JP4604153B2 - Forming functional coatings with excellent anticorrosion properties - Google Patents

Description

  The present invention relates to a method of forming a functional coating having excellent anticorrosion properties on a metal structure exposed to a radiation atmosphere. This method includes, for example, a channel box and a shroud disposed in a nuclear reactor, It can be used for anticorrosion of control rods.

  Among metal structures exposed to a radiation irradiation atmosphere such as a reactor internal structure, particularly a welded portion is likely to undergo corrosion and stress corrosion cracking during use. However, in order to repair or replace structural materials used in nuclear facilities, it is impractical to frequently perform inspections and replacement work because the operation of the nuclear reactor must be temporarily stopped.

  By the way, since most of the corrosion and stress corrosion cracking seen in the metal structures described above depend on the corrosion potential, the structure can be reduced by reducing dissolved oxygen and hydrogen peroxide contained in the water in contact with the structure. If the corrosion potential of the material is lowered, corrosion and stress corrosion cracking can be suppressed. Therefore, as a method for preventing corrosion and stress corrosion cracking, a hydrogen injection method (Patent Document 1) is known in which oxygen and hydrogen peroxide in reactor water are reduced by supplying hydrogen from a water supply system.

  However, this method has a problem that the injected hydrogen reacts with N-16 produced by the nuclear reaction to become volatile ammonia, thereby increasing the dose of the turbine system.

  At this time, it has been studied to attach a noble metal such as platinum to the structure so that a high corrosion inhibiting effect can be obtained with a small hydrogen injection amount. However, in this method, there is a problem that noble metal adheres onto an oxide film of zirconium used as a nuclear fuel, for example, and causes oxidation or hydrogenation deterioration of the nuclear fuel.

  In Patent Document 2, as a method for preventing corrosion and stress corrosion cracking of a welded part, an electron-hole pair is formed by irradiation with ultraviolet rays (Chelenkov light) existing in a nuclear reactor to cause an oxidation-reduction reaction. A method is disclosed in which the surface of a structure is covered with a photocatalytic substance that suppresses stress corrosion cracking by reducing the corrosion potential of the structure. Examples of the photocatalytic substance used include titanium oxide and zirconium oxide. It has been.

  Furthermore, Patent Document 3 discloses a method for reducing stress corrosion cracking by lowering the corrosion potential of the structure by coating the surface of the structure with a catalytic substance excited by radiation existing in the nuclear reactor. . Also in this document 3, titanium oxide and zirconium oxide are disclosed as catalyst substances. However, since higher energy radiation than ultraviolet rays is used, other metal oxides, nitrides and carbides having a larger band gap are also used. It is described that it can be used as a catalyst material.

  If a catalytic substance excited by ultraviolet rays or radiation is used in this way, problems such as an increase in dose due to volatile ammonia generated by the reaction between injected hydrogen and N-16, oxidation deterioration of hydrogen and hydrogenation deterioration due to noble metals. The corrosion potential of the structure can be lowered without accompanying. However, in order to effectively exhibit such effects on a practical scale, oxides such as titanium oxide and zirconium oxide, which are promising as radiation-induced surface active catalysts, must be coated with a thickness of several tens to several hundreds of microns.

By the way, oxidation of a thickness of several tens of microns or more is performed on the surface of a general metal material (for example, SUS304, SUS316L, etc.) that is an object of corrosion prevention by a vapor deposition method such as sputtering or ion plating with a slow deposition rate. Since it is difficult to form a coating, the plasma spraying method in which the thermal spraying raw material powder is melted at a high temperature by plasma and laminated as fine particles on the substrate surface is most suitable, considering practical application on an industrial scale. Seem.
JP 56-72396 A JP 2001-4789 International Publication WO 02 / 090008A1

  In general, it is known that oxide coatings such as titanium oxide and zirconium oxide formed by plasma spraying process have many fine pores and tend to lack electrical contact with the metal surface. . For this reason, the movement of the electrons in the direction of the metal substrate necessary to lower the corrosion potential of the metal material is hindered by the oxidation-reduction reaction caused by the action of the electron-hole pairs generated in the oxide film. This is an obstacle to anticorrosion properties. In addition, since the area where the surface of the metal material is exposed to the solution environment through the pores is enlarged, the potential lowering effect due to the oxidation-reduction reaction is also impaired.

  The present invention has been made by paying attention to such circumstances, and its purpose is to make the surface coating containing the radiation-induced surface active catalyst thick enough to exhibit an anticorrosion effect and a stress corrosion cracking suppression effect. In addition, the present invention provides a method capable of obtaining a functional coating having excellent corrosion resistance and stress corrosion cracking resistance by suppressing deterioration of corrosion resistance and stress corrosion cracking resistance due to pores of the surface coating as much as possible. There is.

The method for forming a functional coating according to the present invention that has solved the above-mentioned problem is that a part or all of a base material constituting a metal structure exposed to a radiation irradiation atmosphere is caused by a decrease in corrosion potential due to radiation irradiation. In forming a functional coating having an anticorrosive effect by plasma irradiation, a raw material powder inlet is provided on the upstream side of the plasma axis extending in the direction to be sprayed, and a plurality of spray guns are arranged at equal intervals around the axis, In addition, it uses a multi-electrode plasma converging sprayer that is arranged so that the plasma projected from each spray gun converges to one point, and is characterized by spraying ceramic powder with an average particle size of 5 to 10 μm. is doing.

  The above-described method of the present invention can be effectively used for corrosion protection of various in-core structures including a channel box, a shroud, a control rod, and the like arranged in the nuclear reactor, taking advantage of its excellent characteristics.

  According to the present invention, by using a multi-electrode plasma convergence type thermal spraying device, it is possible to efficiently form a coating having a thickness necessary for corrosion resistance, and by using a thermal spray material having an appropriate particle size configuration, A dense coating having excellent corrosion resistance can be obtained. Therefore, the present invention uses an oxide such as titanium oxide or zirconium oxide useful as a radiation-induced surface-active catalyst as a coating material to prevent corrosion deterioration of an in-core structure such as a nuclear reactor. Can greatly extend the durable life.

  Moreover, according to the present invention, problems such as an increase in turbine system dose due to the generation of volatile ammonia pointed out by conventional corrosion prevention measures and oxidation / hydrogenation deterioration of nuclear fuel caused by adhesion of noble metals, etc. occur. Therefore, it can greatly contribute to extending the life of various internal reactor structures such as channel boxes, shrouds, control rods and the like disposed in the nuclear reactor.

  In the present invention, as a first requirement, a multi-electrode plasma convergence type thermal spraying apparatus is used for thermal spraying. The multi-electrode plasma convergence type thermal spraying apparatus includes a raw material powder inlet on the upstream side of a plasma axis extending in the direction to be sprayed, a plurality of spray guns arranged at equal intervals around the axis, and each spray gun The spraying device is arranged so that the plasma projected from one point converges to one point, and preferably a three-electrode type using three spray guns is used, but the number of electrodes is not limited to this. A four-electrode type, a five-electrode type, or even a multi-electrode type more than that can be used if necessary.

  In any case, by using a one-point convergence type multi-electrode plasma spraying apparatus and supplying the raw material powder to the central position where the plasma jet flow converges, the axis of the converged plasma jet flow and the supply direction of the raw material powder is Since they are the same, all of the raw material powder can be effectively melted, and the problem that the partially unmelted raw material powder is sprayed onto the substrate surface can be avoided.

  By the way, in the conventional single electrode plasma spraying apparatus, the raw material powder is supplied from the crossing direction with respect to one plasma jet flow, so a part of the raw material powder is projected to the substrate surface in an unmelted state, and the melting efficiency Becomes worse. As a result, what is supplied in an unmelted state is mixed in the coating formed by thermal spraying, and the density of the coating is reduced. As a result, the electrical conductivity between the functional coating and the substrate decreases, and the movement of electrons necessary for the oxidation-reduction reaction caused by the action of electron-hole pairs generated in the coating toward the metal substrate It is inhibited and the anticorrosion performance is impaired. In addition, the formation of gap voids between the thermal spray particles constituting the coating also increases the area where the surface of the metal substrate is exposed to the outside, increasing the loss of the potential lowering effect due to the oxidation-reduction reaction. As a result, the corrosion resistance is significantly reduced.

  However, in the case of a multi-electrode plasma convergence type thermal spraying apparatus, the above problems are all improved, and all of the supplied raw material powder is efficiently heated by the plasma jet converged from the radial direction, in a completely molten state. Sprayed onto the substrate surface. As a result, a dense and highly corrosion-resistant coating without interparticle voids is formed.

  The specific configuration of the multi-electrode plasma convergence type thermal spraying device is not particularly limited including the number of electrodes as described above as long as it has a one-point convergence structure from a plurality of thermal spray guns. A known plasma spray gun may be appropriately combined. However, in order to more reliably melt all of the raw material powder while reliably preventing the scattering of the raw material powder in the plasma converging part, for example, as schematically shown in FIG. 1, three or more spray guns A, A, A are connected to the plasma axis. It is preferable to arrange them around P at equal intervals of 120 ° and to set the inclination angle α in the range of 5 to 65 ° toward the convergence point S direction. The raw material powder may be supplied in the coaxial direction toward the convergence point S from the upstream side of the axis of the plasma convergence flow.

  However, even when the above-mentioned multi-electrode plasma convergence type thermal spraying apparatus is used, in order to form the high-performance corrosion-resistant coating intended in the present invention, it is necessary to use a raw material powder having an appropriate particle size as a second requirement. .

  That is, when a one-point convergence type plasma spraying apparatus is used as described above, the time until the raw material powder charged into the spraying apparatus collides with the substrate surface by the convergent plasma is usually 1 to several milliseconds, During this period, it is necessary to melt the raw material powder completely and uniformly without agglomeration. For that purpose, it turned out that the average particle diameter of raw material powder should be 1 micrometer or more and 20 micrometers or less.

  Incidentally, in the case of coarse particles having an average particle diameter exceeding 20 μm, a part of the raw material powder is sprayed in an unmelted state in spite of plasma convergence, or the temperature distribution of individual particles after melting is relatively wide. Thus, it becomes difficult to form a uniform and dense coating. Further, in the case of a fine powder having an average particle size of less than 1 μm, the raw material powder aggregates before spraying or re-solidifies before forming the sprayed coating, and the uniformity and denseness of the coating are also lowered.

  However, using raw material powder with an average particle size in the range of 1 to 20 μm may cause aggregation before spraying or re-solidification before spray coating formation as described above, or a part of the powder may be sprayed in an unmelted state. In addition, the individual molten particles are sprayed on the surface of the base material with a relatively narrow temperature distribution, and a uniform, dense, and highly corrosion-free coating without voids can be formed.

  From such a viewpoint, the more preferable average particle diameter of the raw material powder is 3 μm or more, more preferably 5 μm or more and 15 μm or less, and further preferably 10 μm or less. In the present invention, it is desirable to use the one having the narrowest possible particle size distribution while satisfying such a suitable average particle size.

  The type of raw material powder used for plasma spraying is not particularly limited, but is preferably used as a functional coating for preventing corrosion deterioration of the reactor internal structure, which is the initial purpose of the present invention, is titanium oxide, oxidation These are metal oxide ceramics such as zirconium and chromium oxide, and examples thereof include single or two or more composite oxides. However, depending on the type of functional coating, it is also possible to use ceramics such as oxides, carbides, nitrides, borides, and any mixtures or composites thereof.

  In addition, when an oxide such as titanium oxide or zirconium oxide is used as a constituent material of the coating, the thicker the coating, the more electron-hole pairs generated by the coating and the higher the corrosion resistance. Is 10 μm or more, preferably 15 μm or more, more preferably 20 μm or more. However, even if it is thicker than that, the corrosion resistance is saturated, and if it is excessively thick, not only the cost including the consumption and formation efficiency of the coating material is increased, but also the coating is easily peeled off by residual stress. The thickness should be 10 mm or less, preferably 5 mm or less, and more preferably 1 mm or less.

  Next, as a metal structure exposed to a radiation irradiation atmosphere to which the present invention is applied, a reactor body member such as a channel box in a nuclear reactor, a shroud, a control rod and other in-core structures, a pressure vessel, a lid, etc. , Water-based cooling piping materials, water-cooled structures in the neutron irradiation section, and the like. In these various metal structures, by forming a functional coating with high corrosion resistance on a part or the entire surface including a welded portion, which is likely to cause local corrosion such as stress corrosion cracking, crevice corrosion, pitting corrosion, etc. Therefore, corrosion and stress corrosion cracking of these metal structures can be effectively suppressed, and their durability life can be greatly extended.

  Hereinafter, the configuration and operational effects of the present invention will be described in more detail with reference to experimental examples.However, the present invention is not limited by the following experimental examples, and is appropriate within a range that can be adapted to the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

Experimental Example A plurality of zirconia (ZrO ₂ ) having an average particle size adjusted in the range of 0.5 to 25 μm is used on the entire front and back surfaces of a pure iron substrate having a size of 20 mm × 20 mm × thickness 1 mm, and plasma spraying is used. Three types of zirconia coatings having a film thickness of 50 μm, 100 μm, and 300 μm constituting the radiation excitation catalyst layer were formed.

  Two types of spray guns were used for the above spraying. In a group of spraying experiments, a conventional single electrode plasma spraying apparatus was used, and a spraying distance of 60 to 120 mm, a voltage of 40 V, and a current of 600 to 800 A using an argon / helium / hydrogen (volume ratio 49/8/2) mixed gas. A film was formed. In another group of thermal spraying experiments, there are spraying material powder inlets on the upstream side of the plasma axis extending in the spraying direction, and there are three spray guns around the axis so that the projected plasma converges at one point. Using the same plasma projection conditions using the same argon / helium / hydrogen mixed gas as described above, using a three-electrode plasma converging sprayer arranged at 120 ° intervals and oriented in the axial direction at an angle of 32 ° The film was formed.

Each of the obtained samples was immersed in a 3% NaCl aqueous solution, and γ rays were irradiated for 20 hours at an irradiation intensity of 600 Gy / h using ⁶⁰ Co as a radiation source. A sample was taken out after this immersion irradiation test, and the rusting state of the thermal spray coating was visually observed by the state of rust oozing out on the surface, and the corrosion resistance was evaluated according to the following criteria.
A: No rusting, B: Rust area ratio 1% to less than 5%, B: Rust area ratio 5% to less than 10%, X: Rust area ratio 10% or more.

  Table 1 shows the corrosion resistance evaluation results of the samples formed using the conventional single electrode plasma spraying apparatus, and Table 2 shows the corrosion resistance evaluation results of the samples formed using the three-electrode plasma convergence type thermal spraying apparatus.

  As is apparent from Table 1, when the film is formed using a conventional single electrode plasma spraying apparatus, when the average particle size of the sprayed material is 5 to 10 μm or less, the merit of particle refining is hardly utilized, and the film forming state However, the film formation itself is difficult. On the other hand, when the average particle size of the raw material powder is 15 μm or more, although the film can be formed, the film becomes rough and the effect of improving corrosion resistance is hardly obtained. Although depending on the film thickness of the zirconia coating, a coating having a certain degree of corrosion resistance can be formed in a very narrow range where the average particle size of the sprayed material is 7 to 13 μm, but the corrosion resistance is not sufficient.

  On the other hand, as is clear from Table 2, when the film is formed using a three-electrode plasma converging type thermal spraying apparatus, the average particle size of the sprayed material is a fine particle region of 0.5 μm or less, or a coarse particle region of 25 μm or more However, it is impossible to form a film, but a corrosion-resistant film can be formed in a relatively wide particle size range of 1 to 20 μm. In particular, in the range of 3 to 15 μm, particularly in the range of 5 to 10 μm, the conventional single particle size can be formed. It can be confirmed that a corrosion-resistant film having superior performance that surpasses the electrode plasma spraying method can be obtained.

  As for the film thickness, there is no substantial difference in corrosion resistance in the range of 50 μm to 300 μm.

1 is a principle diagram of a multi-electrode plasma convergence type thermal spraying apparatus to which the present invention is applied.

Explanation of symbols

A Plasma spray gun P Plasma axis S Convergence point α Inclination angle

Claims (2)

When forming a functional coating that exhibits a corrosion protection effect due to a decrease in corrosion potential by radiation irradiation on a part or all of the base material constituting the metal structure exposed to the radiation irradiation atmosphere, it extends in the direction to be sprayed. A raw material powder inlet is provided on the upstream side of the plasma axis, a plurality of spray guns are arranged at equal intervals around the axis, and the plasma projected from each spray gun is arranged to converge at one point. A method for forming a functional coating excellent in anticorrosion, characterized by spraying ceramic powder having an average particle diameter of 5 to 10 μm using an electrode plasma convergence type thermal spraying apparatus.   The forming method according to claim 1, wherein the metal structure is a reactor internal structure.

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