JPH09184096A - Surface treatment for stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance of a stainless steel to a greater extent without deteriorating surface luster by applying anode electrolytic treatment and cathode electrolytic treatment to the stainless steel under respectively specified conditions. SOLUTION: A stainless steel material such as SUS304, having relatively low corrosion resistance, is subjected to anode electrolytic treatment by using it as anode in an electrolyte containing nitric acid and chromic acid by (50 to 250)g/l and (50 to 250)g/l, respectively, at a relatively small quantity of electricity of (10 to 50)C/dm<2> . Subsequently, cathode electrolytic treatment is performed in the same electrolyte at a quantity of electricity of (50 to 300)C/dm<2> . By this method, the characteristic corrosion resistance of the stainless steel itself can be improved to a greater extent by the strengthening of a passivating film composed essentially of chromium by nitric acid and the electrodeposition action of metallic chromium without deteriorating the surface luster and color tone of the stainless steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method in which a stainless steel is electrolytically treated to substantially improve the corrosion resistance without substantially impairing the surface gloss.

[0002]

2. Description of the Related Art For example, JP-A-59-6398 discloses that corrosion resistance is improved by electrolytically treating a BA material of a ferritic stainless steel sheet in an aqueous nitric acid solution. Further, for the purpose of improving the corrosion resistance of stainless steel, a solution containing phosphoric acid, chromic acid, magnesium oxide, sodium silicate, after cathodic electrolytic treatment, with a solution containing phosphoric acid, molybdate, cathodic electrolytic treatment A method of performing the above is disclosed in Japanese Patent Publication No. 59-12755. Further, as another conventional surface treatment method, there is a chromium plating method. This method is a method of improving corrosion resistance by depositing metallic chromium on the surface of stainless steel.

Among these methods, the electrolytic treatment method using a nitric acid aqueous solution can obtain a certain degree of improvement in corrosion resistance, but cannot expect a significant improvement in corrosion resistance. Further, the method disclosed in JP-B-59-12755 has insufficient corrosion resistance, and the composition of the treatment solution is complicated.
The productivity is poor because two processing solutions are used. In addition, the chromium plating method has a problem in that although the corrosion resistance improving effect is obtained, it consumes a large amount of power, has a high processing cost, and is inferior in productivity.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and provides a surface treatment method capable of significantly improving the corrosion resistance of a stainless steel without substantially impairing the surface gloss. With the goal.

[0005]

The present inventor has conducted various studies on the surface treatment for further improving the corrosion resistance of stainless steel. As a result, an aqueous solution of nitric acid and chromic acid has been used as the first treatment. It was found that by performing the anodic electrolysis treatment as the second treatment after performing the anodic electrolysis treatment with a quantity of electricity, the corrosion resistance is significantly improved with almost no loss of the surface gloss of the stainless steel. Although the details of the mechanism of the effect of improving corrosion resistance are unknown, it is presumed that nitric acid strengthens the passive film mainly composed of chromium and the action of electrodeposition of metallic chromium.

That is, in the present invention based on such knowledge, nitric acid 50 to 250 is used as the first treatment for stainless steel.
After performing an anodic electrolysis treatment with an amount of electricity of 10 to 50 coulombs / dm ² with an aqueous solution containing g / liter and chromic acid of 50 to 250 g / liter, as the second treatment, the amount of electricity is 50 to 300 coulombs / dm ^2. A surface treatment method for stainless steel, characterized by performing a cathodic electrolysis treatment of dm ² .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention will be described in detail below. The material to be treated in the present invention may be stainless steel, but it is effective to apply it to general-purpose steels having relatively low corrosion resistance such as SUS304 and SUS430. Next, the reasons for limiting the conditions of the present invention will be described.

FIG. 1 shows the relationship between rust resistance and the concentrations of nitric acid and chromic acid. In the figure, no rusting was evaluated as ◯, slightly rusting was Δ, and rusting was evaluated as x. As the test material, SU
Using S304, 2B material, an anodic electrolysis treatment was performed at an electrolysis amount of 10 coulombs / dm ² in the first treatment, and 50 coulombs / dm ^{2 in} the second treatment at a solution temperature of 50 ° C. and nitric acid concentration and chromium. As a rust resistance test (salt water spray), the materials treated with different concentrations were repeatedly sprayed for 8 hours and suspended for 16 hours 10 times. Other conditions were JIS (Z23
71).

From FIG. 1, it is understood that the nitric acid in the aqueous solution for electrolytic treatment of the present invention is preferably 50 to 250 g / liter. That is, if it is less than 50 g / liter, a surface film sufficient to improve corrosion resistance is not formed, and if it exceeds 250 g / liter, the effect of improving corrosion resistance is saturated and the amount of nitric acid taken out is large, which is not economical. A desirable range for nitric acid is 100 to 200 g / liter. Also, the concentration of chromic acid is preferably 50 to 250 g / liter. That is, if it is less than 50 g / liter, a sufficient anticorrosion effect cannot be obtained, and if it exceeds 250 g / liter, the effect of improving corrosion resistance is saturated and the amount of chromic acid taken out is large, which is not economical. The preferred concentration range is 100-200 g / liter.

FIG. 2 shows the relationship between rust resistance and the amount of electrolysis.
That is, using SUS304, 2B material as the test material, nitric acid: 100 g / liter, chromic acid 100 g / liter, and solution temperature: 50 ° C. as the treatment solution,
The material treated under the above conditions was subjected to a rust resistance test (salt spray test), where spraying for 8 hours and pause for 16 hours were repeated 10 times, and the other conditions are the results according to JIS (Z2371).

As is clear from the figure, the amount of electrolysis when the anodic electrolysis treatment (first treatment) is carried out in the above aqueous solution is 1
It is preferably 0 to 50 coulombs / dm ² . The reason is that if it is less than 10 coulombs / dm ² , the dirt adsorbed on the surface of the stainless steel cannot be effectively removed.
This is because the anticorrosion effect due to the electrodeposition of metallic chromium in the treatment cannot be obtained, and when it exceeds 50 coulombs / dm ² , the gloss of the stainless steel material is reduced. A desirable range of the amount of electricity is 30 to 40 coulombs / dm ² .

From the figure, the quantity of electricity in the case of performing the cathodic electrolysis after the anodic electrolysis is 50 to 300 coulomb / dm.
It turns out that ² is better. That is, the reason for limiting in this way is that if it is less than 50 coulombs / dm ² , a sufficient surface film having an anticorrosion effect is not formed, and if it exceeds 300 coulombs / dm ² , the anticorrosion effect is saturated and discoloration occurs. Is. The desirable range of electricity is 150
~ 200 coulombs.

The reason why the cathodic electrolysis is carried out after the anodic electrolysis is that the surface of the stainless steel material is slightly dissolved and the dirt adsorbed on the surface is effectively removed. This is to make the electrodeposition of metallic chromium uniform. In addition, the temperature of the electrolytic treatment solution of the present invention is about room temperature, but a temperature of 40 to 60 ° C. is more preferable.

[0014]

EXAMPLES Examples of the present invention will be shown below. 1m of SUS304 which is a typical austenitic stainless steel
An m-thick 2B product stainless steel plate was used as a material to be treated. The product characteristics of the materials obtained in the examples of the present invention and the comparative examples are shown in Tables 1 to 4. The temperature of the treatment solution was constant at 50 ° C.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

The evaluation methods and evaluation criteria for the product characteristics shown in Tables 1 to 4 are based on the following methods. (A) Pitting corrosion potential measurement test The test was performed by the pitting corrosion potential measurement method specified by JIS (G0577) method. The measured potential (mV, vs SCE) is described. (B) Salt spray test The spraying for 8 hours and the rest for 16 hours were repeated 10 times. Other methods were based on JIS (Z2371) method. Rating: ○: No rusting △: Slight rusting ×: Severe rusting

(C) Surface Gloss The surface reflectance of the steel sheet after the surface treatment was evaluated. The surface reflectance is indicated by irradiating a sample surface with a light beam having a diameter of 1 mm using a tungsten lamp as a light source at an incident angle of 45 degrees and setting the intensity of specular reflection light as 100% in the case of silver vapor deposition. Table 5 shows the relationship between the surface reflectance and the gloss rank. (D) Color tone The surface of the stainless steel plate before and after the surface treatment was visually evaluated. ○ indicates no change × indicates discoloration

[0021]

[Table 5]

It can be seen that the stainless steel sheet electrolytically treated by the method of the present invention is excellent in corrosion resistance (pitching corrosion potential measurement result, salt spray test result) and surface gloss, and the color tone does not change. On the other hand, the electrolytic treatment method of Comparative Example is inferior in corrosion resistance, surface gloss, color tone or economy.

[0023]

As described above, according to the method of the present invention, the stainless steel sheet is subjected to the electrochemical surface treatment,
Since it is possible to efficiently produce a stainless steel plate having significantly improved corrosion resistance without impairing the surface gloss and color tone, the industrial effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between rust resistance and nitric acid concentration and chromic acid concentration.

FIG. 2 is a diagram showing the relationship between rust resistance and the amount of electrolytic electricity.

Claims (1)

[Claims] 1. Stainless steel as the first treatment is treated with nitric acid 50 to 250 g / liter and chromic acid 50 to 250 g / liter.
Aqueous solution containing 10 liters of electricity, 10-50 coulomb /
After performing anodic electrolysis treatment of dm ² , as a second treatment, a cathodic electrolysis treatment of 50 to 300 coulomb / dm ² is performed with the aqueous solution, which is a surface treatment method for stainless steel.