Method for detecting ammonia stress corrosion cracking

Abstract

To provide a method by which the ammonia stress corrosion cracking of a metallic material used for a tank for transporting and storing liquid ammonia or the like can be detected simply with satisfactory accuracy.SOLUTION: The corrosion potential of a metal test piece to which external stress is applied or which has residual stress is measured with time while the metal test piece is immersed in liquid ammonia, and ammonia stress corrosion cracking in the metal test piece is detected on the basis of the measurement result of the corrosion potential. Preferably, the presence or absence of ammonia stress-corrosion cracking in the metallic test piece is determined based on the time when the corrosion potential is held in the range of -0. 60V or more and -0. 10V or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for detecting 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 been attracting attention as a clean energy source because it does not emit CO2 when burned, and large-scale demand is expected. This has led to a demand for larger facilities to transport and store liquid ammonia. Generally, when making tanks larger, thinner steel is used to reduce weight and construction costs, and therefore the use of high-strength steel is desirable.

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

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

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

[0006] Such laboratory tests using liquid ammonia are disclosed in Patent Document 1 and Non-Patent Document 1. Patent Document 1 and Non-Patent Document 1 describe a test method in which a test steel piece is anodic polarized in liquid ammonia containing O2 and saturated CO2, thereby accelerating iron dissolution and evaluating ammonia SCC susceptibility in a short period of time. [Prior art documents] [Patent documents]

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

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

[0009] As mentioned above, many laboratory tests for ammonia SCC have been proposed and reported to date. In all of these proposals and reports, a metal test piece is immersed in liquid ammonia for a certain period of time, then removed from the liquid ammonia, and the occurrence of ammonia SCC is determined by destructive testing, in which the macrostructure of the cross section of the metal test piece is observed. However, when attempting to detect ammonia SCC through destructive testing, it is necessary to prepare a large number of metal test pieces and observe the macrostructure of multiple metal test pieces immersed for different periods of time. This has posed a problem in that it requires a great deal of effort to conduct the test.

[0010] Furthermore, in conventional testing methods, ammonia SCC occurs in a shorter time than in actual use environments by anodically polarizing metal test specimens. However, it is not necessarily clear whether ammonia SCC in anodically polarized metal test specimens and ammonia SCC in tanks used in a liquid ammonia environment occur by the same mechanism. Therefore, a method for detecting ammonia SCC under conditions closer to the actual use environment is desired.

[0011] In view of the above circumstances, an object of the present invention is to provide a method capable of easily and accurately detecting ammonia SCC in metal materials used in tanks for transporting and storing liquid ammonia. [Means for solving the problem]

[0012] In order to solve the above problems, the present inventors have conducted a detailed study on the ammonia SCC mechanism of steel sheets in liquid ammonia, and have obtained the following findings.

[0013] In a liquid ammonia environment, corrosion reactions shown in the following chemical formulas 1 and 2 occur on steel plates.

[0014] [C1] Anode reaction: Fe → Fe 2+ +2e - (1)

[0015] [Case 2] Cathode reaction: O2 + 2NH4 + +4e - →2OH - +2NH3(2)

[0016] As a result of the above corrosion reaction, an inactive oxide film forms on the steel plate surface, but pitting corrosion occurs when localized dissolution of the oxide film is accelerated in unstable areas. Because the pits become stress-concentrated areas, film breakdown and corrosion reactions progress at the pit tip, ultimately leading to fracture. Therefore, we envisioned that measuring the corrosion potential and understanding the electrochemical behavior could be used to detect the onset of pitting corrosion and the progression of the corrosion reaction, which are the starting points for ammonia-induced SCC.

[0017] The inventors then measured the corrosion potential of a steel sheet while immersed in liquid ammonia and found that ammonia SCC occurs when the corrosion potential is maintained above a lower limit and below an upper limit for a predetermined period of time, leading to the completion of the present invention.

[0018] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0019] [1] A corrosion potential measuring step of measuring the corrosion potential of a metal test piece to which an external stress has been applied or which has residual stress, over time, while the metal test piece is immersed in liquid ammonia; a detection step of detecting ammonia stress corrosion cracking in the metal test piece based on the measurement result of the corrosion potential; A method for detecting ammonia stress corrosion cracking comprising:

[0020] [2] The liquid ammonia contains 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 25.0 kPa or less. The method for detecting ammonia stress corrosion cracking according to [1] above.

[0021] [3] Prior to the corrosion potential measuring step, an activation treatment is performed in which the metal test piece is cathodically polarized while immersed in the liquid ammonia. The method for detecting ammonia stress corrosion cracking according to [1] above.

[0022] [4] The liquid ammonia contains 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 25.0 kPa or less, prior to the corrosion potential measuring step, an activation treatment is performed in which the metal test piece is cathodically polarized while being immersed in the liquid ammonia. The method for detecting ammonia stress corrosion cracking according to [1] above.

[0023] [5] The temperature of the liquid ammonia is maintained at -40°C or higher and 40°C or lower. The method for detecting ammonia stress corrosion cracking according to any one of [1] to [4] above.

[0024] [5] In the detection step, the presence or absence of ammonia stress corrosion cracking in the metal test piece is determined based on the time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less. The method for detecting ammonia stress corrosion cracking according to any one of [1] to [4] above.

[0025] [6] In the detection step, the presence or absence of ammonia stress corrosion cracking in the metal test piece is determined based on the time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less. The method for detecting ammonia stress corrosion cracking according to any one of [1] to [4] above.

[0026] [7] In the detection step, the presence or absence of ammonia stress corrosion cracking in the metal test piece is determined based on the time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less. The method for detecting ammonia stress corrosion cracking according to [5] above. [Effects of the Invention]

[0027] The ammonia SCC detection method of the present invention makes it possible to easily detect the occurrence of ammonia SCC with high accuracy in ammonia SCC laboratory tests of metal materials applied to tanks and the like used in an ammonia environment. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of a four-point bending jig that applies bending stress to a metal test piece. [Figure 2] FIG. 1 is a schematic diagram showing an outline of a test device for measuring corrosion potential. DETAILED DESCRIPTION OF THE INVENTION

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0030] [Metal test piece] The present invention relates to a method for detecting ammonia SCC by measuring corrosion potential, and therefore can be applied to conductive metallic materials. The metallic material is not particularly limited in terms of its component composition or metal structure. Specific examples of metallic materials include those containing a metallic element (e.g., Fe, Cu, Al, Ni, Ti, etc.) as a main component (i.e., 50 mass% or more), such as steel (iron alloy), copper alloy, aluminum alloy, nickel alloy, and titanium alloy. In particular, it is preferable to apply the present invention to steel materials, since these are generally used in structures exposed to a liquid ammonia environment and resistance to ammonia SCC is important.

[0031] The metal test piece used in the detection method of the present invention may be one taken from a metal material whose ammonia SCC susceptibility is to be evaluated, or one taken from a metal material having the same or similar chemical composition and metallographic structure as the metal material. When a steel material is used as the metal test piece, the carbon equivalent and hardness of the steel material are not particularly limited. It is generally known that the higher the carbon equivalent and hardness of a steel material, the higher the ammonia SCC susceptibility. Therefore, it is preferable that the steel test piece have a carbon equivalent of 0.05% or more and a Vickers hardness of 100 HV or more.

[0032] The shape and size of the metal test piece are not particularly limited and may be determined appropriately depending on the size of the test vessel used and the method of applying external stress, or may be selected from known standards. However, the exposed area (cm) of the metal test piece should be adjusted so that the effect of the corrosion reaction on the solution composition is minimized. 2 The specific liquid volume, which is the ratio of the volume (ml) of liquid ammonia to the volume of 2 The upper limit of the specific liquid volume is not particularly limited, but if the specific liquid volume is excessively large, the cost of test equipment etc. will increase, so it is preferable to set it to 500 ml / cm or more. 2 It is preferable to do the following:

[0033] [Corrosion potential measurement process] The method for detecting ammonia SCC according to the present invention includes a corrosion potential measurement step of measuring the corrosion potential of a metal test piece to which an external stress or which has residual stress has been applied over time while the metal test piece is immersed in liquid ammonia. Because stress corrosion cracking is a reaction accompanied by corrosion, ammonia SCC can be detected by measuring the corrosion potential. The metal test piece to which an external stress has been applied or which has residual stress can be used as the metal test piece for measuring the corrosion potential.

[0034] The method for applying external stress or for creating residual stress to a metal test piece is not particularly limited, but stress can be applied by four-point bending, U-bending, constant load method, etc. Immediately after immersing a metal test piece in a state in which external stress or residual stress has been applied in liquid ammonia, the corrosion potential does not stabilize. Therefore, it is preferable to start measuring the corrosion potential over time after the corrosion potential has stabilized, and for example, measurement can be started 5 minutes after immersing the metal test piece in liquid ammonia.

[0035] The magnitude of the applied external stress or residual stress may be set appropriately depending on the test purpose (for example, the intended use of the material to be evaluated), but it is preferable to apply a load of 20% or more of the yield strength YS of each metal test piece. Since ammonia SCC can be promoted as the applied external stress or residual stress increases, there is no particular upper limit, and a load of up to 120% of the yield strength YS of each metal test piece may be applied as long as damage such as cracks is not caused to the metal test piece itself.

[0036] The corrosion potential can be measured by a two-electrode method or a three-electrode method, although the method is not particularly limited. In these methods, a metal test piece is used as the working electrode or sample electrode. In the two-electrode method, the relative potential of the working electrode to the counter electrode is measured, and in the three-electrode method, the relative potential of the working electrode to the reference electrode is measured. The corrosion potential is determined as the measured relative potential. Platinum electrodes, which are stable in liquid ammonia, are preferably used as the counter electrode in the two-electrode method and as the counter and reference electrodes in the three-electrode method. Unless otherwise specified, the corrosion potential values ​​described herein are those when platinum electrodes are used as the counter and reference electrodes. Conductors for measuring the corrosion potential can be attached to the metal test piece.

[0037] The corrosion potential of the metal test piece in the corrosion potential measurement step is measured over time. That is, in the detection method according to the present invention, the corrosion potential of the metal test piece is not measured instantaneously, but rather the change over time is measured continuously for a certain period of time.

[0038] For measuring the corrosion potential, the internal resistance is 10 8It is preferable to use a measuring device such as a voltmeter or potentiostat with a resistance of Ω or more. If the internal resistance of the voltmeter is large, almost no current flows through the voltmeter, so the effect of voltage drop inside the measuring device can be ignored. This makes it possible to measure the corrosion potential or natural potential of a metal test piece with high accuracy in situ under conditions similar to the actual usage environment of metal materials such as tanks.

[0039] [Detection process] The method for detecting ammonia SCC according to the present invention includes a detection step of detecting ammonia stress corrosion cracking in a metal test piece based on the results of measuring the corrosion potential. The corrosion reactions shown in Chemical Formulas 1 and 2 occur on the surface of a metal test piece immersed in liquid ammonia, and the corrosion potential of the metal test piece changes over time as a result of these reactions. When localized dissolution is promoted on the surface of the metal test piece, pitting corrosion occurs, and cracks propagate, new surfaces are continuously exposed on the surface of the metal test piece, causing the corrosion potential to become less noble. Therefore, ammonia SCC can be easily detected based on the results of measuring the corrosion potential of the metal test piece without using destructive testing.

[0040] In a preferred embodiment, the detection step determines whether or not ammonia stress corrosion cracking occurs in the metal test piece based on the time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less. The occurrence of ammonia SCC is determined when the corrosion potential of the metal test piece is maintained in the range of -0.60 V or more and -0.10 V or less for a predetermined time. On the other hand, when the corrosion potential of the metal test piece exceeds -0.10 V, the surface of the metal test piece is covered with a passive film, and the reaction progresses extremely slowly. Therefore, pitting corrosion cannot occur, and as a result, ammonia SCC cannot occur. On the other hand, when the corrosion potential of the metal test piece is below -0.60 V, the reaction rate is high, resulting in general corrosion in which the entire surface of the metal test piece corrodes uniformly. Since pitting corrosion does not occur and stress concentration does not occur, ammonia SCC cannot occur.

[0041] In a preferred embodiment, the threshold value of the time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less varies depending on the test conditions. Although not particularly limited, under normal test conditions, it can be determined that ammonia stress corrosion cracking has occurred in the metal test piece when the maximum time during which the corrosion potential is maintained in the range of -0.60 V or more and -0.10 V or less is 24 hours or more within the test time.

[0042] During corrosion potential measurement, the corrosion potential may deviate from the range of -0.60 V to -0.10 V due to reasons such as electrical noise in the measuring equipment. Even in such a case, if the corrosion potential abnormality is temporary, measurement of the time during which the corrosion potential is maintained in the range of -0.60 V to -0.10 V can be continued without interruption. Specifically, if the range of the deviated corrosion potential is -0.70 V to 0 V and the deviation lasts for 10 seconds or less, the corrosion potential abnormality can be considered a temporary fluctuation and can be ignored.

[0043] [Ammonium carbamate content in liquid ammonia: 0.01 mass% or more] Liquid ammonia normally contains CO2 as an impurity, and in liquid ammonia, CO2 dissociates and reaches an equilibrium state as shown in chemical formula 3 below.

[0044] [C3] NH4 + +NH2CO2 - ⇔NH4CO2NH2⇔2NH3+CO2(3)

[0045] When ammonium carbamate (NH4CO2NH2) is added to liquid ammonia in equilibrium, a dissociation reaction occurs to form carbamate ions (NH2CO2 -) is formed. Carbamate ions have the effect of destroying the inactive oxide film formed on the surface of metal test pieces, thereby accelerating ammonia SCC. If the ammonium carbamate content in liquid ammonia is less than 0.01 mass%, this effect is small and laboratory tests take an extremely long time. Therefore, it is preferable that liquid ammonia contains 0.01 mass% or more of ammonium carbamate. The ammonium carbamate content in liquid ammonia is more preferably 0.03 mass% or more, and even more preferably 0.05 mass% or more.

[0046] There is no particular upper limit to the amount of ammonium carbamate contained in liquid ammonia. However, since an excessive amount may result in a large deviation from the actual environment, the amount is preferably 5.00 mass% or less, more preferably 1.00 mass% or less, and even more preferably 0.50 mass% or less.

[0047] Although the method for adding ammonium carbamate is not particularly limited, it is preferable to place a predetermined amount of ammonium carbamate in the test vessel before introducing liquid ammonia. The addition of ammonium carbamate can also be replaced by blowing in an amount of CO2 gas or solid CO2 (dry ice) that will provide the predetermined amount of ammonium carbamate.

[0048] [O2 content in liquid ammonia: gas partial pressure 25.0 kPa or less] Because O2 in liquid ammonia has the effect of forming an oxide film, the presence of O2 is preferable when evaluating ammonia SCC susceptibility. If the O2 content in liquid ammonia exceeds 25.0 kPa in gas partial pressure, the film formation rate increases significantly, the film becomes stable, and pitting corrosion does not occur, making it difficult to evaluate ammonia SCC. Therefore, it is preferable that liquid ammonia contains O2 at a gas partial pressure of 25.0 kPa or less, and more preferably 20.0 kPa or less.

[0049] The lower limit of the O2 content in the liquid ammonia is not particularly limited, but is preferably 200 Pa or more in order to increase the film formation rate and improve the efficiency of laboratory tests.

[0050] Although there are no particular limitations on the method for adding O2 to liquid ammonia, it is preferable to inject O2 gas into the test vessel before introducing liquid ammonia in order to ensure a stable supply of O2. Injection of O2 gas can be replaced by injection of air gas.

[0051] Furthermore, the purity of the liquid ammonia is not particularly limited, but if it contains H2O and oil, ammonia SCC is more likely to be suppressed, so it is preferable that the H2O and oil contents are each less than 0.05 mass%.

[0052] [Activation treatment of metal test pieces] In a preferred embodiment, in the detection method according to the present invention, prior to the corrosion potential measurement step, an activation treatment is performed in which the metal test piece is cathodically polarized while immersed in the liquid ammonia. A passive film may be formed on the surface of the metal test piece before immersion in liquid ammonia. By cathodically polarizing the surface of the metal test piece, the passive film formed on the surface of the metal test piece can be removed and the initial surface condition can be homogenized. This enables the ammonia SCC susceptibility to be evaluated with high accuracy. The conditions for cathodic polarization are not particularly limited, but it is preferable to perform the cathodic polarization for 5 minutes or more at a potential equal to or lower than the corrosion potential or a potential lower by −1.0 V than the natural potential. If the corrosion potential of the metal test piece is not stable after cathodic polarization is completed, it is preferable to wait until the corrosion potential stabilizes before starting the time-dependent measurement of the corrosion potential.

[0053] [Liquid ammonia temperature: Maintain above -40°C and below 40°C] Tankers transporting ammonia are generally refrigerated ships. Because the boiling point of ammonia at 1 atmosphere is −33°C, conducting laboratory tests at temperatures close to this temperature can simulate an environment close to the actual environment and evaluate ammonia SCC susceptibility with greater accuracy. Therefore, in the detection method of the present invention, it is preferable to maintain the temperature of liquid ammonia at −40°C or higher. On the other hand, maintaining ammonia in a liquid state at a temperature above its boiling point requires a test vessel that can withstand high pressures. Therefore, in the detection method of the present invention, it is preferable to maintain the temperature of liquid ammonia at 40°C or lower, more preferably at 25°C or lower, even more preferably at 0°C or lower, and even more preferably at −20°C or lower.

[0054] [Combination of preferred embodiments] The method for detecting ammonia stress corrosion cracking according to the present invention can be implemented by combining a preferred embodiment with a basic configuration having a corrosion potential measurement step and a detection step. The preferred embodiment to be combined with the basic configuration can be arbitrarily selected from the multiple preferred embodiments described above.

[0055] [Other test conditions] Test conditions other than those mentioned above are not particularly limited, but liquid ammonia may be stirred during laboratory testing. The corrosion reaction is affected by the composition of the liquid ammonia solution on the surface of the metal test specimen. Stirring the liquid ammonia makes it possible to homogenize the solution composition on the surface of the metal test specimen, enabling ammonia SCC susceptibility to be evaluated with greater accuracy. Furthermore, stirring promotes the supply of ions to the surface of the metal test specimen, thereby shortening the time required to evaluate ammonia SCC susceptibility. [Example]

[0056] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0057] [Test 1: Ammonia SCC test in a liquid ammonia storage tank] In order to evaluate the ammonia SCC susceptibility in actual use environments, eight types of steel, grades A to H, with different chemical compositions, were used as test steels, and an ammonia SCC test was conducted using liquid ammonia stored in a storage tank that is in practical use.

[0058] Table 1 shows the yield strength (YS) and Vickers hardness (HV0.5) of the steel materials subjected to the ammonia SCC test. Yield strength and Vickers hardness measurements were performed at the 1 / 4 position of the plate thickness. Yield strength was measured in accordance with the "Method of tensile testing for metallic materials" specified in Japanese Industrial Standard JIS Z 2241, and the average value of three measurements was used as the measured value. Vickers hardness was measured at 10 points with a load of 500 g in accordance with the "Vickers test" specified in Japanese Industrial Standard JIS Z 2244, and the average value was used as the measured value.

[0059] Next, metal test pieces measuring 5 mm in thickness, 15 mm in width, and 115 mm in length were taken from the 1 / 4 position of each steel specimen A through H. The taken metal test pieces were subjected to ultrasonic degreasing in acetone for 5 minutes, and then the metal test pieces 2 were set in the four-point bending jig 1 shown in Figure 1, and a stress equal to the yield strength YS of each steel type shown in Table 1 (100% YS) was applied using the stress application part 1a.

[0060] Next, the four-point bending jig 1 with the metal test piece 2 set in it was immersed in liquid ammonia stored in a tank maintained at a temperature of 10°C or higher and 30°C or lower, and recovered one year later. The metal test piece 2 was removed from the recovered four-point bending jig 1, and corrosion products on the surface of the metal test piece 2 were removed. The surface and cross section of the metal test piece 2 were then visually inspected for cracks to confirm the occurrence of ammonia SCC. The occurrence of ammonia SCC is shown in Table 1.

[0061] [Table 1]

[0062] As can be seen from Table 1, no ammonia SCC occurred in the metal test pieces of steel types A, B, and C. In contrast, ammonia SCC occurred in the metal test pieces of steel types D to H, which have higher yield strength and Vickers hardness than steel types A, B, and C.

[0063] [Test 2: Laboratory ammonia SCC test] The same steel specimens as those used in Test 1, steel types A to H, were used to evaluate ammonia SCC susceptibility in laboratory tests.

[0064] For each of the test steel materials A to H, multiple metal test pieces 2 measuring 5 mm in thickness, 15 mm in width, and 115 mm in length were taken from the 1 / 4 position of the plate thickness. The taken metal test pieces were subjected to ultrasonic degreasing in acetone for 5 minutes, and an electric wire was welded to one point on the surface. The metal test pieces 2 were then set in the four-point bending jig 1 shown in Figure 1, and a stress equal to the yield strength YS of each steel type shown in Table 1 (100% YS) was applied using the stress application part 1a.

[0065] Next, one four-point bending jig 1 with a metal test piece 2 set therein, a reference electrode 3 made of a platinum electrode, and a counter electrode 4 were placed in a test vessel 6 shown in FIG. 2. The other ends of the electric wires connected to the metal test piece 2, the reference electrode 3, and the counter electrode 4 were connected to a potentiostat (controlled potential electrolysis device) 7. Next, ammonium carbamate and O2 were sealed inside the test vessel 6 so that the contents thereof were as shown in Table 2, and then liquid ammonia 5 with a purity of 99.999% or higher was filled under pressure. The volume of the liquid ammonia 5 filled under pressure was 2.0 L (liters), and the ratio of the volume of liquid ammonia 5 per unit surface area of ​​the metal test piece 2 was 42.1 ml / cm. 2 The temperature of the pressurized liquid ammonia 5 was set to two conditions, 25°C or -30°C, and was maintained at these temperatures until the end of the test.

[0066] Next, five minutes after the liquid ammonia 5 was pressurized and filled, measurement of the corrosion potential of the metal test piece 2 relative to the reference electrode 3 was started by the three-electrode method using a potentiostat 7. However, for some of the metal test pieces 2, activation treatment was performed on the metal test pieces 2 under the conditions shown in Table 2 before starting the corrosion potential measurement, and measurement of the corrosion electrode was started five minutes after the end of cathodic polarization. Furthermore, for some of the metal test pieces 2, the liquid ammonia was stirred during this time using a stirring means (not shown) provided in the test container 6.

[0067] The minimum and maximum corrosion potential values ​​measured from the start of corrosion potential measurement until 1,440 hours had elapsed, as well as the maximum time during which the corrosion potential was maintained within the range between the minimum and maximum values, are shown in Table 2. However, for metal test pieces 2 Nos. 2, 5, 6, 10, and 11, the corrosion potential was always greater than -0.10 V during the test period. Furthermore, for metal test pieces 2 Nos. 7 and 9, the corrosion potential was always less than -0.60 V during the test period.

[0068] Based on the measurement results of the corrosion potential, if the corrosion potential of the metal test piece 2 was maintained at -0.60 V or more and -0.10 V or less for 24 hours or more, it was determined that ammonia stress corrosion cracking had occurred in the metal test piece 2, and if the corrosion potential of the metal test piece 2 was not maintained at -0.60 V or more and -0.10 V or less for 24 hours or more, it was determined that ammonia stress corrosion cracking had not occurred in the metal test piece 2. The results of the determination are shown in Table 2.

[0069] Next, the four-point bending jig 1 after the test was removed from the test vessel 6, the metal test piece 2 was removed from the four-point bending jig 1, and the corrosion products on the surface of the metal test piece 2 were removed. After that, the surface and cross section of the metal test piece 2 were visually inspected for cracks to confirm the occurrence of ammonia SCC. The occurrence of ammonia SCC is shown in Table 2.

[0070] [Table 2]

[0071] As can be seen from Table 2, when the liquid ammonium contained 0.01 mass% or more of ammonium carbamate and an O2 gas partial pressure of 25 kPa or less, the corrosion potential of the metal test specimens was maintained at -0.60 V or more and -0.10 V or less for 24 hours or more. Observation of cracks on metal test specimens No. 12 to 22, which were determined to have undergone ammonia SCC and were judged to have undergone ammonia SCC, confirmed that ammonia SCC had actually occurred. On the other hand, observation of cracks on metal test specimens No. 1 to 11, which were determined not to have undergone ammonia SCC in the above corrosion electrical measurements, confirmed that ammonia SCC had not occurred at all. These results demonstrate that the determinations made by the ammonia SCC detection method according to the present invention closely match the actual occurrence of ammonia SCC.

[0072] Furthermore, the metal test pieces of steel types D to H that were determined to have had ammonia SCC in the 1,440-hour corrosion potential measurement shown in Table 2 also had ammonia SCC in the one-year ammonia SCC test in a liquid ammonia storage tank shown in Table 1. On the other hand, the metal test pieces of steel types A, B, and C that were determined not to have had ammonia SCC in the above corrosion potential measurement also did not have ammonia SCC in the ammonia SCC test in a liquid ammonia storage tank shown in Table 1. From these comparisons, it is clear that the determination results obtained by the ammonia SCC detection method according to the present invention are in good agreement with the occurrence of ammonia SCC in an actual usage environment, and that the occurrence of ammonia SCC can be predicted by a short-term test according to the present invention. [Industrial Applicability]

[0073] The present invention provides a method for easily detecting the ammonia SCC susceptibility of metallic materials with high accuracy. Use of the detection method according to the present invention makes it possible to evaluate ammonia SCC susceptibility and utilize it for relative comparison and prediction of lifespan. The detection method according to the present invention is extremely useful for evaluating the ammonia SCC susceptibility of metallic materials, and can be advantageously used in the selection and development of metallic materials with excellent ammonia SCC resistance that can be suitably used in structures for transporting or storing liquid ammonia. [Explanation of symbols]

[0074] 1 4-point bending jig 1a Stress applying part 2 Metallic test pieces 3 Reference pole 4. Opposite 5. Liquid ammonia 6 Test vessel 7. Potentiostat

Claims

1. a corrosion potential measuring step of measuring the corrosion potential of a metal test piece to which an external stress has been applied or which has a residual stress over time while the metal test piece is immersed in liquid ammonia; a detection step of detecting ammonia stress corrosion cracking in the metal test piece based on the measurement result of the corrosion potential; A method for detecting ammonia stress corrosion cracking comprising:

2. The liquid ammonia contains 0.01 mass % or more of ammonium carbamate and 25.0 kPa or less of O at a gas partial pressure. ２ containing The method for detecting ammonia stress corrosion cracking according to claim 1.

3. prior to the corrosion potential measuring step, an activation treatment is performed in which the metal test piece is cathodically polarized while being immersed in the liquid ammonia. The method for detecting ammonia stress corrosion cracking according to claim 1.

4. The liquid ammonia contains 0.01 mass % or more of ammonium carbamate and 25.0 kPa or less of O at a gas partial pressure. ２ Contains prior to the corrosion potential measuring step, an activation treatment is performed in which the metal test piece is cathodically polarized while being immersed in the liquid ammonia. The method for detecting ammonia stress corrosion cracking according to claim 1.

5. The temperature of the liquid ammonia is maintained at −40° C. or higher and 40° C. or lower.

5. The method for detecting ammonia stress corrosion cracking according to claim 1.

6. In the detection step, the presence or absence of ammonia stress corrosion cracking in the metal test piece is determined based on the time during which the corrosion potential is maintained in the range of −0.60 V or more and −0.10 V or less.

5. The method for detecting ammonia stress corrosion cracking according to claim 1.

7. In the detection step, the presence or absence of ammonia stress corrosion cracking in the metal test piece is determined based on the time during which the corrosion potential is maintained in the range of −0.60 V or more and −0.10 V or less. The method for detecting ammonia stress corrosion cracking according to claim 5.

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