Ammonia stress corrosion cracking acceleration test method

Abstract

This invention provides an accelerated ammonia stress corrosion cracking test method that can evaluate the susceptibility of metallic materials to ammonia stress corrosion cracking with good accuracy and in a short period of time. [Solution] An ammonia stress corrosion cracking acceleration test method characterized by immersing a metal test piece having a soft portion of 1 mm or more 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, while applying an external tensile load in a direction perpendicular to the soft portion and parallel to the surface of the metal test piece, thereby polarizing it to 0 to +1.0 V with respect to the corrosion potential.

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, in order to apply new materials that achieve both high strength and excellent resistance to ammonia SCC (scaling and corrosion cancellation), it is necessary to accurately evaluate 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 such liquid ammonia. 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 slab 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] However, in the test methods described in Patent Document 1 and Non-Patent Document 1 above, the test specimen is 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, making it difficult to say that the susceptibility to ammonia SCC can be accurately evaluated. Furthermore, since 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. Moreover, since many actual corrosion cases have been observed near welds, it is necessary to reproduce the conditions near welds.

[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 sheet. 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 sheet to promote the occurrence of pitting corrosion, which is the starting point for ammonia-mediated SCC. We then found that the oxide film on the surface of the steel sheet becomes unstable by maintaining it at a specific potential in liquid ammonia.

[0014] Furthermore, damage cases in welded joints have shown that crack formation is frequently observed in areas affected by the heat of welding. This is because deformation and corrosion during operation tend to concentrate crack opening and propagation in areas where hardness is inevitably reduced due to the heat of welding. Therefore, we discovered that the ammonia SCC susceptibility of welded joints can be evaluated in a short period of time by exposing the metal test piece to be evaluated to the aforementioned corrosive environment while reproducing a soft area, leading to the completion of the present invention.

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

[0016] [1] An ammonia stress corrosion cracking acceleration test method characterized by immersing a metal test specimen having a soft portion of 1 mm or more in a soft portion of 1 mm in length 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, while applying an external tensile load in a direction perpendicular to the longitudinal direction of the soft portion and parallel to the surface of the metal test specimen, thereby polarizing it to 0 to +1.0 V with respect to the corrosion potential.

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

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

[0019] [4] An ammonia stress corrosion cracking acceleration test method characterized in that, while applying a tensile load by an external force in a direction perpendicular to the longitudinal direction of the soft portion and parallel to the surface of the metal test piece having a soft portion with a length of 1 mm or more, it is immersed in liquid ammonia containing 0.05 to 0.50 mass% of ammonium carbamate and 0.002 to 0.200 bar of O2 in gas partial pressure.

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

[0021] [6] An ammonia stress corrosion cracking acceleration test method according to [4] or [5], wherein the liquid ammonia is stirred during the test.

Advantages 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 the like used in a liquid ammonia environment with good accuracy and in a short period of time.

Brief Description of the Drawings

[0023] [Figure 1] It is a diagram showing the positional relationship between the soft portion and the tensile load according to an embodiment of the present invention. [Figure 2] It is a diagram showing the positional relationship between the soft portion and the tensile load according to an embodiment of the present invention. [Figure 3] It is a schematic diagram according to an embodiment in which a tensile load is applied by four-point bending.

Embodiments for Carrying Out the Invention

[0024] 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, X (numerical value) to Y (numerical value) means X or greater and Y or less.

[0025] [Metal test specimens having a soft portion of 1 mm or more in length] In an embodiment of the test method of the present invention, the metal test specimen to be evaluated has a soft portion of 1 mm or more in length, and an external tensile load is applied in a direction perpendicular to the longitudinal direction of the soft portion and parallel to the surface of the metal test specimen. The soft portion reproduces the unavoidable hardness reduction region due to the heat effect of welding, and is of 1 mm or more in length in order to reproduce the concentration of deformation and corrosion. The soft portion is present on at least one of the front or back surfaces of the metal test specimen to be evaluated, and may penetrate both the front and back surfaces. If the length of the soft portion is less than 1 mm, the concentration of deformation cannot be reproduced due to the surrounding constraints.

[0026] The soft portion on the surface of the metal test specimen is basically a straight line or a shape close to a rectangle (approximately rectangular) with a certain width, but is not necessarily limited to this. The straight line or the shape close to a rectangle does not need to have perfectly straight lines or sides. If the soft portion has the shape close to a rectangle on the surface of the metal test specimen, the length of its longest side (i.e., longitudinal direction) shall be defined as the length of the soft portion. There is no need to specifically define an upper limit to the length of the soft portion, but in practice it is the length from any one end of the test specimen surface where the soft portion exists to any other.

[0027] A soft area is defined as a region where the Vickers hardness measured at five or more locations is 10% or more lower than the average Vickers hardness measured in the base material of the metal specimen (the part that has not undergone any treatment that affects hardness, such as heat treatment or welding). This is because if the reduction in Vickers hardness is less than 10%, the concentration of deformation cannot be reproduced due to the surrounding constraints. The upper limit of the reduction in soft area is not particularly limited, as long as the specimen having the soft area does not crack or fracture immediately after the application of tensile load.

[0028] The method for imparting a soft portion to a metal test specimen is not particularly limited. For example, it can be formed on a metal test specimen by localized heat treatment using a laser or by joining materials of different hardnesses by welding. In addition, test specimens can be taken from materials that already have a soft portion, such as welded joints. The heat treatment and welding methods are not particularly limited, and those commonly used in metal processing can be used as appropriate. The soft portion formed in this way may have residual stress due to the heat treatment or welding. The longitudinal direction of the soft portion is not particularly limited as long as it is parallel to the surface. The position where the soft portion is imparted is not particularly limited other than being a position where the tensile load described above can be applied, but when applying a tensile load by bending test, it is preferable to impart the soft portion to the region where the tensile load (bending stress) is maximum.

[0029] Here, the positional relationship between the soft part and the tensile load will be explained in detail using diagrams.

[0030] Figure 1 shows an embodiment in which the metal test specimen to be evaluated is a flat plate, and the soft portion A penetrates both the front and back surfaces of the plate and has a length equal to the length from one end to the other on both surfaces. In this case, one of the front or back surfaces of the plate is defined as the test specimen surface 1, and an external tensile load B is applied in a direction perpendicular to the longitudinal direction of the soft portion A present on the test specimen surface 1 and parallel to the test specimen surface 1. As a result, as shown in Figure 1, the dotted line along the longitudinal direction of the soft portion A and the dotted line along the tensile load B intersect perpendicularly (at a right angle) on the test specimen surface 1.

[0031] Figure 2 shows an embodiment in which the metal test specimen to be evaluated is a flat plate, and a soft portion A exists on one surface of the specimen in a shape close to a rectangle. An external tensile load B is applied in a direction perpendicular to the longitudinal direction of the soft portion A on the specimen surface 1 and parallel to the specimen surface 1. As a result, as shown in Figure 2, the dotted line along the longitudinal direction of the soft portion A and the dotted line along the tensile load B intersect perpendicularly (at a right angle) on the specimen surface 1.

[0032] In this invention, "perpendicular" to the direction of the tensile load B includes a range of 90 degrees ± 10 degrees with respect to the longitudinal direction of the soft part A. Also, "parallel" to the direction of the tensile load B includes a range of 0 degrees ± 10 degrees in the angle between the surface 1 of the test piece having the soft part A and the tensile load B.

[0033] The specimen surface 1 may be flat or curved. When the specimen surface 1 is curved, "parallel to the surface" means that the angle between the tangential direction at the center of the area to be evaluated on the specimen surface 1 and the longitudinal direction of the soft part A is within the range of 0 degrees ± 5 degrees.

[0034] [Polarizes to 0 to +1.0V relative to the corrosion potential] In one embodiment of the test method of the present invention, the sample 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, and polarized to 0 to +1.0V with respect to the corrosion potential.

[0035] Polarizing the metal specimen to a corrosion potential of 0 to +1.0V denominator 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.

[0036] The corrosion potential is determined by immersing a metal test specimen used to evaluate ammonia SCC susceptibility, or a metal test specimen taken from the same material as the metal material being evaluated, in liquid ammonia for at least 5 minutes while under external tensile load. The method of measuring the potential is not particularly limited, but it can be measured using the two-electrode method or the three-electrode method.

[0037] 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.

[0038] <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.

[0039] [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 -

[0040] 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.

[0041] [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 an external tensile load 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.

[0042] 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%.

[0043] 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.

[0044] 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.

[0045] [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.

[0046] 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.

[0047] [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.

[0048] [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. 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.

[0049] [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.

[0050] <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.

[0051] 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. Therefore, 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.

[0052] The shape of the metal test piece can be appropriately determined according to the test cell to be used and the method of applying tensile load by external force, but it is preferably a flat plate shape with a surface where the soft part exists. A metal test piece including the soft part may be cut out from a metal structure such as a joint where heat treatment or welding has been performed to form the soft part, and used as the evaluation object.

[0053] The size of the metal test piece should be such that the influence on the solution composition due to the corrosion reaction is small. The ratio of the amount of liquid ammonia solution (mL) to the exposed area (cm 2 ) of the metal test piece is preferably 5 mL / cm 2 or more. The upper limit of the ratio of liquid is not particularly limited. However, if the ratio of liquid is made excessively large, the cost of test equipment, etc. will increase. Therefore, it is preferably 500 mL / cm 2 or less.

[0054] [Tensile Load] To simulate the generation environment of ammonia SCC, a tensile stress is applied. The method of applying tensile load by external force is not particularly limited. For example, known methods such as four-point bending, U-bending, constant load method, tensile test, etc. can be used to apply tensile load by external force.

[0055] For example, when applying tensile load by the constant load method or tensile test, if one side of the test piece is fixed and tensile load is applied only to the opposite side, the same magnitude of tensile load will also be applied in the opposite direction of the tensile load, that is, to the fixed side of the test piece.

[0056] When applying tensile load by the constant load method or tensile test, it is preferable to set the shape, size, gripping method, position of the gripping part, etc. of the test piece in accordance with, for example, NACE TM0177 2006 Method A or JIS Z 2241, etc.

[0057] Figure 3 also shows an example of an embodiment in which a tensile load is applied by a four-point bending test. The direction normal to the plane of the paper is the longitudinal direction of the soft part A. The test specimen is positioned with the surface 1 facing downwards, so that the soft part A is located approximately in the center of the two upper four-point bending fixtures 2. In the four-point bending test, as shown in Figure 3, the four-point bending fixtures 2 are brought into contact with the test specimen at two points each on the top and bottom, and the load is applied only to the two upper four-point bending fixtures 2. In this way, a tensile load B is applied as a bending stress in a direction perpendicular to the longitudinal direction of the soft part A on the downward-facing surface 1 of the test specimen, and parallel to the surface 1 of the test specimen.

[0058] When applying a tensile load by a bending test, it is preferable to select the position of the support point on the metal test specimen (the point where the bending jig (support and pressing device) contacts the metal test specimen) so that the tensile load (bending stress) applied to the evaluation area where tensile residual stress exists is maximized.

[0059] The magnitude of the stress applied by the tensile load due to the external force can be set appropriately according to the purpose of the test (for example, the intended use of the material being evaluated), but it is preferable to apply a load of 20% or more of the yield strength YS (MPa) of the metal test specimen being evaluated. From the viewpoint of evaluating ammonia SCC susceptibility, the upper limit of the tensile load is preferably 95% or less of the tensile strength TS of the metal test specimen being evaluated.

[0060] <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.

[0061] 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.

[0062] 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]

[0063] 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.

[0064] [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.

[0065] 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.

[0066] 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.)

[0067] 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.

[0068] Test specimens measuring 5 mm thick × 15 mm wide × 115 mm long were taken from the 1 / 4 position of the plate thickness of the test steel materials A to G. Local heating by laser irradiation was applied to one side surface of the taken test specimens to form a roughly rectangular soft area 2 mm in length. The average Vickers hardness was measured at five or more locations in the formed soft area and it was confirmed that the average Vickers hardness was 10% or more lower than the average Vickers hardness measured similarly in the base material. The Vickers hardness of the base material and the soft area is shown in Table 1. After ultrasonic degreasing in acetone for 5 minutes, the test specimens with the soft areas were subjected to tensile loading of an external force (100% YS) equal to the yield strength of each test specimen by four-point bending. These four-point bent test specimens were immersed in a liquid ammonia tank and removed after one year. After removing corrosion products from the surface of the removed test specimens, the presence or absence of cracks was evaluated by visual inspection of the surface and cross-section of the test specimens.

[0069] 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.

[0070] [Table 1]

[0071] 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.

[0072] [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.

[0073] Table 2 shows the test conditions for the ammonia SCC acceleration test. Test specimens measuring 5 mm thick × 15 mm wide × 115 mm long were taken from the 1 / 4 position of the plate thickness of the test steel materials A to G. Local heating by laser irradiation was applied to one side surface of the collected specimens to form a roughly rectangular soft area of ​​2 mm or 0.5 mm in length. The Vickers hardness was measured at five or more locations in the formed soft area, and the average value was calculated to determine whether it had decreased by 10% or more compared to the hardness of the base material. The Vickers hardness of the soft areas is shown in Table 2. The specimens with the formed soft areas were ultrasonically degreased in acetone for 5 minutes, and then a tensile load equal to the yield strength of each specimen (100% YS) was applied by four-point bending. Ammonium carbamate and O2 were introduced into the test cell containing these four-point bent specimens to the predetermined amounts shown in Table 2, and then 2 L of liquid ammonia was filled. The specific liquid volume was 60 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] [Table 2]

[0078] 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]

[0079] 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. [Explanation of Symbols]

[0080] 1: Surface of the test specimen A: Soft part B: Tensile load 2:4-point bending jig

Claims

1. For a metal test specimen having a soft portion of 1 mm or more in length, an external tensile load is applied in a direction perpendicular to the longitudinal direction of the soft portion and parallel to the surface of the metal test specimen, while adding 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. For a metal test specimen having a soft portion of 1 mm or more in length, an external tensile load is applied in a direction perpendicular to the longitudinal direction of the soft portion and parallel to the surface of the metal test specimen, while 0.05 to 0.50 mass% ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar are used. ２ A method for accelerating ammonia stress corrosion cracking, characterized by immersion in liquid ammonia containing [a specific substance].

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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