JP2004077442A - Method for judging corrosion state, method for selecting corrosion monitoring process and corrosion monitoring system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for judging a corrosion state which can judge a corrosion state of a metal material in a state in which the corrosion does not progress and to provide a method for selecting a corrosion monitoring process utilizing the same and a corrosion monitoring system. <P>SOLUTION: A predetermined potential near a corrosion potential is applied under predetermined conditions. (a) When a polarization resistance is large, a current does not almost flow, but charging current instantaneously flows at the predetermined potential applying time and thereafter the current is rapidly converged to a passive state holding current level, it is judged as a passive state. (b) When the polarization resistance is small, a current flows only by an applied potential part, an influence of an attracting action of an electric charge to the surface of the metal does not almost appear but the current shows an increasing tendency toward a steady state, it is judged to be an overall surface corrosion state. (c) When the charging current flows at the predetermined potential applying time and infinitesimal current continuously flows even after the charging current is converged, it is judged to be a local corrosion state or an intermediate state in which the local corrosion or the overall surface corrosion state is hardly judged. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a method for determining a corrosion state for investigating a corrosion state (corrosion form) of a metal material constituting a facility, a method for selecting a corrosion monitoring method for selecting an appropriate corrosion monitoring method according to a corrosion state, and corrosion. The present invention relates to a corrosion monitoring system using an appropriate corrosion monitoring method according to a state.
[0002]
Problems to be solved by the prior art and the invention
For example, in equipment such as chemical plants that handle corrosive substances, the corrosion resistance of the metal materials that make up the equipment is directly related to the life of the equipment. It is necessary to monitor the corrosion state of the metallic material to be monitored and to grasp the situation.
[0003]
Incidentally, the corrosion state (corrosion form) of a metal is roughly classified into general corrosion and local corrosion (pitting corrosion), and there are effective means for monitoring general corrosion such as an AC impedance method and a polarization resistance method.
[0004]
On the other hand, for the local corrosion, a random pulse method that applies a random potential pulse to the metal material to be determined and analyzes the response current obtained when the random potential pulse is applied to monitor local corrosion is an effective monitoring method. Known as the law.
[0005]
However, in a situation where it is not possible to predict which corrosion form of the metal material that constitutes the equipment will be in the future, prepare corrosion monitoring devices that correspond to both general and local corrosion forms. This is not only uneconomical, but also if the metal material is undergoing microscopic general corrosion and the random pulse method for continuous monitoring is applied, voltage must be continuously applied to the equipment. Therefore, there is a problem that the equipment may be fatally damaged.
Therefore, in order to select an appropriate corrosion monitoring method according to the corrosion state, it is necessary to recognize the corrosion state of the test portion of the metal material to some extent before performing continuous monitoring.
[0006]
However, no appropriate method has been developed to determine the corrosion state (corrosion form) of the test portion of the metal material in advance in a state where no significant corrosion has occurred. In fact, it is not possible to confirm what kind of corrosion state it is in.
[0007]
The present invention has been made in view of such circumstances, and a corrosion state determination method capable of determining the corrosion state (corrosion form) of a metal material in a state where corrosion has not progressed, and an appropriate method according to the corrosion state. A method of selecting a corrosion monitoring method capable of selecting a suitable corrosion monitoring method, and performing highly reliable corrosion monitoring without damaging equipment using the corrosion monitoring method selected by the selection method. It is an object of the present invention to provide a corrosion monitoring system that is capable of performing the following.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, a method for determining a corrosion state according to the present invention (claim 1) includes:
A corrosion state determination method for determining a corrosion state of equipment,
A constant potential that does not destroy the material near the corrosion potential of the metal material to be determined is applied for a short time and for a time sufficient to confirm that the charging current has converged.
(A) Although the polarization resistance is large and almost no current flows, a charge current flows instantaneously when a constant potential is applied due to the adsorption action of electric charge on the metal surface (double layer capacity), and thereafter, the passive state is quickly passed. When converging to about the holding current, it is determined that the protective film is present on the metal surface or the metal material itself is in an inactive passive state,
(B) When the polarization resistance is small and the current flows by the applied potential, the effect of the charge adsorption on the metal surface (double layer capacity) hardly appears, and the current shows a tendency to increase toward the steady state. Is determined to be in a state of general corrosion where corrosion has occurred on the entire surface of the metal,
(C) The charge current flows instantaneously when a constant potential is applied due to the effect of adsorbing electric charge on the metal surface (double layer capacitance). It is characterized in that it is determined that there is a local corrosion state in which a part of the surface film is corroded or an intermediate state in which it is difficult to judge local corrosion and general corrosion.
[0009]
A constant potential that does not destroy the material near the corrosion potential of the metal material is applied for a short time and for a time sufficient to confirm that the charging current has converged. Polarization resistance is large and almost no current flows.However, due to the adsorption of charges to the metal surface, the charging current flows instantaneously when a constant potential is applied, and then quickly converges to the passive state holding current. Determines that the protective film is present on the metal surface or that the metal material itself is in an inactive passive state, and (b) the polarization resistance is small, the current flows by the applied potential, and In the case where the influence of the charge adsorption effect hardly appears and the current shows a tendency to increase toward the steady state, it is determined that the entire metal surface is corroded, and (c) metal Surface charge Due to the adsorption action, the charging current flows instantaneously when the constant potential is applied, but if a small current continues to flow even after the charging current has converged, corrosion has occurred in a part of the film on the metal surface ( The metal surface film is not a completely corrosion-resistant film.) Corrosion of the metal material in a state where corrosion has not progressed by judging that it is in a local corrosion state or an intermediate state where local corrosion and general corrosion are difficult to judge. The state can be determined efficiently and reliably.
Since the method for determining a corrosion state (the method for determining a corrosion state by the film charging characteristic evaluation method) of the present invention (claim 1) is completed in a short time (for example, about 30 seconds) including the data processing time, the equipment Damage to the (sample) can be minimized.
[0010]
Further, the corrosion state determination method according to claim 2 is characterized in that the constant potential is a corrosion potential of the metal material ± 1 to 100 mV, and the application time of the constant potential is 0.1 to 3 seconds. .
[0011]
When a potential of ± 1 to 100 mV of the corrosion potential of the metal material was applied as the constant potential, and the application time of the constant potential was 0.1 to 3 seconds, the charging current converged without destroying the material. It is possible to confirm that the corrosion state of the metal surface is any of the passive state, the general corrosion state, and the local corrosion state (including the intermediate state where it is difficult to judge local corrosion and general corrosion). It is possible to reliably determine whether the equipment is in the state without unnecessarily damaging the equipment, and the present invention can be made more effective.
[0012]
The method for selecting the corrosion monitoring method of the present invention (claim 3) is as follows.
(A) When the corrosion state is determined to be a passive state, a local corrosion state, or an intermediate state in which local corrosion and general corrosion are difficult to determine by the corrosion state determination method of claim 1 or 2, As a corrosion monitoring method, a random pulse method of monitoring a local corrosion by applying a random potential pulse to a metal material to be determined and analyzing a response current obtained when the random potential pulse is applied is applied. When the corrosion state is determined to be a general corrosion state by the corrosion state determination method of item 1 or 2, a polarization resistance method or an AC impedance method is applied as a corrosion monitoring method.
[0013]
If the corrosion state determined by the corrosion state determination method of claim 1 or 2 is determined to be a passive state, a local corrosion state, or an intermediate state in which local corrosion and general corrosion are difficult to determine, When the pulse method is applied and the corrosion state is determined to be the general corrosion state, by applying the polarization resistance method or the AC impedance method, without damaging the equipment by an appropriate corrosion monitoring method, Corrosion state can be monitored accurately.
[0014]
The corrosion monitoring system of the present invention (claim 4)
A system for monitoring the corrosion state of equipment by an appropriate corrosion monitoring method according to the corrosion state of equipment,
When the corrosion state is determined to be a passive state, a local corrosion state, or an intermediate state in which local corrosion and general corrosion are difficult to determine by the corrosion state determination method according to claim 1 or 2, Applying a random potential pulse to a metal material and analyzing the response current obtained when the random potential pulse is applied, monitoring the state of corrosion by applying the random pulse method of monitoring local corrosion,
When the corrosion state is determined to be a general corrosion state by the corrosion state determination method of claim 1 or 2, the corrosion rate is examined by applying a polarization resistance method or an AC impedance method.
[0015]
According to the corrosion monitoring system of the present invention (claim 4), the corrosion state is determined to be a passive state, a local corrosion state, or an intermediate state in which it is difficult to determine local corrosion and general corrosion by the corrosion state determination method of claim 1 or 2. If it is determined that the corrosion state is monitored by applying the random pulse method, and if the corrosion state is determined to be a general corrosion state, the polarization resistance method or AC impedance method Since the corrosion rate is determined by applying the method, in the case of a passive state, a local corrosion state, or an intermediate state where local corrosion and general corrosion are difficult to judge, the subsequent corrosion is determined by the random pulse method. In addition to being able to reliably detect the presence or absence of occurrence and expansion, if the system is totally corroded, equipment was installed by the polarization resistance method or the AC impedance method. Detects a corrosion rate without damage to the al, it is possible to infer accurately equipment life. Therefore, it is possible to reliably prevent the occurrence of an accident due to the corrosion of the equipment, to predict the time for updating the equipment, and to stably operate the equipment.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described, and features thereof will be described in detail.
[0017]
[Judgment of corrosion state]
First, an embodiment of a method for determining a corrosion state (a method for determining a corrosion state using a film charging characteristic evaluation method) of the present invention will be described below.
In general, the corrosion resistance of a metal material greatly changes depending on the state of a passive film formed on the surface. Therefore, it is an indispensable condition for a corrosion monitoring system to quickly determine the state of the passive film and indicate the direction of corrosion monitoring.
[0018]
In the present invention, in determining the corrosion state, a constant potential in the range of ± 1 to 100 mV from the corrosion potential is applied to the test portion of the metal material to be determined for about 0.1 to 3 seconds, The time-dependent current change is examined.
The potential range of ± 1 to 100 mV from the above corrosion potential is a condition selected as a potential range that does not destroy the measurement system, and the application time of 0.1 to 3 seconds indicates that the charging current is sufficient. This is a condition selected as a time range in which convergence can be confirmed.
[0019]
Normally, in an electrode system in a solution, an equivalent circuit in which a resistance component and a capacitor component are connected in parallel as shown in FIG. 1 can be considered. Solution resistance is the voltage drop due to the solution. Further, the polarization resistance and the double-layer capacitance become parameters on the electrode (metal) surface, and the magnitudes of the parameters differ depending on whether the metal surface is in a passive state, a general corrosion state, or a local corrosion state.
Hereinafter, the relationship between the corrosion state and the response current characteristics will be described with reference to the response current characteristic diagrams shown in FIGS. 2 (a), 2 (b) and 2 (c).
[0020]
(1) Passive state In the passive state, since a protective film (passive film) exists on the metal surface or the metal material itself is inactive, the polarization resistance is large and almost no current flows. As shown in FIG. 2 (a), a current (charging current) flows instantaneously due to a charge adsorption action (double layer capacity) on the metal surface, and then quickly converges to a passive state holding current.
In the passive state, the metal material has corrosion resistance, and the metal surface is stable. Then, from the viewpoint of monitoring the presence or absence of future corrosion in such a passive state, it is most desirable to apply the random pulse method as the corrosion monitoring method.
[0021]
(2) General Corrosion State In a general corrosion state in which the entire surface of the metal is corroded, the polarization resistance is small, and a current flows by the applied potential. In this case, as shown in FIG. 2B, the influence of the double-layer capacitance hardly appears, and the current tends to increase toward the steady state. In the case of this general corrosion state, if a constant potential is applied continuously, the current increases, and the corrosion proceeds accordingly, so it is not desirable to apply the random pulse method.For example, the polarization resistance method or the AC impedance method Examining the corrosion rate by applying it is a preferable corrosion monitoring method from the viewpoint of predicting the life of the equipment and the time of renewal.
[0022]
(3) Local Corrosion State In the case of a local corrosion state (including an intermediate state in which it is difficult to judge local corrosion and general corrosion), as shown in FIG. Shows the intermediate behavior of the case. That is, in the case of a localized corrosion state (including an intermediate state where local corrosion and general corrosion are difficult to judge), the current (charging current) ) Flows, but since a complete corrosion resistant film (passive film) is not formed, a minute current continues to flow even after the charging current has converged. However, this behavior is difficult to understand if local corrosion has not progressed to some extent in the case of local corrosion. Therefore, monitoring of local corrosion state (including intermediate states where local corrosion and general corrosion are difficult to judge) is random. The pulse method is most suitable.
[0023]
As described above, since the behavior of the response current characteristic is clearly different between the passive state and the general corrosion state, in the state where the corrosion is not progressing, the general corrosion state (corrosion form) of the metal material is smaller than the behavior of the response current characteristic. ) Is determined, and then an appropriate monitoring method according to each corrosion state (corrosion form) is applied, so that the corrosion state can be accurately monitored.
[0024]
Note that the method for determining a corrosion state according to the present invention is completed in about 30 seconds including the time for data processing, so that damage to equipment (sample) can be minimized.
[0025]
[Monitoring of corrosion state]
(1) In the case of passive state or local corrosion According to the above-mentioned method for determining a corrosion state, it is determined that the corrosion state is in a passive state or a local corrosion state (including an intermediate state in which it is difficult to determine local corrosion and general corrosion). If this is done, corrosion monitoring is performed using the random pulse method as the corrosion monitoring method.
[0026]
In the random pulse method, for example, local corrosion is monitored by analyzing a response current obtained when a random potential pulse is applied by a method described below.
That is, this random pulse method enables monitoring of local corrosion, which has been conventionally difficult to monitor, and differs from the conventional method in that a random potential pulse signal is used as the potential signal.
[0027]
When a metal is immersed in an inorganic solution (electrolytic solution), a potential (natural potential) is generated in the metal. When an electric potential is further applied to this electric potential using an external power supply, an electrode reaction corresponding to the electric potential occurs (corrosion reaction or the like). However, when a potential that is significantly different from the natural potential is continuously applied, a rapid oxidation reaction or reduction reaction occurs, and the natural state of the metal is destroyed, so that the original corrosion state cannot be monitored.
[0028]
Therefore, in the random pulse method, in order to minimize the corrosion promotion due to the electrode reaction, the scanning potential range is divided into, for example, levels 0 to 7 by 8 within a range of about ± 100 mV from the natural potential (for example, ± 7 mV). A fixed pulse train created by combining potential signals divided into eight levels is used as an input signal, and a response current obtained when a random potential pulse is applied is measured.
[0029]
In this random pulse method, E ₀ is the potential at the time of current measurement (current), E ₁ is the potential one time before, and E ₂ is the potential one time before E ₁ . In this case (see FIG. 3), in order to form a fixed pulse array that avoids the occurrence bias of the potential level, not only the current potential E ₀ but also the potentials (E ₁ , E ₂ ), the response current obtained when applying a random potential pulse is measured for 512 potential signals (8 × 8 × 8 = 512) divided into eight levels, and the response behavior Is processed by a potential function (E ₁ , E ₀ ) up to one point in the past.
[0030]
FIG. 4 is a diagram showing the storage location of the response current. In the hatched region R, the potential level (E ₀ ) at the time of current measurement is 3 and the potential (E ₁ ) before that is 2 The response current storage location is shown. However, since the further one previous potential _{E 2} of _{E 1} is _applied, the combination of E 1, _{E 0} is present eight. However, when data is stored, eight data are averaged. Thus, the response current is regularly rearranged to the potential level (0 to 7) at that time.
[0031]
Then, the arranged 8 × 8 response currents (FIG. 4) are processed by a Walsh transform, a mathematical transformation method for feature extraction. Thereby, even in the case of local corrosion (including an intermediate state where local corrosion and general corrosion are difficult to judge), the corrosion state can be accurately known. FIGS. 5A, 5B, and 5C show response current patterns after Walsh conversion. In FIG. 5, (a) shows a response current pattern in a passive state, (b) a local corrosion state, and (c) a response current pattern in a general corrosion state.
[0032]
For example, by performing corrosion monitoring by automatic continuous execution using this random pulse method, it is possible to reliably detect the presence or absence and expansion of subsequent corrosion on the metal surface.
[0033]
(2) In the case of general corrosion When the corrosion state is determined to be a general corrosion state by the above-mentioned corrosion state determination method, the corrosion rate is measured by applying a polarization resistance method or an AC impedance method as a corrosion monitoring method. I do.
[0034]
(A) Polarization resistance method The polarization resistance method is one of the methods generally used to investigate the corrosion rate, and determines the corrosion rate from a minute polarization range within ± 10 mV from the natural potential. It is a method of measuring the potential and current value within ± 10 mV from the natural potential, making a graph, finding the polarization resistance value from the slope, and finding the corrosion rate by substituting it into the existing formula, Because the corrosion rate is determined from the polarization range, the applied potential from the spontaneous potential is small, and the change in the state of the test piece (for example, dissolution of the sample) is small due to measurement. Are suitable.
[0035]
(B) AC impedance method The AC impedance method is one of the methods generally used for examining the corrosion rate, and a resistance component and a capacitor component are connected in parallel as shown in FIG. In an electrode system in a solution represented by an equivalent circuit, a metal surface is a field that causes an electrode reaction (conductivity) and a field that functions to accumulate electric charges (dielectric). This method is used to determine the corrosion rate. In other words, the double-layer capacitance is a capacitor, and when a voltage is applied, current flows only for a short period of time until surface adsorption, and when an AC voltage with a very high frequency is applied, the electric charge of the double-layer portion is reduced. When the frequency is low, the current is hardly flown to the double layer side, and when the frequency is changed, the resistance changes when the frequency is changed. graphed and components, changes in the bilayer components, and solution resistance R _H obtained from the graph (Nyquist plot), the polarization resistance R _P from the sum R _L of the polarization resistance and solution resistance determined, furthermore, the polarization resistance R _P This is a method in which the corrosion current is determined from the corrosion current.
[0036]
As described above, when it is determined that the corrosion state is the general corrosion state, by applying the polarization resistance method or the AC impedance method as the corrosion monitoring method, the equipment that occurs when the random pulse method is applied. It is possible to predict the life of the equipment and the time of renewal without causing damage to the equipment, thereby enabling stable operation of the equipment.
[0037]
[Corrosion monitoring system]
FIG. 6 shows a flow of the corrosion monitoring system of the present invention. As shown in FIG. 3, the corrosion state is determined by applying the corrosion state determination method of the present invention (corrosion state determination method by film charge characteristic evaluation method), and based on the result, an appropriate corrosion monitoring method is selected. As described above, it is possible to accurately monitor the corrosion state of the metal material without damaging the equipment as described above.
[0038]
In the present invention, there is no particular restriction on the type of the metal material to be determined, and the present invention can be applied to the case where various metal materials are used.
Also, the magnitude of the constant potential to be applied and the application time can be variously changed within the scope of the invention.
[0039]
Further, in the above embodiment, when the corrosion state is determined to be a passive state or a local corrosion state (including an intermediate state in which it is difficult to judge local corrosion and general corrosion), a random as a corrosion monitoring method is performed. When the pulse method is applied and the corrosion state is determined to be the general corrosion state, the polarization resistance method or the AC impedance method is applied as the corrosion monitoring method. It is also possible to use the method.
[0040]
The present invention is not limited to the above embodiment in other respects, and various applications and modifications can be made within the scope of the present invention.
[0041]
【The invention's effect】
In order to achieve the above object, a method for determining a corrosion state according to the present invention (claim 1) uses a constant electric potential near the corrosion potential of a metal material that does not destroy the material for a short time and that the charging current converges. By applying the voltage for as long as possible, and examining the change in current over time, (a) the polarization resistance is large and almost no current flows. If the charging current flows instantaneously and then quickly converges to the passive state holding current, it is determined that a protective film exists on the metal surface or that the metal material itself is in an inactive passive state. (B) When the polarization resistance is small, the current flows by the applied potential, the effect of the adsorption of charges on the metal surface hardly appears, and the current shows a tendency to increase toward the steady state. All over It is determined that the entire surface is in a state of corrosion where corrosion has occurred, and (c) the charging current flows instantaneously when a constant potential is applied due to the adsorption of charges to the metal surface. If the gas flows continuously, corrosion has occurred on a part of the coating on the metal surface (the coating on the metal surface is not a completely corrosion-resistant coating). Since the intermediate state is determined, the corrosion state of the metal material can be efficiently and reliably determined in a state where the corrosion is not progressing.
Since the method for determining a corrosion state according to the present invention (claim 1) is completed in a short time (for example, about 30 seconds) including the time for data processing, damage to equipment (sample) is minimized. Can be.
[0042]
Further, as in the method for determining a corrosion state according to claim 2, a potential of ± 1 to 100 mV of the corrosion potential of the metal material is applied as the constant potential, and the application time of the constant potential is set to 0.1 to 3 seconds. In this case, it is possible to confirm that the charging current has converged without destroying the material, and the corrosion state of the metal surface can be determined as a passive state, a general corrosion state, a local corrosion state (determination of local corrosion and general corrosion). (Including an intermediate state that is difficult to adhere to) can be reliably identified without unnecessarily damaging the equipment, making the present invention more effective. it can.
[0043]
The method for selecting a corrosion monitoring method according to the present invention (claim 3) is that the corrosion state determined by the corrosion state determination method according to claim 1 or 2 is a passive state, a local corrosion state, or a local corrosion and general corrosion. When it is determined that the state is an intermediate state that is difficult to judge, the random pulse method is applied, and when it is determined that the corrosion state is a general corrosion state, the polarization resistance method or the AC impedance method is applied. By doing so, the corrosion state can be accurately monitored without damaging the equipment by an appropriate corrosion monitoring method.
[0044]
Further, the corrosion monitoring system according to the present invention (claim 4) uses the corrosion state determination method according to claim 1 or 2 of the present invention to determine whether the corrosion state is a passive state, a local corrosion state, or an intermediate where it is difficult to determine local corrosion and general corrosion. If it is determined that the corrosion state is a normal state, the state of corrosion is monitored by applying the random pulse method, and if it is determined that the corrosion state is a general corrosion state, the polarization resistance method or the AC method is used. Since the corrosion rate is checked by applying the impedance method, in the case of a passive state, a local corrosion state, or an intermediate state where local corrosion and general corrosion are difficult to determine, a random pulse method is used. In addition to being able to reliably detect the presence or absence of expansion of the steel and its expansion, if the entire surface is corroded, the polarization resistance method or the AC impedance method Detects a corrosion rate without unnecessarily damaging the Bei, it is possible to infer accurately equipment life. Therefore, it is possible to reliably prevent the occurrence of an accident due to the corrosion of the equipment, to predict the time for updating the equipment, and to stably operate the equipment.
[Brief description of the drawings]
FIG. 1 is a diagram showing an equivalent circuit of an electrode system in a solution (an equivalent circuit in which a resistance component and a capacitor component are connected in parallel).
FIG. 2 is a diagram showing response current characteristics in a corrosion state determination method (corrosion state determination method using a film charge characteristic evaluation method) of the present invention, wherein (a) is a response current characteristic in a passive state, and (b) is a response current characteristic. FIG. 3C is a diagram illustrating response current characteristics in a state of local corrosion, and FIG.
FIG. 3 is a schematic diagram of potential application of a random pulse.
FIG. 4 is a diagram showing response current storage locations when the potential level (E ₀ ) at the time of current measurement is 3 and the previous potential (E ₁ ) is 2;
FIGS. 5A and 5B are diagrams showing response current patterns after performing Walsh conversion in a random pulse method, wherein FIG. 5A is a passive state, FIG. 5B is a local corrosion state, and FIG. It is a figure showing a pattern.
FIG. 6 is a diagram showing a flow of the corrosion monitoring system of the present invention.

Claims (4)

A corrosion state determination method for determining a corrosion state of equipment,
A constant potential that does not destroy the material near the corrosion potential of the metal material to be determined is applied for a short time and for a time sufficient to confirm that the charging current has converged.
(A) Although the polarization resistance is large and almost no current flows, a charge current flows instantaneously when a constant potential is applied due to the adsorption action of electric charge on the metal surface (double layer capacity), and thereafter, the passive state is quickly passed. When converging to about the holding current, it is determined that the protective film is present on the metal surface or the metal material itself is in an inactive passive state,
(B) When the polarization resistance is small and the current flows by the applied potential, the effect of the charge adsorption on the metal surface (double layer capacity) hardly appears, and the current shows a tendency to increase toward the steady state. Is determined to be in a state of general corrosion where corrosion has occurred on the entire surface of the metal,
(C) The charge current flows instantaneously when a constant potential is applied due to the effect of adsorbing electric charge on the metal surface (double layer capacitance). A method for determining a corrosion state, comprising determining that a part of a surface film has a local corrosion state in which corrosion has occurred or an intermediate state in which local corrosion and general corrosion are difficult to determine. The method according to claim 1, wherein the constant potential is a corrosion potential of the metal material ± 1 to 100 mV, and the application time of the constant potential is 0.1 to 3 seconds. (A) When the corrosion state is determined to be a passive state, a local corrosion state, or an intermediate state in which local corrosion and general corrosion are difficult to determine by the corrosion state determination method of claim 1 or 2, As a corrosion monitoring method, a random pulse method of monitoring a local corrosion by applying a random potential pulse to a metal material to be determined and analyzing a response current obtained when the random potential pulse is applied is applied. A corrosion monitoring method characterized by applying a polarization resistance method or an AC impedance method as a corrosion monitoring method when the corrosion state is determined to be a general corrosion state by the corrosion state determination method of Item 1 or 2. Selection method. A system for monitoring the corrosion state of equipment by an appropriate corrosion monitoring method according to the corrosion state of equipment,
When the corrosion state is determined to be a passive state, a local corrosion state, or an intermediate state in which local corrosion and general corrosion are difficult to determine by the corrosion state determination method according to claim 1 or 2, Applying a random potential pulse to a metal material and analyzing the response current obtained when the random potential pulse is applied, monitoring the state of corrosion by applying the random pulse method of monitoring local corrosion,
3. The corrosion monitoring method according to claim 1, wherein when the corrosion state is determined to be a general corrosion state, the corrosion rate is determined by applying a polarization resistance method or an AC impedance method. system.

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