Ammonia stress corrosion cracking acceleration test method

Abstract

To provide an ammonia SCC acceleration test method capable of evaluating ammonia stress corrosion cracking sensitivity of a metal material with good accuracy in a short period of time.SOLUTION: In an ammonia stress corrosion cracking acceleration test method of the present invention, a metal test piece to which external stress is applied or which has residual stress is immersed in liquid ammonia containing ammonium carbamate of 0.01 mass% or more, O2 of 0.002-2.000 bar in terms of gas partial pressure, and water of 0.05-2.00 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an accelerated ammonia stress corrosion cracking test method for metallic materials such as tanks used in a liquid ammonia environment. [Background technology]

[0002] In recent years, liquid ammonia has been attracting attention as a clean energy source because it does not emit CO2 when burned, and large-scale demand is expected. This has led to a demand for larger facilities to transport and store liquid ammonia. Generally, when making tanks larger, thinner steel is used to reduce weight and construction costs, and therefore the use of high-strength steel is desirable.

[0003] On the other hand, in a liquid ammonia environment, there is concern that carbon steel may suffer from stress corrosion cracking due to liquid ammonia (hereinafter referred to as ammonia SCC). Therefore, for carbon steel structures that handle liquid ammonia, such as piping, storage tanks, tank cars, and line pipes, steel materials with low ammonia SCC susceptibility have been used, and operational measures have been taken to suppress ammonia SCC.

[0004] SCC is a phenomenon that leads to destruction when corrosion reactions and stress overlap, and occurs when certain material, environmental, and stress factors meet certain conditions. For example, ammonia SCC is known to correlate with the strength and hardness of a material. In other words, the higher the strength and hardness, the more likely ammonia SCC occurs. Therefore, when using carbon steel, it is recommended to use a material with a tensile strength of less than 600 MPa.

[0005] Therefore, when applying new materials that combine high strength with excellent ammonia SCC resistance, it is necessary to accurately evaluate ammonia SCC susceptibility.However, since evaluating ammonia SCC susceptibility through exposure tests in actual liquid ammonia tanks requires long-term testing, an accelerated test that can evaluate the ammonia SCC susceptibility of steel materials in a short period of time is desired.

[0006] Such accelerated tests using liquid ammonia are disclosed in Patent Document 1 and Non-Patent Document 1. Patent Document 1 and Non-Patent Document 1 describe a test method for evaluating ammonia SCC susceptibility in a short period of time by anodic polarization of a test steel piece in liquid ammonia containing O2 and saturated CO2 at a gas pressure of 0.3 to 2.0 atmospheres, thereby accelerating iron dissolution. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 60-10575 [Non-patent literature]

[0008] [Non-Patent Document 1] Yoichi Nakai, "Development of an accelerated testing method for stress corrosion cracking in liquid ammonia," Iron and Steel, Vol. 67, No. 14, pp. 2226-2233, 1981 Summary of the Invention [Problem to be solved by the invention]

[0009] In the environments where there is an excess of O2 described in Patent Document 1 and Non-Patent Document 1, a non-protective film is formed, resulting in a corrosion pattern similar to general corrosion, which inhibits the growth of pitting corrosion (localized corrosion) that is the starting point for ammonia SCC, making it difficult to say that ammonia SCC susceptibility can be accurately evaluated. Furthermore, because the growth of pitting corrosion (localized corrosion) is not promoted, it may not be possible to evaluate ammonia SCC susceptibility in materials that are not highly susceptible to ammonia SCC.

[0010] The present invention aims to solve the above problems and provide an accelerated ammonia SCC testing method that can evaluate the ammonia SCC susceptibility of metal materials used in tanks for transporting and storing liquid ammonia with high accuracy and in a short period of time. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted a detailed study on the ammonia SCC mechanism of steel sheets in liquid ammonia, and have obtained the following findings.

[0012] In a liquid ammonia environment, the following corrosion reactions occur on steel plates: Anode reaction: Fe → Fe 2+ +2e - Cathode reaction: O2 + 2NH4 + +4e - →2OH - +2NH3

[0013] As a result of the above corrosion reaction, an inactive oxide film forms on the steel sheet surface. However, pitting corrosion occurs when localized dissolution of the oxide film is accelerated in unstable areas. The pits become stress-concentrating areas, leading to film breakdown and corrosion reactions at the pit tip, ultimately leading to fracture. Therefore, we conceived the idea of ​​destabilizing the oxide film that forms on the steel sheet surface to promote pitting corrosion, which is the initiation point of ammonia-induced SCC.

[0014] The inventors then discovered that by adding a predetermined amount of ammonium carbamate and water to liquid ammonia, the oxide film on the surface of the steel sheet becomes unstable, making it possible to evaluate ammonia SCC susceptibility in a short period of time, which led to the completion of the present invention.

[0015] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0016] [1] An accelerated ammonia stress corrosion cracking test method in which a metal test piece to which external stress has been applied or which has residual stress is immersed in liquid ammonia containing 0.01 mass% or more ammonium carbamate, O2 at a gas partial pressure of 0.002 to 2,000 bar, and 0.05 to 2,000 mass% water.

[0017] [2] The accelerated ammonia stress corrosion cracking test method according to [1], wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized.

[0018] [3] The accelerated ammonia stress corrosion cracking test method according to [1] or [2], wherein the liquid ammonia is stirred during the test.

[0019] [4] An accelerated ammonia stress corrosion cracking test method in which a metal test piece to which external stress has been applied or which has residual stress is immersed in liquid ammonia containing 0.01 mass% or more ammonium carbamate, O2 at a gas partial pressure of 0.002 to 2.000 bar, and 0.05 to 2.00 mass% water, and polarized from 0.0 V to +1.0 V relative to the corrosion potential or platinum reference electrode potential.

[0020] [5] The accelerated ammonia stress corrosion cracking test method according to [4], wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized and then polarized to 0.0 V to +1.0 V relative to the corrosion potential or a platinum reference electrode potential.

[0021] [6] The accelerated ammonia stress corrosion cracking test method according to [4] or [5], wherein the liquid ammonia is stirred during the test. [Effects of the Invention]

[0022] According to the accelerated ammonia stress corrosion cracking test method of the present invention, it is possible to evaluate the ammonia SCC susceptibility of metallic materials used in tanks and the like that are used in a liquid ammonia environment with high accuracy and in a short period of time. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. The present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist thereof.

[0024] In the embodiments of the present invention, A (numerical value) to B (numerical value) means A or more and B or less.

[0025] <Test solution> In the test method of the present invention, liquid ammonia containing 0.01 mass % or more ammonium carbamate, O2 at a gas partial pressure of 0.002 to 2.000 bar, and 0.05 to 2.00 mass % water is used as the test solution.

[0026] [Ammonium carbamate content: 0.01 mass% or more] Liquid ammonia normally contains trace amounts of CO2 as an impurity, and in liquid ammonia, CO2 dissociates and reaches an equilibrium state as shown in the following reaction formula. 2NH3+CO2⇔NH4CO2NH2⇔NH4 + +NH2CO2 -

[0027] Ammonium carbamate is converted to carbamate ion (CO2NH2 -) is formed. Because the carbamate ions have the effect of destroying the inactive oxide film on the surface of a metal test piece, optimizing the content of carbamate ions can destabilize the oxide film without causing anodic polarization, thereby promoting ammonia SCC. If the ammonium carbamate content in liquid ammonia is less than 0.01 mass%, the rate of oxide film regeneration by corrosion reactions is significantly greater than the rate of oxide film destruction by carbamate ions, and this effect cannot be achieved. Therefore, the ammonium carbamate content in liquid ammonia is specified to be 0.01 mass% or more. It is preferably 0.02 mass% or more, and more preferably 0.05 mass% or more. There is no particular upper limit for the ammonium carbamate content in liquid ammonia, and it may be contained up to the saturation amount. For example, it may be 6.00 mass% or less, 1.00 mass% or less, or 0.50 mass% or less.

[0028] The method for adding ammonium carbamate to liquid ammonia is not particularly limited, but it is preferable to place a predetermined amount of ammonium carbamate in the test vessel before introducing the liquid ammonia. The addition of ammonium carbamate can be replaced by blowing in CO2 gas or placing solid CO2 (dry ice) in an amount that will result in the predetermined amount of ammonium carbamate content.

[0029] [O2 content: gas partial pressure 0.002 to 2.000 bar] Because O2 has the effect of forming an oxide film in liquid ammonia, the presence of O2 is essential for evaluating ammonia SCC susceptibility. If the O2 content in liquid ammonia is less than 0.002 bar in gas partial pressure, this effect is not obtained, making ammonia SCC evaluation difficult. On the other hand, if the O2 content in liquid ammonia is more than 2,000 bar in gas partial pressure, the rate of oxide film formation is significantly increased, the oxide film is stabilized, and pitting corrosion does not occur, again making ammonia SCC evaluation difficult. Therefore, the O2 content in liquid ammonia is specified to be 0.002 to 2,000 bar in gas partial pressure. It is preferably 0.005 bar or more, more preferably 0.200 bar or more, and preferably 1,500 bar or less, more preferably 1,250 bar or less.

[0030] Although there are no particular limitations on the method for adding O2 to liquid ammonia, from the viewpoint of a stable supply of O2, it is preferable to blow a predetermined amount of O2 gas into the test vessel before introducing liquid ammonia. The blowing of O2 gas can be replaced by blowing in an amount of air gas that will provide the predetermined O2 gas partial pressure.

[0031] [Water content: 0.05~2.00mass%] Water promotes the formation of an inert oxide film on the surface of a metal test piece in liquid ammonia. This competes with the destruction of the oxide film on the surface of the metal test piece by the carbamate ions in liquid ammonia. This competition promotes the occurrence of pitting corrosion in the oxide film on the surface of the metal test piece, thereby accelerating ammonia SCC. If the water content in liquid ammonia is less than 0.05 mass%, the rate of inert film formation by water is significantly lower than the rate of inert film destruction by carbamate ions, which may result in a corrosion form closer to general corrosion rather than pitting corrosion (localized corrosion). On the other hand, if the water content in liquid ammonia exceeds 2.00 mass%, the rate of inert film formation by water is significantly higher than the rate of inert film destruction by carbamate ions, preventing pitting corrosion and making SCC evaluation difficult. Therefore, the water content in liquid ammonia is specified to be 0.05 to 2.00 mass%. The water content is preferably 0.06 mass% or more, more preferably 0.07 mass% or more. The water content is preferably 1.75 mass % or less, and more preferably 1.50 mass % or less.

[0032] The water to be added is not particularly limited, but from the viewpoint of eliminating the influence of impurities contained in the water, it is preferable that the water be pure water or ultrapure water with an impurity content of 1000 ppb or less.

[0033] The method for adding water to liquid ammonia is not particularly limited, but from the viewpoint of a stable supply of water, it is preferable to introduce a predetermined amount of water into the test vessel before introducing liquid ammonia.

[0034] [Liquid ammonia] The purity of the liquid ammonia used in the test solution is not particularly limited, but if it contains oil, stress corrosion cracking of the metal test piece is more likely to be suppressed. Therefore, it is preferable that the amount of oil contained as an impurity in the liquid ammonia be less than 0.05 mass%.

[0035] [Polarization between 0.0V and +1.0V relative to the corrosion potential or platinum reference electrode potential] In one embodiment of the test method of the present invention, a metal test piece to which external stress has been applied or which has residual stress is immersed in liquid ammonia containing 0.01 mass% or more ammonium carbamate, O2 at a gas partial pressure of 0.002 to 2.000 bar, and 0.05 to 2.00 mass% water, and is polarized between 0.0 V and +1.0 V relative to the corrosion potential or platinum reference electrode potential. This polarization is continued from the start of the test to the end of the test. If cathodic polarization, as described below, is performed at the start of the test, cathodic polarization is performed for a predetermined period of time, and then polarization between 0.0 V and +1.0 V is continued until the end of the test.

[0036] Although the test method of the present invention can be performed without anodic polarization of the metal test specimen, anodic polarization is preferred as described above. By polarizing the metal test specimen between 0.0 V and +1.0 V relative to the corrosion potential or platinum reference electrode potential, the oxide film formed on the surface of the metal test specimen by immersion in liquid ammonia becomes unstable. On the surface of a metal test specimen that has been subjected to external stress or has residual stress, pitting corrosion due to localized corrosion is promoted, allowing for a shorter evaluation of ammonia SCC susceptibility. On the other hand, if the metal test specimen is continuously polarized (cathodic polarization) below 0 V relative to the corrosion potential or platinum reference electrode potential, cathodic reactions continue to be promoted on the metal test specimen surface, making it difficult to evaluate the ammonia SCC susceptibility of the metal test specimen. Furthermore, if the metal test specimen is anodically polarized above +1.0 V relative to the corrosion potential or platinum reference electrode potential, general corrosion is promoted on the test specimen surface compared to localized corrosion, which is the initiation site of ammonia SCC, making it difficult to evaluate the ammonia SCC susceptibility in a short period of time. Furthermore, if general corrosion becomes dominant, localized corrosion, which is the initiation point of SCC, does not occur, and there is a risk that the ammonia SCC susceptibility itself cannot be evaluated. Therefore, it is specified that the metal test piece be anodic polarized at 0.0 V to +1.0 V relative to the corrosion potential or platinum reference electrode potential. The potential applied to the metal test piece relative to the corrosion potential or platinum reference electrode potential is preferably +0.2 V or more, more preferably +0.3 V or more, and preferably +0.9 V or less, more preferably +0.8 V or less.

[0037] The corrosion potential is determined by immersing a metal test piece for evaluating ammonia SCC susceptibility as a working electrode (sample electrode) together with a reference electrode that is not in contact with the working electrode in liquid ammonia for 5 minutes or more while the working electrode is subjected to external stress or while the working electrode has residual stress, and by using the potential of the reference electrode as the reference (0.0 V). The method for measuring the corrosion potential is not particularly limited, and the measurement can be performed by, for example, a two-electrode method or a three-electrode method.

[0038] In the present invention, polarizing a metal test piece from 0.0 V to +1.0 V relative to the platinum reference electrode potential means polarizing the metal test piece (the working electrode) from 0.0 V to +1.0 V, starting from a state in which the potential of the metal test piece, which is the working electrode (sample electrode), is set to 0.0 V relative to the platinum reference electrode potential.

[0039] When performing anodic polarization on a metal test piece in the potential range of 0.0 V to +1.0 V, ammonia SCC can be promoted and the evaluation period for ammonia SCC susceptibility can be shortened regardless of whether the corrosion potential or the platinum reference electrode potential is used as the reference, but when using the platinum reference electrode potential as the reference, it is preferable to measure the corrosion potential in advance.If the corrosion potential is known, it is preferable to use the platinum reference electrode potential as the reference from the viewpoint of test efficiency.

[0040] When polarizing (applying a potential to) a metal test piece with respect to a platinum reference electrode potential, the time from immersion of the metal test piece in liquid ammonia to application of a potential is not particularly limited. From the viewpoint of stabilizing the surface state of the metal test piece, it is preferable to apply a potential to the metal test piece after immersing it in liquid ammonia for 5 minutes or more.

[0041] Measurement of corrosion potential and polarization (application of potential) of a metal test piece can be performed, for example, using a potentiostat (a constant-potential electrolysis device) with the metal test piece being the working electrode (sample electrode). In this case, platinum (Pt) electrodes, which are stable in liquid ammonia, are preferably used as the counter electrode and reference electrode.

[0042] [Cathode polarization] A passive film may be formed on the surface of the metal test piece before immersion in liquid ammonia. By performing cathodic polarization immediately after immersion in liquid ammonia, the passive film on the surface of the metal test piece can be removed, the initial surface condition can be homogenized, and a more accurate evaluation of ammonia SCC susceptibility can be achieved. The conditions for cathodic polarization are not particularly limited, but it is preferable to perform the polarization at a potential between -1.0 V and -3.0 V relative to the corrosion potential or the platinum reference electrode potential for 5 to 30 minutes.

[0043] [Liquid ammonia stirred during testing] The corrosion reaction is affected by the composition of the liquid ammonia solution on the metal test specimen surface. Stirring the test solution (liquid ammonia containing ammonium carbamate, O2, and water) homogenizes the solution composition on the surface of the immersed metal test specimen, enabling more accurate ammonia SCC susceptibility evaluation. Furthermore, stirring promotes ion delivery to the metal test specimen surface, thereby shortening the time required for ammonia SCC susceptibility evaluation. Therefore, it is preferable to stir the liquid ammonia containing ammonium carbamate, O2, and water during the test. Because ammonia SCC susceptibility is affected by nitrogen, carbon dioxide, oxygen, and other elements, stirring is preferably performed using a stir bar. To stabilize the ion delivery to the metal test specimen surface, stirring is preferably performed continuously at 10 rpm or higher.

[0044] <Metal test piece> The metal test piece used in the test method of the present invention may be taken from a metal material whose ammonia SCC susceptibility is to be evaluated, or from a metal material having the same or similar chemical composition and metal structure as the metal material. The chemical composition and metal structure of the metal material are not particularly limited. Specific examples of the metal material include those containing a metal element (e.g., Fe, Cu, Al, Ni, Ti, etc.) as the main component (i.e., 50 mass% or more), such as steel (iron alloy), copper alloy, aluminum alloy, nickel alloy, and titanium alloy.

[0045] When a steel material is used as the metal test piece, the steel material is not particularly limited in terms of its carbon equivalent or hardness. It is generally known that the higher the carbon equivalent or hardness of a steel material, the higher the ammonia SCC susceptibility. Therefore, it is preferable that the steel material test piece has a carbon equivalent of 0.05% or more and a Vickers hardness of 100 HV or more.

[0046] The shape and size of the metal test piece are not particularly limited and may be determined appropriately depending on the test cell to be used and the external stress application method, or may be selected from known standards. However, the exposed area (cm) of the metal test piece should be set so as to minimize the effect of the corrosion reaction on the solution composition. 2 ) is the amount of liquid ammonia solution (mL) relative to the 2 The upper limit of the specific liquid volume is not particularly limited, but if the specific liquid volume is excessively large, the cost of test equipment etc. will increase, so it is preferable to set it to 500 mL / cm or more. 2 It is preferable to do the following:

[0047] [External stress / residual stress] Since the test method of the present invention is an accelerated ammonia stress corrosion cracking test, a metal test piece to which an external stress has been applied or which has residual stress is immersed in a test solution. The method for applying external stress or residual stress is not particularly limited, but external stress can be applied by known methods such as four-point bending, U-bending, or the constant load method, and residual stress can also be applied by heat treatment, welding, processing, etc.

[0048] The magnitude of the applied external stress or residual stress may be appropriately set depending on the test purpose (for example, the intended use of the material to be evaluated), but it is preferable to apply a load of 20% or more of the yield strength YS (MPa) of each metal test piece. Since ammonia SCC can be promoted as the applied external stress or residual stress increases, there is no particular upper limit as long as damage such as cracks is not caused to the metal test piece itself.

[0049] <Other test conditions> The test temperature (temperature of the test solution) at which the test method of the present invention is carried out can be appropriately selected depending on the purpose of the test, but since corrosion reactions tend to be suppressed at low temperatures and the test period tends to be longer at low temperatures, it is preferable to set the temperature at -35 to 60°C. In addition, to further improve test accuracy, it is preferable to set the error between the set test temperature and the actual test temperature within ±5°C.

[0050] The test period for carrying out the test method of the present invention is not particularly limited and can be set appropriately depending on the intended use of the metallic material to be evaluated, but in order to apply new materials, the shorter the test period, the better, and it is preferably within 720 hours. Ammonia SCC testing under an actual tank environment (actual liquid ammonia) generally requires a test period of about one year, so by using the test method of the present invention, the test period can be shortened to about one-tenth or less.

[0051] Test conditions other than those described above are not particularly limited, and known conditions used in corrosion tests can be appropriately selected and used. Test equipment, test cells, etc. that are known in the art, such as those described in Patent Document 1 and Non-Patent Document 1, can also be used. [Example]

[0052] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0053] [Ammonia SCC occurrence in actual tank environment] As a preliminary study to confirm the effectiveness of the test method of the present invention, five types of steel (steel types A to E) with different chemical compositions were used as test steels, and an ammonia SCC test was conducted in an actual tank environment using actual liquid ammonia to evaluate their ammonia SCC susceptibility.

[0054] Table 1 shows the carbon equivalent (CE), yield strength (YS) and Vickers hardness (HV0.5) of the steels subjected to the ammonia SCC test.

[0055] The carbon equivalent (CE) was calculated from the chemical composition of the test steel material using the following formula. CE=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (In the formula, the element symbols indicate the content (mass%) of each element in the test steel material.)

[0056] The yield strength and Vickers hardness were measured at the 1 / 4 position of the plate thickness in accordance with JIS Z 2241 and JIS Z 2244. The Vickers hardness was measured at 20 points using a Vickers test with a load of 500 g, and the average value was used.

[0057] Test specimens measuring 5 mm thick, 15 mm wide, and 115 mm long were taken from the quarter-thickness position of test steels A to E. The taken test specimens were ultrasonically degreased in acetone for 5 minutes, and then subjected to four-point bending to apply an external stress equal to the yield strength of each test specimen (100% YS). The four-point bent test specimens were immersed in a liquid ammonia tank and removed after one year. After removing the corrosion products from the surface of the removed test specimens, the surface and cross section of the test specimens were visually inspected for cracks and evaluated for their presence or absence.

[0058] Those in which cracks occurred were judged as having SCC occurrence ("Good"), and those in which cracks did not occur were judged as having no SCC ("Poor"). The judgment results are also shown in Table 1 as the SCC occurrence status in actual liquid ammonia tanks.

[0059] [Table 1]

[0060] As can be seen from Table 1, in the actual liquid ammonia tank, SCC did not occur in steels A and B. On the other hand, SCC occurred in steels C to E.

[0061] [Ammonia SCC accelerated test] The ammonia SCC susceptibility of the steel specimens A to E, which had been examined for ammonia SCC occurrence in an actual tank environment as described above, was evaluated by carrying out the ammonia SCC accelerated test of the present invention.

[0062] Table 2 shows the test conditions for the ammonia SCC accelerated test. Test specimens measuring 5 mm thick x 15 mm wide x 115 mm long were taken from the 1 / 4 position of the plate thickness of test steels A to E, and ultrasonically degreased in acetone for 5 minutes. An external stress (100% YS) equal to the yield strength of each specimen was applied by four-point bending. Ammonium carbamate and O2 were introduced into the test cell containing the four-point bending specimens in the amounts shown in Table 2, and then 2 L of liquid ammonia was filled. The specific liquid volume was 42 mL / cm. 2 The corrosion potential of the test specimen was then measured using a potentiostat. After performing cathodic polarization, the test was initiated by manipulating the potential to achieve a predetermined potential relative to the corrosion potential or platinum reference electrode potential after one hour. In Table 2, the results are labeled "immersion only" if no anodic polarization was performed. When the polarization value was based on the corrosion potential, the polarization value is listed with "(vs. corrosion potential)." When the polarization value was based on the platinum reference electrode potential, the polarization value is listed with "(vs. Pt reference electrode potential)." After 150 to 720 hours of immersion, the test specimens were removed from the test cell, and the corrosion products on the specimen surface were removed. The surfaces and cross sections were visually inspected for cracks to evaluate their presence or absence.

[0063] The immersion test used liquid ammonia with a purity of 99.999% or higher. Potential measurement and application using a potentiostat were performed using the three-electrode method, with platinum electrodes used as both the reference electrode and counter electrode. The test temperature was set at 25°C. Stirring during the test was performed continuously at 10 rpm using a stirrer placed inside the test cell.

[0064] The occurrence of SCC was assessed by conducting the immersion test 10 times for each test number, and if one or more cracks occurred throughout the entire test, it was assessed as "Yes" for SCC occurrence, and if no cracks occurred, it was assessed as "No" for SCC. For the results showing "Yes" for SCC occurrence, the probability of SCC occurrence was calculated. For the results showing "No" for SCC absence, the probability of occurrence was recorded as 0%.

[0065] For tests that showed a "no SCC" result ("x"), evaluation accuracy was judged to be "good" if the SCC occurrence was the same as the SCC occurrence in an actual liquid ammonia tank for the same steel type (shown in Table 1). For tests that showed a "good" result ("good"), evaluation accuracy was judged to be "good" if the SCC occurrence was the same as the SCC occurrence in an actual liquid ammonia tank for the same steel type (shown in Table 1) and the occurrence probability was 70% or higher. The occurrence probability is expressed as a percentage of the number of tests in which SCC occurrence was confirmed out of 10 immersion tests. Furthermore, for tests that were judged to have a "good" evaluation accuracy, tests with a test period of 720 hours or less were judged to have a "good" evaluation period. Tests other than those judged to have a "good" evaluation accuracy and evaluation period were judged to have a "poor" evaluation. Table 2 shows the SCC occurrence, occurrence probability, evaluation accuracy, and evaluation period as accelerated test results.

[0066] [Table 2] TIFF2025124588000003.tif255135

[0067] As can be seen from Table 2, in the examples of the present invention, the SCC susceptibility could be evaluated within 720 hours, which is the same as the environment simulating an actual liquid ammonia tank. On the other hand, in the comparative examples, the evaluation accuracy was "poor" or the evaluation period was "poor", and the ammonia SCC susceptibility could not be evaluated with good accuracy and in a short period of time. [Industrial Applicability]

[0068] The present invention provides an accelerated ammonia stress corrosion cracking test method that can evaluate the ammonia stress corrosion cracking susceptibility of metallic materials with high accuracy and in a short period of time. Furthermore, by using the test method of the present invention, it becomes possible to evaluate the ammonia stress corrosion cracking susceptibility of metallic materials with high accuracy and in a short period of time. The test method of the present invention is extremely useful for evaluating the ammonia stress corrosion cracking susceptibility of metallic materials, and can be advantageously used in the selection and development of metallic materials with excellent ammonia stress corrosion cracking resistance that can be suitably used for structures for transporting or storing liquid ammonia.

Claims

1. A metal test piece to which external stress has been applied or which has residual stress is subjected to a gas partial pressure of 0.002 to 2.000 bar containing 0.01 mass% or more ammonium carbamate. ２ and an ammonia stress corrosion cracking accelerated test method in which the specimen is immersed in liquid ammonia containing 0.05 to 2.00 mass % water.

2. 2. The ammonia stress corrosion cracking accelerated testing method according to claim 1, wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized.

3. 3. The method for accelerated ammonia stress corrosion cracking testing according to claim 1, wherein the liquid ammonia is stirred during the test.

4. A metal test piece to which external stress has been applied or which has residual stress is subjected to a gas partial pressure of 0.002 to 2.000 bar containing 0.01 mass% or more ammonium carbamate. ２ and an ammonia stress corrosion cracking accelerated testing method in which a specimen is immersed in liquid ammonia containing 0.05 to 2.00 mass % water and polarized to 0.0 V to +1.0 V relative to the corrosion potential or platinum reference electrode potential.

5. 5. The ammonia stress corrosion cracking accelerated testing method according to claim 4, wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized and then polarized to 0.0 V to +1.0 V relative to a corrosion potential or a platinum reference electrode potential.

6. 6. The method for accelerated ammonia stress corrosion cracking testing according to claim 4, wherein the liquid ammonia is stirred during the test.

Images