JPH03215683A - Method for chromating zinc-plated steel sheet - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for applying an alkali degreasing method to the surface of a galvanized steel sheet or a zinc alloy coated steel sheet, which has excellent corrosion resistance, blackening resistance, fingerprint resistance, and treated appearance. The present invention relates to a chromate treatment method for forming a chromate film with less chromium elution during treatment.

[Conventional technology]

Conventionally, galvanized or zinc alloy-plated steel sheets have been subjected to various chromate treatments for the purpose of rust prevention. Chromate treatment includes anti-Gr chromate, which forms a film mainly composed of trivalent chromium, and coated chromate, in which electrolytic chromate and hexavalent chromium are coated with a chromate solution and dried without washing with water.

Performance items required for chromate-treated steel sheets include white rust resistance, blackening resistance, paint film adhesion, corrosion resistance after mounting, weldability, earthing properties, pressability, fingerprint resistance, etc. depending on the application. very diverse. In addition, after being molded, it passes through an alkaline degreasing process in the surface treatment line, so it is required to have chromium elution resistance and alkali resistance, as well as solvent resistance for solvent cleaning after assembly. Furthermore, when used unpainted, uniformity of appearance is important. In particular, chromate-treated steel sheets for home appliances lose their value as a product if they have an unpleasant color due to C.

Various chromate treatments have been carried out to meet these requirements.

[Problem to be solved by the invention]

However, conventional chromate treatment methods have the following problems.

■ When treating with a chromate treatment solution containing only chromic acid, it is necessary to increase the amount of chromium deposited in order to obtain high corrosion resistance, which makes the coloring of the plating surface noticeable and significantly reduces the commercial value.

■ When treating with a treatment solution containing chromic acid and an etching agent such as fluoride, especially when treating hot-dip galvanized steel sheets, A5 Pb contained in the plating composition can be removed.
Elements such as these are segregated on the plated surface, and as a result of this, uniform etching is not achieved, resulting in an uneven film, there is a limit to the level of corrosion resistance that can be obtained.

(2) It is known from Japanese Patent Publications No. 42-14050, No. 52-2851, and No. 61-58552 that adding silica sol to a chromate solution is effective in improving corrosion resistance. The inventions disclosed in these documents add silica sol to chromic acid or a mixture of chromic acid and a chromate reduction product, but if the amount of chromate reduction product added is small, the chromate film will dissolve. Therefore, a poorly soluble chromate film is formed by increasing the mixing ratio of the chromate reduction product to chromic acid. However, with these chromate treatment methods, it is possible to improve fingerprint resistance etc. by increasing the amount of Cr attached.
The coloring caused by r is noticeable and there is a problem with the appearance of the treatment.

Therefore, an object of the present invention is to provide a chromate treatment method for forming a chromate film that improves the paintability, corrosion resistance, treated appearance, etc. of galvanized steel sheets or zinc alloy-plated steel sheets, and reduces chromium elution due to alkaline degreasing. Our goal is to provide the following.

[Means to solve the problem]

This invention relates to chromic acid, chromic acid reduction products, acrylic emulsions, and one or more types of dry type and one type of wet type.
The main component is a silica sol consisting of Crot or more, and the mixing ratio of the chromic acid and the chromic acid reduction product is Crot.
The converted weight ratio is 1:0. 1-1. 5,
The mixing ratio of the total amount of Cr in the chromic acid and the chromic acid reduction product to the resin solid content in the acrylic emulsion is 1:0.05 to 20 by weight, and the silica sol and the chromium The mixing ratio of the acid and the chromic acid reduction product is a weight ratio of the SiOz equivalent value of the silica sol to the Cr equivalent value of the chromic acid and the chromic acid reduction product, and is 0.1 to:l, and the An aqueous solution containing the wet type silica sol and the dry type silica sol in a weight ratio of 0.5 to 1021 in terms of SiOz is prepared, and the aqueous solution is applied to a galvanized steel sheet or a zinc alloy coated steel sheet. It is characterized in that it is attached to the surface and then the steel plate is heat treated at a temperature of 40 to 300°C.

This invention will be explained in detail below.

■ Chromic acid: Chromic acid is chromic anhydride (Crys) dissolved in water, and has the property of adhering to the surface of the galvanized film and exhibiting a self-repairing effect, thereby preventing white rust on zinc. are doing. Due to this property, chromic acid is essential for significantly improving the rust prevention properties of galvanized steel sheets.
Moreover, it is inexpensive.

The required concentration of chromic acid cannot be specified because it changes depending on the method of adhering the chromate treatment solution to the galvanized steel sheet and the amount of adhesion, but the required concentration nR can be obtained in one step using the adhesion method described below. Chromic acid reduction product: The chromic acid reduction product is obtained by adding a reducing agent to chromic acid to reduce chromic acid. The composition of the chromate solution after reduction is mainly composed of unreduced chromic acid and a product reduced to trivalent chromium, and the mixing ratio thereof is adjusted by the amount of reducing agent added. However, if the amount of the reduced product exceeds 15 mol per 1 mol of unreduced chromic acid, precipitation will occur in the chromate solution, making it difficult to deposit it uniformly on the steel plate.

Conventionally, the main purpose of adding a chromate reduction product to chromic acid was to obtain a poorly soluble chromate film, and the higher the mixing ratio of the chromate reduction product, the higher the degree of polymerization of the chromate film. , this increases the low solubility.

However, if the mixing ratio of the chromic acid reduction product to chromic acid is excessively high, the liquid stability will decrease and the operability in continuous processing will be poor. In this invention, the upper limit of the mixing ratio of chromic acid reduction product to chromic acid is 1. The reason why it was set as 5 was because liquid stability was considered.

Furthermore, the chromate film can be made less soluble by applying heat and drying after adhering the chromate solution.

These heating conditions are determined by the reduction rate of chromic acid, and the lower the reduction rate, the higher the temperature needs to be for heating and drying.

However, if the mixing ratio of the chromic acid reduction product to 1 chromic acid is less than 1:0.1 by weight in terms of CrQ, the degree of polymerization of the chromate film is too low and the heat treatment may cause poorly soluble It is difficult to form a film. In this invention, the lower limit of the mixing ratio of chromic acid reduction product to chromic acid is 0. The reason why it is set as 1 is because it is within the range where it is possible to make it difficult to dissolve by heating.

■ Among the characteristics required for acrylic emulsion chromate treated steel sheets, the appearance of the chromate film is one of the important items. In order to solve this problem, 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 effects of various additives for the purpose of preventing chromium coloration. As a result, acids such as hydrofluoric acid and phosphoric acid,
Alternatively, it has been found that coloring of chromium can be prevented by adding an acrylic emulsion. However, when an acid is added, a problem arises in that the chromate-treated galvanized surface tends to turn black. On the other hand, when an acrylic emulsion, which is a water-based resin, was added, the blackening resistance did not decrease, and the effect of preventing chromium coloring was observed. The reason why we add acrylic emulsion is based on the above results.

In addition, in order for the chromate solution to be able to be applied evenly,
It is necessary for the acrylic emulsion to be stable in the chromic acid solution and not to cause gelation. Generally, the mixing stability of chromic acid and water-based resin is poor and gelation occurs quickly. This problem was stabilized by employing an acrylic emulsion resin that was emulsion polymerized using a nonionic emulsifier.

Even with other water-based resins or the same acrylic resin, when no emulsifier was used or when an ionic emulsifier was used, the mixing stability was poor or gelation occurred immediately.

The chromate treatment liquid is applied to the surface of the steel plate by a method that allows continuous and uniform adhesion. As such methods, spray coating, dipping to roll squeezing, dipping to gas squeezing, roll coating, etc. can be used.

As mentioned above, for the purpose of improving the treated appearance of chromate-treated steel sheets, it is effective to uniformly apply an acrylic emulsion containing a nonionic emulsifier to a chromate solution and heat-dry it. be.

The reason why the addition weight ratio of the resin solid content in the acrylic emulsion to the total Cr amount in the treatment liquid is set to 0.05 or more is that if the addition ratio is less than 0.05, coloring of chromium cannot be sufficiently prevented. In addition, the reason why the addition ratio was set to 20 or less was that even if the addition ratio was increased beyond this, there was no effect on other properties such as coloration prevention effect and corrosion resistance, and the economic efficiency of the chromate solution was taken into consideration. It depends.

■ Wet type silica sol: Next, the reason for adding wet type silica sol will be described.

The purpose of adding wet type silica sol is to improve the corrosion resistance of the chromate film. The inventors of the present invention have conducted intensive research and examination on the relationship between the addition of silica sol and the improvement of the corrosion resistance performance of chromate coatings, and as a result, it has become clear that the type of silica sol added has a particularly large effect. There are two main types of silica sol: wet type and dry type. Both types exhibit the effect of improving corrosion resistance when added to the chromate solution, but in particular,
The effect of wet type silica sol is greater.

■ Dry type silica sol: Next, the reason for adding dry type silica sol will be described.

The purpose of adding dry type silica sol is to improve the fingerprint resistance of the chromate film. However, the effect of adding dry type silica sol was observed only when dry type silica sol was further mixed at a certain ratio into a treatment liquid whose main components were chromic acid, a chromic acid reduction product, an acrylic emulsion, and wet type silica sol. The reason for this is not fully understood, but it is presumed that when the chromate film dries, the dry type silica sol affects the state of aggregation of the film, making the surface state in which optically attached fingerprints are less noticeable.

Therefore, the amounts of wet type silica sol and dry type silica sol added are determined based on the balance between corrosion resistance and fingerprint resistance. That is, in order to obtain good corrosion resistance, the total amount of silicate sol of the dry type and wet type must be 0.0000000000000000000000000000 with respect to the total Cr amount of chromic acid and the chromic acid reduction product.
It is necessary to include one or more. However, if this addition ratio exceeds IO, the film becomes brittle and easily separates. In addition, in order to obtain good fingerprint resistance, the mixing ratio of wet type silica sol to dry type silica sol should be 0.5 to
It should be lO. Outside this range, no effect of adding dry type silica sol is observed.

■ Next, we will discuss the steel plate to which the chromate treatment liquid is applied.

The chromate treatment solution developed in the present invention can be used for hot-dip galvanizing, electrolytic galvanizing, hot-dip Fe-Zn alloy plating, and 5%A
It is possible to apply to zinc-based plated steel sheets such as Zn-based alloy plated steel sheets.

In addition, AI steel plate, AI plated steel plate, 55% A 1
-1. Corrosion resistance and paintability can also be imparted to materials such as 5% Si-Zn plated steel plates, stainless steel plates, and turn-plated steel plates.

■ Next, we will discuss how to apply the chromate treatment liquid to the surface of the steel plate.

The chromate treatment liquid is applied to the surface of the steel plate by a method that allows continuous and uniform adhesion. As such methods, spray coating, dipping to roll squeezing, dipping to gas squeezing, roll coating, etc. can be used.

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

If the heat treatment temperature is less 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 of making the steel less soluble can be expected, and on the contrary, the corrosion resistance deteriorates.

〔Example〕

Next, the present invention will be explained by way of examples.

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

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

Chromate treatment procedure: Hot-dip lead-plated steel plate (manufactured by Sennmar Line, lead deposit amount 120g/rr?) Wash with hot water → Chromate treatment ([1
-ru coating) = heat treatment (40°C to 300°C).

Table 1 shows the composition of the chromate treatment solution, the amount of chromate attached, and the heat treatment temperature of each specimen. The total amount of chromium was measured using fluorescent X-rays and expressed as ■/d.

Next, the corrosion resistance, paintability, poor solubility, and treated appearance of each of the prepared specimens were evaluated using the methods shown below, and the results are also shown in Table 1. On the other hand, salt spray test (JIS Z23
71) was carried out, and the incidence of white rust on the galvanized steel sheet after 200 hours was measured and evaluated.

The evaluation criteria are as follows.

10: No white rust occurred, 8. White rust occurrence area is less than 10%, 6: white rust occurrence area is 10% or more and less than 25%, 4: white rust occurrence area is 25% or more and less than 50%, 1: white rust occurrence area is 50% or more.

Poorly soluble. Each specimen was subjected to alkaline degreasing, the amount of chromium deposited before and after degreasing was measured, and the amount of chromium deposited after degreasing was expressed as a percentage of the amount of chromium deposited before degreasing.

The alkaline degreasing conditions are as follows.

Degreasing liquid: CL-N364S (manufactured by Nihon Parkerizing Co., Ltd.), 20 g/l, 60'C0 Degreasing method:
Spray method, spray pressure 0.5 kg/a/2 minutes.

Paint-tightness (primary) A melamine alkite-based material (
The paint adhesion of a sample spray-painted with Delicon #700 white (manufactured by Dainippon Ink) was evaluated by the degree of peeling of the paint film by tape peeling after Erichsen overhang was performed.

Evaluation criteria for paint adhesion. 0・No peeling, 8. Peeling area less than 10%, 6. Less than 25% detachment, 4: Peeling area less than 50%, Peeling area is 50% or more.

? ? ．． Primary adhesion of paint: Evaluate paint adhesion of paint samples immersed in boiling water for 3 hours.

The materials used in this example are shown below.

Dry type silica sol manufactured by Nippon Aeron [Jnor 200] Wet type silica sol゜Snowtex manufactured by Nissan Chemical ■
0, Treated original sheet: GI: Hot-dip galvanized steel sheet (coating amount 6 per side
0g/of, minimized spangle material) GΔ: Alloyed hot-dip galvanized steel sheet (coating amount 45g/of one side)
m), EG, electrogalvanized steel sheet (coating amount 2 per side
0g/m), An-Zn; 5% kl-Zn alloy plated steel sheet (coating amount 60g/rrf per side). As is clear from Table 1, the material of the present invention shown in specimen No. 13 is excellent in all of corrosion resistance, low solubility, paint adhesion, secondary paint adhesion, and treated appearance.

Regarding dirt, the ratio of chromic acid dichromate reduction products in specimens No. 4 and No. 5 was outside the range of the present invention, and they were inferior in terms of corrosion resistance, paint adhesion, secondary paint adhesion, or treatment liquid stability. ing. In specimens Nos. 016 and 17, the ratio of the total Cr content of the dinonionic acrylic emulsion was outside the range of the present invention, and the emulsions were inferior in poor solubility, secondary paint adhesion, treated appearance, and economical efficiency. In specimens No. 8 and 19, the ratio of total Cr content to wet type silica sol was outside the range of the present invention, and the corrosion resistance or adhesion of the chromate film was poor. In specimens Nos. 20 and 21, the ratio of total Cr content to dry type silica sol was outside the range of the present invention, and the paint adhesion and secondary paint adhesion were poor.

〔Effect of the invention〕

As explained above, according to the chromate treatment method of the present invention, chromate is applied to the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet, which has excellent paintability, corrosion resistance, and treated appearance, and which causes less chromium elution during alkaline degreasing. An industrially useful effect of forming a film can be obtained.

Claims (1)

[Claims] 1 The main component is a silica sol consisting of chromic acid, a chromic acid reduction product, an acrylic emulsion, and one or more types of dry type and one or more types of wet type, and the mixing ratio of the chromic acid and the chromic acid reduction product is CrO_3. The weight ratio in terms of conversion is 1:0.1 to 1.5, and the mixing ratio of the sum of the Cr amounts of the chromic acid and the chromic acid reduction product to the resin solid content in the acrylic emulsion is a weight ratio of 1:0.1 to 1.5. and the mixing ratio of the silica sol to the chromic acid and the chromic acid reduction product is 1:0.05 to 20, the SiO_2 equivalent value of the silica sol and the Cr equivalent value of the chromic acid and the chromic acid reduction product. At a weight ratio of 0.
1 to 10:1, and a mixing ratio of the wet type silica sol and the dry type silica sol in a weight ratio of 0.5 to 10:1 in terms of SiO_2, respectively. A method for chromate treatment of a zinc-based plated steel sheet, which comprises adhering the chromate to the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet, and then heat-treating the steel sheet at a temperature of 40 to 300°C.