KR100256361B1 - The solution of the molybdate-based alternatives as a passivation treatment with white color degree and excellent corrosion resistance and method used therefor - Google Patents

Abstract

PURPOSE: A moly-phosphate solution having superior corrosion resistance and whiteness and method for forming film on the surface of a galvanized steel sheet using the same is provided. CONSTITUTION: The method for forming film on the surface of galvanized steel sheets comprises the steps of preparing a moly-phosphate solution consisting of molybdenum 0.4-1.6g/L, phosphate 7.0-14g/L, nitrate 8-50g/L; dipping a galvanized steel sheet in the moly-phosphate solution of which temperature ranges from 55 to 65deg.C; and then baking the galvanized steel sheet coated with moly-phosphate solution.

Description

Molyphosphate solution showing excellent whiteness and corrosion resistance when forming film and film forming method using same

1 is a graph showing changes in whiteness and corrosion resistance according to the content of nitrate groups.

The present invention relates to a molten phosphate solution for surface treatment of a steel sheet and a method for forming a film on the steel sheet using the same, and more specifically, a molybdate solution for a surface-treated steel sheet excellent in whiteness and corrosion resistance containing nitrate groups and a steel sheet using the same The present invention relates to a method of forming a film.

In order to secure the corrosion resistance of galvanized steel sheet, chromate coating has been used. However, chromium hexavalent ions used in chromate solution are toxic. Research has been conducted and as a result the molybdate film has been reported to have relatively good corrosion resistance. Japanese Patent Laid-Open No. 6846/71 is a resin solution containing molybdate and a water-soluble organic compound of 0.05-45wt% or less at 0.5 molar concentration, and Japanese Laid-Open Patent Publication No. 2419/76 discloses a 0.025-0.02M magnesium or sodium molybdate solution solution. 14141/76 disclose a method of forming a molybdate film with a molybdate solution containing ammonium sulfate, respectively, but these methods require a long reaction time. In addition, Japan's Gobae has studied the solution of potassium molybdate (K ₂ MoO ₄ : 10-200g / L), phosphoric acid, sulfuric acid and acetic acid and found that corrosion resistance is improved when phosphate is added. A method of forming a film is disclosed, but no mention is made of surface appearance (whiteness) (GB 2B 2070073). Furthermore, according to Bech-Nielsen WO 93/10278, an oxidation current of -2.5 to +5.0 V (vs SHE) was applied to the subject under a Mo concentration of 2.9-9.8 g / L and a Mo / P molar ratio of 0.2-0.8. A method of forming a morphophosphate coating is disclosed, but this also takes a long reaction time, and nothing is said about the surface appearance (whiteness).

Furthermore, in the case of using the molybdate film as a substitute for the chromate coating treatment widely used to improve the corrosion resistance of the conventional galvanized steel sheet, a method of increasing the adhesion amount has been used to secure corrosion resistance equal to or higher than that of the chromate coating. This also increases the reaction time and increase the whiteness according to the increase in the adhesion amount.

Accordingly, it is an object of the present invention to provide a molten phosphate solution for surface treatment of a steel sheet which exhibits excellent whiteness and corrosion resistance at the same time and can be treated within a short time, and a method for forming a molybdate film having excellent whiteness and corrosion resistance using the same.

According to one aspect of the present invention, there is provided a molten phosphate solution for processing a steel sheet comprising 0.4-1.6 g / l molybdenum, 7.0-14 g / l phosphate group and 8-50 g / l nitrate group.

According to another aspect of the present invention, there is provided a method for forming a molten phosphate coating comprising immersing a galvanized steel sheet in the molybdate solution of the present invention having the composition of 55-65 ° C, followed by baking.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

According to the present invention, by adding nitric acid to the molybdate solution system, the adhesion amount to the steel sheet is increased by the oxidizing power of the nitrate group, so that the corrosion resistance is improved, and the whiteness of the surface of the steel sheet is also improved by the etching effect of the coating layer.

In the molybdate solution of the present invention, the molybdenum is preferably 0.4-1.6 g / l and the phosphoric acid group is 7.0-14 g / l. When the phosphate group is less than the composition range as compared to molybdenum, the corrosion resistance, whiteness, and adhesion amount of the solution are low, and when a large amount of phosphate groups are present, the whiteness is excellent but the solution adhesion amount and corrosion resistance are reduced. As a result, when the phosphoric acid group is added less than 7.0g / ℓ, the corrosion resistance is lowered, and when more than 14g / ℓ is added, the solution adhesion amount and corrosion resistance is reduced, so that the molybdenum and phosphate group is present in the solution in the above composition range.

According to the present invention, nitric acid is added as a reaction accelerator to the molybdate solution having the composition so as to exhibit excellent whiteness and corrosion resistance when forming a film.

When the nitric acid group is added to the molybdate solution, the reactivity is enhanced by the oxidation reaction Zn → Zn ^{+ 2} + 2e due to the large oxidizing power, thereby increasing the amount of adhesion of the solution to the steel sheet. Therefore, the corrosion resistance is improved during film formation, and the whiteness is also improved by the etching effect of the film forming layer. The concentration of the nitric acid group in the silver nitrate to be added is preferably 8-50 g / l, preferably 15-35 g / l. If the concentration of the nitric acid group is less than 8 g / ℓ does not exhibit the desired whiteness, corrosion resistance and solution adhesion amount, whereas if the concentration of the nitric acid group is more than 50 g / ℓ, the zinc plated layer is dissolved in the solution due to the over-aciding, the film is not formed.

In addition, by treating the galvanized steel sheet with the molybdate-based solution of the present invention prepared as described above, it is possible to form a molybdate film excellent in corrosion resistance and whiteness. The coating may be formed by immersing a galvanized steel sheet in a molybphosphate solution at 55-65 ° C., applying the solution to a steel sheet, and then baking it. If the temperature of the molybdate solution is less than 55 ℃ the activation energy of each of the ionic components is low, the adhesion amount is lowered. Therefore, it is preferable to treat in a solution of 55-65 ° C. in order to satisfy both the solution deposition amount and the whiteness.

In addition, when the film is formed at 55-65 DEG C. solution degree using the molybdate solution of the present invention, a film exhibiting sufficient physical properties can be formed by immersing the steel plate in the solution for 30-60 seconds and then treating.

Hereinafter, embodiments of the present invention will be described in detail.

Example 1

After preparing molybdate solution by adding molybdenum and phosphate groups as shown in Table 1 below, the steel sheet was immersed at 60 ° C solution temperature for 40 seconds to form a film, and then the whiteness, corrosion resistance and adhesion of the film were measured. Is shown in Table 1 below.

Whiteness is measured by L ^* , a ^* , b ^* method using Tokyo Densuku's color difference meter C-5120. Corrosion resistance is the area of surface white blue in anodized steel sheet according to ASTM B117, ISO. %) And the adhesion was measured by weight loss method.

When measuring the amount of adhesion, first, cut the surface-treated steel sheet into a circle with a diameter of 50 cm, measure the weight (w1), dissolve it in HCl, measure the weight again (w2), and attach it to the steel sheet according to the following equation (I). The amount of solution was calculated.

Amount of adhesion = (w1-w 2) / specimen area (I)

TABLE 1

The results of evaluating the whiteness, adhesion amount and corrosion resistance were described in Comparative Example 1 to Comparative Example 4 (see Table 1 above). Comparative 1 with less phosphoric acid content showed that the whiteness was less than 50 at low concentrations (molybdenum 0.8-4.0g / ℓ, phosphate 2.0-6.0g / ℓ), resulting in poor surface appearance and poor adhesion and corrosion resistance. As illustrated in Comparative Example 2, when the phosphate group was added in excess of molybdenum, the whiteness was improved to 77-80, but the adhesion amount and the corrosion resistance were not satisfactory. Comparative Example 3 in which the molybdenum is added in excess to the phosphate group shows that the whiteness is low at 56-60 and the adhesion amount is increased, but the corrosion resistance is not satisfactory.

Comparative Example 4, in which an excess of molybdenum and phosphate groups were added, showed a low whiteness of 48-50, but it was found that the adhesion amount and the corrosion resistance were improved compared to other comparative examples.

That is, when a small amount of phosphate group is added (Comparative Example 1) or excessively added (Comparative Example 4), the whiteness of the film is less than 50 due to the imbalance between Mo ions and PO ₄ ions in the solution, and the amount of phosphate groups is higher than that of molybdenum. When added (Comparative Example 2), the whiteness deteriorates the adhesion and corrosion resistance due to the increase in the etch effect due to the increased surface oxidation of the galvanized steel sheet of the phosphate group. When molybdenum is excessively added as compared to the phosphate group (Comparative Example 3), the whiteness is reduced to 60 due to the reduction of the etching effect of the phosphate group, so that the molybdenum composition is preferably 0.4-1.6 g / l and the phosphate group 7.0-14 g / l. In the case of Inventive Example 5 thus constructed, it is possible to satisfy all of the whiteness, adhesion amount and corrosion resistance.

Example 2

This embodiment is for setting an appropriate film forming temperature range using a molybphosphate solution. A molybdenum solution was prepared with the composition shown in Table 2 below, and a film was formed under the conditions of Table 2 to evaluate whiteness, adhesion amount and corrosion resistance, and the results are shown in Table 2 below.

TABLE 2

When treated at low temperature as in Comparative Example 6 (40-55 ℃), the whiteness is improved to about 70, but the amount of adhesion decreases due to the low activation energy of each ionic component, and when treated at high temperature (65-75 ℃) as in Comparative Example 7 The adhesion amount increased with increasing activation energy of each ionic component, but the whiteness was less than 50. Therefore, in order to increase the activation energy of each ionic component to secure whiteness, adhesion amount and corrosion resistance, it was found that the treatment temperature was treated at 55-65 ° C. as shown in Inventive Example 8.

[Example 3]

To prepare a molybdate solution in the composition of Table 3 and then to form a film under the conditions of Table 3 to evaluate the whiteness, adhesion amount and corrosion resistance according to the content of nitrate groups and the results are shown in Table 3 and FIG. .

TABLE 3

As shown in Comparative Example 9, the addition of 2.0-6.0 g / l of nitrate group to the invention of Example 2 without addition of nitrate group compared to 8 in the same treatment conditions (55-65 ° C., 40 seconds) under whiteness, adhesion amount and corrosion resistance Although somewhat improved, when the nitric acid group was added in excess of 60 g / ℓ, as shown in Comparative Example 10, due to the superacid wash, the zinc plated layer was dissolved in the solution, and the film was not formed.

On the contrary, when 8-50 g / l of nitrate group was added as shown in Inventive Example 11, the whiteness was improved to 78-70, and the adhesion amount and corrosion resistance were remarkably improved. The optimum nitric acid addition range, when described at 15-35 g / l as described in Inventive Example 12, showed improved whiteness and corrosion resistance compared to Inventive Example 11 in a relatively stable range. This optimal treatment condition means that there is an optimum treatment condition between the etching effect of oxidation of the nitrate group and the solubility of the film due to super-acid pickling. It is characterized by.

Figure 1 is a graph showing the measurement results of the whiteness and corrosion resistance of the molybdate film according to the nitrate group content change, in order to explain the effect of improving the whiteness and corrosion resistance by the addition of nitrate group Inventive Example 5, a small amount of nitrate group Comparative Example 9 added, Comparative Example 10 added in excess, and Example 12 added in the appropriate amount were compared. Inventive Example 12 shows the best whiteness and corrosion resistance compared to other comparative examples.

Claims (3)

A molten phosphate solution for steel sheet treatment comprising 0.4-1.6 g / l molybdenum, 7.0-14 g / l phosphate group and 8-50 g / l nitrate group. According to claim 1, wherein the concentration of the nitrate group is a molybdate solution for steel sheet treatment, characterized in that 15-35g / l. A method for forming a molten phosphate film, which is immersed in a molten phosphate solution of claim 1 at a temperature of 55 to 65 ° C. to coat the solution on a steel sheet.