JPH06138079A - Electrode for measuring quality of water in gap - Google Patents

Abstract

PURPOSE:To measure the pH of high-temperature water in a gap section by covering a pH-electrode sensor section with a metallic material to be checked for corrosion behavior so that a structural gap can be formed. CONSTITUTION:A zirconia tube 52 is inserted into a body 51 supported by an autoclave cap 21 and the tube 52 is packed with, for example, copper oxide/ copper powder as a filler 53. In addition, a gap-forming tube 55 and sealing section 54 are respectively provided on the outside of the tube 52 and above the body 51. The pH of a solution is measured by measuring the potential appearing at a lead wire 2a as the potential difference between the lead wire 2a and a reference electrode which is not affected by the pH of the solution. It is predicted that the pH of the solution becomes lower than that of bulk water in the gap section due to the hydrolysis of dissolved metallic ions. However, since the metallic ions dissolving from the metallic tube 55 which covers the tube 52 corresponding to a sensor section and forms the gap section deposit in the gap section, the measured pH value becomes the same as that in the gap section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interstitial water quality measuring electrode applied to an advanced water quality control technique for maintaining the soundness of materials in cooling water of a nuclear power plant.

[0002]

2. Description of the Related Art In order to improve the safety of a nuclear power plant and maintain its soundness for a long period of time, it is an important technical subject to prevent stress corrosion cracking (SCC) of metal materials. . The SCC characteristics of austenitic stainless steels and nickel-based alloys, which are often used as pipes and reactor internals of nuclear power plants, greatly depend on the water quality of cooling water. Generally, conductivity and pH are used as indicators of water quality.
And corrosion potential are used.

An example showing the conductivity and corrosion potential dependence of SCC of stainless steel is shown in FIGS. 6 and 7. This FIG. 6 is for explaining the influence of conductivity on SCC of stainless steel, and it can be seen that the SCC susceptibility of sensitized stainless steel increases as the conductivity and corrosion potential increase.

In FIG. 6, the vertical axis represents the crack growth rate at which a crack progresses due to stress corrosion cracking (IGSCC) when the test piece is pulled at a low strain rate, that is, the likelihood of stress corrosion cracking. ing. The abscissa represents the conductivity of the test liquid that underwent the SCC test, and the symbols of the test points and the types of impurities added to increase the conductivity are shown in the figure. The test conditions are a low strain rate stress corrosion cracking test at a temperature of 288 ° C and oxygen of 0.2 ppm.

As is clear from this figure, it is clear that as the conductivity increases, the crack growth rate increases and SCC is promoted.

On the corroded metal surface, there are always an anode site where a metal dissolution reaction takes place and a cathode site where a reduction reaction of an oxidant takes place, cations flow from the anode site to the cathode site, and the cathode site to the anode. The site constitutes a current circuit in which anions flow.

When the resistance of the solution is large, resistance polarization is caused by the resistance of the ionic current, and the corrosion rate decreases. Therefore, in general, it can be said that the corrosion rate increases when impurities are added to pure water to increase the conductivity, but the mechanism of the effect of impurity ions on the SCC of stainless steel in high temperature water is not fully understood. .

FIG. 7 is for explaining the influence of corrosion potential on SCC of stainless steel. In FIG. 7, the left vertical axis indicates stress corrosion cracking (I) when a test piece is pulled at a low strain rate.
GSCC) represents the speed at which a crack progresses, and the right vertical axis represents the proportion of stress corrosion cracking (IGSCC) that appears on the fracture surface when the test piece is pulled and ruptured at a low strain rate.

In the figure, the white dots indicate S in the actual plant.
The results of the CC test are represented by the crack growth rate, and the black circles represent the results of the tests conducted in the laboratory by the fracture rate. The higher the potential, the more easily the corrosion reaction occurs in the anode part, and the IGSCC tends to be accelerated.

It is known that the properties of water change significantly with temperature. Therefore, in order to know the corrosion behavior of metallic materials in high temperature water in a power plant or the like, it is desirable to directly measure the water quality under high temperature and high pressure conditions.

For this purpose, electrodes for high temperature water as shown in FIGS. 8 to 10 have been developed. FIG. 8 shows a pH electrode developed for measuring high temperature water, which utilizes the fact that the potential difference between the inside and outside of the electrode via an oxygen permeable zirconia membrane changes depending on the H ⁺ ion concentration in water. Is.

FIG. 9 shows a conductivity cell for high temperature water, in which an AC voltage is applied between the inner pole and the outer pole, the resistance of the high temperature water is measured from the impedance value between the electrodes, and the conductivity is calculated. It is a thing.

FIG. 10 shows an electrode for measuring corrosion potential in high temperature water,
(A) is a sample electrode and (b) is a reference electrode.

In FIG. 8, reference numeral 1 is a disk made of tetrafluoroethylene resin, 2 is the same seal, 3 is glass wool, 4 is copper wire, and 5 is
Is alumina insulating material, 6 is Vespel disk, 7 is Vespel,
8 is a silver ring, 9 is a zirconia tube, and 10 is a copper / copper oxide powder.

By immersing the zirconia tube 9 portion of this electrode and another reference electrode in high temperature water and measuring the potential difference between the two electrodes, this potential difference changes linearly with the change in pH of the high temperature water. You can know the pH of hot water.

In FIG. 9, reference numeral 1a is an insulating material, 2a is a lead wire, 3a is a spacer, 4a is an outer pole, 5a is an inner pole, 6a is a thermocouple, 7a is a body, 8a is a lead wire mounting member, and 9a.
Each of a indicates a seal cap.

The body 7a has a pressure resistant structure, and the surface of the electrode is covered with an inert metal so as not to be oxidized so that the measurement can be performed in high temperature water.

In FIG. 10, reference numeral 11 is a sample electrode body, 12,
16 is a fluororesin cover, 13 and 14 are copper blocks, 15 is a test piece, 17 is a fluororesin ring, 18 is a sample holder, 19 is a fluororesin bush, 20 is an electrode support tube, 21 is an autoclave lid, 22 is fluororesin Insulation, 23 is a fluorine resin holder, 24 and 25 are fluorine resin filters, 26 and 27 are asbestos liquid junctions, 28 is a fluorine resin lugin tube, 29 is a buffer space, 30 is an asbestos plug, 31 is porous alumina, 32 is an electrode Body, 33 KCl solution, 34 AgCl tip, 35 porous alumina tube, 36 Viton O-ring, 37 silver seal, 38 thrust nut, 39 fitting bolt, 40 fluororesin seal, 41 silver lead Each line is shown.

In (a), a certain surface of the sample metal 7 is exposed to high temperature water, and the other parts are insulated. (B) is an Ag / AgCl reference electrode, but the main part is made of fluororesin, that is, tetrafluoroethylene so that it can be measured in high temperature water.

The corrosion potential of the sample electrode (a) in high temperature water can be measured by measuring the potential difference between the electrodes (a) and (b).

The above-mentioned electrodes are all developed for measuring the water quality of bulk, and it is not possible to measure the water quality change in the gap.

[0022]

By the way, in a corrosive environment such as high temperature water, a structural clearance (hereinafter, referred to as a clearance) in which water stays and the liquid is not exchanged with the outside.
It is said that the corrosive environment is different between the parts that are constantly flowing.

That is, in the gap, dissolved oxygen is consumed by the oxidation reaction of the metal to lower the potential, and a severe corrosive environment is formed due to the hydrolysis of the dissolved metal ions and the concentration of impurities. As a result, it is said that the gap portion becomes an anode portion where corrosion elution easily occurs, and the outside becomes a cathode portion that supports the corrosion reaction.

FIG. 11 shows an example in which the susceptibility of stress corrosion to the gap and the outside is compared. In this Figure 11, the vertical axis is the maximum SC
C depth and the horizontal axis is the potential applied to the SCC specimen during the test. From this figure, it is understood that the potential to be applied to generate SCC is lower by about 250 mV in the case where there is a gap, and SCC is more likely to occur.

From the above, it is considered necessary to grasp the corrosion behavior in the gap in order to maintain the soundness of the structural material of the plant and enhance the long-term stability. Therefore, there is a problem that the water quality in the gap and the corrosive environment cannot be measured while measuring the water quality in bulk.

The present invention has been made in order to solve the above problems, and can accurately measure pH, corrosion potential and conductivity of a gap in high temperature water, and obtain accurate information on water quality and corrosive environment. An electrode for measuring interstitial water quality is provided.

[0027]

According to the present invention, a pH electrode sensor portion that can be used in high-temperature, high-pressure water is covered with a metallic material whose corrosion behavior is to be investigated, and a structural gap is formed to provide high-temperature water. The pH electrode is characterized by being able to measure the pH of the gap portion.

Further, by covering the electrode portion of the conductivity cell which can be used in high temperature and high pressure water with a metal material whose corrosion behavior is to be investigated and providing a structural gap, the conductivity of the gap portion in high temperature water can be improved. The conductivity cell is characterized in that the rate can be measured.

Furthermore, a sample electrode for measuring corrosion potential that can be used in high temperature and high pressure water is covered with a metallic material whose corrosion behavior is to be investigated, and a structural gap is provided, and a liquid junction of the reference electrode is provided in the gap. This is an electrode for measuring corrosion potential, characterized in that the potential of a gap in high temperature water can be measured by inserting a tube.

[0030]

It is said that the corrosive environment is different between the part where the water constantly flows and the gap where the water is retained and the liquid is not exchanged with the outside in the part where the material comes into contact with the high temperature water.

That is, the pH of the gap is expected to be lower than that of the bulk water due to the hydrolysis of the dissolved metal ions.
At the H electrode, the metal ions dissolved from the metal covering the sensor portion are accumulated in the gap, so that the same pH value as in the gap can be measured.

It is expected that the conductivity in the gap becomes higher than that in the bulk water due to the action of concentrating impurities, but in the conductivity measuring cell of the present invention, the metal ions dissolved from the metal covering the sensor are in the gap. The value of the conductivity that is closer to the gap than that of the bulk water can be measured due to the concentration action of the anions from the outside of the electrode.

It is expected that dissolved oxygen will be consumed by the metal oxidation reaction in the gap and the potential will be lowered. However, in the gap potential measuring electrode according to the present invention, dissolved oxygen is consumed by the sample and the metal covering the electrode, Further, since the diffusion of oxygen from the outside is relatively slow, the gap potential simulating a gap different from the outside is measured.

[0034]

EXAMPLE An example of the gap water quality measuring electrode according to the present invention will be described with reference to FIGS. FIG. 1 shows a high temperature interstitial pH electrode in vertical section. In FIG. 1, reference numerals
Reference numeral 51 denotes a body supported by the autoclave lid 21, and a zirconia tube 52 is inserted into the body 51. As the internal filler 53, for example, copper oxide / copper (Cu ₂ O / Cu) powder is inserted into the body 51. Is filled,
A gap forming tube 55 is provided outside the zirconia tube 52. A seal portion 54 is provided on the upper portion of the body 51. The overall configuration is similar to that shown in FIG.

The principle of pH measurement by this electrode is shown in FIG. The description of FIG. 4 will be given later. According to the principle as shown in FIG. 4, the pH of the volume can be measured by measuring the potential appearing on the lead wire 2a as the potential difference with the reference electrode that does not change with the pH of the solution, for example, the Ag / AgCl electrode.

The potential appearing inside the electrode is 2Cu + H ₂ O = Cu ₂ O + 2H ⁺ + 2e, and when measured with the reference electrode as a reference, this potential difference E is expressed as E = A−B · pH, It is a linear expression with respect to pH.

Conventionally, the pH of bulk water was measured in this way, but here, by covering the outer surface of the zirconia tube 52 corresponding to the sensor section with the metal tube 55,
A gap is formed between the sensor / tube and the pH of the gap is detected.

In the gap, the dissolved metal ions and trace impurities in water are concentrated, and the pH and potential are lower than those of bulk water.
It is expected that the conductivity is high. Here, by covering the pH sensitive portion of the pH electrode with a predetermined metal, the water quality in the gap where corrosion is likely to occur can be directly measured.
Various gaps can be simulated by changing the tube material for forming the gap and the clearance.

FIG. 2 shows a vertical cross section of the high temperature interstitial conductivity cell. In FIG. 2, the body 51 has a mounting flange.
56 and a mounting boss 57 are formed, and an inner pole support rod 58 is inserted in the body 51. An insulating seal 59 and a bush 60 are provided on the outer side of the inner pole support rod 58, and these are fixed to the body 51 with a fixed cap 61.

An insulating spacer 62 is provided below the attachment boss 57, an inner pole 63 and an outer pole 64 are attached, and a gap forming tube 65 is provided outside the outer pole 64.

Here, by applying an AC voltage between the inner pole 63 and the outer pole 64, the resistance of water is measured and the conductivity is calculated. Therefore, to measure accurate conductivity,
The distance between the poles must always be constant. In the high-temperature water electrode, both electrodes are fixed by an insulating spacer 62 so that the distance between the electrodes does not change due to temperature changes.

In addition, to withstand high temperature and high pressure water,
The body 51 has a pressure resistant structure made of stainless steel,
The insulating seal is made of tetrafluoroethylene resin so as to seal high temperature and high pressure water.

The outer surface of the inner electrode 63 and the inner surface of the outer electrode 64 must be made of a material that is unlikely to be corroded by high temperature water, such as gold or platinum, or coated with the same material.

Since the gap is formed outside the outer pole 64 and the conductivity is measured inside the outer pole 64, the outer pole 64 has one surface so that the water quality on the outer side is equal to that on the inner side. A hole is provided in the hole or a net-like structure.

FIG. 3 shows a high temperature crevice corrosion potential measuring electrode. In FIG. 3, a reference electrode 72 attached to the autoclave lid 21 and a counter electrode 74 are connected via a liquid junction tube 71. The counter electrode 74 has a lead wire 70 at the bottom, and a sample plate 67 connected to this lead wire 70 has a holder 66 through an insulating material 68.
Held in. In the figure, reference numeral 69 is a holding plate, and 73 is a reference electrode lead wire.

Here, the corrosion potential of the sample can be measured by measuring the potential difference between the lead wire 70 taken out from the sample plate 67 and the reference electrode lead wire 73 with an electrometer or the like.

Here, the sample electrode, the holder 66, and the holding plate 69.
By providing a gap between them with an insulating material 68 and placing the end of the liquid junction tube 71 of the reference electrode 72 close to the surface of the sample plate 67, the potential of the gap can be measured. The holder 66, the sample plate 67, and the holding plate 69 use a metal material whose corrosion behavior is to be investigated.

By installing the end of the liquid junction tube 71 connecting the electrolytic solution of the reference electrode 72 and the high temperature solution close to the surface of the sample plate 67, it is possible to prevent an error in potential measurement due to generation of a local battery or the like. .

Next, the measurement principle of the zirconia membrane type high temperature pH electrode will be described with reference to FIG. FIG. 4 shows the measurement principle of a zirconia membrane type high temperature pH electrode when Cu / Cu ₂ O mixed powder is used as the electrode internal filler. The membrane potential is due to the equilibrium reaction between oxygen ions in zirconia and other substances at the inner and outer zirconia interfaces. The reaction on the inner and outer surfaces of the zirconia film is expressed as follows. Inner surface of the film: 2Cu + O ^2- = Cu ₂ O (1) Outer surface of the film: H ₂ O = 2H ⁺ + O ^2- (2)

Zirconia is an O ² -permeable solid electrolyte, and O ^{2 −} in these reactions is exchanged through the zirconia membrane. All the above reactions become 2Cu + H ₂ O = Cu ₂ O + 2H ⁺ + 2e ⁻ (3), which is the electrode reaction of the pH electrode.

The electrode potential for this reaction is calculated as follows:

[0052]

[Equation 1]

Since Cu and Cu 2 O were mixed in equal amounts, assuming that the activities of both are 0.7 and 0.3, respectively,

[Equation 2] Here, when ΔG ° of 280 ° C. is calculated and substituted into the equation (7), E = 0.360−0.110 pH (vs NHE at 280 ° C.) (8), which is a linear equation of pH.

When measuring the potential, it is measured with reference to an Ag / AgCl reference electrode having a KCl solution as an internal solution. The potential of this electrode is

[Equation 3] Is expressed as Er ° depends on the temperature, and the value at the temperature t ° C. is approximately expressed as E _r ° = 0.23755-5.3783 × 10 ⁻⁴ t−2.3728 × 10 ⁻⁶ t ² (10).

Therefore, when 0.1 NKCl was used as the internal solution, E _r = 0.011 (V _SHE at 280 ° C.).

The actually measured potential is ΔE = E _pH −E _r (11), and when Cu / Cu ₂ O is used, ΔE = 0.349-0.110 pH (12) Hg / HgO When is used, ΔE = 0.845−0.110 pH (13) is calculated.

Next, the operation of the above embodiment will be described. In the pH electrode for measuring water quality of the gap according to the present embodiment, a corrosive environment different from the bulk water quality is formed in the metal tube covering the sensor part, and the pH value simulating the gap can be measured.

Further, in the conductivity measuring cell, metal ions dissolved from the metal covering the sensor portion are stored in the gap, and the value of conductivity near the gap can be measured.

Although it is expected that dissolved oxygen will be consumed by the metal oxidation reaction in the gap and the potential will be lowered, the dissolved oxygen is consumed by the sample and the metal covering the electrode in the gap potential measuring electrode of this embodiment. It is also confirmed that a corrosive environment different from the outside is formed due to the relatively slow diffusion of oxygen from the outside.

The conventional high-temperature water quality measuring electrode can measure only bulk water quality, and no information can be obtained about the gap portion which is said to be in a corrosive environment severer than the outer surface. On the other hand, if the electrode for measuring water quality in the gap according to the present embodiment is used, information on the water quality and the corrosive environment in the gap which has not been obtained in the past can be obtained, which is useful for clarifying the corrosion mechanism in the gap.

FIG. 5 shows an example in which the gap water quality measuring electrode according to the present invention is applied as a water quality monitor of a BWR type nuclear power plant reactor cleaning system. Even if the bulk water quality is maintained at high purity, dissolved oxygen is consumed by the metal oxidation reaction in the gap and the potential drops, forming a local battery with the outside of the gap, and dissolving the dissolved metal ions. After a long time due to decomposition and concentration of impurities, the gap tends to have a more severe corrosive environment than the bulk environment.

When the gap water quality measuring electrode according to the present invention is applied as a water quality monitor or a corrosive environment monitor, it is possible to know the change over time in the corrosive environment of the gap portion, and effective measures can be taken.

[0063]

EFFECTS OF THE INVENTION According to the present invention, the pH, corrosion potential and conductivity of the gap can be accurately measured in high temperature water, which has not been obtained in the past, and therefore it is not only useful for elucidating the corrosion mechanism in the gap. Instead, it can be used as a water quality monitor for gaps and is effective for maintaining long-term health of plants.

[Brief description of drawings]

FIG. 1 is a pH of a gap water quality measuring electrode according to the present invention
FIG. 3 is a vertical cross-sectional view showing an electrode.

FIG. 2 is a vertical cross-sectional view showing a conductivity cell in a gap water quality measuring electrode according to the present invention.

FIG. 3 is an elevational view showing a corrosion potential measuring electrode in a gap water quality measuring electrode according to the present invention.

FIG. 4 is a principle diagram for explaining the measurement principle of the pH electrode in FIG.

FIG. 5 is a system diagram showing an example in which a gap water quality measuring electrode according to the present invention is applied as a water quality monitor to a reactor cleaning system of a BWR power plant.

FIG. 6 is a characteristic diagram showing an example of SCC conductivity dependency of stainless steel.

FIG. 7 is a characteristic diagram showing an example of SCC corrosion potential dependence of stainless steel.

FIG. 8 is a vertical cross-sectional view showing a conventional pH electrode for high temperature water.

FIG. 9 is a vertical cross-sectional view showing a conventional conductivity cell for high temperature water.

FIG. 10A is a vertical sectional view showing a sample electrode in a conventional electrode for measuring a corrosion potential, and FIG. 10B is a vertical sectional view showing a reference electrode as in FIG. 10A.

FIG. 11 is a characteristic diagram showing the ease of occurrence of stress corrosion cracking in the clearance and the outside.

[Explanation of symbols]

51 ... Body, 52 ... Zirconia tube, 53 ... Internal filler, 54 ... Seal part, 55 ... Gap forming tube, 56 ... Mounting flange, 57 ... Mounting boss, 58 ... Inner pole support rod, 59 ... Insulation seal, 60 … Bushings, 61… Fixed caps, 62… Insulating spacers, 63… Inner poles, 64… Outer poles, 65… Gap forming tubes, 66
… Holder, 67… Sample plate, 68… Insulation material, 69… Holding plate, 70
… Lead wire, 71… Liquid junction tube, 72… Reference electrode, 73… Oxygen electrode lead wire, 74… Counter electrode.

Claims (1)

[Claims] 1. A structural gap is formed by covering a pH electrode sensor portion, a conductive cell electrode portion, and a corrosion potential measuring sample electrode portion that can be used in high temperature and high pressure water with a metal material whose corrosion behavior is to be investigated. An electrode for measuring interstitial water quality characterized by being provided.