JPH0768637B2 - Method and apparatus for forming corrosion resistant oxide film on inner surface of pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to oxidation effective for suppressing corrosion of the inner surface of the maximum piping such as reactor primary cooling system piping and suppressing adhesion of radioactive ions to the inner surface. The present invention relates to a method for forming a film and an apparatus therefor.

[Conventional technology]

Impurity ions such as Co and Ni generated by corrosion of the inner surface of the water supply system of a nuclear power plant are irradiated with neutrons in the core along with insoluble iron oxides, and radioactive ions ( ⁶⁰ Co, ⁵⁸
Co ion). These radioactive ions accumulate in the pipes around the core, increasing the dose rate on the pipe surface,
It is the main cause of radiation exposure during regular inspections. Since the accumulation rate of radioactive ions on the inner surface of the pipe is proportional to the growth rate of the oxide film formed by corrosion, in order to further reduce the dose on the surface of the pipe, the inner surface of the structural material, especially the stainless steel pipe, should be reduced. It is an important issue to form an oxide film having a large corrosion inhibiting effect in advance and maintain it stably. To solve this problem, Japanese Patent Laid-Open No. 59-12390 provides a method of preliminarily forming an oxide film by immersing a structural material in high temperature water containing dissolved oxygen. Further, in JP-A-59-65297, a method of providing a reference electrode and a cathode in high-temperature water and anodizing the structural material to accelerate and form an oxide film on the inner surface of the structural material to shorten the pre-oxidation time. Is provided.

[Problems to be solved by the invention]

In the natural immersion oxidation treatment in high temperature water which is the method of the above-mentioned conventional technique, it is possible to form an oxide film having a high anticorrosion effect.
It took 500 to 1000 hours, and there was a problem that the processing took too long. On the other hand, the method of anodizing in high-temperature water, which is the above method, shortens the time, but when this method is applied to the actual reactor primary cooling system piping, it is long and bent. It is necessary to insert the reference electrode and the cathode inside the pipe including the portion, which is not practical.

An object of the present invention is to easily form an oxide film having an excellent anticorrosion effect on the maximum inner surface of a pipe such as a reactor primary cooling system pipe,
Another object is to provide a method capable of forming in a short time.

[Means for solving problems]

The method for forming a corrosion-resistant oxide film of the present invention, when forming a corrosion-resistant oxide film on the inner surface of a closed loop pipe containing a corrosive liquid, electrically insulates the pipe into two parts,
In addition, the separated portion is sealed to be in a liquid-tight state, the separated two pipes are connected to an external power source to be polarized into a cathode and an anode, and an electrolytic solution is circulated in the pipe, thereby corrosion-resistant oxidation. It is characterized by forming a film.
A particularly effective method is characterized by forming an oxide film on the inner surface of the pipe polarized to the cathode by using a circulating electrolytic solution at 150 ° C. or higher as the electrolytic solution. or,
The apparatus of the present invention comprises an electrically insulating member for electrically insulating and separating a closed-loop pipe containing a corrosive liquid into two parts, and sealing the separated parts to make them liquid-tight, and the separated two parts. One of the pipes includes an external power source for polarizing the cathode and the anode, a flow device for circulating the electrolytic solution in the pipe, and a heating device for heating the electrolytic solution.

[Action]

Electrons (+) supplied from the outer surface of the pipe by the external power source are consumed at the contact portion on the inner surface of the pipe for ionization of the metal and oxygen generation (anode reaction) accompanying the electrolysis of water. The metal ions and dissolved oxygen ride on the flow of water inside the pipe and reach the inner surface of another pipe that is electrically insulated and separated, where a metal ion discharge (cathode reaction) occurs,
The emitted electrons (+) are returned to the external power source from the outer surface of the pipe. The surface potential of the portion that causes such a cathode reaction is lower than the natural immersion potential when no current is applied from the outside, and a cathode-polarized state is created. By the way, in the conventional method, when a metal is cathodically polarized in an electrolytic solution, a reduction reaction of an oxide occurs and the once formed oxide film disappears. However, according to the research conducted by the present inventors, in high-temperature water, particularly in water at 150 ° C. or higher, oxide film formation is remarkably observed under cathodic polarization, and the corrosion resistance of the formed film is higher than that when cathodic polarization is not performed. It was found that it was significantly improved. In the present invention, the dissolved oxygen generated in the sword reaction contributes to the formation of an oxide film under such cathodic polarization, and an oxide film having a large corrosion inhibiting effect can be continuously formed.

As described above, it becomes possible to form an oxide film having a large corrosion inhibiting effect on the inner surface of the pipe by taking in and out the electric current from the outer surface of the pipe.

[Examples] Examples of the present invention will be described below with reference to FIG. FIG. 1 is a diagram showing a basic principle and a device configuration in the case of implementation. In FIG. 1, 1 is a polarized metal pipe for forming a corrosion resistant oxide film (hereinafter referred to as main pipe), 2 is an insulating pipe, and 3 is an insulating pipe electrically insulated from the main pipe 1 by insulating pipes at both ends. The counter electrode pipe 4 is an electrolytic solution, 5 is a flow device for circulating the electrolytic solution 4 in a closed loop composed of the main pipe 1, the insulating pipe 2, and the counter electrode pipe 3, 6 is a heating device for the electrolytic solution 4, 7 is a potential inside the main pipe 1 and the counter electrode pipe 3
An external power source for mutually changing the potential on the inner surface of the, and a lead wire 8 for electrically connecting the external power source 7 to the main pipe 1 and the counter electrode pipe 3. By using the apparatus shown in FIG. 1, it is possible to accelerate the formation of an oxide film having excellent corrosion resistance on the inner surface of the main pipe 1 by the following procedure. First, the electrolytic solution 4 in the closed loop is made to flow in one direction using the flow device 5. The flow rate is set to be sufficiently higher than the mobility of ions existing in the electrolytic solution. After that, the external power supply device 7 relatively polarizes the potential on the inner surface of the radiation pipe 3 and the potential on the inner surface of the main pipe 1. In such a polarized state, the temperature of the electrolytic solution is raised by the heating device 6, and then the electrolytic solution is caused to flow in the closed loop for a certain period of time, whereby an oxide film having excellent corrosion resistance is acceleratedly formed on the inner surface of the main pipe.

Hereinafter, more specific examples according to the present invention and their effects will be shown. In this embodiment, stainless steel (SUS304) was used as both the main pipe and the counter electrode pipe in FIG. As the electrolytic solution, high temperature pure water of 150 ° C. or higher was used. Furthermore, as an external power source, a DC constant current power source (galvanostat)
Was used. Figures 2 and 3 show 200 ℃ containing 200 ppb of dissolved oxygen.
2 shows the time change of the corrosion rate of the inner surface of the main pipe when it was continuously treated for 50 hours in the high temperature pure water. In the figure, for comparison, a case where the oxidation treatment is performed naturally without applying an external current is also shown. As is clear from Fig. 2, when the main pipe is polarized so that it has a negative potential relative to the counter pipe (cathode polarization), a minute current of about 1 µA / cm ² is supplied from the external power supply. By doing so, it is possible to form a film having a corrosion inhibiting effect that is about twice as large as that in the case of naturally performing the immersion oxidation treatment in a short time of about 10 hours.

On the other hand, as is clear from FIG. 3, when the main pipe is polarized so that it has a positive potential relative to the counter electrode (anode polarization), a current of about 100 μA / cm ² is supplied from the external power supply. By supplying it, it is possible to accelerate the formation of a film having a large corrosion inhibiting effect. However, in this case, as will be described below (FIG. 4), unless the surface area of the counter electrode piping is made sufficiently larger than the surface area of the main piping, the cathodic polarized counter electrode piping is significantly corroded due to the high current density. As a result, it lacks practicality.

FIG. 4 shows the relationship between the corrosion rate and the cathodic polarization current density when treated in high temperature pure water at 200 ° C. for 24 hours under cathodic polarization. In treatment effect 1~100μA / cm ² is maximized, the hydrogen evolution occurs 200 .mu.A / cm ² or more, in order to oxide film formation is inhibited, the processing effect is remarkably lowered.

FIG. 5 shows the influence of the temperature of the treated water on the treatment effect. 100 μA / c when no external current is applied
Corrosion rates after 24 hours of treatment are shown for anodic polarization at m ² and cathodic polarization at 1 μA / cm ² . In the treatment under anodic polarization, the treatment effect becomes maximum at 200 to 250 ° C, and decreases at higher temperatures. On the other hand, in the treatment under cathodic polarization, the treatment effect becomes remarkable from around 150 ° C., and the treatment effect increases thereafter as the temperature rises.

As described above, according to the present invention, it is possible to accelerate the formation of an oxide film having a high corrosion inhibiting effect regardless of the polarization state under the cathode polarization or the anode polarization. In particular, the treatment under cathodic polarization requires about 1/10 of the current density required to maintain the polarization state compared to the treatment under anodic polarization.
The practicability is higher because it is as low as 0 and the fluctuation of the treatment effect with respect to the treatment temperature is small.

FIG. 6 shows the relationship between the corrosion rate and the dissolved oxygen concentration when treated in high-temperature pure water at 200 ° C. under cathodic polarization current density of 1 μA / cm ² for 24 hours under cathodic polarization. The treatment effect becomes remarkable when the dissolved oxygen concentration is 20 ppb or more, and thereafter, the treatment effect also increases as the dissolved oxygen concentration increases.

In each of the above-mentioned examples, oxygen was supplied from the outside to maintain the dissolved oxygen concentration in the high temperature water at a constant value. However, in the case of performing the oxidation treatment under cathodic polarization, the oxygen supply from the outside does not occur. It is not necessary, but it is also possible for the necessary oxygen to be self-generated in the loop.

Fig. 7 shows the counter electrode when the inner surface area of the main pipe made of SUS304 is kept constant at 1 m ² and the cathode current density is 1 μA / cm ² , and the inner surface area of the counter pipe made of SUS304 is reduced from 1 m ² to 10 cm ^2. Change the amount of oxygen generated from the inner surface of the pipe at 200 ° C in a closed loop
The flow rate of the high temperature pure water is shown as a parameter. As shown in FIG. 7, when the inner surface area ratio of the counter electrode pipe to the main pipe is sufficiently large, oxygen is likely to be generated. The value of the surface area ratio at which oxygen is generated becomes smaller as the flow velocity of the hot water becomes smaller. Therefore, when setting the cathode current density between the external power supply and the main pipe to the optimum value (for example, 1 μA / cm ² ), the ratio of the inner surface area of the counter pipe to the main pipe or the circulation speed of high-temperature water is adjusted. This allows the required amount of oxygen to be self-generated in the loop.

In the above examples, stainless steel was used as the main piping material, but Fe-based and Cr that cause passivation other than stainless steel
It is possible to efficiently form an oxide film having excellent corrosion resistance even on metal pipe materials such as Ni-based and Ni-based alloys. Also, although the main pipe and the counter electrode pipe are made of the same material, the same effect is exhibited even when the two are different.

When the main pipe is subjected to cathodic polarization treatment, metal ions eluted from the counter electrode pipe material adhere to the main pipe surface as oxides. When such a main pipe is naturally dipped thereafter, the adhered oxide is redissolved, so that the high temperature water contains metal ions different from those of the main pipe material. Considering this point, it is more practical to use the metal material contained in the main piping material for the counter electrode piping.

Further, by changing the polarization characteristics, that is, by repeating the cathode polarization process and the anode polarization process alternately, it is possible to prevent metal ions eluted from the counter electrode pipe material from adhering to the main pipe surface. For example, by performing the anode polarization treatment after the cathode polarization treatment, only the oxide attached to the inner surface of the main pipe is selectively dissolved.
Can be removed.

Figure 8 shows the effect of using SUS304 for the main pipe and carbon steel for the counter electrode pipe. 200 ℃, dissolved oxygen concentration 2
Cathodic polarization current density 1μA / cm ² in high temperature pure water of 00ppb
After 24 hours of treatment, the polarization state of the main pipe and the counter electrode pipe is reversed, and the anode polarization treatment is performed at the same current density, which further reduces the corrosion rate and improves the corrosion inhibition effect. This corresponds to a decrease in the surface area of the adhered oxide due to dissolution and disappearance of the adhered oxide. However, if the current density during anodic polarization treatment is extremely high, the adhered oxide will dissolve and disappear quickly, but after that, the corrosion-resistant oxide film formed on the main piping itself during the cathodic polarization treatment will break (minor cracks will occur). ), The corrosion resistance decreases, and the corrosion rate increases again.

FIG. 9 shows the effect when nickel is used for the counter electrode piping. In the case of nickel, unlike the case of carbon steel, when the current density during anodic polarization treatment is as high as 100 μA / cm ^2, it becomes even smaller than 1 μA / cm ² .
The corrosion inhibition effect is improved. Generally, when a metal material having an ionic radius smaller than iron ions, for example, nickel, in addition to nickel, is used for the counter electrode piping, even if the current density during anodic polarization treatment is high, The corrosion-resistant oxide film formed on the main pipe itself during the cathodic polarization process is less likely to be destroyed, and rather, corrosion inhibition is improved.

FIG. 10 shows a concrete example in which the present invention is applied to a feed water heater for a primary boiling water reactor. 10th
In the reactor primary system flow shown in the figure, 9 is a reactor,
10 is a high pressure turbine, 11 is a low pressure turbine, 12 is a condenser, 13
Is a condensate pump, 14 is a demineralizer, 15 is a low-pressure feed water heater, 16
Is a water supply pump, 17 is a high-pressure water supply heater, 18 is a flange for connecting a counter electrode pipe, 19 is an insulating packing, 20 is a galvanostat, 21 is a valve, and 22 is an existing water supply recirculation line.
In the present embodiment, the high-pressure feed water heater 17, which is a main source of corrosion products brought into the furnace from the feed water system at a feed water temperature of 150 ° C. or higher, is targeted for the surface treatment according to the present invention. Water feed pump for stainless steel counter electrode piping 3
The upstream side of 16 and the water supply recirculation line 22 are connected in a form of bypass to form a closed loop. By using electrically insulating sheet packing such as asbestos or electrically insulating O-ring packing 19 such as Teflon as the packing material for the connection flange 18, the high-pressure feed water heater 17 and the counter electrode pipe 3 are electrically connected. Insulate and separate. Using the galvanostat 20, with the high-pressure water heater and the counter electrode piping relatively polarized, operate the water pump 16,
Circulate and raise the temperature of pure water in the loop. Current density 1μA / cm
^{When the} high-pressure water heater part (material: stainless steel, total internal surface area: approx. 1 × 10 ⁴ m ² ) is cathodically polarized in ² , the total current supplied from the galvanostat is approx. 100A. This current is sufficiently smaller than the inner surface area of the high-pressure feed water heater in the inner surface area of the counter electrode piping (1/100 to 1/1000).
By doing so, most of the anode-polarized counter electrode piping is used for oxygen generation by electrolysis of water. The amount of oxygen generated at this time is about 8 mg / s, and the water in the closed loop is 80 kg / s.
When circulating at a flow rate of s (about 5% of the feed water flow during plant operation), the dissolved oxygen concentration in high-temperature water becomes about 100 ppb, and it is possible to maintain the optimum dissolved oxygen concentration level without supplying oxygen from the outside. It is possible.

FIG. 11 shows a concrete example in which the present invention is applied to a recirculation pipe of a boiling water reactor recirculation system.
In this example, radioactive cobalt ions ( ⁶⁰ Co, ⁵⁸ C) generated by the activation of corrosion products brought into the reactor from the water supply system were used.
(Ions) are taken into the piping due to the corrosion of the recirculation system and increase the dose rate on the surface of the pipe, which is the main cause of radiation exposure of workers during regular inspections. The pipe was the object of the surface treatment according to the present invention. 11th
In the figure, 23 is a recirculation system pipe, 24 is a recirculation pump, and 25 is a recirculation pump.
Is a reactor water purification system pipe, 26 is a reactor water purification system pump, and 27 is a reactor water purification device. By bypassing the reactor water purification device 27 from the downstream side of the reactor water purification system pump 26 and immersing the counter electrode pipe in the reactor water purification system pipe 25, a closed loop connecting the reactor and the recirculation system pipe is formed. Similar to the above embodiment, by the galvanostat, in the recirculation pump 24 and the reactor water purification system pump 26 in a state in which the recirculation pipe and the counter electrode pipe are relatively polarized,
The pure water in the closed loop is circulated and the temperature is raised. Current density 10
When the recirculation system piping part (material: stainless steel, total internal surface area: about 150 m ² ) is cathodic polarized with μA / cm ² ,
The total current supplied from the galvanostat is about 15A. At this time, the amount of oxygen generated from the anode-polarized counter electrode piping is about 1 mg / s, and the water in the closed loop is 10 kg / s.
When it is circulated at a flow rate of (about 30% of the reactor water purification system flow rate), the dissolved oxygen concentration in high-temperature water becomes about 100 ppb, and it is possible to maintain the optimum dissolved oxygen concentration level without supplying oxygen from the outside. Is. In addition, the recirculation pump 24 is operated and the flow velocity in the recirculation pipe is 1 m / s (about 1/10 of that during normal operation).
By maintaining the above, it is possible to prevent the cathode current supplied from the galvanostat from being consumed on the metal surface in the furnace vessel, and to selectively surface-treat only the recirculation piping portion.

In the above examples, high temperature pure water was used as the electrolytic solution,
The same effect can be obtained by adding an electrolytic substance such as an acidic, neutral or basic salt to pure water. However, generally, when an electrolyte is added, the corrosion inhibiting effect of the oxide film formed is reduced and the susceptibility to stress corrosion cracking (SCC) increases.

In the above-mentioned embodiment for the actual machine, all of the existing piping,
Cathodic polarization or anodic polarization treatment was carried out by newly installing a counter electrode pipe.
Electrically insulate the high pressure water heater from the water supply pipe,
By electrically insulating the recirculation system piping from the reactor vessel and the water supply piping, the primary system water supply piping itself is used instead of the counter electrode piping, and the high-pressure water heater and recirculation system piping are polarized. It is also possible to carry out the surface treatment with.

[Effect of the Invention] According to the present invention, an oxide film having the same corrosion inhibition performance as that of an oxide film formed by natural immersion in high temperature water for about 1000 hours can be obtained in about 1/50 to 1/100 time. Accelerated formation. As a result, the corrosion products brought into the core due to the corrosion of the structural materials at the beginning of nuclear power plant operation, and radioactive ions due to the activation of the corrosion products brought into the core caused corrosion of the piping around the core. It is possible to suppress the increase of the pipe surface dose rate at the same time,
It is possible to effectively reduce the radiation exposure amount of the worker at the time of regular inspection.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a basic principle and an apparatus configuration for carrying out the present invention, and FIGS. 2 and 3 are corrosion rate (corrosion resistance performance) and treatment time of the present invention. 4 is a graph showing the relationship between the corrosion rate and the cathodic polarization current density, FIG. 5 is a graph showing the relationship between the corrosion rate and the treated water temperature, and FIG. 6 is the corrosion rate and dissolved oxygen. FIG. 7 is a graph showing the relationship with the concentration, and FIG. 7 is a graph showing the relationship between the amount of oxygen generated, the flow rate of treated water and the ratio of the surface area of the cathode polarized metal pipe to the surface area of the anode polarized metal pipe when the present invention is carried out. 8 and 9 are graphs showing the relationship between the corrosion rate and the treatment time when the cathodic polarization treatment and the anodic polarization treatment of the present invention are alternately repeated, and FIG. Of the plant when applied to the inner surface treatment of water heaters Shows the integrated, FIG. 11 is a diagram showing a system of the plant in the case of applying the present invention to the pipe inner surface treatment of the reactor recirculation system. 1 ... Polarized piping for forming a corrosion resistant oxide film, 2
...... Insulation tube, 3 ... Counter electrode piping, 4 ... Electrolyte solution, 5 ... Flow device, 6 ... Heating device, 7 ... External power source, 8 ... Conducting wire, 9 ... Reactor, 10 ... High pressure Turbine, 11 ...... Low pressure turbine, 12 ...... Condenser, 13 ...... Condensation pump, 14 ...... Demineralizer, 15 …… Low pressure water heater, 16 …… Water pump, 17 ……
High-pressure water heater, 18 ... Flange for connecting counter electrode piping, 19 ...
… Insulating packing, 20 …… Galvanostat, 21 ……
Valve, 22 …… Water supply recirculation line, 23 …… Recirculation system piping, 24
...... Recirculation pump, 25 …… Reactor water purification system piping, 26 …… Reactor water purification system pump, 27 …… Reactor water purification device.

Front Page Continuation (72) Inventor Katsumi Okazumi 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi Factory (72) Inventor Motohiro Aizawa 3-2-1, Saiwaicho, Hitachi, Ibaraki Prefecture Within Hitachi Engineering Co., Ltd.

Claims (8)

[Claims] 1. When forming a corrosion resistant oxide film on the inner surface of a closed loop pipe containing a corrosive liquid, the pipe is electrically insulated and separated into two parts, and the separated part is sealed to be in a liquid tight state. The two separated pipes are connected to an external power source to be polarized into a cathode and an anode, and an electrolytic solution is circulated in the pipe to form a corrosion-resistant oxide film on the inner surface of the pipe. A method for forming a corrosion resistant oxide film. 2. The oxide film is formed on the inner surface of the pipe polarized to the cathode by using an electrolytic solution circulating at 150 ° C. or higher as the electrolytic solution. A method of forming a corrosion resistant oxide film on the inner surface of piping. 3. An electrolytic solution is circulated in the loop at a flow rate faster than the mobility of ions in the electrolytic solution, and corrosion resistant oxidation is caused on the inner surface of the pipe according to claim 1 or 2. A method of forming a film. 4. Of the two pipes electrically isolated from each other,
The surface potential of one of the pipes is made higher than the oxygen generation potential, and the surface potential of the other pipe is polarized so as to be a potential lower than the natural immersion potential. 4. A method for forming a corrosion resistant oxide film on the inner surface of a pipe according to any one of 3 above. 5. Of the pipes insulated and separated into two regions,
One is a piping area having a large surface area, the other area is insulated and separated so as to be a piping area having a smaller surface area than the piping of the area, the large surface area side pipe is a cathode, and the small surface area pipe is The method for forming a corrosion-resistant oxide film on the inner surface of a pipe according to claim 4, characterized in that polarization is performed on the anode. 6. The pipe inner surface according to claim 4, wherein a pipe containing a metal element having an ionic radius smaller than that of divalent iron ions is used as a pipe material in a region where the oxygen generation potential is made higher. A method for forming a corrosion-resistant oxide film on a. 7. After the completion of the oxide film formation treatment, the polarization characteristics of the two regions are reversed and an electric current is applied to remove the reduced precipitates adhering to the surface of the oxide film. A method for forming a corrosion-resistant oxide film on the inner surface of the pipe according to claim 4. 8. An electrically insulating member for electrically insulating and separating a closed-loop pipe containing a corrosion resistant liquid into two parts, and sealing the separated part into a liquid-tight state, and the two separated pipes. An external power source for polarizing a cathode and an anode, a flow device for circulating an electrolytic solution in the pipe, and a heating device for heating the electrolytic solution. A device that forms a film.