JPH09291397A - Production of stainless steel improved in antibacterial property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart antibacterial properties to a stainless steel itself by increasing the concn. of Cu in a surface oxidized film and to provide a stainless steel maintaining excellent antibacterial properties oer a long period without deteriorating various characteristics such as beautiful appearance and workability characteristic of the stainless steel. SOLUTION: A stainless steel sheet in which a porous oxidized film is grown on the surface is used as the cathode and is subjected to cathode electrolyzing treatment under the conditions of 0.4 to 5.0A/dm<2> current density, 30 to 60 deg.C electrolyzing soln. temp. and 30 to 600sec treating time in an electrolyte composed of an aq. soln. of phosphoric acid contg. 0.2 to 5.0mol/l chromic acid and 0.4 to 5.0mol/l copper salt. The porous oxidized film can be formed by immersing treatment into a soln. of chrmic acid-sulfuric acid. As the electrolyte, the aq. soln. of 1.0 to 4.0mol/l chromic acid concn. and 0.01 to 0.03mol/l phosphoric acid concn. is preferably used.

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 stainless steel in which the oxide film on the steel surface has an improved antibacterial property.

[0002]

2. Description of the Related Art Stainless steel, which is excellent in corrosion resistance, is used as a hard material used in places where various bacteria are easily propagated or where the generation of various bacteria is undesirable. However, in the present situation where there is an increasing tendency to worry about the adverse effects of contamination, malodor, slime, etc., on the human body, animals, products, etc. due to the propagation of various bacteria, kitchens, medical institutions, and a large number of people who particularly need cleanliness Suppression of various bacteria has come to be required in gathering buildings and the like. It is known that one of the properties of stainless steel is antibacterial property. However, the degree of antibacterial property is not satisfactory, and if the antibacterial property can be further improved, it is preferably used as a building material excellent in hygiene. As a method for improving antibacterial properties, there are methods of coating and laminating a resin containing an antibacterial agent on the surface of stainless steel, plating of a matrix containing an antibacterial agent component, and the like. It is introduced in the -10191 publication.

[0003]

When a resin containing an antibacterial agent is applied and laminated on the surface of stainless steel, the metallic luster peculiar to stainless steel is impaired and the commercial value is lowered. Moreover, the antibacterial film is easily damaged by cracking, chipping, abrasion, etc. during processing or during use. When the defective portion is exposed to a humid atmosphere, the antibacterial component is eluted and the appearance is deteriorated, and the original antibacterial action is impaired. In addition, when the antibacterial agent is exhausted, the remaining film may rather serve as a nutrient for miscellaneous bacteria and promote the propagation of miscellaneous bacteria. In the case of composite plating containing an antibacterial agent, the adhesion of the plating layer is not sufficient,
There is a drawback that the workability is reduced. In addition, not only the appearance may be deteriorated due to dissolution, wear, and defects of the plating layer, but also the antibacterial action may be decreased.

Since each method uses an antibacterial agent, the eluted antibacterial agent may adversely affect the human body and the environment. Therefore, it is desired to impart antibacterial property to the stainless steel itself instead of the method of coating the antibacterial agent component. The present invention has been devised in order to meet such requirements, and by adding Cu to the surface oxide film, the stainless steel itself has antibacterial properties, and has a beautiful appearance and workability peculiar to stainless steel. It is an object of the present invention to provide a stainless steel that maintains excellent antibacterial properties for a long period of time without impairing various properties such as, and is safe for the human body and the environment.

[0005]

In order to achieve the object, the stainless steel manufacturing method of the present invention uses a stainless steel plate having a porous oxide film grown on its surface as a cathode, and chromic acid of 0.2-5. In an electrolytic solution composed of a phosphoric acid aqueous solution containing 0 mol / l and a copper salt of 0.4 to 5.0 mol / l, a current density of 0.4 to 5.0 A / dm ² , an electrolytic solution temperature of 30 to 60 ° C.,
It is characterized in that the cathodic electrolytic treatment is performed under the condition that the treatment time is 30 to 600 seconds. The porous oxide film can be formed by immersion treatment in a chromic acid / sulfuric acid solution. As the electrolytic solution, a chromic acid concentration of 1.0 to 4.0 mol / l and a phosphoric acid concentration of 0.01
An aqueous solution of ~ 0.03 mol / l is preferred. The oxide film formed on the surface of the stainless steel, which is the object of the present invention, is a so-called color-developing film, and an interference color appears when the thickness exceeds a certain level. Further, the antibacterial property can be similarly improved for stainless steel having a colorless oxide film that is thin to the extent that interference color is not developed.

[0006]

When the stainless steel having an oxide film is used as a cathode and the cathode treatment is performed in an electrolytic bath containing chromic acid and a copper salt, Cr and Cu are deposited in the pores of the porous oxide film from the electrolytic bath. The deposited Cr has a function of sealing the oxide film,
It hardens. On the other hand, the precipitated Cu elutes and ionizes a very small amount on the surface of the oxide film in a humid environment where bacteria can easily propagate. Ionized Cu has a high antibacterial effect, and even if it is a very small amount, it efficiently reacts with the respiration and metabolic enzymes of the bacterial cells existing near the surface of the oxide film to inactivate it. As a result, the growth of bacteria is suppressed, an excellent bactericidal action is obtained, and excellent wear resistance and corrosion resistance are exhibited.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Stainless steel to which antibacterial properties are imparted first forms a colorless oxide film or a colored oxide film on the steel surface. In this case, as long as an oxide film is formed, firing method, electrolysis method, molten salt method using chromic anhydride or dichromate, oxidation method using alkali salt, dipping method using sulfuric acid-chromic acid solution Etc., any method may be used. However, oxide scale produced by annealing is not included in the oxide film in the present invention because it is not porous and has low corrosion resistance. Cathodic electrolysis is performed in a electrolytic bath containing chromic acid and a copper salt, using stainless steel on which a colorless or colored oxide film is formed as a cathode. At this time, chromic acid 0.2
The 5.0 mol / l and copper 0.4 to 5.0 mol / l phosphoric acid aqueous solution adjusted to a concentration of use as an electrolytic solution, current density 0.4~5.0A / dm ^2, the electrolytic solution Temperature 30-60
Cathodic electrolysis is performed under conditions of a temperature of 30 to 600 seconds.

Chromic acid concentration in the electrolytic solution is 0.2 mol / l
If it is lower than the above range, the oxide film is not sufficiently hardened and wear resistance is lowered. On the contrary, when the chromic acid concentration is higher than 5.0 mol / l, the amount of chemicals consumed is increased and the treatment cost is increased. When the concentration of the copper salt is lower than 0.4 mol / l, the precipitation of Cu in the oxide film becomes unstable and the antibacterial property varies. On the contrary, 5.0 mol / l
Higher copper salt concentrations increase processing costs only at the expense of chemicals consumption. Further, with an electrolyte solution containing no phosphoric acid, the oxide film is not sufficiently hardened and wear resistance is reduced. In this respect, the phosphoric acid concentration is preferably in the range of 0.01 to 0.03 mol / l in order to obtain the desired wear resistance. At a phosphoric acid concentration of more than 0.03 mol / l, property improvement commensurate with the increase in concentration is not observed, and a large amount of chemicals are consumed, resulting in high treatment cost.

As electrolysis conditions, the current density is set to 0.4 A / dm ² or more in order to stably deposit Cu in the oxide film. At current densities below 0.4 A / dm ² , C
The precipitation of u becomes unstable and the imparted antibacterial property varies. However, if the current density exceeds 5.0 A / dm ² , the amount of Cu deposited will increase rapidly and the surface appearance will be impaired. Further, when the temperature of the electrolytic solution is lower than 30 ° C., gas is remarkably generated and electrolysis efficiency is deteriorated. Although the electrolytic solution temperature of more than 60 ° C. has sufficient effect, it is not industrial because the evaporation rate of water is high and it becomes difficult to maintain the electrolytic solution at a constant concentration. If the treatment time is less than 30 seconds, the oxide film will not be sufficiently hardened and the abrasion resistance will decrease. Conversely, 600
A long-time processing that exceeds a second consumes electric power unnecessarily, and reduces production efficiency.

The antibacterial treated stainless steel produced in this way is no different in appearance from the usual one containing no Cu. In addition, since it can be subjected to the same hardening treatment as usual, it becomes a film that shows excellent characteristics in corrosion resistance and wear resistance,
Highly stable antibacterial property is maintained for a long period of time.
Furthermore, as widely used as tableware, containers, etc.,
Since Cu is harmless to the human body or the like in a normal use environment, the antibacterial treated stainless steel does not adversely affect the human body or the environment.

[0011]

[Example] Stainless steel SUS30 having a plate thickness of 0.38 mm
The BA finishing material of No. 4 and the BA finishing material of stainless steel SUS430 having a plate thickness of 0.35 mm were cut into 50 mm × 50 mm to obtain test pieces. This test piece contains 0.06 wt% Cu of SUS304 and 0.01 wt% Cu of SUS430.
Was included. 2.5 mol of CrO ₃ held at 80 ° C /
The test piece was immersed in a solution of 1, 5 H ₂ SO ₄ 5 mol / l for 10 minutes to grow an oxide film. The oxide film is (Cr, F
e) A porous film mainly composed of spinel type fine crystals represented by ₂ O ₃ .FeNiO.xH ₂ O. The pores of the film consist of a complex sponge-like structure with a diameter of 20 nm.
Below 10 ¹⁵ pieces / m ² exist and have a surface area of 20-30
Account for%. The stainless steel test piece on which the oxide film was formed was immersed in an electrolyte solution having a bath temperature of 40 ° C.
Cathodic electrolysis was performed for a minute. The comparative example 12 in Table 1 is SUS3.
The original plate of No. 04, Comparative Example 13 was cathodic electrolyzed in a Cu-free electrolytic solution, and Comparative Example 14 was cathodic electrolyzed in a Cr-free electrolytic solution. Moreover, SUS430 was used as a test piece in the test numbers 1 to 6 according to the present invention, and SUS304 was used as a test piece in the test numbers 7 to 14.

[0012]

The antibacterial property of each test piece after the cathodic electrolysis treatment was investigated by the following method. Escherichia c
oli IFO3301 (E. coli) and Staphyl
ococcus aureus IFO12732 (Staphylococcus aureus) 3 each in normal broth medium
A culture solution was prepared by culturing at 5 ° C for 16 to 20 hours. A bacterial solution was prepared by diluting this culture solution 20,000 times with a sterile phosphate buffer solution. Bacteria solution on the surface of the test piece 1m
Then, the solution was dropped and stored at 25 ° C. for 24 hours. After storage, the test piece was washed away with SCDLP medium, and the obtained solution was used as a pour plate culture method using standard agar medium (culture at 35 ° C for 2 days).
The viable cell count was measured by. According to this test method, it can be said that a material having a stronger antibacterial property is obtained as the viable cell count after 24 hours is decreased from the initial viable cell count. In addition, in order to confirm that there was no abnormality in the test, the bacterial solution was dropped directly on the petri dish as a reference, and the viable cell count was measured in the same manner. The test results are evaluated as highly reliable when there is no significant increase or decrease in the reference viable cell count.

[0014]

As is clear from Table 2 showing the test results,
Chromic acid concentration in the electrolytic solution is 0.2 mol / l or more, Cu concentration is 0.4 mol / l or more, and current density is 0.4 to 5.0.
In Test Nos. 1 to 11 in which cathodic electrolysis was performed at A / dm ² , it was found that all of the test pieces had a small number of viable cells after 24 hours and exhibited good antibacterial properties. The surface appearance of the test pieces 1 to 11 was also good. On the other hand, in the test plate of test number 12 and the test piece of test number 13 subjected to cathodic electrolysis in the electrolyte solution containing no Cu, the antibacterial property after 24 hours is low. Further, in the test piece of Test No. 14 which was cathodic electrolyzed in a Cr-free electrolytic solution, although good antibacterial property was observed, discoloration was observed because a large amount of Cu was deposited on the surface. Further, as a result of various treatments of the chromic acid concentration and phosphoric acid concentration of the electrolytic solution, when the chromic acid concentration is less than 1.0 mol / l and the phosphoric acid concentration is less than 0.01 mol / l, the wear resistance of the surface is low. It was found to be insufficient, and the antibacterial property was lowered in a relatively short time, and it was found to be impractical.

[0016]

As described above, in the present invention, by subjecting the stainless steel on which the oxide film has been formed to cathodic electrolytic treatment under specific conditions, an oxide film containing Cu and having a sealing treatment is formed. Therefore, an antibacterial treated stainless steel is produced that maintains a stable antibacterial property at a high level for a long period of time without impairing the originally required properties such as appearance, corrosion resistance, and abrasion resistance. The stainless steel thus improved in antibacterial properties is used in a wide range of fields such as various machines and appliances which emphasize hygiene, kitchen supplies and building materials.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akihiro Ando 7-1 Takatani Shinmachi, Ichikawa City, Chiba Nisshin Steel Co., Ltd. Technical Research Laboratory (72) Inventor Hiromitsu Fukumoto 7-1 Takatani Shinmachi, Ichikawa City, Chiba Japan Shin Steel Co., Ltd. Technical Research Center

Claims (3)

[Claims] 1. A phosphor containing 0.2 to 5.0 mol / l of chromic acid and 0.4 to 5.0 mol / l of copper salt as a cathode of a stainless steel plate having a porous oxide film grown on its surface. Current density 0.4 to 5.0 A / d in an electrolytic solution composed of an aqueous acid solution
m ² , electrolyte temperature 30 to 60 ° C., treatment time 30 to 600
A method for producing stainless steel with improved antibacterial properties, characterized by performing cathodic electrolysis under the condition of seconds. 2. A method for producing stainless steel with improved antibacterial properties, which comprises forming the porous oxide film according to claim 1 by immersion treatment in a chromic acid / sulfuric acid solution. 3. The antibacterial property according to claim 1 or 2, wherein an aqueous solution having a chromic acid concentration of 1.0 to 4.0 mol / l and a phosphoric acid concentration of 0.01 to 0.03 mol / l is used as an electrolytic solution. Improved stainless steel manufacturing method.