JPH0518917B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for removing surface metal oxides, and in particular uses a cleaning solution with a pH around neutrality that is less corrosive to the base metal. The present invention relates to a method for removing surface metal oxides in which only the metal oxides are dissolved.

Metal oxides adhere to or grow on the inner surfaces of equipment and piping in plants such as thermal power plants, nuclear power plants, and chemical plants over the course of years of operation. Since such metal oxides may inhibit the functions of equipment and piping, it is desirable to remove them. In particular, in nuclear power plants, radioactive ions in the cooling water are taken in by metal oxides that have adhered to or grown on the inner surfaces of equipment and piping, increasing the surface dose rate and making plant maintenance and inspection difficult. There is a particularly great need to remove metal oxides from the inner surface of pipes.

The method for removing surface metal oxides of the present invention is suitable for dissolving metal oxides on the inner surfaces of such devices and piping without dissolving the base metal.

[Conventional technology]

A conventional method for removing metal oxides from metal surfaces is to use a cleaning solution containing a mixture of an acid, a complexing agent, and a reducing agent. Such a cleaning liquid has strong acidity and therefore tends to corrode the base metal. On the other hand, if a cleaning solution with a weakly corrosive pH close to neutrality is used, its ability to dissolve oxides will be significantly weakened.

Therefore, the present inventors first investigated the weakly corrosive PH
As a method that makes it possible to dissolve surface metal oxides even when using a cleaning solution that is near neutral, the object to be cleaned, which has metal oxides on the surface of the base metal, is immersed in a solution containing a complexing agent, and the external He invented a method for promoting the dissolution of metal oxides by injecting electrons into the metal oxides by supplying energy (see Japanese Patent Application Laid-open No. 85980/1983). More specifically, this method includes a method of irradiating light as a means of injecting electrons into an oxide;
This includes a method of cathodically polarizing the object to be cleaned and a method of improving the reducing power of the cleaning solution by electrolytically reducing it.

Although these methods are excellent, the method of irradiating light and the method of cathodically polarizing the object to be cleaned have the disadvantage that they are difficult to apply to long piping systems, and the method of electrolytically reducing the cleaning solution is difficult to apply to long piping systems. Since it is an indirect method in the sense of injecting electrons into the oxide, it has the disadvantage that the reducing power is somewhat insufficient to inject electrons from the reduced liquid into the oxide.

[Purpose of the invention]

The purpose of the present invention is to provide a method for removing surface metal oxides that can sufficiently dissolve metal oxides on the inner surface of long piping systems while using a near-neutral cleaning solution that is less corrosive to base metals. is to provide.

[Summary of the invention]

The feature of the method for removing surface metal oxides according to the present invention is that a cleaning solution containing a complexing agent or a reducing agent is brought into contact with the object to be cleaned by increasing its reducing power through electrolytic reduction, and the object to be cleaned is used as a cathode electrode and an anode electrode. By directly applying a DC voltage to the counter electrode, the electrolytic reduction of the cleaning solution and the interaction with the cathode polarization of the object to be cleaned significantly promotes the dissolution of only the metal oxide on the surface without dissolving the base metal. It's what I chose to do. If you do this,
Even in the low current density region where the current flowing through the object to be cleaned is extremely small when the object to be cleaned is cathodically polarized and cannot promote the dissolution of metal oxides by itself, it is possible to simply electrolytically reduce the cleaning solution to the object to be cleaned. The dissolution of the metal oxide is significantly promoted compared to the case where only contact is made. This means that when direct current is applied to the piping system as a cathode, the effect of promoting dissolution of metal oxides can be exerted to a part far away from the counter electrode, where only a small current flows which cannot promote dissolution of metal oxides by itself. This means that it is possible. Note that when the cleaning liquid that contacts the object comes into contact with the counter electrode, which serves as an anode for applying a DC voltage with the object to be cleaned as a cathode, and the counter electrode, which acts as an anode for electrolytically reducing the cleaning liquid, the cleaning liquid is oxidized and reduced. In order to prevent this, the liquid that comes into contact with these counter electrodes should be washed with a diaphragm such as an ion exchange membrane so that the cleaning liquid that comes into contact with the object to be cleaned does not come into contact with these counter electrodes, which serve as anodes. It is important to isolate the product from cleaning fluids that come into contact with it.

[Embodiments of the invention]

FIG. 1 shows an example in which the method for removing surface metal oxides according to the present invention is applied to the removal of metal oxides on the inner surface of a pipe. The piping from which metal oxides on the inner surface are to be removed, that is, the object to be cleaned 1, is connected by a connecting pipe 4 to an electrolytic bath 2 for electrolytically reducing the cleaning liquid and an electrolytic bath 3 for cathodically polarizing the object to be cleaned 1. . A pump 5 is inserted in the middle of the connecting pipe 4, and circulates a cleaning liquid 6 containing a complexing agent or a reducing agent. The electrolytic cell 2 has an anode 7 and a cathode 8, and a DC power supply 10 connected to the anode 7 and the cathode 8 supplies a DC current to electrolytically reduce the cleaning liquid 6. The electrolytic cell 3 has an anode 11 as an electrode.
This anode 11 and the object to be cleaned 1 are connected to a DC power supply 13, and the object to be cleaned 1 becomes a cathode, and a cathode current flows through this with the anode 11 as a counter electrode, cathodically polarizing the object to be cleaned 1. . The electrolytic cells 2 and 3 are separated by ion exchange membranes 9 and 12, respectively, so that the cleaning liquid flowing to the object to be cleaned 1 is separated from these anodes so as not to contact the anodes 7 and 11 and lose their reducing power. Isolated.

In the embodiment described above, the electrolytic cell 2 for electrolytically reducing the cleaning liquid and the electrolytic cell 3 for cathodically polarizing the object to be cleaned are connected in series with respect to the flow of the cleaning liquid 6, but as shown in FIG. An embodiment in which these pumps are connected in parallel and a pump 14 is added is also possible, and the object of the present invention can also be achieved by this.

Next, an experimental example conducted to demonstrate the effects of the present invention will be described with reference to FIGS. 3 and 4.

Figure 3 shows the test equipment used in this experiment. The configuration of the equipment is basically the same as that shown in Figure 1, and elements corresponding to those in Figure 1 have the same reference numerals. is attached. In this diagram,
The test piece 16 corresponds to the object to be cleaned 1 in Figure 1.
This was immersed in the dissolution tank 15 and connected to the negative side of the DC power supply 13 to serve as a cathode. Dissolving tank 1
In step 5, the cleaning solution was circulated in the same manner as in FIG.
A heater 17 was inserted into the dissolution tank 15 to heat the cleaning liquid, and a gas injection pipe 18 was inserted into the dissolution tank 15, and argon gas was blown into this to degas the cleaning liquid.

Figure 4 shows the experimental results. In the experiment, a piece cut out from stainless steel piping of a nuclear power plant was used as the test piece 16. This test piece 16
has an oxide attached to it, which incorporates radioactive nuclides. Use pure water as the cleaning liquid
EDTA-2NH ₄ , diammonium citrate and L
- A mixed solution containing 0.002 mol/dissolved ascorbic acid and adjusted to pH 5.5 with ammonium water was used. The temperature was 80°C.

Curves A1 and A2 in FIG. 4 are the results based on the present invention. In other words, these curves show that while the cleaning solution is electrolytically reduced in the electrolytic cell 2 and circulated, the test piece 16 receives 3 μA/cm ² (in the case of curve A1) and 15 μA/cm 2 .
cm ² (in case of curve A2) is applied to the electrolytic cell 3.
The results are shown below. Curves A3, A4,
A5 shows a comparative example with the present invention. Curve A3 shows the case where the test piece was simply immersed in the cleaning solution, Curve A4 shows the case where the cathode current of 15 μA/cm ² was only applied to the test piece, and Curve A5 shows the case where the washing solution was simply electrolytically reduced. As can be seen from Figure 4, the radioactivity of the test piece is hardly removed by mere immersion (curve A3) or by cathodic polarization only (curve A4), and in the case of electrolytic reduction of the cleaning solution, even A5 takes 16 hours. The removal rate is only 30%. However, in the case based on the present invention, when the cathode current is 3 μA/cm ² (curve A1), 60%;
In the case of 15 μA/cm ² (curve A2), a radioactivity removal rate of over 75% was obtained, demonstrating the remarkable effect of the present invention.

On the other hand, in the case of an actual piping system, we calculated how much the current density would be depending on the distance from the counter electrode 11 when a cathode current was passed through the piping as a cathode. The trial calculation results are shown in Figure 5. Here, the calculations are made assuming that the inner diameter of the pipe is 20 cm, the electrical conductivity of the cleaning liquid is 10 ⁴ μ/cm, and the voltage applied between the pipe and the counter electrode 11 is 100 V.

Another experiment revealed that when direct current is simply passed through the pipe as a cathode, a current density of 0.1 mA/cm ² or higher is required to sufficiently remove metal oxides from the inner surface of the pipe. Therefore, from this fact and FIG. 5, it can be seen that in this case, oxides located at a distance of 20 cm or more from the counter electrode cannot be removed.
However, according to the method of the present invention, the density of the flowing cathode current of 3 μA/cm ² is sufficient to remove surface oxides, as described in FIG. 4, so this and FIG. Considering that, it is 30m from the opposite pole.
It is clear that metal oxides on the inner surface of pipes can be removed to remote locations.

〔Effect of the invention〕

According to the present invention, it is possible to promote the dissolution of surface metal oxides even with a near-neutral cleaning solution that is weakly corrosive to base metals, and the dissolution promoting effect extends over a considerable distance. When used for cleaning the piping system, it has the effect of effectively removing metal oxides on the inner surface of the piping system without dissolving the base metal of the piping over a considerable distance.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic diagrams illustrating an example in which the method of the present invention is applied to the removal of metal oxides on the inner surface of piping, and Figure 3 is an experimental diagram illustrating an example in which the method of the present invention is applied to the removal of metal oxides on the inner surface of piping. A schematic diagram illustrating an example, FIG. 4 is a graph of experimental results showing the effects of the present invention, and FIG. 5 is a graph showing trial calculation results of cathode current distribution for estimating the range of effects of the present invention. . 1... Item to be cleaned (piping), 2... Electrolytic cell for electrolytic reduction, 3... Electrolytic cell for cathode polarization, 5... Pump, 6... Cleaning liquid, 7... Anode, 8... Cathode, 9... ...Ion exchange membrane, 10...DC power supply,
11... Anode, 12... Ion exchange resin, 1
3...DC power supply, 14...Pump, 15...Dissolution tank, 16...Test piece, 17...Heater, 18...
Argon gas injection tube.

Claims (1)

[Scope of Claims] 1. A cleaning solution containing a complexing agent and/or a reducing agent is brought into contact with an object to be cleaned, which has an oxide formed on the surface of the base metal, with the reducing power strengthened by electrolytic reduction, and the A method for removing surface metal oxides, which comprises dissolving oxides on the surface of the object without dissolving the base metal by directly applying a DC voltage to the object to be cleaned as a cathode and an anode as a counter electrode. 2 A liquid path for circulating the cleaning liquid is provided in the object to be cleaned, and the cleaning liquid is electrolytically reduced by at least one electrolytic cell equipped with a cathode and an anode provided in the liquid path, and the cleaning liquid is Claim 1, characterized in that at least one other electrolytic cell having only an anode is provided in the path, and the object to be cleaned is used as a cathode, and a DC voltage is applied between this and the latter anode. The method for removing surface metal oxides according to item 1. 3. The method for removing surface metal oxides according to claim 2, wherein the cleaning liquid circulating to the object to be cleaned is separated from each anode by a diaphragm in the two types of electrolytic cells.