Corrosion testing method for CrNi-based alloys

Abstract

To provide a method for testing corrosion of a CrNi-based alloy, which can perform evaluation in a short time, and has safety and reproducibility.SOLUTION: A corrosion test method for a CrNi-based alloy containing Cr of 24 mass% or more and 35 mass% or less, comprises: immersing a test piece into a corrosion test solution containing nitric acid of 45 mass% or more and 75 mass% and hydrofluoric acid of 0.005 mass% or more and less than 1.0 mass% for a certain time; and detecting intergranular corrosion in a CrNi-based alloy.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] This invention relates to a corrosion test method for CrNi-based alloys with a Cr concentration of 24% by mass or more and 35% by mass or less. by law be. [Background technology]

[0002] When higher corrosion resistance is required in nuclear power plants, 690-series alloys with improved stress corrosion cracking (SCC) resistance are used, with a chromium concentration of 27% to 31% by mass (see, for example, Patent Document 1). Furthermore, technologies for evaluating the corrosion resistance and SCC resistance of these alloys are being developed. As for intergranular corrosion tests, in addition to the Striker test and Huey test specified in JIS and the Corio test specified in JSME, a test method using a mixed acid of nitric acid and hydrofluoric acid (hereinafter referred to as nitrophilic acid) has been proposed (see, for example, Patent Documents 2 and 3). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-19981 [Patent Document 2] Japanese Patent Publication No. 2008-291281 [Patent Document 3] Japanese Patent Publication No. 2017-166007 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, the methods described in Patent Documents 2 and 3 above involve the addition of 2 to 3% hydrofluoric acid, requiring management in accordance with the Act on Promotion of Understanding and Management of Emissions of Chemical Substances. Furthermore, the test duration is also long, at 100 hours or 24 hours.

[0005] To solve the above-mentioned problems, the present invention provides a corrosion test method for CrNi-based alloys that can be evaluated in a short time, is safe, and has high reproducibility, and a reactor in-structure made of a CrNi-based alloy whose high corrosion resistance has been evaluated by this method. [Means for solving the problem]

[0006] The corrosion test method for CrNi-based alloys of the present invention is applicable to CrNi-based alloys containing 24% to 35% by mass of Cr. This corrosion test method involves immersing a test specimen in a corrosion test solution containing 45% to 75% by mass of nitric acid and 0.005% to less than 1.0% by mass of hydrofluoric acid for a certain period of time to detect intergranular corrosion of the CrNi-based alloy.

[0007] Furthermore, the reactor internal structure of the present invention is made of a CrNi-based alloy containing 24% to 35% by mass of Cr. The CrNi-based alloy exhibits a corrosion degree of 200 g / m² when immersed for 4.0 hours in a corrosion test solution containing 45% to 75% by mass of nitric acid and 0.005% to less than 1.0% by mass of hydrofluoric acid. 2 It is less than or equal to / h. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a corrosion test method for CrNi-based alloys that can be evaluated in a short time, is safe, and has high reproducibility, and a reactor in-structure made of a CrNi-based alloy whose high corrosion resistance has been evaluated by this method. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows a corrosion testing apparatus related to corrosion testing methods. [Figure 2] This figure shows the correlation between the degree of corrosion and mNBar obtained from a corrosion test at a nitric acid concentration of 45% by mass. [Figure 3] This figure shows the correlation between the degree of corrosion and mNBar obtained from a corrosion test at a nitric acid concentration of 55% by mass. [Figure 4]It is a diagram showing the correlation between the corrosion degree and mNBar by the corrosion test at a nitric acid concentration of 65 mass%. [Figure 5] It is a diagram showing the correlation between the corrosion degree and mNBar by the corrosion test at a nitric acid concentration of 75 mass%. [Figure 6] It is a diagram showing the surface observation result, corrosion degree, test error (%), and evaluation of the corrosion form of the test piece at a HF concentration of 0.01 mass%. [Figure 7] It is a diagram showing the surface observation result, corrosion degree, test error (%), and evaluation of the corrosion form of the test piece at a HF concentration of 0.1 mass%. [Figure 8] It is a diagram showing the surface observation result, corrosion degree, test error (%), and evaluation of the corrosion form of the test piece at a HF concentration of 1.0 mass%. [Figure 9] It is a diagram showing the relationship between the corrosion degree and the maximum crack depth of SCC. [Figure 10] It is a diagram showing the relationship between mNBar of the CrNi-based alloy and the Vickers hardness. [Figure 11] It is a diagram showing the relationship between the Nb addition amount of the CrNi-based alloy and the Vickers hardness.

Embodiments for Carrying out the Invention

[0010] [[ID=*29]]Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. The present invention is not limited to the embodiments described here, and it is possible to appropriately combine with known technologies or improve based on known technologies without departing from the technical idea of the invention.

[0011] [Outline of CrNi-based Alloy and Corrosion Test Method] The corrosion test method for CrNi-based alloys according to the present disclosure is a test method for evaluating the corrosion resistance characteristics, particularly the intergranular corrosion resistance characteristics, of CrNi-based alloys containing 24% by mass or more and 35% by mass or less of Cr. The CrNi-based alloy according to the present disclosure is a Ni-Cr-Fe-based alloy having a chemical composition equivalent to a 690 series alloy mainly composed of Ni, and in addition to the main component Ni, it contains at least 24 to 35% by mass of Cr and one or more of Nb, Ta, and Ti.

[0012] Such CrNi-based alloys are used, for example, in nuclear power plants where high reliability is required, and high corrosion resistance is required. In nuclear power plants, pipes and structures are exposed to high-temperature and high-pressure reactor water reaching about 300 °C for a long period of time. In addition, the reactor water contains dissolved oxygen and various radicals that promote corrosion under the action of radiation. In particular, there is concern about the occurrence of stress corrosion cracking (SCC) in the welded parts inside the reactor due to the influence of welding residual stress and the corrosion environment. Therefore, the CrNi-based alloys used in nuclear power plants require management by evaluating their corrosion resistance and SCC resistance.

[0013] The SCC resistance of CrNi-based alloys in a nuclear reactor can be evaluated, for example, using a low-strain bending test (Creviced Bent Beam test: CBB test). However, since this CBB test is conducted in a high-temperature and high-pressure water environment, it is necessary to use dedicated equipment such as an autoclave. Furthermore, the CBB test requires a very long test time of several hundred hours or more. Therefore, in the present disclosure, the evaluation of the SCC resistance of CrNi-based alloys is performed based on the intergranular corrosion resistance characteristics that are correlated with the SCC resistance.

[0014] The intergranular corrosion resistance properties of Ni-based alloys are evaluated using the Stryker test, specified in JIS G 0572 "Test Method for Sulfuric Acid and Ferric Sulfate Corrosion of Stainless Steel," and the Huey test, specified in JIS G 0573 "Test Method for 65% Nitrate Corrosion of Stainless Steel." Additionally, the Corio test, specified in JSME "Reprocessing Equipment Standards and Design Specifications," is also used. However, the Stryker test is insensitive to CrNi-based alloys with high Cr concentrations. The Huey test has a long test duration of 48 hours. Furthermore, the Corio test uses highly toxic hexavalent Cr, raising safety concerns. Furthermore, as mentioned above, a test method for intergranular corrosion resistance using nitrate-hydrofluoric acid, which is a nitric acid solution to which 2-3% hydrofluoric acid has been added, has been proposed, but the test time is long, at 100 hours or 24 hours.

[0015] Therefore, the corrosion test method for CrNi-based alloys according to this disclosure uses a corrosion test solution containing 45% to 75% by mass of nitric acid and 0.005% to less than 1.0% by mass of hydrofluoric acid. Then, a test specimen is immersed in this corrosion test solution for a certain period of time to detect intergranular corrosion in the corrosion test method for CrNi-based alloys.

[0016] By adding 0.005% by mass or more and less than 1.0% by mass of hydrofluoric acid, the hydrofluoric acid concentration can be reduced, thus exempting it from the regulations of the Chemical Substances Release and Discharge Promotion Act. Furthermore, as a corrosion test solution for CrNi-based alloys, an appropriate amount of hydrofluoric acid can be added relative to the nitric acid concentration, enabling safe and highly reproducible corrosion tests.

[0017] Furthermore, in corrosion tests using the above-mentioned corrosion test solution, intergranular corrosion of CrNi-based alloys is detected by measuring the degree of corrosion as the change in mass per unit area of ​​the test specimen per unit time. By determining the degree of corrosion from the change in mass per unit area of ​​the CrNi-based alloy test specimen, quantitative measurement of intergranular corrosion of CrNi-based alloys becomes possible.

[0018] Furthermore, in the corrosion test described herein, the corrosion test solution is heated, and a CrNi-based alloy test piece is immersed in the corrosion test solution at a temperature between 90°C and 220°C. By heating the corrosion test solution to the above temperature, it becomes possible to evaluate the degree of corrosion in a shorter time, for example, 2 to 4 hours.

[0019] In CrNi-based alloys applicable to the corrosion test of this disclosure, it is preferable that one or more of Nb, Ta, and Ti are added in a concentration range limited based on the stabilization parameter (mNBar) shown in formula (I) described later. Specifically, it is preferable to apply the corrosion test to CrNi-based alloys with an mNBar of 2 or more and 35 or less. By limiting the mNBar of the CrNi-based alloy to a range in which there is a high correlation between the degree of corrosion obtained by the corrosion test and the SCC resistance of the CrNi-based alloy, it becomes possible to evaluate intergranular corrosion resistance and SCC resistance with greater reproducibility and accuracy.

[0020] As described above, the intergranular corrosion properties obtained by the corrosion test method described herein correlate with the SCC resistance of CrNi-based alloys. For this reason, the degree of corrosion obtained by the above corrosion test method and the maximum SCC crack depth of CrNi-based alloys obtained by a gap-filled low-strain bending test are measured in advance, and the [corrosion-SCC property] showing the relationship between the degree of corrosion and the maximum SCC crack depth of CrNi-based alloys is determined. For example, the degree of corrosion and the maximum SCC crack depth are determined for multiple CrNi-based alloys with different compositions, and the correlation between the degree of corrosion and the maximum SCC crack depth is determined as the [corrosion-SCC property]. In this case, it is preferable to determine the [corrosion-SCC property] for each composition of the corrosion test solution. Then, by applying the degree of corrosion of the CrNi-based alloy obtained by the above corrosion test to the [corrosion-SCC properties], the SCC resistance can be estimated. In this way, the corrosion test of this disclosure allows for the evaluation of both the degree of corrosion and the SCC resistance of the CrNi-based alloy.

[0021] In the corrosion test method of this disclosure, it is preferable to use at least one of a fluororesin container and a fluororesin-coated container as the container for introducing the corrosion test solution. Furthermore, it is preferable to use at least one of a fluororesin condenser and a glass condenser with a fluororesin coating as the cooler (condenser) for cooling and condensing (liquefying) the corrosion test solution that has been vaporized by heating. It is preferable that the fluororesin or fluororesin coating used in the above-mentioned test apparatus is at least one of polytetrafluoroethylene (PTFE) and perfluoroalkoxyalkane (PFA). In the corrosion test method described above, hydrofluoric acid is added to the corrosion test solution. Therefore, by using the above-described test apparatus, the elution of Si into the corrosion test solution can be suppressed compared to glass test apparatus commonly used in Stryker and Huey tests. As a result, corrosion tests with high accuracy and reproducibility can be performed.

[0022] Furthermore, by evaluating the corrosion using the above-described corrosion test method, a CrNi-based alloy with excellent resistance to intergranular corrosion and SCC can be obtained. For example, if the corrosion degree after immersion in the above corrosion test solution for 4.0 hours is 200 g / m² 2 If the value is less than / h, CrNi-based alloys can be applied to reactor internal structures in nuclear power plants where high reliability is required. In particular, if the maximum crack depth of SCC, determined using the [corrosion-SCC characteristics] described above, based on the degree of corrosion of the CrNi-based alloy obtained in the corrosion test described above, is 50 μm or less, then it is suitable for application to structures inside a nuclear reactor.

[0023] [Description of the testing equipment] Next, the test equipment (test instruments) used in the corrosion test method of this disclosure will be described. Figure 1 shows a corrosion testing apparatus related to the corrosion testing method of this disclosure.

[0024] The corrosion test apparatus shown in Figure 1 comprises a corrosion test solution 3, a CrNi-based alloy test piece 6 immersed in the corrosion test solution 3, and a test container 1 containing a jig 7 for holding the test piece 6. The corrosion test apparatus also includes a heating device 5 for heating the test container 1 and the corrosion test solution 3. In Figure 1, an example is shown in which an oil bath is used as the heating device 5, and a heat transfer medium 4 such as silicone oil is used. Furthermore, the corrosion test apparatus has a cooler (condenser) 2 located above the test container 1, through which cooling water 8 circulates to cool and liquefy the vaporized steam from the corrosion test solution 3 and return it to the test container 1.

[0025] The hydrofluoric acid used as corrosion test solution 3 in this test has the effect of dissolving glass due to the influence of hydrofluoric acid. For this reason, test container 1 and condenser 2 should be made of corrosion-resistant materials such as fluororesin, or glass or resin containers coated with fluororesin.

[0026] Furthermore, it is preferable to use a heating device 5 for heating the corrosion test solution 3 in the corrosion test apparatus. Generally, corrosion reactions proceed faster at higher ambient temperatures. For this reason, during the corrosion test, the corrosion test solution 3 is held in the test container 1 and maintained at a high temperature using the heating device 5. Thus, it is preferable to raise the temperature of the corrosion test solution 3 using the heating device 5 and conduct the test at a high temperature as a condition for the corrosion test.

[0027] In particular, conducting the test under conditions where the corrosion test solution 3 is boiling makes it easier to adjust the temperature of the heating device 5. Furthermore, by bringing the corrosion test solution 3 to a boil, bubbles are generated within the corrosion test solution 3, and these bubbles agitate the inside of the corrosion test solution. As a result, the solution around the test piece 6 circulates, the corrosion reaction becomes more uniform, and the reproducibility of the corrosion test is improved. Therefore, it is preferable to use a heating device 5 that can heat the corrosion test solution 3 to a boiling state in the corrosion test apparatus.

[0028] In addition, the corrosion test solution 3 may be forcibly stirred and circulated using a stirring device, such as a magnetic stirrer, along with the heating device 5 described above. For this reason, it is preferable to use an oil bath with a stirrer as the heating device 5. In particular, if the corrosion test solution 3 is not brought to a boil, the corrosion test solution 3 will stagnate and the reproducibility will decrease, so the use of a stirring device is effective.

[0029] When heating corrosion test solution 3 to a boiling point, the boiling point of nitric acid varies from approximately 110 to 122°C depending on the nitric acid concentration. At this time, it is necessary to set the temperature of the heating device 5 to around 200°C. For this reason, it is preferable to use PTFE (polytetrafluoroethylene) or PFA (perfluoroalkoxyalkane), which have excellent heat resistance among fluororesins, for the test container 1, jig 7, etc.

[0030] Furthermore, when the nitrate-hydrofluoric acid in corrosion test solution 3 is heated to a high temperature, corrosion test solution 3 vaporizes. Therefore, by using the cooler 2 to cool and liquefy the water vapor of the vaporized solution and return it to the test container 1, it is possible to prevent changes in the concentration of corrosion test solution 3 due to vaporization. The cooler 2 has a structure in which cooling water 8 circulates inside. To check the condition of the cooling water 8 inside the cooler 2, it is preferable to use a cooler 2 made of highly transparent fluororesin or glass coated with highly transparent fluororesin.

[0031] As shown in Figure 1, the test specimen 6 used for the corrosion test is held in a jig 7 and immersed in the corrosion test solution 3. At this time, the liquid ratio is 0.5 m 3 / m 2 The above is preferable. Furthermore, as shown in Figure 1, when a jig 7 for fixing the CrNi-based alloy test piece 6 is placed in the corrosion test solution 3, this jig 7 should also be a fluororesin jig or a jig made of glass or resin with a fluororesin coating.

[0032] In Figure 1, an example is shown in which an oil bath is used as the heating device 5. However, other devices with various heating functions, such as band heaters or stirrers with heaters, can be used as the heating device 5. In corrosion tests, it is desirable to heat the corrosion test solution 3 uniformly, so it is preferable to use an oil bath as the heating device 5, as it can provide a uniform heat source.

[0033] [Explanation of corrosion test solution] Corrosion test solution 3, used in the corrosion test, contains a mixed acid (nitrate-hydrofluoric acid) of nitric acid and hydrofluoric acid. The components of corrosion test solution 3 other than nitric acid and hydrofluoric acid are pure water, and it may also contain unavoidable impurities.

[0034] In corrosion test solution 3, the nitric acid concentration is between 45% by mass and 75% by mass. In particular, if the nitric acid concentration exceeds 69% by mass, it becomes fuming nitric acid, emitting smoke in the air, making it difficult to control the concentration of the corrosion test solution and requiring careful handling. For this reason, the nitric acid concentration of corrosion test solution 3 is preferably less than 69% by mass.

[0035] In corrosion test solution 3, the hydrofluoric acid concentration is 0.005% by mass or more and less than 1.0% by mass. If the hydrofluoric acid concentration exceeds 1.0% by mass, management in accordance with the Chemical Substances Release and Discharge Regulation Law becomes necessary, and careful handling is required. For this reason, a hydrofluoric acid concentration of less than 1.0% by mass in corrosion test solution 3 is preferable.

[0036] [In-reactor structures using nickel-based alloys with excellent resistance to SCC (Steel Crusher Carcinoma)] As described above, the intergranular corrosion properties obtained by the corrosion test method described herein correlate with the SCC resistance of CrNi-based alloys. Therefore, by applying the corrosion test method described herein, it is possible to select CrNi-based alloys with excellent SCC resistance.

[0037] CrNi-based alloys evaluated as having excellent SCC resistance by the corrosion test method of this disclosure are suitable for use as structural materials for nuclear power plants. In particular, they are suitable for in-reactor structures of nuclear reactors and can be used in parts that come into contact with high-temperature, high-pressure reactor water. In-reactor structures using CrNi-based alloys may be welded structures including the base material and welded joints. Furthermore, the welded joints may be machined or ground to remove the hardened surface layer. In addition, welded joints may be subjected to peening or polishing treatments.

[0038] Specific reactor structures in which CrNi-based alloys are used include, for example, the bottom and lower end of the reactor pressure vessel through which neutron instrumentation detector tubes and control rod guide tubes pass, as well as the cladding portion of the lower end of the reactor pressure vessel. Other examples include the support portion of the core shroud, which is composed of the shroud, shroud support cylinder, shroud support leg, and shroud support plate, as well as nozzle portions of feedwater inlets and recirculation water inlets. As described above, by using the corrosion tests related to this disclosure, it is possible to predict the SCC resistance of CrNi-based alloys, select material conditions for CrNi-based alloys with good corrosion resistance, and apply them to structures inside nuclear reactors. [Examples]

[0039] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to these examples. A corrosion test of CrNi-based alloys was conducted using the test equipment configured as shown in Figure 1 above. The corrosion test of CrNi-based alloys was performed using the following materials and following the procedure below.

[0040] [material] (Alloy composition; stabilization parameters) As samples of CrNi-based alloys used in the above corrosion tests, alloys TP1 to TP5, whose chemical compositions are shown in Table 1, were prepared. TP1 to TP4 are CrNi-based alloys containing 24% to 35% by mass of Cr, based on 690 series alloys. TP5 is a CrNi-based alloy based on a 600 series alloy with a lower Cr concentration. TP5 is a comparative sample used to verify the applicable range of Cr concentration for CrNi-based alloys suitable for corrosion tests.

[0041] [Table 1]

[0042] Furthermore, the SCC resistance of the TP1 to TP5 alloys shown in Table 1 was controlled by the stabilization parameter (mNBar) shown in the following formula (I). These alloys were fabricated in a vacuum melting furnace, hot forged at 1200°C, and then subjected to solution heat treatment by holding at 1150°C for 1 hour before water cooling.

[0043]

number

[0044] In equation (I), [Nb] is the Nb concentration (mass%), [Ta] is the Ta concentration (mass%), [Ti] is the Ti concentration (mass%), [C] is the C concentration (mass%), [Cr] is the Cr concentration (mass%), [Cr0] is the critical Cr concentration (=14 mass%), and a is the fitting parameter (=0.5).

[0045] (Heat treatment of alloys) For alloys TP1 to TP5, post-weld heat treatment was performed under conditions mainly used for welded areas. In the corrosion test of this embodiment, in order to evaluate the change in intergranular corrosion resistance due to thermal sensitization of CrNi-based alloys with high Cr concentration, sensitization heat treatment was performed after the post-weld heat treatment. Table 2 shows the treatment conditions for the post-weld heat treatment and the sensitization heat treatment.

[0046] [Table 2]

[0047] [Preparation of corrosion test specimens] Using heat-treated alloys TP1 to TP5, strip-shaped test specimens measuring 40 × 10 × 3 mm were prepared. The surfaces of the test specimens were polished, and finally finished with water-resistant emery paper #320. Next, they were degreased by ultrasonic cleaning in acetone for 5 minutes. After that, the dimensions of the test specimens were measured using calipers and a micrometer. Furthermore, the mass of the test specimens was measured using an electronic balance. Immediately before testing, the test specimens were ultrasonically cleaned in ethanol for 5 minutes.

[0048] [Test conditions] Corrosion test solutions were prepared by adding 0.005% by mass, 0.01% by mass, 0.1% by mass, or 1.0% by mass of hydrofluoric acid to solutions with nitric acid concentrations of 45% by mass, 55% by mass, 65% by mass, or 75% by mass. The prepared corrosion test solution was placed in a fluororesin test container, and a glass cooler coated with fluororesin was attached to the top of the test container. Furthermore, the test specimens were fixed with fluororesin wires, suspended from a cooler, and immersed in the corrosion test solution. The corrosion test solution volume was 0.9 liters, and the specific liquid ratio was 0.8 m³ on average for all test specimens. 3 / m 2 That was the case.

[0049] The corrosion test solution was heated using an oil bath after the test specimen was placed inside. The oil bath was set to 180°C, bringing the corrosion test solution to a boil. Since the boiling point of the corrosion test solution varies depending on the concentration of the solution, the boiling point was confirmed in advance. The solution temperature during the test was measured using a thermocouple, and the holding time was defined as the point when the temperature reached 1°C below the boiling point. The holding time was 4.0 hours from the start of holding until the start of cooling. After 4.0 hours had elapsed from the start of holding, the oil bath power was turned off and cooling began. The number of repetitions for each condition was 7.

[0050] [Evaluation: degree of corrosion, corrosion morphology] After the test, the test specimens were ultrasonically cleaned in pure water for 10 minutes to remove any deposits and detached crystal grains from the surface. Then, they were ultrasonically cleaned in ethanol for 5 minutes, and after drying, the mass of the test specimens was measured using an electronic balance. Then, the change in mass per unit area per unit time was evaluated from the surface area before the test and the change in mass before and after the test. This change in mass per unit area per unit time was defined as the corrosion rate [g / m²]. 2 Defined as / h] Furthermore, to evaluate the corrosion morphology of the test specimens, the surface of the specimens was observed using a microscope.

[0051] [Correlation between corrosion degree and mNBar] Figures 2-5 show the results of tests conducted under the above test conditions using corrosion test solutions prepared by adding 0.01% by mass, 0.1% by mass, or 1.0% by mass of hydrofluoric acid to nitric acid solutions with nitric acid concentrations of 45% by mass, 55% by mass, 65% by mass, or 75% by mass. In Figures 2-5, the horizontal axis represents the mNBar of the alloys TP1-TP5, and the vertical axis represents the degree of corrosion (Corrosion rate) [g / m²]. 2 This indicates / h] Figure 2 shows the corrosion test results for specimens TP1 to TP5 obtained using corrosion test solutions in which the nitric acid concentration was fixed at 45% by mass and the added hydrofluoric acid (HF) concentration was 0.01% by mass, 0.1% by mass, or 1.0% by mass. Similarly, Figure 3 shows the corrosion test results for specimens TP1 to TP5 obtained using corrosion test solutions in which the nitric acid concentration was fixed at 55% by mass, Figure 4 shows the nitric acid concentration at 65% by mass, and Figure 5 shows the nitric acid concentration at 75% by mass, and the added hydrofluoric acid (HF) concentration was 0.01% by mass, 0.1% by mass, or 1.0% by mass. In Figures 2-5, TP1-TP4, with a Cr concentration of approximately 30% by mass, and TP5, a comparative example with a Cr concentration of approximately 20% by mass, are shown with different symbols.

[0052] As shown in Figures 2-5, in the alloys TP1-TP5, the degree of corrosion increased with increasing amounts of hydrofluoric acid added, regardless of the nitric acid concentration. Also, in the alloys TP1 to TP4 with a Cr concentration of about 30% by mass, under any conditions, when mNBar was in the range of 2 to 14, the corrosion rate decreased as the addition amount of hydrofluoric acid increased. Further, in the range of mNBar from 2 to 14 (TP1 to TP3), the corrosion rate decreased as mNBarm increased. In particular, between mNBar of 10 (TP2) and 14 (TP3), the corrosion rate decreased steeply.

[0053] On the other hand, when mNBar increased from 14 (TP3) to 35 (TP4), the corrosion rate turned to increase and the corrosion resistance decreased. At this time, it was clarified that the increase amount of the corrosion rate when the addition amount of hydrofluoric acid was 0.01% by mass was smaller than that when the addition amounts of hydrofluoric acid were 1.0% by mass and 0.1% by mass regardless of the nitric acid concentration.

[0054] Also, in the case of TP5 with a Cr concentration of about 20% by mass, mNBar = 14, but the corrosion rate was higher than that of TP3 (mNBar = 14.1) with a Cr concentration of about 30% by mass. This result was obtained at all nitric acid concentrations and all addition amounts of hydrofluoric acid.

[0055] Thus, the corrosion rate obtained in the corrosion test of the present disclosure has a correlation with the stabilization parameter mNBar. Specifically, in CrNi-based alloys with mNBar of 14 or less, the corrosion rate decreases as mNBar increases. And a correlation was obtained that when mNBar of the CrNi-based alloy exceeds a certain value, the corrosion rate turns to increase. By controlling the composition of the CrNi-based alloy with mNBar, the above-mentioned corrosion test can be stably and reproducibly performed.

[0056] [Observation results of corrosion morphology and test error] Next, after the above corrosion test, the surface observation results of the test piece before and after the corrosion test, and the corrosion rate (g / m 2The quantitative values ​​of ( / h), test error (%), and evaluation of the corrosion morphology are shown. Figure 6 shows the results of a corrosion test using a corrosion test solution prepared by adding 0.01% by mass of hydrofluoric acid to 65% by mass of nitric acid. Similarly, Figure 7 shows the results of a corrosion test using a corrosion test solution prepared by adding 0.1% by mass of hydrofluoric acid to 65% by mass of nitric acid, and Figure 8 shows the results of a corrosion test using a corrosion test solution prepared by adding 1.0% by mass of hydrofluoric acid to 65% by mass of nitric acid.

[0057] As shown in Figure 6, when the hydrofluoric acid concentration was 0.01% by mass, intergranular corrosion and general corrosion occurred in all test specimens except for TP3 (mNBar=14.1). TP3 showed high resistance to intergranular corrosion, with only general corrosion observed in the test specimen, and no intergranular corrosion was detected. As shown in Figure 7, when the hydrofluoric acid concentration was 0.1% by mass, intergranular corrosion was observed on the surface of all test specimens TP1 to TP5 after testing, and grain shedding was observed in all test specimens except TP3. In this case, intergranular corrosion was observed only in some areas of the TP3 test specimen, and no grain shedding occurred. As shown in Figure 8, when the hydrofluoric acid concentration was 1.0 mass%, all of the TP1 and TP2 specimens, which had low mNBar values, dissolved. Furthermore, the TP3 specimen showed no intergranular corrosion and exhibited overall corrosion. The TP4 and TP5 specimens showed both overall and intergranular corrosion.

[0058] Furthermore, as shown in Figures 6-8, the test error [(2σ / mean) × 100%] of the degree of corrosion was examined for each test specimen TP1-TP5 in each corrosion test solution. The test error of the degree of corrosion tended to increase as the amount of hydrofluoric acid added increased. However, the maximum test error was 20% or less. Immersion tests, such as the corrosion test method described above, often have large error margins, sometimes exceeding several tens of percent. Therefore, the above corrosion test method has a smaller error margin compared to conventional corrosion tests.

[0059] Therefore, as shown in Figures 6-8, by using the above corrosion test method, intergranular corrosion of a CrNi-based alloy with a high Cr concentration of approximately 30% by mass could be detected in a short time of 4.0 hours. Furthermore, the degree of corrosion obtained also showed a sufficiently small test error. For this reason, the corrosion test method of this disclosure allows for testing of intergranular corrosion resistance characteristics with greater accuracy and in a shorter time compared to conventional testing methods for CrNi-based alloys such as the Huey test.

[0060] [Correlation between corrosion degree and susceptibility to SCC occurrence] Next, we will explain the relationship between the degree of corrosion obtained from the above corrosion tests and the susceptibility of CrNi-based alloys to SCC (Screw Cracking) formation. Figure 9 shows the relationship between the degree of corrosion and the maximum SCC crack depth. The maximum SCC crack depth indicates the susceptibility to SCC formation and was evaluated by the creased bent beam test (CBB test). Figure 9 shows the degree of corrosion of test specimens TP1 to TP5 using a corrosion test solution of 65% by mass nitric acid with 0.1% by mass hydrofluoric acid added, with the vertical axis representing the maximum crack depth [μm] of SCC determined by the CBB test described below.

[0061] (CBB test) The CBB test was performed in an accelerated environment on a test specimen measuring 10 mm in width, 50 mm in length, and 2 mm in thickness. The test specimen was fitted into a test fixture with a gap created by graphite wool, and then subjected to bending strain. The test conditions were accelerated conditions with a temperature of 288°C, a dissolved oxygen concentration of 40 ppm, a hydrogen peroxide concentration of 20 ppm, and an electrical conductivity of 20 μS / cm. The test duration was 4000 hours, and after the test, the crack depth was measured by microscopic observation of the central cross-section of the test specimen.

[0062] CBB testing revealed that grain boundary type SCC occurred in TP1-TP3 and TP5, excluding TP4. On the other hand, TP4 exhibited intragranular cracking. The occurrence of both grain boundary type SCC and intragranular cracking is attributed to the hardness of the CrNi-based alloy.

[0063] (Vickers hardness) Figure 10 shows the relationship between mNBar and Vickers hardness of CrNi-based alloys. In Figure 10, the vertical axis represents Vickers hardness, and the horizontal axis represents the mNBar of CrNi-based alloys (TP1 to TP5). As shown in Figure 10, except for TP5, which has a Cr concentration of about 20 mass%, Vickers hardness increases with increasing mNBar in TP1 to TP4, which have a Cr concentration of about 30 mass%. Therefore, as shown in Figure 10, TP4, which has a large mNBar, has a high Vickers hardness and thus experienced intragranular cracking. In contrast, for alloys other than TP4, grain boundary type SCC occurred due to their low Vickers hardness.

[0064] Furthermore, Figure 11 shows the relationship between the amount of Nb added to CrNi-based alloys and Vickers hardness. In Figure 11, the vertical axis represents Vickers hardness, and the horizontal axis represents the amount of Nb added (mass%) to CrNi-based alloys (TP1 to TP5). As shown in Figure 11, a relationship can be observed between the amount of Nb added to a CrNi-based alloy and its hardness. According to this relationship between Nb content and hardness, the Vickers hardness begins to increase with increasing Nb content once the Nb content exceeds approximately 3.7 mass%. Here, in the case where the amount of Nb added is approximately 3.7% by mass, if we calculate the mNBar for the average values ​​of the Cr, C, Ta, and Ti concentrations from TP1 to TP4, the mNBar is approximately 25. Therefore, when adjusting the mNBar of a CrNi-based alloy by the amount of Nb added, the CrNi-based alloy exhibits high corrosion resistance in the range of 14 to 25 mNBar. Furthermore, if Nb is added to a value exceeding 25 mNBar, the corrosion resistance of the CrNi-based alloy may decrease. For this reason, it is preferable to adjust the amount of Nb added to the CrNi-based alloy so that the mNBar is less than 25.

[0065] (Maximum crack depth of SCC) Returning to the explanation of Figure 9, focusing on the correlation between the degree of corrosion and the maximum crack depth of SCC, it was shown that the maximum crack depth of SCC, i.e., the susceptibility to SCC, increases with increasing degree of corrosion. In this case, if the maximum crack depth of SCC is 50 μm or less, the crack is unlikely to propagate further, and therefore, it can be evaluated as having high resistance to SCC. As shown in Figure 9, the maximum crack depth of SCC is 50 μm or less when the corrosion degree is 220 g / m². 2 This was the case when the rate was less than / h. Therefore, the corrosion degree obtained from the above corrosion test was 220 g / m 2 CrNi-based alloys with a density of 200 g / m² or less can be evaluated as having excellent resistance to SCC. Preferably, from the range for which data has been confirmed, 200 g / m². 2 CrNi-based alloys with a minimum of / h can be evaluated as having excellent resistance to SCC (Steel Chain Crushing).

[0066] Thus, the degree of corrosion obtained by the above corrosion test method correlates with susceptibility to SCC (Steel Chloride Crust) formation. Therefore, by evaluating the degree of corrosion using the above corrosion test method, the SCC resistance of CrNi-based alloys can be predicted. Note that both SCC susceptibility and corrosion degree vary depending on the test conditions. Therefore, we focus on the fact that when the maximum crack depth of SCC is 50 μm or less, the crack is in a pre-propagation stage. Then, we select material conditions (CrNi-based alloy composition, mNBar, etc.) in the estimated corrosive environment (corrosion test solution, temperature, etc.) where the SCC susceptibility is 50 μm or less. By applying the above corrosion test to these material conditions and evaluating the corrosion degree, we can estimate the SCC resistance under those material conditions.

[0067] Specifically, the degree of corrosion obtained by the above corrosion test method and the maximum SCC crack depth of the CrNi-based alloy obtained by a gap-filled low-strain bending test are measured in advance, and the [corrosion-SCC property] representing the correlation between the degree of corrosion and the maximum SCC crack depth of the CrNi-based alloy, as shown in Figure 9, is determined. For example, the degree of corrosion and the maximum SCC crack depth are determined for multiple CrNi-based alloys with different compositions, and the correlation between the degree of corrosion and the maximum SCC crack depth is determined as the [corrosion-SCC property]. At this time, it is preferable to determine the [corrosion-SCC property] of the CrNi-based alloy, as shown in Figure 9, for each composition of the corrosion test solution and test temperature conditions. Subsequently, the corrosion test described above is applied to the compositional conditions of the CrNi-based alloy to evaluate the degree of corrosion. Then, by applying the obtained degree of corrosion to the [corrosion-SCC properties], the SCC resistance of the CrNi-based alloy can be estimated.

[0068] Furthermore, under the corrosive environmental conditions (corrosion test solution, temperature, etc.) of the CrNi-based alloy, the composition and mNBar of the CrNi-based alloy that result in a maximum SCC crack depth of 50 μm or less are determined from the [corrosion-SCC characteristics]. Thereafter, when evaluating the SCC resistance of a new CrNi-based alloy, if the degree of corrosion of this CrNi-based alloy is below the degree of corrosion of the composition conditions previously determined to have excellent SCC resistance, then the SCC resistance of that new CrNi-based alloy can be judged to be high. By utilizing these composition conditions that result in a maximum SCC crack depth of 50 μm or less, it is possible to predict the susceptibility to SCC occurrence of new CrNi-based alloys through the short-term corrosion test described above.

[0069] Furthermore, the above corrosion test makes it possible to efficiently corrode only the grain boundaries in CrNi-based alloy specimens. The degree of corrosion due to grain boundary corrosion depends on the corrosion length, corrosion width, and corrosion depth of the grains. Therefore, even if cross-sectional observation is performed after the corrosion test and the corrosion is evaluated based on the corrosion width and corrosion length of the grains instead of the degree of corrosion, results equivalent to those in Figure 9 can be obtained. [Examples]

[0070] Next, the effect of test time on the degree of corrosion in the above corrosion test method was investigated. Under the same conditions as in Example 1, a CrNi-based alloy specimen was used as a corrosion test solution with TP1, a nitric acid concentration of 65% by mass, and a hydrofluoric acid concentration of 0.1% by mass. The effect of changing the test time to 4, 8, and 24 hours was investigated. In the corrosion test, the corrosion test solution was kept at a boiling state. Table 3 shows the measurement results for test time, degree of corrosion, and test error.

[0071] [Table 3]

[0072] As shown in Table 3, the degree of corrosion tended to decrease with increasing test time. This is thought to be due to the protective effect of the corrosion products generated by the corrosion. Furthermore, the test error increased with increasing test time. Under the test conditions of Example 2 described above, grain shedding occurred in the CrNi-based alloy specimen, similar to Example 1. Therefore, as the test time increased, the shape change of the specimen became larger due to grain shedding. It is thought that the test error increased because the shape change of the specimen became larger with increasing test time. Based on these results, the above corrosion test method allows for the detection of intergranular corrosion with high accuracy by setting the test time to 4 hours.

[0073] The embodiments and examples described above are explained to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace some of the configurations of the embodiments with configurations that are common knowledge to those skilled in the art, and it is also possible to add configurations that are common knowledge to those skilled in the art to the configurations of the embodiments. In other words, the present invention allows for deletion, substitution with other configurations, and addition of other configurations to some of the configurations of the embodiments and examples specified herein, as long as it does not depart from the technical spirit of the invention. [Explanation of symbols]

[0074] 1. Test container, 2. Cooler, 3. Corrosion test solution, 4. Heat transfer medium, 5. Heating equipment, 6. Test specimen, 7. Fixture, 8. Cooling water

Claims

1. A corrosion test method for CrNi-based alloys containing 24% by mass or more and 35% by mass or less of Cr, A test specimen is immersed for a certain period of time in a corrosion test solution containing 45% to 75% by mass of nitric acid and 0.005% to less than 1.0% by mass of hydrofluoric acid, and the degree of corrosion, which is the change in mass per unit area of ​​the test specimen per unit time, is measured. The relationship between the maximum crack depth of stress corrosion cracking (SCC) of the CrNi-based alloy obtained by a gap-filled low-strain bending test and the degree of corrosion of the CrNi-based alloy, known as the [corrosion-SCC property], was determined. The stress corrosion cracking of the CrNi-based alloy is estimated from the aforementioned degree of corrosion and [corrosion-SCC properties]. Corrosion testing method for CrNi-based alloys.

2. Stabilization parameter [mNBar = (0.065 × ((Nb + Ta + 2Ti) / C) × (Cr / 14)] １．８９ The corrosion degree of the CrNi-based alloy is measured between 2 and 35. The corrosion test method for a CrNi-based alloy according to claim 1.

3. The temperature of the corrosion test solution is heated to between 90°C and 220°C. The corrosion test method for a CrNi-based alloy according to claim 1.

4. The testing equipment is, The system uses at least one of the following: a container made of fluororesin for pouring the corrosion test solution, and a container coated with fluororesin; and at least one of the following: a condenser made of fluororesin, and a condenser made of glass coated with fluororesin. The corrosion test method for a CrNi-based alloy according to claim 1.

5. The fluororesin or the fluororesin coating is at least one of polytetrafluoroethylene (PTFE) and perfluoroalkoxyalkane (PFA). The corrosion test method for a CrNi-based alloy according to claim 4.

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