Ammonia stress corrosion cracking accelerated test method

Abstract

To provide an ammonia stress corrosion cracking (SCC) acceleration test method capable of evaluating ammonia stress corrosion cracking sensitivity of a metal material with excellent accuracy in a short period.SOLUTION: In an ammonia stress corrosion cracking acceleration test method, a metal test piece to which external stress is added or which has residual stress is immersed in ammonium carbamate of 0.01 mass% or more and liquid ammonia containing O2 of 0.002 to 0.200 bar at a gas partial pressure, and is polarized to more than +1.0 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 materials that combine high strength with excellent ammonia SCC resistance, it is necessary to accurately evaluate 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 polarization of a test steel piece in liquid ammonia containing O2 and saturated CO2 at a gas pressure of 0.3 to 2.0 atmospheres, 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 environments where there is an excess of O2 described in Patent Document 1 and Non-Patent Document 1, a non-protective film is formed, resulting in a corrosion pattern similar to general corrosion, which inhibits the growth of pitting corrosion (localized corrosion) that is the starting point for ammonia SCC, making it difficult to say that ammonia SCC susceptibility can be accurately evaluated. Furthermore, because the growth of pitting corrosion (localized corrosion) is not promoted, it may not be possible to evaluate ammonia SCC susceptibility in materials that are not highly susceptible to ammonia SCC.

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

[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 - Cathode reaction: O2 + 2NH4 + +4e - →2OH - +2NH3

[0013] As a result of the above corrosion reaction, an inactive oxide film is formed on the surface of the steel sheet. When cracks occur in this oxide film due to external stress, the dissolution of the exposed steel substrate is accelerated, resulting in pitting corrosion. The pit becomes a stress concentration area, which leads to film breakdown and corrosion reactions at the tip of the pit, ultimately leading to fracture. Therefore, it was thought that the oxide film formed on the steel sheet surface is susceptible to cracking due to external or residual stress, which promotes the initiation of pitting corrosion, which is the starting point for ammonia SCC.

[0014] They found that by controlling the amount of O2 in liquid ammonia, a thick, brittle oxide film was formed on the surface of the steel sheet, making it possible to evaluate ammonia SCC susceptibility in a short period of time.

[0015] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0016] [1] An accelerated ammonia stress corrosion cracking test method, characterized in that 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 ammonium carbamate and O2 at a gas partial pressure of 0.002 to 0.200 bar, and polarized to more than +1.0 V relative to the corrosion potential.

[0017] [2] The accelerated ammonia stress corrosion cracking test method according to [1], wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized and then polarized to a corrosion potential of more than +1.0 V.

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

[0019] 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 with high accuracy and in a short period of time. DETAILED DESCRIPTION OF THE INVENTION

[0020] 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.

[0021] [Polarized to more than +1.0 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 0.002 to 0.200 bar, and polarized to more than +1.0 V relative to the corrosion potential.

[0022] Polarization to more than +1.0 V relative to the corrosion potential promotes the anodic reaction on the surface of a metal test piece immersed in liquid ammonia, resulting in the formation of a thick, brittle oxide film on the surface of the test piece. On the surface of a metal test piece that has been subjected to external stress or has residual stress, the stress causes cracks in the oxide film, promoting the onset of pitting corrosion due to localized corrosion, making it possible to evaluate ammonia SCC susceptibility in a short period of time. On the other hand, if the metal test piece is polarized to less than +1.0 V relative to the corrosion potential, a thick, brittle oxide film is not formed, and the above-mentioned effect cannot be achieved. Therefore, polarization to more than +1.0 V relative to the corrosion potential is specified. Polarization is preferably at least +1.3 V, more preferably at least +1.5 V, relative to the corrosion potential.

[0023] Furthermore, if a metal test piece is anodically polarized to a potential greater than +3.0 V relative to the corrosion potential, excess nitrogen will be generated by the reduction of NH3, resulting in the metal test piece being covered with nitrogen bubbles, preventing the anodic reaction from occurring in those areas, which may make it impossible to evaluate the ammonia SCC susceptibility. Therefore, the electrode range to be polarized is preferably +3.0 V or less relative to the corrosion potential, more preferably +2.7 V or less, and even more preferably +2.5 V or less.

[0024] The corrosion potential is the potential measured by immersing a metal test piece for which ammonia SCC susceptibility is to be evaluated, or a metal test piece taken from the same metal material as the metal material to be evaluated, in liquid ammonia for 5 minutes or more while applying external stress or while maintaining residual stress. There are no particular restrictions on the method for measuring the potential, but it can be measured by the two-electrode method or the three-electrode method.

[0025] The polarization (application of electric potential) and corrosion potential measurement of the 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 electrodes, which are stable in liquid ammonia, are preferably used as the counter electrode and reference electrode.

[0026] <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.

[0027] [Ammonium carbamate content: 0.01 mass% or more] Liquid ammonia normally contains trace amounts of CO2 as an impurity, and in liquid ammonia, CO2 dissociates and reaches an equilibrium state as shown in the following reaction formula. 2NH3+CO2⇔NH4CO2NH2⇔NH4 + +NH2CO2 -

[0028] Liquid ammonia without impurities has a low electrical conductivity. However, when it contains ammonium carbamate (NH4CO2NH2), the electrical conductivity of the liquid ammonia increases due to the dissociation reaction, accelerating the corrosion reaction. If the ammonium carbamate content in liquid ammonia is less than 0.01 mass%, this effect cannot be obtained, and it is difficult to promote ammonia SCC by polarizing the metal test piece. Therefore, the ammonium carbamate content in liquid ammonia is specified to be 0.01 mass% or more. It is preferably 0.03 mass% or more, more preferably 0.10 mass% or more, and even more preferably 0.30 mass% or more.

[0029] 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, 6.00 mass% or less, 1.00 mass% or less, or 0.50 mass% or less.

[0030] 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 vessel before introducing the liquid ammonia. The addition of ammonium carbamate can be replaced by blowing in CO2 gas or placing solid CO2 (dry ice) in an amount that will result in the predetermined amount of ammonium carbamate content.

[0031] [O2 content: gas partial pressure 0.002 to 0.200 bar] Because O2 has the effect of forming an oxide film in liquid ammonia, the presence of O2 is essential for evaluating ammonia SCC susceptibility. If the O2 content in liquid ammonia is less than 0.002 bar in gas partial pressure, this effect is not obtained, making ammonia SCC evaluation difficult. The O2 gas partial pressure is preferably 0.010 bar or higher, and more preferably 0.050 bar or higher. On the other hand, if the O2 content in liquid ammonia exceeds 0.200 bar in gas partial pressure, an oxide film without protective properties is formed, resulting in a corrosion form similar to general corrosion, which suppresses the initiation and growth of pitting corrosion (localized corrosion), making ammonia SCC evaluation difficult. Therefore, the O2 content in liquid ammonia is specified to be 0.002 to 0.200 bar in gas partial pressure. It is preferably 0.175 bar or less, and more preferably 0.150 bar or less.

[0032] Although there are no particular limitations on the method of adding O2 to liquid ammonia, from the viewpoint of a stable supply of O2, it is preferable to inject a predetermined amount of O2 gas into the test vessel before introducing liquid ammonia. The injection of O2 gas can be replaced by injection of air gas in an amount that will obtain the predetermined O2 gas partial pressure.

[0033] [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.

[0034] [Cathode polarization] A passive film may be formed on the surface of a metal test piece before immersion in liquid ammonia. After immersion in liquid ammonia, cathodic polarization is performed prior to anodic polarization to remove the passive film on the surface of the metal test piece, homogenizing the initial surface condition and enabling a more accurate evaluation of ammonia SCC susceptibility. The conditions for cathodic polarization are not particularly limited, but it is preferable to perform the polarization at a temperature between -1.0 V and -3.0 V relative to the corrosion potential for 5 to 30 minutes.

[0035] [Liquid ammonia stirred during testing] The corrosion reaction is affected by the composition of the liquid ammonia solution on the metal test specimen surface. Stirring the test solution, which contains ammonium carbamate and O2, homogenizes the solution composition on the surface of the immersed metal test specimen, enabling more accurate ammonia SCC susceptibility evaluation. Furthermore, stirring promotes ion supply to the metal test specimen surface, thereby shortening the time required for ammonia SCC susceptibility evaluation. Therefore, it is preferable to stir the liquid ammonia containing ammonium carbamate and O2 during the test. Because ammonia SCC susceptibility is affected by nitrogen, carbon dioxide, oxygen, etc., stirring is preferably performed using a stir bar. To stabilize the ion supply to the metal test specimen surface, stirring is preferably performed continuously at 10 rpm or higher.

[0036] <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, the test method is preferably applied to steel materials, since these materials are generally used in structures exposed to a liquid ammonia environment and resistance to ammonia SCC is important.

[0037] When a steel material is used as the metal test piece, the steel material is not particularly limited in terms of its carbon equivalent or hardness. It is generally known that the higher the carbon equivalent or hardness of a steel material, the higher the ammonia SCC susceptibility. Therefore, it is preferable that the steel test piece has a carbon equivalent of 0.05% or more and a Vickers hardness of 100 Hv or more. The metal test piece used in the test method of the present invention may be 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.

[0038] The shape and size of the metal test piece 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 metal test piece should be set so as 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:

[0039] [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.

[0040] 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 from the viewpoint of causing cracks in the thick, brittle oxide film formed on the surface and inducing localized pitting corrosion, it is preferable to apply a load of 100% or more of the yield strength YS (MPa) of each metal test piece, and more preferably a load of 120% or more. Since ammonia SCC can be promoted as the applied external stress or residual stress increases, there is no particular upper limit as long as the metal test piece itself is not damaged, such as by cracking.

[0041] <Other test conditions> The test temperature (temperature of the test solution) at which the test method of the present invention is carried out 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. In addition, to further improve the test accuracy, it is preferable to keep the error between the set test temperature and the actual test temperature within ±5° C.

[0042] 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.

[0043] 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]

[0044] 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.

[0045] [Ammonia SCC test under actual tank environment] As a preliminary study to confirm the effectiveness of the test method of the present invention, five types of steel (steel types A to E) 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.

[0046] Table 1 shows the carbon equivalent (CE), yield strength (YS) and Vickers hardness (HV0.5) of the steels subjected to the ammonia SCC test.

[0047] The carbon equivalent (CE) was calculated from the chemical composition of the test 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 test steel material.)

[0048] The 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. The Vickers hardness was measured at 20 points using a Vickers test with a load of 500 g, and the average value was used.

[0049] 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 E. 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.

[0050] Those in which cracks occurred were judged as having SCC occurrence ("Good"), and those in which cracks did not occur were judged as having no SCC ("Poor"). The judgment results are also shown in Table 1 as the SCC occurrence status in actual liquid ammonia tanks.

[0051] [Table 1]

[0052] As can be seen from Table 1, in the actual liquid ammonia tank, SCC did not occur in steels A and B. On the other hand, SCC occurred in steels C to E.

[0053] [Ammonia SCC accelerated test] The ammonia SCC susceptibility of the steel specimens A to E, which had been 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.

[0054] Table 2 shows the test conditions for the ammonia SCC accelerated test. Test specimens measuring 5 mm thick x 15 mm wide x 115 mm long were taken from the 1 / 4 position of the plate thickness of test steels A to E, and ultrasonically degreasing was performed in acetone for 5 minutes. An external stress of 1.2 times the yield strength of each specimen (120% YS) was applied by four-point bending. Ammonium carbamate and O2 were introduced into the 2-L test cell containing the four-point bending specimens to the amounts shown in Table 2, and then liquid ammonia was added. The specific liquid volume was 42 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 one hour. After 168 to 2160 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 whether or not they were present.

[0055] 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.

[0056] The occurrence of SCC was assessed by conducting the immersion test 10 times for each test number, and if one or more cracks occurred throughout the entire test, it was assessed as "Yes" for SCC occurrence, and if no cracks occurred, it was assessed as "No" for SCC. For the results showing "Yes" for SCC occurrence, the probability of SCC occurrence was calculated. For the results showing "No" for SCC absence, the probability of occurrence was recorded as 0%.

[0057] For tests that showed a result of "×" (no SCC), evaluation accuracy was judged to be "good" if the SCC occurrence status was the same as the SCC occurrence status in an actual liquid ammonia tank for the same steel type shown in Table 1 ("×"). For tests that showed a result of "○" (occurrence of SCC), evaluation accuracy was judged to be "good" if the SCC occurrence status was the same as the SCC occurrence status in an actual liquid ammonia tank for the same steel type shown in Table 1 and the occurrence probability was 70% or higher. The occurrence probability is expressed as a percentage of the number of tests in which SCC occurrence was confirmed out of 10 immersion tests. Furthermore, for tests that were judged to have a "good" evaluation accuracy, tests with a test period of 720 hours or less were judged to have a "good" evaluation period. Tests other than those judged to have a "good" evaluation accuracy and evaluation period were judged to be "poor." Table 2 shows the accelerated test results including the SCC occurrence status, occurrence probability, evaluation accuracy, and evaluation period.

[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, which is the same as the environment simulating an actual liquid ammonia tank. On the other hand, in the comparative examples, the evaluation accuracy was "poor" or the evaluation period was "poor", and the ammonia SCC susceptibility could not be evaluated with good accuracy and 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 with high accuracy and in a short period of time. Furthermore, by using the test method of the present invention, it becomes 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 extremely 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 ammonia stress corrosion cracking resistance that can be suitably used for 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 0.002 to 0.200 bar containing 0.01 mass% or more ammonium carbamate. ２ and polarizing the specimen to a voltage exceeding +1.0 V relative to the corrosion potential.

2. 2. The accelerated ammonia stress corrosion cracking test method according to claim 1, wherein the surface of the metal test piece immersed in the liquid ammonia is cathodically polarized and then polarized to a potential of more than +1.0 V with respect to a corrosion potential.

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

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