JPH03215680A - Method for chromating zinc-plated steel sheet - Google Patents

Abstract

PURPOSE:To form a chromate film excellent in corrosion resistance and suitability for coating by applying an org. soln. having specified contents of chromic acid, the reduction product of chromic acid, dry-type silica sol and wet-type silica sol on a zinc-plated steel sheet and heating the sheet at a specified temp. CONSTITUTION:An aq. soln. contg. chromic acid, the reduction product of chromic acid, >=1 kind of dry-type silica sol and >=1 kind of wet-type silica sol as the essential components and in which the weight ratio of the chromic acid to its reduction product respectively expressed in terms of CrO3 is controlled to 1:(0.10 to 1.5), the weight ratio of the total Cr in the chromic acid and its reduction product to the silica sol expressed in terms of SiO2 to 1:(0.5 to 1.0) and the weight ratio of the dry-type silica sol to wet-type silica sol expressed in terms of SiO2 to 1:(0.05 to 20) is prepared. The aq. soln. is deposited on the surface of a zinc-plated steel sheet or a zinc alloy-plated steel sheet, and the sheet is heated at 40-300 deg.C. Consequently, a hardly soluble chromate film low in electric resistance and high in conductivity is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention provides excellent corrosion resistance, paintability, low solubility, and electrical resistance on the surface of galvanized steel sheets or zinc alloy-plated steel sheets. The present invention relates to a chromate treatment method for forming a chromate film with low conductivity and excellent electrical conductivity.

[Conventional technology]

Chromate treatment is performed as a primary rust prevention method for zinc-based plated steel sheets, and paint is further applied to the chromate-treated steel sheets for the purpose of rust prevention. On the other hand, in recent years, users such as manufacturers of home appliances and building materials have omitted such coating for the purpose of rust prevention, and have often used galvanized steel sheets themselves as they are after processing. For this reason, a chromate treatment method that forms a chromate film with excellent corrosion resistance that can withstand use even in an unpainted state is desired. However, conventional chromate treatment methods have the following problems. There is.

■ 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 causes the plating surface to become more noticeable and the commercial value to decrease significantly. .

■ When treating with a treatment solution containing chromic acid and an ethosing agent such as fluoride, especially when treating hot-dip galvanized steel sheets, Δffi,
Elements such as Pb are segregated on the plating surface, and due to their influence, uniform etching is not achieved, resulting in an uneven film, so 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, Japanese Patent Publication No. 52-2851, Japanese Patent Publication No. 61-58552, etc. that the addition of silica sol to a chromate solution is effective in improving corrosion resistance. The inventions disclosed in these publications involve adding a solitary sol to chromic acid or a mixture of chromic acid and a chromic acid reduction product, but if the amount of the chromic acid reduction product added is small, the chromate film will not form. Because of this problem of easy dissolution, an N-soluble chromate film is formed by increasing the mixing ratio of the chromic acid reduction product to chromic acid.

[Problem to be solved by the invention]

In order to use galvanized steel sheets as part of home appliances, the surface of the steel sheet must be electrically conductive so that it can be grounded. Furthermore, this current conduction property is known to be effective for weldability such as spot welding, and can be said to be one of the main properties of surface-treated steel sheets.

However, in the conventional chromate treatment method, since the mixing ratio of the chromic acid reduction product to chromic acid is large, the electrical resistance of the chromate film is high, and moreover, only a film with poor electrical conductivity can be obtained.

On the other hand, if the amount of chromate film deposited is reduced, the electrical conductivity will be improved, but the desired corrosion resistance performance will not be obtained.

Furthermore, if the mixing ratio of the chromic acid reduction product to chromic acid is made small, a poorly soluble chromate film cannot be obtained as described above.

As described above, with conventional chromate treatment methods, it has been difficult to form a highly corrosion-resistant chromate film that is poorly soluble and maintains electrical conductivity.

Therefore, an object of the present invention is to form a film on the surface of a galvanized steel sheet that has excellent corrosion resistance and paintability, is hardly soluble, has low electrical resistance, and has excellent electrical conductivity. An object of the present invention is to provide a chromate treatment method.

[Means to solve the problem]

This invention has a silica sol consisting of chloro, acid, a chromic acid reduction product, and one or more types of dry type and one or more types of wet type -F as a main component, and the chromic acid and the chromic acid reduction product. The mixing ratio is a weight ratio based on Cr01 conversion, ]: 0.10 to 1.5 The mixing ratio of Cri, which is the sum of the chromic acid and the chromic acid reduction product, and the 8102 equivalent value of the silica sol. , in weight ratio, 1.
05 to 10, the mixing ratio of the dry type silica sol and the wet type silica sol is 1:0 by weight in terms of SiO2. 0 to 20, the aqueous solution is applied to the surface of a galvanized steel plate or a zinc alloy plated steel plate, and the steel plate is then heat-treated at a temperature of 40 to 300°C. It has characteristics.

This invention will be explained in detail below.

■ Chromic acid: Chromic acid is chromic acid anhydride (CrO,) dissolved in water, and it adheres to the surface of the zinc plating film and exhibits a self-repairing effect, and has the property of preventing white rust on zinc. have. 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 varies depending on the method and amount of adhesion of the chromate treatment solution to the galvanized steel sheet, so it may or may not be possible to specify it. Chromic acid reduction product: The chromic acid reduction product is obtained by adding a reducing agent to chromic acid to reduce chromic acid. The chromate 1& composition 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, for each mole of unreduced chromic acid, the reduced product is 1.5
If the amount exceeds the molar amount, precipitation will occur in the state of the chromate solution, making it difficult to deposit it uniformly on the steel plate.

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

However, in such a method, applying heat to the chromate film causes excessive polymerization of the film and reduces corrosion resistance, so that drying can only be carried out to the extent that water adhering to the chromate film is blown off.

Further, in the case of a chromate film in which the chromate reduction product is mixed to an extent suitable for the purpose, the electrical resistance of the film becomes large and becomes an insulating film, which is disadvantageous. Effective methods for reducing the insulation properties of the film include reducing the amount of silica sol, which is the main component of the chromate film, or reducing the amount of chromate film deposited, but both of these methods reduce the corrosion resistance performance. This is not the preferred method as it will result in

In the present invention, the mixing ratio of the chromic acid reduction product in the chromate treatment solution is 1 to 1. The reason why it is set to 5 or less is to lower the electrical resistance of the chromate film to be formed and to make the film excellent in electrical conductivity.

In the chromate treatment method of the present invention, after the chromate treatment liquid is attached to the surface of the steel plate, the heat treatment temperature is 40 to 3
The purpose of heat treatment in the range of 00°C is to make the chromate film containing a large amount of soluble chromic acid difficult to dissolve. The temperature conditions for this heat treatment are determined by the reduction rate of chromic acid.
The lower the reduction rate, the higher the temperature needs to be treated.

However, the mixing ratio of chromic acid reduction product to chromic acid 1 is 1:0.
If it is less than 1, the degree of polymerization of the chromate film is too low, making it difficult to form a poorly soluble film by heat treatment. On the other hand, the mixing ratio of the chromic acid reduction product to chromic acid 1 was 1:1 by weight. If it exceeds 5, the chromate film will polymerize excessively, resulting in a decrease in the self-repairing action of hexavalent chromium or the occurrence of cracks due to the chromate film, resulting in a decrease in corrosion resistance.

Therefore, in order to obtain a chromate film with excellent electrical conductivity, low solubility, and high corrosion resistance, the mixing ratio of the chromate reduction product to chromic acid 1 should be 1:0. 10-1. 5
It should be within the range of .

In the present invention, the reducing agents used for reducing chromic acid include organic substances that are inexpensive and leave almost no decomposition products, such as sugar syrup, methanol, isobrobyl alcohol, and inorganic substances, such as hydrogen peroxide. can be used. Further, it is also possible to use an intermediate produced when these reducing agents and chromic acid react as the reducing agent.

FIG. 1 is a graph showing the results of analyzing a liquid mixture of chromic acid and a chromic acid reduction product using a thermogravimetric analysis-scanning differential calorimeter (TGDSC). NO shown in Figure 1
The mixed solution of I and 2 has a mixing ratio of chromic acid reduction product to chromic acid 1 of 0.33, 0.70, which is within the range of the present invention.
No. 3 is 0.00 (excluding), which is outside the scope of the present invention.

No. 3, which does not contain a chromic acid reduction product, does not lose mass even when subjected to heat treatment and only exhibits a melting reaction at 198°C.

On the other hand, with NO2 in which the mixing ratio of the chromic acid reduction product is within the range of the present invention, mass reduction and endothermic reactions occur from temperatures below 100°C, indicating that dehydration reactions easily occur.

Also, the mixing ratio of the chromic acid reduction product to chromic acid is within the scope of the present invention]: 0. 3 In chromate solution No. 3, rapid mass loss and endothermic reaction both occur at 140 to 180°C, and a dehydration condensation film can be formed by applying an appropriate heat treatment depending on the chromium reduction rate. I understand.

■　Nrikazol・ Next, the reason for adding Nricazol will be described.

The purpose of adding silica sol is to improve the corrosion resistance and paintability of the chromate film. The present inventors have conducted intensive research and examination on the relationship between the addition of chromate sol and the improvement of the corrosion resistance performance of the chromate film.As a result, the extent of the effect of improving the corrosion resistance performance and paintability of the chromate film is
It has been found that the amount and type of sol added determines the amount of chromic acid and the chromic acid reduction product.
The mixing ratio of the SiO2 equivalent value of the silica sol to rn is 1.05 to 10 by weight, and the silica sol to be added is composed of two or more types: one or more types of dry type typon rica sol and one or more types of wet type lica sol. The mixing ratio of type and wet type is 1.0 by weight based on SiO2.
．． It is necessary to carry out the chromate treatment using a chromate treatment solution having a concentration within the range of 0.05 to 20.

The content of silica sol is at a mixing ratio of I:0. If it is less than 5, corrosion resistance performance that meets the objective cannot be obtained.

On the other hand, even if silica sol is contained in a ratio exceeding 1:10, no improvement in corrosion resistance is observed, and moreover, the adhesion between the chromate film and the plating film decreases, and the chromate film easily peels off. Therefore, it is undesirable.

Further, the mixing ratio of silica sol is 1:0. Even within the range of 5 to 10, satisfactory performance can be obtained in terms of bare corrosion resistance if only the warping sol type or dry type is used, and in terms of paintability if only the wet type is used. do not have. Further, even if one or more dry type silica sol and one or more wet type silica sol are mixed, the weight ratio of dry type silica sol and wet type silica sol will be 10. If it is outside the range of 0.05 to 20, the characteristics of each warp sol cannot be fully exhibited.

The reason for this is not clear, but it is presumed that there is an optimal mixing ratio for preferentially concentrating on the surface of the chromate film compared to dry type silica sol or wet type silica sol, which has a large effect on improving paintability. , describes a method for applying a chromate treatment liquid to the surface of a 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 a method, 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 can be expected in making the steel less soluble, and on the contrary, corrosion resistance deteriorates.

■ 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
j! - It is possible to apply to zinc-based plated steel sheets such as Zn-based alloy plated steel sheets.

Furthermore, Al steel plate, Al plated steel plate, 55%Aβ-
Corrosion resistance or paintability can also be imparted to materials such as 1.5% Si-Zn plated steel plates, stainless steel plates, and turn-plated steel plates.

〔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/jl' of chromic anhydride. After the addition, stirring was performed for 2 hours at a liquid temperature of 90 to 95°C to reduce the chromic acid portion.

Next, a chromate treatment solution was prepared by adding the sori sol to the mixed solution of chromic acid and the chromic acid reduction product. 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 specimen No. 11 of the present invention and comparative specimens No. 12 to 18.

Chromate treatment procedure: Hot-dip galvanized steel sheet (manufactured by Sendzimer line, zinc coating amount 120g/resistant) - Wash with hot water → Chromate treatment (roll coating) - Heat treatment (60°C to 200°C).

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

Next, the corrosion resistance, paintability, poor solubility, and electrical conductivity (film resistance) of each of the prepared specimens were evaluated by the methods shown below, and the results are also shown in Table 1.

Corrosion resistance - Salt spray test (JIS 223
71) was carried out, and the incidence of white rust on the galvanized steel sheet after 400 hours was measured and evaluated.

The evaluation criteria are as follows.

lO: No white rust occurrence, 8: White rust occurrence area less than 10%, 6: White rust occurrence area 10% or more and less than 25%, 4: White rust occurrence area 25% or more and less than 50%, l: White rust occurrence area 50 %that's all.

Poor Solubility: 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/f, 60°C.

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

Electrical conductivity (film resistance): Measured using an interlayer resistance measuring device, and the measurement results are expressed in Ω.

Paint adhesion: Melamine alkyd paint (
After spray painting Delicon #700 (white) manufactured by Dainippon Ink Co., Ltd., the sample was immersed in boiling water for 3 hours, processed by Erichsen stretch, and evaluated by the degree of paint film peeling by tape peeling.

The evaluation criteria are as follows.

lO: no peeling; 8: Peeling area less than 10%, 6: Peeling area less than 25%, 4: Peeling area less than 50%, l: Peeling area 50% or more.

The degree of blistering that occurred after immersion in boiling water was also observed.

The materials used in this example are shown below.

Dry type silica sol 2 Aerosil 200 manufactured by Nippon Aerosil ■ Wet type silica sol 2 Snowtex 10 manufactured by Nissan Chemical ■ Processed original plate: GI: Hot-dip galvanized steel sheet (coating amount 6 per side
0 g/ttr, minimized spangle material), G
A. Alloyed hot-dip galvanized steel sheet (coating amount 45g/rd per side), EG. Electrogalvanized steel sheet (coating amount: 2 per side
0 g/rd), Af-Zn; 5% Af-Zn alloy coated steel sheet (coating amount 60 g/rd per side), as is clear from Table 1, the present invention material shown in specimens No. Corrosion resistance,
Excellent in low solubility, electrical conductivity, and paintability. On the other hand, in specimens No. 2 and No. 3, the ratio of the chromate dichromate reduction product was outside the range of the present invention, and they were poor in poor solubility, paint adhesion, or electrical conductivity.

Specimens No. 4, 15, and 16 had a total chromium:total silica sol ratio outside the range of the present invention, and were inferior in corrosion resistance, paint adhesion, and chromate film tightness. In specimens Nos. 7 and 18, the dry type Noricasol wet type/Ricasol C ratio was outside the scope of the present invention, and they were inferior in terms of corrosion resistance or paint system adhesion.

〔Effect of the invention〕

As explained above, according to the chromate treatment method of the present invention, the surface of a galvanized steel sheet or a zinc alloy coated steel sheet has excellent corrosion resistance, paintability, is hardly soluble, and has low electrical resistance. The industrially useful effect of forming a chromate film with excellent electrical conductivity can be obtained.

[Brief explanation of drawings]

FIG. 1 is a graph showing the results of analyzing a liquid mixture of chromic acid and a chromic acid reduction product using a thermogravimetric scanning differential calorimeter (TGDSC).

Claims (1)

[Claims] 1 The main component is chromic acid, a chromic acid reduction product, and a silica sol consisting of one or more dry types and one or more wet types, and the mixing ratio of the chromic acid and the chromic acid reduction product is calculated as CrO_3. The weight ratio is 1:
0.10 to 1.5, the total amount of Cr of the chromic acid and the chromic acid reduction product and the SiO_ of the silica sol
The mixing ratio with the converted value is 1:0.5 to 10 by weight,
The mixing ratio of the dry type silica sol and the wet type silica sol is 1:0 in terms of weight ratio in terms of SiO_2.
．． 05 to 20, the aqueous solution is applied to the surface of a galvanized steel plate or a zinc alloy plated steel plate, and then the steel plate is heat-treated at a temperature of 40 to 300°C. Chromate treatment method for galvanized steel sheets.