JPH07188976A - Surface treatment of stainless steel - Google Patents

Abstract

PURPOSE:To drastically improve the corrosion resistance of a stainless steel material hardly deteriorating the surface gloss by anodizing the stainless steel material in an aq. soln. of specified composition and then cathodizing the material. CONSTITUTION:A stainless steel material is dipped into an aq. soln. contg. 50-250g/l. nitric acid and 50-250g/l molybdate such as ammonium molybdate, the material is firstly anodized at the quantity of electricity of 2-100Asec/dm<2> to slightly dissolve the surface, and the contaminant adsorbed on the material is efficiently cleaned off. The material is then cathodized at the quantity of electricity of 2-100Acm/dm<2> in the soln. of the same composition to uniformly deposit molybdenum on the material surface. The corrosion resistance of the stainless steel material is improved without deteriorating its surface gloss.

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 which can greatly improve the corrosion resistance of stainless steel without substantially impairing its surface gloss.

[0002]

BACKGROUND OF THE INVENTION The present invention relates to a method of improving the corrosion resistance of stainless steel by electrolytic treatment. As a conventional technique, a ferritic stainless steel sheet BA material is electrolytically treated in an aqueous nitric acid solution to obtain corrosion resistance. Can be improved by, for example, JP-A-59-6398.
Japanese Patent Publication No. 61-2760 and Japanese Patent Publication No. 5-8275
It is described in Japanese Patent Publication No. Further, for the purpose of increasing the corrosion resistance of the stainless steel sheet, after cathodic electrolysis treatment in a solution containing phosphoric acid, chromic acid, magnesium oxide, sodium silicate, cathodic electrolysis in a solution containing phosphoric acid and molybdate. The method of processing is Japanese Patent Publication Sho 59-12755
It is proposed in Japanese Patent Publication No. Further, conventionally, there is a chromium plating method as another surface treatment method. This method is a method of improving corrosion resistance by electrodepositing 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 Japanese Patent Publication No. 59-12755 is not sufficient in corrosion resistance, the treatment solution is complicated, and since two treatment solutions are used, the productivity is poor. Further, although the chromium plating method can obtain the effect of improving the corrosion resistance, it has a problem that it consumes a lot 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 for significantly improving the corrosion resistance without substantially impairing the surface gloss of stainless steel. With the goal.

[0005]

Means and Actions for Solving the Problems
In order to solve the conventional problems associated with high corrosion resistance technology for stainless steel, as a result of various studies on surface treatment for further increasing the corrosion resistance of stainless steel, as a result, stainless steel was treated in an aqueous solution of nitric acid and ammonium molybdate. It was found that the corrosion resistance is significantly improved by performing the anodic electrolysis treatment as the first treatment and then performing the cathodic electrolysis treatment as the second treatment, with almost no loss of the surface gloss of the stainless steel. Here, although the details of the mechanism of the effect of improving the corrosion resistance are unknown, it is presumed that the action of strengthening the passivation film mainly composed of Cr by nitric acid and the electrodeposition of molybdenum metal. As the chemical substance that promotes the electrodeposition of molybdenum, molybdate may be used, but among them, it was also found that ammonium molybdate is the most excellent in terms of the corrosion resistance improving effect and the cost. It was also found that the corrosion resistance can be significantly improved by first performing an anodic electrolysis treatment in a solution containing nitric acid and ammonium molybdate and then performing a cathodic electrolysis treatment for electrodeposition of molybdenum. It is estimated that this is because molybdenum can be electrodeposited uniformly.

That is, according to the present invention, nitric acid 5 is used as the first treatment.
0-250 g / l, ammonium molybdate 50-2
Aqueous solution containing 50 g / l, electricity 2-100 Asec
After performing the anodic electrolysis treatment of / dm ² , as the second treatment,
The amount of electricity in the aqueous solution is ^{2 to} 100 Asec / dm ² . The gist is a method of surface-treating stainless steel, which is characterized in that cathodic electrolysis is performed.

The present invention will be described in detail below. The material treated in the present invention may be stainless steel, but S
It is effective to apply it to a material having relatively low corrosion resistance such as US430. Hereinafter, the reasons for limitation of the present invention will be described.
FIG. 1 shows the relationship between the nitric acid and ammonium molybdate concentrations and rust resistance, and FIG. 2 shows the relationship between the electrolysis amount and rust resistance.

The nitric acid concentration of the aqueous solution used for the treatment of the present invention is 50 to 250 g / l, but below 50 g / l,
250g / because it could not form a surface film sufficient to improve corrosion resistance
When it exceeds 1, the corrosion resistance improving effect is saturated and the amount of nitric acid taken out is large, which is not economical. Also, the ammonium molybdate concentration is 50 to 250 g / l,
If it is less than 50 g / l, a sufficient anticorrosion effect cannot be obtained,
When it exceeds g / l, the corrosion resistance improving effect is saturated and
The amount of ammonium molybdate taken out becomes large, making it uneconomical.

The reason why the amount of electricity when performing the electrolytic treatment of the anode and the cathode in the above-mentioned aqueous solution is limited to ² to 100 Asec / dm ² is that a surface film cannot be sufficiently formed at less than ² Asec / dm ² , and 100 Asec / dm ² If it exceeds, the film formed on the surface becomes thick and discoloration occurs. The reason for performing the cathodic electrolysis treatment after performing the anodic electrolysis treatment is that the surface of the stainless steel is dissolved very slightly, the dirt adsorbed on the surface is effectively removed, and the molybdenum electrolysis is performed by the cathodic electrolysis treatment. This is to make the wear uniform.

The preferable quantity of electricity of the present invention is 20 to 6
0 Asec / dm ² (current density 40-60 mA / c
m ² , electrolytic treatment time 5 to 10 sec). Further, the bath temperature of the electrolytic treatment solution of the present invention is about room temperature, but it is more preferable to heat it to 40 to 60 ° C.

[0011]

EXAMPLES Examples of the present invention will be shown below, but the present invention is not limited thereto. A brightly annealed 1 mm thick stainless steel plate of SUS430, which is a typical chromium-based stainless steel, was used as a material to be treated. The product characteristics of the material obtained by performing the electrolytic treatment condition within the range of the present invention are shown in Tables 2 and 3 (continued from Table 2). The temperature of the treatment solution was constant at 50 ° C.

The evaluation methods and evaluation criteria for the product characteristics shown in Tables 2 and 3 were as follows. (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 entered as it is. (B) Salt spray test 8 hours spraying and 16 hours rest were repeated 5 times.

Other methods are based on JIS (Z2371) method. Evaluation: ○: No rusting △: Slight rusting ×: Severe rusting (C) Surface gloss: The surface reflectance of the thin plate 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 the intensity of specular reflection light is 100% in the case of silver vapor deposition. Table 1 shows the relationship between the surface reflectance and the gloss rank.

[0014]

[Table 1]

(D) Color tone The surface of the stainless steel plate after the surface treatment was evaluated by appearance. ○ indicates no change × indicates yellow change.

[0016]

[Table 2]

[0017]

[Table 3]

It can be seen that the stainless steel plate surface-treated by the method of the present invention is excellent in corrosion resistance (results of pitting corrosion potential measurement and salt spray test) and surface gloss, and does not change in color tone. On the other hand, the surface treatment method of Comparative Example is inferior in corrosion resistance, surface gloss, color tone or economical effect.

[0019]

As described above, according to the present invention, a high corrosion-resistant stainless steel sheet having a significantly improved corrosion resistance is obtained by electrochemically treating the stainless steel sheet without impairing the gloss and color tone of the surface. Since it can be manufactured efficiently, it has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between rust resistance and concentrations of nitric acid and ammonium molybdate.・ Test material: SUS430 BA material ・ Rust resistance test: Salt spray test 8 hr spray-16 hr stop, repeated 5 times Other JIS (Z2371) method compliant ・ Electrolytic electricity: (+) 50 Asec / dm ² → (-) 5
0 Asec / dm ²

FIG. 2 is a diagram showing the relationship between rust resistance and the amount of electrolysis electricity.・ Test material: SUS430 BA material ・ Rust resistance test: Salt spray test 8 hr spray-16 hr stop, repeated 5 times Other JIS (Z2371) method compliant ・ Electrolytic solution: Nitric acid 50 g / l, ammonium molybdate 50 g / l

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Claims (1)

[Claims] 1. As the first treatment, nitric acid 50 to 250 g /
1 and an aqueous solution containing 50 to 250 g / l of ammonium molybdate, after performing an anodic electrolysis treatment with an electricity amount of ^{2 to} 100 Asec / dm ^2, a second treatment is a cathode with an electricity amount of ^{2 to} 100 Asec / dm ² . A method for surface treatment of stainless steel, characterized by performing electrolytic treatment.