JP7205126B2 - Corrosion sensors and corrosion evaluation systems - Google Patents

Description

The present invention relates to a corrosion sensor and the like used for grasping the state of corrosion of metal.

Corrosion sensors have been developed to detect corrosion currents (galvanic currents) flowing between noble metals and base metals in order to evaluate the corrosion resistance of members exposed to various weather environments. For example, an ACM sensor (Atmospheric Corrosion Monitor/Atmospheric Corrosion Monitor or Galvanic Corrosion Sensor) is obtained by laminating a conductive layer (Ag, etc.) on a substrate made of a sample metal (Fe, Zn, etc.) with an insulating layer ( _SiO2 , etc.) interposed therebetween. ) are well known.

When a noble metal and a base metal are short-circuited by a thin film of water or condensation due to moisture in the atmosphere, a corrosion current corresponding to the corrosion rate of the base metal flows. Conventional corrosion sensors measure and calculate the corrosion current density (icorr) or corrosion rate (Vcorr) of base metals by detecting the corrosion current.

Descriptions related to such corrosion sensors are found in Patent Documents 1 and 2 below. Further, Patent Documents 3 and 4 propose a method of calculating the corrosion amount or corrosion rate by the AC impedance method.

Japanese Patent Application Laid-Open No. 2009-53205 JP 2003-215024 A JP-A-2002-71616 JP-A-63-259456

The conventional corrosion sensors and methods for calculating corrosion rate, etc., as described in the above patent documents simply focus on the corrosion current (density), and the same corrosion reaction (cell reaction) occurs on the surface of the sample metal. It was assumed that it would occur uniformly.

However, the forms and reactions related to corrosion are not always uniform. Therefore, the occurrence and progress of localized corrosion (pitting corrosion, crevice corrosion, etc.), which is important for judging the life of metal members, cannot be properly understood only by averaging the corrosion that occurs on the metal surface.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and provides a corrosion sensor or the like that is useful for accurately grasping corrosion according to the actual situation, unlike conventional ones.

As a result of intensive research to solve this problem, the inventors of the present invention have found that the potential of corroding metal changes peculiarly in accordance with changes in corrosion morphology and reaction (both are simply referred to as "corrosion morphology"). We have newly found that Developing this result led to the completion of the present invention described below.

《Corrosion sensor》
(1) The present invention comprises a sample electrode made of a sample metal to be evaluated for corrosion, and a reference electrode disposed on the sample electrode via an insulating layer and serving as a reference for the potential of the sample electrode. Corrosion sensor.

According to the corrosion sensor of the present invention, it is possible to measure the potential of the sample electrode made of metal, which is the target of corrosion evaluation. By monitoring this potential (change), it is possible to determine the form (reaction) of corrosion, progress of corrosion, and the like.

The corrosion sensor of the present invention measures the potential of the sample electrode as follows. First, the sample electrode and the reference electrode are insulated by an insulating layer. Therefore, initially, when the corrosion sensor is almost completely dry, the sample electrode and the reference electrode are in a non-energized state, and there is no output based on the potential difference between the electrodes (that is, no potential is measured).

However, when the corrosion sensor is exposed to a corrosive environment (such as an atmospheric environment), the sample electrode and the reference electrode can become energized due to (water) vapor contained in the corrosive environment. The energized state is not limited to the case where dew condensation or the like straddles the insulating layer (instead of the electrolytic solution) and directly connects the sample electrode and the reference electrode. For example, vapor that has penetrated into a breathable or liquid-permeable insulating layer can also cause the sample and reference electrodes to become energized. In any event, the occurrence of such an energized state allows the potential of the sample electrode to be measured with respect to the reference electrode.

Furthermore, after the sample electrode starts to corrode (after corrosive substances (such as rust) are formed), even if the corrosion sensor is exposed to a dry environment without steam, corrosion of the sample electrode can progress. This is because oxides, hydroxides, and the like contained in the corroded material may react with the sample metal to generate new corroded material. At this time as well, the sample electrode and the reference electrode are in a conductive state, and the potential of the sample electrode can be measured.

In this way, the corrosion sensor of the present invention can measure a potential that reflects the corrosive environment, corrosion mode, progress of corrosion, and the like. Conversely, based on the measured potential, various information about corrosion can be obtained.

(2) The corrosion sensor of the present invention may further include an auxiliary electrode which is disposed on the sample electrode via an insulating layer and which is paired with the sample electrode and is energized. This makes it possible to measure and calculate the corrosion current (density), polarization resistance, etc., in addition to grasping the corrosion morphology.

<<Method to be measured>>
(1) The present invention can also be grasped as a corrosion evaluation system using the corrosion sensor described above. For example, the present invention includes the corrosion sensor described above, a potential measuring means for measuring the potential of the sample electrode with respect to the reference electrode, and a judgment method for determining the corrosion form of the sample metal by comparing the potential of the sample electrode with a threshold value. and means for evaluating corrosion.

By comparing the potential of the sample electrode obtained by the potential measuring means with the threshold value, the corrosion form (reaction) occurring in the sample metal can be determined. For example, if the potential exceeds a threshold, it can be determined that the corrosion morphology has changed. This makes it possible to change or correct the method of calculating the corrosion rate or the like. In addition, if accelerated progress of corrosion is expected due to changes in the form of corrosion, the determination results may be used for predicting/announcing the service life or replacement timing of products and parts. Note that the determination means may compare the threshold with not only the potential itself, but also the amount of change in the potential, the rate of change, the time required for a predetermined change, and the like.

(2) Further, in the case where the corrosion sensor comprises the auxiliary electrode described above, the present invention provides, for example, the corrosion sensor, a potential measuring means for measuring the potential of the sample electrode with respect to the reference electrode, and a threshold value for the potential of the sample electrode. a determination means for determining the corrosion form of the sample metal by comparing with the sample metal; a polarization resistance calculation means for calculating the polarization resistance of the sample electrode by energizing between the sample electrode and the auxiliary electrode; and considering the corrosion form and corrosion rate calculation means for calculating an index value of the corrosion rate based on the polarization resistance.

In this case, it is possible to calculate an appropriate corrosion rate (index value) according to the corrosion state (actual situation) of the sample metal. The polarization resistance is calculated by, for example, a DC polarization method, an AC impedance method, or the like. The corrosion rate calculation means may calculate the corrosion rate itself as the index value of the corrosion rate, or may calculate the reciprocal of the polarization resistance approximately proportional to the corrosion rate, or the corrosion current density. In this specification, the term "polarization resistance" is simply used to include "charge transfer resistance" in the sample electrode.

"others"
(1) The present invention can be understood not only as a corrosion evaluation system but also as a corrosion evaluation method by replacing "- means" with "- step" in this specification. The determining means (steps) and the calculating means (steps) are configured as, for example, programs executed by a computer.

(2) Unless otherwise specified, "x to y" as used herein includes the lower limit value x and the upper limit value y. A new range such as “a to b” can be established as a new lower or upper limit of any numerical value included in the various numerical values or numerical ranges described herein.

1 is a cross-sectional view schematically showing an outline of a corrosion evaluation system; FIG. 4 is a graph showing the relationship between the corrosive environment (temperature, humidity) and the potential/polarization resistance measured/calculated using the corrosion sensor. FIG. 4 is a schematic diagram showing polarization curves for each corrosion mode;

In addition to the components of the present invention described above, one or more components arbitrarily selected from this specification may be added. The contents described in this specification can be appropriately applied not only to the corrosion sensor of the present invention, but also to a corrosion evaluation system, a corrosion evaluation method, and the like using the sensor. Which embodiment is the best depends on the target, required performance, and the like.

《Corrosion sensor》
The corrosion sensor comprises at least a sample electrode, a reference electrode and an insulating layer, and preferably further comprises an auxiliary electrode and an insulating layer. Further, the corrosion sensor preferably includes means (for example, wiring such as terminals and leads) for electrically connecting each electrode to an external circuit (measuring device, etc.).

(1) Sample electrode The sample electrode is made of a sample metal that is an evaluation target for corrosion. The form of the sample electrode can be appropriately selected according to the object to be evaluated. For example, in addition to the shape of a flat plate, it may be in the shape of a block, a column, a cylinder, a film, or the like. Usually, a sample metal plate piece may be used as the sample electrode.

The sample metal is selected according to the object to be evaluated, and may be any metal such as iron-based, aluminum-based, magnesium-based, and titanium-based. In addition, "--series" means a pure metal or an alloy. A typical example is steel (carbon steel, stainless steel, etc.), which is a type of iron alloy.

(2) Reference electrode The reference electrode should preferably be made of a substance that has a stable electrode potential and is easy to handle as an electrode of a corrosion sensor. The reference electrode may be of high resistance (low conductivity), and may be made of not only metal but also an oxide semiconductor or a compound. As a representative example, the reference electrode may be an electrode made of silver-silver chloride (Ag/AgCl electrode). In addition, a noble metal such as platinum (Pt) may be used for the reference electrode.

(3) Auxiliary Electrode The auxiliary electrode is preferably made of a material that ensures stable electrical connection with the sample electrode even in a corrosive environment. For the auxiliary electrode, it is preferable to use a material having high corrosion resistance (insoluble or sparingly soluble, standard electrode potential nobler than that of the sample electrode) and excellent electrical conductivity. As a representative example, the auxiliary electrode may be a Pt electrode. In addition, noble metals (Au, Ag, etc.), oxide semiconductors, pnictide conductive materials, etc. may be used for the auxiliary electrodes. Note that the pnictide conductive material is, for example, Ti ₃ P, FeTiP, XTiP (X: metal element), or the like.

(4) Insulating layer Various insulating materials can be used for the insulating layer. For example, ceramics, resin, paper, or the like may be used for the insulating layer. The voltage acting or applied between the sample electrode and the auxiliary electrode is usually several volts at most. A withstand voltage of about 10 V, for example, is sufficient for the insulating layer.

Electricity (or ionic conduction) between the sample electrode and the auxiliary electrode and a decrease in insulation resistance are not limited to the case where the outer peripheral side of them is bridged with liquid (droplet etc.), It can also occur when steam is included. Therefore, depending on the usage environment (corrosive environment) of the corrosion sensor, the insulating layer is preferably made of an insulating material having air permeability (for example, water absorption) or liquid permeability (for example, moisture retention). For example, the insulating layer may be made of a porous body having pores communicating from one side to the other side. For example, various sponges and filters can be used for the porous body.

The insulating layer between the sample electrode and the reference electrode and the insulating layer between the sample electrode and the auxiliary electrode may be of different types or have different forms. is preferred.

(5) Wiring The corrosion sensor preferably has wiring (terminals, leads, etc./these are simply referred to as “wiring”) for electrically connecting each electrode to an external circuit (measuring device, etc.). Various wiring forms are possible. The number of terminals (the number of wires) may be the same as the number of electrodes, or may be larger than that. For example, when the auxiliary electrodes are provided, there may be four terminals in total (two terminals between the sample electrode and the reference electrode+two terminals between the sample electrode and the auxiliary electrode).

《Corrosion Evaluation System》
Potential measuring means for measuring the potential of the sample electrode with respect to the reference electrode is, for example, a voltmeter. The amount of current between the sample electrode and the auxiliary electrode is measured by, for example, a (non-resistive) ammeter. A potentiostat or galvanostat may be used for these measurements. Each measuring means may be composed of a general-purpose product or may be composed of a dedicated product (such as a circuit element developed for corrosion evaluation).

When calculating the polarization resistance of the sample electrode by the AC impedance method, the polarization resistance calculation means includes, for example, a potentiostat, a frequency response analyzer (FRA) or a fast Fourier transform analyzer (FFT), and an arithmetic device (computer). Consists of In this case, the polarization resistance (charge transfer resistance) is determined as, for example, the difference in impedance (difference between real components) between low frequencies (1 mHz to 100 mHz) and high frequencies (1 Hz to 10 kHz).

The determination means and corrosion rate calculation means are implemented by a computer and its execution program. The determination means determines the corrosion mode (corrosion reaction, corrosion stage, etc.) of the sample metal based on the measured potential of the sample electrode. The determination result may be output as it is, or may be reflected in the corrosion rate calculation means. For example, the corrosion rate calculation means may correct or change the coefficient in consideration of the corrosion mode when calculating the index value of the corrosion rate based on the already calculated polarization resistance.

Specifically, for example, polarization resistance: Rp, corrosion rate: Vcorr, proportional coefficient: K, correction coefficient: α, potential of sample electrode: E1 (> s: threshold value), E2 (< s), E1 Vcorr=K/Rp when E2, Vcorr=.alpha..K/Rp when E2. If corrosion is accelerated when the potential becomes base (E2<s), α>1 should be satisfied. Further, α>10 to 100 may be set depending on the degree of acceleration of corrosion.

When energizing between the sample electrode and the auxiliary electrode when calculating the polarization resistance, etc., the potential of the sample electrode may be the potential relative to the reference electrode provided in the corrosion sensor, or the potential of another reference electrode connected to a potentiostat or the like. may be used.

"others"
The corrosion sensor may be one in which a plurality of reference electrodes or auxiliary electrodes are arranged on one sample electrode via an insulating layer. The corrosion evaluation system may comprehensively determine the form of corrosion based on outputs (potentials) from a plurality of corrosion sensors.

《Corrosion Evaluation System》
FIG. 1 schematically shows a longitudinal sectional view of a corrosion evaluation system M that is an embodiment of the present invention. The corrosion evaluation system M includes a corrosion sensor S and a measurement analysis device D. The corrosion sensor S includes a sample electrode 1 , a reference electrode 2 , an auxiliary electrode 3 , an insulating layer 21 interposed between the sample electrode 1 and the reference electrode 2 , and an insulating layer 21 interposed between the sample electrode 1 and the auxiliary electrode 3 . and an insulating layer 31 to be applied. The sample electrode 1, the reference electrode 2, and the auxiliary electrode 3 are electrically connected to the measurement analysis device D by leads 12, 22, and 32 from respective terminals.

The measurement analysis device D measures the potential difference between the sample electrode 1 and the reference electrode 2 (potential measuring means), measures the impedance between the sample electrode 1 and the auxiliary electrode 3, and calculates the polarization resistance based on the impedance (polarization resistance calculation means). Specifically, the measurement analysis device D includes a potentiostat, an FRA that sends a sinusoidal signal (voltage, current) to the potentiostat and receives a response signal therefrom, and a computer that controls the FRA and calculates the polarization resistance. Prepare. By executing a program prepared in advance on the computer, it is possible to determine the corrosion mode based on the potential of the sample electrode 1 (determining means) and calculate the corrosion rate (index value) based on the polarization resistance (corrosion rate calculation means), etc. may be performed.

《Experiment example》
(1) Configuration A corrosion evaluation system M was used to evaluate corrosion occurring in a steel material (specimen metal) in an air atmosphere.

A cold-rolled steel plate (SPC/5 mm×30 mm×1 mm) was used for the sample electrode 1 of the corrosion sensor S. As the reference electrode 2, an Ag/AgCl electrode made of a thin plate-like piece of silver coated with silver chloride was used. A Pt electrode (2 mm×5 mm×0.3 mm) was used as the auxiliary electrode 3 . For the insulating layers 21 and 31, a hygroscopic or water-permeable porous double-sided tape (3M "4390"/thickness 0.05 mm) was used. The sizes of the insulating layer 21 and the insulating layer 31 are matched to the sizes of the reference electrode 2 and the auxiliary electrode 3, respectively.

(2) Corrosion Test The corrosion sensor S was subjected to a combined cyclic corrosion test in which it was alternately exposed to a dry environment and a wet environment simulating an atmospheric environment. The combined cycle corrosion test consists of (a) a drying process in which the corrosion sensor S is held for 6 hours in an air atmosphere of 60°C x about 45% RH, and (b) in an air atmosphere of 50°C x about 85% RH. A wet step in which the corrosion sensor S was held for 6 hours was alternately repeated (one cycle: 12 hours).

The potential of sample electrode 1 (VS. reference electrode 2) was constantly measured during the corrosion test. The polarization resistance of the sample electrode 1 was calculated by the alternating current impedance method by energizing between the sample electrode 1 and the auxiliary electrode 3 . Specifically, the impedance was measured at a low frequency (100 mHz) and a high frequency (1 Hz), and the difference between the real parts was taken as the polarization resistance. Impedance measurements and polarization resistance calculations were performed every 30 minutes to account for the low frequencies.

FIG. 2 summarizes the corrosive environment (temperature×humidity) to which the corrosion sensor S was exposed and the polarization resistance and potential of the sample electrode 1 under each environment.

(3) Evaluation Fig. 2 shows the following. First, since the sample electrode 1 is insulated by the insulating layer 21 at the beginning of the test, its potential is not measured and is almost zero.

However, immediately after the start of the test, the potential of the sample electrode 1 is measured. Then, the potential decreased (became noble) in a wet environment, and the potential increased (became noble) in a dry environment. The variation width of the potential tended to decrease from the initial period (from the start of the test to 20 hours later) to the middle period (125 to 180 hours after the start of the test).

Also, in the initial period, the potential recovered in about 1.5 hours, but in the middle period, it took about 2 to 3 hours to recover the potential. Furthermore, the fluctuation range of the polarization resistance was large in the early period, but the fluctuation became stable in the middle period.

In the later stage, the changes in the corrosive environment almost corresponded to the changes in potential and polarization resistance. However, in the middle of the late period, the potential showed a minimum value. Since then, the potential and polarization resistance waveforms have become different from the previous morphology.

(4) Discussion The reason why the potential of the sample electrode 1 fluctuates due to environmental changes and the passage of time will be discussed with reference to FIG. FIG. 3 is a polarization curve schematically showing a corrosion reaction (oxidation-reduction reaction) that is considered to occur between the sample electrode 1 and the auxiliary electrode 3. As shown in FIG. The horizontal axis of FIG. 3 is potential, and the vertical axis is logarithmic representation of current. The curve above the horizontal axis is the local anodic polarization curve showing the corrosion reaction occurring on the sample electrode 1 side. The curve below the horizontal axis is the local cathodic polarization curve that indicates the corrosion reaction occurring on the auxiliary electrode 3 side.

The polarization curve shown in the upper part of FIG. 3 shows the corrosion response in an initial wet environment. At this stage, a corrosion reaction (oxidation reaction) of mainly Fe→Fe ²⁺ +2e ⁻ occurs on the sample electrode 1 which is the anode. On the auxiliary electrode 3 which is the cathode, vapor (H ₂ O) and oxygen (O ₂ ) taken in from the environment are reduced to produce OH ⁻ .

In a dry environment, it is thought that corrosion reactions such as the polarization curve shown in the lower part of FIG. 3 occur. That is, H ₂ O supplied from the environment decreases, and iron oxyhydroxide (FeOOH) participates in the corrosion reaction. At this time, the potential of the sample electrode 1 is lowered. At such a stage, the average corrosion rate (∝1/polarization resistance) predicted from the polarization resistance cannot be grasped. In other words, it is considered that localized corrosion (pitting corrosion, etc.) begins to progress at an accelerated rate.

The mechanism by which corrosion progresses is complicated, and its details are not necessarily clear. However, the inventor has confirmed that local corrosion such as pitting corrosion begins to occur in the sample electrode 1 when the potential becomes base or exhibits a minimum value. Therefore, it is certain that by monitoring the potential of the sample electrode using the corrosion sensor of the present invention, it is possible to detect corrosion reactions or changes in corrosion morphology.

M Corrosion evaluation system D Measurement analysis device S Corrosion sensor 1 Sample electrode 2 Reference electrode 3 Auxiliary electrode 21, 31 Insulating layer

Claims (8)

a sample electrode made of a sample metal to be evaluated for corrosion in an atmospheric environment;
a reference electrode disposed on the sample electrode via an insulating layer and serving as a reference for the potential of the sample electrode;
an auxiliary electrode disposed on the sample electrode on the same side as the reference electrode with an insulating layer interposed therebetween and paired with the sample electrode to be energized ;
A corrosion sensor in which the insulating layer is made of an insulating material having air permeability from one side to the other side. The corrosion sensor of claim 1, wherein said reference electrode is an Ag/AgCl electrode. 3. The corrosion sensor according to claim 1, wherein said auxiliary electrode is a Pt electrode. The corrosion sensor according to any one of claims 1 to 3, wherein the insulating layer is sponge or filter. The corrosion sensor according to any one of claims 1 to 4, wherein the sample metal is iron or an iron alloy. 6. The corrosion sensor according to claim 5, wherein said sample electrode is a steel plate. a corrosion sensor according to any one of claims 1 to 6;
potential measuring means for measuring the potential of the sample electrode with respect to the reference electrode;
determining means for determining the corrosion form of the sample metal by comparing the potential of the sample electrode with a threshold value;
corrosion rating system. further, polarization resistance calculation means for calculating the polarization resistance of the sample electrode by energizing between the sample electrode and the auxiliary electrode;
Corrosion rate calculation means for calculating an index value of the corrosion rate based on the polarization resistance in consideration of the corrosion mode;
8. The corrosion assessment system of claim 7, comprising:

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