Steel pipes for geothermal power generation

Abstract

To provide a steel pipe for geothermal power generation that has sufficient high temperature resistance and corrosion resistance for use in geothermal power generation, which is installed deep underground, thereby preventing or suppressing deterioration due to high temperature and highly oxidizing environments and enabling an extension of its lifespan. [Solution] A steel pipe 1 for geothermal power generation is configured such that a sprayed base layer 2 is formed by blasting the outer surface of a steel pipe base material 11, a first sprayed layer 3 is formed by spraying either a nickel-based material or an aluminum-based material onto the sprayed base layer, and a sealing treatment layer 6 is formed by sealing the outermost sprayed layer 5.

Description

[Technical Field]

[0001] This invention relates to a steel pipe for geothermal power generation, which is installed from the ground to an underground heat source such as a geothermal reservoir and transports steam or hot water in the geothermal reservoir or a working medium for heat exchange in the heat source such as the geothermal reservoir, and is used in a geothermal power generation apparatus. [Background technology]

[0002] Geothermal power generation is a technology that involves digging a well into a heat source such as a geothermal reservoir located approximately 1,000 to 3,000 meters underground, and using the geothermal heat from this reservoir to turn a turbine on the ground to generate electricity.

[0003] Geothermal power generation technologies include the "flash power generation method," which extracts high-temperature, high-pressure groundwater (steam and hot water) from a geothermal reservoir and uses it to rotate a turbine to generate electricity, and "binary power generation," which heats a working fluid with a lower boiling point than water through heat exchange, evaporating it and using it to rotate a turbine. Research is also being conducted into technology for generating electricity using supercritical water (water that is indistinguishable between liquid and gas due to temperatures of 374°C or higher and pressures of 218 atmospheres or higher), which can exist at greater depths, around 3,000 to 5,000 meters underground.

[0004] As mentioned above, there are various methods for geothermal power generation, but whichever method is adopted, it is necessary to dig a well called a borehole from the surface to the geothermal reservoir (or an even deeper point) and install steel pipes.

[0005] Generally, geothermal reservoirs are hot, exceeding 200-300°C, and the steel pipes used there must be able to withstand high temperatures. Furthermore, volcanic gases (such as sulfur dioxide and hydrogen sulfide) and other acidic substances are present in the geothermal reservoir and underground leading to it, which could cause the steel pipes to rust, so corrosion resistance is also required.

[0006] The technology described in Patent Document 1 proposes that "a metal coating thinner than the body is formed on the side of the medium transfer pipe that comes into contact with the ground," "a coating with high thermal conductivity is formed," and "the thickness of the coating is in the range of 300 μm to 500 μm." This technology makes it possible to suppress corrosion of the steel pipe body through the simulated corrosion protection of the coating, thereby improving the durability of the medium transfer pipe (steel pipe). However, the technique described in Patent Document 1 has the problem that corrosion resistance is insufficient. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2022-19283 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, the object of this invention is to provide a steel pipe for geothermal power generation that has sufficient high temperature resistance and corrosion resistance for use in geothermal power generation, which is installed deep underground, and thereby prevents or suppresses deterioration due to high temperature and highly oxidizing environments, and can extend its lifespan. [Means for solving the problem]

[0009] The above object of the present invention is achieved by the following configuration.

[0010] 1. A steel pipe for geothermal power generation that is used for geothermal power generation and is installed from the surface to the underground heat source, a thermal spray base layer is formed on the outer surface of the steel pipe base material for geothermal power generation by blasting; a first thermal sprayed layer is formed by thermally spraying either a nickel-based material or an aluminum-based material onto the thermal sprayed base layer; A steel pipe for geothermal power generation, characterized in that a sealing treatment layer is formed by subjecting a thermal sprayed layer formed on the outermost surface to a sealing treatment.

[0011] 2. A second sprayed layer is formed by spraying a high-temperature oxidation-resistant material onto the first sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the second sprayed layer, which is the outermost sprayed layer.

[0012] 3. A first sprayed layer is formed by thermally spraying a nickel-based material; an aluminum-based material is sprayed onto the first sprayed layer to form a third sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the third sprayed layer, which is the outermost sprayed layer.

[0013] 4. A first sprayed layer is formed by thermally spraying a nickel-based material; a second thermal sprayed layer is formed by thermal spraying a high-temperature oxidation-resistant material onto the first thermal sprayed layer; an aluminum-based material is sprayed onto the second sprayed layer to form a third sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the third sprayed layer, which is the outermost sprayed layer.

[0014] 5. A steel pipe for geothermal power generation according to claim 2 or 4, characterized in that the high-temperature oxidation resistant material is a material containing at least three of nickel, molybdenum, chromium and iron.

[0015] 6. The film thickness of the first thermal spray layer is 100 to 300 μm, The thickness of the second sprayed layer is 100 to 300 μm, 5. The steel pipe for geothermal power generation according to claim 4, wherein the third sprayed layer has a thickness of 50 to 200 μm. [Effects of the Invention]

[0016] According to the invention shown in claim 1, by performing blasting on the outer surface of a steel pipe base material (a steel pipe that has not been subjected to any work or treatment such as blasting, thermal spraying, or sealing treatment; the same applies below), irregularities are formed on the outer surface of the steel pipe base material, which is the material to be thermally sprayed, and this has the effect of improving the adhesion between the steel pipe base material and the thermal sprayed coating, By using either a nickel-based material or an aluminum-based material as the spray material for the first sprayed layer, it is possible to improve the adhesion between the steel pipe base material and the second sprayed layer, and it is also possible to improve corrosion resistance in high-temperature environments. By applying a sealing treatment to the outermost sprayed layer, it is possible to prevent or suppress initial corrosion of the entire sprayed coating, and further to prevent or suppress penetrative corrosion, thereby providing the effect of protecting the sprayed coating.

[0017] In other words, the overall configuration of the invention shown in claim 1 has the effects of improving the adhesion between the steel pipe base material and the thermal spray coating, improving corrosion resistance in high-temperature environments, and preventing or suppressing initial corrosion of the entire thermal spray coating.As a steel pipe for geothermal power generation that is installed deep underground, it has sufficient high-temperature resistance and corrosion resistance, which makes it possible to prevent or suppress deterioration due to high-temperature and highly oxidizing environments and to extend its lifespan.

[0018] According to the invention shown in claim 2, by using a high-temperature oxidation-resistant material as the spray material for the second sprayed layer, it is possible to improve durability against high-temperature oxidation and also exhibit corrosion resistance against intergranular corrosion.

[0019] Intergranular corrosion is explained in detail below. While the main purpose of conventional thermal spraying (general thermal spray materials) is to prevent or suppress corrosion on the surface, they are weak against corrosion between grains, and the corrosion resistance between grains depends on the effectiveness of sealing treatment. In this case, if the sealing treatment becomes insufficient due to deterioration or other reasons, components from the ground will enter the gaps between the grains, accelerating the progression of corrosion. Therefore, by using a high-temperature oxidation-resistant material that is also resistant to intergranular corrosion as the second sprayed layer, it is possible to improve corrosion resistance in intergranular corrosion.

[0020] According to the invention shown in claim 3, when the first sprayed layer is formed by spraying a nickel-based material, by using an aluminum-based material as the spray material for the third sprayed layer, the sacrificial corrosion protection provided by this third sprayed layer has the effect of further improving the durability and corrosion resistance of the first sprayed layer and the steel pipe base material.

[0021] According to the invention described in claim 4, by adopting a three-layer structure consisting of first to third sprayed layers, it is possible to maximize high temperature resistance and corrosion resistance, thereby preventing or suppressing deterioration due to high temperature and highly oxidizing environments and prolonging the life of the pipe. In other words, by combining sprayed layers, it is possible to achieve an extension of the life of the steel pipe in high temperature and highly oxidizing environments.

[0022] According to the invention shown in claim 5, by using a material containing three or more of nickel, molybdenum, chromium, or iron as the high-temperature oxidation-resistant material that is the spray material of the second sprayed layer, it is possible to form a sprayed layer that is not only resistant to high-temperature oxidation but also resistant to intergranular corrosion.

[0023] According to the invention set forth in claim 6, by specifying the film thickness of each of the first to third sprayed layers, the effects of each can be fully exerted while also contributing to cost savings by not using more material than necessary. [Brief explanation of the drawings]

[0024] [Figure 1] Schematic diagram showing steel pipes for geothermal power generation installed underground [Figure 2] Schematic explanatory diagram showing one embodiment of a steel pipe [Figure 3] FIG. 1 is a schematic partial cross-sectional view showing an example of the configuration of the outer surface of a steel pipe for geothermal power generation according to the present invention. [Figure 4]FIG. 1 is a schematic partial cross-sectional view showing another embodiment of the outer surface configuration of a steel pipe for geothermal power generation according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0026] The present invention relates to a steel pipe 1 for geothermal power generation (also simply referred to as "steel pipe 1"). Steel pipes for geothermal power generation, as shown in Figure 1, are installed from the ground to an underground heat source such as a geothermal reservoir, and are used to transport steam or hot water in the geothermal reservoir, or a working medium (also called a secondary medium) that undergoes heat exchange in the underground heat source such as the geothermal reservoir.

[0027] As shown in Fig. 2, a steel pipe 1 is used by connecting a plurality of steel pipes 1 each having a predetermined length. Since one steel pipe 1 is long, its length is omitted in Fig. 2. Geothermal reservoirs are often located about 1,000 to 3,000 meters underground, and since steel pipes 1 must be installed up to this point, steel pipes 1 of a specified length are connected longitudinally and installed up to the geothermal reservoir.

[0028] Examples of the method for connecting the steel pipes 1 include flange joining, butt welding, and sleeve joining. Flange joining is a method of joining steel pipes by attaching disk-shaped parts called flanges to both ends of the pipe and tightening them with bolts. Butt welding is a method of directly welding the ends of steel pipes together. Sleeve joining is a method of inserting a cylindrical part called a sleeve inside a steel pipe and fixing it in place with welding or adhesive.

[0029] Both of these connection methods are well known, and the method for connecting steel pipes 1 in the present invention can employ any of these well-known and commonly used means without any particular restrictions. It should be noted that the shape and configuration of the steel pipe 1 in FIG. 2 and the above-mentioned connection method are merely examples, and the contents of the present invention are not limited to these configurations.

[0030] There are several known geothermal power generation methods, the most common of which are the flash method and the binary method. The flash method uses geothermal fluid (steam) to turn a turbine, and utilizes the steam and hot water obtained from underground as direct energy. The steam and hot water accumulated in the geothermal reservoir are first taken into a separator where they are separated into high-temperature steam and hot water, with the steam being used to turn the turbine and the hot water being returned to the geothermal reservoir. This flash method requires high-temperature steam, making it suitable for areas where hot water above 200°C can be pumped up from underground.

[0031] The binary system uses a secondary medium between the geothermal reservoir and the turbine. In other words, heat is exchanged between the heat source, such as a geothermal reservoir, and the secondary medium (also called the working medium), and the high-temperature secondary medium generates steam, which drives the turbine and generates electricity. By using a secondary medium with a lower boiling point than water (such as ammonia, pentane, or alternative chlorofluorocarbons), steam can be generated at a lower temperature. This binary system does not require as much high-temperature steam as the flash system, making it suitable for areas with low geothermal temperatures.

[0032] Also attracting attention is the closed-loop (also called closed cycle) method, particularly closed-loop geothermal power generation using double-pipes. The closed-loop system using double pipes uses a double pipe consisting of an inner pipe and an outer pipe as the steel pipe for geothermal power generation. The outer pipe sends pressurized water from above ground deep underground, where it undergoes heat exchange underground (at a heat source such as a geothermal reservoir), and the heated water is collected by the inner pipe, which then reduces the pressure above ground to generate steam that drives a turbine. This closed-loop system using double pipes does not recover hot water or steam from an underground heat source, but rather circulates water within the double pipes. This has the advantage that geothermal power plants can be built in locations with geothermal energy even if there is no hot water reservoir, and there is no need to worry about hot springs drying up.

[0033] FIG. 1 shows an embodiment that utilizes a closed loop system using a double pipe, and the steel pipe 1 for geothermal power generation according to the present invention is also suitable as a steel pipe for use in this system. The above-mentioned power generation methods are merely examples, and the present invention is not limited to these methods. Depending on the power generation method and scale, multiple steel pipes may be installed from the ground to the underground heat source.

[0034] Next, the specific structure of the steel pipe 1 for geothermal power generation will be described with reference to FIG. Fig. 3 is a schematic explanatory diagram (schematic partial cross-sectional view) for explaining the configuration of the outer surface of the steel pipe 1. This is a schematic view showing a part of the cross-section of the steel pipe 1, and the thickness (film thickness), scale, etc. differ from the actual ones. Also, cross-sectional lines are omitted.

[0035] As shown in Figure 3, the steel pipe 1 according to the present invention is formed, in order from the outer surface of the steel pipe base material 11 outwards, by a thermal sprayed base layer 2, a first thermal sprayed layer 3, a second thermal sprayed layer 4, a third thermal sprayed layer 5, and a sealing treatment layer 6. However, as will be described later, the second thermal sprayed layer 4 and the third thermal sprayed layer 5 are not essential components, and it is preferable to use them selectively depending on the application, underground conditions (environment), etc.

[0036] <Steel pipe base material 11> In this specification, the steel pipe base material 11 refers to a steel pipe that has not been subjected to blasting, thermal spraying, or sealing treatment. There is no limitation on the shape or material of the steel pipe base material 11, and any known and commonly used configuration can be used without any particular limitation. Commonly used materials include iron, stainless steel, alloys thereof, carbon steel, nickel alloys, etc.

[0037] <Thermal spray base layer 2> The thermal spray base layer 2 is formed by blasting the outer surface of the steel pipe base material 11 to create irregularities on the outer surface of the steel pipe base material 11, which is the material to be thermal sprayed, in order to improve the adhesion between the steel pipe base material 11 and the thermal spray coating (first thermal spray layer 3 to third thermal spray layer 5).

[0038] First, black scale is removed from the outer surface of the steel pipe base material 11 by shot blasting (cleaning shot blasting). Black scale is an oxide film that mainly exists on the surface of steel materials, and in this process, the black scale is removed by spraying or impacting the outer surface of the steel pipe base material 11 with projectile material. This process removes the black scale, oil, etc. that have adhered to the surface of the steel pipe base material 11, exposing the surface of the normal base material. In this cleaning shot blasting process, it is preferable to use an alumina material as the blast material. Furthermore, it is preferable to use this alumina material with a roughness of #30 to #40, and the roughness is selected from this range depending on the material of the steel pipe base material 11.

[0039] Thereafter, thermal spray base blasting is carried out to form irregularities on the outer surface of the steel pipe base material 11 . In this thermal spray base blasting process, it is preferable to use an alumina material with a roughness of #16 to #20. It is preferable that this step forms irregularities on the outer surface of the steel pipe base material 11 with a roughness of about 100 to 400 μm (about Ra 5 to 8 μm).

[0040] By these blasting processes, a thermal sprayed base layer 2 is formed on the outer surface of the steel pipe base material 11. The sprayed base layer 2 is a portion of the outer surface of the steel pipe base material 11 that has been textured, and is part of the surface of the steel pipe base material 11, but in the explanation in this specification, it will be treated as a layer that forms the steel pipe 1 for geothermal power generation related to the present invention.

[0041] <First sprayed layer 3> The first sprayed layer 3 is formed by spraying either a nickel-based material or an aluminum-based material onto the steel pipe base material 11 (sprayed base layer 2) on which a sprayed base has been formed. As for the specific means of thermal spraying, any known and used means can be used without any particular restrictions. The same applies to the steps for the second thermal sprayed layer 4 and the third thermal sprayed layer 5 described below.

[0042] The nickel-based material used as the thermal spray material is preferably an alloy of nickel and chromium. The aluminum-based material used as the thermal spray material may contain trace amounts of other materials such as magnesium, chromium, and iron in addition to aluminum. By using either a nickel-based material or an aluminum-based material as the first sprayed layer 3, the adhesion between the steel pipe base material 11 and the second sprayed layer 4 or the third sprayed layer 5 can be improved, and there is also the effect of improving corrosion resistance in high-temperature environments.

[0043] The thickness of the first sprayed layer 3 is preferably 100 to 300 μm. Forming the first sprayed layer 3 with a thickness in this range can provide sufficient effect while also contributing to cost savings by not using more material than necessary.

[0044] <Second sprayed layer 4> The second sprayed layer 4 is formed on the first sprayed layer 3 by spraying a high-temperature oxidation-resistant material.

[0045] The high-temperature oxidation-resistant material used as the thermal spray material is preferably a material containing one or more of nickel, molybdenum, chromium, and iron. By using a high-temperature oxidation resistant material for the second sprayed layer 4, it is possible to improve durability against high-temperature oxidation and also exhibit corrosion resistance against intergranular corrosion.

[0046] Intergranular corrosion is explained in detail below. While the main purpose of conventional thermal spraying (general thermal spray materials) is to prevent or suppress corrosion on the surface, they are vulnerable to corrosion between grains and are dependent on the effectiveness of sealing treatment. In this case, if the sealing treatment becomes insufficiently effective due to deterioration, components from the ground will penetrate between the grains, accelerating the progression of corrosion. Therefore, by using a high-temperature oxidation-resistant material that is also resistant to intergranular corrosion as the second sprayed layer 4, it is possible to improve the corrosion resistance to intergranular corrosion.

[0047] The thickness of the second sprayed layer 4 is preferably 100 to 300 μm. By forming the second sprayed layer 4 with a thickness in this range, the effect can be fully exerted, and by not using more material than necessary, it is also possible to contribute to cost savings.

[0048] <Third sprayed layer 5> The third sprayed layer 5 is formed by spraying an aluminum-based material onto the first sprayed layer 3 or the second sprayed layer 4, which has been formed by spraying a nickel-based material. The aluminum-based material used as the thermal spray material may contain trace amounts of other materials such as magnesium, chromium, and iron in addition to aluminum.

[0049] By using an aluminum-based material as the third sprayed layer 5, the sacrificial corrosion protection provided by this third sprayed layer 5 has the effect of further improving the durability and corrosion resistance of the first sprayed layer 3 and the steel pipe base material 11.

[0050] The thickness of the third sprayed layer 5 is preferably 50 to 200 μm. By forming the layer with a thickness in this range, the effect can be fully exerted, and by not using more material than necessary, it is also possible to contribute to cost savings.

[0051] <Sealing treatment layer 6> The sealing treatment layer 6 is formed by applying a sealing treatment to the first sprayed layer 3, the second sprayed layer 4, or the third sprayed layer 5, i.e., the outermost sprayed layer. In other words, the sealing treatment is not applied to each of the first sprayed layer 3, the second sprayed layer 4, or the third sprayed layer 5, but is applied only to the outermost sprayed layer. Sealing is a process used in metal surface treatment to close up tiny holes (pinholes) that remain after treatment, thereby improving corrosion resistance and wear resistance. As for the specific means for sealing the pores, any known and used means can be adopted without any particular restrictions.

[0052] The sealing treatment is preferably carried out by applying a glass-based paint, which is preferably an Si-O2 paint and preferably a moisture-curing solvent. There is no particular limitation on the thickness of the sealing layer 6.

[0053] It is preferable to apply the coating twice in total, once for penetration and once for surface protection. By carrying out the sealing treatment, it is possible to prevent or suppress initial corrosion of the entire thermal spray coating, and further to prevent or suppress penetrative corrosion, thereby having the effect of protecting the thermal spray coating.

[0054] <Embodiments of the present invention> As described above, the steel pipe for geothermal power generation 1 according to the present invention is constructed by forming a thermal sprayed base layer 2, a first thermal sprayed layer 3, a second thermal sprayed layer 4, a third thermal sprayed layer 5, and a sealing treatment layer 6 on the outer surface of the steel pipe base material 11. 3, the thermal sprayed base layer 2, first thermal sprayed layer 3, second thermal sprayed layer 4, third thermal sprayed layer 5, and sealing layer 6 are all formed on the outer surface of the steel pipe base material 11, but the second thermal sprayed layer 4 and the third thermal sprayed layer 5 are not essential components and are used selectively depending on the application, underground conditions (environment), etc. In other words, the thermal sprayed base layer 2, first thermal sprayed layer 3, and sealing layer 6 are essential components, and it is possible to select a configuration in which either or both of the second thermal sprayed layer 4 and the third thermal sprayed layer 5 are formed.

[0055] Fig. 4 shows the configuration of another embodiment described above. In order to prioritize visibility, unlike Fig. 3, cross-sectional lines are shown as parallel lines. 4(a) shows an example in which a thermal sprayed base layer 2, a first thermal sprayed layer 3, and a sealing treatment layer 6 are formed on the outer surface of a steel pipe base material 11. The first thermal sprayed layer 3 is made of either a nickel-based material or an aluminum-based material. 4(b) shows an example in which a thermal sprayed base layer 2, a first thermal sprayed layer 3, a second thermal sprayed layer 4, and a sealing treatment layer 6 are formed on the outer surface of a steel pipe base material 11. The first thermal sprayed layer 3 is made of either a nickel-based material or an aluminum-based material, and the second thermal sprayed layer is made of a high-temperature oxidation-resistant material. 4(c) shows an example in which a thermal sprayed base layer 2, a first thermal sprayed layer 3, a third thermal sprayed layer 5, and a sealing treatment layer 6 are formed on the outer surface of a steel pipe base material 11. The first thermal sprayed layer 3 is made of a nickel-based material, the second thermal sprayed layer is made of a high-temperature oxidation-resistant material, and the third thermal sprayed layer is made of an aluminum-based material. The embodiments shown in FIGS. 4(a) to 4(c) are all included in the examples of the present invention. According to the examples shown in Figs. 3 and 4, the life of steel pipes in high temperature and highly oxidizing environments can be extended by combining thermal sprayed layers.

[0056] In addition, the present invention does not exclude a configuration in which further sprayed layers or treatment layers are formed on the outer surface of the steel pipe base material 11 in addition to the configuration in which the sprayed base layer 2, the first sprayed layer 3, the second sprayed layer 4, the third sprayed layer 5, and the sealing treatment layer 6 are formed (the configuration shown in Figures 3 and 4). [Explanation of symbols]

[0057] 1 Steel pipe 11 Steel pipe base material 12 Connection 2 Thermal sprayed base layer 3 First sprayed layer 4 Second sprayed layer 5 Third sprayed layer 6 Sealing layer G. Power generating equipment P pump

Claims

1. A steel pipe for geothermal power generation that is used for geothermal power generation and is installed from the ground to the underground heat source, a thermal spray base layer is formed on the outer surface of the steel pipe base material for geothermal power generation by blasting; a first thermal sprayed layer is formed by thermally spraying either a nickel-based material or an aluminum-based material onto the thermal sprayed base layer; A steel pipe for geothermal power generation, characterized in that a sealing treatment layer is formed by subjecting a thermal sprayed layer formed on the outermost surface to a sealing treatment.

2. a second thermal sprayed layer is formed by thermal spraying a high-temperature oxidation-resistant material onto the first thermal sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the second sprayed layer, which is the outermost sprayed layer.

3. A first thermal sprayed layer is formed by thermal spraying a nickel-based material, an aluminum-based material is sprayed onto the first sprayed layer to form a third sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the third sprayed layer, which is the outermost sprayed layer.

4. A first thermal sprayed layer is formed by thermal spraying a nickel-based material, a second thermal sprayed layer is formed by thermal spraying a high-temperature oxidation-resistant material onto the first thermal sprayed layer; an aluminum-based material is sprayed onto the second sprayed layer to form a third sprayed layer; A steel pipe for geothermal power generation as described in claim 1, characterized in that a sealing treatment layer is formed by performing a sealing treatment on the third sprayed layer, which is the outermost sprayed layer.

5. 5. A steel pipe for geothermal power generation according to claim 2 or 4, wherein the high-temperature oxidation resistant material is a material containing at least three of nickel, molybdenum, chromium and iron.

6. The film thickness of the first thermal spray layer is 100 to 300 μm, The thickness of the second sprayed layer is 100 to 300 μm, 5. The steel pipe for geothermal power generation according to claim 4, wherein the third sprayed layer has a thickness of 50 to 200 μm.

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