JP5176927B2 - Test method for accelerated corrosion of steel in high humidity environment - Google Patents

Description

  The present invention relates to a corrosion promotion test method for steel materials used for structures such as bridges, particularly in a highly humid environment such as a water leakage portion or a narrow space.

  It is known that structural steel materials in the field of civil engineering, for example, steel materials that are used barely in a highly humid environment such as a leaking portion of a bridge or a narrow space, are difficult to obtain corrosion resistance that can be practically used.

To date, a number of corrosion promotion test methods for steel materials have been developed. As a conventional corrosion acceleration test method for steel materials, a corrosion acceleration test in which a salt adhesion process, a drying process, and a wetting process are combined is performed (for example, see Patent Document 1). Further, in addition to the salt adhesion step-drying step-wetting step, a corrosion acceleration test is performed in which a washing step for washing away the salt accumulated in the rust layer is added (for example, see Patent Document 2).
JP 2007-139484 A Japanese Patent No. 3605630

It is known that the corrosion resistance of steel in a real environment is caused by the composition of rust. In a dry and wet environment, the amount of Fe ₃ O ₄ produced is small, and in an environment where there is a lot of salt, a large amount of β-FeOOH may be produced. know. The process of generating rust in a dry and wet environment is as follows. First,
Fe → Fe ²⁺ + 2e ^―
As a result of the reaction, divalent Fe ions are generated, and γ-FeOOH is generated.

Here, the reduction reaction occurs in the gamma-FeOOH in the wet process, X-rays non-crystalline rust is generated by O ₂ in the air in the drying process, X-rays to an amorphous rust γ- An oxidation reaction to FeOOH occurs. Further, in the wetting step, a reduction reaction from X-ray amorphous rust to Fe ₃ O ₄ occurs. In addition, β-FeOOH is generated from divalent Fe ions in an environment of repeated drying and wetting in the presence of salt. Similarly to γ-FeOOH, β-FeOOH undergoes an oxidation / reduction cycle of rust that is amorphous in X-rays in the drying step and the wetting step. Therefore, in a dry and wet environment, the amount of Fe ₃ O ₄ produced is reduced due to the presence of the drying process. In addition, when a sufficient amount of salt is present, the amount of β-FeOOH produced increases.

In Patent Documents 1 and 2, since the drying process and the wetting process are repeated, the amount of Fe ₃ O ₄ generated is reduced by the rust generation process.

On the other hand, it is known that in a highly humid environment, a large amount of Fe ₃ O ₄ is produced and the amount of β-FeOOH produced is small.

  As described above, Patent Documents 1 and 2 do not generate a rust having the same composition as that of a rust generated in an actual high-humidity environment, and cannot be said to appropriately simulate an actual high-humidity environment.

  The present invention has been made in view of the circumstances as described above, and when evaluating the corrosion resistance of steel materials used in structures such as bridges in a highly humid environment such as a water leakage portion or a narrow space, the actual highly wet An object of the present invention is to provide a corrosion promotion test method for a steel material in a high-humidity environment, in which rust having a composition that matches that generated in the environment is generated, and the corrosion resistance of the steel material can be accurately evaluated.

  In order to achieve the above object, the present inventors presuppose a combined cycle test consisting of a salt adhesion step, a drying step, a wetting step, and a washing step, from the viewpoint of the composition of rust generated in a steel material in a highly humid environment, We conducted an intensive study.

As a result, the composition of rust produced in an actual high-humidity environment when the wetting ratio = (wetting process holding time / (drying process holding time + wetting process holding time)) when the wetting ratio is over 95% and 100% or less. It was found that matching rust was produced, that is, a composition in which Fe ₃ O ₄ was 10 mass% or more and β-FeOOH was less than 10 mass% in the rust.

  The process of generating rust in a highly humid environment is as follows.

At a wetting rate of 100%, there is no drying step, so almost no β-FeOOH is produced. Further, since there is no drying step, almost no oxidation reaction from X-ray amorphous rust to γ-FeOOH occurs, and only X-ray amorphous rust reduction reaction proceeds from γ-FeOOH. Further, Fe ₃ O ₄ is generated from rust that is amorphous in X-rays in the wet process. Therefore, the amount of Fe ₃ O ₄ produced is large and the amount of β-FeOOH produced is small.

Further, when the wetting rate is 95%, β-FeOOH is generated because there is a drying step, but the amount of generation is small because the drying time is short. In addition, since an oxidation / reduction cycle from γ-FeOOH and β-FeOOH to X-ray amorphous rust occurs, the amount of Fe ₃ O ₄ produced is small.

  The present invention is configured based on the above findings and has the following characteristics.

[1] A corrosion promotion test method for a steel material comprising performing the following step (A), the following step (B), and the following step (C) at least once, and in the rust generated on the surface of the steel material Fe ₃ O ₄ is 10 mass% or more, β-FeOOH is less than 10 mass%,
In the following step (A), the salt adhesion amount is 0.1 to 100,000 mg / m ² , and in the following step (B), the drying step and the wetting step are performed within the following condition range. Test method for accelerated corrosion of steel in the environment.
(A) The process of adhering salt containing chloride ions to the surface of the steel material (B) The drying process and the wetting process set by changing the temperature and relative humidity are repeated on the steel material as one cycle, and drying is performed. The relative humidity of the process and the wet process and the wet rate represented by the following formula are performed within the following condition range, and this cycle is performed at least once. Dry process Relative humidity: More than 40% and 70% or less Wet process Relative humidity : 80% or more Wetting rate = (wet process holding time / (drying process holding time + wetting process holding time)): 95% or more and 100% or less

  In the step (B) of [1] above, the wetting rate of 100% is the case where the drying step holding time = 0, and the drying step is not performed.

  In the present invention, when evaluating the corrosion resistance of a steel material used in a structure in a high-humidity environment, a rust having the same composition as that of the rust generated in the actual high-humidity environment is generated, and the corrosion resistance of the steel material is accurately determined. It became possible to evaluate.

An embodiment of the present invention will be described.
FIG. 1 shows a corrosion promotion test method for steel in a highly humid environment according to an embodiment of the present invention. In this embodiment, in order to evaluate the corrosion resistance of a steel material in a highly humid environment, a corrosion acceleration test having a process as shown in FIG. 1 is performed.

  That is, this corrosion promotion test is composed of the following (A) process, the following (B) process, and the following (C) process in which various environmental factors are combined in order to simulate an actual environment. The process consisting of A) process, (B) process, and (C) process is performed once or more. More preferably, the process consisting of the process (A), the process (B), and the process (C) is performed a plurality of times. By performing multiple times, corrosion is promoted, the weight of the generated rust is increased, and quantitative X-ray diffraction with higher accuracy can be performed. The process order is the order of the process (A), the process (B), and the process (C), and starts from the process (A) and ends with the process (B) or the process (C). When the test piece is stored after the corrosion acceleration test, it is preferable that the test piece is finished in the step (C) rather than the step (B) from the viewpoint of suppressing the progress of the corrosion.

(A) A step of attaching a salt containing chloride ions to the surface of a steel material: a salt attachment step (B) One cycle of repeating a drying step and a wetting step set by changing the temperature and relative humidity for the steel material And the wet ratio is more than 95% and 100% or less, and this cycle is performed at least once: drying / wetting step (C) The surface of the steel material is washed with washing water: the washing step

  First, step (A): the salt adhesion step will be described.

In the salt adhesion step (A), the amount of salt containing chloride ions attached to the surface of the steel material is 0.1 to 100000 mg / m ² . By setting it as this range, it is possible to cover areas with a large amount of attached salt, such as a rural area, a mountainous area such as a mountainous area, and a region where an antifreezing agent is sprayed.

In particular, when assuming small areas with relatively airborne salt amount such as rural areas and mountainous areas, salt coating weight in the range of 1 to 100 mg / m ^2, more adherent amount of salt such as areas where spraying antifreeze Area , It is preferable that the amount of adhering salt is in the range of 100 to 100,000 mg / m ² .

  In this embodiment, the method of attaching salt to the surface of the steel material is not particularly limited, but salt water dripping is desirable. By controlling the concentration of salt water and the amount of dripping, the amount of attached salt can be easily controlled.

  Moreover, as salt water used for making salt adhere to the surface of steel materials, sea salt or artificial sea salt, sodium chloride, magnesium chloride, calcium chloride, sodium chloride-magnesium chloride mixture, sodium chloride-calcium chloride mixture, rock salt, etc. An aqueous solution of can be used. In the environment where the bridge is constructed, the incoming sea salt and antifreeze agent will affect the corrosion of the steel material, so the salt water used is sea salt or artificial sea salt, sodium chloride-magnesium chloride mixture, aqueous solution of sodium chloride. Is preferably used.

  Next, step (B): the drying / wetting step will be described.

  The drying step and the wetting step (B) are performed within the following condition range.

Drying process Relative humidity: Over 40% and 70% or less Wetting process Relative humidity: 80% or more Wetting rate 95% or more and 100% or less Here, wetting rate = (wetting process holding time / (drying process holding time + wetting process holding time)) )
The wetting rate of 100% is when the drying process holding time = 0, and the drying process is not performed.

  As described above, the relative humidity in the drying process is more than 40% and 70% or less. In the environment where bridges are constructed, flying sea salt and anti-freezing agents affect the corrosion of steel materials, the main components of which are sodium chloride or sodium chloride and magnesium chloride. The seaside humidity of sodium chloride is about 75-78% relative humidity and dries below 75%, but the seaside humidity of magnesium chloride is about 30-35% relative humidity and is the lowest among the chemicals contained in sea salt. Hard to dry. Therefore, assuming a highly humid environment (such as a water leakage part) in a bridge, the relative humidity in the drying process is set to more than 40% and 70% or less in order to reproduce the corrosion pattern in the actual environment.

  The relative humidity in the wetting process is 80% or more. The seaside humidity of sodium chloride is about 75 to 78% relative humidity. Therefore, when the relative humidity is set to 80% or more, a water film is formed by the incoming sea salt and antifreezing agent adhering to the steel material surface absorbing water, and the steel material surface can be kept in a wet state.

In rust generated in an actual high-humidity environment, Fe ₃ O ₄ , γ-FeOOH, β-FeOOH, α-FeOOH, and amorphous rust are observed. In particular, Fe ₃ O ₄ is contained in an amount of 10 mass% or more, and β-FeOOH is less than 10 mass%.

In the corrosion promotion test in this embodiment, when the wetting rate is 95% or more, the composition of rust to be generated is 10 mass% or more for Fe ₃ O ₄ and less than 10 mass% for β-FeOOH. Therefore, in this embodiment, the wet rate is set to be 95% or more and 100% or less.

  In addition, the holding time of the drying step and the wetting step is not particularly specified, but when the holding time of the drying step is 12 hours or more or the holding time of the wetting step is less than 24 hours, a lot of amorphous rust is generated. The holding time in the drying step is preferably 0 to 12 hours, and the wet time is preferably 24 hours or more.

  The drying process and the wetting process are set to different temperatures and relative humidity. When shifting from the drying process to the wetting process (or moving in the opposite direction), the temperature and relative humidity change. The transition time from the drying process to the wetting process and the transition time from the wetting process to the drying process may be set in advance. This is because when the transition time is not set, the test apparatus may cause a difference in the transition time from the drying process to the wetting process and the transition time from the wetting process to the drying process, resulting in variations in test results. .

  Moreover, when conducting an environmental survey of the actual environment and conducting a corrosion acceleration test based on the result, the drying step and the wetting step may be repeated a plurality of times. Based on the results of the environmental survey in the real environment, it is possible to conduct a corrosion acceleration test closer to the real environment.

  Next, step (C): the cleaning step will be described.

  The washing step (C) is performed in the following range by applying washing water to the steel surface to prevent salt from accumulating in the rust layer.

Flow rate: 0.01 l / min or more and less than 100 l / min Time: 5 seconds or more and less than 600 seconds If the flow rate is less than 0.01 l / min, a sufficient cleaning effect cannot be obtained. On the other hand, if it is 100 l / min or more, the rust layer peels off.

  If the time is less than 5 seconds, a sufficient cleaning effect cannot be obtained. On the other hand, when it becomes 600 seconds or more, elution of the steel material ions inside the rust layer occurs and the cleaning effect is saturated.

  The drying step and the wetting step in the step (A), the step (B), the step (C), and the step (B) may be repeated a plurality of times. This is because, assuming an actual environment, the salt content is not supplied to the steel surface at regular intervals, the amount of which is considered to vary to some extent over time, and the drying and wetting processes vary greatly depending on the environment. This is because it is considered. More preferably, the temperature / humidity, the amount of adhering salt, etc. are investigated in the actual environment, reflected in this corrosion acceleration test, and the corrosion acceleration test is performed under conditions closer to the actual environment.

  Example 1 of the present invention will be described. Note that the present invention is not limited to this.

  First, C: 0.092 mass%, Si: 0.20 mass%, Mn: 0.70 mass%, P: 0.017 mass%, S: 0.0034 mass%, Cu: 0.31 mass%, Cr: 0.52 mass% , Ni: 0.17 mass% steel material equivalent to JIS-SMA was melted and hot-rolled to produce a steel plate having a thickness of 6 mm. A test piece of 35 mm × 35 mm × 5 mm was collected from the obtained steel plate. The test piece was processed into a surface ΔΔΔ finish. The test piece was tape-sealed at a position 5 mm from the edge of the end face, the back face and the front face so that the test face was 25 mm × 25 mm.

  The corrosion acceleration test was performed under the conditions shown in Table 1. That is, the case where it carried out based on one embodiment of the above-mentioned present invention was made into the example of the present invention (test No. 1-20), and the case where it carried out off was made into the comparative example (test No. 21-27). .

  In the salt adhesion step, salt water dropping and salt water spraying were performed. The salt water dripping was performed by applying an artificial seawater solution to the surface of the test piece once a week so that the water film thickness was 5 mm. At this time, the amount of adhering salt was controlled by changing the concentration of the artificial seawater solution. In salt spray, the amount of adhering salt was controlled by controlling salt water concentration and spray time.

  In the cleaning process, tap water was used as the cleaning water, the test piece was held at an angle of 45 degrees from the horizontal plane, and the cleaning water whose flow rate was adjusted was applied to the portion of the test piece surface that was not tape-sealed. The test period was 12 weeks.

Then, after the test, the rust generated on the test piece surface was taken, subjected to quantitative X-ray diffraction, × a case where _Fe 3 _{O 4} content in the rust is not less than 10 mass% ○, less than 10 mass%, beta The case where the amount of -FeOOH was less than 10 mass% was evaluated as ◯, and the case where it was 10 mass% or more was evaluated as x. The evaluation results are shown in Table 1.

As shown in Table 1, in the test conditions of the present invention examples (Test Nos. 1 to 20), the amount of Fe ₃ O ₄ in the rust generated on the surface of the test piece is 10 mass% or more (◯), and the amount of β-FeOOH is Since it is less than 10 mass% (◯), it is considered that the actual highly humid environment is reproduced.

On the other hand, under the test conditions of the comparative example (Test Nos. 21 to 27), the amount of Fe ₃ O ₄ is less than 10 mass% (×), the amount of β-FeOOH is 10 mass% or more (×), and the actual high-humidity environment is reproduced. You can see that they are not.

  From the above, the effectiveness of the present invention could be confirmed.

It is process drawing of the corrosion resistance evaluation test in one Embodiment of this invention.

Claims (1)

A corrosion promotion test method for a steel material comprising performing the following step (A), the following step (B) and the following step (C) at least once , and Fe ₃ in rust generated on the surface of the steel material. O ₄ is 10 mass% or more, β-FeOOH is less than 10 mass%,
In the following step (A), the salt adhesion amount is 0.1 to 100,000 mg / m ² , and in the following step (B), the drying step and the wetting step are performed within the following condition range. Test method for accelerated corrosion of steel in the environment.
(A) The process of adhering salt containing chloride ions to the surface of the steel material (B) The drying process and the wetting process set by changing the temperature and relative humidity are repeated on the steel material as one cycle, and drying is performed. The relative humidity of the process and the wet process and the wet rate represented by the following formula are performed within the following condition range, and this cycle is performed at least once. Dry process Relative humidity: More than 40% and 70% or less Wet process Relative humidity : 80% or more Wetting rate = (wet process holding time / (drying process holding time + wetting process holding time)): 95% or more and 100% or less

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