Corrosion-resistant vehicle bracket and its manufacturing method

Abstract

The vehicle bracket according to the present disclosure comprises a base steel plate and a hot-dip Zn-Al-Mg plating layer formed on the base steel plate, with one or more cut surfaces formed thereon, and an anti-rust coating layer formed on the cut surfaces and the plating layer, the anti-rust coating layer being formed by immersion in an anti-rust coating liquid containing a metal coating liquid containing a metal nitrate and a surfactant in a mass ratio of 2 to 3:1.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to a vehicle bracket and a method for manufacturing the same. More specifically, the present disclosure relates to a vehicle bracket having a cut surface with improved corrosion resistance and a method for preventing the bracket from rusting. [Background technology]

[0002] Cold-rolled steel sheet (SPCC) has high formability and workability, and is therefore easy to bend, press, and draw, and is therefore widely used for parts such as vehicle brackets.

[0003] Cold-rolled steel sheets have high formability and workability, but they oxidize very easily once the oxide film peels off, and they may rust depending on the storage conditions. Therefore, cold-rolled steel sheets are mainly zinc-plated or painted.

[0004] On the other hand, vehicle brackets are exposed to moisture and are affected by the outside air, so they require a high level of corrosion resistance. Therefore, the process of forming the zinc plating layer and painting are carried out separately.

[0005] Vehicle brackets are manufactured in various shapes. WSS brackets for supporting EPB cables have two or more cut surfaces and curves, and are mainly manufactured by galvanizing the surface of a cold-rolled steel sheet to form a plating layer, and then pressing the cold-rolled steel sheet into the shape of the bracket.

[0006] In this case, the exposed portion of the base steel plate is formed into a complex cut surface by the press working, and the plating layer is damaged, resulting in a problem that the corrosion resistance of the entire bracket is reduced, which also affects the durability of the vehicle.

[0007] Therefore, there is an urgent need to develop a vehicle bracket with improved corrosion resistance.

[0008] As background art of the present disclosure, Patent Document 1 discloses a fixing bracket for assembling a vehicle fender panel. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Application Publication No. 10-2005-0089568 Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present disclosure to provide a vehicle bracket having a cut surface with improved corrosion resistance.

[0011] Another object of the present disclosure is to provide a manufacturing method for a vehicle bracket that can improve corrosion resistance by using press processing to form an anti-rust coating layer as a sealing film on the entire cut surface and the entire outer surface of a vehicle bracket having a cut surface.

[0012] All of the above and other objects of the present disclosure can be accomplished by the present disclosure as described below. [Means for solving the problem]

[0013] 1. One aspect of the present disclosure relates to a bracket for a vehicle.

[0014] This vehicle bracket comprises a base steel plate and a hot-dip Zn-Al-Mg plating layer formed on the base steel plate, with one or more cut surfaces formed thereon, and an anti-rust coating layer formed on the cut surfaces and the plating layer, the anti-rust coating layer being formed by immersion in an anti-rust coating liquid containing a metal coating liquid containing metal nitrate and a surfactant in a mass ratio of 2 to 3:1.

[0015] 2. In Example 1, the hot-dip Zn-Al-Mg plating layer may contain 1.5 to 3.0 wt % Zn, 1.5 to 2.5 wt % Al, 1.5 to 2.5 wt % Mg, and the balance being unavoidable impurities.

[0016] 3. In Example 1 or 2, the metal coating solution may contain 3 to 5% by weight of aluminum nitrate, 5 to 10% by weight of silicon dioxide, and the balance water.

[0017] 4. In any one of Specific Examples 1 to 3, the surfactant can contain 3 to 5% by weight of sodium sulfate, with the balance being water.

[0018] 5. In any one of Specific Examples 1 to 4, red rust can be prevented from forming in a salt spray test using conditions of a salt water concentration of 5%, a salt water spray rate of 1.5 ml / h, and continuous spraying for 740 hours or more.

[0019] 6. In any one of Specific Examples 1 to 5, a paint layer may not be formed on the rust-preventive coating layer.

[0020] 7. Another aspect of the present disclosure relates to a method for manufacturing a bracket for a vehicle.

[0021] The method for manufacturing a vehicle bracket includes: (a) a step of manufacturing an alloy steel plate including a hot-dip Zn-Al-Mg plating layer; (b) a step of pressing the alloy steel plate to produce a bracket shape; and (c) a step of immersing the bracket in a rust-preventive coating liquid to form a rust-preventive coating layer, the rust-preventive coating liquid containing a metal coating liquid containing a metal nitrate and a surfactant in a mass ratio of 2 to 3:1.

[0022] 8. In embodiment 7, the metallization solution may contain 3-5% by weight aluminum nitrate, 5-10% by weight silicon dioxide, and the balance water.

[0023] 9. In embodiment 7 or 8, the surfactant can contain 3-5% by weight sodium sulfate, with the balance being water.

[0024] 10. In any one of Specific Examples 7 to 9, in step (c), the rust-preventive coating liquid can be maintained at 40 to 60°C.

[0025] 11. In any one of Specific Examples 7 to 10, in step (c), the rust-preventive coating liquid can be maintained at a pH of 3.0 to 4.0.

[0026] 12. In any one of Examples 7 to 10, in step (c), the immersion time may be 50 to 70 seconds. Effect of the Invention

[0027] According to the vehicle bracket of the present disclosure, even in the case of a complex structure having one or more cut surfaces, an anti-rust coating layer can be added to the top of the cut surface, so that the entire plating layer does not corrode, and as a result, corrosion resistance is greatly enhanced.

[0028] According to the vehicle bracket of the present disclosure, the surface hardness can be increased by using a Zn-Al-Mg ternary plated steel sheet with high corrosion resistance, and an additional rust-proof coating layer can be formed on the cut surface, thereby significantly improving corrosion resistance.

[0029] According to the manufacturing method of the vehicle bracket according to another aspect of the present disclosure, there is no need to form a separate plating layer and an additional coating layer, and furthermore, the corrosion resistance can be improved by a single process of forming an anti-rust coating layer on the outer surface of the Zn-Al-Mg ternary plated steel sheet, thereby significantly improving the efficiency of the manufacturing process of the vehicle bracket. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a schematic diagram of a vehicle bracket according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a process flow diagram of a method for manufacturing a bracket for a vehicle according to another embodiment of the present disclosure. [Diagram 3]FIG. 3 shows scanning electron microscope images of a vehicle bracket with and without an anti-rust coating layer according to an embodiment of the present disclosure. [Figure 4] FIG. 4 shows photographs of a vehicle bracket according to an embodiment of the present disclosure according to salt spray time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Hereinafter, the present disclosure will be described in more detail with reference to the accompanying drawings. Meanwhile, the following drawings are provided merely to facilitate understanding of the present disclosure, and the present disclosure is not limited to the following drawings. In addition, the shapes, sizes, ratios, angles, numbers, and the like shown in the drawings are merely shown as examples, and the present disclosure is not limited to the contents shown in the drawings.

[0032] Throughout this specification, the same reference numerals refer to the same components. Also, in the description of the present disclosure, detailed descriptions of known related art will be omitted if it is determined that such detailed descriptions may unnecessarily obscure the subject matter of the present disclosure.

[0033] As used herein, the terms "comprising," "having," or "including," unless used in conjunction with the term "only," may mean that other elements may be added. Unless otherwise specified, the singular terms used herein are intended to include the plural terms.

[0034] Unless expressly stated otherwise, the component analyses are to be construed as including a margin of error.

[0035] When describing the positional relationship between two components using terms such as "above," "upper," "below," or "to the side," it is possible that one or more components are located between the two components unless the terms "immediately" or "directly" are used.

[0036] The positional relationships of "upper portion", "upper surface", "lower portion", "lower surface", etc. are described based on the drawings and do not indicate absolute positional relationships. In other words, the positions of "upper portion", "upper surface", "lower portion" and "lower surface" may change depending on the observation position.

[0037] In this specification, the numerical range "a to b" is defined as "≧a and ≦b".

[0038] Hereinafter, a vehicle bracket according to a specific example of the present disclosure will be described in detail with reference to the drawings.

[0039] FIG. 1 is a schematic diagram of a vehicle bracket according to an embodiment of the present disclosure.

[0040] Referring to FIG. 1, the vehicle bracket includes a base steel plate 100 , a plating layer 200 and an anti-rust coating layer 300 .

[0041] The base steel sheet 100 may comprise a steel sheet that may be plated with a hot-dip galvanized layer. In particular, the base steel sheet 100 may be a cold-rolled steel sheet.

[0042] When the base steel sheet 100 is a cold-rolled steel sheet, the coating layer has a uniform thickness and a clean surface so that the hot-dip galvanized coating layer can be easily formed.

[0043] If the base steel sheet 100 is a hot-rolled steel sheet, an oxide layer is formed by heat treatment, making it difficult to form a separate plating layer on the upper surface of the base steel sheet 100. In addition, the oxide layer itself has anticorrosive properties, so no additional plating process is required.

[0044] The hot-dip Zn—Al—Mg plating layer 200 may be formed by melting metal elements on the base steel sheet 100 .

[0045] Specifically, Zn-MgZn 2 Binary eutectic structure of Zn-Al-MgZn 2A ternary eutectic structure of Zn-MgZn may be formed. 2 In the process structure of MgZn 2 Since the Zn-Al-Mg content in the plating bath is more actively dissolved and the corrosion resistance of the cut surface is improved, a large amount of binary eutectic structure is formed, which is preferable. In the embodiment of the present disclosure, since the rust-proof coating layer is formed not only on the top surface of the coating layer but also on the cut surface, it is not necessary to adjust the Zn-Al-Mg composition in the plating bath to form a large amount of binary eutectic structure.

[0046] In a specific example, the hot-dip Zn-Al-Mg plating layer 200 may contain 1.5 to 3.0 wt % Zn, 1.5 to 2.5 wt % Al, 1.5 to 2.5 wt % Mg, and the balance being unavoidable impurities.

[0047] In the content range, Zn-MgZn 2 Binary eutectic structure of Zn-Al-MgZn 2 When the coating layer is exposed to a corrosive environment, MgZn 2 has a lower corrosion potential than Zn, so MgZn 2 is first dissolved in a corrosive environment. The dissolved Mg is then converted to Mg(OH) 2 This reduces the alkalinity in the cathode region, resulting in a stable Zn 5 (OH) 8 Cl 2 H 2 The formation of O can improve corrosion resistance.

[0048] The vehicle bracket has one or more cut surfaces, and an anti-rust coating layer is formed on the cut surfaces and the plating layer.

[0049] When the cut surface is formed, the hot-dip Zn-Al-Mg plating layer 200 may be removed, which may result in a deterioration in corrosion resistance. However, since the rust-preventive coating layer 300 is formed on the upper part of the cut surface, the corrosion resistance of the cut surface can be significantly improved.

[0050] If the anti-rust coating layer 300 is not formed on the upper part of the cut surface, corrosion such as white rust and red rust may occur in a corrosive environment.

[0051] The rust-preventive coating layer 300 is formed by immersion in an anti-rust coating liquid containing a metal coating liquid containing metal nitrates and a surfactant in a mass ratio of 2 to 3:1.

[0052] The rust-preventive coating layer 300 is formed by immersing the metal in an anti-rust coating liquid containing a metal coating liquid and a surfactant, and then drying the metal.

[0053] When the anticorrosive coating liquid contains a metal coating liquid containing a metal nitrate within the above range and a surfactant, the degree of dispersion of the metal nitrate in the anticorrosive coating liquid can be increased, so that the anticorrosive coating layer can be formed only by immersion. In addition, the thickness of the anticorrosive coating layer can be made uniform, and the metal coating liquid can be deposited on the plating layer with high durability.

[0054] In a specific example, the metallization solution may contain 3-5% by weight aluminum nitrate, 5-10% by weight silicon dioxide, and the balance water.

[0055] When the metal coating liquid contains aluminum nitrate within the above range, a fine crystal structure can be formed, and as a result, not only the corrosion resistance of the anti-rust coating layer but also the hardness of the anti-rust coating layer can be effectively increased.

[0056] Silicon dioxide can effectively improve the corrosion resistance of the alloy layer containing magnesium. Within the above range, silicon dioxide can improve not only the corrosion resistance but also the wear resistance, and as a result, the hardness of the rust-preventive coating layer can be increased.

[0057] In a specific example, the surfactant can contain 3-5% by weight sodium sulfate and the balance water.

[0058] Sodium sulfate is an anionic surfactant. Within the above range, the crystal structure of aluminum nitrate can be stabilized, and as a result, corrosion prevention can be improved.

[0059] In a salt spray test using conditions of a salt water concentration of 5%, a salt water spray rate of 1.5 ml / h, and continuous spraying for 740 hours or more, red rust can be prevented from forming on the vehicle bracket 1000.

[0060] The vehicle bracket 1000 has a significantly improved corrosion resistance, so that in a continuous salt spray test of 740 hours or more, even if salt water is sprayed continuously for 741 hours at a salt water concentration of 5% and a salt water spray rate of 1.5 ml / h, no red rust is formed. Therefore, the corrosion resistance can be significantly improved.

[0061] In a specific example, there may be no paint layer formed on top of the anticorrosive coating layer 300 .

[0062] The plating layer of the vehicle bracket 1000 does not have a separate paint layer formed thereon, and the rust-preventive coating layer 300 can be substituted for the rust-preventive paint.

[0063] The rust-preventive coating layer 300 is colored dark gray in comparison with the plating layer, which allows visual inspection to determine whether or not the rust-preventive coating layer has been formed.

[0064] Another aspect of the present disclosure relates to a method of manufacturing a vehicle bracket.

[0065] FIG. 2 is a process flow diagram of a method for manufacturing a bracket for a vehicle according to another embodiment of the present disclosure.

[0066] Referring to FIG. 2, the method for manufacturing a vehicle bracket includes the steps of (a) manufacturing an alloy steel plate including a hot-dip Zn-Al-Mg plating layer, (b) pressing the alloy steel plate to manufacture a bracket shape, and (c) immersing the bracket in an anti-rust coating liquid to form an anti-rust coating layer.

[0067] First, a hot-dip galvanized steel sheet is manufactured (S100).

[0068] The hot-dip galvanized steel sheet may be an alloy steel sheet having a base steel sheet and a hot-dip Zn-Al-Mg plating layer formed on the top of the base steel sheet.

[0069] An alloy steel plate is pressed to produce a bracket shape (S200).

[0070] The shape of the vehicle bracket can be determined by pressing, and one or more cut surfaces can be formed.

[0071] The hot-dip Zn-Al-Mg plating layer can be removed from the cut surface, exposing the iron components of the base steel sheet.

[0072] The method may include degreasing the vehicle bracket (S210).

[0073] The degreasing step removes any oily substances remaining on the top of the plating layer, so that the anti-rust coating layer can adhere securely.

[0074] The method may include, after the degreasing step, cleaning the vehicle bracket (S220).

[0075] In the step of cleaning the vehicle bracket, foreign matter on the surface can be removed in advance.

[0076] The anti-rust coating layer is formed by immersing the bracket in an anti-rust coating liquid (S300).

[0077] The anticorrosive coating liquid may contain a metal coating liquid containing a metal nitrate and a surfactant in a mass ratio of 2 to 3:1.

[0078] The anticorrosive coating solution can be prepared by mixing the metal coating solution and a surfactant within the above range, and the anticorrosive coating layer can be formed by adjusting the immersion conditions. The durability of the anticorrosive coating layer can be improved by stabilizing the crystal structure of the anticorrosive coating layer.

[0079] In a specific example, the metallization solution may contain 3-5% by weight aluminum nitrate, 5-10% by weight silicon dioxide, and the balance water.

[0080] The metal coating solution contains aluminum nitrate, which forms a fine crystal structure on the anti-rust coating layer, thereby improving the corrosion resistance and also increasing the hardness of the anti-rust coating layer.

[0081] In a specific example, the surfactant can contain 3-5% by weight sodium sulfate and the balance water.

[0082] The surfactant may contain sodium sulfate and may be used to prepare an anti-rust coating solution. The metal coating solution may be dispersed to stabilize the crystal structure.

[0083] In step S300, the rust-preventive coating liquid can be maintained at 40 to 60°C.

[0084] For example, the temperature is preferably 50 to 60°C, and more preferably 50°C.

[0085] Since the anti-rust coating liquid is maintained within this temperature range, the vehicle bracket can be uniformly coated by immersion.

[0086] In S300, the pH of the anti-rust coating solution can be maintained at 3.0 to 4.0.

[0087] Specifically, the pH is preferably 3.6 to 4.0, and more preferably 4.0.

[0088] The anticorrosive coating layer can be uniformly formed within this pH range, and the smoothness of the surface can be improved. If the pH exceeds the above range, precipitation may occur in the metal coating solution.

[0089] Within this pH range, the color of the anticorrosive coating layer can be dark, for example, the anticorrosive coating layer can have a dark gray color.

[0090] In S300, the immersion time may be 50 to 70 seconds.

[0091] When the immersion is performed within this time range, the thickness of the anti-rust coating layer can be made uniform and the average illuminance can be reduced, thereby forming a smooth surface for the vehicle bracket.

[0092] After immersion, the vehicle bracket is collected and dried at room temperature, thereby producing a vehicle bracket having an anticorrosive coating layer.

[0093] In the vehicle bracket of the present disclosure, a bracket shape having one or more cut surfaces is manufactured by pressing a base steel plate having a hot-dip Zn-Al-Mg plating layer with high corrosion resistance, and then an anti-rust coating layer is formed on the cut surfaces and on the upper surface of the plating layer, thereby significantly improving corrosion resistance and also increasing hardness.

[0094] High corrosion resistance can be ensured by simply immersing the vehicle bracket in an anti-corrosive coating liquid to form an anti-corrosive coating layer, and then drying the vehicle bracket. Therefore, there is no need to perform a separate painting process to deal with color changes in the vehicle bracket after painting, and this significantly improves the manufacturing efficiency of the vehicle bracket.

[0095] Examples are given below to aid in understanding the present disclosure. However, the following examples are merely illustrative of the present disclosure, and the scope of the present disclosure is not limited to the following embodiments.

[0096] Example 1. Manufacturing of vehicle brackets

[0097] A hot-dip galvanized steel sheet (an alloy layer of 1.5 wt % Zn, 1.5 wt % Mg, and 1.5 wt % Al) was produced, and the hot-dip galvanized steel sheet was cut by a press machine to produce a shape of a vehicle bracket.

[0098] A vehicle bracket having a cut surface was degreased, washed twice, and then immersed in a bath containing an anti-rust coating solution.

[0099] A metallization solution was prepared containing 5% by weight aluminum nitrate, 10% by weight silicon dioxide and 85% by weight water, and a surfactant was prepared containing 5% by weight sodium sulfate and 95% by weight water.

[0100] An anti-rust coating solution containing 25% by weight of a surfactant and 75% by weight of a metal coating solution was prepared and charged into a plating bath.

[0101] After determining whether the anti-rust coating liquid had a pH of 4.0 and adjusting the temperature of the anti-rust coating liquid to 50°C, a vehicle bracket having a cut surface was immersed in the anti-rust coating liquid in a plating bath for 60 seconds.

[0102] The vehicle bracket was collected, washed twice, and dried at room temperature (25° C.) for 24 hours or more to collect the vehicle bracket having the rust-proof coating layer.

[0103] Experimental example 1. Identification of anti-rust coating layer

[0104] FIG. 3 shows scanning electron microscope images of a vehicle bracket with and without an anti-rust coating layer according to an embodiment of the present disclosure.

[0105] 3, when the anticorrosive coating layer is not formed, the illuminance of the surface increases and becomes non-uniform as shown in Fig. 3A, whereas when the anticorrosive coating layer is formed on the upper surface of the plating layer again as shown in Fig. 3B, the anticorrosive coating layer is durably adhered to the upper surface of the plating layer, so that the surface is smooth and the crystal structure of the anticorrosive coating layer is stabilized.

[0106] Experimental Example 2. Anti-rust coating liquid composition

[0107] [Table 1]

[0108] Table 1 shows the shape of the anti-rust coating layer determined by the composition of the anti-rust coating solution.

[0109] Referring to Table 1, it can be seen that in Example 1, a smooth anticorrosive coating layer with high smoothness is formed. On the other hand, in Comparative Example 1, when the content of the metal coating liquid is reduced from that in the Examples, the anticorrosive coating layer is hardly formed by immersion alone. In Comparative Example 2, since the amount of the metal coating liquid after immersion is large, the thickness of the anticorrosive coating layer is non-uniform and the smoothness is low, and as a result, an additional painting process is required.

[0110] Experimental example 3: Salt spray test

[0111] In order to determine the corrosion resistance of the vehicle bracket manufactured according to the embodiment, a salt spray test was carried out.

[0112] According to the conditions of KS D 9502, the salt concentration was 5%, pH was 6.7, the temperature during the test was 35°C, the spray method was continuous spray, and the spray pressure was 1.0 kg / cm 2 The salt spray rate was adjusted to 1.5 ml / h.

[0113] Using a saltwater tester (VT ST-200), saltwater was sprayed onto the vehicle bracket manufactured according to Example 1 at an angle of 15 to 30°.

[0114] FIG. 4 shows photographs of a vehicle bracket according to an embodiment of the present disclosure according to salt spray time.

[0115] According to the results of spraying salt water for a total of 741 hours, white rust appeared on the vehicle bracket according to the embodiment after 429 hours. On the other hand, even after spraying salt water for more than 741 hours, no red rust appeared, demonstrating extremely high corrosion resistance. In particular, no red rust appeared on the cut surface.

[0116] Therefore, since the vehicle bracket according to the present disclosure is formed with an anti-rust coating layer and has a high corrosion resistance and significantly improved surface hardness, the vehicle bracket can be used as a WSS bracket for supporting an EPB cable used in an environment where corrosion and vibration are present. In particular, the corrosion resistance of the cut surface can be improved, and the corrosion resistance and hardness can be significantly improved for a WSS bracket having a shape more complicated than that of a simple hot-dip galvanized steel sheet.

[0117] The above describes the embodiments of the present disclosure. Those skilled in the art to which the present disclosure pertains can understand that modifications can be made without departing from the essential features of the present disclosure. Therefore, the disclosed embodiments should be considered as illustrative and not restrictive. The scope of the present disclosure is defined by the claims, not by the above description, and all differences within the scope of the equivalents should be interpreted as being within the scope of the present disclosure. [Explanation of symbols]

[0118] 1000 Vehicle Bracket 100 Base steel plate 200 plating layers 300 Anti-rust coating layer

Claims

1. A base steel plate; A hot-dip Zn-Al-Mg plating layer is formed on the base steel plate, One or more cut surfaces are formed; an anti-rust coating layer is formed on the cut surface and the plating layer; The vehicle bracket has an anti-rust coating layer formed by immersing the bracket in an anti-rust coating liquid containing a metal coating liquid containing a metal nitrate and a surfactant in a mass ratio of 2 to 3:

1.

2. 2. The vehicle bracket according to claim 1, wherein the hot-dip Zn-Al-Mg plating layer contains 1.5 to 3.0 wt % Zn, 1.5 to 2.5 wt % Al, 1.5 to 2.5 wt % Mg, and the balance being unavoidable impurities.

3. 2. The vehicle bracket of claim 1, wherein the metallization liquid contains 3-5% by weight aluminum nitrate, 5-10% by weight silicon dioxide, and the balance water.

4. 2. The vehicle bracket of claim 1, wherein the surfactant comprises 3-5% by weight sodium sulfate and the balance water.

5. 2. The vehicle bracket according to claim 1, which does not develop red rust in a salt spray test using conditions of a salt water concentration of 5%, a salt water spray rate of 1.5 ml / h, and continuous spraying for 740 hours or more.

6. 2. The vehicle bracket according to claim 1, wherein no paint layer is formed on the anticorrosive coating layer.

7. In a method for manufacturing a bracket for a vehicle, (a) producing an alloy steel sheet including a hot-dip Zn—Al—Mg plating layer; (b) pressing the alloy steel plate to produce a bracket shape; and (c) immersing the bracket in an anti-rust coating liquid to form an anti-rust coating layer; The method for manufacturing a bracket for a vehicle, wherein the anti-rust coating liquid contains a metal coating liquid containing a metal nitrate and a surfactant in a mass ratio of 2 to 3:

1.

8. 8. The method of claim 7, wherein the metallization solution contains 3-5% by weight aluminum nitrate, 5-10% by weight silicon dioxide and the balance water.

9. The method of claim 7, wherein the surfactant comprises 3-5% by weight sodium sulfate and the balance water.

10. The method according to claim 7, wherein in step (c), the anticorrosive coating liquid is maintained at 40 to 60°C.

11. The method according to claim 7, wherein in step (c), the anticorrosive coating solution is maintained at a pH of 3.0 to 4.

0.

12. The method of claim 7, wherein in step (c), the immersion time is 50 to 70 seconds.

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