Method for evaluating corrosion resistance of steel material for exhaust gas piping, method for selecting steel material for exhaust gas piping, and method for manufacturing exhaust gas piping

Abstract

Provided is a method for evaluating the corrosion resistance of a steel material for exhaust gas piping, the method being capable of appropriately evaluating the corrosion resistance of a steel material used for exhaust gas piping through which exhaust gas, containing ammonia and at least one among a substance eluting SO4 2- and a substance eluting Cl- into drainage generated in the piping, passes.　In this method for evaluating the corrosion resistance of a steel material for exhaust gas piping, the exhaust gas piping is piping through which exhaust gas containing ammonia and at least one among a substance that elutes SO4 2- and a substance that elutes Cl- into drainage generated in the piping passes, the method including an immersion step for immersing the steel material in a test liquid for 4 hours or longer, wherein the test liquid has a pH of 8.0 or higher and contains at least one among sulfate ions and chloride ions, the concentration of at least one of the sulfate ions and the chloride ions is 1 mg / L or more, the concentration of the sulfate ions is 1,000 mg / L or less, and the concentration of the chloride ions is 100 mg / L or less.

Description

Method for Evaluating Corrosion Resistance of Steel Material for Exhaust Gas Pipe, Method for Selecting Steel Material for Exhaust Gas Pipe, and Method for Manufacturing Exhaust Gas Pipe

[0001] The present invention relates to a method for evaluating the corrosion resistance of a steel material for an exhaust gas pipe, a method for selecting a steel material for an exhaust gas pipe, and a method for manufacturing an exhaust gas pipe.

[0002] In plant facilities, exhaust gas generated by the combustion of various fuels is discharged from a chimney, and a pipe for exhaust gas (exhaust gas pipe) through which the exhaust gas passes until it reaches the chimney may be corroded, which may cause problems.

[0003] Specifically, in a steelworks, for example, high-temperature exhaust gas generated in a heating furnace for steel materials passes through an exhaust gas pipe and is naturally cooled. When the temperature of the exhaust gas drops below the dew point, the exhaust gas becomes drain (concentrated liquid) and accumulates on the bottom surface of the exhaust gas pipe, L-shaped joint parts, etc. In a heating furnace for steel materials, a mixed gas mainly composed of carbon monoxide, methane, hydrogen, etc., which is a by-product gas of a steelworks, is used as fuel. However, the above mixed gas contains sulfur (S) etc. as impurities, and the exhaust gas generated by the combustion of the above mixed gas contains sulfur oxide. This sulfur oxide reacts with moisture in the exhaust gas to generate sulfuric acid, which concentrates on the steel material inside the pipe when cooled below the dew point, causing corrosion of the exhaust gas pipe. Therefore, it is important to evaluate the corrosion resistance of members in contact with exhaust gas in an environment that simulates the environment where drain accumulates.

[0004] Patent Document 1 describes a test method for evaluating the corrosion resistance of a material for a heat exchanger that recovers exhaust gas latent heat. Specifically, an immersion liquid having a composition approximated to the drain of exhaust gas and added with a corrosion-promoting anion is prepared, and then an operation of immersing a test piece of the above material in this immersion liquid and an operation of drying the above test piece with exhaust gas after immersion are repeated a number of times at a constant temperature and a constant cycle. At the same time, a dew point corrosion promotion test method for periodically detecting a predetermined anion concentration in the immersion liquid by ion chromatography during the test period and maintaining the above predetermined anion concentration constant by replenishment or dilution is described.

[0005] In addition, Patent Document 2 describes a method for evaluating the corrosion resistance of a metal material used for a member of an LNG-fired boiler combustion exhaust system. Specifically, it is maintained at a temperature near the dew point of 50 to 70°C, and CO ２ , O ２ , Cl － , SO ４ ２－ dissolved, and it is described that a liquid thin film corrosion environment due to condensed water in a real environment can be reproduced by a Dip&Dry test in which a gas simulating combustion exhaust gas is aerated into the gas phase and the solution. [[ID=第十一条]]

[0006] Furthermore, Patent Document 3 describes a method for accurately evaluating the corrosion resistance of a metal material used for an automotive exhaust system such as a muffler. Specifically, after powdered activated carbon is attached to the surface of a test material heat-treated at a predetermined temperature for a predetermined time in the atmosphere, Cl at a predetermined concentration － , SO ３ ２－ , SO ４ ２－ , CO ３ ２－ , NH ４ ＋ , NO ２ － , NO ３ － , CH ３ COO － , HCOO － , HCHO are included, and it is immersed completely in a solution adjusted to pH 7 to 10, kept at a constant temperature, the solution is completely evaporated, the crystals of the solution adhering to the test material and the activated carbon are removed, the solution and the activated carbon are newly replenished, and the same test is repeated a plurality of times, and the corrosion resistance is evaluated by the degree of corrosion of the test material.

[0007] Japanese Patent Application Laid-Open No. 2000-46724 Japanese Patent Application Laid-Open No. 6-337241 Japanese Patent Application Laid-Open No. 5-72114

[0008] With the recent trend of carbon neutrality, with respect to conventional fuel gas such as LNG and by-product gas in a steel mill (for example, CO, H ２ , CH ４The idea is to add ammonia to a mixed gas (composed of various substances). This is because replacing some of the carbon dioxide produced by combustion in LNG and other fuels with ammonia would reduce carbon dioxide emissions.

[0009] As described above, it was found that the corrosion resistance of exhaust gas piping materials when ammonia is mixed as fuel gas cannot be properly evaluated using the methods described in Patent Documents 1 and 2. Furthermore, it was found that the method described in Patent Document 3 is a method for evaluating the corrosion resistance of metal materials used in automobile exhaust systems such as mufflers, and is not suitable for evaluating the corrosion resistance of exhaust gas piping materials in plants, etc.

[0010] The present invention has been made in view of the above circumstances, and is a method for evaluating the corrosion resistance of steel materials for exhaust gas piping, wherein ammonia and the drain generated in the exhaust gas piping are used. ４ ２－ Substances that leach and Cl － The objective is to provide a method for evaluating the corrosion resistance of steel materials used in exhaust gas piping, which can appropriately evaluate the corrosion resistance of steel materials used in exhaust gas piping through which exhaust gas containing at least one substance that elutes a certain substance passes.

[0011] 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.

[0012] In other words, the gist of the present invention is as follows: [1] A method for evaluating the corrosion resistance of steel materials for exhaust gas piping, wherein the exhaust gas piping is ammonia and SO2 is used in the drain generated in the exhaust gas piping. ４ ２－ Substances that leach and Cl －A method for evaluating the corrosion resistance of steel materials for exhaust gas piping, comprising: a pipe through which exhaust gas containing at least one substance that elutes a substance passes; an immersion step of immersing the steel material to be evaluated in a test solution for 4 hours or more; the test solution having a pH of 8.0 or higher and containing at least one of sulfate ions and chloride ions; the concentration of at least one of the sulfate ions and chloride ions being 1 mg / L or higher; the concentration of the sulfate ions being 1000 mg / L or lower; and the concentration of the chloride ions being 100 mg / L or lower. [2] The method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to [1], wherein the pH of the test solution is adjusted using at least one selected from the group consisting of ammonia, sodium hydroxide and calcium hydroxide. [3] The method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to [1] or [2], further comprising a washing step of washing the steel material after the immersion step. [4] A method for evaluating the corrosion resistance of exhaust gas piping steel materials according to any one of [1] to [3], further comprising a test solution replacement step of replacing the test solution after the immersion step with a new test solution. [5] A method for selecting exhaust gas piping steel materials, comprising an evaluation step of evaluating the corrosion resistance of steel materials using the method for evaluating the corrosion resistance of exhaust gas piping steel materials according to any one of [1] to [4], and a selection step of selecting steel materials based on the evaluation results obtained in the evaluation step. [6] A method for manufacturing exhaust gas piping, comprising a manufacturing step of processing the steel materials selected by the method for selecting exhaust gas piping steel materials according to [5] to manufacture exhaust gas piping.

[0013] According to the present invention, ammonia and the drain generated in the piping are treated with SO ４ ２－ Substances that leach and Cl － This invention provides a method for evaluating the corrosion resistance of steel materials used in exhaust gas piping, which can appropriately evaluate the corrosion resistance of steel materials used in exhaust gas piping through which exhaust gas containing at least one substance that elutes is carried.

[0014] The corrosion resistance evaluation method for exhaust gas piping steel materials of the present invention can reproduce the corrosion patterns in the actual environment of exhaust gas piping through which the exhaust gas passes. Furthermore, it is possible to evaluate the order of corrosion resistance of steel materials in the aforementioned actual environment. Hereinafter, the corrosion resistance evaluation method for exhaust gas piping steel materials of the present invention will also be simply referred to as the corrosion resistance evaluation method of the present invention.

[0015] Figure 1 illustrates one embodiment of the test cycle related to the corrosion resistance evaluation method of the present invention. Figure 2 illustrates another embodiment of the test cycle related to the corrosion resistance evaluation method of the present invention. Figure 3 illustrates another embodiment of the test cycle related to the corrosion resistance evaluation method of the present invention. Figure 4 illustrates another embodiment of the test cycle related to the corrosion resistance evaluation method of the present invention.

[0016] First, we will explain the findings obtained by the inventors. The inventors conducted a detailed investigation into the corrosion behavior of pipes (made of steel) caused by exhaust gas generated when a fuel gas, which is a mixture of ammonia and by-product gases in a steel mill, is burned.

[0017] As a result, it was found that localized corrosion occurs in areas where drain (condensed water) accumulates in the piping. Therefore, a detailed investigation into the mechanism of localized corrosion in the piping revealed the following: (1) Unburned ammonia is dissolved in the drain; (2) The dissolution of ammonia in the drain increases the pH of the drain, causing a passivation film to form on the surface of the piping (steel material) and resulting in passivation; (3) SO4 is present in the drain as an impurity. ４ ２－ or Cl － (4) SO ４ ２－ or Cl － It was found that the presence of [the substance] destroys the passive film formed on the surface of the pipe (steel material), leading to the progression of localized corrosion. Based on the above findings, the inventors conducted various studies and completed the present invention.

[0018] The present invention will be described in detail below. The following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below.

[0019] [Method for evaluating the corrosion resistance of steel materials for exhaust gas piping] A method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to one embodiment of the present invention comprises an immersion step of immersing the steel material to be evaluated in a test liquid.

[0020] <Exhaust Gas Piping> The steel material to be evaluated by the corrosion resistance evaluation method of the present invention is the steel material used in exhaust gas piping. The exhaust gas piping is used for ammonia and for the drain (condensed water) that is produced in the exhaust gas piping, which is treated with SO2. ４ ２－ Substances that leach and Cl － This is a pipe through which exhaust gas containing at least one substance that leaches is passed.

[0021] The exhaust gases mentioned above include LNG used as fuel in plants, etc., and by-product gases (e.g., CO, H) in the case of steel mills. ２ ,CH ４ This is exhaust gas produced when a fuel mixed with ammonia (such as a mixed gas composed of the above) is burned. Examples of the by-product gases include converter gas (LD gas) generated from a converter, blast furnace gas (B gas) generated from a blast furnace, coke oven gas (C gas) generated from a coke oven, and M gas which is a mixture of these. The exhaust gas contains unburned ammonia and at least one of S and Cl as an impurity. The S and Cl included as impurities are SO ４ ２－ and Cl － As a result, it dissolves into the drain in the exhaust gas piping. In other words, in the exhaust gas piping through which the above-mentioned exhaust gas passes, ammonia and SO2 derived from sulfur in the exhaust gas are present. ４ ２－ and Cl derived from Cl － A drain (condensed water) is produced that contains at least one of the following.

[0022] As an example, the composition of the condensate generated in the exhaust gas piping through which the exhaust gas described above passes is such that the ammonia concentration in the condensate is 1 to 1000 mg / L. Also, as an example, the SO2 concentration in the condensate is... ４ ２－ The concentration is 0 to 1000 mg / L. Also, as an example, Cl in the drain － The concentration is 0 to 100 mg / L. However, the drain contains ammonia and SO2. ４ ２－ , Cl － It includes at least one of the following.

[0023] <Steel materials for exhaust gas piping> In this invention, steel materials used in exhaust gas piping as described above are the subject of evaluation. The steel materials to be evaluated are not particularly limited.

[0024] (Immersion Process) In the immersion process, the steel material to be evaluated is immersed in the test solution for at least four hours. Specifically, the steel material is placed in a corrosion test container, the test solution is introduced, and the material is immersed for at least four hours.

[0025] The container for corrosion testing is not particularly limited as long as it is a container of a size and shape that allows the steel material to be immersed in the immersion process. The material of the container is preferably a material that has corrosion resistance to the test liquid. Furthermore, if the immersion process is performed at a temperature higher than room temperature, the material is preferably a heat-resistant material. Specific examples of heat-resistant materials include glass and Teflon (registered trademark, hereinafter the same).

[0026] <Test Solution> The volume of the test solution should be such that the evaluation surface of the steel material to be immersed in the immersion process is submerged in the test solution. The specific liquid volume (mL / cm³) is defined as the volume of the test solution relative to the area of ​​the evaluation surface of the steel material. ２ ) is 20 mL / cm³ ２ The above is preferable. If iron ions generated by the corrosion of steel are present in the test solution, the corrosion of the steel may be suppressed and affect the corrosion rate of the steel. However, the specific liquid volume is 20 mL / cm³. ２ By doing so, the amount of test solution relative to the surface area of ​​the steel material being evaluated increases, which suppresses the rise in the concentration of iron ions in the test solution and eliminates the influence of iron ions on the corrosion rate of the steel material. As a result, the corrosion resistance of the steel material can be evaluated more appropriately. There is no particular upper limit to the amount of specific solution used. As mentioned above, increasing the amount of specific solution used eliminates the influence of iron ions on the corrosion rate of the steel material, but the amount of specific solution used is 50 mL / cm³. ２ Beyond this point, the aforementioned effect saturates. Therefore, from an economic standpoint, the specific volume should be 50 mL / cm³. ２ The following are preferable.

[0027] The pH of the test solution should be 8.0 or higher. Here, the pH should be the pH at 25°C. By setting the pH of the test solution to 8.0 or higher, a highly protective passive film is formed on the surface of the steel material, making it possible to reproduce the corrosion behavior at drain accumulation points in exhaust gas pipes through which ammonia-containing exhaust gas passes. The pH of the test solution is preferably 9.0 or higher, and more preferably 10.0 or higher. On the other hand, there is no particular upper limit to the pH of the test solution. The pH of the test solution may be 14.0 or lower. However, since higher pH levels pose risks in experimental work, it is preferable to set the pH of the test solution to 12.0 or lower.

[0028] The method for adjusting the pH of the test solution is not particularly limited as long as the pH can be adjusted to 8.0 or higher, but it is preferable to adjust it using at least one selected from the group consisting of ammonia, sodium hydroxide, and calcium hydroxide. Ammonia, sodium hydroxide, and calcium hydroxide are each NH in the test solution. ４ ＋ Na ＋ Ca ２＋ The following cations are generated. The drain often contains these cations, and the reproducibility of the real environment can be further improved by adjusting the pH of the test solution using ammonia, sodium hydroxide, and calcium hydroxide. However, if Na is present in the test solution ＋ Ya Ca ２＋ The presence of certain substances can suppress the corrosion of steel materials. As a result, the evaluation time for steel materials may be extended. On the other hand, when using ammonia, there is no concern that such problems will occur. Therefore, it is more preferable to adjust the pH of the test solution using ammonia.

[0029] The test solution must contain at least one of sulfate ions and chloride ions. Furthermore, the concentration of at least one of the sulfate ions and chloride ions must be 1 mg / L or higher. Specifically, the sulfate ion concentration must be 1000 mg / L or lower, and the chloride ion concentration must be 100 mg / L or lower. That is, the test solution must have a sulfate ion concentration of 1 mg / L or higher, and / or a chloride ion concentration of 1 mg / L or higher. If the test solution contains sulfate ions, the sulfate ion concentration must be 1000 mg / L or lower. If the test solution contains chloride ions, the chloride ion concentration must be 100 mg / L or lower. Sulfate ions and chloride ions have the effect of destroying the passive film, causing localized corrosion as corrosion preferentially progresses at the destroyed areas. This allows for the simulation of the corrosion behavior of exhaust gas piping in a real-world environment, which is the target of this invention.

[0030] If the concentration of at least one of sulfate ions and chloride ions is less than 1 mg / L, the passive film formed on the surface of the steel material will not break down. Therefore, the concentration of at least one of sulfate ions and chloride ions should be 1 mg / L or higher. Preferably, the concentration of at least one of sulfate ions and chloride ions should be 10 mg / L or higher. On the other hand, if the concentration of sulfate ions exceeds 1000 mg / L, or if the concentration of chloride ions exceeds 100 mg / L, the corrosion mode of the steel material will be general corrosion rather than local corrosion, and the corrosion mode in the actual environment of the exhaust gas piping targeted by this invention cannot be reproduced. Therefore, if sulfate ions are included, the concentration of sulfate ions should be 1000 mg / L or less, and if chloride ions are included, the concentration of chloride ions should be 100 mg / L or less. Preferably, the concentration of sulfate ions should be 800 mg / L or less, more preferably 500 mg / L or less, and even more preferably 100 mg / L or less.

[0031] The sulfate ions and chloride ions mentioned above can be supplied to the test solution, for example, by adding a substance that supplies sulfate ions or a substance that supplies chloride ions to the test solution. Examples of substances that supply sulfate ions include sulfuric acid, ammonium sulfate, sodium sulfate, calcium sulfate, and iron sulfate. Examples of substances that supply chloride ions include hydrochloric acid, ammonium chloride, sodium chloride, calcium chloride, and iron chloride. The substance that supplies sulfate ions can be appropriately selected while considering the composition and pH of the test solution; for example, one of the sulfate ion-supplying substances may be used alone, or two or more may be used in combination. Similarly, the substance that supplies chloride ions can be appropriately selected while considering the composition and pH of the test solution; for example, one of the chloride ion-supplying substances may be used alone, or two or more may be used in combination. However, if an iron-containing substance such as iron sulfate or iron chloride is used as the sulfate ion-supplying substance or chloride ion-supplying substance in the test solution, iron ions will be supplied to the test solution, suppressing corrosion of the steel material and potentially prolonging the evaluation of corrosion resistance. Therefore, in order to suppress the time required for evaluating corrosion resistance, it is preferable not to use iron-containing substances such as iron sulfate and iron chloride as the substances that supply sulfate ions and chloride ions. The concentrations of sulfate ions and chloride ions in the test solution can be adjusted, for example, by adjusting the amount of the sulfate ion supplying substance and the chloride ion supplying substance added to the test solution.

[0032] The immersion time for the steel material in the test solution should be 4 hours or more. If the immersion time is less than 4 hours, a passive film will not form on the surface of the steel material, and the corrosion behavior in the actual environment of the exhaust gas piping targeted by this invention cannot be reproduced. For this reason, the immersion time should be 4 hours or more. Preferably, the immersion time should be 6 hours or more. On the other hand, the upper limit of the immersion time is not particularly limited and can be set arbitrarily. However, if the immersion time is extended, there is a tendency for more Fe ions to dissolve into the test solution due to the corrosion of the steel material. Also, as mentioned above, Fe ions present in the test solution have the effect of suppressing the corrosion of the steel material. Therefore, as the immersion time increases, the iron ion concentration in the test solution increases due to the iron ions produced by the corrosion reaction of the steel material, and the corrosion rate tends to decrease. As a result, the evaluation time for determining the superiority or inferiority of the corrosion resistance of the steel material increases, which is disadvantageous in material development. Furthermore, the pH and composition of the test solution may fluctuate, and it may not be possible to properly evaluate the corrosion resistance of the steel material. For this reason, the immersion time should preferably be 48 hours or less, and more preferably 24 hours or less.

[0033] The arrangement of the steel material is not particularly limited, as long as the evaluation surface of the steel material is in contact with the test liquid. For example, the evaluation surface of the steel material may be arranged horizontally, diagonally, or vertically. From the viewpoint of reproducing corrosion at drain accumulation points in actual exhaust gas piping, it is preferable to place the steel material horizontally (evaluation surface facing upwards).

[0034] The temperature of the test solution used for immersion is not particularly limited and may be room temperature or set to the actual temperature of the exhaust gas piping being evaluated. Setting the temperature of the test solution to 60°C or higher accelerates the corrosion reaction of the steel, making it possible to evaluate the corrosion resistance of the steel in a short period of time. Therefore, from the viewpoint of evaluating corrosion resistance in a short period of time, it is preferable to set the temperature of the test solution to 60°C or higher. The temperature of the test solution can be controlled by setting up a corrosion test container containing the test solution and the steel in a temperature-controlled environment. Temperature control may be performed, for example, using a constant temperature bath or an electric furnace.

[0035] The corrosion resistance evaluation method according to this embodiment may further include a washing step of washing the steel material with water after the immersion step.

[0036] (Water Washing Process) In the water washing process, the steel material is immersed in the test solution for a predetermined time, and then the surface of the steel material (evaluation surface) is washed with water. When a water washing process is provided, corrosion products (sediments) formed on the surface of the steel material during the immersion process can be removed by washing with water. Therefore, it is possible to suppress the reduction in the corrosion rate due to corrosion products. As a result, the corrosion resistance of the steel material can be evaluated in a short period of time, and the development of steel materials can be promoted. The water washing method is not particularly limited, but the steel material may be removed from the test solution and washed with water. Alternatively, as described later, the test solution may be drained from the corrosion test container and then the steel material may be washed with water. Furthermore, after the water washing process, the immersion process may be repeated in which the steel material after the water washing process is immersed in the test solution used before the water washing process, or an immersion process may be performed in which the steel material after the test solution replacement process described later is immersed in a new test solution.

[0037] The corrosion resistance evaluation method according to this embodiment may further include a test solution replacement step in which the test solution after the immersion step is replaced with a new test solution (unused test solution).

[0038] (Test Solution Replacement Process) In the test solution replacement process, the test solution used after the immersion process is replaced with a new test solution. In the test solution replacement process, after the immersion process, the test solution can be replaced with a new test solution by draining the test solution from the corrosion test container and then introducing a new test solution (unused test solution) into the corrosion test container. Alternatively, in the test solution replacement process, the corrosion test container used after the immersion process may be replaced with a corrosion test container containing a new test solution prepared separately.

[0039] After the test solution replacement step, an immersion step is performed in which the steel material is immersed in the new test solution (unused test solution) that has been replaced as described above. By performing this test solution replacement step, it is possible to suppress the increase in the amount of iron ions eluted into the test solution and the fluctuation of pH, thereby suppressing the influence of fluctuations in the composition and pH of the test solution on the corrosion resistance of the steel material. For this reason, it is preferable to include a test solution replacement step.

[0040] In the immersion process following the test solution replacement process, the steel material may be immersed in a new test solution with the same composition as the test solution used in the immersion process before the test solution replacement process. Furthermore, when evaluating the effects of changes in drain composition over time in a real environment, the steel material may be immersed in a new test solution with a different composition than the test solution used in the immersion process before the test solution replacement process.

[0041] <Test Cycle> The corrosion resistance evaluation method according to one embodiment of the present invention may be carried out by arbitrarily combining an immersion step and a washing step and / or a test solution replacement step. For example, as shown in Figure 1, the immersion step and the test solution replacement step may be considered as one cycle, and this cycle may be carried out once or two or more times. Also, as shown in Figure 2, the immersion step and the washing step may be considered as one cycle, and this cycle may be carried out once or two or more times. Furthermore, the immersion step, the washing step and the test solution replacement step may be considered as one cycle, and this cycle may be carried out once or two or more times. In this case, as shown in Figure 3, the order may be immersion step → washing step → test solution replacement step, or as shown in Figure 4, the order may be immersion step → test solution replacement step → washing step. By using a test cycle that includes a test solution replacement step, the influence of fluctuations in the composition and pH of the test solution can be eliminated as much as possible, the corrosion form of localized corrosion in the actual environment inside the exhaust gas piping that is the target of evaluation in the present invention can be reproduced, and the corrosion resistance of the steel material can be evaluated in a short time. Furthermore, by including a water rinsing step in the test cycle, the influence of corrosion products formed on the steel surface is reduced, making it possible to evaluate the corrosion resistance of the steel in a shorter time. When the cycle is performed two or more times, the water rinsing step and / or test solution replacement step after the immersion step may be omitted in the second and subsequent cycles, and in this case the number of cycles is the number of times the immersion step is performed. There is no particular upper limit to the number of cycles. For example, the number of test days may be predetermined, and the number of cycles may be equivalent to the number of test days. The number of cycles can be set appropriately, for example, taking into consideration the reproduction of the corrosion form in the actual environment inside the exhaust gas piping to be evaluated. As an example, the number of cycles may be set so that the total time of the immersion step is 2500 hours or less, or so.

[0042] In the corrosion resistance evaluation method according to this embodiment, after performing an immersion step for a predetermined immersion time, or after performing the above test cycle for a predetermined number of cycles, the corrosion resistance of the surface (evaluation surface) of the steel material is evaluated (an evaluation step for evaluating the corrosion resistance of the steel material).

[0043] <Evaluation of Corrosion Resistance> The evaluation of the corrosion resistance of steel materials is not particularly limited, but it is preferable to evaluate it using one or more of the corrosion amount and corrosion rate of the steel material. For example, by using the corrosion amount of the steel material, the corrosion form and the order of corrosion resistance of the steel materials can be determined. The type of corrosion amount is not particularly limited. For example, the corrosion amount can be determined from the weight difference of the steel material before and after the immersion process. In this case, it is preferable to use the weight of the steel material after the immersion process as the weight of the steel material after removing the corrosion products formed on the evaluation surface of the steel material by immersion in acid. Furthermore, in order to determine the order of corrosion resistance of the steel materials, it is more preferable to use the corrosion amount obtained by dividing the above weight difference by the area of ​​the evaluation surface of the steel material. Alternatively, the corrosion depth may be measured as the corrosion amount. In this case, after removing the corrosion products formed on the evaluation surface of the steel material by immersion in acid, the surface irregularities (surface shape) of the steel material can be observed with a 3D shape measuring machine or the like, and the corrosion depth can be measured as the corrosion amount. The corrosion depth may be the average value of the corrosion depth over the measurement area (e.g., the entire evaluation surface), or the maximum value of the corrosion depth at the location where localized corrosion occurred. Here, the average value of the corrosion depth may be the average value of the measured corrosion depth, or a value calculated by dividing the weight difference by the area of ​​the evaluation surface of the steel material and the density of the steel material. Furthermore, the corrosion form of the steel material (localized corrosion, general corrosion, etc.) can be evaluated from the distribution of corrosion depth on the evaluation surface. In addition, the corrosion resistance of the steel material may be evaluated using the corrosion rate of the steel material. In this case, the corrosion rate calculated from the amount of corrosion and test conditions (e.g., immersion time in the test solution) can be used. The superiority or inferiority of the corrosion resistance of the steel material can be evaluated from the corrosion rate. Based on the above evaluation, the ranking of the corrosion resistance of the steel material can be determined. The corrosion rate may be the average corrosion rate based on the weight difference, the average corrosion rate based on the average corrosion depth, or the maximum corrosion rate of localized corrosion based on the maximum value of the corrosion depth at the location where localized corrosion occurred. Furthermore, because it offers superior reproducibility in real-world environments, it is preferable to use the maximum corrosion rate of localized corrosion when determining the ranking of corrosion resistance of steel materials. However, if the corrosion morphology and the ranking of corrosion resistance of steel materials can be determined visually, the evaluation of corrosion resistance of steel materials may be performed by visually observing the evaluation surface of the steel material.For evaluation using the amount of corrosion, corrosion rate, and corrosion morphology of steel materials, please refer to the description in the examples.

[0044] [Method for selecting steel materials for exhaust gas piping] The present invention provides a method for selecting steel materials for exhaust gas piping, comprising: an evaluation step of evaluating the corrosion resistance of steel materials using the corrosion resistance evaluation method for exhaust gas piping steel materials described above; and a selection step of selecting steel materials based on the evaluation results obtained in the evaluation step.

[0045] By performing the above evaluation process, for example, the relationship between the steel type and the amount of corrosion of the steel material under predetermined test conditions can be obtained. Then, using the obtained relationship between test conditions, steel type, and amount of corrosion, the steel type can be classified. Then, depending on the environment in which it will be used, a steel type with corrosion resistance that can be used for a predetermined target service life can be selected from the above classification. Note that the above selection process may also be carried out based on whether the steel material subjected to the above evaluation process meets predetermined criteria. In this case, for example, in the above selection process, if the steel material subjected to the above evaluation process meets the predetermined criteria, that steel material can be selected. As the criteria, for example, corrosion resistance that can be used for a predetermined target service life can be set as the selection criterion. Specifically, the amount of corrosion, the immersion time to reach a predetermined amount of corrosion, or the number of test cycles can be set as the selection criteria.

[0046] [Method for Manufacturing Exhaust Gas Piping] The method for manufacturing exhaust gas piping of the present invention comprises a manufacturing process for manufacturing exhaust gas piping by processing steel materials selected by the above-described method for selecting steel materials for exhaust gas piping. The processing is not particularly limited and can be any processing that enables the manufacture of piping. The processing may include one or more types of processing such as pressing, bending, and welding.

[0047] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0048] <Steel Materials> Test specimens were prepared using two levels of flat steel materials (steel material A and steel material B) shown in Table 1. First, steel material A and steel material B were cut to prepare three test specimens each measuring 75 mm in width and 70 mm in length. Next, the obtained test specimens were degreased by immersion in a mixed solvent of toluene and ethanol and ultrasonic cleaning, and the initial weight W0 of the test specimens was measured. After that, the areas of the test specimens other than the evaluation area (evaluation surface) (including the back and sides) were covered with heat-resistant Teflon tape. At this time, the area of ​​the corrosion resistance evaluation surface (evaluation area) was 1.6 × 10⁻⁶. ３ mm ２ It was (40 mm x 40 mm).

[0049]

[0050] The corrosion morphology in a real environment was evaluated using the above-described test specimens. First, the test specimens were placed in locations where condensate accumulated in the exhaust gas piping when fuel was burned in the experimental reactor. The corrosion depth and average corrosion rate at each location were calculated using the method described later, and the corrosion morphology was evaluated. Here, a fuel consisting of carbon monoxide, hydrogen, methane, etc., and a by-product gas containing S and Cl as impurities, was mixed with ammonia.

[0051] Based on the average corrosion rate of the test specimens and the corrosion depth at each position on the evaluation surface, determined by the method described below, it was confirmed that localized corrosion occurred in steel materials A and B. Furthermore, the maximum localized corrosion rate for steel material A was 2.9 mm / year, and the maximum localized corrosion rate for steel material B was 2.8 mm / year, indicating that the corrosion resistance of steel materials A and B were approximately equivalent. The composition of the drain generated in the piping during this test was NH ４ ＋ :302mg / L, SO ４ ２－ :46mg / L, Cl － : <0.5 mg / L. The aforementioned NH ４ ＋ SO ４ ２－ , Cl － The concentration was measured by ion chromatography.

[0052] The corrosion resistance of steel materials A and B, which showed localized corrosion in real-world environments, was evaluated using the corrosion resistance evaluation method of the present invention.

[0053] <Test solution> pH, sulfate ions (SO2) as shown in Table 2 ４ ２－ ) concentration, chloride ion (Cl － The test solution was prepared by adding each reagent to pure water to achieve the specified concentration. Here, sulfuric acid, ammonium sulfate, and iron sulfate were used as the substances that supplied sulfate ions, and hydrochloric acid was used as the substance that supplied chloride ions. The pH of the test solution was measured using a pH meter at 25°C.

[0054]

[0055] (Corrosion Resistance Evaluation Test) First, the test specimen was placed on the bottom of the corrosion test container so that the evaluation surface of the test specimen was facing upward (parallel to the bottom of the container: horizontally placed). 500 mL of the test solution No. listed in Table 2 was introduced, and the test specimen was immersed in the test solution (immersion step). At this time, in order to prevent the test solution from decreasing due to evaporation, a steel lid covered with Teflon tape was placed over the opening of the corrosion test container. After that, the corrosion test container was placed in an electric furnace set to the temperature shown in Table 3 (90°C or 50°C).

[0056] Next, after the immersion time indicated in Table 3 had elapsed, the corrosion test containers were removed from the electric furnace, and the test solution replacement process was performed for all but No. 14 in Table 3. The test solution replacement process was carried out by draining the test solution from the corrosion test container after the immersion process, and then introducing a new test solution (with the same test solution number) into the corrosion test container. Furthermore, only No. 10 in Table 3 underwent a water washing process. In the water washing process, the evaluation surface of the test piece was washed with water to remove the corrosion products that had formed on the evaluation surface. Water washing was performed by pouring pure water over the evaluation surface of the test piece for 20 seconds using a poly washing bottle with a nozzle. After that, the immersion process, test solution replacement process (only for Nos. 1-13, 15, and 17-24), and washing process (only for No. 10) were carried out again under the same conditions as above, for the number of test cycles shown in Table 3. For No. 14 in Table 3, the test solution replacement process and water washing process were not performed, and the immersion process was continued for 48 hours. The test solution replacement and rinsing processes were carried out in a short time (for example, within 2 minutes each).

[0057] The validity of the corrosion resistance evaluation method of the present invention was evaluated by the corrosion morphology. Specifically, on the evaluation surface of the test specimen, localized corrosion was defined as corrosion at a corrosion depth that was twice or more the average corrosion rate based on the corrosion depth of the entire evaluation surface (hereinafter referred to as the average corrosion rate). If localized corrosion occurred on the test specimen, the exhaust gas piping targeted by the present invention (ammonia and the drain generated in the piping, SO2) was evaluated. ４ ２－ Substances that leach and Cl － We determined that the corrosion morphology in a real-world environment (exhaust gas piping through which exhaust gas containing at least one substance that leaches out) could be reproduced.

[0058] Furthermore, the corrosion resistance of the steel materials was evaluated based on the maximum corrosion rate of localized corrosion. Here, the average corrosion rate, the maximum corrosion rate of localized corrosion, and the corrosion depth at each location on the evaluation surface were calculated using the following procedure.

[0059] After removing the Teflon tape from the test specimens following a predetermined test cycle, the specimens were immersed in a pickling solution consisting of concentrated hydrochloric acid and deionized water in a volume ratio of 1:1 with 3.5 g / L of hexamethylenediamine added as an inhibitor to completely remove corrosion products. Next, the specimens were neutralized by immersion in an aqueous sodium bicarbonate solution, washed with deionized water, dried in a hot air dryer, and the weight W1 of the pickled specimens was measured. The average corrosion rate (mm / year) of each specimen was calculated using the following formula (1) from the absolute value of the weight difference (weight loss) between the initial weight W0 and the weight W1 of the pickled specimen. Here, the average of the average corrosion rates of the three obtained specimens was taken as the average corrosion rate for each steel material. The average corrosion rate values ​​for each steel material are shown in Table 3. Average corrosion rate (mm / year) = Weight loss (g) / Area of ​​the evaluation surface of the specimen (mm) ２ ) / Density of the test specimen (g / mm³) ３ ) / Total immersion time (years) ... (1) Here, the density of the test specimen is 7.85 g / cm³ ３ I used it.

[0060] Furthermore, the corrosion depth at each location on the evaluation surface and the maximum corrosion rate of localized corrosion were calculated using the following method. First, the three-dimensional shape information of the evaluation surface of the specimen after pickling was measured using a one-shot 3D shape measuring machine (VR-3200, manufactured by Keyence Corporation). Using the obtained three-dimensional shape information of the evaluation surface, the corrosion depth at each location on the evaluation surface (distance to the bottom of the recess based on the initial position on the specimen surface) was determined, and the presence or absence of localized corrosion was determined. Subsequently, the corrosion depth (corrosion depth of localized corrosion) of the locations where localized corrosion occurred was calculated. Here, localized corrosion was defined as corrosion at locations where the corrosion rate is twice or more than the average corrosion rate based on the corrosion depth of the entire evaluation surface of the specimen. The maximum corrosion depth at the locations where localized corrosion occurred was defined as the maximum corrosion depth (mm), and the maximum corrosion rate of localized corrosion (mm / year) was calculated for each specimen using the following formula (2). Here, the average value of the maximum corrosion rates of localized corrosion of the three obtained specimens was defined as the maximum corrosion rate of localized corrosion for each steel material. The maximum corrosion rate of localized corrosion obtained is shown in Table 3.

[0061] Maximum corrosion rate of local corrosion (mm / year) = Maximum corrosion depth (mm) / Total immersion time (year) ... (2)

[0062]

[0063] The conditions underlined in Table 3 indicate those outside the scope of the present invention. From Table 3, it was confirmed that within the scope of the present invention, local corrosion occurred in both Steel A and Steel B, and the corrosion form in the actual environment of the exhaust gas pipe targeted by the present invention could be reproduced. Thus, according to the present invention, for ammonia and the drain generated in the pipe, SO ４ ２－ a substance that elutes and Cl － the corrosion resistance of the steel material used for the exhaust gas pipe through which the exhaust gas containing at least one of the substances that elutes can be appropriately evaluated.

[0064] Further, the ranking of the maximum corrosion rate of local corrosion in the invention examples, which are the conditions within the scope of the present invention, is almost the same between Steel A and Steel B, and it was confirmed that the ranking of the corrosion resistance of the steel material in the actual environment of the exhaust gas pipe targeted by the present invention could be reproduced.

[0065] Comparing the results of Example No. 1 and 14, it can be seen that the maximum corrosion rate of local corrosion increases due to the presence of the test solution replacement step. This is presumably because by providing the test solution replacement step, an increase in iron ions generated by the corrosion of the steel material into the test solution can be suppressed, and the decrease in the corrosion rate of the steel material due to iron ions is alleviated.

Claims

1. A method for evaluating the corrosion resistance of steel materials for exhaust gas piping, wherein the exhaust gas piping is exposed to ammonia and the drain generated in the exhaust gas piping, and SO2 is used. ４ ２－ Substances that leach and Cl － A method for evaluating the corrosion resistance of steel materials for exhaust gas piping, comprising: a pipe through which exhaust gas containing at least one substance that elutes is passed; an immersion step of immersing the steel material to be evaluated in a test solution for 4 hours or more; the test solution having a pH of 8.0 or higher and containing at least one of sulfate ions and chloride ions; the concentration of at least one of the sulfate ions and chloride ions being 1 mg / L or higher; the concentration of the sulfate ions being 1000 mg / L or lower; and the concentration of the chloride ions being 100 mg / L or lower.

2. The method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to claim 1, wherein the pH of the test solution is adjusted using at least one selected from the group consisting of ammonia, sodium hydroxide, and calcium hydroxide.

3. The method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to claim 1 or 2, further comprising a washing step of washing the steel material after the immersion step.

4. A method for evaluating the corrosion resistance of steel materials for exhaust gas piping according to any one of claims 1 to 3, further comprising a test solution replacement step of replacing the test solution after the immersion step with a new test solution.

5. A method for selecting steel materials for exhaust gas piping, comprising: an evaluation step of evaluating the corrosion resistance of steel materials using the corrosion resistance evaluation method for exhaust gas piping steel materials described in any one of claims 1 to 4; and a selection step of selecting steel materials based on the evaluation results obtained in the evaluation step.

6. A method for manufacturing exhaust gas piping, comprising a manufacturing step of processing steel materials selected by the method for selecting exhaust gas piping steel materials described in claim 5 to manufacture exhaust gas piping.

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