Method for selecting metal materials and method for manufacturing structures

Abstract

To easily and appropriately select metal materials suitable for use in powder deposition environments (submerged environments or repeatedly wet and dry environments) where conductive powders are deposited, water is supplied to the same powder as the deposited powder to prepare a water-containing powder in which corrosive ions are dissolved from the powder into the water. When preparing the water-containing powder, if the powder deposition environment is a 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 matched to the standard concentration (concentration in a wet state). If the powder deposition environment is a repeatedly wet and dry environment (an environment in which the deposited powder repeatedly goes between wet and dry states), the concentration of corrosive ions in the water-containing powder is matched to a saturation concentration higher than the standard concentration. The potential of the metal material is measured while in contact with the water-containing powder, and metal materials with a pitting potential greater than the corrosion potential are determined to be suitable for use in powder deposition environments.

Description

Technical Field

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

Background Art

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

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For example, in a steelworks, structures such as an unloader for unloading steel raw materials (coal, ore, coke, sinter, etc.), a belt conveyor for transporting steel raw materials, and a hopper for storing or discharging steel raw materials are used. In addition, 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 powders are deposited. The powder deposition environment may have different corrosion behaviors of metal materials from those in a normal corrosion environment (a corrosion environment where powders are not deposited). Therefore, it is required to simply and appropriately select a metal material suitable for use in a powder deposition environment.

[0006] This invention has been made in view of the above points, and aims to provide a method for selecting metal materials that can be easily and appropriately selected for use in powder deposition environments. [Means for solving the problem]

[0007] As a result of diligent research, the inventors of this invention discovered that the above objective can be achieved by adopting the following configuration, and thus completed the present invention. In other words, 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 conductive powder is deposited, wherein the powder deposition environment is a submerged environment or a wet-and-dry environment, the submerged environment is an environment in which the powder is in a wet state, the wet-and-dry environment is an environment in which the powder repeatedly goes between a wet state and a dry state, and the powder is SO4 in the wet state. 2- Cl - and NO3 - At least one corrosive ion selected from the group consisting of the above is eluted, and the concentration of the corrosive ion shows a saturation concentration higher than the reference concentration which is the concentration in the wet state between the wet state and the dry state, and moisture is supplied to the same powder as above to prepare a water-containing powder, and 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 match the reference concentration, and if the powder deposition environment is the wet-dry cycle environment, the concentration of the corrosive ion in the water-containing powder is made to match the saturation concentration, and the corrosion potential E of the metal material is measured in contact with the water-containing powder. corr and pitting potential E pit Measure the above corrosion potential E corr The above pitting potential E pit A method for selecting a metal material, which determines that the above-mentioned metal material with a large value is suitable for use in the above-mentioned powder deposition environment. [2] The metal material having a passive film on its surface, the method for selecting the metal material described in [1] above. [3] The method for selecting a metal material as described in [2] above, wherein the metal material is a steel material. [4] The method for selecting a metal material as described in [3] above, wherein the iron material is stainless steel. [5] A method for manufacturing a structure, comprising manufacturing the structure using a metal material selected using one of the metal material selection methods described in any of [1] to [4] above. [Effects of the Invention]

[0008] According to the present invention, a method for selecting metal materials that is suitable for use in powder deposition environments can be provided in a simple and appropriate manner. [Brief explanation of the drawing]

[0009] [Figure 1] This graph shows an example of an anodic polarization curve. [Modes for carrying out the invention]

[0010] <Knowledge gained by the inventors> First, the inventors' findings regarding the corrosion of metal materials used in powder deposition environments (the background leading to the present invention) are described below.

[0011] Powders such as coke and activated carbon are electrically conductive, and galvanic corrosion (corrosion between dissimilar metals) occurs between conductive powders and metal materials. In other words, the conductive powders act as cathode sites with a large surface area, accelerating the anodic reaction (a reaction in which the metal material dissolves) of the metal material, causing the metal material to corrode at a very rapid rate.

[0012] To prevent galvanic corrosion, it is effective to prevent contact between conductive powder and metal materials. For example, applying a coating to the surface of the metal material can be considered. However, the coating deteriorates due to ultraviolet rays, airborne sea salt, airborne 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 years, strict maintenance can prevent galvanic corrosion over a long period if it is frequently carried out. However, considering the situation where structures are used for an extremely long period of several decades or more, it is difficult to perfectly carry out maintenance over such a long period. Also, frequent implementation of strict maintenance is costly. Especially when carrying out maintenance in a place where work cannot be done without erecting scaffolding, the cost is extremely high.

[0013] By the way, in a powder deposition environment, a metal material (for example, a steel material such as stainless steel) with a passive film formed on its surface may be used. The passive film has, for example, a thickness of several nm and exhibits high corrosion resistance. However, depending on the situation of the powder deposition environment and the type of metal material, the passive film may be broken and corrosion may progress at a very high speed. Therefore, when using a metal material in a powder deposition environment, it is important to confirm in advance that the passive film will not be broken. However, to confirm the breakage of the passive film, it is preferable to conduct an exposure test in the powder deposition environment where the metal material is actually used and evaluate the period until rusting occurs, but the exposure test takes a very long time.

[0014] The passive film will not be broken unless it becomes a potential (higher potential) nobler than a certain potential determined by the characteristics of the passive film and the powder deposition environment, etc. Therefore, the corrosion potential E corr obtained under conditions simulating (reproducing) the powder deposition environment pit is lower than the pitting potential E that breaks the passive film, then the metal material can retain the passive film in that powder deposition environment. And while the passive film is retained, the anodic reaction is suppressed by the passive film, so the corrosion rate of the metal material remains very small. corr <Epit Metal materials that satisfy these conditions can be determined to be suitable for use in powder deposition environments.

[0015] This invention is based on the above findings. Preferred embodiments of the present invention will be described below. The metal material selection method of this embodiment is, in general terms, a method of selecting a metal material that will 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 explanation also serves as an explanation of the manufacturing method of the structure.

[0016] <Metal materials> While not limited to metal materials, for the reasons mentioned above, metal materials having a passive film on their surface (more specifically, metal materials in which a passive film is formed on the surface in a powder deposition environment) are preferred, and specifically, steel materials such as stainless steel (e.g., SUS430, SUS304, SUS316, and SUS329J4L) are preferred.

[0017] Metallic materials are used as building materials in structures within powder deposition environments. Examples of structures include those found in steel mills, thermal power plants, and coal or ore mines. If the powder deposition environment is a steel mill (or part of one), examples of structures include unloaders for unloading steel raw materials (coal, ore, coke, sintered ore, etc.); belt conveyors for transporting steel raw materials; and hoppers for storing or transporting steel raw materials. Furthermore, if the powder deposition environment is a coal or ore mining site, 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 the deposited powder, which can cause corrosion of the metal materials. Powder accumulation environments occur spontaneously 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; and powders of coke and sintered ore produced from these mineral resources. Furthermore, these powders contain conductive powders. Examples of conductive powders include coke powder and activated carbon powder used in water filtration.

[0020] The environment in which powder accumulates is either a submerged environment or an environment with repeated wetting and drying cycles. A submerged environment is an environment in which accumulated powder is in a moist state, for example, an environment in which the accumulated powder is constantly wet (contains moisture) due to industrial water used in the surrounding area. In contrast, a wet-and-dry environment is one in which the deposited powder repeatedly goes between wet and dry states. For example, 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, contains SO4 2- Cl - and NO3 - 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) present around the powder. Corrosive ions originate, for example, from impurities mixed into powders. The powder may also leach ions other than corrosive ions, but in this embodiment, we focus on these three types of ions. NO3 - For example, in acidic conditions, it can form a passivation film, but SO4 2- and Cl - This 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 corrosive ions become concentrated. For example, just before the dry state, the concentration of corrosive ions becomes saturated, reaching a "saturation concentration" that is higher than the reference concentration. When the concentration of corrosive ions is at the "saturation concentration," the passivation film on metal materials is more easily destroyed than when it is at the "reference concentration." Note that the "reference concentration" and "saturation concentration" are not fixed values, but rather vary depending on the location and humidity of the powder deposition environment, 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 referred to as "deposited powder") is prepared, and water (for example, deionized water) is supplied to the prepared powder to prepare a water-containing powder.

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

[0026] When preparing water-containing powders, the concentration of corrosive ions in the water-containing powder is 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 adjusted to match the "standard concentration" mentioned above. Furthermore, if the powder deposition environment is a "wet-drying and wetting environment," the concentration of corrosive ions in the water-containing powder should be matched to the "saturation concentration" mentioned above.

[0027] Here, "match" 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 be "matching."

[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 to use it as a hydrated powder.

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

[0032] For measuring the potential of metallic materials, a typical 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, a reference electrode such as a silver-silver chloride electrode (SSE) or a calomel electrode is used, and a counter electrode such as a platinum electrode or a carbon electrode (carbon rod) is used.

[0033] Furthermore, the metallic material whose potential is to be measured is used as the working electrode. In general, when using metal materials as the working electrode, it is common practice to polish the surface of the base material (in a state without a passivation film) before use in order to evaluate its performance. However, in this embodiment, the potential at which the passivation film is destroyed (pitting potential E) 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 electrode is preferable, and this volume is sufficient to fill the working electrode 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. 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 value]. Furthermore, after 10 minutes, the corrosion potential E corrIf the value fluctuates beyond ±10mV 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, from the viewpoint of measurement time, it is 5 mV / min. The potential reaches +1.5V (or the current density reaches 1mA / cm²). 2 By sweeping up to (beyond) a certain point, an anodic polarization curve (vertical axis: current density, horizontal axis: electric 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, which is the natural potential, is 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 materials> And, the 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 It is large (noble), that is, E corr <E pitMetal materials that meet the specified criteria are determined to be suitable for use in powder deposition environments. Then, structures are manufactured using these determined metal materials. This determination may be made by a worker in a powder deposition environment (such as a steel mill), or it may be made by a personal computer (PC) into which the measured potential data has been entered.

[0041] E corr <E pit Metal materials that satisfy these conditions can be considered usable in actual powder deposition environments, however, the potential at which the passivation film is destroyed may change slightly. Therefore, E pit and E corr The difference (E pit -E corr The value of is preferably as large as possible, for example, 100mV (0.1V) 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 Since it cannot be said that this is the case, the metal material can be judged as unusable in actual powder deposition environments. [Examples]

[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, in accordance with the metal material selection method described above. For the corrosive ion concentrations, the standardized concentrations (reference concentrations or saturation concentrations) are 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 test. (200 cm²) 3 Prepare the powder, and the prepared powder is 1 cm 3 In contrast, 0.5 mL / cm³ 3 The following 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 up to (above) a certain value. 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] [Table 1]

[0048] <Summary of Evaluation Results> As shown in Table 1 above, for Nos. 1 to 22, if no rust occurred in the exposure test, the potential of the metal material was "E" in all cases. corr <E pit (No.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 pit " was the case. From this, it was found 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 occurred 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 "standard 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 the selection test for No. 29, 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, which used 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 "saturation concentration" for the selection test, 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 in which conductive powder is deposited, The aforementioned powder deposition environment is a submerged environment or a wet-and-dry environment. The aforementioned submerged environment is an environment in which the powder is in a wet state. The aforementioned wet-dry cycle environment is an environment in which the powder repeatedly goes between the wet state and the dry state. The powder, in the wet state, is SO ４ ２－ , Cl － and NO ３ － Elute at least one corrosive ion selected from the group consisting of the following, The concentration of the corrosive ions shows a saturation concentration higher than the reference concentration, which is the concentration in the wet state, between the wet state and the dry state. A water-containing powder is prepared by supplying water to the same powder as described above. When preparing the water-containing powder, if the powder deposition environment is a submerged environment, the concentration of the corrosive ions in the water-containing powder is made to match the reference concentration, and if the powder deposition environment is a wet-and-dry environment, the concentration of the corrosive ions in the water-containing powder is made to match the saturation concentration. When in contact with the water-containing powder, the corrosion potential E of the metal material ｃｏｒｒ and pitting potential E ｐｉｔ Measure, The corrosion potential E ｃｏｒｒ is greater than the pitting potential E ｐｉｔ A method for selecting a metal material, which determines that the metal material is suitable for use in the powder deposition environment.

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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