JPS5983800A - Dissolution of iron oxide adherent on surface - Google Patents

Abstract

PURPOSE:To effectively and selectively dissolve away an iron oxide film only adherent onto piping or the like, by circulating a liquid electrolyte containing a complexing agent between the piping or the like for supplying water and a long anode provided in said piping or the like, and applying DC between the piping or the like and the anode. CONSTITUTION:In decontaminating piping 1, the valve of said piping 1 in the vicinity of a part connecting both ends of a recirculating system 2 is closed, and a decontaminating liquid, i.e. a predetermined liquid electrolyte, in a reservoir tank 4 is deaerated and circulated through the interior of the piping 1. As said liquid electrolyte, the solution of a complexing agent such as EDTA is suitable, and it is used by adding a reducing agent to it at need. Thereafter, while circulating said electrolyte, DC is let flow from a DC power source 8 between the piping 1 as a cathode 7 and a long anode provided inside the piping 1 in a manner such that anode potential is lower than the corrosion potential of the matrix metal of the piping 1. Hence, without dissolving the matrix material of the piping 1, an iron oxide film only at its inner surface is dissolved as iron ion and separately captured with an ion-recovering device 3 containing an adsorbent in it. Thereafter, the decontaminating liquid is returned to the reservoir tank 4 and recirculated.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a method for dissolving an iron oxide film that has adhered or accumulated on the inner surface of piping through which water is passed, and in particular to a method for dissolving iron oxide coatings that have adhered or accumulated on the inner surface of piping through which water is passed, and in particular to a method for dissolving an iron oxide film that has adhered or accumulated on the inner surface of piping through which water is passed. This invention relates to a method suitable for dissolving and sweating a radioactive ferric acid film attached to and deposited on the inner surface of a pipe system.

[Prior art]

Self-contained pipes used for primary cooling water in nuclear power plants.

Radioactive acid skin on the inner surface of valve 1 equipment, etc.! power;
This is one of the causes of high surface dose rates in plants, so it is worth removing it.

The decontamination technology used to remove this film has not yet been proven in domestic plants, and is currently being used in Canadian plants.

flJ 75 carried out at an American nuclear power plant: It's too powerful. The difficult point of this decontamination technology is that it does not dissolve the carbon steel or stainless steel, which is the base material of the tubes and equipment, and the Unfortunately, only the film of the
In addition to requiring a decontamination method suitable for this purpose, it is also necessary to consider the impact of residual decontamination agents on the base material.

Decontamination methods include a proven chemical decontamination method and an electrochemical decontamination iron oxide film dissolution method (Japanese Patent Application No. 57-94407), which was previously filed by the present applicant. The chemical decontamination method uses a decontamination agent mixed with an acid, a reducing agent, a complexing agent, and an inhibitor selected in consideration of the characteristics of the iron oxide film. Although this method is superior in terms of the dissolution rate of the coating, there is a risk that the base material will also be dissolved and there is a risk of corrosion due to the residual liquid. On the other hand, electrochemical methods can be roughly divided into two types.

One is a cathode polarization method, and the other is a method in which electrons are injected into an oxide using an electrolyte with enhanced reducing power.

The former method involves polarizing the iron oxide film with a counter electrode in order to adjust its potential, but this method requires a counter electrode facing the film to be dissolved, so it does not require large-scale decontamination or multi-H piping. Difficult to decontaminate the system. The latter electron injection method is, in principle, an excellent method that enables selective dissolution of only the iron oxide film, but it is limited by the electrolytic bath that strengthens the reducing power and its cathode material, and the stability of decontamination performance and Reliability remains an issue.

[Purpose of the invention]

Compared to these existing methods or methods under development, the present invention provides a protective effect by arranging a long anode inside a pipe, etc. without dissolving the base material over a predetermined length of the pipe, etc., and effectively forming an iron oxide coating. Provides a stable and reliable method for dissolving iron oxide films on the inner surface of piping, etc., using a long anode that can selectively dissolve iron oxide films on the inner surface of piping, etc., and that can deformably adapt to bends in the target piping. The purpose is to

[Summary of the invention]

The components of the iron oxide film that adheres to the base material piping 1 equipment are generally magnetite (Fe3O4) and hematite (α-Fe
203). The dissolution mechanism involving electrons between these oxides and the base material carbon steel or stainless steel is explained by the following three equations.

Fe +Fe, +2e...
・・・・・・・・・(1) Fe304 +8H+2e
−+ 3Fe +4H20−(2)Fe203 +
6H++ 2'e-→2Fe +3H20- (3) That is, in iron (base material), an oxidation reaction that releases electrons proceeds as shown by the formula (]). On the other hand, in magnetite and hematite, protons (H
+) and electrons are supplied, and the reduction reaction proceeds.

Taking advantage of the fact that iron oxide undergoes a reduction reaction,
If a current is passed under a potential lower than the potential at which the base material corrodes (natural potential) so that a cathode reaction occurs in the base material that has an iron oxide film, the iron oxide film will dissolve without dissolving the base metal iron ions. do. In the case of nuclear power plants, the radioactive iron oxide film is often extremely thin, on the order of microns, so it is only necessary to apply a small amount of current to dissolve it.

The present invention is based on the above-mentioned idea in principle, and is characterized by arranging a long anode extending in the longitudinal direction inside the pipe, etc. from which the iron oxide film on the inner surface is to be removed. By circulating an electrolytic solution containing a complexing agent and using a pipe as a cathode, a direct current is passed between the cathode and the long anode at a cathode potential lower than the corrosion potential of the base metal of the pipe. The iron oxide film is dissolved by applying cathodic potential over a predetermined length of piping, etc.6, and a complexing agent solution such as EDTA solution, which can mask Fe2+ ions during dissolution, is used as the electrolyte.

A reducing agent may be further added to this electrolytic solution if necessary, and it is preferable to use a sufficiently degassed solution so that the reduction reaction of dissolved oxygen in the solution does not occur when the reduction reaction occurs. It's small. The elongated anode is preferably an insoluble anode material, such as platinum wire or carbon fiber, covered with an insulating material that allows liquid to pass through, such as a mesh rubber or synthetic resin, and the elongated anode is made of It is too complicated to be deformable while maintaining a certain degree of rigidity, and it can be inserted into piping for a long time by adapting to the bends of piping, etc. as much as possible.

[Embodiments of the invention]

An example of the iron oxide film removal flow for carrying out the method of the present invention is schematically shown in FIG. 1. A system 2 for recirculating decontamination liquid is connected to piping 1 of a plant whose iron oxide film is to be removed. Between both ends of the recirculation system 2 there is an ion collector 3. Storage tank equipped with a heating source and a diffuser for deaeration4. A liquid feeding pump 5 is arranged.

Inside the pipe 1, a deformable elongated anode 6 extending in the longitudinal direction is arranged, the shape of which can be arbitrarily bent according to the bending of the limb canal. The anode is made of an insoluble anode material such as platinum wire or carbon fiber coated with an insulating and liquid-permeable mesh rubber or synthetic resin, for example by providing suitable flexible legs. It is desirable to arrange them so as to maintain as equal a distance as possible from the inner surface of the pipe 1 to be decontaminated. When decontaminating piping 1, recirculation system 20
Close the valves shown on the piping 1 near the connections at both ends, put a predetermined electrolyte solution as a decontamination liquid into the storage tank 4, and degas it by bubbling with inert gas to lower the dissolved oxygen and at the same time raise the temperature to a predetermined temperature. It is heated and circulated through the pipe 1 using a bonder 5. A complexing agent solution such as EDTA is suitable as the electrolytic solution, and a reducing agent is added thereto as necessary. While circulating the electrolyte in this way, connect the pipe 1 to the cathode 7.
A DC current is passed between this and the anode 6 from a DC current component 8 so that the cathode potential is lower than the corrosion potential (natural potential) of the base metal of the pipe 1.

In this case, the base material of the pipe 1 is not dissolved, and only the iron oxide coating on the inner surface thereof is dissolved as iron-indicating ions. The iron ions dissolved in this liquid are captured and removed by an ion collector 3 containing an adsorbent, and the subsequent decontamination liquid is returned to the storage tank 4 and recirculated.

Next, an experimental example showing the effectiveness of the method of the present invention will be explained.

First, the potential dependence of the dissolution of iron oxide and the base metal was investigated through dissolution experiments. The specimens tested were
The base material is a carbon steel plate, and the iron oxide is a sintered body of magnetite and hematite powder. The sintered body was made by sintering around 1100 tll of each powder, which was attached to a SUS 304 stainless steel substrate with conductive paste, and the area other than the surface of the sintered body was covered with an insulating adhesive. be. The electrode area of each of the above three types of specimens was iJ.

The dissolution experiment was conducted using an apparatus having the configuration shown in FIG.

That is, a glass container 10 is placed in a constant temperature bath 9, and an electrolytic solution and a test piece 11° and a counter electrode 12 are placed in the container.
A DC voltage was applied for a period of 1.12 to polarize the specimen 11.

The polarization potential is determined by measuring the potential from the Luggin capillary tube 13 installed on the surface of the test piece 11 using the agaric electrode as a reference. A potentiostat 14 capable of setting a constant potential was used for polarization.

The electrolyte was degassed by blowing pure argon gas through the diffuser tube 15.

As a result of an experiment using this apparatus, the rate of dissolution of Fe by polarization in a solution of EDTA-2NH40,002M/solution at a temperature of 60 DEG C. was obtained as shown in FIG.

According to this diagram, 1. α-F e 203 and F e3.
It is shown that 04 melts with cathodic polarization, whereas carbon steel melts with anodic polarization. The natural potential of each specimen is E=-0.73V for carbon steel and E=-0.73V for α-Fe 20
3 has E=+0.05'V, and Fe 304 has E=-0.09V. From this, in order to prevent the carbon steel plate from melting, it is necessary to set the potential to -0.73V or less. In addition, iron oxides have different dissolution rates depending on the potential, but Fe5o4 has -0.9V, α-Fe20
3f is -3, the maximum dissolution rate is obtained near OV. From the above, it can be seen that in order to dissolve only the iron oxide film without dissolving the base material carbon steel, the potential should be set to -0.9V or less.

In addition to the above-mentioned dissolution test, a platinum wire covered with a nylon mesh was placed inside a steel pipe to which an actual iron oxide film was attached, and the dissolution characteristics were tested. As a result, the potential of the film was reduced to -1,
When set to Ov, iron oxide film dissolution with a current efficiency of 80% or more was obtained.

It has been found that by adjusting the potential of the iron oxide film in this manner, it is possible to selectively dissolve only the film without dissolving the base material.

〔Effect of the invention〕

According to the method of the present invention, it is possible to efficiently dissolve only the iron oxide film on the inner surface of the metal base material such as piping without dissolving it, and its decontamination performance is stable and reliable. excellent,
There are no restrictions such as installation of a separate electrolytic cell, and concerns about corrosion due to residual liquid can be significantly reduced. It would be especially useful if the long anode inserted into the piping to be decontaminated could be deformed to accommodate bends in the piping, and it would also be applicable to piping systems with complex shapes. be.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing an example of a system for removing iron oxide film on the inner surface of piping by implementing the method of the present invention, and Figure 2 is a special example of polarization of a specimen used to demonstrate the effectiveness of the present invention. Fig. 3 is an experimental graph showing the iron dissolution characteristics of carbon steel, magnetite, and hematite specimens under polarization. 1: Piping, 2: Decontamination liquid recirculation system, 3: Ion collector, 4: Storage tank, 5: Bonder, 6: Anode, 7: Cathode pipe, 8: DC power supply, 9: Constant temperature chamber, 10
11: Sample, 12: Counter electrode, J3: Lugin capillary, 14: Potentiostat), 15: Diffuser tube for deaeration. Figure 1 Figure 3

Claims (1)

[Scope of Claims] 1. Pipes and valves through which water flows; 2. A method for dissolving V (oxide) attached to the inner surface of equipment (referred to as piping, etc.). A long anode extending according to the shape of is arranged, and an electrolytic solution containing a complexing agent is circulated through the piping, etc., and the piping, etc. A method for dissolving iron oxide deposited on a surface, characterized in that the iron oxide is cathodically polarized and dissolved by flowing a direct current with a cathode potential lower than the corrosion potential of the metal matrix.2 The above anode is A method for dissolving surface-adhered iron oxide according to claim 1, characterized in that the anode layer is a long deformable insoluble anode coated with an insulating material through which an electrolyte can pass. 2. The method for dissolving surface-adhered iron oxide according to claim 1, wherein the electrolytic solution further contains a reducing agent.