Ammonia stress corrosion cracking acceleration test method

Abstract

To provide an ammonia stress corrosion cracking acceleration test method capable of evaluating ammonia stress corrosion cracking sensitivity of a metal material with excellent accuracy in a short period.SOLUTION: In an ammonia stress corrosion cracking acceleration test method, a metal test piece to which external stress is added or which has residual stress is immersed in ammonium carbamate of 0.01 mass% or more and liquid ammonia containing O2 of 0.002 to 0.200 bar at a gas partial pressure, and is polarized to 0 to +1.0 V relative to corrosion potential.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a method for accelerating ammonia stress corrosion cracking of metal materials such as tanks used in a liquid ammonia environment. [Background technology]

[0002] In recent years, liquid ammonia has attracted attention as a clean energy source because it does not produce CO2 when burned, and large-scale demand is expected. Consequently, there is a need for larger facilities for transporting and storing liquid ammonia. Generally, when enlarging tanks, the use of thin-walled steel is preferred to reduce weight and construction costs, thus necessitating the use of high-strength steel.

[0003] On the other hand, in a liquid ammonia environment, carbon steel is susceptible to stress corrosion cracking (hereinafter referred to as ammonia SCC) caused by liquid ammonia. For this reason, for structures such as carbon steel pipes, storage tanks, tank cars, and line pipes that handle liquid ammonia, measures have been taken to use steel materials with low susceptibility to ammonia SCC and to implement operational measures to suppress ammonia SCC.

[0004] SCC is a phenomenon in which corrosion reactions and stresses combine to lead to fracture, and it occurs when material factors, environmental factors, and stress factors meet specific conditions. For example, ammonia SCC is known to correlate with the strength and hardness of the material. That is, it is known that the higher the strength and hardness, the more ammonia SCC is generated, and when using carbon steel, it is considered desirable to use materials with a tensile strength of less than 600 MPa.

[0005] Therefore, when applying new materials that combine high strength with excellent resistance to ammonia SCC (scaling and corrosion cancellation), it is necessary to accurately evaluate the ammonia SCC susceptibility. On the other hand, evaluating ammonia SCC susceptibility through exposure tests in actual liquid ammonia tanks requires long-term testing, so accelerated testing that can evaluate the ammonia SCC susceptibility of steel materials in a short period of time is desirable.

[0006] Patent Document 1 and Non-Patent Document 1 disclose accelerated testing using liquid ammonia, as described above. Patent Document 1 and Non-Patent Document 1 describe a test method for evaluating ammonia SCC sensitivity in a short period of time by accelerating the dissolution of iron by anodic polarization of a test steel piece in liquid ammonia containing O2 and saturated CO2. [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 Stress Corrosion Cracking Test Method in Liquid Ammonia," Iron and Steel, 1981, Vol. 67, No. 14, pp. 2226-2233. [Overview of the project] [Problems that the invention aims to solve]

[0009] In the test methods described in Patent Document 1 and Non-Patent Document 1 above, the test specimens are anodically polarized with +2V vs Pt. However, excessive anodic polarization leads to predominantly general corrosion, and pitting corrosion (localized corrosion), which is the starting point for ammonia SCC, does not occur sufficiently. Therefore, it is difficult to say that the susceptibility to ammonia SCC can be accurately evaluated. Furthermore, because the occurrence of pitting corrosion (localized corrosion) is not promoted, long-term testing is required for materials that do not have high susceptibility to ammonia SCC.

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

[0011] To solve the above problems, the inventors of the present invention conducted a detailed study on the ammonia-mediated SCC mechanism of steel sheets in liquid ammonia and obtained the following findings.

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

[0013] As a result of the corrosion reaction described above, an inert oxide film is formed on the surface of the steel plate. However, localized dissolution is accelerated in areas where the oxide film is unstable, leading to pitting corrosion. Since the resulting pitting corrosion becomes a stress concentration area, the film breaks down and the corrosion reaction progresses at the tip of the pitting corrosion, eventually leading to fracture. Therefore, we conceived of destabilizing the oxide film formed on the surface of the steel plate in order to promote the occurrence of pitting corrosion, which is the starting point for ammonia-mediated corrosion chain reaction (SCC).

[0014] We discovered that by maintaining a specific potential in liquid ammonia, the oxide film on the surface of the steel plate becomes unstable, allowing for rapid evaluation of ammonia-mediated scalable corrosion (SCC) sensitivity, thus completing the present invention.

[0015] This invention is based on the above findings, and the gist of this invention is as follows:

[0016] [1] A method for accelerating ammonia stress corrosion cracking test, characterized in that a metal test piece with external stress applied or having residual stress is immersed in liquid ammonia containing ammonium carbamate of 0.01 mass% or more and O2 with a gas partial pressure of 0.002 to 0.200 bar, and polarized to 0 to +1.0 V with respect to the corrosion potential.

[0017] [2] The method for accelerating ammonia stress corrosion cracking test according to [1], wherein the surface of the metal test piece immersed in the liquid ammonia is polarized cathodically and then polarized to 0 to +1.0 V with respect to the corrosion potential.

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

[0019] [4] A method for accelerating ammonia stress corrosion cracking test, characterized in that a metal test piece with external stress applied or having residual stress is immersed in liquid ammonia containing ammonium carbamate of 0.05 to 0.50 mass% and O2 with a gas partial pressure of 0.002 to 0.200 bar.

[0020] [5] The method for accelerating ammonia stress corrosion cracking test according to [4], wherein the surface of the metal test piece immersed in the liquid ammonia is polarized cathodically.

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

[0022] According to the method for accelerating ammonia stress corrosion cracking test of the present invention, it becomes possible to evaluate the ammonia SCC susceptibility of metal materials applied to tanks etc. used in a liquid ammonia environment with good accuracy and in a short period. [Embodiments for Carrying out the Invention]

[0023] The following describes in detail embodiments for carrying out the present invention (hereinafter simply referred to as "this embodiment"). The present invention is not limited to the following embodiments, and can be implemented in various ways within the scope of its gist. In the embodiments of the present invention, A(numerical value) to B(numerical value) means A or greater and B or less.

[0024] [Polarizes to 0 to +1.0V relative to the corrosion potential] In one embodiment of the test method of the present invention, a metal test piece subjected to external stress or possessing residual stress is immersed in liquid ammonia containing 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar, thereby polarizing it to 0 to +1.0V relative to its corrosion potential.

[0025] Polarizing the metal specimen to 0 to +1.0V relative to its corrosion potential destabilizes the oxide film formed on the specimen surface by immersion in liquid ammonia. On the surface of a metal specimen subjected to external stress or possessing residual stress, localized corrosion and pitting corrosion are accelerated, allowing for rapid evaluation of ammonia SCC susceptibility. On the other hand, polarizing the metal specimen to below 0V relative to its corrosion potential (cathode polarization) accelerates the cathode reaction on the specimen surface, making it difficult to evaluate the ammonia SCC susceptibility of the metal specimen. Furthermore, polarizing the metal specimen above +1.0V relative to its corrosion potential accelerates general corrosion on the specimen surface compared to localized corrosion, which is the starting point for ammonia SCC, potentially making it impossible to evaluate ammonia SCC susceptibility in a short period. Moreover, if general corrosion becomes dominant, localized corrosion, which is the starting point for SCC, may not occur, potentially making it impossible to evaluate ammonia SCC susceptibility itself. Therefore, polarization to 0 to +1.0V relative to the corrosion potential is specified. The potential applied to the corrosion potential is preferably +0.2V or higher, more preferably +0.4V or higher, and more preferably +0.8V or lower, and more preferably +0.6V or lower.

[0026] The corrosion potential is defined as the potential measured by immersing a metal test specimen, either one used to evaluate ammonia SCC susceptibility or one taken from the same material as the metal material being evaluated, in liquid ammonia for at least 5 minutes under external stress or residual stress. The method of measuring the potential is not particularly limited, but it can be measured using a two-electrode method or a three-electrode method.

[0027] Polarization (application of potential) and corrosion potential of metal test specimens can be measured, for example, using a potentiostat (potential-constant electrolytic device) with the metal test specimen as the working electrode (sample electrode). In this case, it is preferable to use platinum (Pt) electrodes, which are stable in liquid ammonia, as the counter electrode and reference electrode.

[0028] <Test Solution> In the test method of the present invention, liquid ammonia containing 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar is used as the test solution.

[0029] [Ammonium carbamate content: 0.01 mass% or more] Liquid ammonia typically contains trace amounts of CO2 as an impurity, and in liquid ammonia, CO2 dissociates as shown in the following reaction equation to reach equilibrium. 2NH3 + CO2 ⇔ NH4 CO2 NH2 ⇔ NH4 + +NH2CO2 -

[0030] Liquid ammonia without impurities has low electrical conductivity, but when ammonium carbamate (NH4CO2NH2) is present, the dissociation reaction increases the electrical conductivity of the liquid ammonia, accelerating the corrosion reaction. If the ammonium carbamate content in the liquid ammonia is less than 0.01 mass%, this effect cannot be obtained, and it is difficult to accelerate ammonia SCC by polarization of the metal test piece. Therefore, the ammonium carbamate content in the liquid ammonia is specified to be 0.01 mass% or more. Preferably it is 0.03 mass% or more, more preferably 0.10 mass% or more, and even more preferably 0.30 mass% or more. The upper limit of the ammonium carbamate content in the liquid ammonia is not particularly limited, and it may be included 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.

[0031] [Ammonium carbamate content: 0.05~0.50 mass%] In another embodiment of the test method of the present invention, a metal test specimen subjected to external stress or possessing residual stress is simply immersed in liquid ammonia containing 0.05 to 0.50 mass% ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar, and the potential applied to the metal test specimen can be set to 0V, that is, polarization is not required.

[0032] Ammonium carbamate is released as a carbamate ion (CO2NH2) through the aforementioned dissociation reaction. -) is formed. The carbamate ions have the effect of destroying the inert oxide film on the surface of the metal test piece, so by optimizing the content of carbamate ions, the oxide film can be destabilized without causing anodic polarization, and ammonia SCC can be promoted. If the ammonium carbamate content in liquid ammonia is less than 0.05 mass%, the rate of oxide film regeneration by the corrosion reaction is significantly larger than the rate of oxide film destruction by carbamate ions, so this effect cannot be obtained. On the other hand, if the ammonium carbamate content in liquid ammonia is greater than 0.50 mass%, the rate of oxide film destruction by carbamate ions is large, and the corrosion form becomes closer to general corrosion rather than pitting corrosion (local corrosion), so this effect cannot be obtained. For this reason, when anodic polarization is not performed, the ammonium carbamate content in liquid ammonia is specified to be between 0.05 and 0.50 mass%. Preferably, it is 0.10 mass% or more, more preferably 0.15 mass% or more, even more preferably 0.20 mass% or more, also preferably 0.45 mass% or less, more preferably 0.40 mass% or less, and even more preferably 0.35 mass% or less.

[0033] Furthermore, embodiments in which the ammonium carbamate content in liquid ammonia is 0.05 to 0.50 mass% and the corrosion potential is polarized to above 0V to +1.0V or less are also included within the scope of the test method of the present invention.

[0034] 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 container before introducing the liquid ammonia. Furthermore, the placement of ammonium carbamate can be replaced by blowing in an amount of CO2 gas or solid CO2 (dry ice) that yields the predetermined amount of ammonium carbamate content.

[0035] [O2 content: gas partial pressure 0.002~0.200 bar] O2 has the effect of forming an oxide film in liquid ammonia, and therefore the presence of O2 is essential in evaluating the susceptibility of ammonia to SCC (Scaling Critical Carcinomatosis). If the O2 content in liquid ammonia is less than 0.002 bar in gas partial pressure, this effect is not obtained, and it becomes difficult to evaluate ammonia SCC. On the other hand, if the O2 content in liquid ammonia is greater than 0.200 bar in gas partial pressure, the oxide film formation rate increases significantly, the oxide film stabilizes, and pitting corrosion does not occur, again making it difficult to evaluate ammonia SCC. For this reason, the O2 content in liquid ammonia is specified to be between 0.002 and 0.200 bar in gas partial pressure. Preferably, it is 0.005 bar or more, more preferably 0.020 bar or more, and also preferably 0.175 bar or less, and more preferably 0.150 bar or less.

[0036] The method for adding O2 to liquid ammonia is not particularly limited, but from the viewpoint of stably supplying O2, it is preferable to blow a predetermined amount of O2 gas into the test container before introducing the liquid ammonia. The blowing of O2 gas can be replaced by blowing in an amount of air gas that can obtain the predetermined partial pressure of O2 gas.

[0037] [Liquid ammonia] While there are no particular restrictions on the purity of the liquid ammonia used in the test solution, the presence of H2O and oil tends to suppress stress corrosion cracking in metal test specimens. Therefore, it is preferable that the amount of H2O and oil contained as impurities in the liquid ammonia be less than 0.05 mass% each.

[0038] [Cathode polarization] A passive film may form on the surface of a metal test specimen before immersion in liquid ammonia. By performing cathode polarization immediately after immersion in liquid ammonia, the passive film on the surface of the metal test specimen can be removed, homogenizing the initial surface condition and enhancing the effect of the subsequent anode polarization, enabling more accurate evaluation of ammonia SCC susceptibility. The conditions for cathode polarization are not particularly limited, but it is preferable to perform it at a voltage between -1.0V and -3.0V relative to the corrosion potential for 5 to 30 minutes.

[0039] [Agitate liquid ammonia during the test] The corrosion reaction is affected by the solution composition of liquid ammonia on the surface of the metal test specimen. By stirring the liquid ammonia containing ammonium carbamate and O2, which is the test solution, the solution composition on the surface of the immersed metal test specimen can be homogenized, enabling a more accurate evaluation of ammonia SCC sensitivity. Furthermore, stirring promotes ion supply to the surface of the metal test specimen, thus shortening the time required for ammonia SCC sensitivity evaluation. For this reason, it is preferable to stir the liquid ammonia containing ammonium carbamate and O2 during the test. Since ammonia SCC sensitivity is affected by nitrogen, carbon dioxide, oxygen, etc., it is preferable to perform stirring using a stirring bar. To stabilize the ion supply to the surface of the metal test specimen, it is preferable to perform stirring continuously at 10 rpm or more.

[0040] <Metal test piece> The present invention's test method, which promotes ammonia SCC by applying an electric potential, can be applied to conductive metallic materials. The metallic material is not particularly limited in terms of its component composition or microstructure. Specific examples of the metallic material include those containing metallic elements (e.g., Fe, Cu, Al, Ni, Ti, etc.) as the main component (i.e., 50 mass% or more), such as steel (iron alloys), copper alloys, aluminum alloys, nickel alloys, and titanium alloys. In particular, it is preferable to apply the method to steel materials, as they are commonly used in structures exposed to liquid ammonia environments and resistance to ammonia SCC is important.

[0041] When using steel as a metal test specimen, the carbon equivalent and hardness of the steel are not particularly limited. Generally, it is known that steel with a higher carbon equivalent and hardness is more susceptible to ammonia SCC, and it is preferable that the steel test specimen has a carbon equivalent of 0.05% or more and a Vickers hardness of 100 Hv or more. The metal test piece used in the test method of the present invention may be taken from the metal material whose ammonia SCC sensitivity is to be evaluated, or may be taken from a metal material having the same or similar component composition and metal structure as the metal material.

[0042] The shape and size of the metal test piece may be appropriately determined according to the test cell used and the external stress application method, or may be selected from known standard specifications, and are not particularly limited. However, in order to reduce the influence on the solution composition due to the corrosion reaction, the liquid ammonia solution volume (mL) per exposed area (cm 2 ) of the metal test piece, i.e., the liquid ratio, is preferably 5 mL / cm 2 or more. The upper limit of the liquid ratio is not particularly limited. However, if the liquid ratio is made excessively large, the cost of test equipment and the like will increase. Therefore, it is preferably 500 mL / cm 2 or less.

[0043] [External stress / Residual stress] Since the test method of the present invention is an ammonia stress corrosion cracking acceleration test, a metal test piece under an externally applied stress or having a residual stress is immersed in a test solution. The method of applying the external stress or residual stress is not particularly limited. For example, the external stress can be applied by known methods such as four-point bending, U-bending, and constant load method, and the residual stress can be applied by performing heat treatment, welding, processing, etc.

[0044] The magnitude of the externally applied stress or residual stress to be applied may be appropriately set according to the test purpose (for example, the intended use of the material to be evaluated), etc. However, it is preferable to load 20% or more of the yield strength YS (MPa) of each metal test piece. Since ammonia SCC can be promoted as the externally applied stress or residual stress to be applied increases, the upper limit is not particularly limited as long as the metal test piece itself is not damaged such as cracked.

[0045] [Other test conditions] The test temperature (temperature of the test solution) used in the test method of the present invention can be appropriately selected according to the purpose of the test, but it is preferable to set it to 0 to 60°C, as low temperatures tend to suppress the corrosion reaction and prolong the test period. Furthermore, in order to further improve the accuracy of the test, it is preferable to keep the error between the set test temperature and the actual test temperature within ±5°C.

[0046] The test period for the test method of the present invention is not particularly limited and can be set appropriately according to the intended use of the metal material to be evaluated. However, for the application of new materials, a shorter test period is preferable, and it is preferable that it be within 720 hours. Since ammonia SCC testing in a real tank environment (actual liquid ammonia) generally requires a test period of about one year, the test period can be shortened to about 1 / 10 or less by using the test method of the present invention.

[0047] Other test conditions are not particularly limited and any known conditions used in corrosion testing can be appropriately selected and used. Test apparatus and test cells can also be those known in the art, such as those described in Patent Document 1 or Non-Patent Document 1. [Examples]

[0048] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to the following examples.

[0049] [Ammonia SCC generation status under actual tank conditions] As a preliminary investigation to confirm the effectiveness of the test method of the present invention, seven types of steel materials with different component compositions (steel grades A to G) were used as test materials, and ammonia SCC tests were performed in an actual tank environment with actual liquid ammonia to evaluate the ammonia SCC sensitivity.

[0050] Table 1 shows the carbon equivalent (CE), yield strength (YS), and Vickers hardness (HV0.5) of steel materials A to G subjected to ammonia SCC testing.

[0051] The carbon equivalent (CE) was calculated from the component composition of the tested 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 tested steel material.)

[0052] Yield strength and Vickers hardness were measured at a position 1 / 4 of the plate thickness, in accordance with JIS Z 2241 and JIS Z 2244. Vickers hardness was measured at 20 points using a Vickers test with a load of 500g, and the average value was used.

[0053] Test specimens measuring 5 mm thick × 15 mm wide × 115 mm long were taken from the 1 / 4 position of the plate thickness of each of the test steel materials A to G. The collected test specimens were ultrasonically degreased in acetone for 5 minutes, and then an external stress (100% YS) equal to the yield strength of each specimen was applied by four-point bending. These four-point bent test specimens were immersed in a tank of liquid ammonia 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 specimens were visually inspected for cracks, and the presence or absence of cracks was evaluated.

[0054] We determined that SCC occurred when cracking occurred ("○"), pitting corrosion occurred when cracking did not occur ("△"), and no SCC occurred when neither cracking nor pitting corrosion occurred ("×"). The results of this determination are shown in Table 1 as the SCC occurrence status in an actual liquid ammonia tank.

[0055] [Table 1]

[0056] As can be seen from Table 1, in an actual liquid ammonia tank, steel A and steel B did not develop SCC or pitting corrosion, which is the initiation point of SCC. Steel C did not develop SCC, but pitting corrosion did occur. Steels D to G developed SCC.

[0057] [Ammonia SCC Accelerated Test] As described above, the ammonia SCC acceleration test of the present invention was performed on test steel materials A to G, whose ammonia SCC generation status under actual tank conditions was investigated, and their susceptibility to ammonia SCC was evaluated.

[0058] Table 2 shows the test conditions for the ammonia SCC acceleration test. Test specimens measuring 5 mm thick × 15 mm wide × 115 mm long, taken from the 1 / 4 position of the plate thickness of steel materials A to G, were ultrasonically degreased in acetone for 5 minutes. An external stress (100% YS) equal to the yield strength of each specimen was then applied by four-point bending. The test cell containing these four-point bent specimens was then filled with ammonium carbamate and O2 in the amounts specified in Table 2, followed by 2 L of liquid ammonia. The specific liquid volume was 42 mL / cm³. 2 Subsequently, the corrosion potential of the test specimen was measured using a potentiostat, and the test was started by controlling the potential so that a predetermined potential was applied to the corrosion potential after 1 hour. After 168 to 2160 hours of immersion, the corrosion products on the surface of the test specimen were removed, and the presence or absence of cracks was evaluated by visual inspection of the surface and cross-section.

[0059] In this immersion test, liquid ammonia with a purity of 99.999% or higher was used. Potential measurement and potential application were performed using a three-electrode method with a potentiostat, and platinum electrodes were used as both the reference electrode and the counter electrode. The test temperature was set to 25°C. During the test, stirring was performed continuously at 10 rpm using a stirring bar installed in the test cell.

[0060] The occurrence of SCCs was determined by performing the immersion test 10 times for each test number. If one or more cracks occurred throughout the entire test, it was judged as SCC occurrence ("○"); if no cracks occurred but one or more pitting corrosion occurred, it was judged as pitting corrosion occurrence ("△"); and if neither SCC nor pitting corrosion occurred, it was judged as no SCC ("×"). For those showing results of SCC occurrence ("○") or pitting corrosion occurrence ("△"), the probability of SCC or pitting corrosion occurring was calculated.

[0061] For samples marked "×" where no SCC or pitting corrosion occurred, the evaluation accuracy was judged as "good" if the SCC occurrence status in the actual liquid ammonia tank shown in Table 1 was the same "×" for the same steel type. For samples marked "〇" or "△" where SCC or pitting corrosion occurred, the evaluation accuracy was judged as "good" if the SCC occurrence status in the actual liquid ammonia tank shown in Table 1 was the same ("〇" or "△") for the same steel type, and the probability of occurrence was 70% or higher. The probability of occurrence is expressed as a percentage of the number of tests in which the occurrence was confirmed out of 10 immersion tests. In addition, for tests judged as having good evaluation accuracy, the evaluation period was judged as "good" if the test period was 720 hours or less. Anything other than those judged as "good" for evaluation accuracy and evaluation period was judged as "poor". Table 2 also shows the SCC occurrence status, occurrence probability, evaluation accuracy, and evaluation period as accelerated test results.

[0062] [Table 2]

[0063] As can be seen from Table 2, in the present invention example, the same SCC susceptibility as in an environment simulating an actual liquid ammonia tank can be evaluated within 720 hours. On the other hand, in the comparative example, SCC did not occur in the steel material in an environment simulating an actual liquid ammonia tank, and therefore the SCC susceptibility could not be evaluated. [Industrial applicability]

[0064] 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 good accuracy and in a short period of time. Furthermore, by using the test method of the present invention, it is 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 very 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 resistance to ammonia stress corrosion cracking that are suitable for use in structures for transporting or storing liquid ammonia.

Claims

1. A metal test specimen subjected to external stress or with residual stress is subjected to 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar. ２ A method for accelerating ammonia stress corrosion cracking, characterized by immersion in liquid ammonia containing a substance and polarization to 0 to +1.0 V relative to the corrosion potential.

2. The ammonia stress corrosion cracking acceleration test method according to claim 1, wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized and then polarized to 0 to +1.0 V with respect to the corrosion potential.

3. The ammonia stress corrosion cracking acceleration test method according to claim 1 or 2, wherein the liquid ammonia is stirred during the test.

4. Metal test specimens subjected to external stress or with residual stress are subjected to 0.05-0.50 mass% ammonium carbamate and O2 at a gas partial pressure of 0.002-0.200 bar. ２ A method for accelerating ammonia stress corrosion cracking, characterized by immersion in liquid ammonia containing a substance and without anodic polarization.

5. The ammonia stress corrosion cracking acceleration test method according to claim 4, wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized.

6. The ammonia stress corrosion cracking acceleration test method according to claim 4 or 5, wherein the liquid ammonia is stirred during the test.

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