Hydrogen permeation current measuring device, hydrogen permeation current measuring method, and delayed breakdown characteristic evaluation method

Abstract

This invention provides a hydrogen permeation current measuring device that can accurately measure hydrogen permeation current while tensile stress is applied to a metal material. [Solution] A hydrogen permeation current measuring device comprising a metal material as a test subject, a jig for applying tensile stress to the metal material, and an electrochemical cell disposed on one side of the metal material, wherein the tensile strength σ (MPa) of the metal material and the thickness t (mm) and width W (mm) of the metal material in the portion where the electrochemical cell is installed satisfy the following equation (1). σ × t × W ≤ 10000 …(1)

Description

[Technical Field]

[0001] The present invention relates to a hydrogen permeation current measuring device, a hydrogen permeation current measuring method, and a delayed breakdown characteristic evaluation method. [Background technology]

[0002] From the perspective of energy and resource conservation, the strength of steel materials used in various industrial fields is increasing. Steel sheets used in automobiles are also becoming stronger; for example, high-strength steel sheets with a tensile strength exceeding 1470 MPa are being applied to automotive components.

[0003] Incidentally, delayed fracture can occur in high-strength steel materials. Delayed fracture is a phenomenon in which a metallic material, under static load stress (load stress less than or equal to its tensile strength), suddenly undergoes brittle fracture after a certain period of time, with little apparent plastic deformation. Delayed fracture in steel materials is more likely to occur as the strength of the steel increases, and is particularly more likely to occur in high-strength steel materials with a tensile strength of 1180 MPa or higher.

[0004] Delayed fracture in steel materials is influenced by both applied stress and hydrogen embrittlement. For example, when steel materials are formed by press working, residual stress generated by the processing and hydrogen embrittlement in the processed area where residual stress is concentrated influence delayed fracture.

[0005] The hydrogen that causes hydrogen embrittlement is, in most cases, hydrogen that penetrates and diffuses into the steel material from the external environment (intrusive hydrogen). For example, in a corrosive environment, hydrogen generated by the corrosion of the steel material penetrates and diffuses into the steel material, causing hydrogen embrittlement.

[0006] To date, much research has been conducted on methods for evaluating the delayed fracture characteristics of various metallic materials, including steel.

[0007] For example, Patent Document 1 proposes a method for measuring the amount of hydrogen generated and penetrating into the metal interior of a metal specimen due to corrosion, using an electrochemical hydrogen permeation method.

[0008] Furthermore, Patent Document 2 proposes a method in which a test specimen, which has been subjected to a U-bend process to apply a load stress, is charged with hydrogen in an electrolyte solution, and the time until the test specimen fractures is measured. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2013-044728 [Patent Document 2] Japanese Patent Publication No. 2005-134152 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] According to the method proposed in Patent Document 1, the amount of hydrogen infiltrating a metal material due to corrosion can be measured over time. However, it has been found that the method in Patent Document 1 cannot accurately evaluate the amount of hydrogen infiltrating a metal material under stress.

[0011] Furthermore, while the method proposed in Patent Document 2 allows evaluation under stress, it requires visual inspection to determine the occurrence of cracks. Since a certain amount of time is required between the occurrence of a crack and its subsequent visual detection, this method cannot accurately determine the exact moment of crack occurrence.

[0012] To design metallic materials with excellent delayed fracture characteristics, it is necessary to accurately evaluate the time it takes for a stressed metallic material to crack, and furthermore, to accurately determine the amount of hydrogen that penetrates at the time of cracking.

[0013] The present invention has been made in view of these circumstances, and aims to provide a hydrogen permeation current measuring device that can accurately measure hydrogen permeation current while tensile stress is applied to a metal material.

Means for Solving the Problem

[0014] First, the findings obtained by the present inventors will be described. First, the present inventors focused on the measuring device described in Patent Document 1 that can measure the amount of hydrogen infiltrated over time, and measured the amount of hydrogen infiltrated into a steel plate to which tensile stress was applied using the measuring device.

[0015] When conducting a test by applying tensile stress to a metal in this way, it is common to use a so-called tensile test piece. However, when performing the above measurement using a conventionally widely used tensile test piece, for example, a JIS No. 5 test piece, etc., it was found that the hydrogen permeation current becomes too small to distinguish from noise, and the amount of hydrogen infiltrated cannot be determined.

[0016] As a result of further investigation, it was found that the hydrogen permeation current can be measured without problems by controlling the tensile strength, thickness, and width of the metal material as the test piece so as to satisfy specific conditions.

[0017] The present invention has been completed based on the above findings, and the main configuration is as follows.

[0018] 1. A metal material as a test specimen, A jig for applying tensile stress to the metal material, An electrochemical cell disposed on one surface of the metal material, comprising, A hydrogen permeation current measuring device in which the tensile strength σ (MPa) of the metal material and the thickness t (mm) and width W (mm) of the metal material at the portion where the electrochemical cell is installed satisfy the following formula (1). σ × t × W ≦ 10000 …(1)

[0019] 2. The hydrogen permeation current measuring device according to 1 above, wherein the metal material has a constricted portion having a smaller cross-sectional area than other portions, and the electrochemical cell is installed in the constricted portion.

[0020] 3. A method for measuring hydrogen permeation current using the hydrogen permeation current measuring device described in 1 or 2 above, A method for measuring hydrogen permeation current, comprising measuring the hydrogen permeation current using the electrochemical cell while applying tensile stress to the metal material using the jig in a hydrogen-ingress environment.

[0021] 4. A method for evaluating delayed fracture characteristics of a metal material, which evaluates the delayed fracture characteristics of the metal material based on the hydrogen permeation current measured by the hydrogen permeation current measurement method described in 3 above.

[0022] 5. The delayed fracture characteristic evaluation method described in 4 above, which determines whether or not cracks occur in the metal material in the evaluation of the delayed fracture characteristics. [Effects of the Invention]

[0023] According to the present invention, the hydrogen permeation current can be accurately measured while tensile stress is applied to a metal material. By measuring the hydrogen permeation current, the amount of hydrogen intrusion can be accurately determined through conversion. Furthermore, delayed fracture characteristics, such as crack initiation, can be evaluated based on the measured hydrogen permeation current. Therefore, according to the present invention, the amount of hydrogen intrusion into a metal material subjected to tensile stress before cracking can be measured with high accuracy. [Brief explanation of the drawing]

[0024] [Figure 1] This is a schematic diagram showing the structure of a hydrogen permeation current measuring device in one embodiment of the present invention. [Figure 2] This is a schematic diagram showing the structure of a jig in one embodiment of the present invention. [Figure 3] This graph shows the change over time in the amount of hydrogen intrusion during a test in which a tensile stress equivalent to YP was applied to steel plate A (tensile strength: 1510 MPa, plate thickness at measurement: 0.5 mm, width: 9 mm). [Figure 4] This graph shows the change over time in the amount of hydrogen intrusion during a test in which a tensile stress equivalent to YP was applied to steel plate A (tensile strength: 1510 MPa, plate thickness at measurement: 1.2 mm, width: 10 mm). [Modes for carrying out the invention]

[0025] The present invention will be described in detail below with reference to the drawings. In each figure, corresponding parts are denoted by the same reference numerals. The following description describes examples of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. Furthermore, points not specifically mentioned can be described in the same manner as disclosed in Patent Document 1.

[0026] [Hydrogen permeation current measuring device] Figure 1 is a schematic diagram (side view) showing the structure of a hydrogen permeation current measuring device 100 in one embodiment of the present invention. In this specification, the hydrogen permeation current measuring device 100 with an electrochemical measurement power supply PS added is referred to as the hydrogen permeation current measuring system 200.

[0027] The hydrogen permeation current measuring device 100 comprises a metal material 10 as a test subject, a jig 20 for applying tensile stress to the metal material 10, and an electrochemical cell 30 placed on one side of the metal material (the lower surface in Figure 1).

[0028] [Metal materials] Any metal material can be used as the metal material 10 without any particular limitations, but usually, a metal material that exhibits delayed fracture should be used. Examples of metal materials that exhibit delayed fracture include steel, aluminum alloys, and nickel alloys. The steel may be, for example, carbon steel or stainless steel.

[0029] As mentioned earlier, generally speaking, the higher the strength of the steel, the more likely delayed fracture is to occur. Therefore, when using steel as a metallic material, it is preferable to use high-strength steel with high tensile strength, and in particular, it is preferable to use high-strength steel with a tensile strength of 1180 MPa or higher.

[0030] The shape of the metal material is not particularly limited; it can be any shape as long as it has one surface for attaching the electrochemical cell and the other surface through which hydrogen can enter. Typically, it is preferable to use a plate-shaped metal material as shown in Figure 1. It is preferable that the metal material has a constricted portion 11, but this point will be explained later.

[0031] In this invention, it is important that the tensile strength σ (MPa) of the metal material 10, as well as the thickness t (mm) and width W (mm) of the metal material 10 in the area where the electrochemical cell 30 is installed (hereinafter referred to as the "measurement area"), satisfy the following equation (1). The reason for this is explained below. σ × t × W ≤ 10000 …(1)

[0032] If the thickness (t) of the measurement section of the metal material 10 is excessively large, the time delay between the change in the corrosive environment and the change in the measured hydrogen permeation current increases, and as a result, the relationship between the factors of the corrosive environment and the amount of hydrogen intrusion calculated from the hydrogen permeation current may become unclear. Also, if the hydrogen diffusion coefficient of the metal material is small, it is better to make t thinner. Furthermore, from the perspective that accurate calculation of the amount of hydrogen intrusion is not possible if hydrogen diffusion in the metal material is non-steady, it is better to make t thinner.

[0033] Furthermore, if the width (W) of the measurement section is too large, it becomes difficult to apply tensile stress uniformly. As a result, variations in hydrogen susceptibility occur within the measurement section, leading to variations in the time until cracking occurs, making it impossible to accurately evaluate delayed fracture characteristics.

[0034] The dimensions of such a measurement section are affected by the tensile strength of the metallic material. Therefore, in this invention, the product of σ, t, and W is set to 10,000 or less. By adjusting the dimensions of the measurement section to satisfy this condition, the delayed fracture characteristics of the metallic material can be evaluated with high accuracy.

[0035] The thickness t of the measuring section is not particularly limited, but for the reasons mentioned above, it is preferably 1.50 mm or less, more preferably 1.00 mm or less, and even more preferably 0.50 mm or less. On the other hand, if t is excessively small, corrosion may cause holes to form before the metal material cracks. For this reason, t is preferably 0.20 mm or more, and more preferably 0.25 mm or more.

[0036] The width W of the measurement section is not particularly limited, but for the reasons mentioned above, it is preferable to set it to 30 mm or less, and more preferably to 15 mm or less. On the other hand, if W is excessively small, the hydrogen permeation current measured by the electrochemical cell will also be small, making it impossible to detect the difference with noise, and thus the amount of invading hydrogen may not be measured with high accuracy. For this reason, it is preferable to set W to 4 mm or more, and more preferably to 5 mm or more.

[0037] In this invention, the thickness t of the metal material in the portion where the electrochemical cell is installed is defined as the dimension from the surface of the metal material on which the electrochemical cell is installed to the opposite surface. Typically, the thickness t is the thickness of the metal material in the direction of hydrogen permeation. In this invention, the width W of the metal material in the portion where the electrochemical cell is installed is defined as the dimension in the direction perpendicular to the thickness t in a cross-section of the metal material perpendicular to the direction in which tensile stress is applied. If the metal material has a constricted portion as described later, the width W refers to the width of the constricted portion.

[0038] Furthermore, in this invention, as described later, the hydrogen permeation current can be measured while tensile stress is applied to the metal material using a jig. Strictly speaking, the dimensions of the metal material change when tensile stress is applied, but the amount of change is sufficiently small compared to the dimensions of the metal material. Therefore, it is sufficient that the thickness t and width W of the metal material before the application of tensile stress satisfy the above-mentioned conditions.

[0039] The metal material may have a coating on at least a portion of its surface. The coating is not particularly limited, and any coating can be used. The coating may be made of metal, a nonmetal, or both. Examples of nonmetals include resin, ceramics, and carbon. The coating may be a plating layer or a paint film. The coating may also be a multilayer coating consisting of multiple layers of the same or different types. If the coating interferes with the measurement, the coating present at the location where the electrochemical cell is to be installed may be removed beforehand before installing the electrochemical cell.

[0040] As shown in Figure 1, an electrochemical cell 30 is installed on one surface of the metal material 10 to electrochemically detect hydrogen that has permeated through the metal material 10. Preferably, at least the portion of one surface of the metal material 10 to which the electrochemical cell 30 is attached is provided with a layer (hereinafter referred to as the metal layer) made of a metal that has a high hydrogen diffusion coefficient and promotes the oxidation reaction of hydrogen. Examples of metals constituting the metal layer include Pd, Pd alloys, and Ni. Examples of Pd alloys include Pd-Ni alloys and Pd-Co alloys.

[0041] The presence of the aforementioned metal layer promotes the oxidation reaction of hydrogen, thereby increasing the sensitivity of the anode current due to hydrogen ionization. Furthermore, Pd has a larger hydrogen diffusion coefficient than Ni, and is also highly effective in reducing the temperature dependence of the passivation holding current. For this reason, it is more preferable to use a Pd coating as the aforementioned metal layer.

[0042] Furthermore, if the above-mentioned coating exists on the surface of the metal material 10, it is preferable to remove the coating present at the location where the electrochemical cell 30 is to be installed to expose the surface of the metal material, and then form the metal layer at that location.

[0043] The thickness of the metal layer is not particularly limited, but is preferably 10 to 100 nm. The metal layer is preferably a plated film formed by a plating method.

[0044] When forming a plating film of Pd or a Pd alloy as the metal layer, cathodic electrolysis can be performed in an aqueous solution (plating bath) containing palladium ions. As a palladium ion source in the plating bath, for example, a palladium compound such as [Pd(NH3)4]Cl2·H2O can be used. When forming a Ni plating film, cathodic electrolysis can be performed in any plating bath, such as a Watt bath.

[0045] The metal layer may consist of multiple layers. For example, a Ni plating film formed on the surface of a metal material and a Pd plating film or Pd alloy plating film formed on the Ni plating film can be used.

[0046] The metal material 10 may be pre-deformed by rolling, tensile processing, or other methods to impart strain (plastic deformation). In actual parts, various processes such as bending and drawing are performed, that is, after strain is applied, stress may be applied again. Therefore, by performing measurements using a metal material that has been pre-deformed, evaluation can be performed under conditions closer to those of an actual part.

[0047] [jig] The jig is a fixture for applying tensile stress to a metal material. The structure of the jig is not particularly limited and can be any structure that can apply tensile stress to a metal material. Typically, the jig preferably comprises a pair of holding parts that hold both ends of the metal material and a movable part that adjusts the distance between the pair of holding parts.

[0048] Figure 2 is a schematic diagram (viewed from above) showing the structure of a jig 20 in one embodiment of the present invention. In this example, the jig 20 comprises an outer frame 21, a fixed support part 22 attached to the outer frame 21, a movable support part 23 that can move within the outer frame 21, a fixing pin 24, and a bolt 25.

[0049] Holes are drilled at both ends of the metal material 10 for fixing to the jig, and one end of the metal material 10 is fixed to the fixed support part 22 by passing a fixing pin 24a through these holes. On the other hand, the other end of the metal material 10 is fixed to the movable support part 23 by a fixing pin 24b. A bolt 25 is connected to the movable support part 23 through a hole drilled in the outer frame 21. By rotating the bolt 25, the movable support part 23 can slide in the longitudinal direction of the metal material 10 (in the direction indicated by the arrow), thereby adjusting the tensile stress applied to the metal material 10.

[0050] In addition to the jigs shown in Figure 2, stress may also be applied using jigs with a shape similar to the proof ring of NACE TM0177 Method A.

[0051] [Waist area] As shown in Figure 2, it is preferable that the metal material 10 has a constricted portion 11 whose cross-sectional area is smaller than that of other parts. In other words, it is preferable that the cross-sectional area of ​​the measurement portion of the metal material 10 is smaller than that of other parts. When the metal material 10 has a constricted portion 11 in this way, tensile stress is concentrated in the constricted portion 11, so it is possible to prevent cracking at locations other than the constricted portion 11. Therefore, by installing the electrochemical cell 30 in the constricted portion 11, it becomes possible to evaluate the delayed fracture characteristics more reliably. The cross-sectional area is defined as the area of ​​the cross-section of the metal material perpendicular to the direction in which tensile stress is applied.

[0052] The constricted portion 11 is preferably located in the center of the longitudinal direction of the metal material 10. Furthermore, the metal material 10 is preferably symmetrical with respect to its longitudinal center. The shape of the constricted portion 11 is not particularly limited, but it is preferably plate-shaped or prismatic in shape to facilitate the attachment of the electrochemical cell 30.

[0053] In one embodiment of the present invention, the metal material may be a plate-shaped tensile test specimen. In that case, the metal material 10 has, like a test specimen used in general tensile tests, a "parallel portion" having a constant width provided in the center in the longitudinal direction, "gripping portions" provided at both ends in the longitudinal direction and having a wider width than the parallel portion, and a "shoulder portion" provided between the parallel portion and the gripping portion and having a continuously changing cross-sectional area. The gripping portions, i.e., both ends in the longitudinal direction of the metal material 10, are portions for fixing the metal material 10 to the jig 20, and as shown in Figure 2, through holes are formed for passing fixing pins 24a and 24b through. On the other hand, the parallel portion is the portion to which tensile stress is applied, and an electrochemical cell is installed in this portion.

[0054] [Electrochemical cell] The electrochemical cell 30 is placed on one side of the metal material 10. The method of installing the electrochemical cell 30 is not particularly limited, and any method can be used as long as the electrolyte 32 inside the electrochemical cell 30 is in contact with the surface of the metal material 10. However, it is desirable to use a method that does not affect the tensile stress applied to the metal material 10. For example, the electrochemical cell 30 can be fixed to the surface of the metal material 10 by sandwiching the metal material 10 and the electrochemical cell 30 from above and below with a cell fixing jig. It is preferable to place the electrochemical cell 30 after applying a predetermined tensile stress to the metal material 10 using the jig 20.

[0055] Next, we will describe the structure of the electrochemical cell 30.

[0056] (electrolytic cell) The electrochemical cell 30 includes an electrolytic cell 31. Any electrolytic cell 31 can be used as long as it is capable of containing an electrolyte 32 and bringing the electrolyte 32 into contact with the surface of the metal material 10. Typically, as shown in Figure 1, the electrolytic cell 31 has an opening for bringing the electrolyte 32 into contact with the metal material 10.

[0057] The size of the electrolytic cell 31 is not particularly limited and should be a size that can be installed on the surface of the metal material 10. Here, the area of ​​the part in contact with the metal material 10 by the electrolyte 32 is defined as the measurement area, and the opposite side of the measurement area is the hydrogen intrusion area. Usually, the area of ​​the opening of the electrolytic cell 31 can be considered as the measurement area.

[0058] The shape and dimensions of the opening can be adjusted to match the shape of the measuring portion of the metal material 10. Normally, the size of the opening should be less than or equal to the size of the measuring portion so that the opening of the electrolytic cell 31 is closed by the metal material 10. If the metal material 10 has a constricted portion 11, the opening of the electrochemical cell 30 should be reduced to match the size of the constricted portion 11. For example, only the portion of the top surface of the electrolytic cell 31 that contacts the metal material 10 may be open, with the rest covered by a top plate. Also, the cross-sectional area of ​​the upper part of the electrolytic cell 31 (the side with the metal material 10) may be smaller than that of other parts.

[0059] The material of the electrolytic cell 31 is not particularly limited, but it is preferable that it be made of resin from the viewpoint of durability, processability, ease of work, and chemical resistance to the electrolyte. Furthermore, it is preferable that the electrolytic cell 31 be transparent from the viewpoint of being able to visually check the state of the electrolyte inside the electrolytic cell 31. From the above viewpoints, it is more preferable that the electrolytic cell 31 be made of acrylic resin.

[0060] (electrolyte) The electrolyte 32 is not particularly limited, and any electrolyte can be used. From the viewpoint of keeping the surface of the metal material 10 in a passive state during measurement, it is preferable that the pH of the electrolyte 32 be 9 or higher. On the other hand, if the pH is higher than 13, which is strongly alkaline, leakage due to an accident will cause significant damage to the environment. For this reason, it is preferable that the pH of the electrolyte 32 be 13 or lower.

[0061] The composition of the electrolyte is not particularly limited and may be any composition. From the viewpoint of ensuring a passivation state of the surface of the metal material 10, for example, an aqueous NaOH solution of about 0.1 to 1.0 M (M = mol / L) is preferably used.

[0062] The electrolyte may be in liquid form, but from the viewpoint of preventing leakage of the electrolyte and ease of handling, it is also preferable to use a gel-type electrolyte. To make the electrolyte gel-type, any gelling agent can be added to the electrolyte.

[0063] (electrode) The electrochemical cell 30 is equipped with at least one electrode 33. If the amount of ingress hydrogen is measured using a two-electrode system, the electrochemical cell 30 only needs to be equipped with one electrode 33. In this case, the metal material 10 is used as the working electrode, and the electrode 33 of the electrochemical cell 30 is used as the counter electrode.

[0064] However, from the standpoint of more accurately controlling the potential, it is preferable to use a three-electrode system rather than a two-electrode system. When measuring the amount of invading hydrogen using a three-electrode system, the electrochemical cell 30 only needs to be equipped with two electrodes 33c and 33r, as shown in Figure 1. In this case, the metal material 10 is used as the working electrode, electrode 33c as the counter electrode, and electrode 33r as the reference electrode.

[0065] • Opposite The material of the electrode 33c used as the counter electrode is not particularly limited as long as it is capable of measuring the anode current, but it is generally preferable to use a metal electrode. The metal is not particularly limited, but it is preferable to use an inert metal so as not to inhibit the ionization reaction of hydrogen atoms. Examples of the inert metal include platinum, gold, ruthenium, rhodium, palladium, iridium, and alloys thereof. Among these, platinum and gold are particularly preferred for use as counter electrodes because they are very stable in the electrolyte and have high conductivity. It is also possible to use electrodes in which the inert metal is plated onto the surface of a substrate such as titanium.

[0066] ·Reference electrode The reference electrode is an electrode that serves as the reference for the potential when measuring the anode current, and is also called the reference electrode. The material of the electrode 33r used as the reference electrode is not particularly limited, but it is generally preferable to use a metallic electrode, and more preferably iridium (Ir) or tungsten (W). This is because Ir and W form an oxide film on their surface in the electrolyte, allowing for a stable potential over a long period of time. Electrodes plated with Ir or W on the surface of a substrate such as titanium can also be used.

[0067] The shape of the electrode 33 is not particularly limited and can be any shape, whether it is a counter electrode or a reference electrode. Typically, a rod or wire shape is preferred.

[0068] The electrode 33 is installed so as to penetrate the electrolytic cell 31, with one end located inside the electrolytic cell 31 and the other end located outside the electrolytic cell 31.

[0069] When measuring hydrogen permeation current, the anode current can be measured using the electrochemical cell 30. The polarization conditions for measuring the anode current are not particularly limited, but should be such that sufficient hydrogen ionization occurs on the surface of the metal material 10, while maintaining passivation on the surface of the metal material 10. Generally, polarization conditions of -0.1 to 0.3 V vs. SCE are widely used in a 1 M NaOH aqueous solution. Here, SCE refers to a saturated calomel electrode, and the potential of this SCE relative to the standard hydrogen electrode (SHE) is given as +0.244 V (vs SHE, 25°C). Furthermore, if an SSE is used, measurement can be performed at 0 V.

[0070] For measurement, the other end of the electrode 33, that is, the end located outside the electrolytic cell 31, should be connected to the electrochemical measurement power supply PS using a coated wire 35 or the like. The coated wire 35 is not particularly limited and any type can be used. Typically, a metal wire coated with an insulating material such as resin can be used as the coated wire 35. The metal wire is preferably made of a metal with low electrical resistance, such as silver or copper.

[0071] The connection between the electrode 33 and the insulated wire 35 can be made by any method. Typically, soldering can be used. However, if the material of the electrode 33 is difficult to solder, another metal that is easy to solder may be interposed between the electrode 33 and the insulated wire 35.

[0072] (Hydrogen intrusion area) In this invention, as described above, the electrochemical cell 30 is attached to one side of the metal material 10 for measurement, so the other side of the metal material 10 (the top surface in Figure 1) functions as a hydrogen intrusion area. Therefore, when performing the measurement, it is sufficient to expose the hydrogen intrusion area to the hydrogen intrusion environment.

[0073] For example, when measurements are taken in a corrosive environment, corrosion of the metal material 10 progresses on the surface of the hydrogen intrusion area, and the hydrogen atoms (Had) generated as a result penetrate into the interior of the metal material 10. The penetrated hydrogen diffuses through the interior of the metal material 10 and reaches the opposite surface, where it is oxidized by the electrochemical cell 30, and H + It is released into the electrolyte 32 and detected as an electric current at that time.

[0074] When performing measurements using the apparatus of the present invention, electrochemical measurements are performed with the metal material 10 as the working electrode. Therefore, the metal material 10 is connected to the terminal for the working electrode of the electrochemical measurement power supply PS using a coated wire 36 or the like, and current is applied. The coated wire 36 is not particularly limited and any type can be used. Typically, the same type as the coated wire 35 used for connecting electrodes can be used.

[0075] [Method for measuring hydrogen permeation current] In the hydrogen permeation current measurement method according to one embodiment of the present invention, the hydrogen permeation current is measured using the hydrogen permeation current measuring device 100. Specifically, under a hydrogen-impregnated environment, the hydrogen permeation current is measured by the electrochemical cell 30 while tensile stress is applied to the metal material 10 using the jig 20.

[0076] The hydrogen intrusion environment may be any environment in which hydrogen can penetrate the metal material 10. For example, the hydrogen intrusion environment may be a corrosive environment or a hydrogen gas-containing atmosphere.

[0077] As mentioned earlier, the hydrogen permeation current can be measured using an electrochemical measurement power supply PS connected to the hydrogen permeation current measuring device 100. The measurement can be performed for any period of time, but it is preferable to continue it until cracks appear in the metal material 10.

[0078] [Method for evaluating delayed failure characteristics] In one embodiment of the present invention, the delayed fracture characteristics of the metal material 10 can be evaluated based on the measured hydrogen permeation current. The delayed fracture characteristics can be evaluated, for example, by the time it takes for cracks to occur in the metal material 10 after the introduction of hydrogen, the amount of hydrogen that penetrates at the time the cracks occur, or the minimum stress when the same amount of hydrogen is introduced and cracks occur after a predetermined time has elapsed.

[0079] According to the present invention, the time-dependent change in hydrogen permeation current can be detected with high accuracy. Therefore, the occurrence of cracks can be easily detected as a disturbance in the current value. For example, when abrupt changes in the current value that do not normally occur are observed, such as the current value changing tenfold or to one-tenth in one minute, or the measured current value suddenly becoming negative, it can be determined that a crack has occurred. As a result, it is possible to accurately evaluate the time when a crack occurred based on the hydrogen permeation current. On the other hand, conventional visual evaluation evaluates the time when a crack has progressed to a level that a person can detect as the time when a crack has occurred, which is less accurate.

[0080] Furthermore, since the hydrogen permeation current can be converted into the amount of hydrogen intrusion, according to the present invention, the amount of hydrogen intrusion at the time of cracking can be evaluated with high accuracy.

[0081] An example of a method for determining the amount of hydrogen entering from the hydrogen permeation current is described below.

[0082] In electrochemical hydrogen permeation, the amount of hydrogen that penetrates is the surface hydrogen concentration (adsorbed hydrogen concentration on the hydrogen penetration surface): Cab is represented as. This surface hydrogen concentration C ab is calculated by the following formula (2) using the hydrogen permeation current density: i H . C ab = (i H × t × M H ) / (D × F × d) × 10 6 …(2) C ab : Surface hydrogen concentration (wt. ppm) i H : Hydrogen permeation current density (A / cm 2 ) t: Thickness of the metal material in the measurement part (cm) D: Hydrogen diffusion coefficient in the metal material (cm 2 / s) F: Faraday constant (C / mol) M H : Molar mass of hydrogen atoms (g / mol) d: Density of the metal material (g / cm 3 )

[0083] Hydrogen permeation current density: i H can be obtained by dividing the measured hydrogen permeation current by the measurement area (the area where the electrolyte contacts the metal material).

[0084] Note that since the ease of hydrogen diffusion in the metal material depends on temperature, strictly speaking, the value of the hydrogen diffusion coefficient D is not constant. It is known that there is a linear relationship between the logarithm of the hydrogen diffusion coefficient D and the reciprocal of the temperature, and the relationship between the two can be expressed by the Arrhenius equation. Therefore, if the temperature dependence of the hydrogen diffusion coefficient is formulated in advance, the hydrogen diffusion coefficient at that temperature can be obtained from the temperature of the metal material. By using the hydrogen diffusion coefficient obtained in this way, the amount of intruding hydrogen can be calculated with higher accuracy.

Example

[0085] Next, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

[0086] The hydrogen permeation current was measured using the hydrogen permeation current measuring device 100 shown in Figures 1 and 2, and the amount of hydrogen entering the metal material 10 was calculated from the hydrogen permeation current.

[0087] Two types of steel sheets were used as the metal material 10. Table 1 shows the component composition, tensile strength σ, and yield stress YP of the steel sheets A and B used.

[0088] For each of the steel plates A and B, which initially had a thickness of 1.6 mm, the thickness was reduced to 0.3 to 1.2 mm by mechanical grinding and chemical treatment on one side. Next, a rectangular plate measuring 25 mm x 81 mm was taken from the steel plate and further processed to form a tensile test specimen having parallel sections as shown in Figure 2. The processing of the tensile test specimen was performed by laser cutting and grinding to form parallel sections with predetermined thickness and width. Subsequently, a Pd plating layer (thickness: approximately 100 nm) was formed as a metal layer on one side of the parallel section of the test specimen.

[0089] Next, the test specimen was attached to the jig 20 as shown in Figure 2. At that time, the test specimen was positioned so that the Pd plating layer was facing downwards. After that, the movable support part 23 was moved by rotating the bolt 25, and tensile stress was applied to the metal material 10.

[0090] Next, an electrochemical cell 30 equipped with an electrolytic cell 31 was placed on the lower surface of the test specimen, i.e., on the surface of the Pd plating layer. An acrylic resin electrolytic cell with a circular opening of 3 mm in diameter was used as the electrolytic cell 31. An Ir wire (0.5 mmφ) was used as the counter electrode 33c, and a Pt wire (0.5 mmφ) was used as the reference electrode 33r.

[0091] The counter electrode 33c and reference electrode 33r of the electrochemical cell 30 were connected to the electrochemical measurement power supply PS using insulated wires 35. The test specimen (metal material 10) was also connected to the electrochemical measurement power supply PS using insulated wires 36. A potentiostat was used as the electrochemical measurement power supply PS.

[0092] An electrolyte solution 32 was placed in the electrolytic cell 31 of the electrochemical cell 30 so that the liquid surface was in contact with the surface of the metal material 10. A 1N NaOH aqueous solution was used as the electrolyte solution 32.

[0093] To evaluate the delayed fracture characteristics due to hydrogen generated during corrosion of steel plates A and B under atmospheric corrosive conditions, hydrogen permeation current was measured under a corrosion test cycle consisting of a chloride deposition process (A), a corrosion process (B), and a water washing process (C). Table 2 shows the details of the chloride deposition process (A) and the corrosion process (B).

[0094] In the chloride deposition process (A), a chloride-containing aqueous solution was sprayed onto the surface of the test specimen under the conditions shown in Table 2. The spraying was performed using a two-fluid sprayer, with a distance of 30 cm from the spray nozzle to the surface of the test specimen. The spray pressure of the spray nozzle was 0.2 MPa, and the spraying time of the salt solution was 5 seconds. By spraying under the above conditions, the amount of chloride deposited was 10,000 mg / m². 2 I adjusted it to that.

[0095] In corrosion process (B), a cycle consisting of a drying process (b1), a transition process (b3), a wetting process (b2), and a transition process (b4) was performed 9 or 12 times. The temperature in corrosion process (B) was kept constant at 30°C, with a temperature fluctuation range of ±5°C or less.

[0096] The cycle of chloride deposition (A), corrosion (B) was repeated a predetermined number of times, followed by a water rinsing (C). The evaluation period was 28 days (672 hours), during which the amount of hydrogen intrusion was measured. Specifically, the hydrogen permeation current (anode current) was continuously measured under the aforementioned corrosive environment. The amount of hydrogen intrusion was then calculated from the measured anode current value. For the measurement, the electrochemical cell 30 was anode-polarized so that the potential was 0V vs. SSE.

[0097] Next, the time at which cracks occurred in the metal material 10 was evaluated based on the measured amount of hydrogen intrusion. Specifically, the time at which a significant disturbance occurred in the amount of hydrogen intrusion was determined to be the time at which cracks occurred.

[0098] For example, Figure 3 is a graph showing the change in the amount of hydrogen intrusion over time in a test in which a tensile stress equivalent to YP was applied to steel plate A (tensile strength: 1510 MPa, plate thickness at the measurement point: 0.5 mm, width: 9 mm). In Figure 3, a significant disturbance occurs in the amount of hydrogen intrusion at 334.4 hours, so it can be concluded that cracking occurred at this point. Furthermore, the amount of hydrogen intrusion at the time of cracking (crack time) was 0.0115 wt. ppm.

[0099] For comparison, Figure 4 shows the amount of hydrogen intrusion measured under the same conditions except that the plate thickness of the measurement section was 1.2 mm and the width was 10 mm (σ × t × W = 18120). In this example, it can be seen that the amount of hydrogen intrusion was not measured.

[0100] Next, the parallel section dimensions (thickness t, width W) of the above test specimen were changed as shown in Table 3, and the same test was performed. In these tests as well, a tensile stress equivalent to YP was applied to the test specimen. Table 3 shows the results of measuring the amount of hydrogen intrusion at the cracking time under each condition. When the amount of hydrogen intrusion was measured and the amount of hydrogen intrusion at the cracking time could be quantitatively measured, as shown in Figure 3 above, it was judged as "○", and when the amount of hydrogen intrusion could not be measured, as shown in Figure 4 above, it was judged as "×".

[0101] As can be seen from the results shown in Table 3, when the tensile strength σ (MPa) of the metal material and the thickness t (mm) and width W (mm) of the metal material in the parallel section satisfy the conditions of equation (1), the amount of hydrogen intrusion can be measured without any problems. On the other hand, when the conditions of equation (1) are not satisfied, the amount of hydrogen intrusion cannot be measured.

[0102] Next, using a test specimen with a plate thickness of 0.5 mm and a tensile stress load width of 9 mm, the cracking time and the amount of hydrogen intrusion during the cracking time were evaluated. The results obtained are shown in the "Examples of Inventions" column of Table 4.

[0103] For comparison, the time at which cracks occurred was also evaluated visually, as in the conventional method. Visual evaluation was performed once a day. Furthermore, in order to measure the amount of hydrogen intrusion at the time when cracks occurred visually, the amount of hydrogen intrusion at the time when cracks were visually observed was measured. Specifically, when cracks were observed, the metal material 10 was removed from the jig 20 and stored in liquid nitrogen to prevent the release of intruded hydrogen before measurement. Subsequently, corrosion products adhering to the metal material 10 were mechanically removed while cooling as needed, and the amount of hydrogen contained in the measurement area of ​​the metal material 10 (amount of hydrogen in steel) was measured by temperature-controlled desorption analysis.

[0104] In this manner, the cracking time and the amount of hydrogen intrusion during the cracking time were evaluated for each test specimen using the conventional method. The results obtained are shown in the "Comparative Examples" column of Table 4.

[0105] Table 4 shows that, regarding cracking time, in the inventive example, the cracking time was shorter for test specimens with higher stress, yielding results consistent with theory. In contrast, in the conventional visual evaluation (comparative example), at some levels, test specimens with higher stress had longer cracking times. Similarly, regarding hydrogen penetration, in the inventive example, the amount of hydrogen penetration was lower for test specimens with higher stress, yielding results consistent with theory. In contrast, in the comparative example, at some levels, test specimens with higher stress had higher hydrogen penetration. This is thought to be because the method of the present invention can detect the occurrence of microscopic cracks, whereas visual inspection cannot detect microscopic cracks until they have spread to a certain size.

[0106] Furthermore, the amount of hydrogen impregnated using the conventional method tended to be generally lower than that used in the inventive example. This is thought to be because it took time to measure the amount of hydrogen impregnated, allowing hydrogen to be released from the test specimen.

[0107] Next, the hydrogen permeation current measuring device of the present invention was used to evaluate the crack initiation limit stress, which is one of the evaluation indicators for delayed fracture characteristics. Here, the minimum stress value of the test specimens in which cracking occurred at 672 hours (28 days) of tensile stress loading was defined as the crack initiation limit stress (unit: MPa) and is listed in Table 4.

[0108] The larger the value of the lower limit stress for cracking, the less likely delayed fracture is to occur. From Table 4, the lower limit stress for cracking is (large) steel plate A > steel plate B (small), indicating that steel plate A is less prone to cracking and has superior resistance to delayed fracture than steel plate B. It should be noted that, for steel plate A and steel plate B, it had been confirmed in a separate test beforehand that steel plate B is more prone to delayed fracture and steel plate A is less prone to delayed fracture.

[0109] From the above results, it can be seen that the delayed fracture characteristics can be accurately evaluated according to the present invention. Therefore, according to the present invention, for example, for metal materials used in the corrosive atmospheric environment in which automobiles are driven, the amount of hydrogen that penetrates into the metal material due to atmospheric corrosion in which chlorides are the dominant factor in corrosion can be accurately measured until the metal material subjected to tensile stress cracks, thereby enabling accurate evaluation of the time when cracking occurs and the amount of hydrogen that penetrates at the time cracking occurs. Furthermore, the lower limit stress for cracking can also be appropriately evaluated.

[0110] [Table 1]

[0111] [Table 2]

[0112] [Table 3]

[0113] [Table 4] [Explanation of symbols]

[0114] 10 Metal materials 11 Waist area 12 Hydrogen intrusion section 20 jigs 21 Outer frame 22 Fixed support part 23 Movable support part 24 fixing pins 25 volts 30 electrochemical cells 31 Electrolytic cell 32 Electrolyte 33 electrode 33c opposite pole 33r reference electrode 35 Insulated wire 36 Insulated wire 100 Hydrogen permeation current measuring device 200 Hydrogen Permeation Current Measurement System PS Power supply for electrochemical measurements

Claims

1. Metal material as the test subject, A jig for applying tensile stress to the aforementioned metal material, An electrochemical cell disposed on one side of the aforementioned metal material, Equipped with, A hydrogen permeation current measuring device wherein the tensile strength σ (MPa) of the metal material and the thickness t (mm) and width W (mm) of the metal material in the portion where the electrochemical cell is installed satisfy the following equation (1). σ × t × W ≤ 10000 …(1)

2. The aforementioned metal material has a constricted portion whose cross-sectional area is smaller than that of other parts. The hydrogen permeation current measuring device according to claim 1, wherein the electrochemical cell is installed in the constricted portion.

3. A method for measuring hydrogen permeation current using the hydrogen permeation current measuring device described in claim 1 or 2, A method for measuring hydrogen permeation current, comprising measuring the hydrogen permeation current using the electrochemical cell while applying tensile stress to the metal material using the jig in a hydrogen-ingress environment.

4. A method for evaluating delayed fracture characteristics of a metal material, which evaluates the delayed fracture characteristics of the metal material based on the hydrogen permeation current measured by the hydrogen permeation current measurement method described in claim 3.

5. The delayed fracture characteristic evaluation method according to claim 4, which determines whether or not cracks occur in the metal material in the evaluation of the delayed fracture characteristics.

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