JP2950563B2 - Chromate treatment of zinc-coated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides excellent paintability, corrosion resistance, and treated appearance on the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet, and prevents chromium elution during alkaline degreasing. The present invention relates to a chromate treatment method for forming a small chromate film.

[Conventional technology]

2. Description of the Related Art Conventionally, various chromate treatments have been performed on steel sheets plated with zinc or zinc alloy to prevent rust. The chromate treatment includes a reactive chromate which forms a film containing trivalent chromium as a main component, and a coating type chromate which is applied with a chromate solution containing electrolytic chromate and hexavalent chromium and dried without washing with water.

The performance items required for chromate-treated steel sheet are, for example, white rust resistance, blackening resistance, coating adhesion, corrosion resistance after painting, weldability, grounding properties, pressability, fingerprint adhesion resistance, etc. Wide variety. In addition, after the molding process, it passes through an alkali degreasing step in a surface treatment line, so that chromium elution resistance and alkali resistance are required, and solvent resistance is required for solvent washing after assembly. When used without painting, uniformity of appearance is important. In particular, chromate-treated steel sheets for home appliances lose their commercial value if they are markedly colored by Cr. Various chromate treatments have been performed for these requests.

[Problems to be solved by the invention]

However, the conventional chromate treatment method has the following problems.

In the case of treating with a chromate treatment solution containing only chromic acid, it is necessary to increase the amount of chromium adhered in order to obtain high corrosion resistance, so that the plating surface is conspicuous and the commercial value is significantly reduced.

When processing with a processing solution in which an etching agent such as fluoride is added to chromic acid, particularly when processing a hot-dip galvanized steel sheet, elements such as Al and Pb contained in the plating composition are segregated on the plating surface. As a result, uniform etching is not performed due to the influence, resulting in a non-uniform film, and thus the level of corrosion resistance obtained is limited.

The addition of silica sol to a chromate solution is effective for improving corrosion resistance as described in JP-B-42-14050 and JP-B-52-2
No. 851 and Japanese Patent Publication No. 61-58552. The inventions disclosed therein are those in which silica sol is added to chromic acid or a mixture of chromic acid and a chromate reduction product, but when the amount of the chromate reduction product is small, the chromate film is dissolved. There is a problem that it is easy. For this reason, the hardly soluble chromate film is formed by increasing the mixing ratio of the chromate reduction product to the chromate. However, with these chromate treatment methods, it is difficult to obtain satisfactory performance in both corrosion resistance and paintability.

Accordingly, an object of the present invention is to provide a chromate treatment method for forming a chromate film in which the paintability, corrosion resistance, and treated appearance of a galvanized steel sheet or a zinc alloy-plated steel sheet are improved, and the elution of chromium due to alkali degreasing is reduced. Is to provide.

[Means for solving the problem]

The present invention is mainly composed of chromic acid, a chromic acid reduction product, an acrylic emulsion and a silica sol composed of at least one dry-type silica sol and at least one wet-type silica sol, and the particle size of the wet-type silica sol is 20 nm or less, the mixing ratio of the chromic acid and the chromic acid reduction product each in a weight ratio by CrO ₃ terms is 1: 0.1 to 1.5, the mixing ratio of the acrylic emulsion and the chromic acid and the chromic acid reduction product, By weight ratio of the chromic acid and the chromic acid reduction product of the resin solid content of the acrylic emulsion to the total amount of Cr, 0.05 to 20: 1, the chromic acid and the chromic acid reduction product and the wet type The mixing ratio with the silica sol is such that the chromic acid and the chromate reduction product are in terms of Cr, and the wet type silica sol is in terms of SiO _{2 in} terms of weight ratio. The mixing ratio between the chromic acid and the chromic acid reduction product and the wet type silica sol is 1: 0.1 to 10, and the chromic acid and the chromic acid reduction product are calculated as Cr, and the dry type silica sol is SiO _2. Prepare an aqueous solution containing a weight ratio in the range of 1: 0.1 to 10 by conversion, attach the aqueous solution to the surface of a galvanized steel sheet or a zinc alloy-coated steel sheet, and then heat the steel sheet at a temperature of 40 to 300 ° C. It is characterized by performing heat treatment.

 Hereinafter, the present invention will be described in detail.

Chromic acid: Chromic acid is obtained by dissolving chromic anhydride (CrO ₃ ) in water and has the property of adhering to the surface of a galvanized film to exert a self-repairing action and preventing the generation of zinc white rust. doing. Chromic acid is indispensable for remarkably improving the rust resistance of the galvanized steel sheet due to this property, and is inexpensive.

The required concentration of chromic acid cannot be specified because it varies depending on the method and amount of the chromate treatment liquid applied to the galvanized steel sheet, but the required amount of the chromic acid can be obtained by a single step in the adhesion method described below. It is preferable that the concentration be as high as possible.

Chromic acid reduction product: Chromic acid reduction product is obtained by adding a reducing agent to chromic acid,
Chromic acid is reduced. The composition of the chromate solution after the reduction is mainly composed of unreduced chromic acid and a product reduced to trivalent chromium, and the mixing ratio is adjusted by the amount of the reducing agent added. However, if the amount of the reduced product exceeds 1.5 moles with respect to 1 mole of the unreduced chromic acid, precipitation occurs in the state of a chromate solution, and it is difficult to uniformly deposit on the steel sheet.

Conventionally, the purpose of adding a chromate reduction product to chromic acid is to obtain a hardly soluble chromate film, and the higher the mixing ratio of the chromate reduction product, the higher the degree of polymerization of the chromate film. , Thereby increasing the sparing solubility.

However, if the mixing ratio of the chromic acid reduction product to the chromic acid is excessively high, the liquid stability decreases, and the operability in continuous processing is poor. In the present invention, the upper limit of the mixing ratio of the chromic acid reduction product to chromic acid is set to 1.5 in consideration of liquid stability.

Further, the chromate film can be made hardly soluble by performing heat drying after attaching the chromate solution. The heating conditions are determined by the reduction ratio of chromic acid, and the lower the reduction ratio, the more it is necessary to heat and dry at a higher temperature.

However, when the mixing ratio of the chromic acid reduction product to chromic acid 1 is less than 1: 0.1 in terms of the weight ratio in terms of CrO ₃ , the degree of polymerization of the chromate film is too low to form a hardly soluble film by heat treatment. Is difficult to do. In the present invention, the lower limit of the mixing ratio of the chromic acid reduction product with respect to chromic acid is set to 0.1 because it is in a range where insolubilization by heating is possible.

Acrylic emulsion: Among the required properties for chromate-treated steel sheets, the appearance of the treated chromate film is one of the important items. In order to solve this, it is necessary to use a chemical solution that can be applied uniformly, to require equipment, and to reduce the coloring of chromic acid.

We investigated the effect of various additives to prevent the coloring of chromium. As a result, they found that coloring of chromium can be prevented by adding an acid such as hydrofluoric acid or an acrylic emulsion. However, when an acid is added, there arises a problem that the chromated zinc-plated surface is liable to turn black. On the other hand, when the acrylic emulsion was added, the blackening resistance did not decrease and the effect of preventing chromium coloring was recognized. The reason why we add an acrylic emulsion is based on the above results.

In addition, in order to be a chromate liquid that can be applied uniformly, it is necessary that the acrylic emulsion, which is an aqueous resin, is stable in a chromic acid liquid and does not cause gelation or the like.
In general, the mixing stability between chromic acid and an aqueous resin is poor, and gelation occurs immediately. This problem has been stabilized by employing an acrylic emulsion resin emulsion-polymerized using a nonionic emulsifier. When no emulsifier was used or an ionic emulsifier was used for other aqueous resins or the same acrylic resin, the mixing stability was poor or gelation had already occurred.

The method of attaching the chromate treatment liquid to the surface of the steel sheet is as follows:
A method capable of continuous and uniform deposition is applied. As such a method, spray coating, dipping to roll drawing, dipping to gas drawing, roll coating and the like can be used.

As described above, for the purpose of improving the treated appearance of the chromate-treated steel sheet, a means of uniformly applying a liquid obtained by adding an acrylic emulsion using a nonionic emulsifier to a chromate liquid, and drying by heating is effective. is there.

The reason why the addition weight ratio of the resin solids in the acrylic emulsion to the total Cr amount in the treatment liquid is 0.05 or more is that if the addition ratio is less than 0.05, chromium coloring cannot be sufficiently prevented. In addition, the reason for setting the addition ratio to 20 or less is that even if the addition ratio is increased beyond this, other than the coloring prevention effect, no effect on other properties such as corrosion resistance is recognized.
This is due to the consideration of the economics of the chromate solution.

Silica sol (dry type silica sol, wet type silica sol): Next, the reason for adding silica sol will be described.

The purpose of adding the dry-type silica sol is to improve the paint adhesion of the chromate film. However, liquids to which chromic acid, a chromate reduction product, an acrylic emulsion resin, or a wet-type silica sol are added cannot provide excellent performance particularly in secondary adhesion after coating. In order to obtain excellent secondary adhesion, it is effective to add a dry-type silica sol. Further, the merit of the chromate treatment of chromic acid, a chromate reduction product, an acrylic emulsion resin, a wet-type silica sol, and a dry-type silica sol is that the formed chromate film becomes white and coloring by chromium becomes less noticeable. . To whiten the chromate film, none of the above-mentioned additives can be removed.

As described above, in order to form a chromate film that satisfies all of corrosion resistance, paintability, chromate treatment appearance, and poor solubility, chromic acid, chromate reduction product, acrylic emulsion, dry type silica sol, wet type silica sol It is necessary to form a mixed film of

Wet type silica sol is effective in improving corrosion resistance,
The amount of added SiO ₂ in the wet type silica sol is
On the other hand, if it is less than 0.1, no effect on corrosion resistance is observed, and if it exceeds 10, no particular effect is observed.

In addition, if the addition of SiO ₂ in the dry-type silica sol is less than 0.1 relative to the total Cr amount, no effect on paintability and whitening of the chromate film is observed, and conversely, even if it exceeds 10, the effect is observed. Absent.

Therefore, the addition amount of the wet-type silica sol should be limited to the range of 0.1 to 10 with respect to the total Cr amount of 1 as described above, and the addition amount of the dry-type silica sol should be limited to the range of 0.1 to 10 with respect to the total Cr amount.

Next, the steel sheet to which the chromate treatment liquid is adhered will be described.

The chromate treatment solution developed in the present invention includes hot-dip galvanizing, electrogalvanizing, hot-dip Fe-Zn alloy plating, and 5% Al-
It can be applied to a zinc-based plated steel sheet such as a zinc-based alloy-plated steel sheet.

In addition, Al steel sheet, Al-plated steel sheet, 55% Al-1.5% S
Materials such as i-Zn-plated steel sheets, stainless steel sheets, turn-plated steel sheets, etc. can also improve corrosion resistance and impart paintability.

Next, a method of attaching a chromate treatment liquid to the surface of a steel sheet will be described.

The method of attaching the chromate treatment liquid to the surface of the steel sheet is as follows:
A method capable of continuous and uniform deposition is applied. As such a method, spray coating, dipping to roll drawing, dipping to gas drawing, roll coating and the like can be used.

The reason why the heat treatment temperature is limited to 40 to 300 ° C. is to make the chromate film hardly soluble.

If the heat treatment temperature is lower than 40 ° C., it is difficult to make the chromate film hardly soluble.

On the other hand, if the heat treatment temperature exceeds 300 ° C., no further effect can be expected in the insolubilization, and conversely, corrosion resistance deteriorates.

〔Example〕

 Next, the present invention will be described with reference to examples.

Methanol was added as a reducing agent to an aqueous solution of chromic anhydride 100 g /. After the addition, the mixture was stirred at a liquid temperature of 90 to 95 ° C. for 2 hours to partially reduce chromic acid.

Next, a silica sol or an acrylic emulsion was added to a mixed solution of chromic acid and a chromic acid reduction product to prepare a chromate treatment solution. Then, using the prepared chromate treatment solution, a zinc-plated steel sheet or a zinc alloy-plated steel sheet was subjected to a chromate treatment by the following procedure to prepare specimens Nos. 1 to 13 of the present invention and comparative specimens Nos. 14 to 21.

Chromate treatment procedures: galvanized steel sheet (lozenge Ma line producing zinc coating weight 120 g / m ²⁾ → water washing → chromate treatment (roll coating) → heat treatment (40 ° C. to 300 ° C.).

Table 1 shows the composition of the chromate treatment solution, the amount of attached chromate, and the heat treatment temperature of each specimen. The amount of chromate attached was determined by measuring the total amount of chromium by X-ray fluorescence and expressed in mg / m ² .

Next, the corrosion resistance, paintability of each prepared specimen,
The poor solubility and treated appearance were evaluated by the following methods, and the results are shown in Table 1.

Corrosion resistance: For each specimen, salt spray test (JIS Z2371)
After 200 hours, the rate of occurrence of white rust on the galvanized steel sheet was measured and evaluated.

 The evaluation criteria are as follows.

 10: No white rust generated, 8: White rust generated area less than 10%, 6: White rust generated area 10% to less than 25%, 4: White rust generated area 25% to less than 50%, 1: White rust generated area 50 %that's all.

Poor solubility: Each specimen was subjected to alkaline degreasing, the amount of chromium adhering before and after degreasing was measured, and the amount of chromium adhering after degreasing with respect to the amount of chromium adhering before degreasing was indicated by%.

 The alkaline degreasing conditions are as follows.

Degreasing liquid: CL-N364S (manufactured by Nippon Parkerizing Co., Ltd.), 20
g /, 60 ° C.

Degreasing method: spray method, spray pressure 0.5 kg / cm ² , 2 minutes.

Paint adhesion (primary): The paint adhesion of a sample obtained by spray-coating a melamine alkyd-based material (Delicon # 700 white, manufactured by Dainippon Ink Co., Ltd.) on an alkali-degreased test piece was evaluated by using the Goban-Erichsen overhang. Evaluation was based on the degree of peeling of the coating film due to tape peeling.

Evaluation criteria for paint adhesion: 10: no peeling, 8: less than 10% peeled area, 6: less than 25% peeled area, 4: less than 50% peeled area, 1: 50% or more peeled area.

Secondary paint adhesion: The paint adhesion of a paint sample immersed in boiling water for 3 hours was evaluated.

 The materials used in this example are described below.

Dry type silica sol: Aerosil 200 manufactured by Nippon Aerosil Co., Ltd. Wet type silica sol: Snowtex-0 manufactured by Nissan Chemical Co., Ltd. Processing plate: GI; Hot-dip galvanized steel plate (60 g per coated surface
/ m ² , minimized spangled material), GA; galvannealed steel sheet (20 g / m ² per coating weight per side), EG; electrogalvanized steel sheet (2 per coating weight per side)
0 g / m ² ), Al-Zn; 5% Al-Zn alloy coated steel sheet (60 g / m ² per side of coating weight).

As is evident from Table 1, the materials of the present invention shown in Specimens Nos. 1 to 13 are excellent in all of corrosion resistance, poor solubility, paint adhesion, paint secondary adhesion, and treated appearance. In contrast, the specimen
In No14 and No15, the ratio of chromic acid: chromic acid reduction product was out of the range of the present invention, and corrosion resistance, paint adhesion,
It is inferior in secondary adhesion of paint and stability of processing solution. In specimens Nos. 16 and 17, the ratio of total Cr content: nonionic acrylic emulsion was outside the scope of the present invention,
Poor solubility, poor paint secondary adhesion, treated appearance, or poor economic efficiency. For specimens 18 and 19, the total
The ratio of Cr content: wet type silica sol is out of the range of the present invention, and is inferior in corrosion resistance or chromate film adhesion. In Samples Nos. 20 and 21, the ratio of total Cr to dry silica sol was out of the range of the present invention, and was inferior in paint adhesion and paint secondary adhesion.

[Effects of the Invention] As described above, according to the chromate treatment method of the present invention, on the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet, paintability, corrosion resistance, excellent treatment appearance, and
An industrially useful effect is obtained in which a chromate film with less elution of chromium during alkaline degreasing can be formed.

Continuation of the front page (72) Inventor Takayuki Aoki 1-15-1 Nihonbashi, Chuo-ku, Tokyo Japan Inside Parkerizing Co., Ltd. (72) Inventor Takeharu Umezu 1-15-1 Nihonbashi, Chuo-ku, Tokyo Japan Parkerizing Stock In-company (56) References JP-A-63-145785 (JP, A) JP-A-53-132439 (JP, A) JP-A-55-152184 (JP, A) JP-A-60-153972 (JP, A) (58) Fields surveyed (Int. Cl. ⁶ , DB name) C23C 22/30 C23C 22/28

Claims (1)

(57) [Claims] 1. A silica sol comprising at least one of a chromic acid, a chromic acid reduction product, an acrylic emulsion, and one or more dry-type silica sols and one or more wet-type silica sols, wherein the particle diameter of the wet-type silica sol is 20 nm or less. , in a weight ratio of the mixing ratio by each CrO ₃ in terms of the chromic acid and the chromic acid reduction product, 1: 0.1 to 1.5, the mixing ratio of the acrylic emulsion and the chromic acid and the chromic acid reduction product The weight ratio of the solid content of the resin in the acrylic emulsion and the total amount of Cr of the chromic acid and the chromic acid reduction product was 0.05 to 20: 1, and the chromic acid and the chromic acid reduction product and the wet method were used. The mixing ratio with the type silica sol, the chromic acid and the chromic acid reduction product are in terms of Cr, and the wet silica sol is in a weight ratio in terms of SiO ₂ , 1: 0.1 to 10, the mixing ratio of the chromic acid and the chromic acid reduction product and the wet type silica sol, the chromic acid and the chromic acid reduction product in terms of Cr, the dry type silica sol is
In a weight ratio by the terms of SiO ₂ 1: An aqueous solution containing in the range of 0.1 to 10 were prepared, the aqueous solution is deposited on the surface of the galvanized steel sheet or zinc alloy plated steel sheet, then the said steel plate 40 to 300 ° C. A method for chromate treatment of a galvanized steel sheet, comprising heat-treating at a temperature.