Ammonia stress corrosion cracking acceleration test method

Abstract

To provide an accelerated ammonia stress corrosion cracking (ACC) testing method that can evaluate the susceptibility of metallic materials to Ammonia SCC in a short period of time. [Solution] The ammonia stress corrosion cracking acceleration test method of the present invention involves immersing a metal test piece in liquid ammonia containing 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.002 to 2,000 bar, and anodic polarization is performed either without applying a potential or in a potential range of greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential, for a total of 5 × 10⁻¹⁰ -8 / s or more 1×10 -4 It is characterized by being pulled at a strain rate of less than / s.

Description

[Technical Field]

[0001] This invention relates to a method for accelerating ammonia stress corrosion cracking in metal materials 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, high-strength steel materials such as carbon steel are 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 (Steel Chloride Crushing) 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; specifically, the higher the strength and hardness, the more ammonia SCC is generated. Therefore, when using carbon steel in a liquid ammonia environment, it is considered desirable to select steel with a tensile strength of less than 600 MPa.

[0005] Therefore, in order to apply new steel materials that achieve both high strength and excellent resistance to ammonia SCC (scalding 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] As such liquid ammonia SCC acceleration tests, methods disclosed in Patent Documents 1 and 2 and Non-Patent Document 1 have been proposed to date.

[0007] For example, 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 specimen in liquid ammonia containing O2 and saturated CO2. Patent Document 2 describes a test method for evaluating ammonia SCC sensitivity in a short period of time by anodic polarization of a test specimen in liquid ammonia containing O2 and a large amount of saturated ammonium carbamate CO2. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Special Publication No. 60-010575 [Patent Document 2] Patent No. 7401838 [Non-patent literature]

[0009] [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 Initiative] [Problems that the invention aims to solve]

[0010] In the test methods described in Patent Document 1 and Non-Patent Document 1 mentioned above, there is room for improvement in terms of accuracy and test period with respect to the evaluation results in an actual tank environment. In the test method described in Patent Document 2, it is necessary to prepare a reference test piece for comparison in advance, for which SCC has been confirmed to occur, and there is a problem in that it cannot be tested with only the evaluation test piece.

[0011] An object of the present invention is to solve the above problems and provide an ammonia SCC acceleration test method capable of evaluating the ammonia SCC susceptibility of a metal material used for tanks for transporting and storing liquid ammonia in a short period of time.

Means for Solving the Problems

[0012] In order to solve the above problems, the present inventors have carefully studied the ammonia SCC mechanism of a steel plate in liquid ammonia and obtained the following findings.

[0013] In a liquid ammonia environment, the following corrosion reactions occur on the steel plate. Anode reaction: Fe → Fe 2+ + 2e - (Equation 1) Cathode reaction: O2 + 2NH4 + + 4e - → 2OH - + 2NH3 (Equation 2)

[0014] When tensile stress is applied to a steel plate in liquid ammonia, cracks progress due to so-called APC (Active Path Corrosion). In APC, slip steps are formed by the tensile stress, resulting in the breakdown of the oxide film on the steel plate surface, and the preferential corrosion of the newly formed surface generated accordingly is repeated. When the application of tensile stress is carried out by a constant load test, a constant strain test or a two-hole constrained cracking test as described in the above prior art documents, stress and strain may become insufficient over time, and the formation rate of slip steps may decrease, leading to the possibility of APC stagnation. Therefore, the inventors considered that by appropriately balancing the corrosion rate of the newly formed surface, the repassivation rate, and the formation rate of slip steps, the corrosion reaction can be localized at the crack tip and the crack can continue to progress. In this way, in order to promote ammonia SCC, it was recalled to control the corrosion rate of the newly formed surface, the repassivation rate, and the formation rate of slip steps.

[0015] And it was found that by holding at a specific potential in liquid ammonia containing a predetermined component, the corrosion rate and the repassivation rate of the newly formed surface can be controlled, and furthermore, by controlling the strain rate within a specific range, the formation rate of slip steps can be controlled, thus leading to the completion of the present invention.

[0016] The present invention is based on the above findings, that is, the gist of the present invention is as follows.

[0017] [1] A method for promoting ammonia stress corrosion cracking test, in which a metal test piece 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 2.000 bar, and is anodically polarized in a potential region of more than 0 V to +3.0 V with respect to the corrosion potential or the platinum reference electrode potential without applying a potential, and is pulled at a strain rate of 5×10 -8 / s or more and 1×10 -4 / s or less.

[0018] [2] The method for promoting ammonia stress corrosion cracking test according to [1], wherein the liquid ammonia contains 0.05 to 2.00 mass% of water.

[0019] [3] The ammonia stress corrosion cracking acceleration test method according to [1] or [2], wherein the liquid ammonia is stirred during the test.

[0020] [4] The ammonia stress corrosion cracking acceleration test method according to [1] or [2], wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized, and then no potential is applied, or it is polarized to greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential.

[0021] [5] The ammonia stress corrosion cracking acceleration test method according to [3], wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized, and then no potential is applied, or it is polarized to greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential. [Effects of the Invention]

[0022] According to the ammonia stress corrosion cracking acceleration test method of the present invention, it becomes possible to evaluate the ammonia SCC susceptibility of metal materials applied to tanks and other devices used in ammonia environments with good accuracy and in a short period of time. [Modes 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.

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

[0025] <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 2.000 bar, and optionally 0.05 to 2.00 mass% of water, is used as the test solution.

[0026] [Ammonium carbamate content: 0.01 mass% or more] Normally, CO2 is present as an impurity in liquid ammonia, and in liquid ammonia, CO2 dissociates and reaches an equilibrium state as shown in the following reaction formula. 2NH3 + CO2 ⇔ NH4CO2NH2 ⇔ NH4 + + CO2NH2 - (Equation 3)

[0027] Although the electrical conductivity of liquid ammonia that does not contain impurities is small, when ammonium carbamate (NH4CO2NH2) is contained, the electrical conductivity of liquid ammonia increases due to the dissociation reaction, so the corrosion rate on the fresh surface increases, and at the same time, it becomes possible to control the corrosion rate and the re-passivation rate of the fresh surface due to polarization. Also, ammonium carbamate forms carbamate ions (CO2NH2 - ) by the dissociation reaction. Since the carbamate ions have the effect of breaking the inert oxide film on the surface of the metal test piece, it is possible to reduce the re-passivation rate.

[0028] When the ammonium carbamate content in liquid ammonia is less than 0.01 mass%, such an effect cannot be obtained, and it is difficult to increase the corrosion rate on the fresh surface, reduce the re-passivation rate, and promote ammonia SCC by polarizing the metal test piece. Therefore, the ammonium carbamate content in liquid ammonia is specified to be 0.01 mass% or more. The ammonium carbamate content in liquid ammonia is preferably 0.03 mass% or more, more preferably 0.10 mass% or more, and still more preferably 0.30 mass% or more. The upper limit of the ammonium carbamate content in liquid ammonia is not particularly limited, 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.

[0029] 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.

[0030] [O2 content: gas partial pressure 0.002~2.000 bar] O2 has the effect of increasing the repassivation rate on the newly formed surface of a metal test specimen in liquid ammonia. 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 promote ammonia SCC. On the other hand, if the O2 content in liquid ammonia exceeds 2.000 bar in gas partial pressure, the repassivation rate becomes significantly larger, and it becomes difficult to promote ammonia SCC. Therefore, the O2 content in liquid ammonia is specified to be between 0.002 and 2.000 bar in gas partial pressure. The O2 content in liquid ammonia is preferably 0.005 bar or more, more preferably 0.200 bar or more, and also preferably 1.500 bar or less, and more preferably 1.250 bar or less.

[0031] 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.

[0032] [Water content: 0.05~2.00mass%] Water has the effect of increasing the repassivation rate on the newly formed surface of a metal test specimen in liquid ammonia. This effect cannot be obtained if the water content in the liquid ammonia is less than 0.05 mass%. On the other hand, if the water content in the liquid ammonia exceeds 2.00 mass% the repassivation rate becomes significantly larger, and it may become difficult to promote ammonia SCC. For this reason, it is preferable to have a water content in the liquid ammonia of 0.05 to 2.00 mass%. The water content is more preferably 0.06 mass% or more, and even more preferably 0.07 mass% or more. Furthermore, the water content is more preferably 1.75 mass% or less, and even more preferably 1.50 mass% or less.

[0033] 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 to use pure water or ultrapure water with an impurity content of 1000 ppb or less. The method of adding water to liquid ammonia is not particularly limited, but from the viewpoint of ensuring a stable water supply, it is preferable to introduce a predetermined amount of water into the test container before introducing the liquid ammonia.

[0034] [Liquid ammonia] While there are no particular restrictions on the purity of the liquid ammonia used in the test solution, the presence of oil tends to suppress stress corrosion cracking in metal test specimens. Therefore, it is preferable that the amount of oil contained as an impurity in the liquid ammonia be less than 0.05 mass%.

[0035] [Without applying a potential, or polarized to more than 0V to +3.0V 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 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 2,000 bar, and anodic polarization is performed without applying a potential or in a potential region of greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential, and 5 × 10 -8 / s or more 1×10 -4Pull at a strain rate of less than / s. If polarization is performed, it should be carried out continuously from the start to the end of the test. If cathode polarization, as described below, is performed at the start of the test, after performing cathode polarization for a predetermined time, either stop applying the potential or polarize to a potential range of greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential and continue in that state until the end of the test.

[0036] The test method of the present invention can be performed without anodic polarization of the metal specimen, but anodic polarization with respect to the corrosion potential or platinum reference electrode potential is preferable. By polarizing the metal specimen to above 0V to +3.0V with respect to the corrosion potential or platinum reference electrode potential, the corrosion rate and repassivation rate of the newly formed surface due to tensile stress can be controlled and balanced with the formation rate of the sliding step. As a result, the corrosion reaction is localized at the crack tip, crack propagation is promoted, and ammonia SCC susceptibility can be evaluated in a shorter period of time. On the other hand, if the metal specimen continues to be polarized to below 0V even after arbitrary cathode polarization as described later, the cathode reaction is promoted on the surface of the metal specimen, and the anodic reaction is significantly suppressed. This makes it difficult to control the corrosion rate and passivation rate of the newly formed surface, making it difficult to propagate cracks based on APC, and thus difficult to evaluate ammonia SCC susceptibility. Furthermore, if the metal specimen is anodically polarized beyond +3.0V, the corrosion rate of the newly formed surface is significantly accelerated, which does not balance with the repassivation rate and the formation rate of the sliding step. As a result, crack propagation based on APC becomes difficult, and evaluation of ammonia SCC susceptibility becomes difficult. Moreover, the reduction of NH3 generates an excess of nitrogen, and the surface of the metal specimen is covered with nitrogen bubbles, preventing the anodic reaction from occurring in that area, which may result in the inability to evaluate ammonia SCC susceptibility. For this reason, it is preferable to polarize the specimen to a range of 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential. The lower limit of polarization relative to the corrosion potential or platinum reference electrode potential is preferably +0.2V or higher, more preferably +0.4V or higher, and even more preferably +0.6V or higher. The upper limit of polarization relative to the corrosion potential or platinum reference electrode potential is preferably +2.5V or lower, more preferably +2.0V or lower, and even more preferably +1.0V or lower.

[0037] The corrosion potential is the potential of the working electrode (sample electrode), which is used as the working electrode (sample electrode) to evaluate the ammonia SCC susceptibility. The working electrode is immersed in liquid ammonia with a non-contact reference electrode for at least 5 minutes, and the potential of the reference electrode is measured with the reference potential set to 0 V. The method for measuring the potential is not particularly limited and can be measured, for example, by the two-electrode method or the three-electrode method.

[0038] When polarization (application of potential) is performed using a platinum reference electrode as the reference, there is no particular limit to the time from immersion of the metal test specimen in liquid ammonia to the application of potential. In order to stabilize the surface condition of the metal test specimen, it is preferable to immerse the metal test specimen in liquid ammonia for 5 minutes or more before applying potential.

[0039] When performing anodic polarization on a metal test specimen in the potential range of 0V to +3.0V, either the corrosion potential or the platinum reference electrode potential may be used as the reference. However, if the platinum reference electrode potential is used as the reference, it is preferable to measure the corrosion potential beforehand. 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] Corrosion potential measurement and polarization (potential application) can be performed, for example, using a potentiostat (potential-constant electrolytic device) with the metal test piece under test 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.

[0041] [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 enabling more accurate evaluation of ammonia SCC susceptibility. In addition, since the metal surface is activated, the time required for ammonia SCC susceptibility evaluation can be shortened. 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.

[0042] [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, O2, and water, 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, O2, and water 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.

[0043] <Metal test piece> The metal test specimens used in the test method of the present invention may be taken from a metal material whose ammonia SCC sensitivity is to be evaluated, or from a metal material having the same or similar component composition and microstructure as said metal material. The component composition and microstructure of the said metal material are not particularly limited. Specific examples of said metal material include those containing metal 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.

[0044] 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 (CE) of 0.05% or more and a Vickers hardness of 100 HV or more.

[0045] The carbon equivalent (CE) can be calculated from the component composition of the steel material under test using the following formula 4. CE=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (Formula 4) (In Equation 4, the element symbols indicate the content (mass %) of each element in the tested steel material.)

[0046] The shape and size of the metal test specimen can be appropriately determined according to the test cell and tensile testing machine used, or selected from known standards, and are not particularly limited. However, the exposed area (cm²) of the metal test specimen should be such that the influence of the corrosion reaction on the solution composition is minimized. 2 The specific volume, which is the amount of liquid ammonia solution (mL) relative to the given volume, is 5 mL / cm³. 2 It is preferable to keep the above values. The upper limit of the relative liquid volume is not particularly limited, but if the relative liquid volume is made excessively large, the cost of test equipment, etc. will increase, so 500 mL / cm³ is preferable. 2 The following is preferable.

[0047] <Strain rate> When tensile stress is applied to a metal test specimen in liquid ammonia, the tensile stress causes slip steps to form on the specimen, leading to repeated destruction of the oxide film on the specimen surface and preferential corrosion of the newly formed surface. The present invention's test method is based on the understanding that by balancing the corrosion rate of the newly formed surface, the rate of repassivation, and the rate of slip step formation, it is possible to continuously localize the corrosion reaction at the crack tip and allow the crack to propagate. To appropriately control the rate of slip step formation due to tensile stress, the "strain rate" when applying tensile stress is controlled to a predetermined range throughout the entire test period. Here, "strain rate" refers to the increment of strain per unit time measured using the extensometer gauge length as specified in JIS Z 2241.

[0048] In the test method of the present invention, the strain rate of tensile stress is set to 5 × 10⁻⁶ -8 / s or more 1×10 -4 Control the strain rate to less than / s. -8 If the strain rate is less than 1 / s, the rate of slip step formation becomes significantly slower, and the balance with the corrosion rate of the newly formed surface and the rate of repassivation is disrupted, making it difficult to evaluate SCC. Also, if the strain rate is less than 1 × 10⁻⁶ -4If the strain rate exceeds 5 × 10⁻¹⁰, the rate of slip step formation becomes significantly faster, disrupting the balance with the corrosion rate of the newly formed surface and the rate of repassivation, making SCC evaluation difficult. Therefore, the strain rate should be 5 × 10⁻¹⁰. -8 / s or more 1×10 -4 The strain rate is controlled to a range of less than or equal to / s. The strain rate is preferably 7 × 10 -8 / s or more, more comfortably 1 x 10 -7 / s or more, preferably 1 × 10 -5 / s or less, more preferably 1 × 10 -6 It is less than or equal to / s.

[0049] The means of pulling the metal test piece at the above strain rate is not particularly limited, but it is preferable to use a tensile testing machine. Furthermore, the above strain rate should be kept at the same value throughout the entire test period, but it may be varied within the above range.

[0050] <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 since corrosion reactions tend to be suppressed at low temperatures and the test period tends to be prolonged, it is preferable to set the temperature to -35 to 60°C. 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. [Examples]

[0051] 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.

[0052] [Ammonia SCC generation status under actual tank conditions] As a preliminary investigation to confirm the effectiveness of the test method of the present invention, five types of steel materials with different component compositions (steel grades A to E) were used as test materials, and ammonia SCC tests were performed under actual tank conditions using liquid ammonia to evaluate ammonia SCC sensitivity.

[0053] Table 1 shows the carbon equivalent (CE), yield strength (YS), and Vickers hardness (HV0.5) of the steel subjected to the ammonia SCC test. The carbon equivalent (CE) was calculated from the component composition of the tested steel material using the following formula 4. CE=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (Formula 4) (In Equation 4, the element symbols indicate the content (mass %) of each element in the tested steel material.)

[0054] 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.

[0055] Test specimens measuring 5 mm thick × 15 mm wide × 115 mm long were taken from the 1 / 4 position of the plate thickness of steel materials A to E. The collected specimens were ultrasonically degreased in acetone for 5 minutes, and then subjected to an external stress (100% YS) equal to the yield strength of each specimen by four-point bending. These four-point bent specimens were immersed in a tank of liquid ammonia and removed after one year. After removing corrosion products from the surface of the removed specimens, the presence or absence of cracks was evaluated by visual inspection of the surface and cross-section of the specimens.

[0056] Cracks that occurred were marked as "○" indicating SCC occurrence, and those that did not occur were marked as "×" indicating no SCC. The results of this determination are shown in Table 1, along with the SCC occurrence status in actual liquid ammonia tanks.

[0057] [Table 1]

[0058] As can be seen from Table 1, no SCC occurred in A-steel and B-steel in an actual liquid ammonia tank. On the other hand, SCC occurred in C-E-steel.

[0059] [Ammonia SCC Accelerated Test] As described above, the ammonia SCC acceleration test of the present invention was performed on test steel materials A to E, whose ammonia SCC generation status under actual tank conditions was investigated, and their susceptibility to ammonia SCC was evaluated.

[0060] Table 2 shows the test conditions for the ammonia SCC accelerated test. Tensile test specimens, each with a gauge length of 25.4 mm, a parallel section width of 3.81 mm, and a notch with a radius of curvature of 0.1 mm at a 60-degree angle and a depth of 0.255 mm, were taken from the 1 / 4 position of the plate thickness of the test steel materials A to E. These specimens were ultrasonically degreased in acetone for 5 minutes. Ammonium carbamate, O2, and, in some tests, water were introduced into the test cell containing these specimens in the amounts specified in Table 2, and then 3 L of liquid ammonia was added. Subsequently, the corrosion potential of the specimens was measured using a potentiostat, and after arbitrary cathode polarization, the tensile test was started at a predetermined strain rate, either by not applying a potential or by controlling the application of a predetermined potential to the corrosion potential or platinum reference electrode potential after 1 hour. In Table 2, when the anodic polarization value is based on the corrosion potential, it is indicated as "(vs. corrosion potential)," and when the anodic polarization value is based on the platinum reference electrode potential, it is indicated as "(vs. Pt reference electrode potential)."

[0061] The RA ratio (%) was calculated by dividing the cross-sectional area reduction rate RA after the above test by the cross-sectional area reduction rate RA in air. The cross-sectional area reduction rate RA after the above test is the value expressed as a percentage by dividing the reduction in cross-sectional area before and after the test (cross-sectional area before the test - cross-sectional area after the test) by the cross-sectional area before the test. The cross-sectional area reduction rate RA in air was calculated by preparing a control specimen from the same test steel material used in the test, in the same way as the tensile test specimen described above, and conducting a tensile test in air without applying a potential at the same strain rate and tensile stress, and determining the cross-sectional area reduction rate after the tensile test as described above. When the susceptibility to ammonia stress corrosion cracking is high, cracks occur early and fracture occurs before necking progresses, so the cross-sectional area reduction rate RA after the ammonia SCC accelerated test becomes small, and the RA ratio, which is the ratio of the cross-sectional area reduction rate RA in air to the cross-sectional area reduction rate RA, becomes small. For this reason, an RA ratio of 80% or less was evaluated as indicating that SCC had occurred.

[0062] For tests showing an RA ratio of 80% or less, the evaluation accuracy was judged as "good" if the SCC occurrence status in an actual liquid ammonia tank for the same steel type was "×" as shown in Table 1. For tests showing an RA ratio exceeding 80%, the evaluation accuracy was judged as "good" if the SCC occurrence status in an actual liquid ammonia tank for the same steel type was "〇" as shown in Table 1. In addition, among those with "good" evaluation accuracy, those with a test period of less than 168 hours were judged as having a "good" evaluation period. For those judged as having "poor" evaluation accuracy, the evaluation period was omitted. Table 2 shows the RA ratio, evaluation accuracy, and evaluation period together as accelerated test results.

[0063] [Table 2]

[0064] As can be seen from Table 2, in the example of the present invention, the same SCC susceptibility as in an environment simulating an actual liquid ammonia tank can be evaluated within 161 hours. On the other hand, in the comparative example, the evaluation accuracy was "poor," and ammonia SCC susceptibility could not be evaluated in a short period of time. [Industrial applicability]

[0065] 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. The metal test piece was treated with 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.002 to 2.000 bar. ２ Immerse in liquid ammonia containing and anode polarization is performed either without applying a potential or in a potential range of greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential, 5 × 10 －８ / s or more 1×10 －４ An accelerated ammonia stress corrosion cracking test method that involves pulling at a strain rate of less than 0.5 / s.

2. The ammonia stress corrosion cracking acceleration test method according to claim 1, wherein the liquid ammonia contains 0.05 to 2.00 mass% of water.

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. The ammonia stress corrosion cracking acceleration test method according to claim 1 or 2, wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized, and then no potential is applied, or the surface is polarized to a value greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential.

5. The ammonia stress corrosion cracking acceleration test method according to claim 3, wherein the surface of the metal test piece immersed in the liquid ammonia is cathode-polarized, and then no potential is applied, or it is polarized to a value greater than 0V to +3.0V relative to the corrosion potential or platinum reference electrode potential.

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