JP4982391B2 - Diagnosis method of steel material buried in soil - Google Patents

Description

  The present invention relates to a method for diagnosing a steel material embedded in soil in a state where a part is covered with concrete and the other part is in contact with the soil.

  Conventionally, for example, a steel tower is installed to support a transmission line. Such a steel tower has a base end portion buried in the ground. By the way, in the old steel tower installed more than 50 years ago, although the part near the ground is covered with concrete among the steel materials used as the legs of the steel tower, the base part located deep in the ground is made of concrete. Many steel materials are buried in the soil without being covered. The steel material provided in such a manner is generally called a soil foundation or a steel foundation.

  In this way, steel foundations have been adopted in the old steel towers because the steel towers are often installed in the mountains and there is a problem in transporting concrete, and all parts of the steel materials are the same as the foundations of today's steel towers. The fact that the so-called covered concrete foundation could not be used, the supply of oxygen in the soil was cut off, and it was thought that corrosion would not be a problem. This is due to the fact that it was not so large and even if it was not a concrete foundation.

  However, in recent years, corrosion of steel materials has been discovered, and the number of years since installation has already been long. Therefore, in order to maintain the safety of steel towers, the corrosion status of steel materials has been often confirmed. Yes. The specific method is to actually excavate the foundation portion of the steel tower, and directly confirm the corrosion state of the steel material by visual observation, and measure the dimensions of the steel material.

  However, this method has a problem that excavation may adversely affect the strength of the steel tower. In addition, in order to reduce the adverse effects of excavation, it is necessary to reinforce the steel tower and to prevent the ground from collapsing (referred to as earth retaining support). There is a problem that costs are required.

  In the field of buried pipes, for example, as described in Japanese Patent Laid-Open No. 01-250841 and Japanese Patent Laid-Open No. 02-107947, a method of performing diagnosis without performing actual excavation (trial drilling) work is proposed. However, the actual situation is that the method cannot be applied as it is due to the difference in installation conditions.

  Then, an object of this invention is to provide the diagnostic method of the steel materials which can diagnose the degradation condition of the steel materials embed | buried in soil, without actually performing a trial digging.

  A method for diagnosing a steel material according to the present invention is a method for diagnosing a steel material partly covered with concrete and the other part is in contact with the soil, and is embedded in the soil, at least the ground potential of the steel material and the soil potential The specific resistance is measured, and the pitting corrosion is based on an estimation function indicating the correlation between the pitting depth of the pitting corrosion that can occur in the steel material, the ground potential of the steel material, the specific resistance of the soil, and the embedding period of the steel material. The estimated value of depth is calculated | required, The corrosion cross-sectional area of the steel material in a pitting corrosion occurrence location is calculated | required using the estimated value of this pitting corrosion depth.

  According to the steel material diagnosis method having the above-described configuration, the pitting corrosion depth can be estimated without using actual measurement by using the estimation function. Here, the corrosion cross section has a direct relationship with the strength and life of the steel material, and is useful for various estimations regarding the steel material. Thus, it is possible to easily and accurately perform the deterioration diagnosis of the steel material.

  Further, in the steel material diagnosis method, the estimation function is more than a correlation function obtained by using a plurality of actually measured values of pitting depth, steel ground potential, and measured soil resistivity. In many of the pitting corrosions, a configuration in which the estimated value of the pitting corrosion depth is corrected so as to exceed the actual measurement value is preferable.

  Since the correlation function represents the data that is the basis of the correlation function, the data is distributed around the correlation function. Therefore, in about half of all the data, the pitting corrosion depth obtained from the correlation function is smaller than the actual pitting corrosion depth. It can be estimated to be smaller than actual. Therefore, the safety of the steel material can be determined at a higher level by using the estimation function that is corrected so that the pitting depth exceeds the actual measurement value in more cases.

  Further, in the steel material diagnosis method, the estimated value of the pitting depth is multiplied by a constant value to obtain a pitting corrosion length, and the corrosion breakage is calculated using the pitting corrosion depth and the pitting corrosion length. A configuration requiring an area is preferable.

  As a result of intensive studies, the inventor has come to discover that the length of pitting corrosion and the depth of pitting corrosion can be regarded as having a certain relationship. The corrosion cross section can be obtained by a very simple process.

  Moreover, in the said steel material diagnostic method, the said steel material is used as a support part of a structure, The steel material required in order to support the said strength and the steel material calculated | required from the said corrosion cross-sectional area It is preferable that the strength of the steel material is diagnosed by comparing the strength of the steel.

  Specifically, in the above steel material diagnostic method, to ensure the remaining cross-sectional area of the steel material obtained based on the corrosion cross-sectional area and the strength of the steel material required to support the structure. Compare the required cross-sectional area of the required steel.

  In this way, since the strength of the steel material is quantitatively represented by the index of area, the strength can be easily evaluated.

  Moreover, in the said steel material diagnostic method, the said steel material is used as a support part of a structure, and the steel material required in order for the intensity | strength of the steel material calculated | required from the said corrosion cross section to support the said structure The structure which calculates | requires the period until it becomes this intensity | strength as a remaining life is preferable.

  Specifically, in the method for diagnosing a steel material, a steel material in which the remaining cross-sectional area of the steel material is required to support the structure based on the corrosion cross-sectional area and the corrosion rate obtained from the burial period. The period until the required cross-sectional area of the steel material necessary to ensure the strength of the steel is obtained as the remaining life.

  In this way, since the remaining life of the steel material is quantitatively represented by the index of area, the remaining life can be easily evaluated.

  As described above, according to the present invention, it is possible to provide a steel material diagnosis method capable of diagnosing a steel material buried in soil without actually performing a trial digging.

  Hereinafter, an embodiment of a steel diagnosis method according to the present invention will be described with reference to the drawings.

  First, with reference to FIG. 1, a steel material that is a diagnostic object for which deterioration diagnosis is performed using the diagnostic method according to the present embodiment will be described. The steel material is a member used as a support portion of the structure, and specifically, constitutes a leg portion (or column portion) 2 of the steel tower 1. The steel tower 1 has four legs 2, that is, four steel materials 10 are used for one steel tower 1. Further, as the steel material 10, a member generally called equilateral mountain steel is used, and the cross-sectional shape in the transverse direction perpendicular to the longitudinal direction (or the length direction) has an L shape.

  As shown in FIG. 2, the steel material 10 exists in a state where a part of the steel material 10 is covered with concrete 20 and the other part is embedded in the soil in a state in contact with the soil. Specifically, the steel material 10 is embedded in the ground with a length of about 1.5 to 3.5 m. In some cases, only about 1 m is buried. The concrete 20 is applied from a portion of the steel material above the ground to a portion in the ground, and the concrete 20 is generally called “root-wrapped concrete”. In many cases, the concrete 20 is provided so as to cover up to a depth of about 1 m from the ground. Therefore, the steel material 10 is in a bare state without being covered with concrete at about 0.5 to 2.5 m of the base end. The foundation of the steel tower 1 provided in this manner is generally called a soil foundation or a steel material foundation. In the following, except for the case of referring to FIG. 1 and FIG. 2, terms such as steel towers and steel materials are not particularly denoted.

  Next, the types of corrosion (metal corrosion) that can occur in the steel materials as described above will be described. First, corrosion is broadly classified into natural corrosion caused by moisture present in the soil and electric corrosion caused by electric current from artificial electric equipment. Among these, natural corrosion is further classified into macro cell corrosion and micro cell corrosion. This is shown in Table 1.

<Macro cell corrosion>
Macrocell corrosion is corrosion that can clearly distinguish between corroded parts and non-corroded parts, forming so-called large cells (macrocells) in which current flows from the corroded part to the non-corroded part through the electrolyte (soil, etc.). Is in state. Such macro cell corrosion causes so-called “pitting corrosion” in which the steel material is locally corroded. As typical macrocell corrosion, concrete / soil (C / S) macrocell corrosion, air flow difference macrocell corrosion, and dissimilar metal macrocell corrosion are known.

<C / S macrocell corrosion>
First, C / S macrocell corrosion will be described. Since concrete is strongly alkaline with a pH of about 13, a passive film (an oxide film that maintains the corrosion resistance of the metal) is formed on the steel surface in the concrete, and corrosion is prevented. On the other hand, since the soil is often almost neutral (about pH 5 to 8), the steel material in direct contact with the soil is in a natural corrosive environment. In this case, C / S macrocell corrosion is corrosion caused by the macrocell formed with the steel material in direct contact as the anode and the steel material in the concrete as the cathode. That is, C / S macrocell corrosion can occur at the boundary between the part and the other part. Here, in the steel material constituting the steel tower, because the concrete is unreinforced, that the steel material is galvanized, the steel material area in the soil is larger than the steel material area in the concrete, Although it was considered that the steel material constituting the steel tower is not likely to cause C / S macrocell corrosion, there is actually an example in which C / S macrocell corrosion was confirmed in the steel material constituting the steel tower.

<Ventilation difference macro cell corrosion>
Next, aeration difference macrocell corrosion will be described. In the soil, the soil quality differs depending on the depth.For example, there are parts with high moisture content such as clay and poor air permeability, and sandy soil and other parts with low water content and good air permeability. Differences in oxygen concentration in the soil may occur. In this case, the corrosion caused by the macrocell formed with the steel material in contact with soil having poor air permeability as the anode and the steel material in contact with soil with good air permeability as the cathode is the air flow difference macrocell corrosion. That is, the air flow difference macrocell corrosion can occur due to a difference in oxygen concentration in the soil at each predetermined location in the other part of the steel material. In the steel material constituting the steel tower, heterogeneous soil may be distributed in layers or randomly due to the presence of groundwater or backfilling at the time of installation, and differential air macrocell corrosion may occur.

  In addition, when different types of metal macrocell corrosion come into contact with two different types of metals, they constitute a battery due to the difference in potential between the two, and the base potential metal corrodes. No dissimilar metals are used, and dissimilar metal macrocell corrosion usually does not occur.

<Micro cell corrosion>
Microcell corrosion is a state in which a fine battery (microcell) is formed in which a corroded portion and a non-corroded portion cannot be distinguished. Such microcell corrosion causes so-called “overall corrosion” in which the steel material gradually corrodes entirely. As typical microcell corrosion, bacterial corrosion, acidic soil corrosion, and general soil corrosion are known.

  Among such microcell corrosion, general soil corrosion is caused by oxygen in soil, which is essentially similar to natural water, as the main cathode reactant, and this general soil corrosion in steel materials that make up steel towers. Will occur. The oxide film formed by general soil corrosion is stable, and the corrosion rate Sg of general soil corrosion is very slow compared to the corrosion rate Sp of pitting corrosion. The subscript “g” indicates general corrosion “general corrosion”, and “p” indicates pitting corrosion “pitting corrosion”.

  Bacterial corrosion is a phenomenon often seen in mud and swamps. However, since steel towers are rarely installed in such places, corrosion due to bacterial corrosion is unlikely in steel materials constituting steel towers. In addition, acidic soil corrosion is corrosion caused by strongly acidic soil (pH 4 or lower). However, since Japanese soil with relatively much rain is kept almost neutral, it is difficult to consider corrosion due to acidic soil corrosion.

  From the above, it can be said that in steel materials constituting a steel tower, general corrosion mainly caused by general soil corrosion and pitting corrosion caused by C / S macrocell corrosion and air flow difference macrocell corrosion mainly occur. it can.

  Next, a steel material diagnosis method according to this embodiment will be described. A series of flow of the diagnostic method is as shown in FIG. 3 and will be described in detail below. First, the ground potential (tower ground potential) V of the steel material and the specific resistance ρ of the soil are measured.

<Measurement of ground potential V of steel>
Using a drilling tool such as a hand auger, a measurement hole 30 as shown in FIG. 2 is formed in the soil in the vicinity of the steel material (for example, a distance of up to about 1 m). The measurement hole 30 may be excavated along the vertical direction, or may be excavated at the installation angle of the steel material 10 (that is, the angle of the root of the steel tower). Further, the ground potential measuring device and the steel material 10 are connected, and a reference electrode (reference electrode) connected to the ground potential measuring device is inserted into the measurement hole 30 so that the steel material 10 is covered with the concrete 20. The ground potential V of the steel material 10 is measured at a depth (for example, about 0.5 m) and a depth not covered with concrete (for example, about 1.5 m). Specifically, the measurement is performed every about 0.5 m from the ground to the depth of the lower end portion of the steel material 10 (for example, about 3.5 m). A direct current is used for the measurement. Moreover, it can be grasped | ascertained by checking the position of the lower end part of steel materials beforehand using elastic waves etc. to which depth should be measured.

<Measurement of soil resistivity ρ>
A ground rod connected to a soil resistivity measuring device is inserted into the measurement hole 30, and the ratio of the soil at a depth at which the steel material 10 is not covered with the concrete 20 (for example, a depth of about 1 m or more from the ground). Measure resistance ρ. Specifically, the measurement is performed every about 0.5 m from the ground to the depth of the lower end portion of the steel material 10. The soil specific resistance measuring apparatus uses the principle of an AC bridge. In addition, the normal measurement value is about several kΩ · cm to about 1000 kΩ · cm, but if a value close to 0 kΩ · cm is measured, there is a possibility that the grounding rod is in contact with or close to the buried ground wire. Therefore, change the position in the same hole and perform measurement to confirm.

<Confirmation of other soil corrosion environmental factors>
As other soil corrosion environmental factors, the soil moisture content, pH value, groundwater status, etc. are confirmed. The moisture content of the soil may be measured strictly, or may be determined empirically in a wet state. Empirically, it is known that the moisture content is 20% or less if it easily collapses when picking up the soil taken by hand, and exceeds 20% otherwise.

  When measuring the pH value, soil existing at a depth of about 1 m from the ground is used for the measurement. For example, soil excavated when the measurement hole 30 is excavated may be used. Japanese soil is generally neutral (eg, pH 5.5) close to acidity, so measurement is not necessarily required. However, humic acid is caused by the decay of plants (decomposition rotification). When organic acid such as acid is generated and the acidity of the soil may increase, it is desirable to actually measure.

  Regarding the confirmation of the status of groundwater, check whether the presence of groundwater (arrangement of groundwater) falls under one of the three types of "with or without groundwater". In addition, since the situation of groundwater can fluctuate by rainfall, it is desirable to confirm the situation of groundwater by avoiding it immediately after the rain.

  In addition, as shown in FIG. 1, since the steel material 10 has a L-shaped cross-sectional shape, the measurement is preferably performed on two portions corresponding to each side of the L-shape. Specifically, as shown in FIG. 1, the measurement is performed at two locations P1 and P2 for one steel material.

  Next, the corrosion state of the steel material is diagnosed based on the measurement result.

<Evaluation of overall corrosion>
The overall corrosion is evaluated based on the measurement results of the ground potential V of the steel material and the specific resistance ρ of the soil. Specifically, the degree of the overall corrosion is determined in consideration of the influence on the overall corrosion based on the ground potential V of the steel material and the influence on the overall corrosion based on the specific resistance ρ of the soil.

  The influence on the overall corrosion based on the ground potential V of the steel material is determined based on Table 2 in which the ground potential V of the steel material and the influence degree of the corrosion are associated with each other and graded in stages. In addition, in the said measurement, in order to subtract the influence of concrete, the measured value in the depth (for example, depth of about 1 m or more from the ground) which is not covered with the concrete of the said steel material is used. In addition, among the plurality of measurement values measured at different depths, the one having the highest influence is used. In the case where measurement is performed at two locations on one steel material, the one having the highest influence is used among the measured values at these two locations.

  The influence on the overall corrosion based on the specific resistance ρ of the soil is determined based on Table 3 in which the specific resistance ρ of the soil and the influence degree of the corrosion are associated with each other and graded in stages. In addition, in order to subtract the influence of concrete, the measured value in the depth (for example, depth of about 1 m or more from the ground) which is not covered with the concrete of the said steel material is used. In addition, among the plurality of measurement values measured at different depths, the one having the highest influence level (that is, the one having a large measurement value) is used. In the case where measurement is performed at two locations on one steel material, the one having the highest influence is used among the measured values at these two locations.

  Then, based on each degree of influence determined above, the degree of the overall corrosion is determined. Specifically, Table 4 or Table 5 is used. Tables 4 and 5 show substantially the same contents although the notation method is different. Then, if the total of the above effects is 7 or more, it is determined as “significant overall corrosion”, if the total is 6, it is determined as “total corrosion”, and if the total is 5, it is determined as “surface corrosion”. If it is 4 or less, it is determined as “healthy”. More specifically, the evaluation is made based on whether or not the sum is 6 or more. When the sum is 6 or more, the degree of corrosion is evaluated as high, and when the sum is 5 or less, the degree of corrosion is evaluated. Assess low. In this case, a total condition of 6 or more is a predetermined condition set for the evaluation related to the general corrosion.

<Evaluation of pitting corrosion due to C / S macrocell corrosion>
Based on the potential difference of the ground potential V of the steel material in the part and the other part, evaluation on C / S macrocell corrosion of the steel material that can occur at the boundary part between the part and the other part is performed. In the evaluation of pitting corrosion (that is, C / S macrocell corrosion and aeration difference macrocell corrosion), the evaluation result is given by the possibility (or probability) of corrosion. Specifically, the ground potential V of the steel material having a depth of the steel material covered with concrete (for example, about 0.5 m) and a depth not covered with concrete (for example, about 1.5 m). A potential difference (or a gradient of ground potential V) is obtained. And it determines based on Table 6 which matched the potential difference and the possibility of generation | occurrence | production of a pitting corrosion, and gave the grade in steps. In this case, the rating is 3 or more (or the potential difference of the ground potential V of the steel material is −100 mV or less) is a predetermined condition set for the evaluation on the C / S macrocell corrosion. When measurement is performed at two locations on one steel material, a value having a larger absolute value is used among the potential differences of the ground potential V at each location.

<Evaluation of pitting corrosion due to air flow difference macrocell corrosion>
Evaluation of the air flow difference macrocell corrosion can be performed according to DIN 50929 (1985) of the German Standards Association. Table 7 shows DIN 50929 (1985) of the German Standards Association.

  Here, in the general soil of the mountainous region where the steel tower is usually installed, the items related to items 1, 5, 6, 7, 8, and 10 out of all items shown in Table 7 have a rating of around zero. Therefore, the items related to items 2, 3, 4, 9, and 11 indicated by the dark color among the items are examined. Specifically, as items corresponding to the above items 2, 3, 4, 9, and 11, (a) Influence of soil resistivity (see Table 8), (b) Influence of moisture content (see Table 9) (C) Influence of pH value (see Table 10), (d) Influence of groundwater (see Table 11), (e) Influence of homogeneity of backfill soil (see Table 12), (f) Gradient of resistivity (See Table 13).

  In addition, regarding the item 11, the matter equivalent to the item 11 is evaluated as a technique suitable for the steel material which comprises a steel tower. Specifically, the influence of the resistivity gradient is determined by the difference (Δa) between the adjacent depths of the rating regarding the resistivity ρ of the soil for each depth determined in item (a) based on Table 8 above. Desired. Moreover, (a) In the influence of the specific resistance of soil, when measuring at two places P1 and P2 (see FIG. 1) for one steel material, the influence degree is large for each depth (ie, The measured value of the smaller part is used.

  And from these each item (a)-(f) shown in Table 8-Table 13, the grade (possibility) of aeration difference macrocell corrosion is evaluated. Specifically, the scores for each of the items (a) to (f) are summed, and the total value is collated with Table 14 and determined. In this case, the total score is −5 or less is a predetermined condition set for the evaluation on the aeration difference macrocell corrosion.

<Comprehensive evaluation of steel corrosion>
Finally, the corrosion state of the steel material is diagnosed using the above-described evaluation results of each corrosion. Specifically, the evaluation related to each corrosion is performed based on whether or not the state of corrosion satisfies a predetermined condition set for each type of corrosion. Further, when at least one of the evaluation results satisfies the predetermined condition, the steel material is diagnosed as being deteriorated. More specifically, as shown in FIG. 3, the score in the evaluation of the overall corrosion is 6 or more, the score in the evaluation of pitting corrosion by C / S macrocell corrosion is 3 or more, and the difference in macroscopic cell corrosion. If any one of the scores in the evaluation of pitting corrosion by 3 is 3 or more, it is determined that some measures are necessary for the steel material. Table 15 shows the overall evaluation regarding the corrosion of this steel material. That is, the portion shown in dark color in Table 15 is a case that requires some measures.

  Even if it is determined that the corrosion requires some measures in the comprehensive evaluation on the corrosion of the steel materials, the content and timing of the measures vary depending on the degree of corrosion of the steel materials. Therefore, it is necessary to quantitatively grasp the degree of corrosion of the steel material. Next, a method for quantitatively evaluating the degree of corrosion of steel materials will be described.

  The quantitative evaluation of the degree of corrosion is performed by evaluating the residual strength of the steel material and evaluating the remaining life, and the residual strength and remaining life of these steel materials are determined by the corrosion cross section (that is, loss due to corrosion). Calculated on the basis of the loss cross-sectional area ΔD of the steel material in the cross-sectional direction. If it demonstrates roughly, the residual area D of steel materials can be grasped | ascertained from corrosion cross-sectional area (DELTA) D. Note that the corrosion cross-sectional area ΔD is obtained in consideration of each of those caused by overall corrosion and those caused by pitting corrosion. If the remaining area D is used, the remaining strength can be obtained by using, for example, the allowable stress degree Sa. On the other hand, since the strength of the steel material required to support the steel tower can be obtained separately, it is possible to evaluate how much the residual strength is by comparing with the strength.

  Specifically, the evaluation of the residual strength is performed by obtaining the magnitude of the residual strength with respect to the required strength required for the steel material (that is, residual strength / required strength) as the safety factor Rs. The residual strength is evaluated for each of tension and compression. If the safety factor Rs is 1 or more, it is immediately determined that some countermeasure is necessary, and if the safety factor Rs is less than 1, it is determined that some countermeasure will be required in the future.

  Further, if the residual strength and the strength of the steel material required to support the steel tower are known, it is possible to evaluate how much the remaining life is based on the rate of decrease in the strength of the steel material. That is, the remaining life is performed by calculating a period until the remaining strength reaches the required strength (a period until the margin for the required strength disappears). In addition, although the remaining life is calculated | required regarding each of tension | tensile_strength and compression, Preferably, it determines based on the one with the smaller safety factor Rs among tension | tensile_strength and compression. Moreover, although the remaining life is calculated | required per year, it is not limited to this. Below, each of evaluation of residual strength of steel materials and evaluation of remaining life is demonstrated in detail.

<Calculation of corrosion amount due to overall corrosion>
The corrosion cross-sectional area ΔDg of the steel material due to the general corrosion is obtained based on the loss thickness ΔT of the steel material lost due to the general corrosion. More specifically, if it is assumed that the steel material corrodes at the same rate over the entire surface in the overall corrosion, the cross-sectional shape after corrosion is considered to be that the entire surface is reduced by the same thickness from the cross-sectional shape before corrosion. If the cross-sectional shape and dimensions before corrosion are known, the cross-sectional shape and dimensions after corrosion can be grasped using the loss thickness ΔT of the steel material due to overall corrosion. Therefore, the corrosion cross-sectional area ΔDg of the steel material can be obtained as a difference between the cross-sectional area D ₀ before the corrosion and the cross-sectional area D after the corrosion (current state). That is, ΔDg = D ₀ −D is established. Further, the loss thickness ΔT of the steel material is obtained based on the corrosion rate Sg of the overall corrosion and the steel material burying period Yp. That is, ΔT = 2 × Yp × S is established. Note that ΔT is obtained as a loss thickness when the steel material corrodes from both sides.

Further, the steel material constituting the steel towers, since it is a member called a above as equilateral mountain-steel, as shown in FIG. 4, the corrosion before the steel edge length of the (outer) and L _0, a thickness T _{If 0} , the cross-sectional area D ₀ (see FIG. 4A) before corrosion can be expressed by the following equation.
D ₀ = T ₀ × (2L ₀ −T ₀ )
On the other hand, when the overall corrosion occurs, the entire surface is reduced evenly. Therefore, the remaining cross-sectional area D (see FIG. 4B) can be expressed by the following equation.
D = (T ₀ −2ΔT) × (2L ₀ −2ΔT−T ₀ )
Therefore, the corrosion cross section ΔDg can be expressed by the following equation.
ΔDg = 4ΔT × (L ₀ −ΔT)

  Note that the corrosion rate Sg of the overall corrosion is determined based on the specific resistance ρ of the soil. Specifically, the relationship shown in the following Table 16 is known as the relationship between the corrosion rate Sg due to the overall corrosion and the specific resistance ρ of the soil, and the corrosion rate Sg is determined using this. In each numerical range, the corrosion rate Sg and the specific resistance ρ of the soil are considered to have a linear proportional relationship, and the specific resistance ρ of the soil corresponding to the intermediate point of each numerical range is determined at both ends of each numerical range. Is obtained using a linear function of the line segment passing through.

<Calculation of corrosion due to pitting>
The amount of corrosion due to pitting corrosion is obtained based on the estimated value of the pitting depth H. Moreover, the estimated value of the pitting corrosion depth H shows the correlation among the pitting corrosion depth H that can occur in the steel material, the ground potential V, the specific resistance ρ of the soil, and the embedding period Yp of the steel material. It is obtained based on the estimation function. First, these correlation functions are generally based on the function used for the pitting depth generated in the buried pipe, and are expressed by the following equations.
H / √Yp = a + b × 1 / √ρ + c × V
A specific correlation function is obtained by performing multiple regression analysis using H / √Yp as a dependent variable, 1 / √ρ and V as independent variables, and determining coefficients a, b, and c.

By the way, the correlation function is representative of the data that is the basis of the correlation function, so that the data is distributed around the correlation function. Therefore, in about half of all data, the pitting depth H obtained from the correlation function is smaller than the actual pitting depth H, that is, the pitting depth H is estimated to be smaller than the actual depth. Could be. Therefore, the estimated function is more pitting corrosion than the correlation function obtained by using the measured values of a plurality of pitting depths H, the ground potential V of the steel material, and the measured values of the specific resistance ρ of the soil. Is corrected so that the estimated value of the pitting depth H exceeds the actually measured value. Preferably, the estimation function is obtained by increasing the value of a which is an intercept of the correlation function by α, and is given by the following equation.
H / √Yp = (a + α) + b × 1 / √ρ + c × V
Specifically, the value of α is set so that the pitting depth H exceeds the actually measured value in all cases on which the correlation function is based. Note that even after the estimation function has been performed once, the estimation function may be updated based on new new data.

  Next, the amount of steel material lost due to pitting (that is, the corrosion cross-sectional area ΔDp due to pitting) is obtained. Here, as a result of examining a large number of samples, the inventors have found that pitting corrosion is formed in an elongated shape. Furthermore, the cross-sectional shape of the pitting corrosion can be regarded as a triangular shape as shown in FIG. 5, and it has also been discovered that the pitting corrosion can be regarded as a trapezoidal shape when the pitting corrosion progresses to penetrate the steel material. Moreover, it has been discovered that the elongated pitting corrosion has a certain length relative to the pitting depth H. Specifically, it was found that the length of the pitting corrosion is about 7 times the pitting corrosion depth H.

  By the way, pitting corrosion is formed so as to extend in various directions depending on the situation where it occurs, and the loss cross-sectional area that affects the strength of the steel material is a projection of the pitting corrosion cross-sectional area in the lateral direction. It becomes. Therefore, when the pitting corrosion is formed so as to intersect in the horizontal direction, if the pitting corrosion cross section is directly adopted as the corrosion cross section ΔDp, the influence on the strength of the steel will be greatly estimated. This is preferable from the viewpoint of determining the property at a higher level. Therefore, in the following, for any pitting corrosion, it is assumed that the pitting corrosion is formed so as to extend along the transverse direction of the steel material, and the pitting corrosion cross section is used as the corrosion cross section ΔDp. However, it is not limited to this.

From the above, the corrosion cross-sectional area ΔDp due to the substantially triangular pitting corrosion that does not penetrate can be expressed by the following equation using the pitting depth H.
ΔDp = 7 × H ² ÷ 2
Further, the corrosion cross-sectional area ΔDp due to the penetrating trapezoidal pitting can be expressed by the following equation using the thickness T ₀ of the steel material.
ΔDp = 7 × T ₀ × (2 × H−T ₀ ) / 2

<Total corrosion amount>
From the above, the amount lost due to corrosion occurring in the steel material (total corrosion cross-sectional area ΔDt) can be determined by the sum of the amount of corrosion due to overall corrosion and the amount of corrosion due to pitting corrosion. That is, it can be expressed as ΔDt = ΔDg + ΔDp.

<Evaluation of residual strength>
Next, a method for obtaining the residual strength from the above-described total corrosion sectional area ΔDt will be described. The residual strength is represented by the allowable stress La of the current steel material. Here, the allowable stress is a load that the steel material can withstand at a certain point in time, and is a value set for the steel material. The allowable stress La is determined for each of tension and compression. The allowable stress La can be expressed by the product of the allowable stress degree Sa (allowable stress per unit area) and the remaining cross-sectional area D. That is, it can be expressed as La = Sa × D. The allowable stress Sa is shown in Table 17 below. In addition, since allowable stress degree Sa changes with time, the value at the time of construction of a steel tower is used.

  On the other hand, the required strength of the steel material required to support the steel tower is set according to the foundation load Lb. Here, the basic load Lb is the load resistance of the steel material required at least to support the steel tower, and is a value determined for each steel tower. The required strength may be set with a margin with respect to the basic load Lb, but here, the basic load Lb is used as the required strength. The basic load Lb is determined for each of tension and compression. As for the foundation load Lb, if there is a steel tower design document, the value described in it should be used. If there is no steel tower design document, etc., the allowable stress table issued by the Japan Steel Tower Association, etc. Is used to estimate the basic load Lb. Finally, the safety factor Rs of the steel material is obtained by the residual strength / required strength.

Note that when the basic load Lb is estimated, the compressive load is a buckling stress due to the buckling length of the lowermost main column material. For example, when L ₀ = 100, T ₀ = 10, and the buckling length is 120 cm, allowable compressive stress = compressive load = 245.6 (kN). The tensile load is an allowable tensile load excluding bolt holes. For example, in the case of L ₀ = 100, T ₀ = 10, and subtraction of 20 bolt holes M20, allowable tensile stress = tensile load = 236.3 (kN). However, in this case, the estimated load may be 1.2 times or more than the actual load. Therefore, take the opportunity to sketch and measure the basic dimensions, design, and re-evaluate.

<Calculation of remaining life>
The remaining life is obtained based on a cross-sectional area (required cross-sectional area) Dn necessary for ensuring strength. The required cross-sectional area Dn is obtained as a value obtained by multiplying a cross-sectional area necessary for securing the basic load Ls by a predetermined safety factor Rs. Here, when the safety factor Rs is 1, the required cross-sectional area Dn is a cross-sectional area necessary for securing the basic load Ls. 1.01). First, the cross-sectional area required to secure the basic load Ls is obtained by dividing the basic load Ls by the allowable stress Sa. The required cross-sectional area Dn is obtained by multiplying the cross-sectional area necessary for securing the basic load Ls by the safety factor Rs as a coefficient. That is, Dn = Rs × Ls ÷ Sa. Then, by subtracting the required cross-sectional area Dn from the cross-sectional area D ₀ before corrosion, an allowable corrosion cross-sectional area ΔDa that does not cause a problem even when corroded is obtained. That is, ΔDa = D ₀ −Dn. Next, the corrosion rate St up to the present is obtained from the total total corrosion sectional area ΔDt and the burying period Yp. Specifically, it is expressed as S = ΔDt ÷ Yp. Finally, the remaining life Yf is obtained based on the allowable corrosion cross-sectional area ΔDa and the corrosion rate St. Specifically, Yf = ΔDa ÷ S−Yp.

  The corrosion rate St calculated in the remaining life calculation is a proportional straight line obtained by dividing the current calculated corrosion amount by aging. However, since the thinning due to corrosion occurs after the disappearance of the plating, the actual corrosion rate is further increased. It can be fast. Therefore, measures are taken within a period of half or less of the remaining life for those having a remaining life of a predetermined value (for example, 20 years) or less. In addition, if the remaining life is longer than the predetermined value, it is processed that it is necessary to conduct a reinvestigation after an appropriate period (for example, 10 years). These can be summarized as shown in Table 18 below.

  As concrete measures, concrete foundations can be considered, but temporary equipment for transporting materials is necessary, and it will not lead to the extension of life other than foundations. The repair method will be decided after an economic comparison including the rebuilding of the tower. As concrete foundation forms, concrete foundations can be installed directly above the steel foundation floor, and concrete foundations can be installed in the middle. However, the soil foundation tower is small and has a small root opening and small site area. Determine the foundation depth and floor width according to the installation environment. In addition, when installing foundations in clay-rich soils or existing embankments, there is concern about the occurrence of uneven displacement due to consolidation settlement of the foundations.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

<Determination of estimation function of pitting corrosion depth H>
In operating the steel diagnosis method according to the example, first, an estimation function used for calculating the pitting depth H was determined. The data used in determining the estimation function is shown in Table 19 below.

Based on this, the correlation function was determined as follows.
H = (0.808996 + 26.367614 × 1 / √ρ + 0.001707 × V) × √Yp
In addition, when the estimated value of the pitting depth H was calculated | required using (rho), V, and Yp based on this correlation function, the result as shown in following Table 20 was obtained. As can be seen from this result, in the sample numbers 2, 7 to 9, and 11 to 14 of the steel material, the actually measured values are equal to or more than the estimated values. A graph showing the relationship between the estimated value of the pitting corrosion depth H and the actual measurement value is as shown in FIG.

For this reason, the correlation function was corrected so that the actual measurement value was less than or equal to the estimated value in all the sample steel materials. Specifically, since the difference between the estimated value and the actually measured value is the largest in the sample number 12 of the steel material, the intercept a of the correlation function is corrected by α = 0.086201 based on this. As a result, the estimation function was determined as follows.
H = (0.895197 + 26.367614 × 1 / √ρ + 0.001707 × V) × √Yp
According to this estimation function, as shown in Table 21 and FIG. 7, it can be seen that the measured values are less than or equal to the estimated values in all the sample steel materials.

<Operation example>
The steel materials shown in Table 22 below were subjected to deterioration diagnosis using a steel material diagnosis method. Table 23 shows the specific resistance ρ of the steel material soil, the ground potential V and the pH value of the steel material for each predetermined depth. Moreover, it measured in two places with respect to one steel material. The measurement results at the two locations are indicated by I and II in Table 23, respectively.

  First, the overall corrosion evaluation is as follows. Among the measured values of steel ground potential V, the smallest absolute value is -443 mV of the measured value at I at a depth of 2.0 m, so the rating of the effect on the overall corrosion based on the ground potential V of the steel material Is from Tables 2 to 3. Moreover, since the largest measured value of the specific resistivity ρ of the soil is 7.0 kΩ · cm of the measured value of II at a depth of 2.5 m, the influence on the overall corrosion based on the specific resistivity ρ of the soil. The scores are from Tables 3 to 4. Therefore, since the total of these scores is 7, the degree of overall corrosion was determined as “significant overall corrosion” from Table 4 or Table 5.

  Second, the evaluation of pitting corrosion due to C / S macrocell corrosion will be described. The difference in ground potential V of steel materials at depths of 0.5 m and 1.5 m is 493-456 = 37 mV, and C / S macrocell corrosion The score of pitting corrosion by is from Table 6 to 1. That is, it was determined that the possibility of pitting corrosion due to C / S macrocell corrosion was small.

  Third, the evaluation of pitting corrosion due to aeration-difference macrocell corrosion is as shown in Table 23. (a) The score of the influence of the specific resistance of the soil was 0 at each depth. (B) The score of the influence of moisture content was -1 at each depth. (C) The score of the influence of the pH value was 0 at each depth. (D) The groundwater impact score was 0 at each depth. (E) The score of the influence of the homogeneity of the backfill soil was 0 at each depth. (F) The rating of the effect of the resistivity gradient was -1 at a depth of 1.0 m and 0 at other depths. Therefore, the sum of the scores for each item (a) to (f) is 3 at a depth of 1.0 m and 2 at other depths. Was determined to be large at a depth of 1.0 m and small at other depths.

  Moreover, the pitting corrosion depth H was calculated | required for every depth from the specific resistance (rho) of the soil of steel materials, and the ground potential V of steel materials using the estimation function of the said pitting corrosion depth H. The results are also shown in Table 24.

<Confirmation of residual strength>
From the above examination, it was found that the steel material that was the object of measurement corresponds to a steel material that needs some countermeasures because it exceeds the standard in the evaluation of overall corrosion and the evaluation of pitting corrosion due to air flow difference macrocell corrosion. Therefore, next, the residual strength was confirmed.

Based on Table 15, the corrosion rate Sg of the overall corrosion was determined to be about 0.0098 mm / year. Based on this, the corrosion sectional area ΔDg due to the overall corrosion was determined to be about 255.5 mm ² . The maximum pitting corrosion depth H at each depth is 3.4 mm with a depth of -2.0 m (see Table 23). Was found to be about 39.4 mm ² . Accordingly, the total corrosion sectional area ΔDt was about 295 mm ² and the remaining sectional area D was about 795 mm ² . The residual strength was 96 kN for tensile and 96 kN for compression. Therefore, the safety factor Rs was 1.171 for tension and 1.006 for compression. These results are shown in Table 25. That is, since the safety factor Rs exceeded 1, it was determined that this steel material would require countermeasures in the future.

Next, the remaining life was calculated. In addition, when the safety factor Rs of tension and compression is compared, the safety factor Rs is smaller in the compression, so the remaining life is determined based on the compression. Here, from S = ΔDt ÷ Yp, the corrosion rate St up to the present time was about 3.78 mm / year. In calculating the remaining life, the required cross-sectional area Dn was obtained not only when the safety factor Rs was 1, but also when the error was taken into consideration and 1.01. First, when the safety factor Rs is 1, the required cross-sectional area Dn is 782 mm ² for compression from the basic load Ls and the allowable stress Sa, and the allowable corrosion cross-sectional area ΔDa is 308 mm ² for compression. The lifetime was 81 years, and the remaining lifetime was calculated as 3 years. When the safety factor Rs is 1.01, the required cross-sectional area Dn is compression 790 mm ² , the allowable corrosion cross-sectional area ΔDa is compression 300 mm ² , and the life of the steel is 79 years. Lifespan was calculated as 1 year. Therefore, the remaining life was determined to be 1 to 3 years. These results are shown in Table 26. Moreover, from Table 4, it was judged that measures were necessary for this steel material within 0.5 to 1.5 years.

  The effect of the steel material diagnosis method will be verified. 35 measurement surveys and trial drilling surveys on steel materials were conducted, and the accuracy of diagnosis by the above-described steel material diagnosis method was verified. As shown in Table 27, the state of the steel material was classified into healthy, pitting corrosion due to full-surface corrosion, C / S macrocell corrosion, or pitting corrosion due to aeration difference macrocell corrosion. It was possible to classify it into individual cases, and the number of occurrences in each case was as shown in the table. That is, it was found that among 23 cases where corrosion occurred, there were 19 cases where general corrosion occurred and 4 cases where only pitting corrosion occurred. From this result, it was confirmed that the steel material in which the corrosion occurred can be detected with higher accuracy and more comprehensively than in the case of measuring only the overall corrosion.

  As described above, according to the steel material diagnosis method according to the present embodiment, a steel material embedded in soil can be diagnosed without actually performing a trial digging.

  That is, the steel material diagnosis method according to the present embodiment diagnoses the corrosion state of the steel material using the evaluation result related to the overall corrosion, the evaluation result related to the C / S macrocell corrosion, and the evaluation result related to the air flow difference macrocell corrosion. Therefore, it is possible to more accurately diagnose the corrosion state of the steel material without overlooking the occurrence of corrosion by using the evaluation results regarding different types of corrosion in combination.

  The steel material diagnosis method according to the present embodiment is particularly useful as a steel material diagnosis method used as a support portion of a structure as described above. That is, the steel material used as the support portion of the structure is embedded along the vertical direction, and is different from the embedded pipe such as a gas pipe in the direction and depth of the embedded pipe. Therefore, it is difficult to apply a conventionally known method for diagnosing the corrosion state of a buried pipe as it is to a method for diagnosing a steel material used as a support portion of a structure. The above embodiment is a diagnosis that has not been provided in the past. A method is provided.

  The evaluation relating to each corrosion is performed based on whether or not the corrosion state satisfies a predetermined condition set for each type of corrosion, and at least one of the evaluation results satisfies the predetermined condition. If this is the case, the steel material is diagnosed as being in a deteriorated state. Although each of the above types of corrosion can occur almost at the same time, although the principles are different, if the above configuration is adopted, even if each corrosion occurs separately, any of them can be detected, and the steel It becomes possible to grasp the state of corrosion more accurately.

  The evaluation result regarding the overall corrosion is given depending on whether the degree of corrosion is high or low with respect to a predetermined standard, and the evaluation result regarding the C / S macrocell corrosion and the air flow difference macrocell corrosion indicates that there is a possibility of corrosion. It is given by whether it is high or low relative to a given standard. Therefore, since the evaluation result can be expressed in a binary manner, it is possible to simply evaluate the corrosion of the steel material.

  Further, the measurement of the ground potential V of the steel material is performed at every predetermined depth, and the evaluation relating to the overall corrosion is performed based on the maximum value of the ground potential V of the steel material measured at every predetermined depth. . Therefore, the accuracy of diagnosis can be improved by performing the measurement at every predetermined depth. Moreover, since the evaluation is performed using the maximum value of the measured values of the ground potential V (that is, the portion measured as having a higher degree of influence on corrosion), the safety of the steel material is increased to a higher level. Can be determined.

  In addition, the measurement of the specific resistance ρ of the soil is performed at every predetermined depth, and the evaluation on the overall corrosion is performed based on the minimum value of the specific resistance ρ of the soil measured at every predetermined depth. . Therefore, the accuracy of diagnosis can be improved by performing the measurement at every predetermined depth. Moreover, since the evaluation is performed using the minimum value (that is, the portion measured as having a higher degree of influence on corrosion) among the measured values of the specific resistance ρ, the safety of the steel material is raised to a higher level. Can be determined.

  Moreover, the evaluation regarding the air flow difference macrocell corrosion is performed based on the specific resistance ρ of the soil, the moisture content, the pH value, and the state of groundwater. Therefore, it is possible to evaluate the aeration difference macrocell corrosion without directly measuring the oxygen concentration in the soil, which is considered to be the cause of the aeration difference macrocell corrosion.

  Moreover, the steel material diagnosis method according to the present embodiment provides a correlation between the pitting corrosion depth H that can occur in the steel material, the ground potential V of the steel material, the specific resistance ρ of the soil, and the embedding period Yp of the steel material. Based on the estimated function indicating the relationship, an estimated value of the pitting corrosion depth H is obtained, and the estimated cross-sectional area ΔDp of the steel material at the pitting corrosion occurrence position is obtained using the estimated value of the pitting corrosion depth H. That is, by using the estimation function, the pitting corrosion depth H can be estimated without using actual measurement. Here, the corrosion cross section ΔDp (and thus ΔDt) has a direct relationship with the strength and life of the steel material, and is useful for various estimations regarding the steel material. By obtaining the corrosion cross-sectional area ΔDp, it is possible to easily and accurately diagnose deterioration of the steel material.

  In addition, the estimation function is more pitting corrosion with respect to a correlation function obtained by using a plurality of actually measured values of the pitting depth H, steel ground potential V and soil resistivity ρ. Is corrected so that the estimated value of the pitting depth H exceeds the actually measured value. Here, since the correlation function represents the data that is the basis of the correlation function, the data is distributed around the correlation function. Therefore, in about half of all data, the pitting corrosion depth H obtained from the correlation function is smaller than the actual pitting corrosion depth H, and when the correlation function is used as an estimation function as it is, The pitting depth H may be estimated to be smaller than the actual depth. Therefore, the safety of the steel material can be determined at a higher level by using the estimation function corrected so that the pitting depth H exceeds the actually measured value in more cases.

  Moreover, in the said diagnostic method of steel materials, the estimated value of the said pitting corrosion depth H is multiplied by a fixed value, the length of pitting corrosion is calculated | required, and the said pitting corrosion depth H and the length of pitting corrosion are used. The corrosion cross section ΔDp is obtained. As a result of intensive studies, the inventor has come to discover that the pitting corrosion length and the pitting corrosion depth H can be regarded as having a certain relationship. For example, the corrosion cross section ΔDp (and thus ΔDt) can be obtained by an extremely simple process.

  Further, the steel material is used as a support portion of a structure, and the strength of the steel material obtained from the corrosion cross section ΔDt is compared with the strength of the steel material required to support the structure. Thus, the strength of the steel material is diagnosed.

  Specifically, in the method for diagnosing a steel material, the remaining cross-sectional area D of the steel material obtained based on the corrosion cross-sectional area ΔDt and the strength of the steel material required to support the structure are ensured. The required cross-sectional area Dn of the steel material necessary for this is compared. Therefore, since the strength of the steel material is quantitatively represented by the index of area, the strength can be easily evaluated.

  In addition, the steel material is used as a support portion of a structure, and a period until the strength of the steel material obtained from the corrosion cross-sectional area ΔDt becomes the strength of the steel material required to support the structure. Is obtained as the remaining life Yf.

  Specifically, in the steel material diagnosis method, the remaining cross-sectional area D of the steel material is necessary to support the structure based on the corrosion rate St obtained from the corrosion cross-sectional area ΔDt and the burying period Yp. The period until the required cross-sectional area Dn of the steel material necessary to ensure the strength of the steel material is obtained as the remaining life Yf. Therefore, since the remaining life Yf of the steel material is quantitatively represented by the index of area, the remaining life Yf can be easily evaluated.

  The steel material diagnosis method according to the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

  For example, the various values such as the boundary value of the measurement value, the boundary value of the score, and the coefficient value used in the evaluation of each corrosion are not limited to those described above, and these various values may be changed as appropriate. Naturally, a method for diagnosing steel can be used.

  Moreover, in evaluation regarding aeration difference macrocell corrosion, you may evaluate by measuring the oxygen concentration in soil.

  Further, in the above embodiment, the estimation function is corrected so that the pitting depth exceeds the actual measurement value in all cases on which the correlation function is based. However, the present invention is not limited to this. Instead, any correction may be made so that the estimated value of the pitting depth exceeds the actual measurement value in the pitting corrosion more than the correlation function before correction. That is, in addition to correcting the difference between the measured value and the estimated value of the pitting depth to be zero, the pitting depth is determined based on the estimated value of the pitting depth. The estimated value may be corrected so as to be smaller than 0.

The top view showing arrangement | positioning of the steel tower and steel materials in which deterioration diagnosis is performed using the diagnostic method of the steel materials which concerns on embodiment of this invention is shown. The side view of the steel material of the steel tower in which deterioration diagnosis is performed using the diagnostic method of the steel material which concerns on the embodiment is shown. The flow of the diagnostic method of the steel materials concerning the embodiment is shown. The schematic sectional drawing of steel materials for demonstrating the method of calculating | requiring the corrosion cross-sectional area of steel materials in the diagnostic method of the steel materials which concerns on the embodiment is shown. The expanded schematic sectional drawing of the steel materials for demonstrating the method of calculating | requiring a pitting corrosion depth in the diagnostic method of the steel materials which concerns on the embodiment is shown. The Example which shows the relationship between the estimated value of pitting depth at the time of using a correlation function and the measured value in the Example of the embodiment is shown. The Example which shows the relationship between the estimated value of the pitting corrosion depth H at the time of using the estimated function obtained by correct | amending a correlation function in the Example of the embodiment, and an actual measurement value is shown.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Steel tower, 10 ... Steel material, 20 ... Concrete, 30 ... Measurement hole, P1, P2 ... Measurement location

Claims (7)

A method for diagnosing a steel material partly covered with concrete and the other part in contact with the soil in the soil,
Measure at least the ground potential of the steel and the specific resistance of the soil,
Based on an estimation function indicating the correlation between the pitting depth of the pitting corrosion that can occur in the steel material, the ground potential of the steel material, the specific resistance of the soil, and the embedding period of the steel material, an estimated value of the pitting depth is obtained. ,
A method for diagnosing a steel material, characterized in that the corrosion cross-sectional area of the steel material at a pitting corrosion occurrence location is obtained using the estimated value of the pitting corrosion depth.   The estimation function includes a plurality of pitting corrosion depths in relation to a correlation function obtained by using a plurality of measured values of pitting depths, measured values of steel ground potential and soil specific resistance. The steel material diagnosis method according to claim 1, wherein the estimated value is corrected so as to exceed the actually measured value. Multiply the estimated value of the pitting depth by a certain value to obtain the length of pitting corrosion,
The steel material diagnosis method according to claim 1, wherein the corrosion cross-sectional area is obtained using the pitting corrosion depth and the pitting corrosion length. The steel material is used as a support portion of a structure,
The strength of the steel material is diagnosed by comparing the strength of the steel material obtained from the corrosion cross-sectional area and the strength of the steel material required to support the structure. The diagnostic method of the steel materials as described in any one of these.   Comparing the residual cross-sectional area of the steel obtained based on the corrosion cross-sectional area with the required cross-sectional area of the steel necessary to ensure the strength of the steel necessary to support the structure. The method for diagnosing a steel material according to claim 4, wherein: The steel material is used as a support portion of a structure,
The period until the strength of the steel material obtained from the corrosion cross-sectional area becomes the strength of the steel material required to support the structure is obtained as the remaining life. The method for diagnosing a steel material according to Item.   Based on the corrosion cross-sectional area and the corrosion rate obtained from the burial period, the necessary cross-section of the steel material required to ensure the strength of the steel material required for the remaining cross-sectional area of the steel material to support the structure. The method for diagnosing a steel material according to claim 6, wherein a period until the area is reached is obtained as a remaining life.

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