JPH11158665A - Method for preventing corrosion of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method for preventing the corrosion of a steel independently of the conventional physical method, capable of being easily applied to the jog site and excellent in rust preventing effect as the method of protecting the steel of the structure submerged in the seawater or fresh water or intermittently covered with the waters and wetted, the structure irradiated with artificial light having <=400 nm wavelength or the outdoor structure exposed to sunlight. SOLUTION: A resin coating film 2 contg. anatase-structure titanium oxide is formed on the surface of the steel 1 submerged in seawater of freash water, the steel intermittently covered with the waters and wetted or the steel material exposed to or irradiated with the artificial light having <=400 nm wavelength or sunlight to cathodically prevent the corrosion of the steel by the photoelectrochemical reaction of the coating film. Besides, stainless steel is preferably used as the steel, an acrylic resin is used as the resin as a binder for forming the resin coating film, and the weight ratio of the anatase-structure titanium oxide to the resin forming the film is preferably controlled to 0.1 to 0.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for irradiating an artificial light having a wavelength of 400 nm or less in an ocean or a freshwater liquid or an intermittently wetted state with these liquids, The present invention relates to a method for preventing corrosion of a steel material of a structure exposed to light.

[0002]

2. Description of the Related Art Generally, steel materials for harbor structures in marine or freshwater liquids or intermittently wetted by these liquids and in a wet state, for example, in a dry and wet environment such as a splash zone, Alternatively, there is no other method for preventing corrosion of steel such as a structure such as a freshwater dam weir where seawater or freshwater exists so as to form a thin water film on the surface of the steel except for painting or coating. Was impossible to do. The reason is that a thin water film is formed on the steel surface in the above environment, and even if an attempt is made to supply current to the steel material through this thin water film, the polarization resistance of the steel material is low, and the thickness of the water film is too thin. This is because the corrosion resistance is not supplied over a wide range because the circuit resistance increases.

On the other hand, recently, a technique for preventing corrosion of steel by applying an electrochemical reaction of titanium oxide has been proposed and attracted attention. For example, the invention of Japanese Patent Application Laid-Open No. Hei 8-201578 discloses that a titanium oxide semiconductor layer having an oxygen deficient structure is formed by performing powder spraying of titanium oxide on a steel material surface in a reducing atmosphere to form a titanium oxide semiconductor layer. It is intended to prevent corrosion of nuclear reactor structures in radiation and underwater environment. Also, “Materials and Environment” 43, (P482-486, 1994
Issued by the Corrosion and Corrosion Protection Association of Japan in 1989).
A paper has been disclosed that covers _two films and discusses light irradiation and anticorrosion effects.

[0004]

However, in the prior art as described above, the titanium oxide film formed on the surface of the steel material has a titanium oxide semiconductor layer having an oxygen-deficient structure by DC magnetron sputtering or thermal spraying in a reducing atmosphere. The former method is to form an extremely thin sputtering film by physical means, and the applied steel material is limited to relatively small steel materials to which these physical film forming methods can be applied. In addition, large equipment and the like are required and the cost is extremely high, so that the construction on site is complicated and difficult.
The latter method is intended to form a coating having an oxygen-deficient structure applied in a limited thermal spray atmosphere with the intention of preventing corrosion of a reactor structure used in a special environment of radiation irradiation.

The present inventors have proposed a harbor structure in the above-mentioned marine or freshwater liquid or in a wet state intermittently wetted by these liquids, for example, in a dry or wet environment such as a splash zone. We have been studying the corrosion prevention of steel products, such as seawater or freshwater that forms a thin water film on the surface of steel products, such as structures such as freshwater dam weirs. We have focused on whether electrochemical properties can be used for the corrosion prevention of relatively large steel structures as described above, and have continued to study. If an anticorrosion method for a steel material that can be applied by simple means without relying on a physical method and has an excellent anticorrosion effect can be proposed, the above-described relatively large equipment and the like will be required, resulting in extremely high costs and on-site Problems such as complicated and difficult construction work are eliminated, and the technical and economic effects are extremely useful in this field.

[0006]

SUMMARY OF THE INVENTION The present invention is directed to an artificial light or sunlight having a wavelength of 400 nm or less which is in a marine or freshwater liquid or intermittently wetted by such a liquid and is in a wet state. Or on the steel surface to be irradiated,
The present invention is a method for preventing a steel material from being subjected to cathodic cathodic protection by forming a resin coating film containing anatase-type titanium oxide and performing a photoelectrochemical reaction of the coating film, thereby achieving the above object. Preferably, the steel material is a stainless steel material, and a resin as a binder for forming a resin coating film is an acrylic resin, and the coating film has anatase type titanium oxide with respect to a weight of 1.0 of a coating film forming resin to be formed. Preferably, the weight is in the ratio of 0.1 to 1.2.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, as a method for preventing corrosion of steel, a resin coating containing titanium oxide is applied to the surface of the steel by brushing or spray coating, and the coating is coated with sunlight or a wavelength of 400 nm or less. The artificial steel is irradiated with artificial light having the following properties to prevent corrosion of the steel material by a photoelectrochemical reaction of titanium oxide contained in the coating film. Titanium oxide is of an anatase type, and is usually a commercially available, preferably spherical product, and is contained in the resin. As the coating film forming substance (binder), an acrylic resin is mainly used, but a fluororesin or a silicone resin having excellent oxidation characteristics, weather resistance and water resistance can be used. For details, alkyd-modified silicone resin for silicone resin,
An acrylic-modified silicone resin, an epoxy-modified silicone resin, and the like are included. Examples of the fluorine-based resin include a polyisocyanate-cured fluorine resin. These are all cold-setting resins, and among them, acrylic resins are most preferable for use in the present invention. The anatase-type titanium oxide contained in these film-forming substances has a weight ratio of the titanium oxide of 0.1 to 1.2 with respect to a resin weight of 1.0. If the weight of titanium oxide is less than 0.1 with respect to the resin weight of 1.0, the photoelectrochemical reaction is not sufficient, and if the weight of titanium oxide exceeds 1.2 with respect to the weight of the resin of 1.0, acrylic resin, fluororesin, and silicone resin will not be obtained. The viscosity of the coating increases,
Brush coating and spray coating become difficult, and a good coating film cannot be obtained. Titanium oxide is contained in the above resin and kneaded by further adding an organic solvent as appropriate. The titanium oxide-containing resin paint is preferably applied to the surface of a steel structure by a method such as coating or spray coating, preferably to have a thickness of 40 to 80 μm. The thicker the coating, the lower the cathode potential for the photochemical reaction, but 60 μm is sufficient.

In the present invention, the steel material to be protected against corrosion is preferably stainless steel, but other inconel materials, low alloy steels and the like can be applied. These steel materials are applied to the present invention in the marine or freshwater liquid or in a part where the liquid is intermittently wetted by these liquids and is in a wet state, specifically, in a dry or wet environment such as a splash zone. It is used for steel materials where seawater or freshwater forms a thin water film on the steel material surface, such as for harbor structures below or for structures such as freshwater dam weirs. The resin coating containing titanium oxide formed on the surface of the steel material is exposed to sunlight outdoors, through a water film existing on the surface of the steel structure for the above-mentioned use,
Exposure to artificial light below 400nm.

The anatase-type titanium oxide contained in the resin, when exposed or irradiated with light such as sunlight or artificial light having a wavelength of 400 nm or less, causes a photoelectrochemical reaction to cause cathodic protection of the steel material. Can be. Hereinafter, the principle will be described with reference to a conceptual model shown in FIG. When the surface of the titanium oxide is irradiated with light having energy equal to or greater than the band gap, electrons in the valence band are excited to the conduction band according to the formula 1, and holes are generated in the valence band.

(Equation 1) The holes generated in the valence band come into contact with water on the surface of the titanium oxide, thereby causing an oxygen generation reaction represented by Formula 2.

(Equation 2) The electrons excited in the conduction band move from the titanium oxide to the steel material, and generate a hydrogen generation reaction represented by Formula 3 on the surface of the steel material.

(Equation 3)

When the above reaction is carried out on the surface of titanium oxide and the surface of steel material, electrons are transferred from titanium oxide to steel material to form an electric circuit, whereby the titanium oxide side becomes an anode and the steel side becomes a cathode. It will be anti-corrosion. Regarding the electrochemical stability of the titanium oxide itself in the oxidation and reduction reactions of the titanium oxide, the energy level ε ° (D) corresponding to the standard electrode potential of the oxide decomposition reaction and the standard value of the water oxidation reaction It is determined by the relative relationship of the energy level ε ° (O ₂ / H ₂ O) corresponding to the electrode potential. In titanium oxide, ε ° (D) = − 5.92 eV, which is lower than ε ° (O ₂ / H ₂ O) = − 5.73 eV,
The oxidation reaction of water takes precedence, and titanium oxide itself is electrochemically stable. Thus, titanium oxide becomes a non-sacrificial anode, enabling cathodic protection of steel. Since the titanium oxide has a bandgap energy of 3.0 eV in the electrochemical reaction of titanium oxide, the reaction proceeds by light having a wavelength of 400 nm or less, particularly sunlight, and the corrosion prevention effect of the steel material is exhibited.

In the present invention, since the anatase-type titanium oxide contained in the coating film formed on the surface of the steel material is present on the entire surface of the steel material, the titanium oxide is contained in sunlight or artificial light having a wavelength of 400 nm or less. The supply of electrons from titanium oxide excited by the irradiation of light energy occurs on the entire surface of the steel material to be protected, and the electrons can be supplied even in a thin water film. To achieve the overall corrosion protection of steel.

[0012]

According to the present invention as described above, the marine or freshwater liquid or a portion which is intermittently wetted by these liquids and is in a wet state, specifically, such as a splash zone, etc. Brushing or coating at room temperature on steel surfaces where seawater or freshwater forms a thin water film on the steel surface, such as for harbor structures in dry or wet environment, or for structures such as freshwater dam weirs, etc. It can be applied by spray coating, etc., it can be applied without the need for high temperature and complicated film forming means such as baking process or physical thin film forming technology, the process is extremely simple and sufficient corrosion protection can be performed for a long time Becomes

[0013]

EXAMPLE 1 FIG. 1 shows a coating film formed according to the method of the present invention. In FIG. 1, a resin coating film 2 containing titanium oxide having photoelectrochemical reactivity (manufactured by Kanto Chemical Co., Ltd.) is formed on the surface of a SUS 304 steel plate 1. The constituent material of the coating film 2 is obtained by adding and mixing an acrylic resin as a coating film forming substance (binder), changing the powder weight ratio of titanium oxide (anatase type) to the weight of the coating film forming substance 1.0. This material was applied to a SUS 304 steel plate by dip coating one time, at a thickness of 60 μm, and dried at room temperature to form a titanium oxide-containing resin coating film having photoelectrochemical reactivity. When this coated steel sheet was immersed in natural seawater (room temperature) at a depth of 1 to 2 cm and irradiated with artificial light (100 W ultraviolet lamp) having a wavelength of 365 nm, the results shown in FIG. 4 were obtained. From this, the potential of the coated steel sheet was 30 mV before light irradiation vs. the saturated calomel reference electrode (hereinafter, SCE).
Was abbreviated), but the weight ratio of titanium oxide to the weight of the film-forming substance became 1.0, and the weight ratio of titanium oxide to the weight of the film-forming substance became 1.0. It was confirmed that in the range of 0.8, the weight ratio of titanium oxide was -440 or less, and especially when the weight ratio of titanium oxide was 0.5, -630 mV vs. SCE was lowered. From this, since the potential after light irradiation is lower than the repassivation potential E (R, crev) of SUS 304 steel in seawater, crevice corrosion of SUS 304 steel can be suppressed, and the electrolytic protection of steel material is improved. It turns out that it is possible.

[0014]

Embodiment 2 Next, a conceptual diagram of the case where the method of the present invention is carried out using a titanium oxide having photochemical reactivity on the surface of SUS 304 steel (a resin coated steel sheet containing a product made by Kanto Chemical Co., Ltd.) as a test body. The composition of the coating film 2 in this example is an acrylic resin as a coating material (binder), and a weight ratio of 0.3 to the weight 1.0 of the coating material. This material was added to and mixed with a considerable amount of titanium oxide powder, and this material was applied to SUS 304 steel material 1 and dried at room temperature to form a resin coating film 2 containing titanium oxide having photoelectrochemical reactivity (dip). Coat thickness 45μ
m) was formed. A water film 3 having a thickness of 1 mm was formed on the coated steel sheet, and the potential was measured with a potential measuring device 5 using a silver chloride electrode 4 by irradiating sunlight outdoors.
A decrease in the potential of ~ -450 mV vs SCE was confirmed. Thereafter, the test was continued for 100 days, but rust was generated on the normal coated steel sheet containing no titanium oxide, whereas no rust was observed on the coated steel sheet containing titanium oxide. From these results, it was confirmed that the electrolytic protection by the titanium oxide-containing resin coating film was effective for steel materials in an environment where a thin water film was present.

[0015]

Example 3 A resin coated steel sheet containing titanium oxide having photochemical reactivity (manufactured by Kanto Chemical Co., Ltd.) was formed on the surface of SUS 304 steel as a test piece, and the test piece was exposed to sunlight in the air. The coating film potential of the coated steel sheet was measured when exposed, using a coating film forming substance (binder) of an acrylic resin and a titanium oxide equivalent to a weight ratio of 0.5 to a weight of 1.0 of the film forming substance. Was added, mixed, and dip-coated to form a dip-coated film having a thickness of 60 μm.First, when no sunlight was applied, seawater was sprayed on the surface of the coated steel plate using a spray, and SCE was used. When the potential of the coated steel sheet was measured, it was 20 to 30 mV vs. SCE, and then exposed to the air and irradiated with sunlight, and the generated potential was measured every hour, as shown in FIG. Potential is observed, especially within a few hours after noon, 350mV~450mV vs SCE
Met. From this result, it was confirmed that a photochemical reaction occurred even in the coating film in which a thin film layer of extremely thin water just sprayed with seawater was formed. From this, in the present invention, it was found that when a thin film layer of water was formed, an electric circuit was formed between the steel sheet and the coating film, an electric current was supplied, and the steel sheet was sufficiently subjected to cathodic protection. .

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a coated steel sheet when the method of the present invention is performed.

FIG. 2 is a theoretical explanatory diagram of a photochemical reaction in the method of the present invention.

FIG. 3 is a conceptual explanatory diagram of potential measurement by sunlight irradiation when the method of the present invention is performed.

FIG. 4 is a diagram showing the relationship between the titanium oxide content ratio and the photo potential when the titanium oxide content ratio is changed in the example of the present invention.

FIG. 5 is a graph showing the relationship between the photopotential of the titanium oxide-coated stainless steel plate sprayed with seawater spray and measured every hour in the example of the present invention.

[Explanation of symbols]

 Reference Signs List 1 steel sheet 2 resin coating 3 water film 4 silver chloride electrode 5 potential measuring device

Claims (4)

[Claims] 1. A steel surface exposed to or irradiated with artificial light or sunlight having a wavelength of 400 nm or less in an ocean or freshwater liquid or in an intermittently wetted state by these liquids and in a wet state. A method of forming a resin coating containing anatase-type titanium oxide, and cathodic cathodic protection of the steel by photoelectrochemical reaction of the coating. 2. The method according to claim 1, wherein the steel material is a stainless steel material. 3. The method according to claim 1, wherein the resin as a binder forming the resin coating film is an acrylic resin. 4. The coating film has a weight of anatase type titanium oxide of 0.1 to 1.2 with respect to a weight of 1.0 of a coating film forming resin to be formed.
The method for preventing corrosion of steel according to any one of claims 1 to 3, wherein