JP2008139138A - Electrochemical noise measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more accurately measure the polarization resistance R<SB>p</SB>related to electrode reaction in an electrochemical noise measuring method for measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system, wherein three electrodes arranged in parallel to each other are brought into contact with or immersed in a solution, to calculate noise resistance Rn. <P>SOLUTION: In the electrochemical noise measuring method for measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system, wherein three electrodes arranged in parallel to each other are brought into contact with or immersed in a solution 11, to calculate noise resistance Rn, the distances D from one center electrode 23 to two electrodes 21 and 22 of both ends adjacent to the electrode 23 are changed while made equal to measure the electrochemical potential noise ΔE and electrochemical current noise ΔI between the electrodes 21 and 22 at the respective distances D and the noise resistance Rn in the center line distance L between two electrodes 21 and 22 of both ends is calculated. The center line distance L and the noise resistance Rn are made approximate to calculate a formula (1) and the center line distance L is extrapolated in L=0 to calculate the polarization resistance R<SB>p</SB>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for measuring electrochemical noise, and more particularly to a method for measuring the polarization resistance of an electrode and the solution resistance between the electrodes.

  In recent years, an electrochemical noise measurement method has been used as a method for detecting and analyzing the occurrence of corrosion and the progress of corrosion. In this method, physical properties in the vicinity of an electrode are examined by measuring a weak potential difference and current generated when two electrodes made of the same material are exposed to the same water, solution, or other environment. As a specific measurement method, for example, as described in Patent Document 1, three electrodes are used, and a potential difference and a current are measured between one electrode and the other two electrodes, respectively, to corrode. A method for measuring the polarization resistance of the reaction is mentioned.

JP 2001-208713 A

  However, the resistance of the entire system measured as noise resistance by the three-electrode electrochemical noise measurement method is only the resistance due to the polarization resistance 3 of the oxidation-reduction reaction of the electrode 1 and the electric double layer capacitor 4 as shown in FIG. However, since the solution resistance 6 of the solution 5 between the electrodes is also included, if the solution resistance 6 is large, it becomes difficult to detect the polarization resistance 3, which is insufficient to accurately capture the reaction on the electrode surface. there were. Since it is impossible to directly measure only the solution resistance 6 by a noise measurement method without applying an external current to the system, the polarization resistance 3 is calculated by dividing the solution resistance 6 by the measured resistance of the entire system. It was also difficult.

Therefore, the present invention is an electrochemical method in which a noise resistance Rn is obtained by measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system in which three electrodes arranged in parallel are contacted or immersed in a solution. in noise measurement method aims to measure the polarization resistance R _p in accordance with the electrode reaction more accurately.

The present invention is an electrochemical noise which obtains a noise resistance Rn by measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system in which three electrodes arranged in parallel are contacted or immersed in a solution. In the measurement method,
The distance D from the center one electrode to the two electrodes at both ends adjacent to it is changed while being equalized,
The electrochemical potential noise ΔE between the electrode at one end and the one electrode at the center and the electrochemical current noise ΔI between the two electrodes at both ends are measured, and the respective distances are measured. A noise resistance Rn at a distance L between the two center lines at both ends corresponding to D is determined;
A value obtained by obtaining an equation (1) or (1 ′) in which the relationship between the distance L and the noise resistance Rn is modeled by a solution resistance S (L), and extrapolating the centerline distance L to L = 0. Request polarization resistance R _p is, by electrochemical noise measurement method, it was to solve the above problems.

Rn = S (L) + R _p (1)
(1 / Rn) = S (L) + (1 / R _p ) (1 ′)

  According to the present invention, the polarization resistance of an electrode exposed to water or a solution can be obtained, and information for knowing the state of the surface of the metal electrode can be obtained from this polarization resistance. In addition, the solution resistance of the electrochemical system can be obtained.

Hereinafter, the present invention will be described in detail.
The present invention is an electrochemical noise which obtains a noise resistance Rn by measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system in which three electrodes arranged in parallel are contacted or immersed in a solution. in the measurement method, an electrochemical noise measurement method for determining the polarization resistance R _p.

  First, an electrode system for performing measurement will be described. Said three electrodes 21-23 consist of the same metal material used as the object which measures the polarization resistance in the said solution 11, and the area and shape of the part which is contacting at least solution are the same. In order to equalize the state of each electrode, it is preferable that the size and shape of the electrodes are the same as shown in FIGS. 1 (a) and 1 (b). Electrochemical current noise ΔI and electrochemical potential noise ΔE generated between the electrodes are measured. FIG. 1B is a view of the electrode of FIG. 1A viewed from below.

  As a specific method for measuring the electrochemical current noise ΔI and the electrochemical potential noise ΔE, the electrochemical current noise ΔI between the two electrodes 21 and 22 at both ends of the three electrodes is determined. A current measuring means 24 for measuring is provided, and a voltage measuring means 25 for measuring the electrochemical potential noise ΔE between the electrode 22 at one end and the central electrode 23 is provided, and as shown in FIG. There is a method of measuring with these measuring means in such a wiring configuration.

  As the current measuring means 24 used here, for example, a resistance resistance ammeter which is an ammeter whose resistance value and impedance are close to zero can be cited. As the voltage measuring means 25, for example, a resistance value and an input impedance are very large. A voltmeter is mentioned. In addition, these measuring means preferably measure high sensitivity because they measure electrochemical noise, which is a weak change. Furthermore, it is more preferable to provide a band pass filter for cutting low frequency changes other than electrochemical noise.

With these measuring means, changes in potential and current are continuously recorded. This change in potential and current is analyzed, and a change corresponding to a frequency of about 0.01 to 10 Hz is extracted. Particularly preferably, a change corresponding to a frequency around 1 Hz is set to electrochemical potential noise ΔE and electrochemical current, respectively. Obtained as noise ΔI. From these values, the noise resistance Rn is calculated by the following equation (2). The noise resistance Rn has a polarization resistance R _p of the electrode surface vicinity of the system, a solution resistance S (L) and the combined corrosion resistance of the solution portion is diffused layer.

  Rn = ΔE / ΔI (2)

In the measurement of the noise resistance Rn as described above, the center lines of the electrodes at both ends are made equal to the distance D from the one electrode at the center shown in FIG. The distance L between the center lines, which is the distance between them, is changed a plurality of times. A plurality of pairs of the center line distance L and the noise resistance Rn, determine the polarization resistance R _p by performing an approximation of the following equation (1). This polarization resistance R _p is the sum of the polarization resistances of two electrodes, S (L) is a solution resistance that is a function of the distance L between the center lines, and in a general first type electrode system, The following equation (3), which is a quadratic function of L, can be approximated.

Rn = S (L) + R _p (1)
Rn = a × L ² + R _p (3)

Here, as an approximation method, for example, a plurality of pairs of measurement data sets are obtained by measuring a plurality of centerline distances L and noise resistances Rn, and this is substituted into equation (3). There is a method of preparing a plurality, calculating Rp assuming a, and calculating until the variation in Rp reaches a minimum value. As a specific example of how to obtain, first, n pairs of centerline distance L and noise resistance Rn pair (L _i , Rn _i ) are substituted into equation (3), and n groups of the following equations ( 4) is obtained.

[Equation 1]
Rn ₁ = a × L ₁ ² + R _p
Rn ₂ = a × L ₂ ² + R _p
・ ………… (4)
・
Rn _n = a × L _n ² + R _p

The _{R p} obtained in this respective equations and _{R pn,} determine the Rn 'by the following equation (5).

(R _p1 + R _p2 +... + R _pn ) / n = Rn ′ (5)

By substituting this Rn ′ into any of the above formula group (4), a constant a is obtained, and the obtained value is set as an assumed value of a. By repeating this, a and R _p can be obtained.

As another approximate expression, 'for each of the reciprocal of Rn and R _p on the basis of the approximate expression of the following formula is a linear function of L (3 formula (1)') in can also be approximated.

(1 / Rn) = S (L) + (1 / R _p ) (1 ′)
(1 / Rn) = a × L + (1 / R _p ) (3 ′)

[Equation 2]
(1 / Rn ₁ ) = a × L ₁ + (1 / R _p )
(1 / Rn ₂ ) = a × L ₂ + (1 / R _p )
・ ………… (4 ')
・
(1 / Rn _n ) = a × L _n + (1 / R _p )

In this case as well, R _p obtained in each equation is set to R _pn , Rn ′ is obtained by the above equation (5), and is substituted into any of the equation group (4 ′) to obtain the constant a. .

Here, the polarization resistance R _p is a resistance component that is irrelevant to the distance L between the center lines in the corrosion resistance, and is therefore a resistance component derived from an electrode reaction or the like in the vicinity of the electrode surface in the electrode system.

  An example of an apparatus for performing the method according to the present invention is shown in FIG. Plate electrodes 21 to 23 having the same material and the same size and shape as much as possible to the solution 11 that can cause an electrode reaction to be analyzed or to investigate the solution resistance, are the electrodes 21 and 23, Further, the distances D between the electrodes 22 and 23 are made equal to each other, and the electrodes are immersed so that they are parallel to each other. Further, the positions of the electrodes 21 to 23 can be changed while the distance D between the respective electrodes remains equal to each other.

  The electrode 21 and the electrode 22 are connected by a non-resistance ammeter which is a current measuring means 24 having a resistance value and an impedance close to zero, and the electrochemical current a generated during this period is measured. The electrode 21 and the electrode 23 are voltmeters which are voltage measuring means 25 having a very large resistance value and input impedance in order to measure the electrochemical potential difference b so that the reaction does not proceed excessively between the electrodes. In connection, the electrochemical potential difference b between the electrodes is measured. However, it is preferable that the non-resistance ammeter and the voltmeter have high sensitivity in order to measure electrochemical noise.

The electrochemical current a and the electrochemical potential difference b include not only electrochemical noise but also a coupling current I _mean corresponding to the progress of corrosion on each electrode surface and fluctuations due to other than reactions. Therefore, it is necessary to perform extraction in order to extract electrochemical noise. As an extracting method, for example, as shown in FIG. 2, before sending data to the computer 40, the electrochemical current noise ΔI and the electrochemical potential noise ΔE are passed through band-pass filters 26 and 27 as analysis means in advance. There is a way to extract. When the bandpass filters 26 and 27 are used, by extracting fluctuations in the low frequency region, particularly in the frequency region of 1 Hz or less, preferably in the frequency region of about 0.01 to 1 Hz, by using these bandpass filters, The electrochemical current noise ΔI can be obtained from the chemical current a, and the electrochemical potential noise ΔE can be obtained from the electrochemical potential difference b. If the band-pass filter is not passed, the coupling current I _mean can be obtained from the electrochemical current a, and the potential difference V _mean can be obtained from the electrochemical potential difference b. Further, it is desirable that these are converted by the converter 30 so as to be data that can be recognized by the computer 40.

FIG. 3 shows an example when the processing circuit until the voltage and current measurement data signals are input to the computer 40 is constituted by an analog circuit. In this case, first, the current data, that is, the electrochemical current a between the first electrode 21 and the second electrode 22 is measured by the current measuring means 24 as shown in FIG. The frequency component of about 1 Hz or less is extracted by the band pass filter 26, and then the RMS circuit for obtaining the mean square of the signal → the DC circuit for converting the obtained signal into direct current → the signal converted into direct current is converted into a logarithm. The computer 40 is converted logarithmically by a converter (hereinafter referred to as “logarithmic converter”) 31 composed of a LOG circuit, and further digitally converted by an analog / digital converter (hereinafter referred to as “A / D converter”) 33. Is input as electrochemical current noise ΔI. On the other hand, the data of the electrochemical current a can be passed through the logarithmic converter 31 and the A / D converter 33 without passing through the bandpass filter 26 and input to the computer 40 as the coupling current _Imean .

Next, the voltage data, that is, the electrochemical potential difference b between the first electrode 21 and the third electrode 23 is first measured by the voltage measuring means 25 as shown in FIG. . From this signal, a frequency component of 0.01 Hz or more and about 1 Hz or less is extracted by the band pass filter 27, and then logarithmically converted by the logarithmic converter 32 in the same manner as the current data, After digital conversion, it is input to the computer 40 as electrochemical potential noise ΔE. On the other hand, the data of the electrochemical potential difference b can be passed through the logarithmic converter 32 and the A / D converter 34 without passing through the band pass filter 27 and input to the computer 40 as the potential difference V _mean .

  On the other hand, FIGS. 4A and 4B are examples when the data processing circuit is configured by a digital circuit corresponding to the analog circuit configuration of FIGS. 3A and 3B. Are the same or corresponding parts, and the same operation can be obtained by the above digital circuit configuration. 2 shows the case where the analog circuit shown in FIG. 3 is used. In the case where the digital circuit shown in FIG. 4 is used, the logarithmic converters 31 and 32 are omitted. Will be.

When obtaining the data of the electrochemical current noise ΔI, in addition to the method of using the bandpass filter as the analysis means as described above, the data of the coupling current I _mean is directly used as the analysis means. It can also be obtained by calculating and obtaining the standard deviation. Similarly, when obtaining the data of the electrochemical potential noise ΔE, the data of the potential difference V _mean between the electrodes 21 and 23 can be directly calculated by the computer 40 and the standard deviation thereof can be obtained. . The electrochemical current a and the electrochemical potential difference b and the electrochemical current noise ΔI and the electrochemical potential noise ΔE analyzed therefrom are in a relationship as shown in FIG.

Also, the data of the electrochemical current noise ΔI and the electrochemical potential noise ΔE input to the computer 40 constituting the electrochemical sensor, or the data of the coupling current I _mean and the potential difference V _mean are Recorded in the recording means 50 in the order of series. The recording means 50 includes not only a storable medium for storing these data, but also a semiconductor memory or the like that is a temporary recording means for handling the data by the calculation means 60.

Each data recorded in the recording means 50 is analyzed by the calculation means 60. When the bandpass filters 26 and 27 are not used, the coupling current I _mean and the potential difference V _mean are analyzed by the calculation means 60 to calculate the electrochemical current noise ΔI and the electrochemical potential noise ΔE. Good. Both when calculated by the calculation means 60 and when extracted beforehand using the bandpass filters 26 and 27, the calculation means 60 allows each of the electrochemical current noise ΔI and the electrochemical potential noise ΔE to be small. From the changes ΔI and ΔE, the noise resistance Rn is obtained by a calculation step of calculating according to the above equation (2) according to Ohm's law. In this calculation, it is necessary to match the time series. The calculated noise resistance Rn is recorded in the recording unit 50 in time series.

  The noise resistance Rn calculated in this way is averaged with respect to the time axis, whereby the value of the noise resistance Rn corresponding to the value of the distance L between the center lines that has been measured can be obtained. A plurality of data pairs of the distance L between the center lines and the noise resistance Rn obtained in this way are obtained by changing the distance L between the center lines. The center line distance L can be automatically input by providing a sensor for automatically detecting the set center line distance L in addition to the method of inputting the distance L between the center lines from the input means 100 to the recording means 50 by hand. Good. Further, when the calculation of the noise resistance Rn is finished, the measurement method according to the present invention can be performed simply by automatically moving the electrode and changing the distance L between the center lines to perform the next measurement.

  Further, instead of performing the measurement by changing the distance L between the center lines in this way, five or more electrodes are immersed in the solution in advance as shown in FIG. 6, and among these, the electrode to be measured is changed. Thus, when the measurement is performed under the condition that the distance L between the center lines is different (L1, L2, and L3 in the figure, each of which uses the center electrode and the electrode that is equidistant from the center electrode), it is predetermined. The measurement at the center line distance L can be performed by quickly switching, and the measurement can be performed more efficiently.

When a plurality of pairs of centerline distances L and noise resistances Rn are measured and recorded in the recording means 50, the expression (1) is obtained from the above expression (3) or automatically according to the instruction of the measurer from the input means 100. performs approximation, the center line distance L between the noise resistance Rn is calculated the polarization resistance R _p is extrapolated value to L = 0. This value corresponds to double the polarization resistance derived from the reaction in the vicinity of the electrode surface of one electrode, excluding the solution in the solution portion that is the diffusion layer.

These calculated values are sent to the output means 90, and output and displayed on the screen of the display 91 such as a CRT or liquid crystal or on the printout of the printer 92. Further, measurement data such as noise resistance Rn, electrochemical current noise ΔI, electrochemical potential noise ΔE, coupling current I _mean , potential difference V _mean may be output and used for more detailed analysis.

  Hereinafter, the present invention will be described more specifically with reference to examples.

[Secondary function approximation]
(Example 1)
Solution 11 (ethanol (Wako Pure Chemical Industries, Ltd.) of electrodes 21 to 23 made of SPCC (according to JIS G 3141) having a width W of 2.0 mm, a height H of 30.0 mm, and a thickness T of 2 mm Ltd. special grade reagent): 98.38Wt%, distilled water: 1.61wt%, HCl (manufactured by Wako Pure Chemical Industries, Ltd.: first grade reagent): the polarization resistance _{R p} in 0.0064Wt%) in FIG. 1 It measured using the apparatus as described in 1 above. In addition, the terminal part of each electrode is insulated with an insulator so as not to come into contact with the solution. As the current measuring unit 24 and the voltage measuring unit 25, 1280B manufactured by Solartron Co. was used.

The center line distance L between the electrodes 21 and 22 at both ends was changed to 5 mm, 6 mm, 10 mm, and 14 mm, and the electrochemical potential noise ΔE and the electrochemical current noise ΔI were measured for 60 minutes each. The values of the noise resistance Rn calculated from the average values of ΔE and ΔI obtained by averaging are shown in Table 1, and plotted in the graph of FIG. This plot is determined as a quadratic function of L expressed by the above equation (3). By substituting these into the above equation (4) to obtain the above equation (5) and approximating the Rn centerline distance L extrapolated to L = 0, the polarization resistance R _p is 1162Ω. Calculated as cm ²

Originally, the unit of the polarization resistance R _p and the noise resistance Rn is “Ω”, but apparently “Ω · m ² ”. Since the rate of the corrosion reaction of the electrode is proportional to the anode current value per unit area, the measured current value is a current on an unmeasurable area, so the unit is “A / m ² ” and is calculated from it. Rn (= ΔE / ΔI) is apparently treated as “Ωm ² ”. However, since the value is small in this unit, it is represented by “Ω · cm ² ” which is a value obtained by multiplying “Ωm ² ” by 10,000. The same applies hereinafter.

(Example 2)
Instead of the solution of Example 1, ethanol: 98.38wt%, distilled water 1.61wt%, HCl: a 0.013% solution was used to similarly measured polarization resistance _{R p.} Similarly, when it was judged as a quadratic function of L expressed by the above formula (4) and approximated by the least square method, the polarization resistance R _p was calculated to be 958 Ω · cm ² .

(Example 3)
Instead of the solution of Example 1, ethanol: 98.33wt%, distilled water 1.61wt%, HCl: a 0.064Wt% solution was used to similarly measured polarization resistance _{R p.} Similarly, when it was judged as a quadratic function of L expressed by the above equation (4) and approximated by the least square method, the polarization resistance R _p was calculated to be 634 Ω · cm ² .

Example 4
Instead of the solution of Example 1, ethanol: 98.26wt%, distilled water 1.61wt%, HCl: a 0.13 wt% solution was used to similarly measured polarization resistance _{R p.} Similarly, when it was judged as a quadratic function of L expressed by the above formula (4) and approximated by the least square method, the polarization resistance R _p was calculated to be 459 Ω · cm ² .

[Reciprocal function approximation]
(Example 5)
The reciprocal number (1 / Rn) was calculated for the value of Rn for each centerline distance L obtained in Example 1. This value is shown in Table 2. The graph was plotted in the graph of FIG. 8 with the distance L between centerlines on the horizontal axis and (1 / Rn) on the vertical axis. This plot is determined by assuming that (1 / Rn) is a linear function of L expressed by the equation (3 ′), approximated by the least square method, and polarization resistance R _p as the reciprocal of the intercept extrapolated to L = 0. Was calculated to be 1567 Ω · cm ² .

(Example 6)
For the value of Rn for each centerline distance L obtained in Example 2, the reciprocal (1 / Rn) was calculated in the same manner as in Example 5, and the values shown in Table 2 were plotted in the graph of FIG. Similarly, was approximated by the least square method to determine a linear function of L represented by formula (3 '), the polarization resistance R _p is calculated as 906Ω · cm ^2.

(Example 7)
For the value of Rn for each centerline distance L obtained in Example 3, the reciprocal (1 / Rn) was calculated in the same manner as in Example 5, and the values shown in Table 2 were plotted in the graph of FIG. Similarly, when it was judged as a linear function of L expressed by the above formula (3 ′) and approximated by the least square method, the polarization resistance R _p was calculated to be 555 Ω · cm ² .

(Example 8)
For the value of Rn for each centerline distance L obtained in Example 4, the reciprocal (1 / Rn) was calculated in the same manner as in Example 5, and the values shown in Table 2 were plotted in the graph of FIG. Similarly, when it was determined to be a linear function of L expressed by the above equation (3 ′) and approximated by the least square method, the polarization resistance R _p was calculated to be 427 Ω · cm ² .

Schematic diagram of electrode arrangement for performing measurement method according to this invention The block diagram which shows the structural example of the electrochemical measuring apparatus which performs the measuring method concerning this invention The block diagram which shows an example when the circuit of the electrochemical measuring device of FIG. 1 is comprised with an analog circuit. The block diagram which shows an example when the circuit of the electrochemical measuring apparatus of FIG. 1 is comprised with a digital circuit. Conceptual diagram of electrochemical current noise and electrochemical potential noise Conceptual diagram of measurement by changing the distance L between the center lines with five or more electrodes The graph which shows the quadratic function correlation of the noise resistance and the distance between electrodes in an Example The graph which shows the inverse function correlation of the noise resistance and the distance between electrodes in an Example Conceptual diagram of solution resistance, polarization resistance, and electric double layer capacitor in a two-electrode system

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode 2 Electric double layer 3 Polarization resistance 4 Electric double layer capacitor 5 Solution 6 Solution resistance 11 Solution 21 1st electrode 22 2nd electrode 23 3rd electrode 24 Current measuring means (non-resistance ammeter)
25 Voltage measuring means (voltmeter)
26, 27 Band pass filter 30 Converter 31, 32 Logarithmic converter 33, 34 A / D converter 40 Computer 50 Recording means 60 Calculation means 90 Output means 91 Display 92 Printer 100 Input means

Claims (3)

In an electrochemical noise measurement method for determining noise resistance Rn by measuring electrochemical potential noise ΔE and electrochemical current noise ΔI in a three-electrode system in which three electrodes arranged in parallel are contacted or immersed in a solution,
The distance D from the center one electrode to the two electrodes at both ends adjacent to it is changed while being equalized,
The electrochemical potential noise ΔE between the electrode at one end and the one electrode at the center and the electrochemical current noise ΔI between the two electrodes at both ends are measured, and the respective distances are measured. A noise resistance Rn at a distance L between center lines of the two electrodes at both ends corresponding to D is obtained,
A value obtained by obtaining an equation (1) or (1 ′) in which the relationship between the distance L and the noise resistance Rn is modeled by a solution resistance S (L), and extrapolating the centerline distance L to L = 0. An electrochemical noise measurement method for obtaining a polarization resistance R _p which is
Rn = S (L) + R _p (1)
(1 / Rn) = S (L) + (1 / R _p ) (1 ′) From the above polarization resistance R _p and the noise resistance Rn, or from the constant of the above formula (1), obtaining the solution resistance S (L) at the distance L when measuring the above noise resistance Rn, according to claim 1 Electrochemical noise measurement method. A voltage measuring means for measuring the electrochemical potential noise ΔE is provided between the one electrode at the center and one of the two electrodes at both ends adjacent to the center electrode,
A current measuring means for measuring electrochemical current noise ΔI is provided between the two electrodes at both ends,
The calculation step of calculating the noise resistance Rn from the electrochemical potential noise ΔE and the electrochemical current noise ΔI measured by the current measuring unit and the voltage measuring unit according to the formula (2). Or the electrochemical noise measuring method of 2.
Rn = ΔE / ΔI (2)

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