KR940004904B1 - Chromate solution for galvinizing steel sheets with an excellent black stain resistance and corrosion resistance - Google Patents

Abstract

This invention relates to chromate treatment solution which has good black stain resistance and anticorrosion and does not damage a surface of zinc plated steel. The solution manufacturing method comprises: A) adding a reducing agent to chromic anhydride solution having 1-20 g/l of Cr+6 ion conc and making the Cr+3 ion 5-15 % in the total chromium ion; and B) adding phosphoric acid 1.5-5.0 g/l and fluorine compound 0.01-0.5 g/l.

Description

Chromate treatment solution for galvanized steel with excellent black and corrosion resistance

The present invention relates to a chromate treatment solution for galvanized steel sheet, and more particularly, to a chromate treatment solution for galvanized steel sheet which is excellent in black blue resistance and corrosion resistance and does not damage the surface appearance of the galvanized steel sheet.

Conventionally, galvanized steel sheets, such as zinc and aluminum, have been used without coating or painting. However, chromate treatment is performed using a solution containing chromic anhydride as a main component on the surface of the coated steel sheet to improve corrosion resistance and paint adhesion. In the case of galvanized steel sheets, corrosion products in the form of black stain or white rust are generated on the surface in response to the atmosphere, and chromate treatment is performed as a post-treatment during the manufacturing process of the galvanized steel sheet to prevent this. Doing. Corrosion resistance of the galvanized steel sheet depends on the Cr adhesion amount in the chromate coating and the compactness of the coating.

Until now, a method for increasing the corrosion resistance of steel sheet has been formed by using chromate ion as a main component, a solution containing mineral acid such as sulfate ion, and forming a chromate film, and removing the remaining solution on the surface by washing with water. Responsive methods are known. This method is most often used because of the large increase in the amount of film deposited by the mine and the resultant film shows excellent film properties.However, when the amount of sulfate ion is added, the amount of film adhesion increases depending on the concentration of sulfate ion in the solution. Even if the amount of sulfuric acid ion is changed in the furnace solution, the film deposition amount is greatly changed, resulting in a problem of uniformity of product quality and loss of gloss peculiar to galvanized steel sheet.

To date, the chromate treatment solution composition used for the chromate treatment of galvanized steel sheet mainly uses a large number of fluorine compounds due to the above reasons. However, fluoride alone has limitations in improving corrosion resistance and surface appearance. Therefore, after chromate treatment, a bright chromate treatment is additionally performed. A method of forming a dense film by adding a compound such as the above has been proposed, but in the method of performing a chromate chromate treatment after chromate treatment, an organic substance such as polyacrylamide in the gloss chromate treatment solution is added, so that it is possible to change the solution over time. There is a possibility that a problem may occur, and in the method of adding heavy metals such as titania and arsenic, it is difficult to manage the solution because these heavy metals are likely to exist as oxides in the solution.

As a method to solve this problem, a method of improving the corrosion resistance of steel sheets by adding a reducing agent to chromic acid to reduce chromium hexavalent to chromium trivalent and applying chromate treatment solution containing phosphoric acid and hydrofluoric acid to the steel sheet and then drying them It is known from Japanese Patent Laid-Open No. 57-35685, No. 57-29681, and the like. These methods are recognized by the addition of phosphoric acid and chromium trivalent in solution to increase the amount of chromate coating without impairing the gloss of the steel sheet surface. There is a problem that is likely to cause black stains to be damaged (Japanese materials and processes (vol. 2, 1681, 1989)).

The present inventors conducted research and experiments to solve the above-described problems, and based on the results, the present invention proposes the present invention, which is excellent in black blue resistance and corrosion resistance, and damages the gloss characteristic of galvanized steel sheet. An object of the present invention is to provide a chromate treatment solution for galvanized steel sheet that is not intended.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention adds a reducing agent to the chromium anhydride solution having a chromium hexavalent ion concentration of 1-20 g / l so that the concentration of chromium trivalent reduced from chromium hexavalent is 5-15% relative to the total chromium concentration of the solution, It relates to a chromate treatment solution for galvanized steel sheet excellent in black blue resistance and corrosion resistance formed by adding 1.5-5.0 g / l phosphoric acid and 0.01-0.5 g / l fluorine compound to the solution.

The reason for the numerical limitation will be described below.

If the chromium hexavalent ion concentration in the chromate treatment solution is 1 g / l or less, the thickness of the chromate film is thin and the improvement of corrosion resistance is not sufficient. Although there is a problem that the surface is easily colored yellow, it is preferable to limit the chromium hexavalent ion concentration in the solution to 1-20 g / l. As the reducing agent for reducing chromium hexavalent in the solution to chromium trivalent, any of water-soluble and reducing materials such as methyl alcohol and ethylene glycol may be used.

When the chromium trivalent is 15.0% or more of the total chromium concentration, black blue is more likely to occur, and there is a risk of sludge in the solution due to a decrease in the stabilization of the solution. It is preferable to limit the concentration of chromium trivalent to 5.0-15.0% of the total chromium concentration since the decrease of the surface is increased or the risk of coloring the surface is increased, thereby increasing the amount of chromium deposition is not enough to improve the corrosion resistance.

The phosphoric acid has the effect of suppressing the coloration to yellow by chromium hexavalent. When the amount of phosphoric acid is added more than 5g / l, the possibility of black blue is high, and when the 0.5g / l or less, the possibility of black blue is reduced, It is suggested to increase the amount of chromium deposition by increasing the risk that the surface is colored yellow, so that the amount of phosphoric acid added is preferably limited to 1.5-5.0 g / l.

The fluorine compound has the effect of uniformly etching the surface to form a uniform chromate coating, but when the addition amount is 0.01g / l or less, the chromate coating is not uniformly localized unsatisfactory corrosion resistance, 0.5g / l or more Since the effect of improving the corrosion resistance is not obvious and the surface of the plating layer is excessively etched and there is a risk of damaging the gloss of the galvanized steel sheet itself, the amount of the fluorine compound added is preferably limited to 0.01-0.5 g / l.

Examples of the fluorine compound include HF, H ₂ SiF ₆ , NaF, Na ₂ , and SiF _{6. In the} present invention, one or more of these fluorine compounds is added.

As a method of chromate treatment using the chromate treatment solution of the present invention as described above, a method of dipping a steel sheet in a solution and a method of spraying a solution on the steel sheet may be preferably applied.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The specimen collected from the hot-dip galvanized steel sheet was immersed in a chromate treatment solution at 60 ° C. for 10 seconds, as shown in Table 1 below, taken out and pressed with a rubber roll to adjust the coating amount to 20-30 mg / m 2, and then the specimen was subjected to 100 ° C. It dried with hot air and chromate-treated.

Corrosion resistance, black blue resistance, and surface appearance were evaluated for the chromate treated specimens as described above, and the evaluation criteria are shown in Table 1 below.

TABLE 1

Test method and evaluation method for the corrosion resistance, black blue resistance and surface appearance in Table 1 are as follows.

1) corrosion resistance

The corrosion resistance of the chromate treated specimens was tested by the salt spray test according to JIS Z-2371. The specimens were observed every 12 hours to investigate the start time of white rust.

2) Black Blue Resistance

The black rust resistance was evaluated by visual observation of black rust after storing the specimen for 140 hours in a high temperature and high humidity environment with a relative humidity of more than 90% and a temperature of 50 ° C. The criteria are as follows.

○: No black rust

△: black blue

×: blackish black

3) Surface appearance

The appearance of the chromated specimens was visually evaluated and evaluated as follows.

○: Good (no coloring)

△: gloss loss, slightly coloring

X: Coloring severe.

As shown in Table 1, the inventive material (a-h) according to the present invention can be seen that the white rust generation time is more than 24 hours, excellent in corrosion resistance, and also excellent in black rust resistance and surface appearance. On the other hand, in the addition amount of phosphoric acid, the surface appearance is not good when it is less than the range (Comparative Example 2) in the present invention, and when it is larger (Comparative Example 3), it can be seen that the black blue resistance is not good.

In addition, it can be seen that the surface appearance is not good when no fluorine compound (SiF ₆ ^-2 ) is added at all (Comparative Example 4) or when more fluorine compounds are added (Comparative Example 5) than the range of the present invention. .

In addition, it can be seen that in the case of not being reduced to Cr at all (Comparative Example 6) or in a case where more amount is reduced than in the scope of the present invention (Comparative Example 7), the black-blue resistance and surface appearance are not good.

In addition, when Cr ^{+ 6} is smaller than the scope of the present invention (Comparative Example 1), the corrosion resistance is poor, and when more than the scope of the present invention (Comparative Example 8), it can be seen that the black black resistance and the surface appearance are not good.

As described above, the present invention is a chromate treatment by using a chromate treatment solution having a composition range presented in the present invention in the method of manufacturing a galvanized steel sheet is excellent in black blue resistance and corrosion resistance, while the distinctive luster of the galvanized steel sheet The effect is to produce a galvanized steel sheet that is not damaged.

Claims (3)

A reducing agent is added to the chromium anhydride solution having a chromium hexavalent ion concentration of 1-20 g / l so that the concentration of chromium trivalent reduced from chromium hexavalent is 5-15% relative to the total chromium concentration of the solution. A chromate treatment solution for galvanized steel sheet having excellent black blue resistance and corrosion resistance, comprising 1.5-5.0 g / l phosphoric acid and 0.01-0.5 g / l fluorine compound. The chromate treatment solution for galvanized steel sheet having excellent black-blue resistance and corrosion resistance according to claim 1, wherein the reducing agent is ethyl alcohol or ethylene glycol. The chromate for galvanized steel sheet having excellent black and blue resistance and corrosion resistance according to claim 1 or 2, wherein the fluorine compound is at least one member selected from the group consisting of HF, H ₂ SiF ₆ , NaF, and Na ₂ SiF ₆ . Treatment solution.