Method for selecting metal material and method for manufacturing structure

Abstract

The purpose of the present invention is to simply and appropriately select a metal material suitable for use in a powder deposition environment (submerged environment or dry-wet cyclic environment) in which a powder containing an electroconductive powder is deposited. For this purpose, water is supplied to the same powder as the deposited powder to prepare a water-containing powder in which corrosive ions are eluted from the powder into the water. When the water-containing powder is prepared, in a case where the powder deposition environment is the submerged environment (an environment in which the deposited powder is in a wet state), the concentration of corrosive ions in the water-containing powder is allowed to coincide with a reference concentration (concentration in a wet state), or in a case where the powder deposition environment is the dry-wet cyclic environment (an environment in which the deposited powder alternates between a wet state and a dry state), the concentration of corrosive ions in the water-containing powder is allowed to coincide with a saturation concentration higher than the reference concentration. The electric potential of the metal material is measured in a state of being brought into contact with the water-containing powder, and it is determined that a metal material having a pitting potential higher than a corrosion potential is suitable for use in the powder deposition environment.

Description

Method for Selecting Metal Material and Method for Manufacturing Structure

[0001] The present invention relates to a method for selecting a metal material and a method for manufacturing a structure.

[0002] Conventionally, metal materials are used as materials for structures in social infrastructure such as bridges, roads, railways, rivers, ports, etc. or various factories such as steelworks. One of the causes of deterioration of such used metal materials is corrosion. For example, Patent Document 1 discloses a technique for evaluating the corrosion of a metal material in consideration of the amount of sea salt particles.

[0003] Japanese Patent Application Laid-Open No. 2016-197102

[0004] For example, in a steelworks, structures such as an unloader for unloading steel raw materials (coal, ore, coke, sintered ore, etc.), a belt conveyor for transporting steel raw materials, and a hopper for storing or discharging steel raw materials are used. Further, as the metal material constituting such a structure, for example, a steel material such as stainless steel is used.

[0005] By the way, steel raw materials may be handled in a powder state. In this case, the metal materials constituting the above-described structures are used in a corrosion environment (powder deposition environment) where the powder is deposited. The powder deposition environment may have different corrosion behaviors of metal materials from a normal corrosion environment (a corrosion environment where no powder is deposited). Therefore, it is required to simply and appropriately select a metal material suitable for use in a powder deposition environment.

[0006] The present invention has been made in view of the above points, and an object thereof is to provide a method for selecting a metal material that can simply and appropriately select a metal material suitable for use in a powder deposition environment.

[0007] As a result of intensive studies, the present inventors have found that the above object can be achieved by adopting the following configuration, and have completed the present invention. That is, the present invention provides the following [1] to [5]. [1] A method for selecting a metal material to be used in a powder deposition environment in which a powder containing conductive powder is deposited, wherein the powder deposition environment is a submerged environment or a dry-wet repeating environment, the submerged environment is an environment in which the powder is in a wet state, the dry-wet repeating environment is an environment in which the powder repeats the wet state and the dry state, the powder elutes at least one corrosive ion selected from the group consisting of SO ４ ２－ , Cl － and NO ３ － , the concentration of the corrosive ion shows a saturation concentration higher than a reference concentration which is the concentration in the wet state between the wet state and the dry state, water is supplied to the same powder as the powder to prepare a water-containing powder, when preparing the water-containing powder, if the powder deposition environment is the submerged environment, the concentration of the corrosive ion in the water-containing powder is made to coincide with the reference concentration, if the powder deposition environment is the dry-wet repeating environment, the concentration of the corrosive ion in the water-containing powder is made to coincide with the saturation concentration, in a state of contact with the water-containing powder, the corrosion potential E ｃｏｒｒ and the pitting potential E ｐｉｔ of the metal material are measured, and the metal material in which the pitting potential E ｃｏｒｒ is larger than the corrosion potential E ｐｉｔ is determined to be suitable for use in the powder deposition environment. A method for selecting a metal material. [2] The method for selecting a metal material according to [1], wherein the metal material has a passive film on its surface. [3] The method for selecting a metal material according to [2], wherein the metal material is a steel material. [4] The method for selecting a metal material according to [3], wherein the steel material is stainless steel. [5] A method for manufacturing a structure, wherein the structure is manufactured using the metal material selected by using the method for selecting a metal material according to any one of [1] to [4].

[0008] According to the present invention, it is possible to provide a method for selecting a metal material that can simply and appropriately select a metal material suitable for use in a powder deposition environment.

[0009] It is a graph showing an example of an anodic polarization curve.

[0010] <Findings obtained by the present inventors> First, regarding the corrosion of metal materials used in a powder deposition environment, the findings obtained by the present inventors (the background leading to the present invention) will be described below.

[0011] Powders such as coke and activated carbon have conductivity, and galvanic corrosion (dissimilar metal contact corrosion) occurs between the conductive powder (conductive powder) and the metal material. That is, the conductive powder becomes a cathode site having a large surface area, and the anodic reaction of the metal material (the reaction in which the metal material dissolves) is promoted, and the metal material corrodes at a very high speed.

[0012] To prevent galvanic corrosion, it is effective to prevent contact between the conductive powder and the metal material. For this purpose, for example, it is conceivable to apply a coating to the surface of the metal material. However, the coating deteriorates due to ultraviolet rays, flying sea salt, flying soil, etc., and gradually, the area where the metal material contacts the conductive powder increases. Since the deterioration of the coating progresses over a period of several months to a year, if strict maintenance is frequently carried out, galvanic corrosion can be prevented for a long period. However, considering the situation where the structure is used over an extremely long period of several decades or more, it is difficult to perfectly carry out maintenance over such a long period. In addition, frequent implementation of strict maintenance is costly. In particular, when maintenance is carried out in a place where work cannot be done without scaffolding, the cost is extremely high.

[0013] Incidentally, in powder deposition environments, metal materials with a passive film formed on their surface (for example, steel materials such as stainless steel) are sometimes used. The passive film has a thickness of, for example, several nanometers and exhibits high corrosion resistance. However, depending on the conditions of the powder deposition environment and the type of metal material, the passive film may be destroyed, and corrosion may progress at a very rapid rate. For this reason, when using metal materials in a powder deposition environment, it is important to confirm in advance that the passive film will not be destroyed. Ideally, to confirm the destruction of the passive film, it is preferable to conduct an exposure test in the powder deposition environment in which the metal material will actually be used and evaluate the time until rust occurs, but exposure tests are very time-consuming.

[0014] The passive film will not be destroyed unless the potential becomes nobler (higher potential) than a certain potential determined by the properties of the passive film and the powder deposition environment. Therefore, the corrosion potential E obtained under conditions that simulate (reproduce) the powder deposition environment is not destroyed. ｃｏｒｒ However, the pitting potential E destroys the passivation film. ｐｉｔ If the potential is lower than E, the metal material can maintain a passive film in that powder deposition environment. And while the passive film is maintained, the anodic reaction is suppressed by the passive film, so the corrosion rate of the metal material remains very low. That is, E ｃｏｒｒ <E ｐｉｔ Metal materials that satisfy these conditions can be determined to be suitable for use in powder deposition environments.

[0015] The present invention is based on the above findings. Preferred embodiments of the present invention will be described below. The method for selecting a metal material in this embodiment is, in general terms, a method for selecting a metal material to be used in a powder deposition environment in which conductive powder is deposited (a metal material suitable for use in a powder deposition environment) in advance (before actually using it in a powder deposition environment). The following description also serves as a description of the manufacturing method of the structure.

[0016] <Metallic Materials> While there are no limitations on the metallic materials, for the reasons mentioned above, metallic materials having a passive film on their surface (more specifically, metallic materials in which a passive film is formed on the surface in a powder deposition environment) are preferred. Specifically, steel materials such as stainless steel (e.g., SUS430, SUS304, SUS316, and SUS329J4L) are preferred.

[0017] Metallic materials are used as building materials for structures in powder deposition environments. Examples of such structures include those found in steel mills, thermal power plants, and coal or ore mines. When the powder deposition environment is a steel mill (or part of one), examples of structures include unloaders for unloading iron and steel raw materials (coal, ore, coke, sintered ore, etc.); belt conveyors for transporting iron and steel raw materials; and hoppers for storing or transporting iron and steel raw materials. When the powder deposition environment is a coal or ore mine, examples of structures include coal mining equipment such as drum cutters and shield frames; and mining machinery such as continuous miners.

[0018] <Powder deposition environment> A powder deposition environment is a corrosive environment in which the metal materials constituting a structure are in contact with deposited powder, which can cause corrosion of the metal materials. Powder deposition environments are naturally generated in at least a portion of areas in places such as steel mills, thermal power plants, and coal or ore mines.

[0019] In a powder deposition environment, examples of deposited powders include powders of mineral resources such as coal and ore; powders of coke and sintered ore produced from these mineral resources; and so on. These powders also contain conductive powders. Examples of conductive powders include coke powder and activated carbon powder used in water filtration; and so on.

[0020] Powder deposition environments are either submerged environments or wet-dry / wet-and-dry environments. A submerged environment is one in which the deposited powder is in a wet state, for example, an environment in which the deposited powder is constantly wet (contains moisture) due to industrial water used in the surrounding area. In contrast, a wet-dry / wet-and-dry environment is one in which the deposited powder repeatedly goes between wet and dry states, for example, an environment in which the deposited powder is exposed outdoors and becomes wet during rainy weather but dry during sunny weather.

[0021] The deposited powder, in a wet state, is SO ４ ２－ , Cl － and NO ３ － At least one corrosive ion (an ion that affects the corrosion of metal materials), selected from the group consisting of the above, is eluted into the water (industrial water, rainwater) surrounding the powder. The corrosive ion originates, for example, from impurities mixed in the powder. Note that the powder may also elute ions other than corrosive ions, but in this embodiment, we focus on these three types of ions. NO ３ － For example, in acidic conditions, it can form a passivation film, but SO ４ ２－ and Cl － It destroys the passivation film.

[0022] For convenience, the concentration of corrosive ions in a wet state is referred to here as the "reference concentration." When deposited powder changes from a wet state to a dry state, the moisture content decreases and the corrosive ions become concentrated. For example, just before the dry state, the concentration of corrosive ions becomes saturated, reaching a "saturated concentration" higher than the reference concentration. When the concentration of corrosive ions is at the "saturated concentration," the passivation film of metal materials is more easily destroyed than when it is at the "reference concentration." Note that the "reference concentration" and "saturated concentration" are not fixed values, but vary depending on the location of the powder deposition environment and the wetness conditions, but the relationship reference concentration < saturation concentration is always satisfied.

[0023] Corrosive ions leached from powders can be identified and quantified, for example, using ICP (inductively coupled plasma) emission spectroscopy.

[0024] <Supplying moisture (preparation of water-containing powder)> In this embodiment, in order to simulate (reproduce) a powder deposition environment before conducting the test, the same powder that is deposited in a powder deposition environment (for convenience, also called "deposited powder") is prepared, and water (for example, deionized water) is supplied to the prepared powder to prepare water-containing powder.

[0025] The powder prepared does not have to be the same as the deposited powder; it may be a simulated powder. The simulated powder is a powder that has the same characteristics as the deposited powder. These characteristics include, for example, the type and amount of corrosive ions that leach; conductivity (electrical conductivity); specific surface area; etc.

[0026] When preparing water-containing powder, the concentration of corrosive ions in the water-containing powder should be adjusted depending on whether the powder deposition environment in which the metal material is used is a "submerged environment" or a "wet-and-dry environment." Specifically, if the powder deposition environment is a "submerged environment," the concentration of corrosive ions in the water-containing powder should be matched to the "standard concentration" mentioned above. If the powder deposition environment is a "wet-and-dry environment," the concentration of corrosive ions in the water-containing powder should be matched to the "saturation concentration" mentioned above.

[0027] Here, "agreement" is a concept that allows for a certain degree of error. Specifically, if the concentration of corrosive ions is within ±5% of the reference concentration (or saturation concentration), it is considered to "agree."

[0028] The amount of water supplied to the prepared powder should preferably be enough to completely immerse the powder, from the viewpoint of preventing changes in the properties of the water-containing powder during the potential measurement described later. Specifically, 1 cm of the prepared powder ３ The amount of water supplied to it is 0.5 mL / cm³. ３ The above is preferable. The upper limit is not particularly limited, for example, 1.5 mL / cm³. ３ That is the case.

[0029] Furthermore, when preparing the water-containing powder, if the concentration of corrosive ions in the water-containing powder does not reach the desired concentration, additional corrosive ions may be added to the water-containing powder in addition to water.

[0030] Alternatively, powder in a wet state (or a state between wet and dry) may be collected from the powder accumulation environment, and water or corrosive ions may be added as needed before using it as a hydrated powder.

[0031] <Measurement of Potential> Next, a metal material intended for use in a powder deposition environment is brought into contact with the prepared water-containing powder, and the potential of the metal material (corrosion potential E) is measured. ｃｏｒｒ and pitting potential E ｐｉｔ The potential is measured. The metal material used to measure the potential is not limited to one type, but two or more types are preferable.

[0032] For measuring the potential of metallic materials, a general electrochemical cell is used, for example, which has a reference electrode, a counter electrode, a working electrode, and an electrolytic cell (container). In this case, the reference electrode may be, for example, a silver-silver chloride electrode (SSE) or a calomel electrode, and the counter electrode may be, for example, a platinum electrode or a carbon electrode (carbon rod).

[0033] Furthermore, the metal material whose potential is to be measured is used as the working electrode. Normally, when using a metal material as the working electrode, its surface is often polished before use in order to evaluate the performance of the base material (in a state without a passivation film). However, in this embodiment, the potential at which the passivation film is destroyed (pitting potential E) is used. ｐｉｔ To measure this, a metal material is used as the working electrode without polishing its surface (as it would be in a real-world powder deposition environment).

[0034] The amount of water-containing powder (apparent amount including voids, etc.) introduced into the electrolytic cell is not particularly limited, but for example, 1 cm of working electrode (metal material) ２ A volume of approximately 100 mL per unit is preferable, and the amount is such that the working electrode is buried by approximately 10 mm.

[0035] The potential of a metallic material is measured by polarizing the metallic material at the anode (by passing an electric current through the metallic material as the anode). The conditions for anodic polarization are the corrosion potential E of the metallic material. ｃｏｒｒ and pitting potential E ｐｉｔ While there are no particular limitations as long as the conditions for measurement are met, for example, anodic polarization is performed using the following procedure.

[0036] First, without applying an external voltage (current), the natural potential (i.e., corrosion potential E) of the metal material that is the working electrode is determined. ｃｏｒｒ ) Measure the corrosion potential E ｃｏｒｒ It often stabilizes in about 10 minutes. Therefore, after bringing the working electrode (metal material) into contact with the water-containing powder, the corrosion potential E is determined at least 10 minutes later. ｃｏｒｒ It is preferable to measure the corrosion potential E after 10 minutes. ｃｏｒｒ If the value of fluctuates beyond ±10 mV from the initial value, wait another 10 minutes before checking the corrosion potential E ｃｏｒｒ It is preferable to measure this.

[0037] Corrosion potential E ｃｏｒｒ After the measurement, the potential of the working electrode (metal material) is swept towards the anode. The sweep rate is preferably 50 mV / min or less, and more preferably 20 mV / min or less, because there is no significant difference in the results. The lower limit is not particularly limited, but for example, 5 mV / min from the viewpoint of measurement time. The potential reaches +1.5 V (or the current density reaches 1 mA / cm²). ２ By sweeping up to (above) a certain value, an anodic polarization curve (vertical axis: current density, horizontal axis: potential) is obtained. In the obtained anodic polarization curve, the potential at which the current density increases sharply is the pitting potential E at which the passivation film is destroyed. ｐｉｔ We will seek it as follows.

[0038] Figure 1 is a graph showing an example of an anodic polarization curve. In the graph shown in Figure 1, the corrosion potential E is the natural potential. ｃｏｒｒ This is approximately 0V (vs. SSE), and the pitting potential E is such that the current density increases rapidly. ｐｉｔ This is approximately 0.4V (vs. SSE).

[0039] When performing the anode polarization described above, any device including a potentiostat function for controlling the potential can be used. Preferably, this device is further connected to (or includes the function of) a logger capable of recording instantaneous values ​​of current and potential.

[0040] <Selection of metal material> And, the electric potential (corrosion potential E ｃｏｒｒ and pitting potential E ｐｉｔ Among the metal materials whose corrosion potential E was measured, ｃｏｒｒ Pitting potential E ｐｉｔ Large (noble), that is, E ｃｏｒｒ <E ｐｉｔ Metal materials that meet the specified criteria are determined to be suitable for use in a powder deposition environment. Structures are then manufactured using these determined metal materials. This determination may be made by workers in the powder deposition environment (such as a steel mill) or by a personal computer (PC) that has the measured potential data entered into it.

[0041] E ｃｏｒｒ <E ｐｉｔ If a metal material satisfies the criteria, it can be judged to be usable in an actual powder deposition environment, but the potential at which the passivation film is destroyed may change slightly. For this reason, E is chosen because it is judged to have superior corrosion resistance. ｐｉｔ and E ｃｏｒｒ The difference (E ｐｉｔ -E ｃｏｒｒ The value of is preferably as large as possible, for example, 100 mV (0.1 V) or more is preferred.

[0042] In the potential measurement described above, if the current density increases significantly immediately after the start of anodic polarization, the pitting potential E ｐｉｔ It is not measured. In this case, the corrosion potential E ｃｏｒｒ Pitting potential E ｐｉｔ Large (E ｃｏｒｒ <E ｐｉｔ Therefore, it cannot be said that the metal material is suitable for use in actual powder deposition environments.

[0043] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0044] First, as the metal material, we prepared one of the following stainless steels: SUS430, SUS304, SUS316, and SUS329J4L, as shown in Table 1 below.

[0045] The prepared metal materials were placed in multiple different locations within an actual factory, and a one-year exposure test was conducted under the conditions shown in Table 1 below. Corrosive ions were identified and quantified using ICP emission spectrometry. After the exposure test, the appearance of the metal materials was observed to check for rust formation. The results are shown in Table 1 below.

[0046] Next, using the prepared metal materials, selection tests were conducted under the conditions shown in Table 1 below, following the metal material selection method described above. For the corrosive ion concentration, the matched concentration (reference concentration or saturation concentration) is listed in Table 1 below. Other conditions included the use of a platinum electrode as the counter electrode and a silver-silver chloride electrode (SSE) as the reference electrode in the selection tests. 200 cm² ３ Prepare the powder, and the prepared powder is 1 cm ３ In contrast, 0.5 mL / cm³ ３ A certain amount of water (deionized water) was supplied. At a sweep rate of 20 mV / min, the potential reached +1.5 V (or the current density reached 1 mA / cm²). ２ The anodic polarization curve was obtained by sweeping until it exceeded (). Thus, for each metal material, the corrosion potential E ｃｏｒｒ and pitting potential E ｐｉｔ We calculated the following. The relationship between the two is shown in Table 1 below.

[0047]

[0048] <Summary of Evaluation Results> As shown in Table 1 above, for Nos. 1 to 22, if there was "no" rust formation in the exposure test, the potential of the metal material was "E" in all cases. ｃｏｒｒ <E ｐｉｔ (Nos. 3-4, 7-8, 10-12, 15-16, 18-22) and if rusting was "present" in the exposure test, the potential of the metal material was "E" in all cases. ｃｏｒｒ ≥E ｐｉｔThis indicates that by conducting selection tests under conditions that satisfy the requirements of the present invention, it is possible to identify and select metal materials suitable for use in powder deposition environments (materials that are less prone to rusting even when used in powder deposition environments) in advance.

[0049] In contrast, in samples No. 23-24, which were selected without conductive powder, even though rust was present in the exposure test, the potential was "E". ｃｏｒｒ <E ｐｉｔ " was the case.

[0050] Furthermore, in tests No. 25-26, where the powder deposition environment was a "repeated wet-dry environment" but the concentration of corrosive ions was set to the "reference concentration" rather than the "saturation concentration," even though rust formation was "present" in the exposure test, the potential was "E ｃｏｒｒ <E ｐｉｔ " was the case.

[0051] Furthermore, in selection tests No. 27-28, where conductive powder was "absent" and the powder deposition environment was a "submerged environment," but the concentration of corrosive ions was set to "saturation concentration," even though rusting was "present" in the exposure test, the potential was "E ｃｏｒｒ <E ｐｉｔ Similarly, in No. 29, which was also selected and tested, even though there was "no" rust formation in the exposure test, the potential was "E ｃｏｒｒ ≥E ｐｉｔ " was the case.

[0052] Furthermore, in No. 30, a selection test was conducted using different corrosive ions than those used in the exposure test, and where the powder deposition environment was a "submerged environment" but the concentration of corrosive ions was set to the "saturation concentration." Even though there was "no" rust formation in the exposure test, the potential was "E ｃｏｒｒ ≥E ｐｉｔ " was the case.

Claims

1. A method for selecting a metal material to be used in a powder deposition environment where a powder containing conductive powder is deposited, wherein the powder deposition environment is a submerged environment or a dry-wet repeated environment, the submerged environment is an environment where the powder is in a wet state, the dry-wet repeated environment is an environment where the powder repeats the wet state and the dry state, and the powder elutes at least one corrosive ion selected from the group consisting of SO ４ ２－ , Cl － and NO ３ － , and the concentration of the corrosive ion shows a saturation concentration higher than a reference concentration which is the concentration in the wet state between the wet state and the dry state. Water is supplied to the same powder as the powder to prepare a hydrated powder. When preparing the hydrated powder, if the powder deposition environment is the submerged environment, the concentration of the corrosive ion in the hydrated powder is made to coincide with the reference concentration, and if the powder deposition environment is the dry-wet repeated environment, the concentration of the corrosive ion in the hydrated powder is made to coincide with the saturation concentration. The corrosion potential E ｃｏｒｒ and the pitting potential E ｐｉｔ of the metal material are measured in a state of contact with the hydrated powder, and the metal material in which the pitting potential E ｃｏｒｒ is greater than the corrosion potential E ｐｉｔ is determined to be suitable for use in the powder deposition environment. A method for selecting a metal material.

2. The method for selecting a metal material according to claim 1, wherein the metal material has a passivation film on its surface.

3. The method for selecting a metal material according to claim 2, wherein the metal material is a steel material.

4. The method for selecting a metal material according to claim 3, wherein the steel material is stainless steel.

5. A method for manufacturing a structure, comprising manufacturing the structure using a metal material selected using the method for selecting a metal material described in any one of claims 1 to 4.

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