet: A salt spray test specified in JIS-Z-2371 was performed for 150 hours before and after alkali washing on a test piece (size 70 × 150 mm), and the white rust generation state was observed over the entire area of the test piece. The corrosion resistance was evaluated.

◎: Area ratio of white rust occurrence 0% ○: Same as above, less than 10% △: Same as above, 10% or more and less than 30% ×: Same as above, 30% or more Electric zinc-nickel alloy-plated steel sheet: Specimen before and after alkali washing, salt spray 4 hours, Dry (60 ° C) 2 hours, wet (50 ° C, RH 95% or more) 2
A 50-cycle test was performed by a composite corrosion test method in which the time was set to one cycle, and the corrosion resistance was evaluated from the state of occurrence of red rust over the entire area of the test piece.

◎: Red rust occurrence area ratio 0% ○: Same as above, less than 10% △: Same as above, 10% or more and less than 30% ×: Same as above, 30% or more (c) Paint scratches reaching the base metal with a cutter and spraying with salt water Test the galvanized steel sheet is 20
The electro-zinc-nickel alloy plated steel sheet was carried out for 0 hour and 300 hours. After execution, the adhesive tape (cellotape) was peeled off, and the judgment was made by measuring the maximum one-sided peeling width from the scratch (unit: mm).

(D) Coating adhesion Goban eye test: For a test piece painted without alkali washing,
A square of 1 mm square was cut with a cutter so as to reach the base metal, an adhesive tape (cellotape) was stuck to one surface of the test piece, and rapidly peeled off, and the degree of peeling of the coating film was observed.

Ericssen extrusion test: For test specimens painted without alkali cleaning
It was extruded by an Erichsen extruder by 6 mm, and was quickly peeled off by attaching a cellophane tape, and the degree of peeling of the coating film was observed.

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

◎: Peeling of coating film 0% ○: Same as above, less than 10% △: Same as above, 10% or more and less than 30% ×: Same as above, 30% or more (e) Weldability test Spot welding is continuously performed on electro-zinc nickel alloy plated steel sheet under the following conditions When the method is performed in a proper manner, the weld terminal gradually deteriorates and the weldability deteriorates, so that the weldability can be determined based on the degree of deterioration. In other words, another test piece of 30 × 100 mm was welded every 100 test points, and the number of test points until the tensile strength of the test piece could be maintained at 400 kg was recorded.

Welding surface: painted surface-unpainted surface Pressure: 200kg Current: 8.5kA Energizing time: 10 cycles Electrode: R40 (radius type), material, chrome-copper [Effects of the Invention] As is clear from the above Examples and Comparative Examples, according to the present invention, zinc-coated steel sheets have excellent corrosion resistance, alkali resistance, weld resistance, and coatability even at low temperature and short time drying. A chromate film having coating film adhesion and corrosion resistance after coating) can be formed.

On the other hand, Comparative Examples 3, 4, 6, 7, 8, and 9 are clearly inferior in alkali resistance and coating film adhesion. Comparative Examples 1, 2,
5, 6, and 7 are inferior in corrosion resistance, painted board corrosion resistance, and coating film adhesion.

Claims (2)

(57) [Claims] 1 to 30 g / l of hexavalent chromium ion, 1 to 30 g / l of trivalent chromium ion, 1 to 59 g / l of hydrofluoric acid, and 1 to 30 g / l of phosphoric acid
Contains 59 g / l, and has a hexavalent / 3-valent chromium ion weight ratio of 0.1 to
2.0, the sum of phosphoric acid and hydrofluoric acid is 2 to 60 g / l, and the sum of phosphoric acid and hydrofluoric acid is (fluorine ion +
An aqueous chromate solution with a weight ratio of 0.5 to 3.5 (phosphate ion) / 3 chromium ions is applied to the surface of the galvanized steel sheet and then dried, and the amount of chromium adhered to the surface of the galvanized steel sheet is 10 to 150 mg / m. ^2. A method for chromate treatment of a galvanized steel sheet, wherein a chromate film is formed. 2. The method according to claim 1, wherein said aqueous chromate solution contains at least one of 0.1 to 10 g / l of zinc ions and nickel ions.