Method for preventing steel material from corroding, and storage container

Abstract

To provide: a method for preventing a steel material from corroding, capable of suppressing the ammonia stress corrosion cracks of high strength steel compared with when using a sacrificial anode related to conventional knowledge; and a storage container including a high strength steel tank material capable of suppressing ammonia stress corrosion cracks compared with when using the sacrificial anode related to conventional knowledge.SOLUTION: A method for preventing a steel material 3 contacting liquid ammonia 7 from corroding comprises coating zinc aluminum alloy having an aluminum content of 0.1wt% or more and 20wt% or less on the surface of the steel material 3 contacting the liquid ammonia 7 by a thermal spray method. The steel material 3 has a tensile strength of 440 MPa or more; and the method for preventing a steel material from corroding is applied to the tank material of a storage container including a high strength steel tank material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a corrosion prevention method for steel materials and storage containers, and more particularly to a corrosion prevention method for steel materials that come into contact with liquid ammonia (for example, steel materials used in containers for liquid ammonia) and a storage container including the steel materials. [Background technology]

[0002] Ammonia is expected to be a next-generation fuel, and currently a demonstration experiment is underway in which ammonia is mixed with coal-fired power plants at a 20% ammonia combustion rate. Mixing ammonia with 20% ammonia alone will significantly reduce CO2 emissions from coal-fired power plants, but to further increase the reduction in CO2 emissions, "mono-fuel combustion," which uses only ammonia as fuel, is expected to begin in the future (see Non-Patent Documents 1 and 2).

[0003] On the other hand, the calorific value of ammonia per unit volume is about 60% of that of methane, the main component of LNG (liquefied natural gas). Therefore, in order to use ammonia as an alternative fuel to LNG, the storage volume of ammonia would need to be about 1.7 times that of methane (see Non-Patent Document 3).

[0004] Currently, the amount of ammonia tanks used in the chemical industry in Japan is around 10,000 to 20,000 tons, and if the existing technology is extended, it will be 40,000 tons (equivalent to a volume of approximately 60,000 m 3 ) is said to be the limit. For LNG, it is 200,000 m 3 Considering that the above tanks are mainstream, the storage capacity is small, and therefore it is thought that larger tanks are essential to produce and utilize ammonia as fuel (see Non-Patent Document 3).

[0005] In order to increase the size of ammonia tanks, it is possible to select high-strength steel for the steel material that makes up the tank, but it is known that the higher the strength of the steel, the more susceptible it is to ammonia stress corrosion cracking (see Non-Patent Document 3).

[0006] Here, it is known that in order to prevent ammonia stress corrosion cracking of steel materials, it is effective to use a sacrificial anode and utilize the sacrificial anticorrosion effect (see Patent Documents 1 to 3 and Non-Patent Document 4). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 56-47584 [Patent Document 2] Japanese Patent Application Publication No. 56-51566 [Patent Document 3] Japanese Patent Application Publication No. 56-58962 [Non-patent literature]

[0008] [Non-Patent Document 1] "Can ammonia become a fuel?! (Part 1) ~ Uses of ammonia that are familiar but you may not know about," Agency for Natural Resources and Energy, Ministry of Economy, Trade and Industry, January 15, 2021, [Retrieved February 19, 2024].<https: / / www.enecho.meti.go.jp / about / special / johoteikyo / ammonia_01.html> [Non-patent document 2] "Can Ammonia Become a 'Fuel'?! (Part 2) ~ Carbon-Free Ammonia Thermal Power Generation," Agency for Natural Resources and Energy, Ministry of Economy, Trade and Industry, January 29, 2021, [Retrieved February 19, 2024].<https: / / www.enecho.meti.go.jp / about / special / johoteikyo / ammonia_02.html> [Non-patent document 3] "Realization of Large Ammonia Tanks and Establishment of Test Methods," IHI Technical Report Vol. 63 No. 1 (2023), [Retrieved February 19, 2024].<https: / / www.ihi.co.jp / technology / techinfo / contents_no / __icsFiles / afieldfile / 2023 / 07 / 06 / 06.pdf> [Non-patent document 4] "Prevention of corrosion cracking of liquid ammonia tanks by zinc spraying", Hiroyuki Imagawa, Kenzo Matsuno, Toshio Konishi, Boshoku Gijutsu, 38, 321-326 (1989) Summary of the Invention [Problem to be solved by the invention]

[0009] However, in order to realize the enlargement of, for example, ammonia tanks, the corrosion protection methods for steel materials using sacrificial anodes according to conventional knowledge as described in Patent Documents 1 to 3 and Non-Patent Documents 1 to 4 are insufficient as countermeasures against ammonia stress corrosion cracking of high-strength steel, and the inventors have considered that it is necessary to develop a corrosion protection method for steel materials that can suppress ammonia stress corrosion cracking of high-strength steel more effectively than when sacrificial anodes according to conventional knowledge are used.

[0010] The present invention has been made in view of the above points, and aims to provide a method for protecting steel materials from corrosion that can suppress ammonia stress corrosion cracking of high-strength steel more effectively than when sacrificial anodes according to conventional knowledge are used, and a storage container equipped with a high-strength steel tank material that suppresses ammonia stress corrosion cracking more effectively than when sacrificial anodes according to conventional knowledge are used. Note that, in this application, high-strength steel means a steel material with a tensile strength of 440 MPa or more. [Means for solving the problem]

[0011] The present invention is an invention that solves the above-mentioned problems and provides the following corrosion prevention method for steel materials.

[0012] That is, a first aspect of the method for protecting steel materials according to the present invention is a method for protecting steel materials that come into contact with liquid ammonia, the method comprising: coating, by a thermal spraying method, a zinc-aluminum alloy having an aluminum content of 0.1 wt% or more and 20 wt% or less, on a surface of the steel material that comes into contact with liquid ammonia, wherein the steel material has a tensile strength of 440 MPa or more.

[0013] Here, the "zinc-aluminum alloy" used in the method for corrosion protection of steel according to the present invention not only includes an alloy consisting of zinc and aluminum and containing no elements other than zinc and aluminum except for unavoidable impurities, but also includes an alloy that contains zinc and aluminum and also contains elements other than zinc and aluminum substantially, rather than as unavoidable impurities, so long as it can solve the problem that the method for corrosion protection of steel according to the present invention is trying to solve (to provide a method for corrosion protection of steel that can suppress ammonia stress corrosion cracking of high-strength steel more than when a sacrificial anode according to conventional knowledge is used). The same interpretation shall be used in similar descriptions elsewhere in this application.

[0014] Furthermore, the "steel material that comes into contact with liquid ammonia" refers to a steel material that has a portion that comes into contact with liquid ammonia when not coated with the zinc-aluminum based alloy, and also includes steel material that has been coated with the zinc-aluminum based alloy and thus no longer comes into contact with liquid ammonia. Similar descriptions in other parts of this application should be interpreted in the same way.

[0015] Furthermore, "the surface of the steel material that comes into contact with liquid ammonia" includes not only the surface of the steel material that is in constant contact with liquid ammonia, but also the surface that may come into temporary contact with liquid ammonia. This same interpretation applies to similar descriptions in other parts of this application.

[0016] A second aspect of the corrosion protection method for steel materials according to the present invention is the method for corrosion protection for steel materials of the first aspect, characterized in that the zinc-aluminum alloy has an aluminum content of 0.1 wt% or more and 1 wt% or less.

[0017] A third aspect of the corrosion protection method for steel materials according to the present invention is the method for corrosion protection for steel materials according to the first or second aspect, characterized in that the zinc-aluminum alloy contains 3 wt% or less of magnesium.

[0018] A fourth aspect of the corrosion protection method for steel materials according to the present invention is the method for corrosion protection for steel materials according to the first or second aspect, characterized in that the zinc-aluminum alloy contains 2 wt% or more and 3 wt% or less of magnesium.

[0019] A fifth aspect of the corrosion protection method for steel materials according to the present invention is an aspect characterized in that, in the corrosion protection method for steel materials of any of the first to fourth aspects, the tensile strength of the steel materials is 490 MPa or more.

[0020] A first aspect of the storage container according to the present invention is a storage container capable of storing liquid ammonia as a storage item, characterized in that it has a tank material formed from steel having a tensile strength of 440 MPa or more, and the surface of the tank material that comes into contact with the storage item is coated by a thermal spraying method with a zinc-aluminum alloy having an aluminum content of 0.1 wt% or more and 20 wt% or less.

[0021] Here, the "zinc-aluminum alloy" used in the storage container of the present invention not only includes an alloy consisting of zinc and aluminum and containing no elements other than zinc and aluminum except for unavoidable impurities, but also includes an alloy that contains zinc and aluminum and also contains elements other than zinc and aluminum substantially, rather than as unavoidable impurities, as long as it can solve the problem that the storage container of the present invention is intended to solve (to provide a storage container equipped with a high-strength steel tank material that is more inhibited from ammonia stress corrosion cracking than when a sacrificial anode according to conventional knowledge is used). The same interpretation will be used in similar descriptions elsewhere in this application.

[0022] Furthermore, "the surface of the tank material that comes into contact with the stored material" refers to the surface of the tank material that comes into contact with the stored material when it is not coated with the zinc-aluminum alloy, and also includes the surface of the tank material that is no longer in contact with liquid ammonia when liquid ammonia is stored as the stored material by being coated with the zinc-aluminum alloy. Similar descriptions in other parts of this application will be interpreted in the same way.

[0023] Furthermore, the term "surface of the tank material that comes into contact with the stored material" refers to not only the surface of the tank material that is in constant contact with the stored material, but also the surface that may come into temporary contact with the stored material. This same interpretation applies to similar descriptions in other parts of this application.

[0024] A second aspect of the storage container according to the present invention is the storage container of the first aspect, characterized in that the zinc-aluminum alloy has an aluminum content of 0.1 wt% or more and 1 wt% or less.

[0025] A third aspect of the storage container according to the present invention is the storage container of the first or second aspect, characterized in that the zinc-aluminum alloy contains 3 wt % or less of magnesium.

[0026] A fourth aspect of the storage container according to the present invention is the storage container of the first or second aspect, characterized in that the zinc-aluminum alloy contains 2 wt% or more and 3 wt% or less of magnesium.

[0027] A fifth aspect of the storage container according to the present invention is the storage container of any one of the first to fourth aspects, characterized in that the tensile strength of the steel material is 490 MPa or more. [Effects of the Invention]

[0028] According to the present invention, it is possible to provide a method for protecting steel materials from corrosion, which can suppress ammonia stress corrosion cracking of high-strength steel more effectively than when sacrificial anodes according to conventional knowledge are used, and a storage container equipped with a high-strength steel tank material in which ammonia stress corrosion cracking is suppressed more effectively than when sacrificial anodes according to conventional knowledge are used. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a cross-sectional view schematically showing a cross section of a wall 2 of a storage facility for liquid ammonia to which a corrosion prevention method for steel according to an embodiment of the present invention is applied. [Figure 2]FIG. 1 is a plan view schematically illustrating a test sample 10. [Figure 3] FIG. 1 is a diagram schematically illustrating a state in which a test specimen sample 10 is fixed to a four-point bending jig 20, and is a side view of the state in which the test specimen sample 10 is fixed to the four-point bending jig 20. [Figure 4] Schematic diagram showing the ammonia stress corrosion cracking acceleration experiment [Figure 5] FIG. 1 is a side view of the autoclave 30, showing the overall shape of the autoclave 30. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, with the description taking into consideration corrosion prevention of inner tank steel materials used in liquid ammonia storage facilities and coming into contact with liquid ammonia. However, the application of the corrosion prevention method for steel materials according to the present invention is not limited to corrosion prevention of the inner tank steel materials, and the corrosion prevention method for steel materials according to the present invention is widely applicable to corrosion prevention of steel materials coming into contact with liquid ammonia, and can also be applied to corrosion prevention of, for example, liquid ammonia transport piping, etc.

[0031] On the other hand, the storage container according to the embodiment of the present invention is a storage container in which the method for preventing corrosion of steel according to the embodiment of the present invention is applied to the steel material in the inner tank that comes into contact with liquid ammonia. Therefore, by explaining the method for preventing corrosion of steel according to the embodiment of the present invention, the main contents of the storage container according to the embodiment of the present invention will be explained. Therefore, in the following explanation of the embodiment of the present invention, the explanation will be given with the method for preventing corrosion of steel according to the embodiment of the present invention in mind, and only necessary explanations will be given of the storage container according to the embodiment of the present invention in the explanation of the method for preventing corrosion of steel according to the embodiment of the present invention.

[0032] 1 is a cross-sectional view schematically showing a cross section of a wall 2 of a liquid ammonia storage facility to which a corrosion prevention method for steel according to an embodiment of the present invention is applied. The wall 2 is configured by arranging, from the inside to the outside, a sprayed metal coating 1, an inner tank steel material 3, a cold insulation material 4, and a liquid barrier 6 in that order, and the inner surface of the inner tank steel material 3, except for the area where the sprayed metal coating 1 is provided, is in direct contact with liquid ammonia 7.

[0033] In a method for protecting steel from corrosion according to an embodiment of the present invention, as shown in Fig. 1, a sprayed metal coating 1 is provided on at least a portion of the surface of inner tank steel material 3 to be protected from corrosion and which comes into contact with liquid ammonia 7. The inner tank steel material 3 is a steel material having a tensile strength of 440 MPa or more, and may be a steel material having a tensile strength of 490 MPa or more.

[0034] The metal forming the sprayed metal coating 1 is a zinc-aluminum alloy with an aluminum content of 0.1 wt% to 20 wt%. Thermal spraying is performed on the surface of the inner vessel steel material 3 to form the sprayed metal coating 1 made of a zinc-aluminum alloy with an aluminum content of 0.1 wt% to 20 wt%. By setting the aluminum content in the sprayed metal coating 1 made of a zinc-aluminum alloy to 0.1 wt% to 20 wt%, ammonia stress corrosion cracking is suppressed even in high-strength steel materials with a tensile strength of 440 MPa or more, as demonstrated in the examples described below. Furthermore, in order to further enhance the sacrificial corrosion protection effect of the zinc-aluminum alloy (sprayed metal coating 1), it is preferable to set the aluminum content to 0.1 wt% to 1 wt%, as demonstrated in the examples described below.

[0035] The spraying method used to form the sprayed metal coating 1 is not particularly limited, and specifically, wire flame spraying, powder flame spraying, high-velocity flame spraying, arc spraying, plasma spraying, cold spray, etc. can be used.

[0036] When wire flame spraying is used, a linear spray material (in this embodiment, a wire of a zinc-aluminum alloy of a predetermined composition) is mechanically fed into a combustion flame of oxygen and fuel, and the molten material in the flame is blown away with compressed air to form a sprayed metal coating 1 on the surface of the target object (in this embodiment, the inner tank steel material 3).

[0037] When powder flame spraying is used, oxygen and acetylene (propane) flames are used as a heat source, and a specified powder material (in this embodiment, a mixed powder of zinc powder and aluminum powder in a specified ratio) is melted and sprayed to form a sprayed metal coating 1 on the surface of the target object (in this embodiment, inner tank steel material 3).

[0038] Cold spraying is a technology that uses compressed gases such as air, nitrogen, or helium to accelerate metal particles of several tens of micrometers to subsonic or supersonic speeds, allowing the particles to collide with a substrate in a solid state without melting, forming a coating. Cold spraying has a lower application temperature than conventional thermal spraying (which exceeds 5000 K in the case of plasma spraying), with a working gas temperature of 500 to 1000°C. The working gas temperature in cold spraying is 500 to 1000°C, but this high temperature results from expanding the gas and delivering the particles (in this embodiment, a mixed powder of zinc powder and aluminum powder in a specified ratio) used to form the coating at high speed. The temperature of the particles themselves is lower than the melting point of the material, which suppresses oxidation more than other thermal spraying methods.

[0039] As demonstrated in the examples described below, the metal forming the sprayed metal coating 1 may be a zinc-aluminum alloy containing 0.1 wt% to 20 wt% aluminum, and may further contain 3 wt% or less of magnesium. Magnesium has a high ionization tendency and can enhance the sacrificial corrosion protection effect. From the perspective of enhancing the sacrificial corrosion protection effect, the magnesium content contained in the zinc-aluminum alloy is preferably 2 wt% to 3 wt%. As demonstrated in the examples described below, if the magnesium content contained in the zinc-aluminum alloy is 3.5 wt% or more, the sprayed metal coating 14 becomes more likely to fall off in powder form, and the sacrificial corrosion protection effect is actually reduced. [Example]

[0040] (1) Overview of the study Examples and comparative examples are provided below to support the present invention relating to a method for corrosion protection of steel materials. In this study, test specimen samples 10 (see FIG. 2) were prepared by forming sprayed metal coatings 14 using different metal compositions for the spraying. The prepared test specimens 10 were attached to four-point bending fixtures 20 as shown in FIG. 3 and placed in autoclaves 30, and an accelerated ammonia stress corrosion cracking experiment was performed as shown in FIG. 4. FIG. 4 is a schematic diagram showing the conditions of the accelerated ammonia stress corrosion cracking experiment performed in this study.

[0041] When a zinc-aluminum alloy or a mixed powder of zinc powder and aluminum powder is sprayed to form a sprayed metal coating 14 of the zinc-aluminum alloy and used as a sacrificial anode, the influence of the aluminum content in the sprayed metal coating 14 on the corrosion prevention effect was investigated. Also, when a mixed powder of zinc powder, aluminum powder, and magnesium powder is sprayed to form a sprayed metal coating 14 of the zinc-aluminum-magnesium alloy and used as a sacrificial anode, the influence of the magnesium content in the sprayed metal coating 14 on the corrosion prevention effect was investigated.

[0042] (2) Experimental method The test piece steel plate 12 used in this study was a 3 mm thick carbon steel plate SLA325A for low-temperature pressure vessels cut into a long, narrow rectangle with a length (a1) of 70 mm and a width (a2) of 15 mm, as shown in Figure 2, and had a yield stress of 409 MPa and a tensile strength of 518 MPa. A metal of a predetermined composition was sprayed onto one surface of this test piece steel plate 12 to a target film thickness of 300 μm, forming a sprayed metal coating 14. Thereafter, as shown in FIG. 2 , the surface of the test piece steel plate 12 on which the sprayed metal coating 14 was formed had a length (b1) of 60 mm and a width (b2) of 10 mm, except for a central portion 12A (an edge portion 12B near the edge of the surface on which the sprayed metal coating 14 was formed and the surface opposite the side on which the sprayed metal coating 14 was formed) (these portions may be referred to as “predetermined portions”), was covered with a silicone-based sealant 16, so that only the central portion 12A of the surface on which the sprayed metal coating 14 was formed was exposed, thereby producing a test specimen sample 10. Before covering the predetermined portion of the test piece steel plate 12 with the silicone-based sealant 16, a vinyl chloride-coated conductor 60A (see FIG. 4 ) connected to a potentio-galvanostat 60 was spot-welded to the edge of the side of the test piece steel plate 12, and then the predetermined portion was covered with the silicone-based sealant 16.

[0043] As shown in FIG. 3, the prepared specimen sample 10 was fixed to a four-point bending jig 20 with the sprayed metal coating 14 facing outward (tension side). Both ends of the specimen sample 10 were restrained from above by two upper support points 22B (the distance L1 between the two upper support points 22B was 60 mm) provided on the underside of an upper protrusion 22A of an outer frame 22 of the four-point bending jig 20, and the central portion 12A of the specimen sample 10 was lifted from below by two central pressure points 24A (the distance L2 between the two central pressure points 24A was 30 mm) provided on the upper surface of a central pressure member 24, which was lifted upward by a shaft 26A of a bolt 26 that penetrated a bottom plate 22C of the outer frame 22 and was adjustable in vertical movement. In this way, the specimen sample 10 was fixed to the four-point bending jig 20 in the bent state as shown in FIG. 3. The upward protrusion of the shank 26A of the bolt 26 was adjusted so that, in this bending strain state, a yield stress was applied to the tensile surface of the test piece steel plate 12 of the test specimen sample 10. During the accelerated ammonia stress corrosion cracking experiment, the test specimen sample 10 was placed in the autoclave 30 with a constant bending strain applied as shown in Figure 3 (as mentioned above, a vinyl chloride-coated conductor 60A connected to a potentio-galvanostat 60 was spot-welded to the end face of the side of the test specimen steel plate 12).

[0044] FIG. 5 is a diagram schematically illustrating the overall shape of autoclave 30, showing a side view of autoclave 30. Autoclave 30 has an inner pipe 32 (3 mm thick) made of SUS316L and an outer pipe 34 (3 mm thick) made of SUS316L. The inner pipe 32 is positioned inside the outer pipe 34, and the lower ends of each pipe are welded to a circular bottom plate 36 (thickness t0: 14 mm) to form a double-pipe structure. A donut-shaped upper plate 38 is attached to close the upper end of the space between the inner pipe 32 and the outer pipe 34, forming a refrigerant circulation space 46. An inlet pipe 34A is attached near the upper end of the outer periphery of the outer pipe 34, and an outlet pipe 34B is attached near the lower end of the outer periphery of the outer pipe 34. The refrigerant discharged from the chiller (not shown) enters the refrigerant circulation space 46 through the inlet pipe 34A, and after circulating in the refrigerant circulation space 46, the refrigerant is discharged to the outside through the outlet pipe 34B and returns to the chiller (not shown) where it is cooled again. The refrigerant repeats this circulation, and the inner space 30A inside the autoclave 30 is cooled.

[0045] The opening flange 40 has a through-hole 40A with an inner diameter equal to the inner diameter of the inner pipe 32, and as shown in Fig. 5, the upper end of the inner pipe 32 is attached to the opening flange 40 by welding so as to coincide with the through-hole 40A of the opening flange 40. A closing flange 42 is disposed on top of the opening flange 40, and the opening flange 40 and the closing flange 42 are fastened together by bolts (not shown) inserted through bolt through-holes (not shown) provided on the outer edges of each flange, thereby sealing the space inside the inner pipe 32 (the inner space 30A of the autoclave 30). As shown in FIG. 5, the inner diameter φ1 of the inner pipe 32 is 77 mm, the outer diameters of the opening flange 40 and the closing flange 42 are the same outer diameter φ2 (=155 mm), the thickness t1 of the opening flange 40 and the thickness t2 of the closing flange 42 are both 14 mm, the distance h1 from the upper surface of the bottom plate 36 to the lower surface of the opening flange 40 is 108 mm, and the height h2 of the outer pipe 34 is 95 mm.

[0046] As shown in Fig. 4, an ammonia pipe 44 is attached to pass through the closing flange 42, and ammonia gas can pass through the ammonia pipe 44 to enter and exit the inner space 30A of the autoclave 30. The ammonia pipe 44 is made of SUS316L. Note that the ammonia pipe 44 is not shown in Fig. 5.

[0047] The four-point bending jig 20, to which the test specimen sample 10 is fixed as shown in FIG. 3, is placed in this state inside the autoclave 30 as shown in FIG. 4, and a closing flange 42 is attached with bolts (not shown) to seal the inner space 30A of the autoclave 30. Then, ammonia gas is introduced into the inner space 30A of the autoclave 30 through the ammonia piping 44. The inner space 30A of the autoclave 30 is cooled to −20°C by the refrigerant circulating in the refrigerant circulation space 46, and the ammonia gas introduced into the inner space 30A of the autoclave 30 through the ammonia piping 44 is liquefied into liquid ammonia 50. The pressure in the inner space 30A is 0.19 MPa (the pressure at which ammonia gas and liquid ammonia are in equilibrium at −20°C is 0.19 MPa; the ammonia gas was introduced into the inner space 30A at a pressure of 0.2 MPa, which is slightly higher than the pressure at −20°C). During the 14 days of the accelerated ammonia stress corrosion cracking experiment in this study, the test specimen sample 10 was immersed in liquid ammonia 50 at -20°C while being fixed to a four-point bending jig 20. In this accelerated ammonia stress corrosion cracking experiment, the liquid ammonia 50 in the inner space 30A of the autoclave 30 contained 100 wtppm of oxygen, 1 wt% of water, and 100 wtppm of carbon dioxide.

[0048] As mentioned above, a vinyl chloride-coated lead wire 60A connected to a potentio-galvanostat 60 was spot-welded to the side edge of the steel specimen 12. As shown in Figure 4, a counter electrode 62 connected to a vinyl chloride-coated lead wire 60B connected to the potentio-galvanostat 60 was immersed in liquid ammonia 50, and a reference electrode 64 connected to a vinyl chloride-coated lead wire 60C connected to the potentio-galvanostat 60 was immersed in liquid ammonia 50. This electrical connection allowed the potentio-galvanostat 60 to control the potential of the test specimen 10. Both the counter electrode 62 and the reference electrode 64 were platinum. The potential of the test specimen 10 was maintained at 0.5 V during the 14-day accelerated ammonia stress corrosion cracking experiment in this study. The potential of the test specimen sample 10 before potential control by the potentio-galvanostat 60 was -0.3 V, and in the ammonia stress corrosion cracking acceleration experiment in this study, a voltage of +0.8 V was applied to maintain the potential of the test specimen sample 10 at 0.5 V, so that the metal of the test specimen sample 10 became ions and became more easily dissolved, thereby accelerating ammonia stress corrosion cracking.

[0049] After the 14-day accelerated ammonia stress corrosion cracking experiment was completed, the cooling by the chiller (not shown) was stopped, the temperature of the inner space 30A of the autoclave 30 was raised to room temperature, the liquid ammonia was vaporized and discharged as ammonia gas from the ammonia piping 44, and the test specimen sample 10 after the 14-day accelerated ammonia stress corrosion cracking experiment was completed was removed from the autoclave 30, and it was confirmed whether or not cracks had occurred in the test piece steel plate 12 of the test specimen sample 10. For some of the test specimen samples 10 taken out of the autoclave 30 after the completion of the 14-day ammonia stress corrosion cracking acceleration experiment, the silicone-based sealant 16 was removed, and the weight (total weight of the test specimen steel plate 12 and the sprayed metal coating 14) was measured after the vinyl chloride-coated conductor 60A was removed from the test specimen steel plate 12. The weight loss of the sprayed metal coating 14 due to the experiment was calculated from the difference between this weight and the weight measured before the ammonia stress corrosion cracking acceleration experiment (total weight of the test specimen steel plate 12 and the sprayed metal coating 14 after the test specimen steel plate 12 was thermally sprayed to form the sprayed metal coating 14).

[0050] (3) Experimental Results (Examples 1 to 22, Comparative Examples 1 to 5) A test piece steel plate 12 was thermally sprayed to form a sprayed metal coating 14 of a zinc-aluminum alloy, which was used as a sacrificial anode, and the accelerated ammonia stress corrosion cracking experiment described in "(2) Experimental Method" was conducted to examine the effect of the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) on the ammonia stress corrosion cracking suppression effect. Also, a test piece steel plate 12 was thermally sprayed to form a sprayed metal coating 14 of a zinc-aluminum-magnesium alloy, which was used as a sacrificial anode, and the accelerated ammonia stress corrosion cracking experiment described in "(2) Experimental Method" was conducted to examine the ammonia stress corrosion cracking suppression effect of forming a sprayed metal coating 14 from a zinc-aluminum-magnesium alloy, which was obtained by further adding magnesium to the zinc-aluminum alloy.

[0051] Specifically, zinc-aluminum alloys were examined by varying the aluminum content from 0 wt% to 100 wt%. The aluminum content was varied in small increments in the low aluminum content region and in large increments in the high aluminum content region. Test specimen samples 10 were fabricated by thermal spraying one surface of a steel plate 12, and sprayed metal coatings 14 were formed with varying aluminum contents as described above (Examples 1 to 15, Comparative Examples 1 to 3). Wire flame spraying, powder flame spraying, and cold spraying were used as the spraying methods. Note that the case where the aluminum content in the sprayed metal coating 14 is 0 wt% (zinc alone, Comparative Example 1) and the case where the aluminum content in the sprayed metal coating 14 is 100 wt% (aluminum alone, Comparative Example 3) are also considered to be zinc-aluminum alloys in the following description.

[0052] In the zinc-aluminum-magnesium alloys, the aluminum content was fixed at 0.5 wt%, and the magnesium content was varied in 0.5 wt% increments within the range of 0.5 to 4 wt%. Test specimen samples 10 were prepared by thermal spraying one surface of a steel plate 12, and forming a thermal sprayed metal coating 14 with varying magnesium contents as described above (Examples 16 to 22, Comparative Examples 4 and 5). Powder flame spraying and cold spraying were used as the thermal spraying methods.

[0053] The accelerated ammonia stress corrosion cracking experiment described in "(2) Experimental Method" was performed for two weeks, and the results are shown in Table 1 below. After the two-week accelerated ammonia stress corrosion cracking experiment, the test specimen samples 10 were removed from the autoclave 30, and the test specimen steel plates 12 of the test specimen samples 10 were checked for cracks. Those in which cracks were observed are marked with "Yes" in the "Crack Occurrence" column in Table 1, and those in which cracks were not observed are marked with "No" in the "Crack Occurrence" column in Table 1. As mentioned above, for some test specimen samples 10, the total weight of the test specimen steel plates 12 and the sprayed metallic coatings 14 was measured before and after the accelerated ammonia stress corrosion cracking experiment, and the weight loss of the sprayed metallic coatings 14 due to the experiment was calculated. The calculated results are shown in the "Weight Loss" column in Table 1. (Those in which the weight loss of the sprayed metallic coatings 14 due to the experiment was not measured are marked with "-" in the "Weight Loss" column in Table 1.) In addition, in Comparative Examples 4 and 5, it was observed that a portion of the sprayed metal coating 14 had fallen off in powder form below the test specimen sample 10 after the experiment was completed, and therefore this was recorded as "powder-like falling off" in the "Notes" column of Table 1 (for those in which "powder-like falling off" was not observed, a "-" was recorded in the "Notes" column of Table 1).

[0054] [Table 1]

[0055] The following can be inferred from the experimental results shown in Table 1. (a) From the results of Examples 1 to 15 and Comparative Examples 1 to 3, when a zinc-aluminum alloy was used as the sprayed metal coating 14, when the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) was 0.1 to 20 wt%, no cracks occurred in the steel plate specimen 12, ammonia stress corrosion cracking was suppressed, and favorable results were obtained. On the other hand, when the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) was 0 wt% (Comparative Example 1 (when the sprayed metal coating 14 was zinc alone)), when the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) was 100 wt% (Comparative Example 3 (when the sprayed metal coating 14 was aluminum alone)), and when the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) was 25 wt% (Comparative Example 2), cracks occurred in the steel plate specimen 12, and ammonia stress corrosion cracking was not suppressed.

[0056] In Comparative Examples 1 to 3, the weight loss of the sprayed metal coating 14 in the accelerated ammonia stress corrosion cracking experiment was 3 to 5 mg, whereas in Examples 1 to 15, the weight loss of the sprayed metal coating 14 in the accelerated ammonia stress corrosion cracking experiment was large, at 17 to 22 mg. This suggests that the zinc-aluminum alloy (sprayed metal coating 14) sprayed onto the test piece steel plate 12 exhibited good sacrificial corrosion protection, and that this is why ammonia stress corrosion cracking did not occur in Examples 1 to 15.

[0057] Furthermore, when the aluminum content in the zinc-aluminum alloy (sprayed metal coating 14) was 0.1 to 1 wt% (Examples 1 to 4, Example 7), the weight loss of the sprayed metal coating 14 in the accelerated ammonia stress corrosion cracking experiment was 19 mg or more, and it is believed that the zinc-aluminum alloy (sprayed metal coating 14) exhibited better sacrificial corrosion protection.

[0058] For the above reasons, when a zinc-aluminum alloy is used as the sprayed metal coating 14, the aluminum content in the zinc-aluminum alloy is preferably 0.1 to 20 wt %, and more preferably 0.1 to 1 wt %.

[0059] Furthermore, when the aluminum content in the zinc-aluminum alloy (thermal-sprayed metal coating 14) was 0 wt% (Comparative Example 1), the weight loss of the thermal-sprayed metal coating 14 in the ammonia stress corrosion cracking acceleration experiment was 4 mg, whereas when the aluminum content in the zinc-aluminum alloy (thermal-sprayed metal coating 14) was 0.1 wt% (Example 1), the weight loss of the thermal-sprayed metal coating 14 in the ammonia stress corrosion cracking acceleration experiment suddenly increased to 19 mg. Furthermore, when the aluminum content in the zinc-aluminum alloy (thermal-sprayed metal coating 14) was 15 wt% (Example 14), the weight loss of the thermal-sprayed metal coating 14 in the ammonia stress corrosion cracking acceleration experiment was 17 mg, whereas when the aluminum content in the zinc-aluminum alloy (thermal-sprayed metal coating 14) was 25 wt% (Comparative Example 2), the weight loss of the thermal-sprayed metal coating 14 in the ammonia stress corrosion cracking acceleration experiment suddenly decreased to 5 mg. These facts are thought to indicate the critical significance of the aluminum content in the zinc-aluminum alloy (thermal-sprayed metal coating 14).

[0060] (b) From the results of Examples 16 to 22 and Comparative Examples 4 and 5, when a zinc-aluminum-magnesium alloy was used as the sprayed metal coating 14, when the magnesium content in the zinc-aluminum-magnesium alloy (sprayed metal coating 14) was 0.5 to 3 wt% (Examples 16 to 22), no cracks occurred in the steel plate specimen 12, ammonia stress corrosion cracking was suppressed, and favorable results were obtained. However, when the magnesium content in the zinc-aluminum-magnesium alloy (sprayed metal coating 14) was 3.5 wt% or more (Comparative Examples 4 and 5), cracks occurred in the steel plate specimen 12, and ammonia stress corrosion cracking could not be suppressed. When the magnesium content in the zinc-aluminum-magnesium alloy (sprayed metal coating 14) was 3.5 wt % or more (Comparative Examples 4 and 5), when the accelerated ammonia stress corrosion cracking experiment was completed, it was confirmed that part of the sprayed metal coating 14 had fallen off in powder form below the test specimen 10. This is thought to have prevented the sacrificial corrosion protection effect of the sprayed metal coating 14 from being fully exerted, and ammonia stress corrosion cracking could not be suppressed, resulting in the occurrence of cracks in the test specimen steel plate 12.

[0061] On the other hand, in Examples 16 to 22, in which the magnesium content in the zinc-aluminum-magnesium alloy (sprayed metal coating 14) was 0.5 to 3.0 wt%, the weight loss of the sprayed metal coating 14 in the accelerated ammonia stress corrosion cracking experiment was large, at 18 to 24 mg (for Examples 19 and 21, the weight loss of the sprayed metal coating 14 in the experiment was not measured, but judging from the measurement results of Examples 18, 20, and 22, the weight loss of Examples 19 and 21 is thought to be within the range of 18 to 24 mg). Furthermore, after the accelerated ammonia stress corrosion cracking experiment was completed, no detachment of the sprayed metal coating 14 was confirmed below the test piece sample 10. Therefore, it is thought that the sacrificial corrosion protection effect was well exerted, and therefore it is thought that ammonia stress corrosion cracking did not occur in Examples 16 to 22.

[0062] Furthermore, when the magnesium content in the zinc-aluminum-magnesium alloy (sprayed metal coating 14) was 2 wt % and 3 wt % (Examples 20 and 22), the weight loss of the sprayed metal coating 14 in the accelerated ammonia stress corrosion cracking experiment was large, at 24 mg and 22 mg, respectively, and no detachment of the sprayed metal coating 14 was confirmed below the test piece sample 10 after the accelerated ammonia stress corrosion cracking experiment was completed. Therefore, it is believed that the zinc-aluminum-magnesium alloy (sprayed metal coating 14) exhibited a better sacrificial corrosion protection effect.

[0063] Therefore, when a zinc-aluminum-magnesium alloy is used as the sprayed metal coating 14, the magnesium content in the zinc-aluminum-magnesium alloy is preferably 0.5 to 3 wt %, and more preferably 2 to 3 wt %.

[0064] (c) In both Examples 4 and 5, a zinc-aluminum alloy with an aluminum content of 0.5 wt % was used as the sprayed metal coating 14, and in Example 4, the spraying method was cold spraying, while in Example 5, the spraying method was powder flame spraying. In both Examples 4 and 5, no cracks occurred in the test piece steel plate 12, ammonia stress corrosion cracking was suppressed, and good results were obtained.

[0065] In addition, both Examples 13 and 14 are examples in which a zinc-aluminum alloy with an aluminum content of 15 wt % was used as the sprayed metal coating 14, and Example 13 was an example in which the spraying method was wire flame spraying, while Example 14 was an example in which the spraying method was cold spraying. In both Examples 13 and 14, no cracks occurred in the test piece steel plate 12, ammonia stress corrosion cracking was suppressed, and good results were obtained.

[0066] In addition, in both Examples 16 and 17, a zinc-aluminum-magnesium alloy having an aluminum content of 0.5 wt % and a magnesium content of 0.5 wt % was used as the sprayed metal coating 14, and in Example 16, cold spraying was used as the spraying method, while in Example 17, powder flame spraying was used as the spraying method. In both Examples 16 and 17, no cracks were generated in the test piece steel plate 12, ammonia stress corrosion cracking was suppressed, and good results were obtained. [Explanation of symbols]

[0067] 1, 14...sprayed metal coating 2…Wall body 3…Inner tank steel material 4...Ice pack 6…Liquid dike 7, 50...Liquid ammonia 10...Test specimen sample 12...Test piece steel plate 12A…Central part 12B…Edge 16...Silicone sealant 20...4-point bending jig 22...Outer frame 22A…Upper protrusion 22B…Upper support point 22C…Bottom plate 24...Central pressure member 24A...Central pressure point 26...Volts 26A…Shaft part 30...Autoclave 30A…Inner space 32...Inner pipe 34...Outer pipe 34A…Inlet pipe 34B…Outlet pipe 36…Bottom plate 38...Top board 40...Opening flange 40A...Through hole 42...Blind flange 44...Ammonia piping 46…Refrigerant circulation space 60...Potentio-galvanostat 60A, 60B, 60C...Vinyl chloride coated conductor 62...opposite 64...Reference electrode a1, b1...length a2, b2…width L1…Distance between upper support points L2: Distance between central pressure points

Claims

1. A method for preventing corrosion of a steel material in contact with liquid ammonia, comprising: The tensile strength of the steel material is 440 MPa or more, A method for preventing corrosion of steel material, comprising coating a zinc-aluminum alloy having an aluminum content of 0.1 wt % or more and 20 wt % or less on the surface of the steel material that comes into contact with liquid ammonia by a thermal spraying method.

2. 2. The corrosion prevention method for steel materials according to claim 1, wherein the zinc-aluminum alloy has an aluminum content of 0.1 wt % or more and 1 wt % or less.

3. 2. The method for preventing corrosion of steel materials according to claim 1, wherein the zinc-aluminum alloy contains 3 wt % or less of magnesium.

4. 2. The method for preventing corrosion of steel materials according to claim 1, wherein the zinc-aluminum alloy contains 2 wt % to 3 wt % of magnesium.

5. 5. The corrosion prevention method for steel material according to claim 1, wherein the tensile strength of the steel material is 490 MPa or more.

6. A storage container capable of storing liquid ammonia as a storage item, The tank is made of a steel material having a tensile strength of 440 MPa or more. A storage container characterized in that the surface of the tank material that comes into contact with the stored material is coated with a zinc-aluminum alloy having an aluminum content of 0.1 wt% or more and 20 wt% or less by a thermal spraying method.

7. 7. The storage container according to claim 6, wherein the zinc-aluminum alloy has an aluminum content of 0.1 wt % or more and 1 wt % or less.

8. 7. The storage container according to claim 6, wherein the zinc-aluminum alloy contains 3 wt % or less of magnesium.

9. 7. The storage container according to claim 6, wherein the zinc-aluminum alloy contains 2 wt % to 3 wt % of magnesium.

10. 10. The storage container according to claim 6, wherein the tensile strength of the steel material is 490 MPa or more.

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