JPH02236291A - Method and device for preventing stress corrosion crack and hydrogen crack of metallic member - Google Patents

Abstract

PURPOSE:To prevent the stress corrosion crack and hydrogen crack of metallic members in contact with a corrosive soln. and to improve the safety and life of in-plant apparatus by holding the boundary potential of the above-mentioned metallic members within a specific range. CONSTITUTION:Zircaloy fuel cladding pipes 31 of a fuel assembly 30 of a boiling water reactor plant, with which pipes the hydrogen crack is anticipated and stainless steel pipes 32 to serve as reference electrodes are electrically insulated via 'Teflon(R) spacers 33 and terminals 12b are connected to the cladding pipes 31 and terminals 12a to the pipings 32 in the case of the above-mentioned assembly 30. The potential thereof is introduced to a potential controller 11 to maintain the potential baser than the potential at which the stress corrosion crack is no longer generated in the cladding pipes 31 and to maintain the potential nobler than the potential at which the hydrogen crack is no longer generated.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method and apparatus for preventing stress corrosion cracking and hydrogen cracking in metal members, and particularly to a method and apparatus for preventing stress corrosion cracking and hydrogen cracking in metal members of nuclear power plants. Regarding the device.

[Conventional technology]

Stress corrosion cracking and hydrogen cracking are fractures that occur in metallic materials after a certain period of time due to the interaction of stress and electrochemical reactions in a corrosive environment. When the fracture is mainly due to metal dissolution due to the anodic reaction at the metal interface, it is called stress corrosion cracking, and when the fracture is mainly due to hydrogen generated by corrosion due to the cathodic reaction, it is called hydrogen cracking.

Conventionally, it has been necessary to prevent stress corrosion cracking (hereinafter referred to as SCC) and hydrogen cracking of metal members placed in a corrosive environment. Particularly in nuclear power plants, it is necessary to prevent this cracking in order to ensure plant safety.

Conventionally, the following methods exist to prevent stress corrosion cracking.

As a method for preventing SCC in reactor internal structural materials of nuclear power plants,
JP-A-57-70499, JP-A-57-3086
As shown in the publication, the corrosion potential of metal materials is -2
A corrosion prevention method is known in which the dissolved oxygen concentration is controlled to be below 50 mVsH+.

There is also a method of injecting hydrazine or the like to create a reducing atmosphere in the environment surrounding the metal material. These various conventional examples are methods for controlling SCC, and the former conventional example maintains the corrosion potential of the metal material on the base side. In the latter, the corrosion potential is maintained on the less noble side by adding hydrazine or the like to reduce the dissolved oxygen concentration to 10 ppb or less, which corresponds to complete aspiration.

Conventional methods for preventing hydrogen cracking include measures to prevent hydrogen cracking (hydrogen embrittlement) in chemical plants using high-strength steel and the like. That is, this is a method in which N-CoCo·β-aminopropionic acid is added as an inhibitor to suppress hydrogen cracking. It is generally known that adding such drugs as inhibitors is effective in suppressing hydrogen cracking. On the other hand, a method using three electrodes, a counter electrode, a reference electrode, and a working electrode to be protected against corrosion, is generally well known as a method for cathodic protection of metal materials in thermal power plants. This is mainly aimed at preventing general corrosion, but does not prevent localized corrosion such as stress corrosion cracking.

[Problem to be solved by the invention]

The above inhibitors used in chemical plants to prevent hydrogen cracking may decompose due to the effects of radiation when used in nuclear plants, and various organic substances that are decomposition products may appear in the reactor water. On the contrary, it has the contradictory aspect of being an inhibitor that tends to promote corrosion of structures.

Furthermore, in conventional examples for preventing SCC, there has been concern about hydrogen absorption on the surface of the metal member. In other words, when the dissolved oxygen concentration in the solution containing the metal material decreases due to hydrogen injection, the anions of the metal react with the oxygen in the water, increasing the amount of hydrogen, and as the corrosion potential becomes more base, corrosion increases. As a result, the amount of hydrogen generated increases. Therefore, in the conventional example that attempts to prevent SCC, from the viewpoint of preventing hydrogen cracking, it actually accelerates hydrogen cracking.

Therefore, in the conventional example described above, it was not possible to prevent hydrogen cracking while preventing SCC of the metal member.

SUMMARY OF THE INVENTION In order to solve these problems, it is an object of the present invention to provide a method and apparatus capable of preventing hydrogen cracking while preventing stress corrosion cracking of metal members.

[Means to solve the problem]

In order to achieve the above object, the method for preventing stress corrosion cracking and hydrogen cracking of a metal member according to the present invention reduces the interfacial potential of a metal member made of a corrosive alloy in contact with a corrosive solution to prevent stress corrosion cracking from occurring in the metal member. Keep the potential less base than the potential that disappears,
This method also maintains the potential at a higher potential than that at which hydrogen cracking will no longer occur. When the metal member is SUS304 stainless steel, the interfacial potential is set to -100 to -1000 mVso.
In the p(7) range, -1 for SUS316 stainless steel
It is effective to maintain it in the range of 0 to -1000 mVsHa, and for Inconel 600, in the range of O to -1000 mVsH2.

A device for preventing stress corrosion cracking and hydrogen cracking of metal members includes a metal member made of a corrosion-resistant alloy that is in contact with a corrosive solution, and a pair of terminals with at least one insulated.
A current flowing through the corrosive solution between the terminals is controlled to set the interfacial potential of the metal member to a potential that is less base than a potential at which stress corrosion cracking will not occur in the metal member, and a potential that is less than a potential at which hydrogen cracking will not occur. This device is equipped with a constant current electrolyzer that maintains a high potential.

This device for preventing stress corrosion cracking and hydrogen cracking of metal members is applicable when the metal member is a metal member inside a nuclear reactor, when it is a reactor primary piping system, or when it is a main steam system piping for a boiler. used effectively.

In addition, in this device for preventing stress corrosion cracking and hydrogen cracking of metal members, the metal members in the nuclear reactor include a B4C tube made of SUS304 filled with powder of control material B4C that controls the output of the reactor, and The B4C control rod consists of a control rod sheath with a tube disposed therein, and an insulating Teflon tube inserted between the control rod sheath and the B4C tube. In case C, one of the pair of terminals from the constant current electrolyzer is connected to the B4G tube, and the other terminal is preferably used as the pressure vessel of the nuclear reactor. and when the metal member in the nuclear reactor is a Zircaloy fuel cladding tube loaded with fuel for the nuclear reactor, one of the pair of terminals is connected to the Zircaloy fuel cladding tube,
As the other terminal, it is preferable to use a stainless steel tube that covers the Zircaloy fuel cladding tube via an electrically insulating Teflon spacer. [Operation] In the crack prevention method configured as described above, in a corrosive environment such as a corrosive aqueous solution, the interfacial potential of a metal member made of a corrosion-resistant alloy is set to a potential lower than the potential at which stress corrosion cracking will not occur in the metal member. Since the surface of the metal member is maintained at
This prevents the occurrence of stress corrosion cracking and suppresses the generation of hydrogen, and also maintains the above-mentioned interfacial potential at a potential higher than the potential at which hydrogen cracking does not occur in metal parts, so hydrogen cracking does not occur. A stable passive film is formed on the anode side of the lower limit hydrogen concentration, that is, the potential that is the critical hydrogen concentration, to prevent hydrogen from penetrating into the metal, and to prevent corrosion because the metal surface environment is in an oxidized state. Even if hydrogen in the environment attempts to enter the metal member, it is immediately oxidized to harmless water molecules, preventing hydrogen cracking from occurring.

For the above reasons, the interfacial potential of the metal member is
For stainless steel, -100 to -1000 mVsHE, for SUS 3 1 6 stainless steel, -1
In the range of 0 to -1000 mVsHp, in the case of Inconel 6oO, O to -1000 mVs
HF! It becomes effective by keeping it within the range of .

In addition, the water splitting prevention device configured as described above has terminals connected to both ends of a region where stress corrosion cracking or hydrogen cracking of a metal member made of a corrosion-resistant alloy may occur in a corrosive environment. The potential between the terminals is controlled by a constant current electrolyzer to a potential that is base and a potential that does not cause stress corrosion cracking in metal members, and a potential that is nobler than the potential that does not cause hydrogen cracking. Hold.

〔Example〕

This example describes how metal members can This is a method of maintaining the interfacial potential at a potential lower than the potential at which stress corrosion cracking no longer occurs and at a potential more noble than the potential at which hydrogen cracking does not occur, and a device utilizing this method.

To maintain the potential in the specific potential range, electrolytic protection consisting of two electrodes, a counter electrode and a working electrode, is used. In other words, the potential of the metal surface of the metal parts or primary piping materials in the reactor that is in contact with water is on the positive side of the potential at which hydrogen cracking no longer occurs, and on the base side of the potential at which stress corrosion cracking does not occur. In addition, it forms an oxide film with excellent corrosion and cracking resistance on the metal surface. Here, the metal member is a corrosion-resistant alloy made of an alloy such as austenitic stainless steel, nickel-based alloy, or zircaloy, and is a metal that may cause stress corrosion cracking or hydrogen cracking.

More detailed information is as follows.

When the interfacial potential of the metal is anodically polarized, the metal surface becomes a passive region and a passive film is formed on the metal surface. In the passive region, the metal surface is difficult to dissolve in a solution, and in other words, a dense oxide film is formed on the metal surface.

A film in this state is called a passive film.

However, when the metal surface is in the passive region, internal or external stress may cause damage to the coating, which may lead to the occurrence of SCC. On the other hand, when the metal surface is significantly polarized from its natural potential toward the cathode side, hydrogen is generated, which enters the metal member and causes hydrogen brittleness, causing hydrogen cracking. Therefore, the present inventors conducted various studies on the potential and surface condition of metal surfaces, and found that
It focuses on the passive region. Here, the active region refers to a state in which the metal surface is easily melted, and refers to a state in which the metal base or oxide film itself is melted.

A passive film is formed on the metal surface in the passive region, and SCC can be prevented on the low potential side of this passive region. Further, in the passive region, even if a small amount of hydrogen is generated, hydrogen cracking does not occur.

Therefore, by maintaining the interfacial potential of the metal member in the passive region in this way, it is possible to prevent hydrogen cracking while preventing SCC. Moreover, a stable passive film exists in this passive region.

That is, the presence of an oxide film is generally effective in preventing hydrogen cracking because it prevents hydrogen from penetrating into the metal. Furthermore, even if hydrogen in the aqueous solution attempts to enter the metal, since the environment is in an oxidized state (anode), it is immediately oxidized to harmless water molecules, thereby preventing hydrogen cracking.

In other words, the potential of the metal surface is maintained so that the surface state of the metal member becomes a passive region, and this interfacial potential is
This method is to prevent hydrogen cracking in a metal member by maintaining the hydrogen concentration at the lower limit at which hydrogen cracking occurs, that is, to be closer to the anode than the potential at which the limit hydrogen concentration occurs.

Below, details of the embodiments of the present invention will be explained in Figures 1 to 6.
This will be explained using figures.

Example 1 In this example, in order to investigate the effect of suppressing hydrogen cracking due to cathodic polarization on the metal surface, the potential dependence of hydrogen cracking in SUS304 as an example of austenitic stainless steel was investigated. When measuring the potential, it is necessary to measure the reference electrode potential in advance in the system. Reference electrodes for measuring the reference potential include, in addition to silver monochloride electrodes and hydrogen electrodes, precious metals such as platinum, base metals such as iron and silver, stainless steel, nickel-based alloys, iron-based metals, and the like.

Stainless steel SUS30 subjected to solution treatment shown in Figure 1
4 SUS3 (JIS standard) when immersed in pure water at 288°C with a dissolved oxygen concentration of 0.01 ppm or less
The relationship between the corrosion form and potential of 04 steel is shown. The horizontal axis of the figure shows the time until the test piece breaks due to stress corrosion cracking or hydrogen cracking, and the vertical axis shows the potential of the metal surface, which is the reference electrode potential with respect to the hydrogen electrode: VS}IE. In the figure, an X mark indicates that the breakage occurred at that time, and an O mark indicates that the breakage did not occur by that time. Hydrogen cracking was observed in the potential range below -1000 mVsoE. Furthermore, SCC was observed in the potential range of O m VSHE or higher. However, -100
10 in the range of ~-1000 mVsop,
No cracks were observed even after 000 hours had passed.

When the interfacial potential of SUS304 is greater than O m VSHE, the surface of SUS304 is in the passive region, so SCC
occurs. On the other hand, if it is below -1100mVsoE, hydrogen will be generated exceeding the limit hydrogen concentration, so
Hydrogen cracking occurs. Therefore, according to this embodiment, SCC
It has become clear that by maintaining the potential in a specific potential region (hereinafter referred to as a transition region) in which neither hydrogen cracking nor hydrogen cracking occurs, it is possible to prevent the occurrence of hydrogen cracking while preventing SCC.

Example 2 Next, as another example of austenitic stainless steel,
The potential dependence of SCC and hydrogen cracking on SUS316 was investigated.

FIG. 2 shows the relationship between the corrosion form and potential of a solution-treated SUS 316 stainless steel material in pure water at 288° C. from which dissolved oxygen has been degassed. As is clear from the figure, −
Hydrogen cracking was observed in the potential range of 1100 mVsoE or lower, but no cracking was observed in the range of -100 to -1000 mVsoE even after 10,000 hours. Further, SCC was observed at QmVsHE or higher. Therefore, according to this embodiment, the potential range for preventing hydrogen cracking while preventing SCC is -100 to -1.
0 0 0 mVsHI:.

Example 3 Next, as an example of a nickel-based alloy, Inconel 600
The potential dependence of SCC and hydrogen cracking was investigated.

FIG. 3 shows the relationship between the corrosion form and potential of a solution-treated Inconel 600 material in deaerated pure water at 288°C. As is clear from the figure, hydrogen cracking was observed in the potential range below -1000 mVsHp;
No cracking was observed in the range of Vso+: even after 10,000 hours. Furthermore, SCC was observed at 100 mVsoE or higher. Therefore, according to this example, it was revealed that the potential range for preventing hydrogen cracking while preventing SCC is 0 to -900 mVsHa. Example 4 An example in which the method of the present invention is applied to a boiling water nuclear power plant will be described.

A boiling water reactor (hereinafter referred to as BWR) is a power reactor that generally uses boiling light water as a coolant, and the overall configuration of the BWR to which the present invention is applied is shown in FIG. Due to the operation of the recirculation pump 3, the cooling water absorbed from the reactor 1 is guided to the jet pump in the reactor 1 via the recirculation system piping 4, and the cooling water spouted from the jet pump 2 is , absorbs water from the upper part of the reactor and distributes it to the lower part of the reactor core.

This cooling water is heated in the fuel assembly housed in the reactor core 5 and becomes a steam-water mixture, which is separated by an upper steam-water separator 6, and the steam is separated from the main steam by passing through an upper steam dryer 7. It is guided to the system piping 8 and sent to the turbine. Note that the water in the upper part of the reactor that is absorbed by the jet pump 2 includes water supplied from the main water supply system piping 9, and hydrogen is injected into the water from the hydrogen injection device 10 as necessary. reactor 1
When adjusting the output of the reactor or stopping the reactor, the reactor core 5
The control rod 20 is inserted into the core 5 from below.

As shown in FIG. 5, the control rod 20 includes a control member B. A B4C tube 21, which is a thin SUS304 tube filled with C powder, is placed inside a cross-shaped SUS304 sheath 23. A Teflon tube 22 is inserted between the B4C tube 21 and the control sheath 23, and electrically insulates the B4C tube 21 and the control sheath 23.
To attach the terminals, attach one terminal 12 as shown in Figure 4.
ati - connected to the pressure vessel of the reactor 1, the other terminal 12
b is connected to the B4C tube 21 made of SUS304, which is concerned about hydrogen cracking, and its potential is led to the potential control device 11, and the potential between the terminals is controlled by this device.

In this embodiment, by measuring the corrosion potential of the control rod sheath 23 made of SUS304 in advance, the potential of the B4C tube 21 is adjusted to the preferable value described in the first embodiment using the potential of the control rod sheath 23 as a reference. -310 to -10
It can be held at the value of QmVsoE. Therefore, B
．． Hydrogen cracking can be prevented without causing SCC of the C tube 21.

Example 5 FIG. 6 shows an example in which the method of the present invention is applied to a fuel cladding tube of a BWR plant.

In this embodiment, the Zircaloy fuel cladding tube 31 of the fuel assembly 30 where hydrogen cracking is a concern and the stainless steel pipe 32 serving as the reference electrode are electrically insulated via a Teflon spacer 33, and one terminal 12b is connected to the fuel cladding tube 31, the other terminal 12a is connected to the stainless steel pipe 32, and its potential is guided to the potential control device 11. Fuel cladding tube 31
The potential can be maintained at a potential in the transition region by the potential control device 11.

In this example, nuclear heating was started while polarizing the fuel cladding tube 31, and after being maintained at 288° C. (steady operating temperature) for 100 hours, the temperature was lowered to room temperature.

Thereafter, the fuel cladding tube 31 was immediately taken out, and the amount of hydrogen absorbed into the fuel cladding tube was measured.

The results are shown in Table 1 in comparison with the results without polarization.

From this result, the polarized Zircaloy fuel cladding tube contains almost no hydrogen, but the water temperature is 288°C, DO is 200ppb, and the holding potential is -0.
It can be seen that the 2Vshe principle contains 20PPI11 of hydrogen. Therefore, by maintaining the fuel cladding tube 31 at a specific potential, it is possible to prevent hydrogen from penetrating into the base material.

As described in Examples 4 and 5 above, the present invention is effective in preventing hydrogen cracking in reactor internal structural materials of nuclear power plants. The same applies to thermal power plants. Of these power plants, the method of the present invention can be applied to selected parts where hydrogen cracking is particularly likely to occur. In the case of thermal power plants, it is effective for main steam system piping for boilers.

〔Effect of the invention〕

Since the present invention is configured as described above, it produces the effects described below.

Maintaining the interfacial potential of a metal component made of a corrosion-resistant alloy in contact with a corrosive solution at a potential less noble than the potential at which stress corrosion cracking will no longer occur in the metal member, and at a potential more noble than the potential at which hydrogen cracking will no longer occur. This suppresses the generation of hydrogen that accompanies corrosion of metal parts, and also prevents hydrogen in a corrosive environment from penetrating into metal parts.As a result, stress corrosion cracking is prevented while hydrogen cracking is prevented. It can be prevented.

Furthermore, if the metal member is SUS304 stainless steel, the interfacial potential is -100 to -1000 mVs+u:
-100 to -10 for S316 stainless steel
0 0 mVsHE, Inconel 600 has 0 to -
By maintaining each at 900 mVsoE, stress corrosion cracking and hydrogen cracking can be effectively prevented.

Then, the stress corrosion cracking and hydrogen cracking prevention device connects the terminals connected to both ends of the area where stress corrosion cracking or hydrogen cracking may occur, and the interfacial potential of the metal member through these terminals.
It is equipped with a potential control device that maintains the potential at a potential lower than the potential at which stress corrosion cracking does not occur in the metal member and more noble than the potential at which hydrogen cracking does not occur, making it easy to control the interfacial potential of the metal member. can be controlled and maintained within the appropriate potential ranges mentioned above. Furthermore, the above effects can be obtained even when this device is applied to metal members in a nuclear reactor, reactor primary system piping, and main steam piping for a boiler.

When the method and device of the present invention are applied to various plants,
This improves the safety and lifespan of equipment within the plant, and prevents unexpected accidents in the plant.

[Brief explanation of drawings]

Figure 1 shows the potential dependence of stress corrosion cracking and hydrogen cracking in stainless steel T[sUs304, Figure 2 shows the potential dependence of stress corrosion cracking and hydrogen cracking in stainless steel IIIsUs 3 1 6, and Figure 3 4 is a diagram showing the potential dependence of stress corrosion cracking and hydrogen cracking in Inconel 600, which is a nickel-based alloy, and FIG.
This figure is an overall perspective view of the control shaft used in the BWR nuclear reactor shown in FIG. 4, and FIG. 6 is a perspective view showing an embodiment in which the device of the present invention is used in a fuel assembly of a BWR type nuclear reactor. DESCRIPTION OF SYMBOLS 1... Nuclear reactor, 11... Potential control device, 12... Terminal, 20... Control rod, 21... B4C tube, 22... Teflon tube, 23... Control rod sheath, 31...Fuel cladding tube, 32...Stainless steel tube for counter electrode, 33...Teflon baser

Claims (1)

[Claims] 1. Maintaining the interfacial potential of a metal member made of a corrosive alloy in contact with a corrosive solution at a potential lower than the potential at which stress corrosion cracking will not occur in the metal member, and hydrogen cracking will no longer occur. A method for preventing stress corrosion cracking and hydrogen cracking of metal members by maintaining them at a potential nobler than the electric potential. 2. A metal member made of a corrosion-resistant alloy that is in contact with a corrosive solution has a pair of terminals, at least one of which is insulated, and a current flowing between the terminals through the corrosive solution is controlled to adjust the interfacial potential of the metal member. A device for preventing stress corrosion cracking and hydrogen cracking of metal members, comprising a constant current electrolyzer that maintains the potential at a potential lower than that at which stress corrosion cracking does not occur and more noble than a potential at which hydrogen cracking does not occur.