KR100345707B1 - Preparation method of chromate solution and method for forming chromate coating on hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE: A preparation method of chromate solution is provided, and also a method for manufacturing a chromated galvanized steel sheet with excellent corrosion resistance, surface quality and chromium elution resistance by using the chromate solution is provided. CONSTITUTION: The method for preparing a chromate solution comprises preparing a subject solution (a) of which chromium concentration is 5 to 30 g/L, wherein Cr¬3+ ratio in total chromium is 0.4 to 0.6; based on the total amount of chromium in the subject solution, adding colloidal silica 50-80 wt.%, fluoric acid 20-40 wt.%, phosphoric acid 40-80 wt.%, sulfuric acid 5-15 wt.% to the subject solution (b); adding phosphoric acid to 2-10 wt.% of aqueous epoxy-based silane solution; adjusting the above solution at pH 2 to 3; adding 1 to 5 wt.% of chromic acid to the above solution, based on the weight of silane and subsequently heating it for 1 to 2 hours to prepare a hardener solution; mixing 10 to 20 parts by weight of the hardener solution with 100 parts by weight of the subject solution (b). The method for manufacturing chromated galvanized steel sheet using the chromate solution includes step of depositing the chromate solution on a galvanized steel sheet, whose zinc-plated amount is in the range of 60 to 120 g/m¬2, in an amount of 30 to 100 mg/m¬2 based on the adhesion amount of dry coat; and baking the chromated galvanized steel sheet at 40 to 190°C.

Description

Preparation method of chromate solution and manufacturing method of chromate-treated hot dip galvanized steel sheet {Preparation method of chromate solution and method for forming chromate coating on hot dip galvanized steel sheet}

The present invention relates to a method for producing a chromate solution used for the resin treatment of the hot-dip galvanized steel sheet, and to a method for producing a chromate-treated hot-dip galvanized steel sheet using the solution, and more specifically, the surface color and chromium elution resistance, A method of producing a chromate solution from which a surface of a hot-dip galvanized steel sheet excellent in corrosion resistance and the like is obtained, and a method of coating on a hot-dip galvanized steel sheet using a chromate solution obtained by this method.

Hot-dip galvanized steel sheet has been developed for the purpose of improving the corrosion resistance of steel plate by sacrificial method of thick film galvanized layer, and for the purpose of home appliance and construction materials. It is used. Chromate solution has been used mainly for temporary rust prevention to prevent rust during product transportation. However, recently, many products have been developed to form a permanent film through the curing process after application, such as resin coated hot dip galvanized steel sheet. have. There are three types of chromate treatment methods: reactive, electrolytic, and coated. The reactive and electrolytic types cause water treatment problems by chromate treatment, whereas the coated chromate solution can suppress the occurrence of such water treatment as much as possible. In terms of the environment, coating chromate treatment is becoming more common. In this case, the chromate solution is used by dissolving chromic anhydride in water and then reducing it with an appropriate reducing ratio using a reducing agent. The formed chromate film should have excellent corrosion resistance and chromium elution resistance. In addition, since the chromate coating of the underlying layer is the most influential factor in the surface appearance of the product, securing the surface color and quality is also considered as an important technology. In order to satisfy the aforementioned properties, various additives are added to the chromate solution to satisfy the aforementioned properties. Attempts are made to make improvements.

Accordingly, the present inventors have repeatedly studied and experimented to obtain a chromate film having excellent physical properties and based on the results, and the present invention proposes the present invention by properly controlling the composition of ingredients and mixing the curing agent solution at an appropriate ratio. It is an object to provide a method for producing an excellent coated chromate solution.

In addition, the present invention provides a method for producing a chromate-treated hot dip galvanized steel sheet excellent in corrosion resistance, chromium elution resistance, surface color and surface quality by using the chromate solution prepared by the above method, and by controlling the coating method appropriately. The purpose is.

The present invention for achieving the above object,

The chromium concentration in the solution is 5-30 g / l, the trivalent chromium component ratio (chromium reduction ratio) to the total chromium amount is 0.4-0.6, and the colloidal silica is 50-80% by weight and the hydrofluoric acid is 20- To obtain a main solution added with 40% by weight, 40-80% by weight phosphoric acid, 5-15% by sulfuric acid,

Phosphoric acid was added to an aqueous solution of epoxy silane at a concentration of 2 to 10% by weight to adjust the pH to 2 to 3, and then 1-5% by weight of chromic acid was added to the silane and heated for 1-2 hours. Gained,

It relates to a method for producing a chromate solution prepared by administering 10 to 20% by weight of the curing agent to the main solution.

In addition, the present invention is applied to the chromate solution prepared as described above on a hot-dip galvanized steel sheet of 60-120g / m ² galvanized coating amount of 30 ~ 100mg / m ² on the basis of the dry film adhesion amount of 140-190 ℃ The present invention relates to a method for producing a chromate-treated hot dip galvanized steel sheet produced by baking at a steel plate temperature.

Next, the chromate solution preparation method of this invention is demonstrated in detail.

In the present invention, the chromium concentration in the solution is 5-30 g / l, the trivalent chromium component ratio (chromium reduction ratio) to the total amount of chromium is 0.4-0.6, and the colloidal silica is 50-80 wt% based on the chromium component, A main solution is added which has 20-40% by weight of hydrofluoric acid, 40-80% by weight phosphoric acid and 5-15% by weight sulfuric acid.

When the trivalent chromium reduction ratio is less than 0.4, it is difficult to obtain the effect of improving the corrosion resistance by trivalent chromium, and the chromium elution resistance is deteriorated due to the increase of the soluble hexavalent chromium. On the contrary, when the reduction ratio is higher than 0.6, Due to the gelation of the solution, the solution storage is poor. An example of a method for obtaining such a reduction ratio is as follows. In other words, dissolve the chromic anhydride to 50-150g / l in pure water and then add ethylene glycol to the dissolved chromate, the hexavalent chromium trivalent chromium compound so that the trivalent chromium component ratio to the total chromium component is 0.4-0.6 Reduced to. Reduction reaction was carried out using a reactor made in the laboratory, the cooling water flowed to the reactor wall to cool the heat generated during the reaction and stirred for a sufficient time until no further reaction occurs.

The chromium component concentration is 5-30 g / l by pure water administration.

The phosphate addition can improve the solution stability, wettability and workability by changing the trivalent chromium salt into Cr (PO ₄ ) ₃ · 6H ₂ O and can improve the whiteness of the surface color when applied to steel sheet. . Phosphoric acid dose was added at 40-80% by weight with respect to chromium component. When it is lower than 40% by weight, the effect of improving the solution property and surface properties is not sufficient, and when it is higher than 80% by weight, solution stability is decreased by increasing the reduction ratio and It causes a decrease in corrosion resistance due to excessive phosphoric acid component.

The addition of hydrofluoric acid may improve the corrosion resistance and smoothing of the coating by the fluorine component, and it is preferable to add 20-40% by weight based on the chromium component. If the added amount is less than 20% by weight does not exhibit a sufficient corrosion resistance effect, when the content is higher than 40% by weight deteriorates solution stability due to the sludge generation in solution.

In the silica addition, a colloidal state having a pH in the range of 2-4, such as silicon oxide (SiO ₂ ), was added at 50-80 wt% based on the chromium component. Since silica forms crosslinks during baking, it suppresses the formation of zinc oxide in the base metal and is hydrophobic, thus improving resistance to corrosion environments such as moisture, thereby improving corrosion resistance and contributing to film adhesion. If the amount of silica added is less than 50% by weight, such an effect of improving the physical properties is not sufficient, and when the content is higher than 80% by weight, solution stability is lowered and coating film adhesion is deteriorated.

The addition of sulfuric acid has a great effect on the color of the solution can improve the surface quality of the steel sheet and serves to improve the solution stability by improving the flow of the solution. The addition amount is preferably 5 to 15% by weight based on the chromium component. When the amount is less than 5% by weight, the effect of improving the surface properties is insignificant, and when the amount is more than 15% by weight, the solution stability and the corrosion resistance are adversely affected.

On the other hand, in the above-described description of the main solution of the present invention, "chromium component" means the ratio of chromium in the total main solution, that is, the total amount of chromium (total weight of trivalent chromium component + hexavalent chromium component) in the main solution.

In addition, in the present invention, dilute the epoxy silane in pure water to prepare an aqueous solution of epoxy silane at a concentration of 2 to 10% by weight, and then add phosphoric acid to adjust the pH to 2-3, followed by chromic acid. 1-5% by weight of the silane is added and heated for 1-2 hours to obtain a curing agent solution.

The epoxy silane aqueous solution is prepared by administering an epoxy silane to a pure water at a concentration of 2-10% by weight, and adding a small amount of phosphoric acid to prevent gelation when mixed with the main solution. Adjust to 2-3 similar to the solution.

In addition, 1-5% by weight of chromic acid was added thereto, followed by heating for 1-2 hours to form a small amount of self-crosslinking of the silane molecules in the curing agent solution. This is to make the reaction easily occur when forming the film structure through cross-linking reaction of the small bush chromate film. If the amount of chromic acid is less than 1% by weight, the self-crosslinking reaction does not occur well. This will occur.

In the present invention, the curing agent solution is administered in 10-20% by weight based on the main solution.

If the added amount is less than 10% by weight, sufficient crosslinking reaction does not occur, and when it exceeds 20% by weight, solution stability is lowered.

Next, a method of manufacturing a chromate-treated hot dip galvanized steel sheet using the chromate solution prepared as described above will be described in detail.

In the present invention, a zinc coating weight by coating the 60-120g / m ² in the molten zinc is applied to the 30-100mg / m ² dry film adhesion amount based on Geum plate.

When the adhesion amount is less than 30, the effect of improving the corrosion resistance by the chromate coating is not sufficient, and when the adhesion amount is higher than 100, the cost of the solution increases and the chromium elution property becomes worse.

In addition, in this invention, baking after application is performed at the steel plate temperature of 140-190 degreeC.

The steel sheet temperature should be heated to more than 140 ℃ for a sufficient curing reaction, if the temperature exceeds 190 ℃ micro-cracks occur on the surface with a decrease in hexavalent chromium in the film, the corrosion resistance is rather reduced.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

A main solution A containing 60 wt% of colloidal silica, 30 wt% of hydrofluoric acid, 60 wt% of phosphoric acid, and 10 wt% of sulfuric acid was prepared based on the chromium component. In the main solution, a main solution B without sulfuric acid, a main solution C without phosphoric acid, a main solution D without silica and sulfuric acid, and a main solution E without hydrofluoric acid and sulfuric acid were prepared. .

Further, phosphoric acid was added to the aqueous solution of epoxy silane at a concentration of 7% by weight to adjust the pH to 2.5, and then 3% by weight of chromic acid was added to the silane and heated for 2 hours to prepare a curing agent.

Using the prepared main solution (A-E) and a curing agent to prepare a chromate solution as shown in Table 1.

The prepared chromate solution was applied on a hot dip galvanized steel sheet having an adhesion amount of 90/90 g / m ² , and then dried, and the surface color and the component ratios of trivalent and hexavalent chromium were measured, and the results are shown in Table 1 below. Chromium elution was measured by punching the specimen at 50 ° and immersing it in boiling water for 10 minutes and measuring the color difference and chromium elution amount before and after immersion, and the results are shown in Table 1 below.

Chromate solution Chromaticity Chromium content (mg / m ² ) Color difference (ΔE) Chromium content ratio (%) Chromium Elution (%) Whiteness Redness Color Cr ⁺³ Cr ⁺⁶ Comparative Material 1 Main solution B + Curing agent 15% 53.92 -1.49 7.84 69.24 6.1 59.83 40.17 62 Invention 1 Main solution A + Curing agent 15% 56.12 -1.08 -0.84 59.58 2.9 77.07 22.93 0.57 Comparative Material 2 Main solution C + Curing agent 15% 55.34 0.67 3.11 56.91 5.7 68.27 31.73 17.7 Comparative Material 3 Main solution D + Curing agent 15% 57.85 0.66 7.57 77.30 8.5 57.62 42.38 23.8 Comparative Material 4 Main solution E + Curing agent 15% 56.62 -0.72 8.17 69.12 8.3 64.07 35.93 18.2 Comparative Material 5 Main solution A, hardener 0% 53.56 0.03 1.79 57.12 5.5 61.05 30.79 31.9

As can be seen in Table 1, in the case of the surface color, the yellowness was reduced by the addition of sulfuric acid, so that the surface of the treated specimen was blue. In this case, the surface color was reduced and the surface color was improved during the resin treatment. Effect. In the case of the chromium dissolution rate, the invention material showed a superior chromium dissolution rate as compared with the comparative material, and it was confirmed that the chromium dissolution rate was also excellent in the surface color difference before and after boiling water immersion. In terms of the chromium component ratio, in the case of the invention, it was found that the component ratio of the minor hardened portion insoluble trivalent chromium was relatively higher than that of the soluble hexavalent chromium to inhibit chromium elution.

Example 2

A conventional reactive chromate solution and chromate solutions of the components shown in Table 2 were prepared.

Using such a solution, a specimen was prepared by forming a film on a hot dip galvanized steel sheet having a zinc plating amount of 90/90 g / cm ² .

After the specimen was prepared, a salt spray test according to JIS-Z 2371 was performed, and an initial white rust was generated. The results are shown in Table 2 below.

Specimen Solution composition (% by weight) Training spray (white rust occurrence time) Chromium deposition amount (mg / m ² ) Phosphoric Acid Sulfuric acid Foshan Silica Hardener Comparative Material A Responsive Chromate 134hr 41.33 Invention Material A 60 10 30 70 7 1272hr 72.58 Comparative material B 60 10 30 30 - 350 hr 88.67 Comparative Material C 60 10 10 10 One 300 hr 56.64 Comparative material D 10 - 10 10 7 1000hr 82.39

As can be seen in Table 2, in the case of the inventive material, the corrosion resistance was shown to be superior to that of the comparative material, and in the case of the comparative chromate solution, the corrosion resistance was lower than that of the coating type. It was greatly affected.

A hardening solution prepared by administering phosphoric acid, hydrofluoric acid, sulfuric acid, and silica to a chromate solution of reduced chromic anhydride as in the method of the present invention, and mixing the epoxy silane with a small amount of chromic acid in an aqueous solution. In the case of manufacturing hot dip galvanized steel sheet by applying chromate solution mixed in proper ratio to the surface, it can contribute to quality improvement by securing excellent surface appearance, corrosion resistance and chrome dissolution resistance.

Claims (2)

In the method of producing a coated chromate solution, The chromium concentration in the solution is 5 to 30 g / l, the trivalent chromium component ratio (chromium reduction ratio) to the total amount of chromium is 0.4 to 0.6, and the colloidal silica is 50 to 80% by weight and the hydrofluoric acid is 20 to the chromium component. To obtain a main solution added to 40% by weight, 40-80% by weight phosphoric acid, 5-15% by sulfuric acid, Phosphoric acid was added to an aqueous solution of epoxy silane at a concentration of 2 to 10% by weight to adjust the pH to 2 to 3, and then 1-5% by weight of chromic acid was added to the silane and heated for 1-2 hours. A method of producing a chromate solution obtained by administering 10 to 20% by weight of the curing agent to the main solution. Steel plate temperature of 140 ~ 190 ℃ after applying the chromate solution prepared as described in claim ^{1 on} the hot-dip galvanized steel sheet with a zinc coating adhesion amount of 60-120g / m ² based on the dry coating amount of 30-100mg / m ² Method for producing a chromate-treated hot dip galvanized steel sheet characterized in that the baking by