JP2925211B2 - Chromate treatment of zinc-coated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a galvanized steel sheet or aluminum
The present invention relates to a chromate treatment method for forming a chromate film on a surface of a zinc alloy-plated steel sheet, which is excellent in corrosion resistance, blackening resistance, treated appearance, and has little chromium elution during alkali degreasing.

[Conventional technology]

Conventionally, various chromate treatments have been performed on steel sheets plated with zinc or aluminum-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 segregate 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-52-285.
No. 1, JP-B-61-58552 and the like.
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, in these chromate treatment methods, if the amount of Cr attached is increased to further increase the corrosion resistance, coloring by Cr becomes conspicuous, and the treated appearance deteriorates.

Therefore, an object of the present invention is to improve the corrosion resistance, blackening resistance, treatment appearance, and the like of a galvanized steel sheet or an aluminum-zinc alloy-coated steel sheet, and to form a chromate film in which elution of chromium due to alkali degreasing is reduced. An object of the present invention is to provide a chromate treatment method.

[Means for solving the problem]

The present invention comprises chromic acid, a chromate reduction product, an acrylic emulsion, and one or more wet-type silica sols as main components, and a mixing ratio of the chromic acid and the chromic acid reduction product is each calculated as CrO ₃ . 1: 0.1 by weight
~ 1.5, the mixing ratio of the acrylic emulsion and the chromic acid and the chromic acid reduction product, the resin solid content in the acrylic emulsion and the Cr equivalent value of the total amount of the chromic acid and the chromic acid reduction product Weight ratio of
0.05 to 20: 1, the mixing ratio between the wet type silica sol and the chromic acid and the chromic acid reduction product, the wet type silica sol is calculated as SiO ₂ , the value and the chromic acid and the chromic acid reduction product Is the weight ratio with the Cr conversion value,
An aqueous solution containing 0.1 to 10: 1 is prepared, the aqueous solution is adhered to the surface of a galvanized steel sheet or an aluminum-zinc alloy-coated steel sheet, and then the steel sheet is 40 to 30.
It is characterized by performing heat treatment at a temperature of 0 ° C.

 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 the chromate solution on the steel sheet.

Conventionally, the purpose of adding a chromate reduction product to chromic acid is to obtain a sparingly soluble 6 chromate film, and the degree of polymerization of the chromate film increases as the mixing ratio of the chromate reduction product increases. , 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, it has been found that coloring of chromium can be prevented by adding an acid such as hydrofluoric acid or an acrylic emulsion which is an aqueous resin. However, when an acid is added, there arises a problem that the zinc plating surface subjected to chromate treatment tends to have a blackening ratio. 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.

Also, in order to be a chromate solution that can be applied uniformly, the acrylic emulsion is stable in the chromic acid solution,
It is necessary that gelation does not occur. 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 water-based resins or the same acrylic resin, the mixing stability was poor or gelation occurred immediately.

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 contained 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.

Wet type silica sol: Next, the reason for adding the wet type silica sol will be described. The first object of the addition of the wet type silica sol is to improve the corrosion resistance of the chromate film. The present inventors have conducted intensive studies and studies on the relationship between the addition of silica sol and the improvement in the corrosion resistance of the chromate film, and as a result, it has been found that the type of silica sol to be added has a particularly large effect. Silica sols are roughly classified into two types: a wet type and a dry type. Both types exhibit an effect of improving corrosion resistance when added to a chromate solution, but the wet type silica sol is particularly effective.

The second purpose of adding the wet type silica sol is to improve the appearance of the treatment. As described above, by mixing the acrylic emulsion with the chromate solution, it is possible to prevent coloring by chromium. However, an interference pattern is generated by the formed chromate film, and a new problem is caused in terms of the processing appearance. It has been clarified that the addition of a wet-type silica sol has the effect of eliminating the interference pattern in a state where coloring by chromium is prevented.

As described above, the purpose of adding the wet type silica sol is to improve the corrosion resistance and the appearance of the treatment. The addition amount of the wet-type silica sol necessary for obtaining such an effect is suitably from 0.1 to 10 per 1 of the total Cr amount. If the total Cr content is less than 0.1 with respect to 1, the effect on the corrosion resistance and disappearance of the interference pattern of the film is not recognized, and even if added over 10, no further effect is recognized.

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

The chromate treatment solution developed in the present invention can be applied to galvanized steel sheets such as hot-dip galvanized, electrogalvanized, and 5% Al-Zn alloy-coated steel sheets.

In addition, Al steel sheet, Al-plated steel sheet, 55% Al-1.5% S
Corrosion resistance can also be improved for materials such as i-Zn plated steel sheets, stainless steel sheets, and turn plated steel sheets.

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 100 g / l of the anhydrous chromium film. 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, chromate treatment is performed on a galvanized steel sheet or a zinc alloy-plated steel sheet according to the following procedure to prepare specimens Nos. 1 to 7 and 10 of the present invention and comparative specimens Nos. 11 to 19. did.

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, poor solubility,
The appearance of the treatment and the resistance to blackening were evaluated by the following methods, and the results are shown in the first vote.

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 /
l, 60 ° C.

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

Blackening resistance: In a 50 ° C, 90% RH atmosphere, samples are stored in a stacked state for 240 hours to promote blackening of plated steel sheets,
Each sample was evaluated based on the degree of black change.

 The evaluation criteria are as follows.

5: Black change is not recognized, 4: Level is slightly black but not a problem, 3: Black is slightly changed, 2: Black is considerably changed, 1: Black is changed considerably on the entire surface I have.

 The materials used in this example are shown 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 base plate: GI; hot-dip galvanized steel plate (60 g / m per coated amount
^2. Minimized spangled material), GA: galvannealed steel sheet (45g / m ² per coating weight per side), Al-Zn; 5% Al-Zn alloy coated steel sheet (60g / m ² per coating weight per side) ).

As is evident from Table 1, the materials of the present invention shown in the specimens Nos. 1 to 7 and 10 are excellent in all of the corrosion resistance, the poor solubility, the blackening resistance and the treated appearance. On the other hand, in Samples No. 11 and No. 12, the ratio of chromic acid: chromic acid reduction product was out of the range of the present invention, and was inferior in terms of poor solubility, corrosion resistance, and treatment solution stability. For specimens Nos. 13 and 16, the total amount of Cr:
The ratio of the acrylic emulsion is out of the range of the present invention, and it is inferior in processing appearance or economic efficiency. Specimen No. 14, 15
In the above, although the appearance of the treatment is improved by additives other than the acrylic emulsion, the blackening resistance is inferior. In Samples Nos. 17 and 18, the ratio of the total amount of Cr to the wet type silica sol was out of the range of the present invention, and was inferior in processing appearance or economic efficiency. Specimen No. 19 was inferior in corrosion resistance to Specimen No. 2 in which the wet type silica sol was added at the same composition ratio because the dry type silica sol was added instead of the wet type silica sol.

[Effects of the Invention] As described above, according to the chromate treatment method of the present invention, the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet has excellent corrosion resistance, blackening resistance, excellent treatment appearance, and alkali degreasing. An industrially useful effect of forming a chromate film with less chromium elution can be obtained.

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-59-197575 (JP, A) JP-A-55-152184 (JP, A) JP-A-61-584 (JP, A) JP-A-3-100180 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 22/00-22/86 B05D 3/10

Claims (1)

(57) [Claims] A chromic acid, a chromic acid reduction product, an acrylic emulsion, and one or more wet-type silica sols as main components, wherein the mixing ratio of the chromic acid and the chromic acid reduction product is CrO ₃ conversion. 1: 0.1 ~
1.5, the mixing ratio of the acrylic emulsion and the chromic acid and the chromic acid reduction product, the resin solid content in the acrylic emulsion and the total amount of the chromic acid and the chromic acid reduction product in terms of Cr By weight ratio, 0.
05-20: 1, the mixing ratio of the wet type silica sol and the chromic acid and the chromic acid reduction product, wet type silica sol in terms of SiO ₂ values, the chromic acid and the chromic acid reduction product and its value Is the weight ratio to Cr conversion value, 0.
An aqueous solution containing 1 to 10: 1 is prepared, the aqueous solution is adhered to the surface of a galvanized steel sheet or an aluminum-zinc alloy-coated steel sheet, and then the steel sheet is 40 to 300
A method for chromate treatment of a galvanized steel sheet, which comprises performing a heat treatment at a temperature of ° C.