JP5835455B2 - Method for surface treatment of metal material - Google Patents

Description

  The present invention relates to a surface treatment method of a metal material for imparting a new function to the surface of the metal material, and a metal material surface-treated by this surface treatment method.

  When using a metal material surface as a functional material, the wettability of the metal material surface, that is, the hydrophilicity and water repellency of the metal material are important factors, and various properties can be imparted to the metal material by controlling this. Can do. For example, a metal material used in a heat exchanger is required to have an affinity between the inner and outer surfaces of a metal heat transfer tube and a medium such as water, that is, the hydrophilicity of the metal surface, in order to improve heat conduction. Further, imparting hydrophilicity to the surface has many advantages such as the expectation of a self-cleaning effect in which dirt attached with water flows down. For this reason, as a technique for imparting hydrophilicity to the metal surface, for example, Patent Document 1 discloses a technique of forming a porous oxide layer using corona discharge, and Patent Document 2 discloses etching involving film formation on the surface. A method of removing the film after application and forming a hydrophilic film is disclosed.

  However, the technique using corona discharge is a technique for forming an oxide layer on the surface, and the function is lost when the oxide layer is removed by peeling or the like. A method combining etching and a hydrophilic film is costly because the process is complicated and involves film formation. In addition, if the hydrophilic paint falls off during use, there is a problem in that the effect is reduced and it does not recover. On the other hand, regarding the water repellency of the surface of the metal material, for example, in an environment where moisture exists, metal materials such as steel materials are corroded by reacting with water. For this reason, make the metal material surface water-repellent (hydrophobic) so that the metal material surface is less wet with water, or even if water comes into contact with the metal surface, water can easily flow down from the metal surface. In recent years, a technique for imparting water repellency to suppress corrosion of a metal material has been proposed.

  For example, Patent Document 3 describes a technique for imparting water repellency to a steel sheet surface by applying an alkoxide such as Al or Zr to the steel sheet surface and heating to 100 ° C. or higher. Patent Document 4 describes a technique for imparting water repellency to a plated steel sheet surface by forming a coating layer of a metal coupling treatment compound on the surface of the plated steel sheet. Patent Document 5 describes a technique for imparting water repellency to a metal plate surface by applying a water-repellent paint to the metal plate surface. However, any of the above-mentioned methods forms a film with an expensive drug on the surface. If the film layer falls off due to peeling or the like, the water repellency is impaired, or the film formation process is complicated and expensive. There are problems such as.

On the other hand, in recent years, in addition to various performances inherent to steel sheets, attempts have been made to provide steel sheets with new functions such as stain resistance and deodorization using a photocatalyst (see Patent Documents 6-9). The technology that is the basis of such an attempt is to disperse photocatalytically active particles in a surface coating material or treatment layer. As the coating material, resin-based (see Patent Documents 6 and 7) or inorganic-organic A composite (see Patent Document 8) has been studied. In addition, as an attempt to impart a photocatalyst directly to a steel sheet, a technique for producing a TiO ₂ thin film on the steel sheet surface using a plasma-enhanced atomic layer deposition using plasma has been proposed. (Refer nonpatent literature 1).

JP-A-5-179419 JP 2002-53977 A Japanese Patent Laid-Open No. 01-68477 Japanese Unexamined Patent Publication No. 09-20983 JP 2008-75064 A JP 2000-14755 A JP 2001-131768 A JP 2007-268761 A JP 2002-53978 A

Chang-sooLee et al., Thin Solid Films 518 (2010) 4757-4761.

  However, the conventional hydrophilicity imparting technology and water repellency imparting technology are technologies that impart water repellency and hydrophilicity to the metal surface by forming a surface film on the surface of the metal material. Because it needs to be granted, extra effort and costs are required. As for the water repellency, it is not sufficient to simply provide a water repellent layer on the surface of the metal material, and it is necessary to perform a special treatment such as applying fine particles.

On the other hand, the conventional photocatalytic function imparting technique has a photocatalytic function on the surface of a steel sheet by dispersing photocatalytic active particles in a coating material or a treatment layer or by forming a film of a photocatalytic active substance. However, since the coating material and the treatment layer mainly composed of organic substances are decomposed by the photocatalytic active particles, it cannot be expected to maintain the photocatalytic function for a long period of time. Further, since a photocatalytically active substance or an organic material is used, the manufacturing process is complicated and the cost is increased. In addition, the method for forming a TiO ₂ thin film at the atomic level requires advanced techniques, is expensive, and is difficult to industrialize.

  The present invention has been made in view of the above problems, and the object thereof is a surface treatment method for a metal material capable of imparting a new function to the surface of the metal material without requiring much labor and cost, And it is providing the metal material surface-treated by this surface treatment method.

  The surface treatment method of a metal material according to the present invention includes a step of immersing a material to be treated as a cathode electrode made of a metal material having a surface to be treated and an anode electrode in an electrolytic solution, and applying a first voltage to the cathode electrode. Applying between the anode electrode, and applying a second voltage different from the first voltage between the cathode electrode and the anode electrode.

  In the metal material surface treatment method according to the present invention, in the above invention, the first voltage is 70 V or more and is in a voltage range in which the cathode electrode is not oxidized or melted, and the second voltage is in the voltage range. And different from the first voltage by 5V or more.

  In the metal material surface treatment method according to the present invention, in the above invention, the metal material is a stainless steel material, the first voltage is 60 V or more, and is within a voltage range in which the cathode electrode is not melted. The voltage is within the voltage range and is different from the first voltage by 5V or more.

  The surface treatment method for a metal material according to the present invention is characterized in that, in the above invention, the second voltage is smaller than the first voltage.

  In the surface treatment method for a metal material according to the present invention, in the above invention, after the treatment with the second voltage, a treatment with a voltage 5 V or more lower than the second voltage is further performed once or more, and the voltage of the subsequent treatment is set immediately before. It is characterized in that the voltage is 5 V or more lower than the processing voltage.

  The surface treatment method for a metal material according to the present invention is characterized in that, in the above-described invention, the surface of the cathode electrode is subjected to a water repellent treatment after applying the first voltage and the second voltage.

  The metal material according to the present invention is characterized by being surface treated by the surface treatment method for a metal material according to the present invention.

  According to the metal material surface treatment method and the metal material according to the present invention, a new function can be imparted to the surface of the metal material without much labor and cost.

FIG. 1 is a flowchart showing a flow of surface treatment of a metal material according to the first embodiment of the present invention. FIG. 2 is a schematic diagram showing one configuration example of an apparatus used in the metal material surface treatment method according to the first embodiment of the present invention. FIG. 3 is a SEM photograph showing the surface of the surface-treated SUS316 stainless steel. FIG. 4 is a photograph showing the state in which distilled water is dropped on the sample surface shown in FIG. 3 as observed from the lateral direction. FIG. 5 is a view of the state in which distilled water is dropped after the water-repellent treatment is performed on the stainless steel surface shown in FIG. 3 from the lateral direction. FIG. 6 is a view of a state in which distilled water is dripped onto a stainless steel surface that has not been surface-treated without being subjected to water repellent treatment, as observed from the lateral direction. FIG. 7 is a flowchart showing a flow of surface treatment of a metal material according to the second embodiment of the present invention. FIG. 8 is a diagram showing a secondary electron image on the surface of stainless steel 316 after the processing in step S12 shown in FIG. FIG. 9 is a diagram showing a secondary electron image on the surface of the stainless steel 316 after the process of step S13 shown in FIG. FIG. 10 is a diagram illustrating an example of an absorbance spectrum.

  Hereinafter, a surface treatment method for a metal material according to first and second embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a flowchart showing a flow of surface treatment of a metal material according to the first embodiment of the present invention. FIG. 2 is a schematic diagram showing one configuration example of an apparatus used in the metal material surface treatment method according to the first embodiment of the present invention. As shown in FIG. 1, in the surface treatment of a metal material according to the first embodiment of the present invention, first, a material to be treated as a cathode electrode, which is a metal material, and an anode electrode are immersed in an electrolytic solution, A voltage A is applied between the cathode electrode and the anode electrode (step S1). Next, a voltage B different from the voltage A is applied between the cathode electrode and the anode electrode (step S2). By these two steps, a fine structure having a large specific surface area can be formed on the surface of the material to be treated. Specifically, as shown in FIG. 2, the anode electrode 3 and the material to be treated 4 are immersed in the electrolytic solution 2 in the container 1, and the anode electrode 3 and the anode electrode 3 are connected from the power source 6 through a conductive wire 5 such as a copper wire. By applying voltage A and voltage B to the material to be processed 4, a fine structure is formed on the surface of the material to be processed 4. Although it is efficient to perform the process of step S1 and the process of step S2 continuously, time is taken after the process of step S1, or the process of step S2 is performed after changing an apparatus, an electrolytic solution, etc. May be.

The electrolytic solution 2 is not particularly limited, and has electrical conductivity, and when the surface treatment of the material to be treated 4 is performed, the surface of the material to be treated 4 is excessively etched, or the anode electrode 3 and the material to be treated are treated. It is a solution that hardly adheres to or precipitates on the surface of the material 4 or forms a precipitate. Examples of the electrolyte of the electrolytic solution 2 include potassium carbonate (K ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), sodium hydrogen carbonate (NaHCO ₃ ), ammonium carbonate ((NH ₄ ) ₂ CO ₃ ), water Lithium oxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH ₄ OH), sodium chloride (NaCl), potassium chloride (KCl), ammonium chloride (NH ₄ Cl), sulfuric acid Such as sodium salt, sulfuric acid potassium salt, sulfuric acid ammonium salt, nitric acid sodium salt, nitric acid potassium salt, nitric acid ammonium salt, sodium citrate (NaH ₂ (C ₃ H ₅ O (COO) ₃ )) Acid sodium salt, citric acid potassium salt, citric acid ammonium salt, nitric acid, hydrochloric acid, etc. It can be illustrated.

  The electrolytic solution 2 can have any pH and concentration as long as the surface of the material to be treated 4 can be modified. For example, when a potassium carbonate aqueous solution is used as the electrolytic solution 2, the concentration thereof is not particularly limited and can be 0.001 mol / L or more, more preferably 0.005 mol / L or more. This is because if the concentration of the electrolytic solution 2 is too low, it may be difficult to maintain a suitable discharge state when a voltage is applied between the anode electrode 3 and the workpiece 4. The upper limit of the concentration of the electrolytic solution 2 is not particularly set, but can be set to 0.5 mol / L or less, for example. Further, the pH of the electrolytic solution 2 can be set to any value as long as excessive corrosion and etching of the electrode are not caused, for example, pH 10 to 12.

  The anode electrode 3 is formed of a material that is thermally and chemically stable during discharge. Examples of such an anode electrode 3 include Pt, Ir, and graphite.

  The material to be treated 4 is not particularly limited as long as it is a metal material, and a cold-rolled material, a hot-rolled material, a cast material, and a processed product thereof (including welding) can be used if it is a steel material. The steel type is not particularly limited, and carbon steel, low alloy steel, stainless steel, or the like can be used. Also, plated steel sheets including electrogalvanized steel sheets can be used. Moreover, the shape of the to-be-processed material 4 is not specifically limited, A plate shape, linear shape, rod shape, a pipe shape, or a processed component can be utilized. Moreover, the to-be-processed material 4 needs to be immersed in the electrolyte solution 2, and needs to be deeper than 1 mm from a liquid level at least.

  As the discharge condition, a range from a partial plasma state to a complete plasma state where unevenness is formed on the surface of the material to be processed 4 can be used. However, it is necessary to implement in the voltage range lower than the voltage which the to-be-processed material 4 melts. Specifically, when the discharge voltage is increased, light emission that can be confirmed with the naked eye in the dark starts, and is a state from a voltage indicating orange point light emission to immediately before the entire material becomes red hot. The applied voltage is preferably in the range of about 70 to 200 V, more preferably in the range of 80 to 150 V, when the size of the workpiece 4 is 1 mm × 1 mm × 20 mm. This voltage range is applicable to most steel materials including alloy steels such as stainless steel. However, since this voltage range changes depending on the type and arrangement of the material to be processed 4, it is preferable to determine the surface of the material 4 to be processed by changing the voltage condition by observing with a scanning electron microscope (SEM).

  It is a necessary condition that the discharge voltage is a voltage for forming fine protrusions on the surface of the steel material. If the voltage is lower than the lower limit voltage, fine protrusions are not formed on the surface. Therefore, it can be determined by confirming the presence or absence of the fine protrusions by SEM. If the upper limit is exceeded, the surface to be treated will melt. Therefore, the upper limit can be determined as the voltage at which the surface melts. When it is desirable not to oxidize the surface, the voltage at which the surface is oxidized can be easily determined by examining the SEM and an energy dispersive X-ray spectrometer (EDS) attached to the SEM. When oxygen is detected with the same X-ray intensity as the oxide of the material 4 to be processed, it can be determined that the surface is oxidized. Further, for the X-ray intensity of oxygen normalized by the intensity of the Fe L-line of the oxide of the material to be processed 4 (for example, it means an oxide of Fe in a cold-rolled steel sheet or low alloy steel), the material to be processed 4 The X-ray intensity normalized with the Fe-L line intensity of oxygen at 1 is required to be 1/3 or less. The surface inspection is performed by changing the voltage and discharging for 30 minutes, taking out the material 4 to be treated, washing it with water, drying it, introducing it into the SEM, and observing it.

  The inventors of the present invention have found that the fine protrusion becomes larger as the voltage is higher within a desirable voltage range. In order to increase the surface area, it is advantageous to provide finer irregularities on the larger irregularities, so that the voltage B is preferably lower than the voltage A. If there is a difference of 5V between the voltage A and the voltage B, a difference is seen in the size of the protrusions to be formed. Therefore, the voltage difference between the voltage A and the voltage B is preferably 5V or more. . Further, since it is advantageous to form large irregularities in the process of step S1, as the voltage A, a voltage close to the upper limit within a desired voltage range may be selected.

  The discharge processing time is required to be 3 seconds or more in the processing of step S1 and step S2. However, the discharge treatment time can be as long as, for example, 60 minutes. However, if the discharge treatment time is too long, the material to be treated 4 may be worn out, so a treatment time of 30 minutes or longer is not preferable. Further, it is not preferable to lengthen the discharge processing time in the process of step S2. This is because if the discharge treatment time is long in the process of step S2, fine protrusions depending on the voltage B are formed on the surface. For this reason, the discharge processing time in the process of step S2 is preferably 5 minutes or less.

FIG. 3 shows an example in which a SUS316 stainless steel plate having a thickness of 0.8 mm is processed. This SUS316 stainless steel plate was cut to a width of 2.5 mm and a length of 30 mm, and was made conductive with a copper wire to form a cathode electrode. The anode electrode used was a 50-cm long Pt wire bent so as not to contact each other and formed into a planar shape. The connecting portion between the SUS316 stainless steel plate and the copper wire was heat-pressed with a heat-resistant resin so that the copper wire did not touch the electrolytic solution, and the 20 mm long portion of the electrode was immersed in the electrolytic solution. The electrolytic solution is an aqueous solution of 0.1 mol / L K ₂ CO ₃ , the voltage is set to 140V and discharged for 10 minutes (Step 1), and then the voltage is set to 110V and discharged for 3 minutes (Step 2). ), The cathode electrode was pulled up from the electrolytic solution and immediately washed with water.

  As a result, as shown in FIG. 3, it was confirmed that relatively large protrusions were formed on the surface of the SUS316 stainless steel plate, and fine protrusions having an average diameter of 1 μm or less were formed on the relatively large protrusions. It was done. Moreover, it was confirmed by the elemental analysis by EDS that the surface of the SUS316 stainless steel plate was not oxidized. In addition, at an applied voltage exceeding 160 V, the tip of the SUS316 stainless steel plate was melted. For this reason, the upper limit value of the applied voltage could be obtained as 160V. In addition, since it was confirmed by EDS elemental analysis that the surface of the SUS316 stainless steel sheet was not oxidized at an applied voltage of 140 V or less, the upper limit of the applied voltage that does not oxidize the surface is 140 V in this experimental condition and test material. I understood. On the other hand, the lower limit value of the applied voltage could be determined to be 80 V from the presence or absence of the protrusion structure. Therefore, the most preferable condition for the voltage A could be determined to be 140V.

  Returning to FIG. Once the fine structure is formed on the surface of the material to be treated 4 as described above, the material to be treated 4 is then taken out from the electrolytic solution 2 and the material to be treated 4 is washed as necessary (step S3). In this state, a hydrophilic surface can be obtained. The cleaning method is performed for the purpose of removing the electrolytic solution on the surface, and includes a method of immersing in pure water or spraying. In addition to pure water, a weak acid or an alkaline solution may be used as long as the fine structure on the surface is not broken. At that time, electrolysis can be applied. After washing, it may be dried, or when performing a water repellent treatment, it may be advanced to the next step without drying.

  FIG. 4 is a photograph showing the state in which distilled water is dropped on the sample surface shown in FIG. 3 as observed from the lateral direction. As shown in FIG. 4, it can be seen that a very small contact angle is obtained and a superhydrophilic surface is obtained. The contact angle was 52 degrees in the discharge process for 15 minutes set to the same voltage 140 V as in Step 1, and the contact angle of the surface subjected to the discharge process in 15 minutes set to the same voltage 110 V as in Step 2 was 70 degrees. From FIG. 4, it can be seen that in order to obtain super hydrophilicity with a contact angle of water of about 10 degrees as shown in FIG. 4, two steps of processing of step 1 and step 2 are necessary.

  In order to obtain a water repellent surface, a water repellent treatment is performed on the treated surface of the cleaned material 4 (step S4). As a water repellent treatment method, a method of applying a water repellent spray, a method of adsorbing an organic substance having a water repellent function such as a fluorine-based resin in a liquid phase or a gas phase, and the like can be adopted. In the present embodiment, nanopro (component: fluorocarbon resin, silicon resin) manufactured by Coronyl Co., Ltd. is sprayed on the surface of the material to be treated 4 and dried for 12 hours or more, so that the surface of the material to be treated 4 is subjected to water repellent treatment. did.

  FIG. 5 is a photograph of the sample surface shown in FIG. 3 that has been subjected to a water repellent treatment and observed from the lateral direction in which distilled water has been dropped. As a result of observation, the contact angle of water was measured to be 170 °, and it was confirmed that super water repellency was realized. When the same water-repellent treatment was performed on the material that was not subjected to plasma discharge in solution, the contact angle of water was 125 °. Moreover, the contact angle of water in the sample not subjected to the water repellent treatment was 77.2 ° (see FIG. 6). Therefore, in order to obtain a super water-repellent surface, it was confirmed that both plasma discharge in solution and water-repellent treatment having Step 1 treatment and Step 2 treatment were necessary.

  The technique of performing plasma discharge twice in a solution under different conditions according to the present invention can be extended to three or more in-solution plasma discharge treatment techniques. From the viewpoint of processing time and cost, it is advantageous that the number of times is small, but when a higher effect is required, three or more plasma discharge treatments in solution can be applied.

〔Example〕
A commercially available 0.8 mm-thick stainless steel SUS316 steel plate was cut into 2.5 mm width and 50 mm length, degreased by dipping in dilute hydrochloric acid, and then conducted through a copper wire to obtain a cathode electrode. The upper part of the electrode including the connection part with the copper wire was coated with a heat-resistant resin, and the length of the treated part where the stainless steel was exposed was 20 mm. This electrode was immersed in an electrolytic solution. The anode electrode was formed by bending a 0.5 mmφ Pt wire having a length of 50 cm so as not to contact each other and forming it into a planar shape. The electrolytic solution is an aqueous solution of K ₂ CO ₃ having a concentration of 0.1 mol / L, the applied voltage is set within the range of 110 to 140 V, the discharge is performed under the conditions shown in Table 1, and after completion, it is washed with pure water and dried. It was. Thereafter, water repellent treatment was performed on some samples by spraying Coronyl Nanopro on the surface of the material to be treated and drying for 12 hours or more, and the water wettability was investigated. Water wettability was obtained by dropping 6 drops of distilled water at 1 μm at 6 locations on the electrode surface with a micropipette at equal intervals and shooting from the side using a Canon digital camera EOS Kiss X2. The contact angle was measured from the photograph and evaluated by taking the average of 6 locations. Distilled water 049-16787 manufactured by Wako Pure Chemical Industries, Ltd. was used. Table 1 shows the test results.

  On the untreated surface (Comparative Example 1), the contact angle is 77 °, and the contact angle rises only to 125 ° even when the water repellent treatment is performed. On the other hand, it can be seen that the invention examples show high hydrophilicity of 8 to 42 °. In addition, in the inventive example subjected to the water repellent treatment, a super-water repellency of 172 ° (Inventive Example 2) was obtained. Even in the comparative example in which one voltage is applied, the hydrophilicity is improved, or the water repellency is expressed by performing the water repellent treatment (Comparative Examples 2 to 13). 147 °. From this, it was confirmed that what was processed with two different voltages has a high effect on hydrophilicity or water repellency. However, in Invention Example 6 in which the processing time in Step 2 is as long as 15 minutes, both hydrophilicity and water repellency are not so improved, and therefore, the processing time in Step 2 is more preferably 5 minutes or less.

[Second Embodiment]
The inventors of the present invention have conducted intensive research aiming at providing a photocatalytic function to a stainless steel material by a simple method and not using TiO _2. As a result, plasma treatment is performed on the stainless steel material in an electrolytic solution. As a result, it was found that a fine uneven structure appeared on the surface of the stainless steel material, and as a result, the photocatalytic function appeared in the stainless steel material. Further, the inventors of the present invention have found that the photocatalytic function of a stainless steel material is greatly improved by performing plasma treatment twice by changing the voltage.

  FIG. 7 is a flowchart showing a flow of surface treatment of a metal material according to the second embodiment of the present invention. The processing apparatus used for the metal material surface treatment method according to the second embodiment of the present invention has the same configuration as the processing apparatus shown in FIG. As shown in FIG. 7, in the surface treatment of the metal material according to the second embodiment of the present invention, first, a material to be treated as a cathode electrode made of a stainless steel material having a surface to be treated and an anode electrode are subjected to an electrolytic solution. It is immersed in (step S11). Then, a fine structure is formed on the surface of the material to be processed by applying a voltage A within a voltage range of 60 V or more and not dissolving the cathode electrode between the cathode electrode and the anode electrode (step S12). More specifically, as shown in FIG. 2, the anode electrode 3 and the material to be treated 4 are immersed in the electrolytic solution 2 in the container 1, and the power source 6 is connected via a conductive wire 5 such as a Cu wire or a Pt wire. By applying a voltage to the anode electrode 3 and the material to be treated 4, a fine structure is formed on the surface of the material to be treated 4. FIG. 8 is a diagram showing a secondary electron image on the surface of the stainless steel 316 after the processing in step S12. As shown in FIG. 8, it can be seen that fine irregularities are formed on the surface of the stainless steel 316 after the processing in step S12.

The electrolytic solution 2 is not particularly limited, and has electrical conductivity, and when the surface treatment of the material to be treated 4 is performed, the surface of the material to be treated 4 is excessively etched, or the anode electrode 3 and the material to be treated are treated. It is a solution that hardly adheres to or precipitates on the surface of the material 4 or forms a precipitate. Examples of the electrolyte of the electrolytic solution 2 include potassium carbonate (K ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), sodium hydrogen carbonate (NaHCO ₃ ), ammonium carbonate ((NH ₄ ) ₂ CO ₃ ), water Lithium oxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH ₄ OH), sodium chloride (NaCl), potassium chloride (KCl), ammonium chloride (NH ₄ Cl), sulfuric acid Such as sodium salt, sulfuric acid potassium salt, sulfuric acid ammonium salt, nitric acid sodium salt, nitric acid potassium salt, nitric acid ammonium salt, sodium citrate (NaH ₂ (C ₃ H ₅ O (COO) ₃ )) Acid sodium salt, citric acid potassium salt, citric acid ammonium salt, nitric acid, hydrochloric acid, etc. It can be shown.

  The electrolytic solution 2 can have any pH and concentration as long as the surface of the material to be treated 4 can be modified. For example, when a potassium carbonate aqueous solution is used as the electrolytic solution 2, the concentration thereof is not particularly limited and can be 0.001 mol / L or more, more preferably 0.005 mol / L or more. This is because if the concentration of the electrolytic solution 2 is too low, it may be difficult to maintain a suitable discharge state when a voltage is applied between the anode electrode 3 and the material to be processed 4. The upper limit of the concentration of the electrolytic solution 2 is not particularly set, but can be set to 0.5 mol / L or less, for example. The pH of the electrolytic solution 2 can be set to any value as long as the electrode is not excessively corroded or etched, for example, pH 10 to 12.

  The anode electrode 3 is formed of a material that is thermally and chemically stable during discharge. Examples of such an anode electrode 3 include Pt, Ir, and graphite.

  The material to be treated 4 is a stainless steel material containing 12% by weight or more of Cr, and any of ferrite, austenite, and two-phase systems can be used. Any surface finish can be used. Moreover, the shape of the to-be-processed material 4 is not specifically limited, Foil shape, plate shape, wire shape, rod shape, pipe shape, or a processed product or the assembled component can be utilized. The to-be-processed material 4 needs to be immersed in the electrolyte solution 2, and needs to be deepened at least 1 mm or more from the liquid level.

  As the discharge condition, a voltage range in which the material to be treated 4 is not melted at a voltage higher than or equal to a voltage exhibiting a partial plasma state in which irregularities are formed on the surface of the material to be treated 4 can be used. The partial plasma state refers to a state between a voltage at which light emission that can be confirmed with the naked eye in the dark when the discharge voltage is raised and a complete plasma state that exhibits orange point light emission. Specifically, the voltage range can be determined by performing discharge and confirming that a fine structure is formed on the surface by SEM or the like. In general, the applied voltage is preferably in the range of 90 to 150 V, and processing is performed by adjusting within the voltage range. The application time can be between 5 seconds and 30 minutes. The fine structure means a concavo-convex structure composed of protrusions and holes having a shape that does not exist on the surface before processing.

  It is a necessary condition that the discharge voltage is a voltage that can form a fine structure on the surface of the workpiece 4. If the discharge voltage is lower than the lower limit, no fine structure is formed on the surface of the material 4 to be processed. The upper discharge voltage is determined in consideration of the processing time. That is, when the discharge voltage is increased, the surface unevenness increases, but melting occurs and it is difficult to form a good microstructure. Therefore, the upper limit discharge voltage may be determined by predetermining the time t to be processed, changing the voltage and discharging only for the time t, and confirming that the fine structure is formed by SEM. It was found that the photocatalytic function is higher as the discharge voltage is higher in the desired voltage range. Therefore, the most preferable discharge voltage is to select a discharge voltage close to the upper limit in the preferable voltage range.

  Next, a voltage B different from the voltage A is applied between the anode electrode 3 and the workpiece 4 (step S13). A fine structure having a large specific surface area can be formed on the surface of the material 4 to be processed by the processes of the two steps S12 and S13 (steps 1 and 2). Conditions such as an electrolytic solution may be changed from the conditions in step S12. The inventors of the present invention have found that the fine structure increases as the discharge voltage increases within a desirable voltage range. In order to increase the surface area, it is advantageous to provide finer irregularities on the larger irregularities. For this reason, it is desirable that the voltage B be lower than the voltage A. If there is a difference of 5V between the voltage A and the voltage B, a difference is seen in the size of the protrusions to be formed. Therefore, the voltage difference between the voltage A and the voltage B is preferably 5V or more. . Further, since it is advantageous to form large irregularities in the process of step S12, it is preferable to select a voltage close to the upper limit within the desired voltage range as the voltage A.

  The discharge processing time is required to be 3 seconds or more in the processing of step S12 and step S13. However, the discharge treatment time can be as long as, for example, 60 minutes. However, if the discharge treatment time is too long, the material to be treated 4 may be worn out, so a treatment time of 30 minutes or longer is not preferable. In the process of step S13, it is not preferable to lengthen the discharge processing time. This is because if the discharge processing time is increased in the process of step S13, a fine structure depending only on the voltage B is formed on the surface, and the effect of processing in two stages is lost. For this reason, the discharge process time in the process of step S13 is preferably 5 minutes or less.

FIG. 9 is a diagram showing a secondary electron image on the surface of the stainless steel 316 after the processing in step S13. In the example shown in FIG. 9, the anode electrode 3 is Pt, the voltage A is set to 140 V in a 0.1 mol / L K ₂ CO ₃ aqueous solution, discharge is performed for 10 minutes, and then the voltage B is set to 110 V. After discharging for 3 minutes, the stainless steel 316 as the material to be treated 4 was pulled up from the K ₂ CO ₃ aqueous solution and immediately washed with water. As shown in FIG. 9, it can be seen that a relatively large protrusion structure is formed on the surface of the stainless steel 316, and further, a fine protrusion having an average diameter of 1 μm or less is formed on the relatively large protrusion.

  When a methylene blue decoloring reaction test was performed on the stainless steel 316 shown in FIGS. 8 and 9, the photocatalytic performance could be confirmed, and further, the stainless steel 316 shown in FIG. 9 was confirmed to have higher photocatalytic performance. This shows that the photocatalytic performance is higher when the plasma treatment is performed twice in the electrolytic solution. This can be extended to a plasma discharge treatment technique in an electrolytic solution having three or more stages. From the viewpoint of processing time and cost, it is advantageous to reduce the number of plasma treatments. However, when higher photocatalytic performance is required, plasma discharge treatment in an electrolytic solution having three or more stages can be selected. .

〔Example〕
A commercially available stainless steel SUS316 steel plate having a thickness of 0.8 mm was cut into a 2.5 mm width and a length of 30 mm, degreased by being immersed in dilute hydrochloric acid, and then conducted through a copper wire to obtain a cathode electrode. The upper part of the electrode including the connection part with the copper wire was coated with a heat-resistant resin so that the copper wire did not come into contact with the electrolytic solution, and the length of the treated part where the stainless steel was exposed was 20 mm. This electrode was immersed in an electrolytic solution. The anode electrode was formed by bending a 0.5 mmφ Pt wire having a length of 50 cm so as not to contact each other and forming it into a planar shape. The electrolytic solution is a 0.1 mol / L K ₂ CO ₃ aqueous solution, the applied voltage is set within the range of 90 to 140 V, discharge is performed under the conditions shown in Table 2, and immediately after completion, the plate is washed with pure water and dried. It was. Further, as a comparative example, an untreated stainless steel plate (degreasing was performed by immersing in dilute hydrochloric acid) used for the base material was used. These samples were subjected to a methylene blue decolorization reaction test in order to examine the photocatalytic performance.

Next, the methylene blue decolorization reaction test will be described. First, prior to the methylene blue decolorization reaction test, only a 0.1% by mass methylene blue aqueous solution was placed in the cell, and a measurement start wavelength of 720 nm and an end wavelength of 500 nm using a spectrophotometer made by JASCO Corporation, model: V630. The absorbance of the methylene blue aqueous solution was measured. Absorbance A _XS at a wavelength (X) at which the absorbance is maximum at an absorption peak of about 660 nm having a relatively large absorbance was obtained and used as a reference.

In the methylene blue decolorization reaction test, 4 ml of a 0.1% by mass methylene blue aqueous solution was placed in a cell, and a test piece of 2.5 mm × 20 mm × 0.8 mm (plate thickness) (stainless steel plate after performing discharge treatment) Etc.) was immersed, and ultraviolet light (wavelength 365 nm) was irradiated to the cell containing the aqueous solution. An aluminum foil was installed to surround the cell so that the entire cell was irradiated with ultraviolet rays. A test piece was taken out of the cell 24 hours after irradiation with ultraviolet rays, and the absorbance of the remaining aqueous solution was measured by the method described above to determine the absorbance A _XP at the wavelength (X) described above. Then, a A _{XP /} A _XS as methylene blue solution absorbance change was evaluated the degree of decolorization of methylene blue. The smaller the value of A _XP / A _XS, the more the decolorization progresses, indicating that the photocatalytic performance of the test piece is large. An example of the obtained absorbance spectrum and the evaluation results are shown in FIG. 10 and Table 2, respectively.

  Inventive Examples 1 to 3 show a higher decolorization rate than untreated stainless steel (Comparative Examples 1 to 5). In addition, it was confirmed that the performance was remarkably improved when discharging was performed twice while changing the voltage.

  According to the present invention, there is provided a metal material surface treatment method capable of imparting a new function to a metal material surface without much labor and cost, and a metal material surface-treated by this surface treatment method. can do.

DESCRIPTION OF SYMBOLS 1 Container 2 Electrolytic solution 3 Anode electrode 4 Material to be processed (cathode electrode)
5 Conductor 6 Power supply 7 Thermometer

Claims (5)

A step of immersing a material to be treated as a cathode electrode made of a metal material having a surface to be treated and an anode electrode in an electrolytic solution;
Applying a first voltage between the cathode electrode and the anode electrode;
Applying a second voltage lower than the first voltage between the cathode electrode and the anode electrode,
A surface treatment method of a metal material, wherein a surface area of the surface of the metal material is increased by applying the first voltage and the second voltage.   The first voltage is 70 V or more and is in a voltage range where the cathode electrode is not oxidized or melted, and the second voltage is in the voltage range and is different from the first voltage by 5 V or more. The metal material surface treatment method according to claim 1.   The metal material is a stainless steel material, the first voltage is 60 V or more and is in a voltage range where the cathode electrode is not melted, the second voltage is in the voltage range, and the first voltage and The surface treatment method for a metal material according to claim 1, wherein the surface treatment method is different by 5 V or more. The process using the second voltage is further performed once or more by a voltage that is 5 V or more lower than the second voltage, and the voltage of the subsequent process is made 5 V or more lower than the voltage of the immediately preceding process. The surface treatment method of the metal material of 1 or 2 . 5. The surface treatment method for a metal material according to claim 1, further comprising a step of applying a water repellent treatment to the surface of the cathode electrode after applying the first voltage and the second voltage.