JPH03215681A - Method for chromating zinc-plated steel sheet - Google Patents

Abstract

PURPOSE:To form a chromate film excellent in resistance to corrosion and blackening by applying an aq. soln. having specified contents of chromic acid, the reduction product of chromic acid, an acrylic emulsion and a 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 (A), the reduction product of chromic acid (B), an acrylic emulsion (C) and >=1 kind of wet-type silica sol as the essential components and in which the weight ratio of A to B expressed in terms of CrO3 is controlled to 1:(0.1 to 1.5), the weight ratio of the solid resin in C to A plus B expressed in terms of Cr to (0.05 to 20):1 and the weight ratio of D expressed in terms of Si to A plus B expressed in terms of Cr to (0.1 to 10):1 is prepared. The soln. is deposited on a zinc-plated steel sheet, and the sheet is heated at 40-300 deg.C. Consequently, a chromate film excellent in appearance and with the elution of chromium reduced when the acrylic resin is removed is formed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention provides excellent corrosion resistance, blackening resistance, and treated appearance on the surface of galvanized steel sheets or zinc alloy-plated steel sheets, and also prevents chromium elution during alkaline degreasing. The present invention relates to a chromate treatment method for forming a chromate film with a small amount of chromate.

[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 uses reactive chromate and electrolytic chromate, which form a film mainly composed of trivalent chromium, and 6
There is a paint-on type chromate that coats a chromate solution containing valent chromium and dries without washing with water.

[ ] The performance items required for meth-treated steel sheets are white rust resistance, blackening resistance, paint film adhesion, corrosion resistance after painting, weldability, earthing properties, pressability, and fingerprint resistance, depending on the application. Adhesive properties etc. are extremely diverse. Furthermore, since the product passes through an alkali degreasing process in a surface treatment line after being molded, it is required to have chromium elution resistance and alkali resistance, and also to have 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 commercial value if they suffer from significant chromium staining.

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, Af and Pb contained in the plating composition may 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, 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 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, in these chromate treatment methods, if the amount of Cr deposited is increased in order to further improve corrosion resistance, the coloration due to Cr becomes noticeable and the treated appearance deteriorates.

Therefore, an object of the present invention is to provide a chromate treatment for forming a chromate film that improves the corrosion resistance, blackening resistance, treated appearance, etc. of galvanized steel sheets or zinc alloy coated steel sheets, and reduces the elution of chromium caused by alkaline degreasing. The purpose is to provide a method.

[Means to solve the problem]

The present invention comprises chromic acid, a chromic acid reduction product, an acrylic emulditane, and one or more types of wet type silica sol as main components, and the mixing ratio of the chromic acid, the chromic acid reduction product, and the acid reduction product are respectively The weight ratio in terms of CrOz is 1:O. 1 to 1.5, the mixing ratio of the acrylic emulsion to the chromic acid and the chromic acid reduction product is Cr of the total amount of the resin solid content and the chromic acid and the chromic acid reduction product in the acrylic emulsion. The weight ratio with the converted value is 0.05 to 20:1, and the mixing ratio of the wet type silica sol to the chromic acid and the chromic acid reduction product is S1.
02 equivalent value, and the weight ratio of the value to the Cr equivalent value of the chromic acid and the chromic acid reduction product is 0, 1 to 10:
The method is characterized in that an aqueous solution containing within the range of 1 is prepared, the aqueous solution is applied to the surface of a galvanized steel sheet or a zinc alloy-plated steel sheet, and then the steel sheet is heat-treated at a temperature of 40 to 300 ° C. It is.

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 cannot be determined because it varies depending on the method and amount of adhesion of the chromate treatment solution to the galvanized steel sheet, but it is possible to obtain the required amount of adhesion in one step using the adhesion method described below. (2) 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, for 1 mole of unreduced chromic acid, the reduced product is 1. 5
If the amount exceeds the molar amount, precipitation will occur in the form of a chromate solution, making it difficult to deposit it uniformly on a steel plate.

Conventionally, the main purpose of adding a chromic acid reduction product to chromic acid was to obtain a poorly soluble 6-chromate film, and the higher the mixing ratio of the chromic acid 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 the chromic acid reduction product to chromic acid is set at 1.5 in consideration of liquid stability.

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 chromate reduction product to chromate l is less than 1°0.1 in terms of weight ratio in terms of Cr03, the degree of polymerization of the chromate film is too low and the heat treatment produces a poorly soluble film. It is difficult to In the present invention, the lower limit of the mixing ratio of the chromic acid reduction product to chromic acid is set to 0.1 because it is within the range where it is possible to make the mixture hardly soluble by heating.

■ Acrylic emulsion: Among the characteristics required for 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,
Alternatively, it has been found that coloring of chromium can be prevented by adding an acrylic emulsion, which is a water-based resin.

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 acrylic emulsion 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 kelization 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 solids contained in the acrylic emulsion to the total amount of Cr in the treatment liquid is set to be 0.05 or more is that if the addition ratio is less than 0.05, coloring of chromium cannot be sufficiently prevented. The reason why the ratio was set to 20 or less was that even if the addition ratio was increased beyond this, no effect on other properties such as anti-coloration effect or corrosion resistance was observed, and we considered the economic efficiency of the chromate solution. .

■ Wet type silica sol: Next, we will discuss the reason for adding wet type silica sol. The first 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
Wet type silica sol is particularly effective.

The second purpose of adding wet type silica sol is to improve the appearance of the treated product. As mentioned above, by mixing the acrylic emulsion with the chromate solution, it is possible to prevent coloring due to chromium, but the chromate film that is formed causes an interference pattern, which causes a new problem in terms of the appearance of the process. It has become clear that by further adding a wet type silica sol, it is possible to obtain the effect of eliminating interference patterns while preventing coloration due to chromium.

As mentioned above, the purpose of adding wet-type silica sol is to improve corrosion resistance and improve the treated appearance.The amount of wet-type silica sol added to obtain these effects is 0.000 to 100% of the total Cr amount l. 1-10 is suitable. 0 per total Cr amount 1. If it is less than 1, no effect on corrosion resistance or elimination of interference patterns on the film will be observed, and if it is added in excess of 10, no further effect will be 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 applied to hot-dip galvanizing, electrogalvanizing, hot-dip Fe-Zn alloy plating, zinc-based plated steel sheets such as 59A A1-Zn alloy-plated steel sheets.

Furthermore, Al@@, AI plated steel plate, 55% A f
-1. Corrosion resistance can also be improved for 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, a greater effect on making the steel less soluble can be expected, but on the contrary, corrosion resistance will deteriorate.

〔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/l 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 No. 1 to IO of the present invention and comparative specimens Mail to 19.

Chromate treatment procedure: Hot-dip galvanized steel sheet (manufactured by Sendzimer line, zinc coating amount 120g/rrf) → hot water wash → chromate treatment (roll coating) → heat treatment (40℃ ~ 300℃)
℃).

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

Next, the corrosion resistance, poor solubility, treated appearance, and blackening resistance of each of the prepared specimens were evaluated by the methods shown below, and the results are also shown in Table 1.

Each corrosion-resistant specimen was subjected to a 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 less than 10%, 6 White rust occurrence area 10% or more and less than 25%, 4 White rust occurrence vI 25% or more and less than 50%, 1. White rust occurrence area 50%
that's all.

Poorly soluble: Each specimen was subjected to alkaline degreasing, and chlorine I before and after degreasing.
- The amount of chromium adhesion was measured and the amount of chromium adhesion after degreasing was expressed as a percentage of the amount of chromium adhesion before degreasing.

The alkaline degreasing conditions are as follows.

Degreasing liquid: CL-N364S (manufactured by Nihon Parkerizing Co., Ltd.), 20 g/j2, 60°C.

Degreasing method: Spray method, spray pressure 0.5kg/aJ2
minutes.

Resistance to blackening: The samples were stored in a stacked state for 240 hours in an atmosphere of 50° C. and 90% RH to promote blackening of the plated steel sheets, and each sample was evaluated based on the degree of blackening.

The evaluation criteria are as follows.

5: No black change observed. 4: Slight black discoloration, but at a level that does not pose a problem. , 3. Slightly turning black, 2: The color has changed considerably, 1: There is a considerable black change over the entire surface.

The materials used in this example are shown below.

Dry type Norikasol: Aeronol 200 manufactured by Nippon Aeronle, Wet type Norikasol: Snow Tenotasu 0 manufactured by Nissan Chemical ■; Treated base plate/G; Hot-dip galvanized steel sheet (coating amount uniformly 6)
0g/m, minimized spangle material) GA: Alloyed hot-dip galvanized steel sheet (with plating a Gf 45g per side
/d), T'. G. Electrogalvanized steel sheet (coated weight: 20 g/m per piece), Δ1-Zn; 5% AA-Zn alloy plated steel sheet (coated weight: 60 g/m per piece).

As is clear from Table I, the materials of the present invention shown in specimens No. 1 to 1O are excellent in all of corrosion resistance, low solubility, blackening resistance, and treated appearance. In contrast, the specimens Noll and N
In ol2, chromic acid. The ratio of the chromic acid reduction product is outside the range of the present invention, and the composition is poor in poor solubility, corrosion resistance, and stability of the processing solution. In specimens No. 3 and 16,
The ratio of total Cr content to acrylic emulsion is outside the scope of the present invention, and the treated appearance or economic efficiency is poor. In specimens No. 4 and 15, although the treated appearance was improved by additives other than the acrylic emulsion, the blackening resistance was poor. In specimens No. 7 and 18, the ratio of total Cr content to wet type silica sol was outside the range of the present invention, and the treated appearance and economic efficiency were poor. In specimen No. 9, since dry type silica sol was added instead of wet type silica sol, it was inferior in terms of corrosion resistance to specimen No. 2, which had the same composition ratio and added wet type silica sol.

〔Effect of the invention〕

As explained above, the chromate treatment method of the present invention provides excellent corrosion resistance, blackening resistance, and treated appearance on the surface of a galvanized steel sheet or zinc alloy coated steel sheet, and
The industrially useful effect of forming a chromate film with little chromium elution during alkaline degreasing can be obtained.

Claims (1)

[Claims] 1 The main components are chromic acid, a chromic acid reduction product, an acrylic emulsion, and one or more types of wet type silica sol, and the mixing ratio of the chromic acid and the chromic acid reduction product is a weight ratio in terms of CrO_3, 1:0.1
~1.5, the mixing ratio of the acrylic emulsion, the chromic acid, and the chromic acid reduction product is based on the Cr equivalent of the total amount of the resin solid content, the chromic acid, and the chromic acid reduction product in the acrylic emulsion. The weight ratio of the wet type silica sol to the chromic acid and the chromic acid reduction product is 0.05 to 20:1, and the wet type silica sol is a SiO_2 equivalent value, and the weight ratio of the wet type silica sol and the chromic acid is 0.05 to 20:1. The acid and the chromate reduction product are Cr
An aqueous solution containing a weight ratio of 0.1 to 10:1 to the converted value is prepared, the aqueous solution is applied to the surface of a galvanized steel sheet or a zinc alloy plated steel sheet, and then the steel sheet is A method for chromate treatment of zinc-based plated steel sheet, characterized by heat treatment at a temperature of 300°C.