JP2002294474A - Method and apparatus for analyzing alternating current corrosion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus for analyzing alternating current corrosion, which can acquire condition of corrosion caused by alternating current, based on a reasonable process and in a comparatively simple process. SOLUTION: The method comprises regarding a possibly corroded interface between a buried tube and the soil as a RC equivalent circuit consisting of resistance for a dummy reaction and an electric double layer capacitor, determining values of the alternating current flowing in the resistance for the dummy reaction, by utilizing nonlinear behavior in reaction resistance of metal composing buried tubes and alternating current values measured in the site, and assuming the corrosion speed from the determined values.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for analyzing corrosion caused by an alternating current that may be generated in a pipe under the influence of an external alternating current, for example, in a buried pipe buried in soil. About.

[0002]

2. Description of the Related Art AC power transmission in a high-voltage transmission line, an AC railway, or the like induces an electromagnetic induction voltage in a buried pipe buried underground. Here, when there is a defect in the coating of the buried pipe, a current flows between the buried pipe and the soil, so that the defect becomes a corrosion interface, and there is a concern that corrosion may occur at this location.

Heretofore, in the evaluation of such corrosion, an on-site method has been adopted. That is, a method has been adopted in which a test piece simulating a defect is placed near a buried pipe, and an electrical state (for example, an anticorrosion state) is grasped from a DC current density and an AC current density flowing through the test piece.

In this method, as a criterion for judging the electrical state, a test piece is buried for a long time while an AC current or a DC current is applied thereto, and after a certain period of time, the corrosion rate is calculated from the weight loss of the test piece. Then, the evaluation is performed based on an evaluation criterion created as a relationship between the value of the added AC current or DC current and the corrosion rate.

[0005]

However, it takes a long time for an experiment to determine the weight loss, and it is practically difficult to maintain a constant environment over such a long period of time. For this reason, the evaluation criterion may include a large error as a result, and the reliability is not good enough. In addition, it is possible that the surface condition at the corrosion interface may change due to changes in soil quality or soil resistance during the test period. Therefore, it is necessary to evaluate in various environments when adopting such a method. It takes a lot of time, and there is a limit to the state that can be used for the experiment.

Further, there is a problem that it is difficult to obtain reproducibility with the evaluation criteria created from these experimental results.

Therefore, for example, there is a demand for a method for immediately grasping the degree of corrosion by a simple method from the AC current value and the DC current value actually flowing through the test piece. However, such a technique is available. Not.

An object of the present invention is to provide a method and an apparatus for analyzing AC corrosion in which a corrosion state caused by an AC current can be obtained based on a rational method and by a relatively simple method.

[0009]

SUMMARY OF THE INVENTION According to the present invention, there is provided, in a predetermined galvanic environment,
The characteristic means of the method for analyzing alternating current corrosion in a structure made of a metal material includes, as described in claim 1, a non-linear behavior of a reaction resistance of the metal material in an environment equivalent to the electrically corrosive environment. And a real AC current measuring step of measuring an AC current flowing in the structure as a real AC current, and generating between the electrically corrosive environment and the structure. A certain corrosion interface is regarded as an RC equivalent circuit composed of a pseudo reaction resistance and an electric double layer capacitor capacitance, and the nonlinear behavior of the reaction resistance collected in the reaction resistance collection step is satisfied, and the entire RC equivalent circuit is In the electrical state where the total alternating current flowing is the actual alternating current measured in the actual alternating current measuring step, the alternating current flowing through the pseudo-reaction resistance Performing a corrosion-related current deriving step of obtaining a corrosion-related current that is a flow, based on the corrosion-related current derived by the corrosion-related current deriving step, estimating a corrosion rate by an alternating current in the structure, The operation and effect are as follows.

In this method, the reaction resistance of a metal material as a constituent material of a structure is obtained in advance in a reaction resistance collection step. Such a reaction resistance usually has a non-linear behavior as shown in FIG.

On the other hand, for a structure to be analyzed or evaluated, for example, an actual AC current flowing through the structure is obtained at a site through an actual AC current measurement step. In the present application, the actual alternating current is induced by, for example, an alternating current flowing through a transmission line or the like in the environment around the structure.

After obtaining these data, a corrosion-related current is derived in a corrosion-related current deriving step. This current is a current flowing on the side of the pseudo reaction resistance in the RC equivalent circuit, and the corrosion interface that may occur between the electrically corrosive environment and the structure is formed by the pseudo reaction resistance and the electric double layer capacitor capacitance. In this circuit, the electric state in which the non-linear behavior of the separately obtained reaction resistance is satisfied and the total current flowing through the RC equivalent circuit becomes the actual AC current is uniquely defined. Then, the AC current flowing through the pseudo-reaction resistance is estimated as a corrosion-related current by estimating the presence of a corrosion interface in this electrical state.

Here, since the corrosion-related current is an alternating current flowing through the reaction resistance, if this value is known, the corrosion rate of corrosion can be substantially estimated.

An apparatus for analyzing alternating current corrosion in a structure made of a metal material, which is provided in a predetermined electrically corrosive environment in performing such an analysis, is described in claim 5 according to the present invention. Storage means for storing a non-linear behavior of a reaction resistance of the metal material in an equivalent environment with respect to the electrically corrosive environment; and input means for receiving an alternating current flowing through the structure as an actual alternating current A corrosion interface that may occur between the electrically corrosive environment and the structure is regarded as an RC equivalent circuit including a pseudo reaction resistance and an electric double layer capacitor capacitance, and is stored in the storage unit. In the electrical state that satisfies the non-linear behavior of the reaction resistance and that the total AC current flowing through the entire RC equivalent circuit becomes the actual AC current input from the input means, And a corrosion-related current deriving means for deriving the corrosion related current is an alternating current through a pseudo reaction resistance, it is sufficient to constitute a possible output corrosion related current derived by the corrosion related current deriving means.

In this device, data relating to the non-linear behavior of the reaction resistance collected in the reaction resistance collection step described in the method section is accumulated and stored in the storage means.

On the other hand, with respect to the actual AC current, the above-described actual AC current measuring step is performed, for example, by a worker at the site, and the obtained actual AC current can be taken into the analyzer by input means. Keep it.

By adopting such a configuration, the conditions for the analysis of the corrosion-related current deriving step in the apparatus described above in the method section are made uniform.

Now, in the analysis apparatus, by the function of the corrosion-related current deriving means, the total AC current is substantially equal to the actual AC current, which satisfies the two conditions (which satisfy the nonlinear behavior of the reaction resistance) described in the method section. A corrosion-related current that satisfies (equals) is derived. And by outputting this, the worker can
By observing this state, the state of corrosion (for example, corrosion rate) can be estimated, and the state of corrosion of the structure can be appropriately determined.

In the analyzer having this configuration, the nonlinear behavior of the reaction resistance of the metal material forming the structure under the predetermined environment, which is collected in advance, and the AC generated in the structure to be analyzed are determined. By simply obtaining the current (actual AC current), it is possible to obtain the AC current flowing through the reaction resistor by using a predetermined nonlinear numerical analysis method. As a result,
With this AC current, it is possible to immediately analyze and evaluate corrosion on site or the like.

In such an analyzer, a corrosion rate for deriving an estimated value of the corrosion rate of the structure from the corrosion-related current derived by the corrosion-related current deriving means is defined in claim 6. Estimation value deriving means is provided,
It is preferable to have a structure capable of outputting the corrosion rate estimated value derived by the corrosion rate estimated value deriving means.

Since the alternating current flowing through the reaction resistor has a fixed relationship with the corrosion rate, the corrosion rate estimation value deriving means derives the corrosion rate estimation value from the corrosion-related current obtained so far. By doing so, it is possible to analyze and evaluate the estimated value criterion.

In the above-described analysis method according to the first aspect, as described in the second aspect, the RC equivalent circuit is provided by connecting the pseudo reaction resistor and the electric double layer capacitor in parallel. It is preferable to use a connection circuit.

This is because such a parallel circuit can best represent the state of the corrosion interface.

According to an analysis apparatus using this analysis method, the RC equivalent circuit is a parallel connection circuit of the pseudo reaction resistance and the capacitance of the electric double layer capacitor. By using the RC equivalent circuit, the corrosion interface can be approximated and appropriate analysis can be performed.

Further, in the analysis method according to the first or second aspect, as described in the third aspect, it is preferable to estimate a corrosion rate due to an alternating current in the structure from the corrosion-related current. Preferably, among the corrosion-related currents, the waveform of the anode region is integrated, and the corrosion rate is estimated from its effective value.

Since it is known that the corrosion rate due to an alternating current is present between a metal material and a buried environment in which the metal material is buried and is in an anode state, the AC current waveform on the anode side is changed. By integrating, the integrated value becomes a value that can represent the corrosion rate, and the corrosion rate or the degree of corrosion to be analyzed in the present application can be well analyzed and evaluated.

Further, in the analysis method described above, an AC current component and a DC current component are obtained for the actual AC current measured in the actual AC current measuring step. Along with, from the corrosion-related current, to estimate the corrosion rate due to AC current in the structure, after shifting the corrosion-related current in the cathode direction by the DC current component, integrate the waveform of the anode region, It is preferable to estimate the corrosion from the effective value.

For example, when considering a buried pipe buried in the soil, there is a case where the pipe is in a state of cathodic protection. In such a case, the current measured in the actual AC current measuring step is as follows.
The DC current component is superimposed on the AC current component. Here, when there is such a direct current component, this component normally works to suppress corrosion due to the alternating current component.

Therefore, an anode component of AC, which is a component that truly contributes to the corrosion rate, is cut out and obtained as an integrated value from an AC current waveform. This makes it possible to satisfactorily estimate the true corrosion rate and analyze and evaluate the corrosion.

Further, in the analysis apparatus, in the corrosion-related current deriving step, the electric state is derived by using the Newton method as a nonlinear numerical analysis method. preferable.

The behavior of the reaction resistance constituting the RC equivalent circuit in the present application is non-linear, and it is necessary to find a state where the pseudo AC current matches the actual AC current. The Newton method is the simplest and most practical.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. 1. Application target of the corrosion analysis method of the present application FIG. 1 shows a site situation where the corrosion analysis method according to the present application is applied. In the figure, reference numeral 1 denotes a buried pipe made of a metal material to be subjected to anticorrosion measures, and a counter electrode 3 is provided to the buried pipe 1 via a DC power supply 2 to achieve cathodic protection. .

Since the transmission line 4 and the AC railway 5 exist on the ground, an induced current caused by an AC current flowing through these facilities 4 and 5 flows through the buried pipe 1. There is a case where a defect 6 such as a coating film exists in the buried pipe 1, and since the metal material is in direct contact with the soil 7 at this portion, a current flows between the buried pipe 1 and the soil 7, and this induced current causes It becomes a corrosion interface where the occurrence of corrosion is concerned.

2. Analyzing Apparatus According to the Present Application The analyzing apparatus 8 is configured such that an interface which is subjected to corrosion by such an alternating current (specifically, an induced current) is connected to an electric double layer capacitor 9 as shown in FIG. Is assumed to be an RC parallel equivalent circuit 11 of the pseudo-reaction resistor 10 involved in the pseudo-resistance 10 based on the previously determined nonlinear behavior of the pseudo-reaction resistor 10 and the measurement result of the AC current actually flowing through the buried pipe 1 at the site. The AC current flowing through the reaction resistor 10 is obtained by numerical analysis of a nonlinear equation to obtain the corrosion rate at the corrosion interface.

In order to fulfill this function, the analyzer 8
As shown in FIG. 2, storage means 12, input means 13, corrosion-related current deriving means 14, and corrosion rate estimated value deriving means 15
And output means 16. The above numerical analysis is executed by the corrosion-related current deriving means 14.

The storage means 12 stores the non-linear behavior of the reaction resistance of a metal material constituting various buried pipes in various environments, and the capacitance of a capacitor which can represent a corrosion interface. Here, the reaction resistance is obtained by taking the current ip on the horizontal axis and the voltage value V (ip) when the current value enters and exits the metal material surface, as shown in FIG.
There is a non-linear relationship between the two. This measurement is separately performed as described later. Further, the capacitor capacity is obtained in advance in a simulation test having a typical corrosion interface. Therefore, the storage unit 12 plays a role of a database in the analysis device 8 of the present application.

The input means 13 is constituted by an input device (not shown) provided in an ordinary computer. In the case of the present invention, the input means 13 is taken in from the buried pipe 1 at the site as described later. An AC current (AC waveform) can be taken in as an actual AC current.

The corrosion-related current deriving means 14 calculates the nonlinear behavior of the reaction resistance from the storage means 12,
Derives the corrosion-related current ip referred to in the present application based on the actual alternating current from

This means 14 reduces the corrosion interface which may occur between the buried pipe 1 and the environmental soil 7 in which the buried pipe 1 is buried by converting the pseudo-reaction resistor 10 and the electric double layer capacitor 9 shown in FIG. And an analysis of the system is performed numerically.

In this analysis, the pseudo reaction resistance 10
The non-linear behavior of the electric double-layer capacitor 9 is based on the non-linear behavior of the reaction resistance from the storage means 12 described above, and the electric double-layer capacitor capacity 9 is also based on the capacity of the capacitor from the storage means. The combination of a plurality of nonlinear behaviors and capacitor capacities stored in the storage means 12 is adapted to the burial condition while referring to the material of the buried pipe 1 and the state of the surrounding soil 7. Is selected and specified.

In the analysis, under the condition that the total AC current ia flowing through the entire RC parallel equivalent circuit 11 is the input actual AC current I0, a pseudo reaction constituting the RC parallel equivalent circuit 11 is performed. Current i flowing through resistor 10
With respect to p, the current ic flowing in the capacitor 9, and the voltage V applied to the pseudo reaction resistor 10, the relationship between ip and V satisfies the nonlinear behavior of the input reaction resistor.
This is searched for as the electrical state actually occurring in this equivalent circuit.

More specifically, the electrical state is specified according to the flowchart shown in FIG. When the means 14 is started, the nonlinear behavior of the pseudo reaction resistance, the capacitance of the electric double layer capacitor, and the actual AC current described above are specified.

The analysis first proceeds at the instantaneous value level at each time t as follows. An initial value a (1) of the current candidate value a is given (step 1). Initial value a
The method of (1) is arbitrary, but since the time required for the nonlinear analysis to converge varies depending on the value, the calculation time should be shortened by predicting and setting a value as close as possible to the result. Is possible.

Hereinafter, steps 2, 3-1, 3-2, 4,
The rationale for the iterative flow through 5-1 and 5-2 is shown. This iterative flow is shown in step 4.
This is a flow for searching for an electrical state satisfying certain conditions among ia, ip, and ic.

The current candidate value a is regarded as the current ip flowing through the pseudo reaction resistor 10 (step 2), and the voltage V (a) applied across the pseudo reaction resistor in this state is referred to the non-linear nonlinear behavior. Is converted (step 3-
1). This voltage V (a) becomes the voltage applied to the electric double layer capacitor 9 as it is (step 3-2).

Now, in the state where the current candidate value flows,
On the capacitor side, ic = jωCV (a) (first complex formula) based on the capacitance C of the capacitor. Further, in FIG. 3, ia = ip + ic
ic = 0 (second complex expression) is obtained. Also, since ia is shunted by the reciprocal of impedance, ip = ia / jωC
V (a) +1 (third complex expression).

Therefore, when the above-mentioned second complex expression is substituted, the third complex expression is converted as shown in Expression 1, and the following fourth complex expression is obtained.

[0048]

## EQU1 ## jωCRia−jωCRic-ic = 0

Further, by substituting the first complex expression, the following fifth complex expression can be obtained as shown in Expression 2.

[0050]

## EQU2 ## Ria-jωCRV (a) -V (a) = 0

Here, since R (reaction resistance) is represented by the slope V 'of the polarization curve, the above equation is converted as follows.

[0052]

V '(a) ia-jωCV' (a) V (a) -V (a) = 0

This equation can be expressed by using a sine function: ia =
At time t satisfying Iasin (ωt),

[0054]

V '(a) Iasin (ωt) + CV' (a) V (a) sin
(Ωt + π / 2) −V (a) sin (ωt) = 0

This is equivalent to finding a that satisfies. That is,

[0056]

F (a) = V '(a) Iasin (ωt) + CV' (a) V
(a) sin (ωt + π / 2) -V (a) sin (ωt)

In other words, a that satisfies f (a) = 0 is obtained. f (a) is a nonlinear equation, and this nonlinear equation can be solved iteratively. In this case, the Newton method is used. The iteration is

[0058]

A (n + 1) = a (n) -f (a (n)) / f '(a (n))

Based on the above, the calculation is performed so that the error gradually decreases from the initial value a (1) of the current candidate value. Here, n is the number of iterations, and f is a function shown in Equation 5 for finding a nonlinear equation.

As described above, the end of the iterative processing is executed in the judgment step of step 4. If the current candidate value a (n) one step before satisfies the equation (4), this iterative processing is performed. The process ends. If not satisfied, a new current candidate value a (n + 1) is obtained according to the equation shown in Equation 6 (Steps 5-1 and 5-2), the calculation is performed again, and the result is set to Step 4 as a judgment step A (2),... Until the judgment error (the difference between the left and right sides of Equation 5) falls within a predetermined value.
.., A (n) is repeatedly calculated.

In the analysis so far, the values of the candidate current value a and the voltage V are all treated as instantaneous values specified by the time t.

In this way, with the solution converged, the instantaneous value is sequentially obtained using the candidate current value a at the time point t as the alternating current ip, and the calculation is repeated for a plurality of predetermined time steps to obtain one cycle. The AC current flowing through the pseudo reaction resistor is obtained as an AC waveform. In this way, a corrosion-related current, which is an alternating current flowing through the pseudo reaction resistor 10, is obtained as an alternating waveform.

The corrosion rate estimation value deriving means 15 calculates the pseudo reaction resistance 10 out of the AC current flowing through the pseudo reaction resistance 10 based on the corrosion-related current obtained as described above.
Then, the half cycle of the alternating current flowing through the anode region (positive region) is integrated, and the corrosion rate estimated value is calculated from its effective value (step 6).

More specifically, the current density is determined from the obtained effective value with reference to the size of the corrosion interface. When the current density becomes 0.1 mA / cm ² , the estimated corrosion rate is calculated as 1. An estimated corrosion rate is determined in proportion to the effective value so as to be 16 mm / year. The above is the outline of the configuration and operation of the analysis device 1 of the present application.

When analyzing the AC corrosion of the buried pipe 1 buried in a predetermined environment and made of a metal material, the apparatus is used as follows.

1 Reaction Resistance Collection Step Since the metal material used as the buried pipe 1 and the soil environment in which such a buried pipe 1 is buried are known in advance, they are electrically equivalent to such a buried environment. A pseudo test facility (not shown) is prepared, and the nonlinear behavior of the reaction resistance in each state is collected in the test facility. This test can be performed based on JIS G0579. Further, the capacitance of the capacitor at the corroded interface in this state is also collected using empirical values and the like.

In this way, a database provided in the storage means 12 of the analyzer 8 of the present invention can be constructed.

The non-linear behavior and the capacitance of the capacitor may greatly change depending on the environment. In the test, the conditions, such as the material of the test piece, soil, moisture content, soil resistivity, temperature, etc., are as close to the site as possible. It is important to simulate Therefore, these conditions become the parameters of the database. When the analysis device 8 is used, the corresponding physical quantity is measured on site, and when this parameter is specified, the nonlinear behavior of the reaction resistance and the capacitor capacity satisfying the on-site condition are determined. Can be identified and used in the device 8.

2. Actual AC Current Collection Step The AC current (actual AC current) flowing through the above-described corrosion interface is measured for the buried pipe 1 at the site. in this case,
As shown in FIG. 1, a test piece 17 called a pseudo-defect piece is arranged near the buried pipe 1, and the test piece 17 and the buried pipe 1 are arranged.
The actual AC current is measured by connecting the terminal 18 connected to the terminal 18 and plotting a current waveform flowing between the terminals and extracting a specific frequency component. Normal,
The frequency of interest is 50 or 60 Hz (commercial frequency). This actual AC current is input to the analysis device 8 of the present application via the input means 13.

When an anticorrosion current is present, a direct current component can be obtained from this waveform. In many cases, a method of flowing a direct current into the buried pipe 1 is adopted for the purpose of preventing the buried pipe 1 from being corroded, and a direct current component is often observed.

3 Corrosion-Related Current Deriving Step This step is a step executed by the corrosion-related current deriving means 14 provided in the analyzer described above. In the means, the corrosion interface is regarded as an RC parallel equivalent circuit 11 composed of a pseudo reaction resistor 10 and an electric double layer capacitor 9, and the non-linear behavior of the reaction resistance collected in the reaction resistance collection step described above is considered. The electrical state that satisfies the pseudo AC current flowing through the RC equivalent circuit is the actual AC current measured in the actual AC current measurement step,
In this state, a corrosion-related current which is an alternating current flowing through the pseudo reaction resistor 10 is obtained. This corrosion-related current is sent to the output means 16 and output.

4 Corrosion Rate Estimation Step This step is a step executed by the corrosion-related current deriving means 15 provided in the analyzer described above. In this step, a corrosion rate estimated value is derived based on the corrosion-related current by the structure of the corrosion-related current deriving means 15 described above, and is sent to the output means 16 and output.

As described above, the actual alternating current referred to in the present application is measured on-site by the analysis apparatus 8 of the present invention using the analysis method of the present invention, and the constituent material of the buried pipe 1 as a kind of structure is used. By specifying and analyzing the electrical physical state of the buried environment, the corrosion state of the buried pipe 1 on site can be analyzed and evaluated quickly and easily.

[Another Embodiment] (1) In the above embodiment, the analysis target was a buried pipe, but the buried part such as a steel structural material buried in the soil, a steel structure in water, etc. A system in which corrosion due to alternating current in such a case poses a problem can be analyzed. (2) In the above embodiment, in deriving the estimated corrosion rate, the waveform of the alternating current in the anode region is simply integrated to determine the effective value, and the corrosion rate is determined. When the target system is used, the effect of the anticorrosion current can be properly evaluated by taking the following method.

That is, as described above, the actual AC current is inputted from the input means 13, and the AC component and the DC current component are divided with respect to the actual AC current. Then, using the DC current component obtained by such processing, the corrosion-related current obtained separately is shifted by the DC current component in the cathode direction (shifting the zero point of the current value). Then, the waveform of the alternating current flowing through the anode region is integrated, and the corrosion rate is obtained from the effective value. Then, from this value, the corrosion state of the buried pipe can be determined.

In this method, regarding the actual AC current measured in the actual AC current measurement step, the AC current component and the DC current component are obtained in advance, and from the corrosion-related current,
To estimate the corrosion rate due to AC current in buried pipes,
By shifting the corrosion-related current in the cathode direction by the DC current component, and integrating the shifted corrosion-related current with the AC current waveform flowing through the anode region, and estimating the corrosion rate from the effective value, The anticorrosion can be removed.

[0077]

According to the present invention, it is possible to analyze the influence of corrosion due to the alternating current in a short time from the measured alternating current value. Further, the analysis can be performed using a theoretical system that assumes that the value of the reaction resistance changes nonlinearly.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a target to which an analysis method of the present application is applied.

FIG. 2 is a functional block diagram of the analysis device of the present application.

FIG. 3 is a diagram showing an equivalent circuit of a corrosion interface.

FIG. 4 is a diagram showing an example of a nonlinear behavior of a reaction resistance.

FIG. 5 is a flowchart showing a procedure for deriving a corrosion rate estimation value.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Buried pipe 2 DC power supply 3 Counter electrode 4 Transmission line 5 AC railway 6 Defect 7 Soil 8 Analysis device 9 Electric double layer capacitor capacity 10 Reaction resistance (pseudo reaction resistance) 11 RC parallel equivalent circuit 12 Storage means 13 Input means 14 Corrosion related current Derivation means 15 Corrosion rate estimated value derivation means 16 Output means 17 Test piece

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K060 AA03 BA02 CA06 DA10 EA12 FA07 FA10

Claims (8)

[Claims] 1. A method for analyzing alternating current corrosion in a structure made of a metal material, which is provided in a predetermined electrically corrosive environment, wherein said metal is in an environment equivalent to said electrically corrosive environment. A reaction resistance collecting step of collecting a non-linear behavior of a reaction resistance in a material, and an actual alternating current measuring step of measuring an alternating current flowing through the structure as a real alternating current, and the electric corrosion environment and the structure The corrosion interface that may occur between the reaction resistance is regarded as an RC equivalent circuit composed of a pseudo reaction resistance and an electric double layer capacitor capacitance, and satisfies the nonlinear behavior of the reaction resistance collected in the reaction resistance collection step, and , In an electrical state in which the total AC current flowing through the entire RC equivalent circuit is the actual AC current measured in the actual AC current measuring step, Performing a corrosion-related current deriving step of obtaining a corrosion-related current that is an alternating current flowing through the resistive resistor; estimating a corrosion rate of the structure by an alternating current based on the corrosion-related current derived in the corrosion-related current deriving step; Analysis method for AC corrosion. 2. The method for analyzing AC corrosion according to claim 1, wherein said RC equivalent circuit is a parallel circuit comprising said pseudo-reaction resistor and said electric double layer capacitor capacitance. 3. A method for estimating a corrosion rate due to an alternating current in the structure from the corrosion-related current, wherein the waveform of an anode region of the corrosion-related current is integrated, and the corrosion rate is estimated from an effective value. Item 7. The method for analyzing alternating current corrosion according to item 1 or 2. 4. An AC current component and a DC current component are obtained for the actual AC current measured in the actual AC current measuring step, and a corrosion rate of the structure by the AC current is estimated from the corrosion-related current. 3. The analysis of AC corrosion according to claim 1, wherein the corrosion-related current is shifted in the cathode direction by the DC current component, and the waveform of the anode region is integrated to estimate a corrosion rate from an effective value. Method. 5. An apparatus for analyzing an alternating current corrosion in a structure made of a metal material, which is disposed in a predetermined electrically corrosive environment, wherein said apparatus is in an environment equivalent to said electrically corrosive environment. Storage means for storing a non-linear behavior of a reaction resistance in a metal material; and input means for inputting an alternating current flowing through the structure as an actual alternating current, wherein the current is generated between the electrically corrosive environment and the structure. A corrosion interface which may be regarded as an RC equivalent circuit composed of a pseudo reaction resistance and an electric double layer capacitor capacity, satisfies the non-linear behavior of the reaction resistance stored in the storage means, and has a total flowing through the entire RC equivalent circuit. A corrosion-related current that derives a corrosion-related current that is an AC current flowing through the pseudo-reaction resistance in an electrical state in which the AC current is the actual AC current input from the input means. An AC corrosion analysis apparatus comprising a derivation unit, and capable of outputting a corrosion-related current derived by the corrosion-related current derivation unit. 6. A corrosion rate estimation value deriving means for deriving a corrosion rate estimation value of the structure from the corrosion-related current derived by the corrosion-related current deriving means, and derived by the corrosion rate estimation value deriving means. The AC corrosion analysis apparatus according to claim 5, wherein the estimated corrosion rate value can be output. 7. The AC corrosion analysis apparatus according to claim 5, wherein the RC equivalent circuit is a parallel circuit including the pseudo reaction resistance and the electric double layer capacitor capacitance. 8. The analysis apparatus for AC corrosion according to claim 5, wherein a Newton method is used as a nonlinear numerical analysis method to derive the electric state.