Ammonia stress corrosion cracking accelerated test method

Abstract

To provide an ammonia stress corrosion cracking acceleration test method capable of evaluating ammonia stress corrosion cracking sensitivity of a metal material in a short period.SOLUTION: In an ammonia stress corrosion cracking acceleration test method, a metal test piece to which external stress is added or which has residual stress is immersed in ammonium carbamate of 0.01 mass% or more and liquid ammonia containing O2 of 0.25 bar or less at a gas partial pressure, and is cathodally polarized in a potential region of up to -3.0 V relative to corrosion potential.SELECTED DRAWING: None

Description

[Technical Field]

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

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

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

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

[0005] Therefore, when applying new steel materials that combine high strength with excellent ammonia SCC resistance, 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, an accelerated test that can evaluate the ammonia SCC susceptibility of steel materials in a short period of time is desired.

[0006] Such accelerated tests using liquid ammonia are disclosed in Patent Document 1 and Non-Patent Document 1. Patent Document 1 and Non-Patent Document 1 describe a test method for evaluating ammonia SCC susceptibility in a short period of time by anodic polarizing a test specimen in liquid ammonia containing O2 and saturated CO2, thereby accelerating iron dissolution. [Prior art documents] [Patent documents]

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

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

[0009] In the test methods described in Patent Document 1 and Non-Patent Document 1, the test specimens are polarized anodically at +2V vs. Pt. However, the cathodic reaction, which is the counter reaction to the anodic reaction, is significantly suppressed, and the amount of hydrogen generated, which is an important factor in promoting cracking due to hydrogen embrittlement, is significantly suppressed. As a result, ammonia SCC is not promoted, and it is difficult to say that ammonia SCC susceptibility can be evaluated in a short period of time.

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

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

[0012] In a liquid ammonia environment, the following corrosion reactions occur on steel plates: Anode reaction: Fe → Fe 2+ +2e - (Formula 1) Cathode reaction: 2NH4 + +2e - →H2+2NH3 (Formula 2)

[0013] As a result of the above corrosion reaction, hydrogen generated by the cathodic reaction is absorbed into the steel from the steel surface, and then diffuses and accumulates at the tip of the crack, accelerating the crack. Therefore, in order to promote ammonia SCC, it was thought that accelerating the cathodic reaction and increasing the amount of hydrogen generated would be necessary.

[0014] Liquid ammonia also contains trace amounts of CO2 as an impurity. In liquid ammonia, CO2 dissociates and reaches an equilibrium state according to the following reaction formula: 2NH3+CO2⇔NH4CO2NH2⇔NH4 + +NH2CO2 - (Formula 3)

[0015] When ammonium carbamate (NH4CO2NH2) is added to liquid ammonia, the dissociation reaction causes the NH4 + It was recalled that the ammonia SCC is promoted because hydrogen generation is promoted by the increase in the cathode reaction of the above-mentioned formula 2.

[0016] Furthermore, when O2 is added to liquid ammonia, the following reaction occurs: O2+2NH4 ++4e - →2OH - +2NH3 (formula 4)

[0017] When O2 is added to liquid ammonia, NH4 is produced by the reaction shown in Equation 4. + It was recalled that ammonia SCC is suppressed because the hydrogen evolution reaction of Equation 2 is suppressed by consuming the NH3.

[0018] From the above, we found that by performing cathodic polarization in liquid ammonia, adding ammonium carbamate to the liquid ammonia, and limiting the O2 content in the liquid ammonia, the hydrogen evolution reaction is promoted rather than inhibited. Therefore, we found that ammonia SCC is promoted and ammonia SCC susceptibility can be evaluated in a short period of time.

[0019] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0020] [1] An accelerated ammonia stress corrosion cracking test method, characterized by immersing a metal test piece to which external stress has been applied or which has residual stress in liquid ammonia containing 0.01 mass% or more ammonium carbamate and O2 at a gas partial pressure of 0.25 bar or less, and cathodically polarizing it in a potential range up to -3.0 V relative to the corrosion potential.

[0021] [2] The accelerated ammonia stress corrosion cracking test method according to [1], wherein the liquid ammonia is stirred during the test. [Effects of the Invention]

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

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

[0024] [Cathodic polarization in the potential range up to -3.0 V relative to the corrosion potential] In the test method of the present invention, a metal test piece to which external stress has been applied or which has residual stress is immersed in liquid ammonia containing 0.01 mass% or more of ammonium carbamate and O2 at a gas partial pressure of 0.25 bar or less, and is cathodically polarized in a potential range up to -3.0 V relative to the corrosion potential. Note that "up to -3.0 V" refers to a range from -3.0 V to less than 0 V.

[0025] Cathodic polarization up to a potential range of -3.0 V relative to the corrosion potential generates hydrogen through a cathodic reaction on the surface of a metal test specimen in liquid ammonia. The generated hydrogen is absorbed into the metal from the test specimen surface and diffuses and accumulates at the crack propagation tip, accelerating cracking and enabling ammonia SCC susceptibility to be evaluated in a short period of time. On the other hand, if a metal test specimen is held at the corrosion potential or anodically polarized relative to the corrosion potential, cathodic polarization on the test specimen surface is not promoted, making it difficult to evaluate the ammonia SCC susceptibility of the test specimen in a short period of time. Furthermore, cathodic polarization of a metal test specimen above a potential range of -3.0 V relative to the corrosion potential promotes excessive hydrogen generation, covering the test specimen surface with hydrogen bubbles, which inhibits the cathodic reaction of Equation 2 in those areas and may result in an inability to evaluate ammonia SCC susceptibility. For this reason, cathodic polarization up to a potential range of -3.0 V relative to the corrosion potential is specified. The potential region for cathodic polarization relative to the corrosion potential is preferably −2.5 V or higher, more preferably −2.0 V or higher, and preferably −0.2 V or lower, more preferably −0.5 V or lower, and even more preferably −1.0 V or lower.

[0026] The corrosion potential is the potential measured by immersing a metal test piece to be evaluated for ammonia SCC susceptibility or a metal test piece taken from the metal material to be evaluated in liquid ammonia for 5 minutes or more while applying external stress or while maintaining residual stress. The method for measuring the potential is not particularly limited, but it can be measured by the two-electrode method or the three-electrode method.

[0027] The polarization (application of electric potential) and corrosion potential measurement of a metal test piece can be performed using, for example, a potentiostat (a constant-potential electrolysis device) with the metal test piece as the working electrode (sample electrode). In this case, platinum (Pt) electrodes, which are stable in liquid ammonia, are preferably used as the counter electrode and reference electrode.

[0028] <Test solution> In the test method of the present invention, liquid ammonia containing 0.01 mass % or more of ammonium carbamate and O2 at a gas partial pressure of 0.25 bar or less is used as the test solution.

[0029] [Ammonium carbamate content: 0.01 mass or more] Liquid ammonia normally contains a small amount of CO2 as an impurity, and in liquid ammonia, CO2 dissociates as shown in equation 3 above and reaches an equilibrium state.

[0030] When ammonium carbamate (NH4CO2NH2) is added to liquid ammonia in equilibrium, the NH4 + The amount of ammonium carbamate in the liquid ammonia increases, and the cathode reaction of formula 2 described above is promoted, which in turn promotes hydrogen generation and thus ammonia SCC. If the ammonium carbamate content in the liquid ammonia is less than 0.01 mass%, this effect cannot be obtained. Therefore, the ammonium carbamate content in the liquid ammonia is specified to be 0.01 mass% or more. It is preferably 0.03 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.10 mass% or more, and may be, for example, 0.50 mass%, 1.00 mass%, or 5.00 mass%.

[0031] The upper limit of the ammonium carbamate content in the liquid ammonia is not particularly limited, and may be up to the saturated amount, for example, 5.00 mass% or less, 1.00 mass% or less, or 0.50 mass% or less.

[0032] Although the method for adding ammonium carbamate to liquid ammonia is not particularly limited, it is preferable to place a predetermined amount of ammonium carbamate in the test vessel before introducing the liquid ammonia. The placement of ammonium carbamate can be replaced by blowing in an amount of CO2 gas or placing solid CO2 (dry ice) so that the predetermined amount of ammonium carbamate is obtained.

[0033] [O2 content: gas partial pressure 0.25 bar or less] In liquid ammonia, O2 reacts as shown in formula 4. When liquid ammonia contains O2, the reaction produces NH4 + is consumed, suppressing the hydrogen generation reaction of formula 2, thereby suppressing ammonia SCC. If the O2 content in liquid ammonia is set to 0.25 bar or less in terms of gas partial pressure, this effect does not occur. Therefore, the O2 content in liquid ammonia is specified to be 0.25 bar or less in terms of gas partial pressure. It is preferably 0.20 bar or less, more preferably 0.15 bar or less, and even more preferably 0.10 bar or less.

[0034] The lower limit of the O2 content in liquid ammonia may be 0 bar, but 0.002 bar is preferable. Ammonia stress corrosion cracking begins at the destruction of the surface film due to stress load. On the other hand, O2 has the effect of forming an oxide film on the surface of a metal test piece in liquid ammonia, so adding an appropriate amount of O2 has the effect of promoting stress corrosion cracking. Since this effect cannot be obtained if the O2 content is less than 0.002 bar in gas partial pressure, the lower limit of the O2 content is preferably 0.002 bar, and more preferably 0.01 bar or more. Although there are no particular limitations on the method for adding O2 to liquid ammonia, from the viewpoint of a stable supply of O2, it is preferable to blow a predetermined amount of O2 gas into the test vessel before introducing liquid ammonia. The blowing of O2 gas can be replaced by blowing in an amount of air gas that will provide the predetermined O2 gas partial pressure.

[0035] [Liquid ammonia] Although there are no particular restrictions on the purity of the liquid ammonia used in the test solution, if it contains HO and oil, stress corrosion cracking of the metal test piece is more likely to be suppressed. Therefore, it is preferable that the HO and oil contained as impurities in the liquid ammonia be less than 0.05 mass% each.

[0036] [Liquid ammonia stirred during testing] The corrosion reaction is affected by the composition of the liquid ammonia solution on the surface of the metal test piece. As the corrosion reaction continues, the reactants on the surface of the metal test piece decrease. In other words, as the corrosion reaction continues, the cathodic reaction 2NH4 + The amount of hydrogen produced also decreases as the amount of NH4 on the surface of the metal test piece decreases. + This promotes the supply of ions, maintains the promotion of hydrogen generation, and further shortens the time required for ammonia SCC susceptibility evaluation. Therefore, it is preferable to stir the liquid ammonia during the test. Since ammonia SCC susceptibility is affected by nitrogen, carbon dioxide, oxygen, etc., stirring is preferably performed using a stirrer. To stabilize the supply of ions to the test specimen surface, it is desirable to stir continuously at 10 rpm or more.

[0037] <Metal test piece> The test method of the present invention is a method for promoting ammonia SCC by applying a potential, and therefore can be applied to conductive metallic materials. The metallic materials are not particularly limited in terms of their composition or metal structure. Specific examples of metallic materials include those containing a metal 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 method to steel materials, since these materials are generally used in structures exposed to a liquid ammonia environment and resistance to ammonia SCC is important.

[0038] The steel materials to which this accelerated ammonia stress corrosion cracking test method can be applied are not particularly limited. However, high-strength steels with a surface Vickers hardness of 280 HV10 or more tend to be particularly susceptible to cracking due to hydrogen embrittlement, and are therefore suitable for evaluating ammonia SCC susceptibility using the test method of the present invention. The metal test piece used in the test 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 component composition and metal structure as the metal material.

[0039] The shape and size of the test specimen are not particularly limited and may be determined appropriately depending on the test cell to be used and the external stress application method, or may be selected from known standards. However, the exposed area (cm) of the test specimen should be set to minimize the effect of the corrosion reaction on the solution composition. 2 ) is the amount of liquid ammonia solution (mL) relative to the 2 The upper limit of the specific liquid volume is not particularly limited, but if the specific liquid volume is excessively large, the cost of test equipment etc. will increase, so it is preferable to set it to 500 mL / cm or more. 2 It is preferable to do the following:

[0040] [External stress / residual stress] Since the test method of the present invention is an accelerated ammonia stress corrosion cracking test, a metal test piece to which an external stress has been applied or which has residual stress is immersed in a test solution. The method for applying external stress or residual stress is not particularly limited, but external stress can be applied by known methods such as four-point bending, U-bending, or the constant load method, and residual stress can also be applied by heat treatment, welding, processing, etc.

[0041] 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 (MPa) of each test specimen. Since ammonia SCC can be promoted as the applied external stress or residual stress increases, there is no particular upper limit as long as damage such as cracks is not caused to the metal test specimen itself.

[0042] <Other test conditions> The test temperature (temperature of the test solution) used in the test method of the present invention can be appropriately selected depending on the purpose of the test, but since corrosion reactions tend to be suppressed at low temperatures and the test period tends to be longer at low temperatures, it is preferable to set the temperature at 0 to 60°C.

[0043] The test period for carrying out the test method of the present invention is not particularly limited and can be set appropriately depending on the intended use of the metallic material to be evaluated, but in order to apply new materials, the shorter the test period, the better, and it is preferably within 720 hours. Ammonia SCC testing under an actual tank environment (actual liquid ammonia) generally requires a test period of about one year, so by using the test method of the present invention, the test period can be shortened to about one-tenth or less.

[0044] Test conditions other than those described above are not particularly limited, and known conditions used in corrosion tests can be appropriately selected and used. Test equipment, test cells, etc. that are known in the art, such as those described in Patent Document 1 and Non-Patent Document 1, can also be used. [Example]

[0045] 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.

[0046] [Ammonia SCC occurrence in actual tank environment] As a preliminary study to confirm the effectiveness of the test method of the present invention, four types of steel (steel types A to D) with different chemical compositions were used as test steels, and an ammonia SCC test was conducted in an actual tank environment using actual liquid ammonia to evaluate their ammonia SCC susceptibility.

[0047] 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 were measured at the 1 / 4 position 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 500 g, and the average value was used.

[0048] Test specimens measuring 5 mm thick, 15 mm wide, and 115 mm long were taken from the quarter-thickness position of test steels A to D. The taken test specimens were ultrasonically degreased in acetone for 5 minutes, and then subjected to four-point bending to apply an external stress equal to the yield strength of each test specimen (100% YS). The four-point bent test specimens were immersed in a liquid ammonia tank and removed after one year. After removing the corrosion products from the surface of the removed test specimens, the surface and cross section of the test specimens were visually inspected for cracks and evaluated for their presence or absence.

[0049] Those that developed cracks were rated as SCC occurrence "Good", and those that did not developed cracks were rated as no SCC "Poor". The results are shown in Table 1 as the SCC occurrence status in actual liquid ammonia tanks.

[0050] [Table 1]

[0051] As can be seen from Table 1, in the actual liquid ammonia tank, SCC did not occur in steels A and B. SCC did occur in steels C and D.

[0052] [Ammonia SCC accelerated test] The ammonia SCC susceptibility of the steel specimens A to D, which were examined for ammonia SCC occurrence in an actual tank environment as described above, was evaluated by carrying out the ammonia SCC accelerated test of the present invention.

[0053] Table 2 shows the test conditions for the ammonia SCC accelerated test. Test specimens measuring 5 mm thick x 15 mm x 115 mm were taken from the 1 / 4 position of the plate thickness of test steels A to D, and ultrasonically degreasing was performed in acetone for 5 minutes. An external stress (100% YS) equal to the yield strength of each test specimen was applied by four-point bending. Ammonium carbamate and O2 were introduced into the test cell containing the four-point bending specimens in the amounts shown in Table 2, and then 2 L of liquid ammonia was filled. The specific liquid volume was 42.1 mL / cm. 2 The corrosion potential of the test piece was then measured using a potentiostat, and the test was started by controlling the potential so that a predetermined potential was applied relative to the corrosion potential after 1 hour. After 168 to 1440 hours of immersion, the test piece was removed from the test cell, and the corrosion products on the surface of the test piece were removed. The surface and cross section were visually inspected for cracks to evaluate the presence or absence of cracks.

[0054] The immersion test used liquid ammonia with a purity of 99.999% or higher. Potential measurement and application using a potentiostat were performed using the three-electrode method, with platinum electrodes used as both the reference electrode and counter electrode. The test temperature was set at 25°C. Stirring during the test was performed continuously at 10 rpm using a stirrer placed inside the test cell.

[0055] The occurrence of SCC was evaluated by conducting the immersion test 10 times for each test number, and if one or more cracks occurred throughout the test, it was judged as "O" (SCC occurrence), and if no cracks occurred, it was judged as "X" (no SCC occurrence).

[0056] If the SCC occurrence status matched the SCC occurrence status in an actual liquid ammonia tank for the same steel type shown in Table 1, the evaluation accuracy was judged to be "good." For tests where the evaluation accuracy was judged to be good, those with a test period of 720 hours or less were judged to have a "good" evaluation period. Tests other than those judged to be "good" in terms of evaluation accuracy and evaluation period were judged to be "poor."

[0057] Table 2 shows the SCC occurrence status, evaluation accuracy, and evaluation period as accelerated test results.

[0058] [Table 2]

[0059] As can be seen from Table 2, in the examples of the present invention, the SCC susceptibility could be evaluated within 720 hours to the same level as in the environment simulating an actual liquid ammonia tank. On the other hand, in comparative examples 10 and 12, SCC did not occur in the steel material in which SCC had occurred in the environment simulating an actual liquid ammonia tank, and SCC susceptibility could not be evaluated. Furthermore, in comparative example 11, SCC did occur in the steel material in which SCC had occurred in the environment simulating an actual liquid ammonia tank, but the test period was too long, and ammonia SCC susceptibility could not be evaluated in a short period of time. [Industrial Applicability]

[0060] The present invention provides an accelerated ammonia stress corrosion cracking test method that can evaluate the ammonia stress corrosion cracking susceptibility of metallic materials in a short period of time. By using the test method of the present invention, it becomes possible to evaluate the ammonia stress corrosion cracking susceptibility of metallic materials 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 steel materials, and can be advantageously used for selecting and developing metallic materials that have excellent ammonia stress corrosion cracking resistance and are suitable for use in structures for transporting or storing liquid ammonia.

Claims

1. A metal test piece to which external stress has been applied or which has residual stress is subjected to a gas partial pressure of 0.25 bar or less containing 0.01 mass % or more of ammonium carbamate. ２ and cathodically polarizing the specimen in a potential range up to -3.0 V relative to the corrosion potential.

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

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