JP2011220717A - Method of measuring polarization resistance, method of monitoring corrosion speed, and polarization resistance measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of measuring polarization resistance capable of directly calculating a corrosion current, a method of monitoring a corrosion speed using the method of measuring polarization resistance, and a polarization resistance measuring device used for the method of measuring polarization resistance.SOLUTION: A sample electrode 12 is made of a metal with which to measure a corrosion current. A counter electrode 14 is made of a solid having favorable electrical conductivity and a different equilibrium potential in a natural corrosion state from that of the metal. After the sample electrode 12 and counter electrode 14 are short-circuited for an extremely minute time, they are immediately opened and polarization resistance is obtained from voltage current response during this period therebetween. Using the polarization resistance thus obtained, a corrosion current can be calculated directly. Since polarization can be measured without significantly disturbing a natural corrosion state of the sample electrode 12, this method can be preferably used as a corrosion speed monitoring method capable of monitoring on a regular basis.

Description

  The present invention relates to a polarization resistance measurement method capable of directly calculating a corrosion current, a corrosion rate monitoring method and a polarization resistance measurement device using the polarization resistance measurement method.

  As a method for obtaining a corrosion current when a metal is placed in a water-based environment, there is a three-electrode method using a metal as a sample electrode and using a sample electrode, a counter electrode, and a reference electrode. In the present invention, the water-based system refers to water or an aqueous solution containing water as a main component.

  There is a polarization resistance method as a method for obtaining a corrosion current from polarization measurement using a three-electrode method. The reference electrode is used to measure the corrosion potential in the natural state of the sample electrode. The counter electrode and the sample electrode are set to the same potential, and the counter electrode potential is adjusted to + or-from that state to polarize the potential and current. To determine the polarization resistance. Furthermore, the corrosion current is obtained from the polarization resistance using the correlation equation. When the polarization is from several mV to several tens of mV, the polarization can be measured with almost no change in the corrosion state of the surface, which is suitable for corrosion monitoring.

  Moreover, there exists an alternating current impedance method as a method of calculating | requiring a corrosion current from the polarization measurement using a 3 electrode method. Usually, using a potentiostat, etc., keeping the counter electrode at the same potential as the sample electrode, and applying an alternating current with a different frequency, the impedance between the sample electrode and the counter electrode changes, and the resistance component is taken out to determine the corrosion resistance. . Further, the corrosion current is obtained from the polarization resistance using a correlation equation (see, for example, Patent Document 2).

  Many proposals have been made regarding methods and apparatuses for monitoring corrosion of metal materials. For example, Patent Document 1 discloses an apparatus for measuring polarization resistance using a three-electrode method for monitoring the corrosion status of boiler steel. Patent Document 2 discloses a corrosion monitoring sensor and measurement method used for estimating a corrosion rate in a molten salt adhesion environment such as a boiler and a gas turbine. Patent Document 3 discloses an electrochemical noise method using three electrodes in an aqueous process management system. Patent Document 4 discloses a method for monitoring corrosion by measuring a natural corrosion potential among methods for preventing corrosion of metals in an aqueous system. Further, Patent Document 5 discloses a metal monitoring method and apparatus for causing local corrosion by supplying an anode current to a sample electrode using a potentiostat and three electrodes.

Japanese Patent Laid-Open No. 10-318962 JP 2003-14682 A Japanese Patent Laid-Open No. 2005-3635 JP 11-118703 A JP 2004-101349 A

  In the polarization resistance method or AC impedance method, which is a method for obtaining the corrosion current from the polarization measurement using the three-electrode method, the corrosion rate and the corrosion current cannot be obtained directly from the obtained polarization resistance, applied voltage, and current value. Therefore, it is necessary to obtain the corrosion rate using a coefficient correlated with the corrosion rate converted from the obtained polarization resistance and the amount of thinning of the material. It is preferable if the direct corrosion rate and corrosion current can be obtained from the obtained polarization resistance, applied voltage, and current value, but the method for obtaining the direct corrosion rate and corrosion current from the polarization resistance, applied voltage, and current value so far is Not found. In addition, the corrosion rate and corrosion current can be easily acquired, and if it can be performed without using a potentiostat or an external power source, the device configuration becomes simple and the corrosion rate monitoring device preferable.

  In addition, the conventional corrosion monitoring method using the three-electrode method uses a general-purpose reference electrode that uses a solution such as a silver / silver chloride electrode as a reference electrode, which may cause environmental pollution due to leakage of the sealing liquid. It is not suitable as a corrosion monitoring method in pure water environments such as boiler environments.

  An object of the present invention is to provide a polarization resistance measurement method capable of directly calculating the corrosion current, a corrosion rate monitoring method using the polarization resistance measurement method, and a polarization resistance measurement device used in the polarization resistance measurement method. .

  The present invention according to claim 1 is a polarization resistance measuring method capable of directly calculating a corrosion current, wherein a metal for which the corrosion current is obtained is used as a sample electrode, and an equilibrium potential in a natural corrosion state is different from that of the metal. A polarization resistor characterized in that a solid having good electrical conductivity is used as a counter electrode, the sample electrode and the counter electrode are short-circuited for a very short time, then immediately opened, and the polarization resistance is obtained from the potential-current response during this period. This is a measurement method.

  According to a second aspect of the present invention, in the polarization resistance measuring method according to the first aspect, a solid made of the same material as the counter electrode is used as a reference electrode, and a very short time is short-circuited between the sample electrode and the counter electrode. At the same time, a potential difference between the sample electrode and the reference electrode is measured at the same time.

  According to a third aspect of the present invention, in the polarization resistance measuring method according to the first or second aspect, instead of short-circuiting the sample electrode and the counter electrode for a very short time, the sample electrode and the counter electrode An extremely minute time voltage is applied between the sample electrode and the counter electrode by using an external power source so that a predetermined current flows through the electrode.

  According to a fourth aspect of the present invention, in the polarization resistance measuring method according to the first or second aspect, the sample electrode and the counter electrode are short-circuited for a very short time and then immediately opened, and the sample is immediately opened. It is characterized in that a very short time is released from a state in which the pole and the counter electrode are short-circuited, and then short-circuited immediately thereafter.

  According to a fifth aspect of the present invention, in the polarization resistance measurement method according to the third aspect, an external power source is used so that a predetermined current flows between the sample electrode and the counter electrode. Instead of immediately opening the gap between the sample electrode and the counter electrode after applying a very short time voltage between them, apply the voltage using an external power supply so that a predetermined current flows between the sample electrode and the counter electrode From this state, the sample electrode and the counter electrode are opened for a very short time, and then a voltage is applied using an external power source so that a predetermined current flows between the electrodes immediately thereafter.

  The present invention described in claim 6 is characterized in that the sample electrode and the counter electrode, or the sample electrode, the counter electrode, and the reference electrode are placed in an aqueous environment in which a corrosion current is desired to be obtained, and the polarization resistance according to any one of claims 1 to 5 It is a corrosion rate monitoring method characterized in that the polarization resistance of a metal material is measured using a measurement method and the corrosion rate is monitored.

  According to a seventh aspect of the present invention, in the corrosion rate monitoring method according to the sixth aspect, the reference electrode is a reference electrode made of a solid material that does not use a solution or a salt solution. It is characterized by excellent property stability and shape stability.

  The present invention according to claim 8 is the corrosion rate monitoring method according to claim 6 or 7, wherein the counter electrode, or the counter electrode and the reference electrode are made of magnetite, and the aqueous environment is boiler feed water of 100 ° C or higher. It is characterized by being.

  The present invention described in claim 9 includes a sample electrode made of a metal for which a corrosion current is obtained, a counter electrode made of a solid having good electrical conductivity and a different equilibrium potential in a natural corrosion state from the sample electrode, and the sample electrode. And the counter electrode are short-circuited for a very short time and then opened, a voltmeter for detecting a potential difference between the sample electrode and the counter electrode, and a current flowing between the sample electrode and the counter electrode are detected. A polarization resistance measuring device comprising: an ammeter for measuring the potential difference; and a data processing device capable of capturing a potential difference and a current value online.

  A tenth aspect of the present invention is the polarization resistance measuring apparatus according to the ninth aspect, further comprising a reference electrode made of the same solid material as the counter electrode, wherein the switch further includes the sample electrode, the reference electrode, and the reference electrode. The voltmeter detects the potential difference between the sample electrode and the reference electrode instead of detecting the potential difference between the sample electrode and the counter electrode. Features.

  In the polarization resistance measuring method according to the present invention, a high current is instantaneously passed between a sample electrode and a counter electrode to measure the polarization resistance. Since the potential-current response obtained in this manner approximates the internal polarization curve, the corrosion current and the corrosion rate can be obtained directly from the obtained polarization resistance. Further, since the polarization time is extremely short, the corrosion state of the sample electrode can be maintained almost the same as the natural corrosion state. Since this method can easily measure the polarization resistance, it can be suitably used for monitoring the corrosion rate.

It is a figure which shows the electric potential-current diagram and polarization curve for demonstrating the polarization resistance measurement principle of this invention. It is a figure which shows the structure of the corrosion rate monitoring apparatus 1 of the 3 electrode system which can perform corrosion rate monitoring using the 3 electrode short circuit method or the 2 electrode short circuit method of this invention. It is a figure which shows the structure of the polarization resistance measuring apparatus 2 which measures polarization resistance using the 2 electrode external power supply method of this invention. It is a figure which shows the structure of the polarization resistance measuring apparatus 3 which measures polarization resistance using the 3 electrode external power supply method of this invention. It is a block diagram of the experimental apparatus of the 2 electrode short circuit method used in Example 1 of this invention. It is a figure which shows the experimental result of Example 1 of this invention, Comprising: It is a figure which shows a time-dependent change of an electric potential and an electric current response when polarization time is 4, 8, 50 ms. It is an experimental result of Example 2 of this invention, Comprising: It is a figure which shows a time-dependent change of the polarization resistance calculated | required from the electric potential and the electric current response, and the corrosion current. It is an experimental result of Example 2 of this invention, Comprising: The corrosion integrated quantity calculated | required from the corrosion current, the mass change of the actual test piece, and the mass change obtained from ICP spectroscopy analysis were compared. It is an experimental result of Example 3 of this invention, Comprising: It is a figure which shows the time-dependent change of the cumulative corrosion amount calculated | required using the corrosion current obtained from the electric potential, electric current response, and the corrosion current in boiler water environmental conditions. It is an experimental result of Example 4 of this invention, Comprising: It is a figure which shows the electric potential of a pure iron sample electrode of a tap water environment by a 2 electrode short circuit method, and an electric current response. It is an experimental result of Example 6 of this invention, Comprising: It is a figure which shows the electric potential and electric current response of the mild steel sample pole of a tap water environment by a 3 electrode short circuit method.

  The first polarization resistance measurement method according to the present invention is a polarization resistance measurement method capable of directly calculating the corrosion current, and the polarization measurement is performed by the two-electrode method or the three-electrode method without using an external power source. The external power source mentioned here is a power source for applying a voltage or current from the outside to the sample electrode and the counter electrode. The first method for measuring polarization resistance according to the present invention utilizes the fact that the potential of a metal or conductor in a certain environment is determined by a redox reaction occurring on the surface, and the natural corrosion potentials of two different metals are different. . By using this potential difference, the sample electrode and the counter electrode are short-circuited for an extremely short time, and the potential and current change between them are measured to obtain the polarization resistance and further the corrosion current. When the sample electrode and the counter electrode are short-circuited, a high current instantaneously flows between the two electrodes. Since the potential-current response obtained in this manner approximates the internal polarization curve, the corrosion current can be obtained directly from the obtained polarization resistance. Corrosion current is directly converted to corrosion rate according to Faraday's law. By shortening the polarization time, external disturbance is hardly caused. The first polarization resistance measurement method according to the present invention can be said to be a polarization resistance measurement method in which a corrosion phenomenon called dissimilar metal contact corrosion is taken in reverse. Details will be described below.

  In the first polarization resistance measuring method according to the present invention, a metal for which a corrosion current is to be obtained is used as a sample electrode, and a solid having good electrical conductivity that is different from the metal in an equilibrium potential in a natural corrosion state is used as a counter electrode. After short-circuiting the counter electrode for a very short time, it is immediately opened, and the polarization resistance is obtained from the potential-current response during this period. The polarization resistance obtained in this way is different from the polarization resistance obtained from the conventional polarization resistance method, and the corrosion current can be calculated directly from the polarization resistance. Hereinafter, this method is referred to as a two-electrode short-circuit method.

  In the two-electrode short-circuit method, the counter electrode is a solid having good electrical conductivity that is different from the sample electrode in the natural corrosion state with respect to the aqueous environment. The component and shape are not limited, and for example, magnetite or platinum which is a stable substance in an aqueous environment can be used.

  In the two-electrode short-circuit method, the short-circuit time between the sample electrode and the counter electrode is made as short as possible in order to measure the potential difference and the short-circuit current between the sample electrode and the counter electrode without greatly disturbing the natural corrosion state of the sample electrode. In the conventional three-electrode method, the potentials of the counter electrode and the sample electrode are set to be the same, and the potential and current are measured by manipulating the potential of the counter electrode to + or-from that state to cause polarization. For this reason, in the conventional three-electrode method, the current flowing between the electrodes is relatively small. However, in the two-electrode short-circuit method, the sample electrode and the counter electrode having different potentials are short-circuited. Flows. For this reason, the time for short-circuiting the sample electrode and the counter electrode is an extremely short time. As shown in the examples described later, the short-circuit time is several milliseconds (ms) to several tens of milliseconds (ms). In the two-electrode short-circuit method, the short-circuit current and potential difference between the sample electrode and the counter electrode are measured separately. At this time, the measurement interval of potential and current measurement is shortened.

  In the first polarization resistance measuring method according to the present invention, the two-electrode short-circuit method can be a three-electrode short-circuit method using a reference electrode. The three-electrode short-circuit method is basically the same as the two-electrode short-circuit method, but since the reference electrode is provided, the potential difference between the reference electrode and the sample electrode is directly measured while measuring the short-circuit current between the sample electrode and the counter electrode. Can do. The reference electrode used in the three-electrode short-circuit method is a solid made of the same material as the counter electrode. For the reference electrode, for example, magnetite or platinum, which is a stable material with respect to an aqueous environment, can be used.

The principle of measuring polarization resistance according to the present invention is assumed as follows. FIG. 1 is a diagram showing a potential-current diagram and a polarization curve for explaining the principle of measuring polarization resistance of the present invention. The vertical axis in FIG. 1 is the relative potential, and the horizontal axis is the logarithm of the current. The sample electrode reaches an equilibrium state that is an aqueous environment. The potential at this time is the corrosion potential. The counter electrode also has an equilibrium potential which is the same aqueous system environment. When these two electrodes are short-circuited, a short-circuit current I ₁ corresponding to the potential difference V _{1 between the} two electrodes instantaneously flows. Immediately thereafter, the two electrodes are polarized so that the potential difference is zero, and the short-circuit current is also reduced. At this time, the potential difference is V ₂ and the short-circuit current is I ₂ . When you open a short-circuit state to the next moment, the potential difference V ₂ is gradually to return to the original state V _1. In the two-electrode short-circuit method, since the potential of the short-circuited sample electrode itself is not measured, the polarization resistance obtained from these values represents the slope of the straight line of the combined virtual internal anode polarization curve and virtual internal cathode polarization curve. In the three-electrode short-circuit method, the potential difference (V ₁ -V ₂ ) between the reference electrode and the sample electrode is directly measured while measuring the current difference (I ₁ -I ₂ ) in the short-circuit state between the sample electrode and the counter electrode. .

In the two-electrode short-circuit method, the polarization resistance is represented by the slope of a straight line that is a combination of the virtual internal anode polarization curve and the virtual internal cathode polarization curve. Therefore, the polarization resistance value that is the slope of the virtual internal anode polarization curve is expressed as the average polarization resistance Rp. ₁ is represented by Formula 1. The average polarization resistance Rp ₂ is also expressed as Equation 2 using a potential difference (V ₃ −V ₂ ) and a current difference (I ₁ −I ₃ ) that change immediately after the potential and current are switched. From these average polarization resistances Rp ₁ and Rp ₂ , the potential difference at one point, and the current value, the corrosion current I _corr can be obtained using Equation 3. On the other hand, in the three-electrode short-circuit method, since the potential of the sample electrode itself in a short-circuit state is measured, the right side (1/2) of Equation 1 and Equation 2 is not necessary.

Rp ₁ = (1/2) × (V ₁ −V ₂ ) / log (I ₁ / I ₂ ) (1)
Rp ₂ = (1/2) × (V ₃ −V ₂ ) / log (I ₁ / I ₃ ) (2)
I _corr = I ₁ × 10 ^{− (V1 / Rp)} = I ₂ × 10 ^{− (V2 / Rp)} (3)
Where Rp = Rp ₁ or Rp = Rp ₂

The corrosion rate R (mm / Year) of the sample electrode undergoing corrosion is expressed by Equation 3, which represents the corrosion current, Faraday constant F = 96500 C / eq in Faraday's law, metal atomic weight M, ion valence n, density ρ, and Using the exposed area A of the sample electrode, calculation is performed using Equation 4.
R = I _corr × ((365 × 24 × 3600) / (96500 × n) × (M / ρ / A)
... (4)

FIG. 2 is a diagram showing a configuration of a three-electrode type corrosion rate monitoring apparatus 1 that can perform corrosion rate monitoring using the three-electrode short-circuit method or the two-electrode short-circuit method. Here, the corrosion rate monitoring of the high-temperature and high-pressure boiler feed pipe 11 used in a power plant or the like is taken as an example. The boiler feed pipe 11 is generally made of carbon steel or low alloy steel. Magnetite (Fe ₃ O ₄ ) is formed on the surface of carbon steel or low alloy steel in an environment of ultrapure water at a pH of around 9 and an oxygen concentration of 7 ppb or less. The corrosion rate monitoring apparatus 1 includes a reference electrode 13 made of magnetite, which is a solid material having good electrical conductivity, and the sample electrode 12 and the sample electrode 12 made of the same material as the boiler feed pipe 11 have different equilibrium potentials in a natural corrosion state. A counter electrode 14 is provided. These electrodes are insulated by an insulator 15 and part of the boiler water supply pipe 11 is removed so that the surface of each electrode is flush with the inner surface of the boiler water supply pipe 11 so as to be in direct contact with the boiler water supply. The electrode fixing tool 16 is fixed to the boiler water supply pipe 11.

  The corrosion rate monitoring device 1 further includes a voltmeter 18 for detecting a potential difference between electrodes and a current, an ammeter 19, an A / D converter 20 for taking in the potential difference and the current value, a changeover switch 21, and a personal computer 22. Each electrode is connected to a changeover switch 21, a voltmeter 18, and an ammeter 19 via a conductor 17, and the voltmeter 18 and ammeter 19 are connected to a personal computer 22 via an A / D converter 20. Controls the changeover switch 21 and the A / D converter 20. The voltmeter 18 has a high input impedance, the ammeter 19 is a non-resistance current measuring instrument, and a measuring instrument with a fast response time is used. Needless to say, a dedicated measuring instrument incorporating a device for controlling the switch 21, the voltmeter 18, the ammeter 19, the A / D converter 20, the selector switch 21, and data processing may be used. If the reference electrode 13 is not used, the corrosion rate monitoring device 1 can be used as a corrosion rate monitoring device using the two-electrode short-circuit method, and if the reference electrode 13 is removed from the corrosion rate monitoring device 1, the corrosion rate monitoring device 1 can be used. Corrosion rate monitoring device using the electrode short-circuit method. The corrosion rate monitoring apparatus using the two-electrode short-circuit method cannot measure the short-circuit current and the potential difference at the same time, but has a very simple structure.

  In the corrosion rate monitoring device 1, the changeover switch 21 is instantaneously operated from the potential measurement state to measure the short-circuit current between the sample electrode 12 and the counter electrode 14 and the potential difference with the reference electrode 13 in this state. The polarization resistance is calculated from the current response, and the corrosion current is calculated from the polarization resistance value. Since the sample electrode 12 performs polarization measurement without instantaneously disturbing the natural corrosion state by instantaneous switching, it is excellent as a corrosion rate monitoring device capable of monitoring a long-time corrosion state. Moreover, since the corrosion rate monitoring apparatus 1 applies a voltage and an electric current between electrodes by short-circuiting the sample electrode 12 and the counter electrode 14, an external power supply is unnecessary. For this reason, installation is easy, and polarization resistance measurement and corrosion current calculation can be easily performed.

  Moreover, in the corrosion rate monitoring apparatus 1, since magnetite is used for the reference electrode 13, unlike the general-purpose reference electrode using a solution such as a silver / silver chloride electrode, there is no concern about contamination due to leakage of the sealing liquid. If the reference electrode 13 is partially dissolved and these are mixed into the boiler feed water as impurities, the process will be adversely affected. However, since magnetite is used for the reference electrode 13, a part of the reference electrode 13 is temporarily dissolved and included in the boiler feed water. However, the boiler water supply pipe 11 is generally made of carbon steel or low alloy steel, and iron oxide such as magnetite or hematite is formed on the surface of the carbon steel or low alloy steel in an environment of ultrapure water having a pH of around 9 and an oxygen concentration of 7 ppb or less. Because it is formed, it is considered that the process is not adversely affected. Further, since magnetite is used for the reference electrode 13, when good magnetite is formed on the surface of the sample electrode 12, the interelectrode potential between the reference electrode 13 and the sample electrode 12 approaches 0, so that the magnetite is formed on the surface of the sample electrode 12. Can be detected. Magnetite is stable in terms of components, physical properties and shape in a water-based environment at any temperature, and magnetite has good electrical conductivity, so that it can be suitably used as a reference electrode. Note that platinum can be used for the reference electrode 13 and the counter electrode 14 depending on the use environment.

  In the second polarization resistance measuring method according to the present invention, a predetermined current flows between the sample electrode and the counter electrode instead of short-circuiting and opening the sample electrode and the counter electrode in the two-electrode short-circuit method and the three-electrode short-circuit method. After applying an extremely small time voltage between the sample electrode and the counter electrode using an external power source, the electrodes are immediately opened. Hereinafter, this method is referred to as a two-electrode external power source method and a three-electrode external power source method.

  FIG. 3 is a diagram showing a configuration of a polarization resistance measuring device 2 that measures polarization resistance using the two-electrode external power source method, and FIG. 4 is a configuration of a polarization resistance measuring device 3 that measures polarization resistance using the three-electrode external power source method. FIG. The same components as those in the corrosion rate monitoring apparatus 1 shown in FIG. Unlike the corrosion rate monitoring device 1 using the three-electrode short-circuit method, the polarization resistance measuring device 2 and the polarization resistance measuring device 3 are external power sources for applying a predetermined voltage between the sample electrode 12 and the counter electrode 14 from the outside. The external power supply 23 and the ammeter 18 are connected in series. In the two-electrode external power supply method and the three-electrode external power supply method, instead of short-circuiting the sample electrode 12 and the counter electrode 14 performed by the two-electrode short-circuit method and the three-electrode short-circuit method, the sample electrode 12 and the counter electrode 14 are A predetermined current (high current) is allowed to flow during Except this point, the corrosion current and the corrosion rate can be obtained in the same manner as the two-electrode short-circuit method and the three-electrode short-circuit method. 2 and 3, reference numerals 24, 25, 26, and 27 indicate a test surface, a coating, a counter electrode working surface, and a solution. In the two-electrode short-circuit method and the three-electrode short-circuit method, the potential difference is determined by the natural corrosion potential of two dissimilar metals, but in the two-electrode external power method and the three-electrode external power method, an arbitrary voltage can be applied between the electrodes. Therefore, it can be suitably used when the natural corrosion potential difference between the sample electrode 12 and the counter electrode 14 is small.

  The third polarization resistance measuring method according to the present invention is to short-circuit the sample electrode 12 and the counter electrode 14 instead of short-circuiting and opening the sample electrode 12 and the counter electrode 14 of the two-electrode short-circuit method and the three-electrode short-circuit method. After being released from the state for a very short time, it is immediately short-circuited, and the polarization resistance is obtained from the potential current response during this time, and the corrosion current is directly calculated from the polarization resistance. The third polarization resistance measurement method is a method for obtaining the polarization resistance by the polarization phenomenon of the sample electrode 12 as in the two-electrode short-circuit method and the three-electrode short-circuit method. However, it takes advantage of the same polarization behavior. Since the two-electrode short-circuit method and the three-electrode short-circuit method instantaneously short-circuit and open the electrodes, they can be kept in a natural corrosion state. On the other hand, since this method opens and short-circuits again for a very short time from the short-circuited state, the natural corrosion state cannot be maintained, but corrosion can be accelerated. This is suitable when it is not necessary to maintain the natural corrosion state and when it is desired to quickly simulate the corrosion state.

  A fourth polarization resistance measuring method according to the present invention is such that a predetermined current flows between the sample electrode 12 and the counter electrode 14 using the external power source 23 of the two-electrode external power source method and the three-electrode external power source method. After applying a very small time voltage between the electrode 12 and the counter electrode 14, a predetermined current flows between the sample electrode 12 and the counter electrode 14 instead of immediately opening the space between the sample electrode 12 and the counter electrode 14. Thus, the voltage is applied using the external power source 23 so that a predetermined current flows between the electrodes immediately after the sample electrode 12 and the counter electrode 14 are opened for a very short time after the voltage is applied using the external power source 23. The polarization resistance is obtained from the potential current response during this period, and the corrosion current is calculated directly from the polarization resistance. The fourth polarization resistance measurement method is a method for obtaining the polarization resistance by the polarization phenomenon of the sample electrode 12 as in the two-electrode external power supply method and the three-electrode external power supply method. Even if polarization is performed, the fact that the polarization behavior is the same is utilized. In the two-electrode external power source method and the three-electrode external power source method, a very short time is required between the sample electrode 12 and the counter electrode 14 so that a predetermined current flows between the sample electrode 12 and the counter electrode 14 using the external power source 23. Since the gap between the sample electrode 12 and the counter electrode 14 is immediately opened after the voltage is applied, it can be kept in a natural corrosion state. On the other hand, in this method, between the sample electrode 12 and the counter electrode 14 from a state where a voltage is applied using the external power source 23 so that a predetermined current flows between the sample electrode 12 and the counter electrode 14. Since the voltage is applied using the external power source 23 so that a predetermined current flows between the electrodes immediately after opening, the natural corrosion state cannot be maintained, but the corrosion can be accelerated. This is suitable when it is not necessary to maintain the natural corrosion state and when it is desired to quickly simulate the corrosion state.

Example 1
Using the experimental apparatus shown in FIG. 5, the polarization resistance method was performed using the two-electrode short-circuit method. The same components as those in the corrosion rate monitoring apparatus 1 shown in FIG. The solution was a hydrochloric acid environment having a water temperature of 21 ° C. and a pH of 1. A pure iron plate as a sample electrode 12 and a magnetite plate as a counter electrode 14 were opposed to each other to form a square test surface 24 and a counter electrode working surface 26 each having a side of 5 mm. Switching between the voltmeter 18 and the non-resistance ammeter 19 was controlled in the range of 4 to 100 ms. In the potential mode, the potential when the sample electrode 12 is corroded in this environment is measured. Next, it switched to the current mode, measured current for several ms, and switched to potential measurement again. The potential difference and current that change at this time were converted by the A / D converter 20 and stored in the personal computer 22. It should be noted that the polarization of several ms hardly disturbs the corrosion state of the sample surface.

FIG. 6 is an experimental result of Example 1, and shows a potential difference and a short-circuit current response result when short-circuiting and opening are performed for a very short time from the state of natural corrosion potential of the sample electrode 12 with respect to magnetite. The short-circuit (polarization) time was 3 times of 4 ms, 8 ms and 50 ms. It can be seen that by short-circuiting the two electrodes, the current value gradually decreases from a high state, and the polarization proceeds. The potential difference between the two electrodes is reduced by the polarization, but when the short circuit is stopped and opened, the potential difference gradually increases and tries to approach the original potential difference. The potential before the short circuit is V ₁ , the current immediately after the short circuit is I ₁ , the current immediately before the opening is I ₂ , and the potential immediately after the opening is V ₂ . V ₃ and I ₃ are behind V ₂ and I ₁ , respectively.

Example 2
The same apparatus as in Example 1 was used. A pure iron plate as a sample electrode 12 and a magnetite plate as a counter electrode 14 are opposed to each other to form a circular test surface 24 and a counter electrode working surface 26 having a diameter of 4 mm. A 6 hour corrosion test was conducted by measuring the potential and current response. One polarization time was 8 ms. Changes with time in the polarization resistance value Rp ₂ and the corrosion current value I _corr are shown in FIG. The corrosion rate was calculated from the corrosion current of FIG. 7 using Equation 4, the result of integrating the corrosion amount, and the ICP emission analysis when a 6 hour corrosion test was performed using pure iron of the same material with a surface area of 455 mm ² The accumulated corrosion amount obtained from the test piece mass reduction amount is shown in FIG. When a 2-hour corrosion test was performed using pure iron of the same material having a surface area of 455 mm ² , the test piece mass reduction amount was 0.2 mg, and the average corrosion rate determined from ICP emission analysis was 66.1 × 10 ⁻⁸ mg / The average corrosion current of the circular surface having a diameter of ² mm and a diameter of 4 mm was 2.6 μA using Equation 4, and the cumulative amount of corrosion in the same area over 6 hours was 0.017 mg. The average corrosion current obtained by the polarization measurement shown in FIG. 7 was 2.2 μA, and the average corrosion currents were in good agreement. Further, the integrated amount of corrosion in 6 hours obtained from the average corrosion rate obtained by the polarization measurement shown in FIG. 7 was 0.014 mg, and the integrated amounts of corrosion were in good agreement.

Example 3
A test solution having a boiler water environment of 140 ° C., pH 9, and dissolved oxygen concentration of 20 ppb or less is placed in a high-pressure vessel, and the electric resistance of the pure iron sample electrode 12 is measured for the polarization resistance by measuring the short circuit time 8 ms for 60 minutes. FIG. 9 shows the change over time in the cumulative corrosion amount obtained from the calculated corrosion current at intervals. The surface area of the sample electrode 12 was 400 mm ² , the surface area of the magnetite counter electrode 14 was 900 mm ² , and the area ratio between the sample electrode 12 and the counter electrode 14 was changed. It can be seen that the amount of corrosion gradually decreases because the magnetite film formed on the surface of the sample electrode 12 grows. The mass reduction amount of the sample electrode 12 after the test was 0.2 mg, which was in good agreement with the cumulative corrosion amount obtained from the corrosion current according to the present invention.

Example 4
The counter electrode 14 is platinum, the sample electrode 12 is pure iron, the test surface 24 and the counter electrode working surface 26 have a diameter of 4 mm, and the periphery is covered 25, then immersed in a tap water environment at 21 ° C., and short-circuited by the two-electrode short-circuit method. FIG. 10 shows the results of viewing the potential and current response of the pure iron sample electrode 12 when it was opened for a very short period of time from the state in which it was applied. In the potential-current response with an opening time of 8 ms and 4 ms, the polarization resistance of Equation 2 was obtained from the potential value V ₃ and current value I ₃ after 0.8 ms, and the corrosion current obtained from Equation 3 was 3 μA. It was found that the potential response when released was complex and reflected the complex corrosion state of the sample surface. In this method, it was confirmed that the corrosion state of the sample surface can be observed by instantaneous switching.

Example 5
Corrosion current of pure iron in a 0.5 wt% saline solution environment when a magnetite plate, carbon plate, martensitic stainless steel is used as the counter electrode 14 by the two-electrode short-circuit method using the same apparatus as in Example 1. Asked. The results are shown in Table 1. The polarization time was 8 ms. Corrosion currents of pure iron when using a magnetite plate, a carbon plate, and martensitic stainless steel as the counter electrode 14 are 2.16 μA, 2.93 μA, and 1.45 μA, respectively, and these values are shown in Example 2. It was in good agreement with the corrosion current of 2.6 μA obtained from the test piece mass loss by the corrosion test, confirming the validity of this method.

Example 6
FIG. 11 shows the results of determining the potential and current response of the mild steel sample electrode 12 using the platinum counter electrode 14 and the reference electrode 13 using a three-electrode short-circuit method in a tap water environment. The surface of the counter electrode 14 and the reference electrode 13 was circular with a diameter of 4 mm, and the surface of the sample electrode 12 was circular with a diameter of 12 mm. The potential of the counter electrode 14 and the sample electrode 12 is measured, and then the counter electrode 14 and the sample electrode 12 are short-circuited instantaneously (4 ms) by the conducting wire 17, and the current flowing through the counter electrode 14 and the sample electrode 12 and the counter electrode 14 are short-circuited. The potentials of the sample electrode 12 and the reference electrode 13 were measured simultaneously. The potential at this time is reversed between plus and minus on the circuit. With this test, the corrosion current was calculated to be 0.02 μA using Equation 3.

DESCRIPTION OF SYMBOLS 1 Corrosion current measuring apparatus 2 Polarization resistance measuring apparatus 3 Polarization resistance measuring apparatus 11 Boiler feed pipe 12 Sample electrode 13 Reference electrode 14 Counter electrode 15 Insulator 16 Electrode fixture 17 Conductor 18 Voltmeter 19 Ammeter 20 A / D converter 21 Changeover switch 22 Personal Computer 23 External Power Supply 24 Test Surface 25 Coating 26 Counter Electrode Working Surface 27 Solution

Claims (10)

A method for measuring polarization resistance capable of directly calculating the corrosion current,
The metal for which the corrosion current is to be obtained is used as a sample electrode, and the solid electrode having a good electrical conductivity different from the equilibrium potential in the natural corrosion state is used as a counter electrode, and the sample electrode and the counter electrode are short-circuited for a very short time. A method of measuring polarization resistance, which is immediately opened and the polarization resistance is obtained from the potential-current response during this period. Furthermore, a solid made of the same material as the counter electrode is used as a reference electrode,
2. The polarization resistance measuring method according to claim 1, wherein when the sample electrode and the counter electrode are short-circuited for a very short time, a potential difference between the sample electrode and the reference electrode is measured simultaneously.   Instead of short-circuiting between the sample electrode and the counter electrode for a very short time, an extremely small amount is used between the sample electrode and the counter electrode using an external power source so that a predetermined current flows between the sample electrode and the counter electrode. 3. The polarization resistance measuring method according to claim 1, wherein a time voltage is applied.   Instead of immediately opening the sample electrode and the counter electrode after short-circuiting for a very short time, instead of opening the sample electrode and the counter electrode short-circuited from the state of being short-circuited, and immediately short-circuiting 3. The polarization resistance measuring method according to claim 1, wherein the polarization resistance is measured.   After applying a very short time voltage between the sample electrode and the counter electrode using an external power source so that a predetermined current flows between the sample electrode and the counter electrode, the gap between the sample electrode and the counter electrode is immediately opened. Instead, the gap between the sample electrode and the counter electrode is opened for a very short time from the state in which a voltage is applied using an external power supply so that a predetermined current flows between the sample electrode and the counter electrode, and then immediately between the electrodes. 4. The polarization resistance measuring method according to claim 3, wherein a voltage is applied using an external power supply so that a predetermined current flows.   The sample electrode and counter electrode, or the sample electrode, counter electrode, and reference electrode are placed in an aqueous environment where the corrosion current is desired to be obtained, and the polarization resistance of the metal material is measured using the polarization resistance measurement method according to claim 1. A corrosion rate monitoring method characterized by measuring the corrosion rate and monitoring the corrosion rate.   The corrosion according to claim 6, wherein the reference electrode is a reference electrode made of a solid material that does not use a solution or a salt solution, and is excellent in property stability and shape stability in a water-based environment to be measured. Speed monitoring method.   The corrosion rate monitoring method according to claim 6 or 7, wherein the counter electrode, or the counter electrode and the reference electrode are made of magnetite, and the water-based environment is boiler feed water at 100 ° C or higher. A sample electrode made of a metal for which a corrosion current is required;
A counter electrode made of a solid having good electrical conductivity, which is different from the equilibrium potential of the natural corrosion state from the sample electrode;
A switch that opens after short-circuiting the sample electrode and the counter electrode for a very short time,
A voltmeter for detecting a potential difference between the sample electrode and the counter electrode;
An ammeter for detecting a current flowing between the sample electrode and the counter electrode;
A data processing device capable of capturing potential difference and current value online;
A polarization resistance measuring device comprising: Furthermore, a reference electrode made of the same solid material as the counter electrode is provided,
The switch is further opened after the sample electrode and the reference electrode are short-circuited for a very short time,
The polarization resistance measurement according to claim 9, wherein the voltmeter detects a potential difference between the sample electrode and the reference electrode instead of detecting a potential difference between the sample electrode and the counter electrode. apparatus.