JPH06235092A - Method for electrolytically chromating steel sheet excellent in color tone stability - Google Patents

Abstract

PURPOSE:To stabilize the developed color tone by bringing the surface of an electrogalvanized steel sheet into contact with an acidic soln. and then electrolytically chromating the sheet. CONSTITUTION:A steel sheet having <= about 3.2mm thickness is used as the starting material. The sheet is electrogalvanized. The surface of the electrogalvanized steel sheet is brought into contact with an acidic soln. at pH 0.5-4 contg. <=1g/l of Cr<6+> ion to dissolve off the zinc oxide layer of the surface plating layer and then electrolytically chromated. An inorg. acid such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid and hydrofluoric acid, an org. acid such as formic acid, acetic acid and oxalic acid, a chelating material such as an EDTA soln. and an metallic salt soln. such as the solns. of ferric chloride, nickel sulfate and cobalt chloride are exemplified as the acidic soln. The steel sheet is electrolytically chromated with the color tone stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an electrolytic type chromate-treated electrogalvanized steel sheet mainly used for home appliances.

[0002]

2. Description of the Related Art Chromate-treated electrogalvanized steel sheets are widely used for home electric appliances such as chassis and motor covers because of their excellent corrosion resistance and lubricity during processing. The chromate treatment method on the electrogalvanized steel sheet is roughly classified into the following three methods.

A) Coating type chromate b) Reactive type chromate c) Electrolytic type chromate The coating type chromate is formed by applying a chromate solution onto an electrogalvanized steel sheet with an air knife or a roll coater, and then using an oven without washing with water. It is baked and dried to form a chromate film. According to this method, since the amount of reaction between the chromate treatment liquid and the zinc plating layer is small, there is an advantage that the treatment liquid is less deteriorated by eluted zinc or the like and liquid management is easy. However, in order to form a sound chromate film, 100 ℃
Since the baking process at the above temperature is indispensable, an oven is required, which causes a high cost required for equipment for this and is not economical.

The second reactive chromate was produced by reducing galvanized steel sheet by immersing or spraying a galvanized steel sheet in a reactive chromate solution containing hexavalent Cr ions to dissolve the metallic zinc plating layer. A chromate film is formed on the plating by precipitating a chromium chromate consisting of valent Cr ions and hexavalent Cr ions in a gel form. Unlike coating type chromate, high temperature drying after chromating is not required, so the production cost is reduced, and reactive type chromate-treated electrogalvanized steel sheets with various Cr deposits are produced depending on the purpose and application. However, since the galvanized layer is dissolved in the film formation, Zn ions are inevitably accumulated in the treatment liquid, which causes a problem in the liquid control and a variation in the quality due to the change in the treatment liquid. Furthermore, since a chromate film is formed by a chemical reaction, it has a problem that it is difficult to control the amount of adhesion.

The third electrolytic chromate treatment is to form a chromate film by electrolytically treating a Zn-plated steel sheet as a cathode in a chromate treatment liquid containing hexavalent Cr ions. According to this method, the elution of zinc into the processing solution is reduced, and the solution management becomes easier,
The amount of electricity applied makes it possible to control the amount of adhesion, and is widely adopted industrially. For example, a method of adding a cationic silica sol in a treatment bath to improve corrosion resistance is disclosed in JP-A-60-1108, and a method of cathodic electrolysis followed by anodic electrolysis is disclosed in JP-A-62-263997. Many techniques relating to electrolytic chromating are disclosed.

However, although electrolytic chromated steel sheets usually produced by these methods can obtain good corrosion resistance and coating adhesion, they often have uneven color on the surface, and a stable appearance cannot be obtained. was there. Further, JP-A-3-223493 discloses a method of performing electrolytic chromate treatment with a uniform amount of deposit by immersing for 0.5 seconds or more in the absence of electric current prior to electrolytic chromate treatment in an acidic bath. According to this method,
Dirt and impurities on the plating surface are removed and the surface is made uniform. However, at this time, the chromate solution produces trivalent and hexavalent chromium chromates (chemical conversion chromates). It has been found that this chemical conversion chromate layer is affected by the state of the underlying plating layer. When the oxide film is thick on the surface of the plating layer, the film structure is rich in hexavalent chromium, and when the oxide film is thin and in an active state, the reduction reaction proceeds and the film structure is highly trivalent. The ratio of hexavalent to trivalent affects the performance of chromate, and particularly the b value, which is an index showing yellowness, greatly fluctuates. In other words, according to this method, although the amount of adhesion is uniform, a chromate film having different performance, especially a color tone, is formed by a manufacturing opportunity, which is not suitable for industrial production. Furthermore, it was eluted by etching the plating surface
Zn ions are accumulated, resulting in instability of the chromate treatment.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing an electrolytic chromate-treated electrogalvanized steel sheet having a stable color tone. is there.

[0008]

In order to achieve the above object, according to the present invention, during electrolytic chromate treatment on an electrogalvanized steel sheet, the plating surface has a pH of 0.5 or more and 4 or less after plating and before electrolytic chromate treatment. And in contact with an acidic solution having an amount of Cr ⁶⁺ ions of 1 g / l or less,
An object of the present invention is to provide a method for electrolytic chromating a galvanized steel sheet having a stable color tone.

[0009]

The present invention will be described in more detail below. As the starting material of the present invention, a thin steel plate having a plate thickness of 3.2 mm or less is used. After heat treatment for obtaining the required material, electrogalvanization is performed. Electrogalvanizing is generally performed in a plating bath used for industrial production. That is, it is composed of an additive that contains Zn ions and is added as needed for the purpose of improving conductivity and imparting gloss. The counter ion of Zn ion may be a generally used sulfate ion or chlorine ion, or a bath in which both are mixed may be used.

As the plating bath, any of carrousel plating which is one-sided sequential plating, horizontal cell which is simultaneously plated on both sides, and vertical type cell such as Gravidel can be used. A predetermined amount of zinc plating is applied in these plating tanks. In the present invention, the steel sheet thus electrogalvanized is subsequently brought into contact with an acidic solution having a pH of 0.5 or more and 4 or less before being subjected to electrolytic chromate treatment.

[0011] Generally, electrolytic chromate-treated electrogalvanized steel sheets produced are rinsed with neutral water immediately after plating to remove the plating solution on the surface, dried with a drier, and electrolytic chromate-treated, but often, There was a case where remarkable color unevenness was generated, and the appearance was impaired. As a result of diligent research conducted by the present inventors on this cause, it has been found that it is due to the oxide of zinc formed during plating. The mechanism of occurrence of uneven color tone will be described below.
During electrogalvanizing, the pH of the electrode interface rises because hydrogen is generated along with the reduction reaction of zinc ions to metallic zinc at the cathode. With this increase in pH, zinc hydroxide is deposited on the plating surface. This thrust layer is not removed by washing with neutral water, and zinc oxide is generated due to corrosion by the oxygen concentration battery in the process of drying with a dryer at a temperature of 80 ° C or higher. When electrochromic chromate is applied to plating with such a surface condition, there is a difference in electrode reactivity between the area where zinc oxide is attached and the area where metallic zinc is exposed, which causes uneven color tone. is there.

The inventors of the present invention have made further studies as to a method for removing zinc oxide on the plated surface for the purpose of improving the color unevenness of the electrolytic chromate-treated steel sheet. Further, they have found that the method of bringing the solution into contact with an acidic solution having an amount of Cr ⁶⁺ ions of 1 g / l or less is effective, and arrived at the present invention. The acidic solution used in the present invention includes hydrochloric acid, sulfuric acid, nitric acid,
Inorganic acids such as phosphoric acid, boric acid and hydrofluoric acid, organic acids such as formic acid, acetic acid and oxalic acid, chelating agents such as EDTA solution, metal salt solutions such as ferric chloride, nickel sulfate and cobalt chloride solution. These can be used alone or in combination.

When a metal salt which is more electrochemically noble than zinc is used here, the metal is deposited by substitution on the plating. However, the present invention is not intended to deposit such a metal and removes the oxide. Is the purpose. It is not desirable that the metal deposits on the zinc plating by contacting it with liquid more than necessary and the electrode reaction during electrolytic chromate is rather hindered.

When a phosphoric acid-based solution is used, a phosphate film is formed on the plating, and excessive contact with liquid is also undesirable. Attention is necessary in any case.
The method of contacting these acidic solutions may be either dipping treatment or spraying treatment. It is not always necessary to use special equipment, and it is possible, for example, to carry out exposure for several seconds in a non-energized state after plating in an acidic plating treatment bath.

The acidic liquid used in the present invention has a pH of 0.5 or more and 4 or less. This is because a strong acid having a pH of less than 0.5 is not suitable because not only the plating layer surface layer but also the plating layer itself is remarkably dissolved, and if the pH exceeds 4 it is not sufficient to dissolve and remove the zinc oxide layer of the plating surface layer. . The effects of the present invention are sufficiently exhibited in the range of pH 0.5 or more and 4 or less, preferably in the range of pH 0.8 to 2.

The amount of Cr ⁶⁺ contained in the acidic liquid is 1 g.
/ L or less. This is because if Cr ⁶⁺ is contained in an amount of more than 1 g / l, the chemical conversion chromate layer is formed on the underlayer as described above, and the performance, especially the color tone becomes unstable. The temperature of the acidic solution and the liquid contact time are not particularly limited because they vary depending on the type of solution used as described above, but zinc oxide on the plating can be sufficiently removed and the plating layer itself is unnecessarily dissolved. It must be done without any unnecessary coating of metal or the like. From this viewpoint, the temperature of the acidic solution is preferably 20 to 80 ° C., and the liquid contact time is preferably 0.1 to 30 seconds.

If necessary, washing and drying may be carried out after the contact with the acidic solution and before the electrolytic chromate treatment. However, as described above, when the drying temperature reaches a high temperature of 80 ° C. or higher, zinc oxide is used. May be formed, which is not preferable. The electrolytic chromate treatment is not particularly limited as long as it forms a chromate film on the plating by cathodic electrolysis treatment. Usually, hexavalent chromic acid is the main component, and if necessary, metal ions such as Zn, Ni and Co, anions such as chloride ions, sulfate ions, nitrate ions, inorganic acids such as silica sol and alumina sol, organic resins, etc. Is added.

Hereinafter, the present invention will be described more specifically based on examples.

[0019]

【Example】

Example 1 A cold rolled steel plate SPCC having a plate thickness of 0.7 mm was used as a material.
Electroplating with zinc was carried out in the following plating bath using a fluidized bed cell with platinum as the counter electrode. Plating bath; ZnSO ₄ / 7H ₂ O 430 g / l K ₂ SO ₄ 2 g / l Na ₂ SO ₄ 4 g / l pH 1.5 (adjusted with H ₂ SO ₄ ) Bath temperature 60 ° C Electrolytic degreasing → 5% sulfuric acid Washing → Electroplating → Washing 1
→ Dry 1 → Acidic solution treatment → Washing 2 → Drying 2 → Electrolytic chromate, in order, using the acidic solution shown in Table 1,
The treatment was performed under the conditions shown in Table 2.

The current density of electrogalvanizing is 100 A / dm ²
The coating amount was 20 g / m ² . The electrolytic chromate treatment includes CrO ₃ 50 g / l, Na ₃ SiF ₅ 1 g / l, cationic silica sol {Nissan Chemical Co., Ltd., trade name: Snowtex AK} 8 g / l as SiO _{2 at} 60 ° C. The solution was electrolyzed under the conditions of 10 A / dm ² and 10 C / dm ² . Immediately after the electrolytic chromate treatment, it was washed with water and dried with a dryer.

Appearance after chromate treatment is optional for sample
Color difference measurement for 10 points (TC-180 manufactured by Nippon Denshoku Industries Co., Ltd.)
0 SZ-Σ80 type) and the b value, which is an index of yellowness, was measured. The standard deviation σ of the b value was calculated to determine the color unevenness of the appearance, and the evaluation was performed as follows. σ ≦ 0.5: ◎ 0.5 <σ ≦ 1.0: ○ 1.0 <σ ≦ 3.0: Δ3.0 <σ: × The results are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

As is apparent from the table, all the chromate-treated electrogalvanized steel sheets of the present invention show a stable color tone.

[0025]

Since the present invention is constructed as described above, the electrolytic chromate treatment method for electrogalvanized steel sheet of the present invention can easily produce a product having a stable color tone, and is industrially widely used. Can be used.

Claims (1)

[Claims] 1. When electrolytically chromating a galvanized steel sheet, the plating surface has a pH of 0.5 to 4 and a Cr ⁶⁺ ion amount of 1 after plating and before electrolytic chromate treatment.
A method for electrolytic chromate treatment of an electrogalvanized steel sheet having a stable color tone, which comprises contacting with an acidic solution of g / l or less.