JPH0735588B2 - Chromate treatment method for hot dip galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention provides a chromate film having excellent corrosion resistance, poor solubility, low electrical resistance and excellent electrical conductivity on the surface of a galvanized steel sheet. The present invention relates to a chromate treatment method for forming.

[Conventional technology]

Chromate treatment is performed as a primary anticorrosion method for steel sheets with hot-dip galvanized steel, and coating is applied on the chromate-treated steel sheets for the purpose of preventing rust. On the other hand, in recent years, users such as home electric appliances and building material manufacturers often omit such coating for the purpose of rust prevention and use the zinc plated steel plate itself as it is after processing. Therefore, there is a demand for a chromate treatment method for forming a chromate film having excellent corrosion resistance enough to withstand use even in an unpainted state.

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

When treating with a chromate treatment liquid containing only chromic acid, it is necessary to increase the amount of chromium deposited in order to obtain high corrosion resistance, and this causes conspicuous coloring on the plating surface, resulting in a marked decrease in commercial value. .

When treating with a treatment liquid in which an etching agent such as fluoride is added to chromic acid, especially when treating a hot-dip galvanized steel sheet, elements such as Al and Pb contained in the galling composition are plated surface. Segregation occurs, and due to this effect, uniform etching is not performed and a non-uniform film is formed, so the level of corrosion resistance obtained is limited.

The fact that the addition of silica sol to the chromate solution is effective for improving the corrosion resistance is shown in Japanese Examined Patent Publication Nos. 42-14050 and 52-2.
It is known from literatures such as 851 and Japanese Patent Publication No. 61-58552. In the inventions disclosed in these, silica sol is added to chromic acid or a mixture of chromic acid and a chromic acid reduction product.However, when the addition amount of the chromic acid reduction product is small, the chromate film is not dissolved. There is a problem that it is easy. Therefore, by increasing the mixing ratio of the chromic acid reduction product to chromic acid, a poorly soluble chromate film is formed.

[Problems to be solved by the invention]

In order for the zinc plated steel sheet to be used for a part of home electric appliances, it is necessary to make the surface of the steel sheet electrically conductive so that it can be grounded. Further, it is known that this energization characteristic is also effective for weldability such as spot welding, and can be said to be one of the main characteristics of the surface-treated steel sheet.

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

On the other hand, if the amount of chromate film adhered is reduced, the electrical conductivity is improved, but the corrosion resistance performance suitable for the purpose cannot be obtained.

Further, if the mixing ratio of the chromic acid reduction product to chromic acid is made small, it is impossible to obtain the hardly soluble chromate film as described above.

As described above, with the conventional chromate treatment method, it was difficult to form a highly corrosion-resistant chromate film having poor solubility and electrical conductivity.

Therefore, an object of the present invention is to provide a chromate treatment capable of forming a film having excellent corrosion resistance, sparingly soluble and small electric resistance, and having excellent electrical conductivity on the surface of a galvanized steel sheet. To provide a method.

[Means for Solving the Problems]

This invention, chromic acid,
Chromic acid reduction product, silica sol with an average particle size of 20 mμ or less, and one or more metal ions as main components from Mg, Ca, Zn, Co, Sr carbonates, oxides or hydroxides And, and the mixing ratio of the chromic acid and the chromic acid reduction product are each a weight ratio in terms of CrO ₃ , 1: 0.10 to 0.55, the total amount of chromic acid and the chromic acid reduction product and the mixture with silica sol. The total amount of chromic acid and chromic acid reduction products is Cr
In terms of O ₃ , silica sol is 1: 1 to 1 by weight ratio in terms of SiO _2.
0, the total amount of chromic acid and chromic acid reduction products and the mixing ratio of metal ions is chromic acid and the total amount of chromic acid reduction products in terms of CrO ₃ , molar ratio with metal ions 1: 0.05 ~.
It is characterized in that an aqueous solution contained within the range of 0.50 is attached, and then the steel sheet is heat-treated at a temperature of 100 to 300 ° C.

Hereinafter, the present invention will be described in detail.

Chromate treatment method of the present invention, chromic acid, chromic acid reduction product, silica having an average particle size of 20 mμ or less, and Mg,
An aqueous solution containing at least one metal ion selected from carbonates, oxides or hydroxides of Ca, Zn, Co and Sr as a main component is used as a chromate treatment liquid.

Chromic acid: Chromic acid is a solution of chromic anhydride (CrO ₃ ) dissolved in water and has the property of adhering to the surface of the zinc plating film and exerting a self-repairing action, preventing white rusting of zinc. Have By virtue of this property, chromic acid is indispensable for significantly improving the rust preventive property of a zinc plated steel sheet and is inexpensive.

The required concentration of chromic acid cannot be specified because it changes depending on the method and amount of adhesion of the chromate treatment solution to the zinc plated steel sheet, but the required amount of adhesion is the It is preferable that the concentration be obtained by the process.

Chromic Acid Reduction Product: 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 the reduction is mainly composed of unreduced chromic acid and the product reduced to trivalent chromium, and the mixing ratio thereof is adjusted by the addition amount of the reducing agent. However, if the amount of the reduced product exceeds 1 mol with respect to 1 mol of unreduced chromic acid, precipitation will occur in the state of the chromate solution, and it will be difficult to uniformly adhere the product onto the steel sheet.

Conventionally, the purpose of adding a chromic acid reduction product to chromic acid is to obtain a poorly soluble chromate film.The higher the mixing ratio of the chromic acid reduction product, the higher the degree of polymerization of the chromate film. However, this increases the poor solubility.

However, in such a method, heat is applied to the chromate film, whereby the film is excessively polymerized and corrosion resistance is deteriorated. Therefore, only drying to remove water adhering to the chromate film has been possible.

Further, in the case of the chromate film in which the chromic acid reduction product is mixed to such an extent as described above, the electric resistance of the film becomes large, which is an insulating film, which is inconvenient. As a method of reducing the insulating property of the coating, it is effective to reduce the addition amount of silica sol, which is the main component of the chromate coating, or to reduce the amount of chromate coating attached, but both of them reduce the corrosion resistance performance. This is not the preferred method because it will occur.

In the present invention, the reason for setting the mixing ratio of the reduction product of chromic acid in the chromate treatment liquid to 0.55 or less relative to chromic acid 1 is to reduce the electric resistance of the formed chromate film and to make the film excellent in electrical conductivity. Is.

In the chromate treatment method of the present invention, after the chromate treatment liquid is attached to the steel sheet surface, the heat treatment temperature is 100 to 300.
The purpose of heat treatment in the range of ° C is to make the chromate film that does not contain much chromic acid reduction product insoluble. The temperature condition of this heat treatment is determined by the reduction rate of chromic acid, and the lower the reduction rate, the higher the temperature must be.

However, when the mixing ratio of the chromic acid reduction product to chromic acid 1 is less than 1: 0.1 in terms of weight ratio in terms of Cro ₃ , the degree of polymerization of the chromate film is too low, and thus the heat treatment makes it difficult to dissolve. It is difficult to form a film. on the other hand,
When the mixing ratio of the chromic acid reduction product to chromic acid is 1: 0.6 or more by weight, the chromate film may be excessively polymerized and the self-repairing action of hexavalent chromium may be reduced, or the chromate film may be cracked. Therefore, the corrosion resistance decreases.

Therefore, in order to obtain a chromate film having excellent electrical conductivity and poor solubility and high corrosion resistance, the mixing ratio of the chromic acid reduction product to chromic acid 1 should be within the range of 1: 0.10 to 0.55. .

In the present invention, as the reducing agent used for the reduction of chromic acid, an organic substance which is inexpensive and has almost no decomposition products left, for example, sucrose, methanol, isopropyl alcohol, and an inorganic substance such as hydrogen peroxide are used. it can. It is also possible to use an intermediate produced when these reducing agents react with chromic acid as a reducing agent.

In the reduction of chromic acid, as the mixing ratio of the chromic acid reduction product increases, the liquid changes from strong acidity to weak acidity, and thus does not easily react. Therefore, depending on the type of the reducing agent or the reduction rate of the chromate-treated solution, a part of the unreacted reducing agent in the chromate-treated solution or a potentially reducing agent has reacted with the chromic acid. Since intermediates that sometimes occur are present in the chromate treatment liquid, a reduction reaction occurs during the heat treatment of the chromate film, which results in a film with a higher mixing ratio of chromic acid reduction products than the target, resulting in a film with poor electrical conductivity. .

Therefore, in order to prevent such a state, it is necessary to select, as the reducing agent, one that leaves almost no decomposition products in the reduced chromate treatment liquid.

Thus, the selection of a reducing agent that does not leave decomposition products is possible when the mixing ratio of chromic acid reduction product to chromic acid is 0.55 or less. On the other hand, when the mixing ratio of the chromic acid reduction product to chromic acid is 0.60 or more, the reduction reaction does not easily proceed and the reaction intermediate of the reducing agent or the unreacted reducing agent itself remains.

Therefore, the mixing ratio of the chromic acid reduction product to the chromic acid 1 should be 0.55 or less so that the decomposition product of the reducing agent is not contained in the film.

FIG. 1 is a graph showing the results of analysis of a liquid obtained by mixing chromic acid and a chromic acid reduction product by a thermogravimetric analysis-scanning differential calorimeter (TG-DSC). The No. 1 mixed solution shown in FIG. 1 has a mixing ratio of the chromic acid reduction product to the chromic acid of 0.33 within the range of the present invention, and the No. 2 mixed solution is 0.70 outside the range of the present invention. , No. 3 is 0.00 (not included).

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

No. 2 in which the mixing ratio of the chromic acid reduction product is outside the range of the present invention is
It can be seen that the mass reduction and the endothermic reaction occur from a temperature of 100 ° C. or less, and the dehydration reaction easily occurs.

On the other hand, the chromate solution N in which the mixing ratio of the chromic acid reduction product to chromic acid is 1: 0.33 within the scope of the present invention.
At o.1, both rapid mass loss and endothermic reaction were 140
It occurs at from ℃ to 180 ℃, it can be seen that the dehydration condensation film is formed by performing an appropriate heat treatment.

Silica sol: Next, the reason for adding silica sol will be described.

The purpose of adding silica sol is to improve the corrosion resistance of the chromate film. The present inventors have earnestly studied and investigated the relationship between the addition of silica sol and the improvement of the corrosion resistance of the chromate film, and as a result, the degree of the effect of improving the corrosion resistance of the chromate film is determined by the addition amount of silica sol and the average particle size. We have found that it depends on the diameter.

That is, in order to obtain a preferable corrosion resistance performance, the mixing ratio of silica sol to the total amount of chromic acid and chromic acid reduction products is a CrO ₃ conversion weight ratio, and a SiO ₂ conversion weight ratio.
1: 1 to 10, and the average particle size of the silica sol to be added is 2
It is necessary to perform chromate treatment with a chromate treatment liquid having a particle size of 0 mμ or less.

If the content of the silica sol is less than 1: 1 in the mixing ratio, the corrosion resistance performance that meets the purpose cannot be obtained. On the other hand, even if the silica sol is contained in excess of 1:10, the corrosion resistance is not improved, and the adhesion between the chromate film and the plating film is reduced, and the chromate film is easily peeled off, which is not preferable. .

Even if the mixing ratio of the silica sol is in the range of 1: 1 to 10, if the average particle diameter of the silica sol exceeds 20 mμ and becomes 30 to 40 mμ or more, the corrosion resistance performance is significantly deteriorated.
The reason for this is not clear, but the pore size of the chromate film increases due to the increase in particle size of the silica sol,
Or, it is presumed that the number increases and the barrier property of the chromate film deteriorates.

Metal Ions: Next, the reason for adding metal ions will be described.

Even if the mixing ratio of the silica sol is within the above range, the chromate film having a higher mixing ratio of the silica sol has a lower solubility. In the dissolution of the chromate film, chromium is preferentially generated over silica, and it is considered that chromic acid, which is hexavalent chromium, is the main one. Therefore, it is necessary to form a hardly soluble chromate film without impairing the anticorrosive effect of chromic acid and the electrical conductivity. Addition of metal ions is effective as a means for solving this problem.

In the results of the study conducted by the inventors, the additional elements that have been found to have a particularly remarkable effect are Mg, Ca, Zn, Co, and Sr, and even if one or more of these elements are mixed, the same effect can be obtained. The effect is obtained.

In order to obtain such a preferable hardly soluble film, the mixing ratio of metal ions to chromium, the total amount of chromate acid and chromic acid reduction products in terms of CrO ₃ , is 1: 0.05 in molar ratio with metal ions. It should be in the range of ~ 0.50.

If the amount of metal ion is less than 0.05 mol relative to 1 mol of chromium, the desired effect cannot be obtained. On the other hand, if the amount of metal ion added is greater than 0.50 mol relative to 1 mol of chromium, the effect is saturated. As a result, the chromate solution gels. Therefore, the amount of metal ions added is 0.05 per 1 mol of chromium.
It is preferably in the range of 0.50 mol.

The above-mentioned addition of metal ions is preferably carried out as a carbonate, oxide or hydroxide. These salts readily dissolve in the chromic acid solution.

However, when chromate treatment is performed with a treatment liquid containing metal ions added as sulfates or nitrates, the plated surface is apt to turn black and the commercial value is significantly reduced, which is not desirable. When treated with a chromate treatment liquid to which metal ions are added as carbonates, oxides or hydroxides, it is possible to improve only the poor solubility of the chromate film without affecting the blackening of the plating surface. .

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

As a method of attaching the chromate treatment liquid to the surface of the steel sheet, a method capable of continuous and uniform attachment is applied. As such a method, spray coating, dipping-roll squeezing, dipping-gas squeezing, roll coating and the like can be used.

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

It is desirable that the chromate treatment liquid developed in the present invention is applied to a hot-dip galvanized steel sheet. The reason is that the surface of the hot dip galvanized steel sheet is generally smoother than the surface of the electrogalvanized steel sheet when viewed microscopically, and is suitable for forming a uniform film.

[Embodiment] Next, the present invention will be described with reference to an embodiment.

Methanol was added as a reducing agent to an aqueous solution of chromic anhydride 100 g / l. After the addition, stirring was performed for 2 hours at a liquid temperature of 90 to 95 ° C to reduce a part of chromic acid. FIG. 2 is a graph showing the relationship between the mixing ratio of chromic acid reduction products and the amount of methanol added. As shown in FIG. 2, it can be seen that when the mixing ratio of the chromic acid reduction product is 0.55 or less, methanol is converted into carbon dioxide and water by the reaction with chromic acid. Then, it can be seen that the compound having a potential reducing ability does not remain in the aqueous solution, so that the reduction reaction of the chromate film does not easily proceed after being attached to the surface of the steel sheet.

Then, silica sol and metal ions were added to a mixed liquid of chromic acid and a chromic acid reduction product to prepare a chromate treatment liquid. Then, using the prepared chromate treatment solution, the chromate treatment is performed on the molten zinc plated steel sheet in the procedure shown below, and the specimens No. 1 to 11 of the present invention and the specimens for comparison
Nos. 1 to 10 were prepared.

Chromate treatment procedure: Steel plate with molten zinc plating (Zenzimer line production, zinc adhesion amount 120 g / m ² ) → wash with water → chromate treatment (roll coating) → heat treatment (60 ° C or 200 ° C).

Table 1 shows the composition of the chromate treatment liquid, the amount of chromate adhered and the heat treatment temperature of each sample. The amount of chromate adhered was measured by measuring the total amount of chromium by fluorescent X-ray and displayed in mg / m ² .

Next, the corrosion resistance, poorly soluble property, 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: A salt spray test (JIS Z2371) was performed on each of the test pieces, and the white rust occurrence rate of the zinc plated 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: 〃 10% to less than 25%, 4: 〃 25% to less than 50%, 1: 〃 50% or more.

Poorly soluble: Each sample was subjected to alkaline degreasing, the chromium adhesion amount before and after degreasing was measured, and the chromium adhesion amount after degreasing to the chromium adhesion amount before degreasing was shown by a percentage.

The alkaline degreasing conditions are as follows.

Degreasing liquid: PC-364S (Nippon Parkerizing Co., Ltd.), 20g /
l, 60 ℃.

Degreasing method: spraying method, spraying pressure 0.5 Kg / cm ² , 2 minutes.

Conductivity (film resistance): Measured by an inter-layer resistance measuring device, and the measurement result is shown in Ωcm ² .

FIG. 3 is a graph showing the relationship between the amount of chromic acid reduction product, corrosion resistance, poor solubility, and film resistance. As shown in FIG. 3, when the mixing ratio of the chromic acid reduction product to chromic acid 1 was in the range of 1: 0.10 to 0.55, good results were obtained in terms of corrosion resistance, poor solubility and film resistance.

As shown in Table 1, a comparative specimen N containing no chromic acid reduction product or having a low mixing ratio outside the range of the present invention
In o.1 and 2, the chromate film was easy to dissolve.

Comparative sample No. 3 in which the amount of metal ions added was outside the range of the present invention and was low in solubility was insufficient. Comparative sample No. 4, in which the mixing ratio of the chromic acid reduction product was out of the range of the present invention, was inferior in corrosion resistance and electrical conductivity.

Comparative sample No. 5 in which the mixing ratio of the chromic acid reduction product was high outside the range of the present invention and the heat treatment temperature was low outside the range of the present invention had poor electrical conductivity.

Comparative sample No. 6 in which the mixing ratio of silica sol was out of the range of the present invention was inferior in corrosion resistance.

Comparative sample No. 7 in which the mixing ratio of silica sol was out of the range of the present invention was inferior in corrosion resistance, and the chromate film was easily peeled off.

Comparative sample No. 8 in which the average particle size of the silica sol was out of the range of the present invention was inferior in corrosion resistance.

Specimens for comparison whose heat treatment temperature is low outside the range of the present invention
In No. 9, the chromate film was easy to dissolve.

Comparative sample No. 10 to which no metal ion was added had a slightly insufficient solubility.

On the other hand, all of the test pieces Nos. 1 to 11 of the present invention were excellent in corrosion resistance, poor solubility and electric conductivity.

[Effects of the Invention] As described above, by the chromate treatment method of the present invention, if a chromate film is formed on the surface of a steel sheet with hot dip galvanized steel, the solubility of the hot-dip galvanized steel sheet, electrical conductivity, and corrosion resistance are significantly improved. Industrially useful effects that can be improved are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the results of analysis of a liquid obtained by mixing chromic acid and a chromic acid reduction product with a thermogravimetric analysis-scanning differential calorimeter (TG-DSC). 2 is a graph showing the relationship between the mixing ratio of chromic acid reduction products and the amount of methanol added,
FIG. 3 is a graph showing the relationship between the amount of chromic acid reduction product, corrosion resistance, poor solubility, and film resistance.

Claims (1)

[Claims] 1. A chromic acid,
Chromic acid reduction product, silica sol with an average particle size of 20 mμ or less, and one or more metal ions as main components from Mg, Ca, Zn, Co, Sr carbonates, oxides or hydroxides And, and the mixing ratio of the chromic acid and the chromic acid reduction product are each a weight ratio in terms of CrO ₃ , 1: 0.10 to 0.55, the total amount of chromic acid and the chromic acid reduction product and the mixture with silica sol. The total amount of chromic acid and chromic acid reduction products is Cr
In terms of O ₃ , silica sol is 1: 1 to 1 by weight ratio in terms of SiO _2.
0, the total amount of chromic acid and chromic acid reduction products and the mixing ratio of metal ions is chromic acid and the total amount of chromic acid reduction products in terms of CrO ₃ , molar ratio with metal ions 1: 0.05 ~.
A method for chromate treatment of a steel sheet with hot dip galvanized, which comprises depositing an aqueous solution contained within a range of 0.50 and then heat treating the steel sheet at a temperature of 100 to 300 ° C.