JP2019007851A - Method for predicting corrosion of weather-resistant steel and method for predicting corrosion of steel structure - Google Patents

Abstract

To provide a method for predicting corrosion of a weather-resistant steel and a method for predicting corrosion of a steel structure.SOLUTION: A period of time when the ratio of the corrosion amount of a weather-resistant steel to the corrosion amount of a steel defined by JIS G 3106:2015 is constant is determined from the above ratio, the corrosion prediction expression is calculated using the corrosion amount of the weather-resistant steel in the period of time when the ratio of the corrosion amount is constant, and the corrosion amount of the weather-resistant steel is predicted from the corrosion prediction expression.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a corrosion prediction method for weathering steel and a corrosion prediction method for steel structures.

  Although it is extremely important to use a bridge, which is one of the steel structures that are social capital, for a long period of time, these steel structures and steel structures have problems such as deterioration. Corrosion is one of the main causes of deterioration, and it is important from the viewpoint of maintenance and maintenance to predict the amount of corrosion of bridges used for a long time.

  In bridges, elements such as Cu, Ni, and Cr are added to steel so that the deterioration of the load bearing performance of the bridge does not cause any engineering problems (corrosion rate of about 0.01 mm / year or less). There are weathering steels that form “protective rust” that can inhibit corrosion. Weather-resistant steel is used for bridges of about 1,900,000t until 2015. This accounted for about 9% of the total bridge, and this figure is increasing. Weatherproof steel is often used bare without being subjected to anticorrosion treatment such as painting. Therefore, it is more important to know the amount of corrosion and perform maintenance and maintenance.

  As a technique for predicting corrosion of weathering steel, for example, Non-Patent Document 1 describes an emblem type exposure test. In the emblem type exposure test, a emblem test piece in which a test material is formed in a small rectangular plate shape is used. When there is an existing bridge or a bridge to be managed nearby, an actual bridge emblem test is performed in which a patch is attached directly to the part that seems to be the most corrosive. In this case, the emblem test using a 100-box or cylindrical exposure container as the exposure container and installing a patch inside the container is conducted. As an actual bridge emblem test, for example, 50 mm × 50 mm × 2 mm emblem weather-resistant steel (hereinafter referred to as emblem test piece) is installed in each part of the bridge and collected every year to reduce the amount of corrosion. Measure and predict the amount of corrosion in the future from the years since the start of the test and the amount of corrosion in each year.

  As other weathering steel corrosion prediction techniques, in Patent Documents 1 and 2, environmental factors such as temperature, wetting time, sulfur oxide amount, amount of incoming salt, and material factors such as weathering steel components are used. A technique for predicting the amount of corrosion is described.

Japanese Patent No. 3990957 Japanese Patent No. 4143018

“JSSC Technical Report No. 73: Possibility of Weatherproof Steel Bridge and New Technology”, Japan Steel Structure Association, October 2006

  However, according to the weathering steel prediction method using the above-mentioned exposure test results with the emblem test piece, the results of long-term exposure tests over 5 to 10 years are required to accurately predict the future corrosion amount. It becomes. Therefore, there is a problem that it takes a long time before the corrosion amount can be predicted. The amount of corrosion in the collected emblem is measured by removing the weight with a pickling solution standardized by ISO 8407, measuring the weight, and calculating the amount of corrosion from the difference from the initial weight. For this reason, when the exposure test time is short and the amount of corrosion is small, the skill of the tester greatly affects the rust removal, and the calculated corrosion amount varies and often lacks accuracy.

  In addition, in the prediction methods described in Patent Documents 1 and 2, since the flow of wind and the ease of accumulation of deposits differ for each part of the bridge, the variation in the corrosion amount becomes large, and the long-term corrosion amount is accurately predicted. There is also a problem that is difficult.

  In view of the above problems, an object of the present invention is to provide a corrosion prediction method for weathering steel and a corrosion prediction method for steel structure, which can predict the corrosion amount of weathering steel in a short test time.

The present invention has been completed by intensive studies to solve the above problems, and has the following gist.
[1] From the corrosion amount ratio of the weathering steel to the steel specified in JIS G 3106: 2015, the period during which the corrosion amount ratio is constant is determined,
Calculate the corrosion prediction formula using the corrosion amount of the weathering steel in the period when the corrosion amount ratio is constant,
A corrosion prediction method for weathering steel, wherein the corrosion amount of the weathering steel is predicted from the corrosion prediction formula.
[2] The method for predicting corrosion of weatherable steel according to [1], wherein an extrapolation period used for calculating the corrosion prediction formula is selected from a period in which the corrosion amount ratio is constant. .
[3] The method for predicting corrosion of weatherable steel according to [2], wherein the extrapolation period is 180 days or longer.
[4] The period in which the corrosion amount ratio is constant is a period from the first day in which a change of less than ± 0.01 with respect to the corrosion amount ratio on the previous day has continued for 30 days or more. The method for predicting corrosion of weatherable steel according to any one of [1] or [2] above.
[5] Corrosion prediction of a steel structure characterized by predicting the corrosion amount of the steel structure using the weathering steel corrosion prediction method according to any one of [1] to [4] above. Method.

  ADVANTAGE OF THE INVENTION According to this invention, the corrosion prediction method of a weathering steel which can predict the future corrosion amount of a weathering steel in a short test time can be provided. The corrosion prediction method of the present invention can be suitably used for predicting the future corrosion amount of steel structures such as bridges, and has an industrially beneficial effect.

FIG. 1A is a graph showing the results of monitoring the amount of corrosion of weathering steel using an electrical resistance type corrosion sensor. FIG. 1 (B) is a graph showing the corrosion amount ratio of the weathering steel to the structural steel material for welding. FIG. 2A is a plan view schematically showing an example of an electrical resistance type corrosion sensor. FIG. 2B is a cross-sectional view taken along line AA in FIG. 3A to 3C show the corrosion prediction formula calculated by changing the period during which the corrosion amount ratio is constant in the bridge A, its prediction curve, the corrosion amount by the electric resistance type corrosion sensor, and the actual bridge emblem test. It is the graph which showed the amount of corrosion. FIG. 4 is a graph showing an error between the prediction result by the prediction method according to the present embodiment and the actual bridge emblem test result for the bridge A with respect to a test time of 17 years. FIGS. 5A to 5F are graphs showing the calculated corrosion prediction formula and its prediction curve, the corrosion amount by the electric resistance type corrosion sensor, and the corrosion amount by the actual bridge emblem test in the examples.

  Hereinafter, the present invention will be described in detail. Note that the present invention is not limited to this embodiment.

  First, the knowledge that led to the present invention will be described.

  The corrosion behavior of weathering steel in an environment where steel structures (for example, bridges using weathering steel) are constructed includes the initial period during which corrosion proceeds at a high corrosion rate without corrosion inhibition by protective rust. There is a corrosion inhibition period due to protective rust formation (hereinafter referred to as protective rust formation period). As described above, the weather resistant steel exhibits its corrosion resistance due to protective rust. Therefore, in evaluating the corrosion resistance of the weathering steel, it is important to grasp the degree of corrosion rate due to protective rust formation rather than the initial corrosion rate.

  FIG. 1A is a graph showing an example of an exposure test result of weathering steel. The horizontal axis shows the time (years) elapsed from the start of the exposure test, and the vertical axis shows the amount of corrosion (μm). Time is 0 years from the start of the test and point a. The amount of corrosion is obtained by converting the result of an electrical resistance type corrosion sensor (see FIG. 2) described later into a corrosion depth. In addition, although the test result has described only to c point, as shown by a dotted line, the actual test result exists to a long term. FIG. 1 (A) shows the period (a to b period) in which corrosion proceeds at a large corrosion rate from the start of the test (point a: time = 0 years), and the influence of the rust layer compared to this period a to b. There is a period (b-c period) during which the corrosion rate is suppressed. From the corrosion behavior of the weather resistant steel described above, it can be estimated that the period ab is the initial period and the period bc is the protective rust formation period.

  Here, as a result of intensive studies to achieve the above-described problems, the present inventors have obtained the following new knowledge.

  For the steel specified in JIS G 3106: 2015 (hereinafter referred to as structural steel for welding or ordinary steel), which will be described later, the same environment, the same time period, and the same corrosion sensor as the exposure test that obtained the result of FIG. Was monitored for corrosion. And from the corrosion monitoring results of ordinary steel and weathering steel, the present inventors have found that the ratio of the corrosion amount of the weathering steel to the corrosion amount of the ordinary steel is characteristic. FIG. 1B shows an example of the relationship between the time from the start of the test and the corrosion amount ratio of the weathering steel to the corrosion amount of the ordinary steel. The horizontal axis indicates the time (years) elapsed from the start of the exposure test, and the vertical axis indicates the corrosion amount ratio of the weathering steel to the normal steel. In order to facilitate comparison with FIG. 1A, the horizontal axis is shown to be the same.

  That is, from the results shown in the graphs of FIGS. 1 (A) and 1 (B), a period in which the protective rust formation period is estimated and a period in which the corrosion amount ratio is constant (periods b to c in FIG. 1 (B)). ) Was found to be almost the same. After that, it was also found that the difference in the corrosion amount between normal steel and weathering steel spreads with a constant corrosion amount ratio. The reason why the corrosion amount ratio is constant is considered to be that the corrosion resistance of the weathering steel is stabilized by the formation of protective rust on the weathering steel. Therefore, the period in which the corrosion amount ratio that matches the estimated period of protective rust formation is constant is captured, and the corrosion amount of the weathering steel and the time from the start of the test in the constant corrosion amount ratio period. The inventors have come up with the idea that the use of data will lead to faster corrosion predictions.

  Therefore, in the present invention, the corrosion test is performed on the weathering steel and the ordinary steel, the corrosion amount with respect to the time from the start of the test is obtained for each steel, and the corrosion amount of the ordinary steel is obtained from the obtained time and corrosion amount data. Taking the period when the corrosion rate ratio of the weathering steel is constant, calculate the corrosion prediction formula using the corrosion amount of the weathering steel during the period when this corrosion rate ratio is constant, and from this corrosion prediction formula, Predict the amount of corrosion. This makes it possible to accurately predict the future corrosion amount of the weather-resistant steel, especially when it has passed for a long time, even in a short test.

  In the present invention, “short time” or “short test time” means less than one year from the start of the test. On the other hand, “long time” means three years or more from the start of the test.

In general, a time period is referred to as a “period”. However, in the present specification, an arbitrary time elapsed from the start of the test also becomes a “period”. Therefore, the time elapsed from the start of the test is referred to as “time” or “test time”, and the period between the elapsed time and the elapsed time from the start of the test is referred to as “period” for distinction.
[Method of predicting corrosion of weathering steel]
Next, the method for predicting corrosion of weathering steel according to the present invention will be described.

  First, a corrosion test is performed using a material to be tested, and a corrosion amount for an arbitrary time is obtained from the start of the corrosion test (see FIG. 1A). As a test material for determining the amount of corrosion, at least one kind of weathering steel and at least one kind of steel (structural steel for welding or ordinary steel) specified by JIS G 3106: 2015 are used. As the weather-resistant steel and ordinary steel here, for example, the weather-resistant steel and ordinary steel described in “Test Material” described later can be used. Moreover, the thing as described in the "corrosion test" mentioned later can be used for the corrosion test here.

  Next, the corrosion amount ratio of the weathering steel to the ordinary steel is determined for each hour using the corrosion amount of the ordinary steel and the corrosion amount of the weathering steel obtained from the corrosion test (see FIG. 1B).

  Next, a period during which the corrosion amount ratio is constant is determined from the result of the obtained corrosion amount ratio. Here, the period during which the corrosion amount ratio is constant can be determined by the method described in “period during which the corrosion amount ratio is constant” described later.

  Next, a corrosion prediction equation is calculated from a predetermined corrosion equation using the corrosion amount Y of the weather-resistant steel and the time X from the start of the corrosion test in a period in which the corrosion amount ratio is constant. In addition, about the calculation method of a predetermined | prescribed corrosion type | formula and corrosion prediction type | formula, the thing as described in the below-mentioned "calculation method of a predetermined | prescribed corrosion type | formula and corrosion prediction type | formula" can be used.

  Next, the corrosion amount of the weather resistant steel at an arbitrary time is predicted from the calculated corrosion prediction formula.

  As described above, in the present invention, the “corrosion amount” is predicted as the corrosion prediction. In general, there are two types of corrosion predictions: one that predicts “amount of corrosion” and one that predicts “corrosion rate”. When the “corrosion rate” is predicted, it is obtained by differentiating the amount of corrosion. That is, the corrosion rate can also be predicted by the corrosion prediction method of the present invention.

<Test material>
Test materials to which the corrosion prediction method of the present invention is applied will be described.
In the present invention, two types of steels, weatherproof steel and ordinary steel, are used as test materials. It is most desirable that the weather resistant steel for the test material has the same composition and the same characteristics as the weather resistant steel subject to corrosion prediction, because the predicted result can be obtained accurately. For example, a weather resistant steel having the same composition and the same characteristics as the weather resistant steel used for a bridge to be subject to corrosion prediction can be used. However, if necessary, it is possible to select a weathering steel to be subjected to corrosion prediction and an equivalent composition and / or weathering performance. Further, depending on the case, a weathering steel to be subjected to corrosion prediction may be selected which has a similar composition and / or weathering performance.

As the weather-resistant steel, a weather-resistant hot rolled steel material for welded structure defined in JIS G 3114: 2016 can be used. Among them, the present invention can be preferably used particularly for SMA490A. More specifically, the following steels are more suitable as weather resistant steels. As a composition of the said weathering steel, C: 0.18 mass% or less, Si: 0.15-0.65 mass%, Mn: 1.40 mass% or less, P: 0.035 mass% or less, S: 0.035 mass% or less, Cu: 0.30 to 0.50 mass%, Cr: 0.45 to 0.75 mass%, Ni: 0.05 to 0.30 mass%, the balance being Fe and inevitable Consists of impurities. In addition, as the mechanical properties, YS: 355N / mm ² or ^{more, TS: 490~610N / mm 2,} El: a 19% or more. However, as long as the weather resistance is not hindered, an element other than the essential elements may be included as a selection element as necessary.

  On the other hand, as the normal steel, a rolled steel material for welded structure defined in JIS G 3106: 2015 can be used. Among these, SM490A can be particularly preferably used. More specifically, the following steels are more suitable as ordinary steels.

<Corrosion test>
In the present invention, a corrosion test is performed on weathering steel and ordinary steel, and the time from the start of the corrosion test and the amount of corrosion at that time are measured for each steel material.

  It is most preferable that the corrosion test is an exposure test in an environment similar to or the same as that of the weathering steel that is the object of corrosion prediction. This is because when the purpose of corrosion prediction is to grasp the amount of corrosion of a steel structure installed in an outdoor environment after a long period of time, accurate prediction is possible. However, other test methods may be used as long as data on the corrosion amount of the test material and the time from the start of the test can be obtained. If necessary, it can be selected from other known test methods (various corrosion acceleration tests, various gas corrosion tests, various corrosion resistance tests, various weather resistance tests, etc.).

  On the other hand, it is most preferable that the corrosion amount is obtained by using an electrical resistance type corrosion sensor every predetermined time from the start of the test. Since the electrical resistance type corrosion sensor can detect a small amount of corrosion and because it can sample corrosion even if the time interval is short, the “period during which the corrosion amount ratio is constant” will be described later. This is because it can be surely obtained. However, as long as this “period during which the corrosion amount ratio is constant” can be calculated, other methods of measuring the corrosion amount may be used. Depending on the corrosion rate of the test material under the corrosion test, it can be selected from known corrosion amount measurement methods (methods using various corrosion sensors such as ACM type corrosion sensors, patch tests, etc.). Whichever corrosion amount measurement method is used, in order to adapt to the corrosion prediction method of the present invention, it is necessary to obtain two types of results, that is, weather resistant steel and ordinary steel as test materials.

<Electrical resistance type corrosion sensor>
In the present invention, it is most preferable to use an electrical resistance type corrosion sensor for measuring the amount of corrosion in the above-described corrosion test. An example of the structure of an electrical resistance type corrosion sensor that can be used in the corrosion prediction method according to the present invention and an example of a method for calculating the amount of corrosion will be described below.

  FIG. 2A is a plan view schematically showing an example of an electrical resistance type corrosion sensor. FIG. 2B is a cross-sectional view taken along line AA in FIG. As shown in FIG. 2A, the electrical resistance type corrosion sensor 1 includes a sensor unit 11 exposed to an arbitrary environment and a reference unit 21 blocked from an arbitrary environment to which the sensor unit 11 is exposed. Have. As shown in FIG. 2B, the sensor unit 11 and the reference unit 21 are arranged in parallel on one surface of a flat substrate 31 via an insulating sheet 41. Both side surfaces of the sensor unit 11 and the reference unit 21 are covered with an insulating resin 51, and the upper surface of the reference unit 21 is covered with an insulating cover 61. In particular, when the corrosion sensor 1 is in a corrosive environment, the sensor portion 11 whose upper surface is exposed progresses in the thickness direction (the direction from the upper surface side to the lower surface side).

  When used in the corrosion prediction method according to the present invention, at least two corrosion sensors shown in FIG. 2 are used to determine a period during which the corrosion amount ratio described later is constant. One corrosion sensor is made of weathering steel and the other corrosion sensor is made of plain steel. Specifically, the sensor part 11 of one corrosion sensor is formed of weathering steel that is a corrosion prediction target, and the sensor part 11 of the other corrosion sensor is formed of plain steel. The material constituting each reference portion 21 is preferably the same material as the material constituting each sensor portion 11.

  For example, as shown in FIG. 2A, the sensor unit 11 and the reference unit 21 are connected to a current source 71, a voltage measurement unit 81 is connected to both ends of the sensor unit 11, and a voltage measurement is performed to both ends of the reference unit 21. Unit 91 is connected. In such a corrosion sensor 1, a constant current is supplied from the current source 71, and the voltage is measured by the voltage measuring unit 81 and the voltage measuring unit 91, thereby obtaining the electric resistance values of the sensor unit 11 and the reference unit 21.

In the corrosion sensor 1, the electrical resistance values of the sensor unit 11 and the reference unit 21 are obtained at arbitrary constant intervals, and the corrosion amount (corrosion depth) of the sensor unit 11 is measured (converted) based on the obtained electrical resistance value. . More specifically, the conversion formula for the corrosion amount is expressed by the following formula (1).
CD = t _init {(R _{ref, init} / R _{sens, init} ) − (R _ref / R _sens )} Expression (1)
CD: Corrosion amount (corrosion depth) [μm]
t _init : initial thickness of sensor part [μm]
R _{ref, init} : Initial resistance value of the reference part [Ω]
R _{sens, init} : Initial electrical resistance value of sensor unit [Ω]
R _ref : Electric resistance value [Ω] at the time of measuring the reference part
R _sens : Electric resistance value [Ω] when measuring the sensor
<Calculation method of predetermined corrosion formula and corrosion prediction formula>
The corrosion prediction formula is calculated using a corrosion amount Y and a time X from the start of the corrosion test in a “period in which the corrosion amount ratio is constant” described later. As the corrosion amount Y and the time X from the start of the corrosion test, the data of the corrosion amount measured in the above-described “corrosion test” and the time from the start of the corrosion test can be used as they are.

The most preferred embodiment is a method of calculating a corrosion prediction formula by extrapolating to a predetermined corrosion formula. As the predetermined corrosion formula, the relational expression between the corrosion amount (Y) and the test time (X) shown in the formula (2) is often used and has a track record, and is most preferable.
Y = A · X ^B (2)
However, Y: Corrosion amount (μm), X: Test time (days).
As the corrosion amount (Y), the corrosion amount in the period when the corrosion amount ratio is constant is extrapolated as the test time (X), and the time from the start of the corrosion test is extrapolated to determine A and B in the above equation (2). Thereby, a corrosion prediction formula can be calculated. In addition, the predetermined corrosion formula may be used other than the above formula (2) regardless of whether it is known or not known.

<Period during which the corrosion amount ratio is constant>
As described above, in the corrosion prediction method of the present invention, it is important to set a period during which the corrosion amount ratio of the weathering steel to the ordinary steel is constant.

  For this reason, in the present invention, the data of the corrosion amount and the time from the start of the corrosion test in the period in which the corrosion amount ratio is constant is extrapolated to a predetermined corrosion equation to obtain a corrosion prediction equation. Even more preferably, an extrapolation period for creating a corrosion prediction formula is selected from the period in which the corrosion amount ratio is constant, and the data of the corrosion amount and the time from the start of the corrosion test within the extrapolation period are used. Thus, extrapolation to a predetermined corrosion equation is made as a corrosion prediction equation. The selection of the extrapolation period will be described using the measurement result of the corrosion amount using the electrical resistance type corrosion sensor of FIG.

  FIG. 3 shows the results of an exposure test on bridge A in the examples described later. The horizontal axis shows the time from the start of the exposure test (unit is year), and the vertical axis shows the corrosion depth (unit is μm). The formula in each graph is the calculated corrosion prediction formula, the prediction curve plotting the dotted corrosion prediction formula, the gray circle sign is the corrosion amount by the electric resistance type corrosion sensor, the diamond sign is the corrosion by the emblem type exposure test Amount. Corrosion prediction formulas were created using the amount of corrosion by the electrical resistance type corrosion sensor and the time from the start of the exposure test, as in FIGS. 5 (A) to (D) of the example.

The extrapolation period is set by creating a graph of the corrosion amount ratio in FIG. 1B and confirming the period during which the corrosion amount ratio is constant (b to c), and is the starting point of the constant corrosion amount ratio period. Three types of 210 days, 180 days, and 90 days were selected starting from point b. For example, when the extrapolation period is 210 days, the test time included in the period of 210 days starting from point b and the corrosion amount for the test time are selected, and the selected test time is X and the corrosion amount is Y. Extrapolated to equation (2), constant A and constant B were determined and used as corrosion prediction equations. The obtained corrosion prediction formula was Y = 18.222 · X ^0.3238 . The extrapolation period is the same for 180 days and 90 days, and the obtained result is the corrosion prediction formula of FIG.

  The extrapolation period (unit: days) was 210 days in FIG. 3A, 180 days in FIG. 3B, and 90 days in FIG. 3C. Comparing the prediction curves in FIGS. 5A to 5D with the emblem type exposure test results, it can be seen that the corrosion amount ratio agrees very well when the predetermined period is 180 days or more. In the case of 90 days in FIG. 3C, if the time to be predicted is up to 1.5 years, the allowable range is reached.

  Therefore, the error of the corrosion amount prediction result when the extrapolation period changed was investigated. As the data of the corrosion amount used in the investigation, the results of the exposure test of the examples described later were used. The bridge A is an object, and the same test results as in FIG. 3 are used. The corrosion prediction formula was calculated in the same manner as in FIG. 3 using the amount of corrosion by the electrical resistance type corrosion sensor 1 of an example described later. From this calculated corrosion prediction formula, a predicted corrosion amount in 17 years was obtained. An error (unit:%) was obtained by subtracting the corrosion amount in 17 years of the emblem type exposure test from this predicted corrosion amount and further dividing by the corrosion amount in 17 years of the emblem type exposure test. In addition, since there were three emblem test pieces, the arithmetic average value of the three pieces was determined as the amount of corrosion in 17 years. The extrapolation period was set to 90 days, 120 days, 150 days, 180 days, 210 days, 240 days, 270 days, and 300 days.

  The survey results are shown in FIG. FIG. 4 is a graph showing an error between the prediction result by the prediction method according to the present embodiment and the actual bridge emblem test result for the bridge A with respect to a test time of 17 years. The horizontal axis indicates the extrapolation period (days), and the vertical axis indicates the error (%). The dotted line in the figure indicates the variation of the three emblem test pieces. From the results shown in the graph of FIG. 4, the extrapolation period is 180 days or more when predicting a long-term corrosion amount of 10 years or more, such as 17 years, even if the variation of the emblem specimen is taken into consideration. It turns out that it is preferable.

  From the above results, in the corrosion prediction method of the present invention, it is possible to select the extrapolation period used to calculate the corrosion prediction formula from the period in which the corrosion amount ratio is constant. This is preferable because it can be obtained. In the corrosion prediction method of the present invention, the extrapolation period can be 180 days or longer. If it is less than 180 days, if the test time is short and the amount of corrosion is small, it may be difficult to accurately predict the amount of corrosion. Therefore, it is more preferable to set it as 180 days or more.

Further, the period during which the above-described corrosion amount ratio is constant is, for example, from the inventors' experience, a change of less than ± 0.01 with respect to the corrosion amount ratio (0 to 1) of the previous day (24 hours ago). The period from the first day that has continued for 30 days or more is most preferable. Note that the period during which the corrosion amount ratio is constant can be obtained by methods other than those described above.
[Method of predicting corrosion of steel structures]
Next, a method for predicting corrosion of a steel structure according to the present invention will be described.

  The method for predicting corrosion of a steel structure according to the present invention can be applied, for example, to a steel structure using at least part of a weathering hot-rolled steel material for welded structure defined in JIS G 3114: 2016. Steel structures include steel bridges, buildings, and other structures. As materials used for the steel structure, there are, for example, weather resistant steel and ordinary steel described in the above-mentioned materials.

  In the corrosion prediction method for a steel structure according to the present invention, the long-term corrosion amount of the steel structure is predicted using the above-described corrosion prediction method for weathering steel.

[Example]
Hereinafter, the effects of the present invention will be described by way of examples. However, the present examples are merely examples for explaining the present invention, and do not limit the present invention.

An exposure test was conducted on bridges A, B, and C in different regions. The amount of corrosion was determined by monitoring the amount of corrosion using an emblem type exposure test and an electrical resistance type corrosion sensor.
<Wappen type exposure test>
The actual bridge emblem test described above was performed. The emblem test piece uses a commercially available weather resistant steel SMA490A (steel specified in JIS G 3114: 2016) as a test material, processed into a shape of 50 mm × 50 mm × 2 mm, washed with ethanol, Affixed to Bridge C using double-sided tape. Each test piece was collected in a predetermined period, rusted with a pickling solution standardized by ISO 8407, weighed, and the amount of corrosion was calculated from the difference from the initial weight. In this example, the amount of corrosion was converted from the weight to the corrosion depth (unit: μm). Three test pieces are collected in each period. The exposure test period was a maximum of 17 years for Bridge A, a maximum of 15 years for Bridge B, and a maximum of 12 years for Bridge C.
<Monitoring the amount of corrosion using an electrical resistance type corrosion sensor>
Two electrical resistance type corrosion sensors described in FIG. 2 were used. One uses SMA490A similar to the emblem type exposure test for the sensor unit 11 and the reference unit 21, and the other uses ordinary steel SM490A (steel specified in JIS G 3106: 2015) for the sensor unit 11 and the reference unit 21. Used for. Both of these corrosion sensors 1 were installed at the same place as the emblem type exposure test. The amount of corrosion was determined as the corrosion depth (unit: μm) using the above equation (1). From the obtained corrosion amount, the period during which the corrosion amount ratio of the weathering steel to the ordinary steel was constant was determined. Judgment whether the corrosion amount ratio of the weathering steel to the ordinary steel is a “constant corrosion amount ratio” is less than ± 0.01 with respect to the corrosion amount ratio (0 to 1) of the previous day (24 hours ago). Judgment is based on whether or not it is a change. A period from the first day (for example, period b to c shown in FIG. 1A) in which this change has continued for 30 days or more is determined as “a period during which the corrosion amount ratio is constant”. A plurality of extrapolation periods were set from the measurement results within a fixed period of the determined corrosion amount ratio. The monitoring period during the exposure test was a maximum of one year for Bridge A, a maximum of 0.74 years for Bridge B, and a maximum of 0.74 years for Bridge C. On the other hand, the sampling period is approximately 1 minute (1.9 × 10 ^-6 years) 60 minutes intervals (1.1 × 10 ^-4 years) as either spacing, for example, the sampling period was 10 minutes.

<Comparison with emblem exposure test results for corrosion after a long time>
The corrosion amount (Y) obtained by the above two types of corrosion amount measurement methods and the time (X) from the start of the exposure test are extrapolated to Equation (2), constant A and constant B are determined, and the corrosion prediction formula is Prepared and predicted the amount of corrosion after a long time. A graph of the results for bridge A is shown in FIG. The horizontal axis shows the time from the start of the exposure test (unit is year), and the vertical axis shows the corrosion depth (unit is μm). The equation in each graph is the calculated corrosion prediction formula, the prediction curve plotting the dotted corrosion prediction formula, the gray circle sign is the corrosion amount by the electric resistance type corrosion sensor, the diamond sign is the corrosion by the emblem type exposure test Amount.

  5A to 5D show a case where a corrosion prediction formula is created using the amount of corrosion by an electrical resistance type corrosion sensor and the time from the start of the exposure test. As the extrapolation period, a plurality of types were set in the same procedure as that shown in FIG. 3 described in the “period in which the corrosion amount ratio is constant”. The extrapolation period (in days) was 90 days in FIG. 5A, 150 days in FIG. 5B, 180 days in FIG. 5C, and 210 days in FIG. 5D. Comparing the prediction curves in FIGS. 5 (A) to (D) and the emblem type exposure test results, as explained above, the corrosion amount ratio agrees very well when the fixed period is 180 days or more. I understand. In particular, the effect becomes remarkable for a long time, specifically, 5 years or more. On the contrary, if the time to be predicted is not so long, for example, if the corrosion amount is predicted after three years, the effect can be obtained even if the corrosion amount ratio is a fixed period of 150 days.

  FIGS. 5E and 5F show a case where a corrosion prediction formula is created using the amount of corrosion by the emblem type exposure test and the time from the start of the exposure test. The extrapolation period (in days) was 1.5 years (corresponding to 550 days in Table 1) in FIG. 5 (E) and 3 years (corresponding to 1091 days in Table 1) in FIG. 5 (F). Comparing the prediction curves in FIGS. 5 (E) and 5 (F) with the emblem type exposure test results, it can be seen that the results are consistent with the measurement results for 5 years or more only when the external insertion period is 3 years.

Table 1 summarizes the extrapolation period and prediction results for determining the corrosion prediction formula for bridges A, B, and C. The amount of corrosion in the emblem exposure test at each test time of the longest time for each bridge as the longest time and 2 years as the shorter time was used as a comparison target. Judgment whether the prediction result using the calculated corrosion prediction formula coincides with the amount of corrosion of the emblem type exposure test piece was determined by whether a prediction curve was included in the variation of the three test pieces. When it is included, it is indicated by a symbol ○, and when it is off, it is indicated by a symbol ×. As an example of the present invention, prediction was made using the corrosion prediction formula calculated by the corrosion prediction method of the present invention, and as a comparative example, the results of the emblem type exposure test of weathering steel without setting a period during which the corrosion amount ratio was constant or Corrosion prediction formulas were calculated and predicted only from the results of monitoring the amount of corrosion using an electrical resistance type corrosion sensor. For the case where the test time is long (the longest time), the error (%) between the corrosion amount prediction result using the calculated corrosion prediction formula and the corrosion amount of the emblem type exposure test piece was calculated by the following equation.
Error = (estimated from the prediction formula in comparison time (A is 17 years, B is 15 years, C is 12 years)-comparison time (A is 17 years, B is 15 years, C is 12 years) Average value of 3 points of emblem test) / Comparison time (A is 17 years, B is 15 years, C is 12 years) Average value of 3 points of emblem test results x 100 (%)

  Corrosion amount prediction results by the corrosion prediction method of the present invention are very good with the results of the emblem exposure test in the case of short-term corrosion for 2 years despite the different exposure conditions and extrapolation period of less than 1 year. You can see that they match. In addition, when the extrapolation period was set to 180 days or more, it became clear that the results of the emblem type exposure test in the case of long-term corrosion of 10 years or more were in good agreement. This is also because the error between the corrosion amount prediction result using the calculated corrosion prediction formula and the corrosion amount of the emblem type exposure test piece is as small as ± 3.0%. When calculating the corrosion prediction formula by selecting an extrapolation period from a period when the corrosion amount ratio is constant, combine the period when the corrosion amount ratio is constant for about half a year and the initial period until the protective rust formation period. It is understood that the amount of corrosion can be accurately predicted in less than one year.

  On the other hand, in the case of the comparative example in which the corrosion prediction formula is calculated only by the results of the weather-resistant steel emblem exposure test without setting the period during which the corrosion amount ratio is constant, the corrosion after a long period of 10 years or more It can be seen that 2 or 3 year test results are required to accurately predict the quantity. This can also be said from the fact that the error becomes smaller in the test results of 2 years or 3 years. If the extrapolation period is longer than 544 days, the test results for the short-term corrosion of 2 years are relatively consistent, but conversely, real-time test results are required even for a short test. It can be said that. This is because, after confirming the period during which the corrosion amount ratio is constant, the extrapolation period is not selected, and because the test time is short and the corrosion amount is small, the corrosion amount is accurately predicted. This is considered to be difficult.

  From these results, it can be seen that the corrosion amount can be predicted in a short period of time by the corrosion prediction method of the present invention. In particular, when the extrapolation period is 180 days or longer, it can be seen that corrosion prediction for a long time of 10 years or longer is possible.

DESCRIPTION OF SYMBOLS 1 Corrosion sensor 11 Sensor part 21 Reference part 31 Base 41 Insulation sheet 51 Resin 61 Insulation cover 71 Current source 81 Voltage measurement part 91 Voltage measurement part

  The corrosion prediction method of the present invention can accurately measure the change over time of the corrosion amount of weathering steel in an atmospheric corrosion environment.

Claims (5)

From the corrosion amount ratio of the weathering steel to the steel specified in JIS G 3106: 2015, the period during which the corrosion amount ratio is constant is determined,
Calculate the corrosion prediction formula using the corrosion amount of the weathering steel in the period when the corrosion amount ratio is constant,
A corrosion prediction method for weathering steel, wherein the corrosion amount of the weathering steel is predicted from the corrosion prediction formula.   The method for predicting corrosion of weatherable steel according to claim 1, wherein an extrapolation period used for calculating the corrosion prediction formula is selected from a period in which the corrosion amount ratio is constant.   The method for predicting corrosion of weatherable steel according to claim 2, wherein the extrapolation period is 180 days or more.   The period in which the corrosion amount ratio is constant is a period from the first day in which a change of less than ± 0.01 with respect to the corrosion amount ratio on the previous day has continued for 30 days or more. The method for predicting corrosion of weatherable steel according to either 1 or 2.   A corrosion prediction method for a steel structure, wherein the corrosion amount of the steel structure is predicted using the weathering steel corrosion prediction method according to any one of claims 1 to 4.