KR20020029151A - Cromate coating solution having superior corrosion resistance in high curing condition after forming for automobile fuel tank and Method for manufacturing steel sheet by using it - Google Patents

Abstract

PURPOSE: A chromate solution is provided which has superior corrosion and fuel resistance after a chromate solution for automobile fuel tanks is dried at a high temperature, and then formed, and a method is provided to manufacture a plated steel sheet having superior corrosion and fuel resistance using the chromate solution. CONSTITUTION: The chromate solution for automobile fuel tanks having superior corrosion resistance after high temperature baking and forming comprises a base solution in which 300 to 2000 wt.% colloidal silica, 20 to 40 wt.% of fluoric acid, 50 to 80 wt.% of phosphoric acid, 5 to 15 wt.% of sulfuric acid and 25 to 40 wt.% of sodium phosphate are contained for chromium in an aqueous chromium solution where a composition of trivalent chromium in 7 to 50 g/L of chromium is 0.4 to 0.8; and a curing agent solution which contains 5 to 30 wt.% of titanium having alkoxy group, and a pH of which is controlled to 2 to 3, wherein the curing agent solution is mixed in an amount of 20 to 50 wt.% for the base solution.

Description

Chromate coating solution having superior corrosion resistance in high curing condition after forming for automobile fuel tank and Method for manufacturing steel sheet by using it}

The present invention relates to a chromate solution used for the resin treatment of lead-free plated steel sheet used in automobile fuel tanks and the like. More particularly, the present invention relates to a chromate solution capable of maintaining excellent post-process corrosion resistance even after high temperature drying in an oven for curing the chromate film.

Materials used in fuel tanks, one of the main parts of automobiles, need not only excellent corrosion resistance and fuel resistance, but also various properties such as processability and weldability. Conventionally, as a fuel tank material, lead-tin alloy plated steel sheets in which hot-lead molten or electroplated lead-tin is widely used, and this steel sheet is commonly referred to as a turnne sheet.

However, in the case of pulverizing a fuel container made of turn steel sheet when the car is abandoned, there is a problem that dust is contained in a large amount of lead is released into the atmosphere causing air pollution. For this reason, Japan is tightening lead regulation. This movement is expanding worldwide, and there is a growing demand for developing alternative materials that do not use lead, which is harmful to the environment. In response to these demands, chrome-treated plated steel sheets using a new plating system that does not use lead have been developed and attempted to be applied to automobiles. Since chromate treatment is often used mainly for temporary rust prevention to prevent rust during transportation of products, there is a lot of development of products that are formed into a permanent film through a curing process after application by chromate treatment for resin treatment. .

As an example of a technique for applying chromate treatment to an automotive fuel tank, there is a special chromate-treated zinc-nickel plated steel sheet proposed in Korean Patent Application No. 97-703448 and Japanese Patent Application Laid-Open No. 9-59783. Special chromate treatment is a technique of forming a fine crack in the zinc-nickel alloy plating layer and applying a chromate solution to prevent corrosion degradation after processing. However, this technique brings a process load because a process for generating a fine crack in the steel sheet is added, and in particular, the fuel resistance deteriorates due to chromium elution by water contained in the fuel.

Accordingly, the present inventors have limited the chromate solution which improved the fuel resistance and corrosion resistance according to chromium elution without further processing to Korean Patent Application No. 98-39059 (Korean Patent Publication No. 2000-20448). The chromate solution proposed here is

(a) a main solution containing colloidal silica, hydrofluoric acid, phosphoric acid and sulfuric acid in an aqueous chromium solution,

(b) It is comprised by mixing the hardening | curing agent solution of an epoxy silane.

Furthermore, the present inventors have developed a chromate solution that can mitigate the film cracking during deep processing in consideration of the characteristics that deep processing is applied to a fuel tank, and has proposed in Korean Patent Application No. 1999-47845. This chromate solution is a new addition of sodium phosphate to the chromate solution proposed in 98-39059 to improve the properties of the coating.

The chromate solution is excellent in corrosion resistance and fuel resistance even without additional process. However, when the hydrophobic resin coating is applied to the inner surface of the fuel tank and the baking treatment is performed at a high temperature of 200 ° C. or higher, the initial corrosion resistance of the film is rapidly deteriorated. Occurred. Hydrophobic resin coating is applied to the inner surface of the fuel tank to prevent the fuel tank from being immersed in the fuel for a long time. Small dry powder at a high temperature of 200 ° C or more for sufficient curing reaction of the resin film. Will be carried out.

The present invention has been made to solve the problem of the chromate solution of the prior art, the object of the present invention is to provide a chromate solution excellent in corrosion resistance and fuel resistance after high-temperature drying of the chromate solution for the fuel tank.

Furthermore, an object of the present invention is to provide a method for manufacturing a plated steel sheet for fuel tank having excellent corrosion resistance and fuel resistance after processing at high temperature drying using the chromate solution.

The chromate solution of the present invention for achieving the above object is (a) 300 to 2000% by weight of chromium in the aqueous solution in a chromium solution of 0.4 to 0.8 of the trivalent chromium component in the chromium component of 7 to 50 g / l A main solution containing% colloidal silica, 20-40% by weight hydrofluoric acid, 50-80% by weight phosphoric acid, 5-15% by weight sulfuric acid and 25-40% by weight sodium phosphate,

(b) 5 to 30% by weight of a titanium having an alkoxy group and composed of a curing agent solution having a pH adjusted to 2-3;

(c) It is comprised by mixing the said hardening | curing agent solution at 20-50 weight% with respect to the said main solution.

The method for producing a chromate-treated steel sheet of the present invention, the coating of the chromate solution of the present invention to zinc or zinc-nickel alloy electroplated steel sheet 30 ~ 150mg / ㎡ based on the dry coating amount of coating steel plate temperature of 180 ~ 250 ℃ It is comprised including what was baked at.

Hereinafter, the present invention will be described in detail.

The chromate solution of the present invention,

First, using a titanium-based curing agent having an alkoxy group with excellent thermal stability, it can exhibit excellent post-processing corrosion resistance even after high temperature drying by preventing crosslinking with sufficient crosslinking even at high temperature drying.

Second, by adding a large amount of hydrophobic colloidal silica into the chromate solution, the hydrophobicity of the coating can be increased to reduce the dissolution of soluble hexavalent chromium, thereby ensuring excellent fuel resistance.

Third, through the structural improvement of the coating through the administration of additives in the chromate solution, there is a feature that can prevent the crack of the coating due to friction with the die surface during deep processing to obtain excellent post-processing corrosion resistance and fuel resistance. The present invention will be described by dividing into a chromate solution and a method for producing a steel sheet using the solution.

[Chromate solution]

The chromate solution of the present invention is mainly composed of a main solution and a curing agent solution, and the main solution contains an additive such as phosphoric acid, hydrofluoric acid, colloidal silica, sulfuric acid, sodium phosphate, and the curing agent solution is an aqueous titanium solution having an alkoxy group. to be.

Chrome solution

The aqueous chromium solution contained in the main solution is preferably dissolved so that the chromium component is 7 to 50 g / l, and the component ratio (chromium reduction ratio) of trivalent chromium in the chromium component is 0.4 to 0.8.

In the chromium aqueous solution of the present invention, when the composition ratio of trivalent chromium is lower than 0.4, it is difficult to obtain the effect of improving the corrosion resistance by trivalent chromium, and the dissolution of chromium is deteriorated due to the increase of the soluble hexavalent chromium. If the component ratio of is greater than 0.8, the solution storage is poor due to the gelation of the solution.

In the present invention, the concentration of the chromium component in which the component ratio of trivalent chromium is adjusted is set in consideration of the coating method of the chromate solution of the present invention. In the present invention, the coated chromate treatment method is preferable. In this case, the coated chromate treatment by the roll coating method is required to satisfy the conditions required for the control of the roll circumferential ratio, which is a control factor of the target deposition amount of chromium, and the pressure between the rolls. That is, due to the characteristics of the facility, there is a limited operating range in the roll circumferential ratio and the pressure control between the rolls, and when the chromium concentration is lower than 7 g / l, the target adhesion amount is not reached even through the limit work. The high case results in a decrease in surface stability along with a decrease in surface appearance due to spreading deterioration of the solution.

In one embodiment, the aqueous solution of chromium is dissolved in pure water at about 70-200 g / l of chromic anhydride, and then ethylene glycol is added to the dissolved chromate to give a trivalent chromium component ratio of 0.4-0.8. After reducing chromium to trivalent chromium compound, pure water can be prepared by administering chromium to a concentration of 7-50 g / l.

An additive is added to the chromium solution to improve the physical properties of the chromate solution. At this time, additives include phosphoric acid, hydrofluoric acid, colloidal silica, sulfuric acid, and sodium phosphate.

Phosphoric Acid

Phosphoric acid is preferably added at 50-80% by weight based on the chromium component of the aqueous chromium solution. If the amount of phosphoric acid is lower than 50% by weight, the effect of improving the solution properties and surface properties is not sufficient, and if the amount of phosphoric acid is higher than 80% by weight, the solution storage capacity decreases due to the increase of the component ratio of trivalent chromium and the corrosion resistance decrease due to excessive phosphoric acid component. Because it is caused.

Foshan

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

Silica

Silica (SiO ₂ ) is added in the colloidal state, which refers to silica having a pH in the range 2-4. Since colloidal silica forms crosslinks in the baking process, it suppresses the formation of zinc oxide in the base metal and is hydrophobic, thereby improving resistance to corrosive environments such as moisture, thereby improving corrosion resistance and contributing to the film-tightness. The addition amount is 300-2000% by weight based on the chromium component, since the effect of improving the physical properties is not sufficient when the amount of silica added is less than 300% by weight, and when the content is higher than 2000% by weight, the solution stability is lowered and the coating film adhesion is deteriorated. .

Sulfuric acid

Sulfuric acid has a great effect on the color of the solution, which can improve the surface quality of the steel sheet and improve the solution stability by improving the flowability of the solution. If the amount is less than 5% by weight of the chromium component, the effect of improving the surface properties is insignificant, and if more than 15% by weight will adversely affect the solution stability and corrosion resistance.

Sodium Phosphate

Sodium phosphate improves the properties of the chromate coating to prevent cracking of the coating that may occur during deep processing, thereby ensuring corrosion resistance and fuel resistance after processing. The amount of sodium phosphate is preferably 25 to 40% by weight. Do. If the added amount of sodium phosphate is less than 25% by weight, the effect of improving the film structure is insufficient. If the amount of the sodium phosphate is more than 40% by weight, precipitation in the solution occurs and solution stability is deteriorated.

Curing agent solution

The curing agent promotes the crosslinking reaction of the chromate to form a chromium film and also maintains the bonding force between the chromate film and the material metal. For this, it is important to select an appropriate curing agent. In case of drying at high temperature, thermal stability should be excellent, so that cracking should not occur during processing due to breakage of connective tissue.

In the case of using the silane-based curing agent as in the prior art, such cracks occur during processing, and the zinc component elutes into crack cracks in a wet environment, thereby rapidly deteriorating initial corrosion resistance. Therefore, the present invention is characterized by using a titanium-based curing agent having an alkoxy group so as to effectively crosslink the chromate film while excellent thermal properties in order to prevent such a phenomenon. Alkoxy groups are largely mono alkoxy groups and neo alkoxy groups. Neo alkoxy groups are recommended in view of excellent thermal stability.

The curing agent solution is administered with a titanium-based curing agent having an alkoxy group at a concentration of 5 to 30% by weight in pure water, and a small amount of phosphoric acid is added to prevent gelation when mixed with the main body, and the pH is similar to that of the main solution (about 2 to 3). Adjust with PH adjustment of the curing agent solution is possible in a variety of ways, more preferably with phosphoric acid. Phosphoric acid is considered that solution stability is good and main solution is phosphoric acid.

If the curing agent is less than 5%, the content of the curing agent to be administered when mixed with the subject to form chromate crosslinking at an appropriate temperature and time range increases, so that the total amount of chromium in the overall chromate solution is reduced, the target amount of adhesion during roll coating It is difficult to reach, and when it exceeds 30% by weight, the solution storage property is lowered, and stability such as precipitation occurrence in the solution is lowered when the curing agent solution for mixing with the main solution is stored alone. In addition, in order to maintain the bonding strength with the zinc or zinc-nickel alloy electroplated steel sheet material, a titanium-based curing agent having an alkoxy functional group should be used, which has a significant effect on the bonding strength between the material and the effective crosslinking of the chromate coating depending on the functional group type. Because you receive.

Mixing ratio of main solution and hardener solution

In the present invention, the curing agent solution is preferably mixed at 20 to 50% by weight based on the main solution. This is because when the curing agent solution is less than 20% by weight, the crosslinking reaction of the chromate does not occur sufficiently during film formation, and when it exceeds 50% by weight, the solution stability is lowered.

The chromate solution of the present invention may be applied as a fuel tank material by chromate coating on a plated steel sheet, or may be applied as a fuel tank material by resin treatment on the chromate film again. In addition, the chromate of the present invention can be applied to zinc-based galvanized steel sheet such as Zn, Zn-Ni, but is not limited thereto.

In addition, the chromate treatment method is largely divided into three types: reactive, electrolytic, and electrolytic. Reactive and electrolytic chromate may cause environmental problems due to dissolution of hexavalent chromium. It can be minimized and to ensure excellent properties in terms of corrosion resistance is most suitable for the present invention.

[Method of manufacturing chromate treated steel sheet]

The above-described chromate solution of the present invention is applied to a steel sheet, which is most suitable for zinc or zinc-nickel alloy electroplating trap plates. The coating may use a method such as a three-stage roll coater. The coating amount is applied on the steel sheet as 30-150mg / m2 on the basis of the adhesion amount of the dry coating, which is not enough to improve the corrosion resistance by the chromate coating when the adhesion amount is lower than 30mg / m2, and the solution is higher than 150mg / m2. This is because the cost rise of and the chromium solubility worsens.

After applying the chromate solution to the steel sheet and baked at a steel sheet temperature of 180 ~ 250 ℃, this is because the curing agent of the present invention occurs at a high temperature and should be heated to 180 ℃ or more for a sufficient curing reaction, in the case of 250 ℃ or more This is because the initial corrosion resistance is deteriorated because the hexavalent chromium component transitions to trivalent with the increase in manufacturing cost due to the increase in calories.

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

Example 1

After coating the chromate solution, as shown in Table 1 below on a plated steel sheet, the corrosion resistance and fuel resistance were evaluated, and the results are shown in Table 2.

Corrosion resistance was shown by performing a salt spray test and a cyclic corrosion test in accordance with JIS Z 2371 on the chromated steel sheet, and comparing the evaluation of the occurrence of white blue and red blue. Here, the salt spray test was sprayed with salt water (5%, NaCl solution) at a spray pressure of 1kg / ㎡, the temperature in the chamber was 35 ℃, the spray amount was 1ml per hour, and the composite corrosion test was carried out with salt spray (4 hours), drying ( 60 ℃, 4 hours), 1 day of the wet test (95% humidity, 50 ℃, 16 hours) was evaluated as 1 cycle. In addition, after the cup was processed into a gasoline (30 ml) / sodium chloride (5% NaCl, 1 ml) of chloride fuel was evaluated for fuel resistance.

division Topic solution Hardener solution Chromium concentration (reduction rate 0.5) (g / ℓ) Phosphoric Acid Sulfuric acid Foshan Colloidal silica Sodium phosphate Comparative A 24 60 6 25 800 35 30 weight% of silane system Comparative solution B Trimethoxy-titanium based 30% by weight Comparative solution C Alkoxy-titanium based 10% by weight Invention Solution 1 20 60 8 35 1000 30 Alkoxy titanium system 30 wt% Invention Solution 2 10 80 11 30 1300 40 Alkoxy titanium system 20 wt% Invention Solution 3 24 70 10 30 1800 27 50% by weight of alkoxy titanium system Phosphoric acid, sulfuric acid, hydrofluoric acid, colloidal silica, and sodium phosphate are added in weight% with respect to chromium, the curing agent solution is added in weight% with respect to main solution, and alkoxy uses neoalkoxy.

As shown in Table 2, in the case of the inventive material (1), the corrosion resistance and fuel resistance were superior to those of the comparative materials (2 to 4) at the baking temperature of 220 ° C., which is a high temperature. Excellent results were obtained compared to Comparative Material 1, which is a raw material, and Comparative Material 8, which was not treated with the chromate solution of the present invention.

In addition, in the case of the invention material (2) coated with the present invention solution on a pure zinc electroplated steel sheet, the corrosion resistance and fuel resistance after processing were superior to those of the comparative material. The comparative materials (5, 6) were treated outside the conditions specified in the present invention, and the corrosion resistance was lower than that of the inventive material, and in the case of the comparative material (7) whose chromate adhesion amount was lower than the conditions of the present invention. It has been shown that corrosion resistance is lowered.

As described above, the present invention has a useful effect of ensuring excellent corrosion resistance and fuel resistance after processing even at high temperature load.

Claims (4)

(a) 300-2000% by weight of colloidal silica and 20-40% by weight of hydrofluoric acid in a chromium solution containing 0.4 to 0.8 of a chromium solution in a chromium component of 7 to 50 g / L. A main solution containing 50-80% by weight phosphoric acid, 5-15% by weight sulfuric acid and 25-40% by weight sodium phosphate, (b) 5 to 30% by weight of a titanium having an alkoxy group and composed of a curing agent solution having a pH adjusted to 2-3; (c) A chromate solution for automobile fuel tanks having excellent corrosion resistance after high-temperature baking after mixing the hardener solution at 20 to 50 wt% with respect to the main solution. According to claim 1, wherein the pH of the curing agent solution is chromate solution for automobile fuel tanks excellent corrosion resistance after high temperature bushing processing, characterized in that the pH is adjusted. The chromate solution for automobile fuel tanks according to claim 1, wherein the alkoxy group is a neoalkoxy group. A method for producing a chromate-treated steel sheet, which is applied to a zinc or zinc-nickel alloy electroplated steel sheet at 30 to 150 mg / m 2 on the basis of the dry coating amount, followed by baking at a steel sheet temperature of 180 to 250 ° C.