Corrosion evaluation method for metal materials, and metal material

Abstract

Provided are: a corrosion evaluation method for metal materials, capable of reproducing the corrosion behavior of a metal material in an environment of low airborne salt content and high sulfur oxide content; and a metal material.　Provided is a corrosion evaluation method for metal materials, for evaluating the state of corrosion of a metal material by performing a process including the following process (A) and the following process (B) 30 times or more, wherein the state of corrosion of a non-plated, non-coated steel plate, serving as the metal material, is evaluated by, in the process (A), setting the concentration of chloride ions in an aqueous solution to be 0.01-0.20 mass% inclusive, and setting the concentration of sulfate ions in the aqueous solution to be 0.01-0.50 mass% inclusive, and in the process (B), setting a variation in dew point when transitioning between a drying process and a wetting process to be within ±10°C.　Process (A): A process in which an aqueous solution containing chloride ions and sulfate ions is applied to the surface of a metal material for a period of 20 minutes or less Process (B): A process in which the metal material to which the aqueous solution has been applied in process (A) is subjected once to a drying process and a wetting process in which the temperature and relative humidity are set to be different

Description

Corrosion evaluation method for metallic materials and metallic materials

[0001] The present invention relates to a corrosion evaluation method for metallic materials that can reproduce the corrosion behavior of metallic materials in an environment with low airborne salt content and high sulfur oxide content, and to the metallic materials.

[0002] Vehicle sales within the Association of Southeast Asian Nations (ASEAN), which has a total population of approximately 670 million, are expected to reach approximately 3.4 million units in 2022, equivalent to roughly two-thirds of the number of units sold in the Japanese market in 2019 (approximately 5 million units). The ASEAN automobile market is attracting the expectations and attention of the Japanese automobile industry, due to the high market share held by Japanese automakers and the room for growth in each country's market.

[0003] In the low airborne salt and high sulfur oxide environments of Southeast Asia, the amount of chloride adhesion to metal materials is low. Therefore, when unplated and unpainted steel sheets are exposed to such environments, the weight loss of the metal material due to corrosion (corrosion weight loss) is not proportional to the exposure period. In other words, the corrosion progression of unplated and unpainted steel sheets stagnates once the corrosion state reaches a predetermined corrosion weight loss. On the other hand, when galvanized and unpainted steel sheets are exposed to such environments, the corrosion weight loss of the metal material increases in proportion to the exposure period without stagnation.

[0004] Furthermore, the development of corrosion test methods for metallic materials in a wide variety of environments has been widely studied. For example, Non-Patent Document 1 discloses corrosion evaluation methods using corrosion tests such as salt spray tests, combined cyclic corrosion tests, and exposure tests, in relation to corrosion test methods for surface-treated steel sheets that are internationally standardized. Non-Patent Document 1 also discloses a combined cyclic corrosion test using a solution containing sulfuric acid, which is targeted at environments with advanced acidity due to acid rain. Furthermore, taking into account the influence of seawater components, a combined cyclic corrosion test using artificial seawater containing sulfate ions is also disclosed.

[0005] Surface Technology Vol. 62 No. 1 P. 30 (2011)

[0006] However, even corrosion assessment methods that take into account the effects of acid rain and seawater have difficulty reproducing corrosion behavior in environments with low airborne salt content and high sulfur oxides. This is because the simulated environment, which takes into account the effects of pH due to acid rain and seawater components, differs from the low airborne salt content and high sulfur oxides environment in Southeast Asia. A corrosion assessment method that can reproduce a low airborne salt content and high sulfur oxide environment without being affected by the effects of pH and seawater components has not yet been developed.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a corrosion evaluation method for metallic materials, and a metallic material, which can reproduce the corrosion behavior of metallic materials in an environment with low airborne salt content and high sulfur oxides.

[0008] [1] A corrosion evaluation method for metallic materials, which evaluates the corrosion state of a metallic material by performing a process having the following step (A) and the following step (B) 30 or more times, wherein in the following step (A), the concentration of chloride ions in the aqueous solution is set to 0.01 mass% or more and 0.20 mass% or less, and the concentration of sulfate ions is set to 0.01 mass% or more and 0.50 mass% or less, and in the following step (B), the fluctuation of the dew point during the transition between the drying step and the wetting step is set to within ±10°C, and the corrosion state of an unplated, unpainted steel sheet is evaluated as the metallic material.

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[0040] [3] A metallic material that is selected by evaluating the corrosion state using the corrosion evaluation method for metallic materials according to [1] and that satisfies the condition of the corrosion weight loss formula (1). (Fe Ｌ（ｎ＋１０） -Fe Ｌｎ ) / (Fe Ｌ１０ -Fe Ｌ０ )≦0.30 (1) where Fe Ｌｎ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet after the process including the step (A) and the step (B) has been performed n times. －２ ) where n is a positive integer other than 0. [4] A metallic material that is selected by evaluating the corrosion state using the corrosion evaluation method for metallic materials described in [2] and that satisfies the condition of the corrosion weight loss formula (2). Ｌ３０（ｍ＋１） / Fe Ｌ３０（ｍ＋１）)-(Zn Ｌ３０ｍ / Fe Ｌ３０ｍ ) <0.10 (2) where m is a positive integer other than 0, and Fe Ｌ３０ｍ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet every 30 times, with 30 times of the process including the step (A) and the step (B) being one unit. －２ ) and Zn Ｌ３０ｍ is the corrosion weight loss (g m) of the galvanized unpainted steel sheet every 30 times, with 30 times of the process including the step (A) and the step (B) being one unit. －２ )

[0009] According to the present invention, it is possible to reproduce the corrosion behavior of metal materials in an environment with low airborne salt content and high sulfur oxides.

[0010] Hereinafter, embodiments of the present invention will be described. In the following description, "relative humidity" refers to the ratio of the water vapor partial pressure actually contained in air at a given temperature to the maximum water vapor partial pressure that can be contained in air at that temperature. "Dew point" refers to the temperature at which the pressure of water vapor in the air is equal to the saturated vapor pressure. "Corrosion behavior" refers to the behavior of the corrosion state of a metal material changing over time when the metal material is exposed to a given environment. "Corrosion weight loss" refers to the weight loss of a metal material that progresses due to corrosion behavior when the metal material is exposed to a given environment. "Low airborne salt content" refers to an environmental state in which the amount of sea salt particles flying to land areas is low compared to the average amount of airborne salt dispersion in a simulated environment in an internationally standardized corrosion evaluation method. "High sulfur oxides" refers to an environmental state in which the amount of SO floating in the air is high compared to the average amount of sulfur oxide dispersion in a simulated environment in an internationally standardized corrosion evaluation method. Ｘ This refers to an environmental condition in which there is a high amount of sulfur oxides, such as

[0011] The following embodiments are intended to exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0012] As a result of extensive research conducted by the present inventors to solve the above-mentioned problems, the present inventors have obtained the following findings regarding a corrosion evaluation method that simulates the environmental conditions of low airborne salt and high sulfur oxides found in Southeast Asia.

[0013] Corrosion is accelerated by subjecting a metal material to a drying process and a wetting process multiple times at different temperatures and relative humidities. When the aqueous solution is applied to an uncoated steel sheet as the metal material, the corrosion-accelerating effect on the metal material increases as the amount of the aqueous solution applied increases. That is, the corrosion weight loss of the metal material increases as the test time for corrosion evaluation increases.

[0014] On the other hand, they found that by adjusting the chloride ion concentration in the aqueous solution to a predetermined range and also adding sulfate ions to the aqueous solution to a predetermined concentration range, the corrosion state of unplated, unpainted steel sheets stagnates when a predetermined corrosion weight loss is reached. One of the reasons for this phenomenon is that the corrosion-accelerating effect of sulfate ions causes denser corrosion products to protect the exposed surfaces of the metal material.

[0015] The inventors have also confirmed that when the concentrations of chloride ions and sulfate ions in the aqueous solution are outside the above-mentioned ranges, the corrosion state of the unplated, unpainted steel sheet does not stagnate but increases in proportion to the exposure period. One of the reasons for this phenomenon is that dense corrosion products are not formed due to the influence of the speed at which corrosion products are formed.

[0016] Therefore, the inventors investigated a method in which an aqueous solution having predetermined concentrations of chloride ions and sulfate ions is applied to a metal material, and the metal material is subjected to drying and wetting processes multiple times at different temperatures and relative humidity settings. They found that by performing the drying and wetting processes multiple times, it is possible to reproduce the corrosion behavior of a metal material in an environment with low airborne salt content and high sulfur oxides.

[0017] Furthermore, the inventors have discovered that in a corrosion evaluation method, by adjusting the chloride ion and sulfate ion concentrations in the aqueous solution applied to the metal material to within a predetermined range, the corrosion state of uncoated, unpainted steel sheets can be halted when a predetermined corrosion weight loss is reached. On the other hand, they have also discovered that the corrosion state of uncoated, zinc-plated steel sheets does not halt but increases in proportion to the exposure period. This phenomenon is partly due to the formation of basic zinc sulfate corrosion products caused by sulfate ions.

[0018] Next, the corrosion evaluation method for metallic materials of the present invention will be specifically described. The corrosion evaluation method for metallic materials of the present invention includes steps (A) and (B). Step (A) is a step of applying an aqueous solution containing chloride ions and sulfate ions to the surface of a metallic material. Step (B) is a step of performing a drying step and a wetting step once on the metallic material to which the aqueous solution has been applied in step (A).

[0019] In step (A), the chloride ion concentration of the aqueous solution is set to 0.01% by mass or more and 0.20% by mass or less. By including chloride ions in the aqueous solution to be applied to the metal material, corrosion of the metal material to which the aqueous solution is applied can be accelerated. Furthermore, by setting the chloride ion concentration to 0.01% by mass or more, corrosion evaluation tests for metal materials can be conducted in a short period of time. Note that if the chloride ion concentration exceeds 0.20% by mass, the corrosion state of the unplated, unpainted steel sheet cannot be stopped when a predetermined corrosion weight loss is reached. Therefore, the chloride ion concentration of the aqueous solution is set to 0.20% by mass or less.

[0020] In step (A), the sulfate ion concentration of the aqueous solution is set to 0.01% by mass or more and 0.50% by mass or less. By including sulfate ions in the aqueous solution to be applied to the metal material, corrosion of the metal material to which the aqueous solution is applied can be accelerated. Furthermore, by setting the sulfate ion concentration to 0.01% by mass or more, corrosion evaluation tests for metal materials can be conducted in a short period of time. Note that if the sulfate ion concentration exceeds 0.50% by mass, the corrosion state of the unplated, unpainted steel sheet cannot be stopped when a predetermined corrosion weight loss is reached. Therefore, the sulfate ion concentration of the aqueous solution is set to 0.50% by mass or less.

[0021] In step (A), the time for which the aqueous solution containing chloride ions and sulfate ions is applied to the surface of the metal material should be within 20 minutes. If the aqueous solution is applied to the surface of the metal material for a time exceeding 20 minutes, corrosion will progress in step (A), making it difficult to recreate an environment with low airborne salt content and high sulfur oxides.

[0022] Furthermore, the method for applying the aqueous solution to the surface of the metal material is not limited as long as it is a method that can apply the aqueous solution to the surface of the metal material, such as immersing the metal material in the aqueous solution or spraying the surface of the metal material with a spray or the like.

[0023] In step (B), the metal material to which the aqueous solution has been applied is subjected to a drying step and a wetting step once. Step (B) aims to reproduce an environment with low airborne salt content and high sulfur oxides, and also simulates the nighttime condensation phenomenon caused by the temperature difference between day and night in that environment. Specifically, the daytime environmental conditions are reproduced in the drying step, and the nighttime environmental conditions are reproduced in the wetting step.

[0024] In step (B), the dew point fluctuation during the transition between the drying step and the wetting step is limited to within ±10°C. This is to suppress excessive fluctuations in temperature and relative humidity within the test equipment while aiming to reproduce an environment with low airborne salt content and high sulfur oxides.

[0025] In step (B), the drying step and the wetting step are set to have different temperatures and relative humidities. To more accurately reproduce the daytime and nighttime environmental conditions in a low airborne salt and high sulfur oxide environment, the time required for transition from the drying step to the wetting step and the time required for transition from the wetting step to the drying step may be individually and specifically set in advance. For example, the transition time between the drying step and the wetting step may be set to "immediate" to quickly reproduce the daytime and nighttime environmental conditions in the testing device. Alternatively, the transition time between the drying step and the wetting step may be set to "60 minutes" to enhance the reproducibility of daytime and nighttime conditions in an actual environment.

[0026] The purpose of step (B) is to reproduce the daytime and nighttime environmental conditions in a low airborne salt and high sulfur oxide environment. The daytime environmental conditions are reproduced in the drying step, and the nighttime environmental conditions are reproduced in the wetting step. The retention time for the wetting step is 2 hours or more and 12 hours or less. If the retention time for the wetting step is less than 2 hours, the corrosive environment in the test equipment will be unstable, and the accuracy of the evaluation results will not be ensured depending on the installation position of the metal material in the test equipment. Furthermore, if the retention time for the wetting step is more than 12 hours, the temperature and relative humidity will differ from those of an actual low airborne salt and high sulfur oxide environment, making it impossible to ensure reproducibility.

[0027] The retention time of the drying step is the time obtained by subtracting the sum of the time required for the wetting step and the transition time between the drying step and the wetting step from the total time of one day (24 hours). Therefore, the retention time of the drying step is 12 hours or more and 22 hours or less.

[0028] The temperature in the drying step is set to 20°C or higher and 60°C or lower. This is because a temperature lower than 20°C reduces the effect of promoting corrosion of the metal material, resulting in a longer time required for the corrosion evaluation test. Furthermore, a temperature higher than 60°C in the drying step changes the corrosion resistance ranking of the metal material, resulting in corrosion behavior different from that in an environment with low airborne salt and high sulfur oxides. This phenomenon is caused in particular by the sacrificial dissolution of iron relative to zinc occurring in unpainted galvanized steel sheets when the temperature exceeds 60°C. The temperature in the drying step may be set based on the temperature in the wetting step, which is set separately, in order to keep the dew point fluctuation during the transition between the drying step and the wetting step within ±10°C.

[0029] The relative humidity during the drying process is set to 70% or less. In environments with low airborne salt content and high sulfur oxides, and in actual environments where metal materials such as automotive steel sheets are used, airborne salt (sea salt particles) affects corrosion, and the saturated critical vapor pressure of sodium chloride, its main component, is approximately 75 to 78% in relative humidity terms. Therefore, in order to simulate corrosion behavior in actual environments, the relative humidity during the drying process is set to 70% or less.

[0030] The temperature in the wetting step is set to 20°C or higher and 60°C or lower. This is because a temperature lower than 20°C has little effect on promoting corrosion of the metal material, and the time required for the corrosion evaluation test increases. Furthermore, a temperature higher than 60°C in the wetting step changes the corrosion resistance ranking of the metal material, resulting in corrosion behavior that differs from the corrosion behavior in an environment with low airborne salt and high sulfur oxides. The temperature in the wetting step may be set based on the temperature in the drying step, which is set separately, in order to keep the dew point fluctuation during the transition between the drying step and the wetting step within ±10°C.

[0031] The relative humidity during the wetting step is set to 80% or more and 100% or less. If the relative humidity during the wetting step is less than 80%, the metal material will not be sufficiently wetted, and the corrosion evaluation test will take a long time. The saturated critical vapor pressure of sodium chloride, which is contained in the aqueous solution as chloride ions, is approximately 75-78% in terms of relative humidity. Therefore, by maintaining the relative humidity at 80% or more, the surface of the metal material can be kept wet by chemical condensation. However, there is no upper limit to the relative humidity. Therefore, the relative humidity during the wetting step is set to 100% or less.

[0032] In the corrosion evaluation method for metallic materials of the present invention, the process including steps (A) and (B) is performed 30 or more times, because this is the number of times necessary to reproduce the corrosion behavior of an unplated, unpainted steel sheet as a metallic material, which stagnates when a predetermined corrosion weight loss is reached.

[0033] The corrosion weight loss of a metal material (unplated, unpainted steel sheet) subjected to the corrosion evaluation method for a metal material of the present invention satisfies the conditions of formula (1). Ｌｎ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet after the process including step (A) and step (B) has been carried out n times. －２ ), where n is a positive integer other than 0. (Fe Ｌ（ｎ＋１０） -Fe Ｌｎ ) / (Fe Ｌ１０ -Fe Ｌ０ ) ≦0.30 ... (1)

[0034] Corrosion loss Fe Ｌｎ indicates the state of the unplated, unpainted steel sheet at the time when the process including step (A) and step (B) has been performed n times. Specifically, the corrosion weight loss at that time may be calculated as the difference between the weight of the unplated, unpainted steel sheet after removing the corrosion products using hydrochloric acid containing an inhibitor and the weight of the unplated, unpainted steel sheet before corrosion evaluation.

[0035] Equation (1) shows the relationship that, in a process of performing a process including steps (A) and (B) multiple times, the ratio of the corrosion weight loss at the time when the process including steps (A) and (B) is performed 10 times to the corrosion weight loss at the time when the number of times the process is performed is further increased is 0.3 or less. In other words, it shows a state in which, as the number of times the process is performed is increased, the corrosion weight loss of the unplated, unpainted steel sheet stagnates when it reaches a predetermined corrosion weight loss.

[0036] For example, when a process including step (A) and step (B) is performed 30 times, the corrosion weight loss of an unplated, unpainted steel sheet at each time point of n = 30, 20, and 10 is calculated by the relationship of formula (1) "(Fe Ｌ３０ -Fe Ｌ２０ ) / (Fe Ｌ１０ -Fe Ｌ０ )≦0.3” is satisfied.

[0037] As described above, by performing the corrosion evaluation method for metallic materials according to the present invention, it is possible to reproduce the corrosion behavior of metallic materials in an environment with low airborne salt content and high sulfur oxide content. That is, it is possible to reproduce the corrosion behavior of an uncoated, unplated steel sheet in an environment with low airborne salt content and high sulfur oxide content, where the corrosion state stagnates when a predetermined corrosion weight loss is reached.

[0038] The corrosion evaluation method for metallic materials of the present invention aims to reproduce the corrosion behavior of metallic materials in an environment with low airborne salt and high sulfur oxides. Furthermore, during the corrosion evaluation method for metallic materials, the progression of corrosion of uncoated, unpainted steel sheets can be arrested when the corrosion state reaches a predetermined corrosion weight loss. Furthermore, when the corrosion evaluation method for metallic materials of the present invention evaluates galvanized, unpainted steel sheets, the progression of corrosion of the galvanized, unpainted steel sheets can be increased in proportion to the exposure period without arresting the progression of corrosion.

[0039] In this case, it is preferable to repeat the process including step (A) and step (B) 60 times or more, because by repeating the process 60 times or more, the stagnation of corrosion progression in the unplated and unpainted steel sheet can be significantly reproduced.

[0040] The sulfate ion concentration of the aqueous solution to be applied to the metal material is preferably 0.10 mass % or more and 0.50 mass % or less. If the sulfate ion concentration is less than 0.10 mass %, the corrosion weight loss of the galvanized and unpainted steel sheet will be smaller than the corrosion weight loss of the unplated and unpainted steel sheet, making it difficult to accurately reproduce an environment with low airborne salt and high sulfur oxides.

[0041] The corrosion weight loss of a metal material subjected to the corrosion evaluation method for metal materials of the present invention, in which a process including steps (A) and (B) is performed 60 times or more and the concentration of sulfate ions in the aqueous solution applied to the metal material is set to 0.10 mass % or more and 0.50 mass % or less, satisfies the condition of formula (2). In this case, the metal material includes an unplated unpainted steel sheet and a zinc-plated unpainted steel sheet. Formula (2) expresses the corrosion weight loss of an unplated unpainted steel sheet after m times of the process including steps (A) and (B) as Fe Ｌｍ The corrosion loss of the zinc-plated unpainted steel sheet after the process has been carried out m times is expressed as Zn Ｌｍ When Ｌｍ / Fe Ｌｍ In formula (2), m is a positive integer other than 0. Ｌ３０ｍ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet every 30 times, with 30 times of the process including step (A) and step (B) being one unit. －２) Zn Ｌ３０ｍ is the corrosion weight loss (g m) of the galvanized unpainted steel sheet every 30 times, with 30 times of the process including step (A) and step (B) being one unit. －２ ) 0.03≦(Zn Ｌ３０（ｍ＋１） / Fe Ｌ３０（ｍ＋１） )-(Zn Ｌ３０ｍ / Fe Ｌ３０ｍ ) <0.10 ... (2)

[0042] Fe, which is corrosion weight loss of unplated and unpainted steel sheet Ｌ３０ｍ may be calculated every 30 times the process including step (A) and step (B) is performed. Specifically, every 30 times the process is performed, the corrosion resistance may be calculated from the difference between the weight of the unplated, unpainted steel sheet after removing corrosion products from the unplated, unpainted steel sheet using hydrochloric acid containing an inhibitor and the weight of the unplated, unpainted steel sheet before corrosion evaluation.

[0043] In addition, the corrosion loss of zinc-plated unpainted steel sheets, Zn Ｌ３０ｍ may be calculated every time the process including step (A) and step (B) is performed 30 times. Specifically, every time the process is performed 30 times, the corrosion resistance may be calculated from the difference between the weight of the galvanized unpainted steel sheet after removing zinc corrosion products with chromic acid and the weight of the galvanized unpainted steel sheet before corrosion evaluation.

[0044] Formula (2) specifies the change in the relationship between the corrosion weight loss of the unplated and unpainted steel sheet and the zinc-plated and unpainted steel sheet, with 30 operations of the process of performing the process including the steps (A) and (B) multiple times as one unit. Specifically, the corrosion weight loss of the unplated and unpainted steel sheet, Fe Ｌ３０ｍ and corrosion weight loss of galvanized unpainted steel sheet Zn Ｌ３０ｍ Relationship with Zn Ｌ３０ｍ / Fe Ｌ３０ｍ For example, the change in Zn after the process including the step (A) and the step (B) is performed 60 times is Ｌ６０ / Fe Ｌ６０ The value of Zn when the process was performed 30 times Ｌ３０ / Fe Ｌ３０It is preferable that the difference between the value of (hereinafter simply referred to as "difference") and the value of (hereinafter simply referred to as "difference") is 0.03 or more and less than 0.10.

[0045] A difference of less than 0.03 means that the corrosion of the uncoated, unpainted steel sheet in the corrosion test did not stop when a predetermined corrosion weight loss was reached. A difference of 0.10 or more means that the corrosion of the galvanized, unpainted steel sheet in the corrosion test progressed excessively faster than the corrosion of the uncoated, unpainted steel sheet. In other words, this indicates that an environment with low airborne salt and high sulfur oxides was not accurately reproduced.

[0046] Therefore, the corrosion evaluation method for metallic materials may be performed by repeating the process including steps (A) and (B) 60 times or more, and setting the sulfate ion concentration of the aqueous solution to be applied to the metallic material to 0.10 mass% or more and 0.50 mass% or less. In this case, the corrosion state of the uncoated, uncoated steel sheet can be stagnated when a predetermined corrosion weight loss is reached. On the other hand, the corrosion state of the galvanized, uncoated steel sheet can be increased in proportion to the exposure period without stagnating.

[0047] Here, in the present invention, a selection method may be adopted in which the corrosion state of a metallic material is evaluated using a corrosion evaluation method for metallic materials, and a metallic material is selected based on the evaluation results. Specifically, for an unplated, unpainted steel sheet as the metallic material, the corrosion state may be evaluated using the corrosion evaluation method for metallic materials, and a metallic material (unplated, unpainted steel sheet) may be manufactured that has been determined to have been "evaluated" because its corrosion weight loss satisfies the condition of formula (1). Alternatively, for an unplated, unpainted steel sheet and a galvanized, unpainted steel sheet as the metallic material, the corrosion state may be evaluated using the corrosion evaluation method for metallic materials, and a metallic material (unplated, unpainted steel sheet and galvanized, unpainted steel sheet) may be manufactured that has been determined to have been "evaluated" because its corrosion weight loss satisfies the condition of formula (2).

[0048] The galvanized unpainted steel sheet of the present invention may have a coating layer mainly composed of an unalloyed η-Zn phase. The manufacturing method of the galvanized unpainted steel sheet is not limited as long as it is a manufacturing method that can form a coating layer, such as hot-dip galvanizing or electroplating.

[0049] Next, the results of carrying out the corrosion evaluation method for metal materials according to the present invention will be described. In the examples, a process including step (A) and step (B) was carried out 60 times on unplated and unpainted steel sheets and galvanized and unpainted steel sheets with an exposed area of ​​50 mm x 120 mm. In step (A), the unplated and unpainted steel sheets and galvanized and unpainted steel sheets were immersed in an aqueous solution containing chloride ions and sulfate ions for 15 minutes or more. In step (B), a drying step was followed by a wetting step. The specific conditions for the corrosion evaluation method for metal materials in each example are shown in Table 1.

[0050]

[0051] For each example, after the corrosion evaluation method for metal materials was performed, the corrosion weight loss (g m －２ For uncoated, unplated steel sheets, corrosion products were removed using hydrochloric acid containing an inhibitor, and then corrosion weight loss was measured. For uncoated, zinc-plated steel sheets, corrosion products were removed using chromic acid, and then corrosion weight loss was measured.

[0052] The corrosion weight loss was measured every 10 times the process including step (A) and step (B) was performed. Then, in each example, the measured corrosion weight loss was substituted into formulas (1) and (2) to determine whether these formulas were valid.

[0053] Specifically, in formula (1), when a process including step (A) and step (B) was performed 60 times, each term was calculated using the corrosion weight loss of the unplated and unpainted steel sheet at each time point of the number of times n = 30, 20, and 10. Ｌ３０ -Fe Ｌ２０ ", "Fe Ｌ１０ -Fe Ｌ０ " and "(Fe Ｌ３０ -Fe Ｌ２０ ) / (FeＬ１０ -Fe Ｌ０ The value of (1) was calculated. If the formula (1) was true, it was judged as "Good", and if the formula was not true, it was judged as "Poor".

[0054] In addition, for formula (2), when a process including steps (A) and (B) was performed 60 times, each term was calculated using the corrosion weight loss of the unplated and unpainted steel sheet and the zinc-plated and unpainted steel sheet at each time point when the number of times m was 60 and 30. Ｌ６０ / Fe Ｌ６０ ", "Zn Ｌ３０ / Fe Ｌ３０ " and "(Zn Ｌ６０ / Fe Ｌ６０ )-(Zn Ｌ３０ / Fe Ｌ３０ The value of (2) was calculated. If the formula (2) was true, it was judged as "Good", and if the formula was not true, it was judged as "Poor". The results of the judgment are shown in Table 2.

[0055]

[0056] As can be seen from Tables 1 and 2, in Comparative Example 1, the chloride ion concentration of the aqueous solution to be adhered to the metal materials (unplated unpainted steel sheet and zinc-plated unpainted steel sheet) was set to 0 mass %. As a result, corrosion of the metal materials was not promoted, and the results for the application of formulas (1) and (2) were both "×".

[0057] In Comparative Examples 2 and 3, the chloride ion concentration of the aqueous solution to be applied to the metal material was set to a concentration of more than 0.20 mass %, which excessively promoted corrosion of the metal material, and the results of the evaluations of the formulas (1) and (2) were both "X."

[0058] In Comparative Example 4, the concentration of sulfate ions in the aqueous solution to be attached to the metal material was set to 0 mass %, which did not promote corrosion of the metal material, and the results of the evaluations of both formulas (1) and (2) were "X."

[0059] In Comparative Examples 5 and 6, the sulfate ion concentration of the aqueous solution to be applied to the metal material was set to a concentration of more than 0.50 mass %, which excessively promoted corrosion of the metal material, and the results of the evaluations of the formulas (1) and (2) were both "X".

[0060] In Comparative Example 7, the metal material was immersed in the aqueous solution for more than 20 minutes, which resulted in the corrosion of the metal material progressing in the step (A), making it impossible to carry out a proper corrosion test, and the results of the application of the formulas (1) and (2) were both "X."

[0061] In Comparative Example 8, the temperature in the drying step of step (B) exceeded 60° C. Therefore, the corrosion behavior differed from the behavior in an actual environment, and the results of the application of formulas (1) and (2) were both "X."

[0062] In Comparative Example 9, the relative humidity in the drying step of step (B) exceeded 70%, which resulted in the results of both the application of formulas (1) and (2) being "x" due to the influence of the saturation behavior of chloride ions, which contribute to corrosion behavior.

[0063] In Comparative Example 10, the relative humidity in the wetting step of step (B) was less than 80%, which resulted in the results of both the application of formulas (1) and (2) being "X" due to the influence of the saturation behavior of chloride ions, which contribute to corrosion behavior.

[0064] In Comparative Example 11, the dew point fluctuation in step (B) exceeded 10°C. As a result, the temperature and relative humidity in the test equipment could not be properly controlled, and an environment with low airborne salt and high sulfur oxides could not be accurately reproduced. As a result, the evaluation results for the application of formulas (1) and (2) were both "X."

[0065] In Comparative Example 12, the holding time in the drying step in step (B) was less than 12 hours, and the holding time in the wetting step was more than 12 hours. Therefore, the actual environment could not be accurately reproduced in the drying step and the wetting step, and the evaluations regarding the application of formulas (1) and (2) were both "X".

[0066] On the other hand, for Inventive Examples 1 to 5 and 7 to 12, all of the Examples satisfied the conditions of the corrosion evaluation method for metallic materials, and therefore the assessment of the applicability of Equations (1) and (2) was "Good." In other words, it was confirmed that the corrosion behavior of metallic materials in an environment with low airborne salt and high sulfur oxides can be reproduced by performing the corrosion evaluation method for metallic materials of the present invention. Furthermore, it was also confirmed that the corrosion state of unplated, unpainted steel sheets stagnates when a predetermined corrosion weight loss is reached, while the corrosion state of galvanized, unpainted steel sheets can increase in proportion to the exposure period without stagnation.

[0067] In Inventive Example 6, the sulfate ion concentration of the aqueous solution applied to the metal material was set to 0.01% by mass. As a result, the corrosion state of the unplated, unpainted steel sheet was able to stagnate when a predetermined corrosion weight loss was reached, and the application of formula (1) was judged to be "Good." On the other hand, because the sulfate ion concentration of the aqueous solution was less than 0.10% by mass, the corrosion weight loss behavior of the galvanized, unpainted steel sheet was equal to or smaller than the corrosion weight loss behavior of the unplated, unpainted steel sheet, and the application of formula (2) was judged to be "Poor."

Claims

1. A corrosion evaluation method for metallic materials, which evaluates the corrosion state of a metallic material by performing a process comprising the following steps (A) and (B) 30 or more times: in step (A), the chloride ion concentration in the aqueous solution is set to 0.01 mass% or more and 0.20 mass% or less, and the sulfate ion concentration is set to 0.01 mass% or more and 0.50 mass% or less; and in step (B), the fluctuation of the dew point during the transition between the drying step and the wetting step is set to within ±10°C; and the corrosion evaluation method for metallic materials evaluates the corrosion state of an unplated, unpainted steel sheet as the metallic material. Step (A): A step of applying an aqueous solution containing chloride ions and sulfate ions to the surface of a metal material for a period of 20 minutes or less. Step (B): A step of performing a drying step and a wetting step once on the metal material to which the aqueous solution has been applied in step (A), the drying step and the wetting step being set within the following condition ranges and at different temperatures and relative humidity. Drying step: A step of setting the temperature at 20°C or higher and 60°C or lower, the relative humidity at 70% or lower, and the holding time at 12 hours or higher and 22 hours or lower. Wetting step: A step of setting the temperature at 20°C or higher and 60°C or lower, the relative humidity at 80% or higher and 100% or lower, and the holding time at 2 hours or higher and 12 hours or lower.

2. A corrosion evaluation method for metallic materials according to claim 1, wherein a process including the steps (A) and (B) is carried out 60 times or more, the concentration of the sulfate ions in the step (A) is set to 0.10 mass% or more and 0.50 mass% or less, and the corrosion state of an unplated and unpainted steel sheet and a zinc-plated and unpainted steel sheet as the metallic materials is evaluated.

3. A metallic material that is selected by evaluating the corrosion state using the corrosion evaluation method for metallic materials described in claim 1 and whose corrosion weight loss satisfies the condition of formula (1). (Fe Ｌ（ｎ＋１０） -Fe Ｌｎ ) / (Fe Ｌ１０ -Fe Ｌ０ )≦0.30 (1) where Fe Ｌｎ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet after the process including the step (A) and the step (B) has been performed n times. －２ ) where n is a positive integer other than 0.

4. A metallic material that is selected by evaluating the state of corrosion using the corrosion evaluation method for metallic materials described in claim 2 and that satisfies the condition of the corrosion weight loss formula (2). Ｌ３０（ｍ＋１） / Fe Ｌ３０（ｍ＋１） )-(Zn Ｌ３０ｍ / Fe Ｌ３０ｍ ) <0.10 (2) where m is a positive integer other than 0, and Fe Ｌ３０ｍ is the corrosion weight loss (g m) of the unplated and unpainted steel sheet every 30 times, with 30 times of the process including the step (A) and the step (B) being one unit. －２ ) and Zn Ｌ３０ｍ is the corrosion weight loss (g m) of the galvanized unpainted steel sheet every 30 times, with 30 times of the process including the step (A) and the step (B) being one unit. －２ )