JP6848277B2 - Local corrosion control method for stainless steel and storage method for metal containers - Google Patents

Description

The present invention relates to a method for suppressing local corrosion of stainless steel and a method for storing a metal container.

Stainless steel is widely used as a constituent material for structures and the like where corrosion is not tolerated. Stainless steel has a dense protective film (passivation film) formed on the surface of the material and has high corrosion resistance.

However, depending on the usage environment, stainless steel may be corroded. For example, in a moist environment in which a structure made of stainless steel (hereinafter, may be simply referred to as “structure”) comes into contact with seawater (moisture), chloride contained in the seawater that comes into contact with the surface of the structure. Ions act on the passivation film of stainless steel. As a result, the passivation film may be locally damaged and the structure may be locally corroded.

Here, the term "local corrosion" as used herein refers to a phenomenon in which the passive film on the surface of stainless steel is locally damaged and the exposed new surface is rapidly corroded (dissolved) at the damaged portion of the passive film. Point to. Local corrosion is a phenomenon that contrasts with "total corrosion (a form in which the entire surface is corroded almost uniformly)" found in iron and carbon steel. In local corrosion, the progress rate in the depth direction is significantly higher than that in total corrosion because the area of the corroded portion is small. In the present specification, local corrosion includes pitting corrosion, crevice corrosion, stress corrosion cracking (SCC), and intergranular corrosion.

Local corrosion progresses quickly. For example, when pitting corrosion occurs, corrosion spreads from the surface of the structure to the depth of the structure at a rate of several mm to several tens of mm per year, and penetration damage may occur in several days to several months. Therefore, when stainless steel is used for the structure, suppressing local corrosion of the stainless steel is one of the important issues. Various studies have been made to solve the problem.

For example, at the design stage of a structure, (1) select a stainless steel having high corrosion resistance that can sufficiently withstand the environment in which it is used from among various stainless steels, and (2) easily cause local corrosion. It is being studied to have a design that does not have a clearance structure, and (3) a structure that suppresses adhesion to the surface so as not to generate a clearance.

In addition, (4) it is also being considered to add equipment for anticorrosion to the structure made of stainless steel.

Furthermore, for the aqueous solution that comes into contact with the structure, (5) the aqueous solution can be removed and sufficiently dried, (6) chloride ions can be removed from the aqueous solution, and (7) dissolved oxygen can be removed from the aqueous solution. It is being considered. This is because water, chloride ions, and oxygen in the aqueous solution can cause local corrosion.

In addition to this, (8) cooling the aqueous solution and (9) adding a chemical (corrosion inhibitor) for suppressing local corrosion to the aqueous solution with which the structure comes into contact are being studied (for example, patent). Refer to Documents 1 to 3).

Japanese Unexamined Patent Publication No. 2009-270131 Japanese Unexamined Patent Publication No. 2014-218729 Japanese Unexamined Patent Publication No. 2015-67859

However, of the above methods, the methods (1) to (3) can be reflected only at the design stage of the structure. Therefore, it is not possible to suppress local corrosion of structures and equipment that are already in use.

The method (4) is known to be a very effective method for suppressing local corrosion. However, it cannot be applied when it is not possible to install anticorrosion equipment due to location restrictions. In addition, when the number of structures is large, installing anticorrosion equipment in each structure requires a large amount of equipment.

The methods (5) to (9) can be expected to be effective if they can be applied to a structure, but they are often difficult to apply due to various restrictions. Moreover, it is not always useful in a special environment where radiation is applied. In a special environment where radiation is applied, unlike a normal environment where radiation is not applied, unexpected problems may occur. For example, a new corrosive substance may be generated by irradiation with radiation, making it difficult to suppress corrosion, or the added corrosion inhibitor may be decomposed or altered by radiation to lose the corrosion suppressing effect. In some cases, the decomposed or altered corrosion inhibitor may promote local corrosion on the contrary.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for suppressing local corrosion of stainless steel, which can easily and effectively suppress local corrosion even in an environment irradiated with radiation. And.

As a result of diligent studies, the present inventors have found that local corrosion of stainless steel can be suppressed by setting the pH of water to a predetermined value or higher in an environment irradiated with radiation. It was also found that the amount of hydrogen peroxide produced by irradiating water with radiation and having a corrosion promoting action is remarkably reduced by setting the pH of water to a predetermined value or higher.
That is, the present invention provides the following means for solving the above problems.

The method for suppressing local corrosion of stainless steel according to one aspect of the present invention is a method for suppressing local corrosion of stainless steel in a moist environment irradiated with radiation, in which the pH of water in contact with the stainless steel is set to 12.5 or higher. How to do it.

In the method for suppressing local corrosion of stainless steel according to the above aspect, a substance containing a hydroxide is added to the water content based on the step of measuring the pH of the water content and the measured value of the measured pH, and the water content is changed. It may have a step of raising the pH to 12.5 or higher.

In the method for suppressing local corrosion of stainless steel according to the above aspect, the concentration of chloride ions contained in the water may be 6000 ppm or less.

In the method for suppressing local corrosion of stainless steel according to the above aspect, the irradiation dose rate of the radiation may be 10 kGy / h or less.

In the method for suppressing local corrosion of stainless steel according to the above aspect, the pH of the water is set to 12.5 or more, the pH decrease in the corroded part is suppressed, and the water content of hydrogen peroxide, which is a corrosion promoting substance generated by irradiation with radiation, is the water content. It may be configured to reduce the concentration in the medium.

The method for storing a metal container according to one aspect of the present invention sets the pH of water stored or retained in a stainless steel metal container irradiated with radiation to 12.5 or higher.

In the method for storing a metal container according to the above aspect, the metal container may be an adsorption tower filled with a radionuclide adsorbent.

According to the method for suppressing local corrosion of stainless steel according to one aspect of the present invention, local corrosion can be easily and effectively suppressed even in an environment irradiated with radiation.

It is a schematic diagram which shows the test piece used in the crevice corrosion occurrence test. It is a schematic diagram which shows the test method of the crevice corrosion occurrence test.

(First Embodiment: Method for suppressing local corrosion of stainless steel)
The method for suppressing local corrosion of stainless steel according to the first embodiment of the present invention is a method for suppressing local corrosion of stainless steel in a moist environment irradiated with radiation.

In the method for suppressing local corrosion of stainless steel of the present embodiment, the "wet environment" refers to an environment in which moisture is in constant contact with a structure made of stainless steel. For example, when water is flowing in a pipe made of stainless steel, when water is stored in a metal container made of stainless steel, when water stays in a metal container made of stainless steel, stainless steel It refers to each environment, such as when a pedestal made of steel is splashed with water. The method for suppressing local corrosion of stainless steel of the present embodiment can be widely applied in these environments.

If the moisture in contact with the stainless steel contains chloride ions exceeding the permissible limit concentration, the passive film on the surface of the stainless steel is damaged by the action of the chloride ions. When the passivation film is damaged, local corrosion occurs on the surface of the stainless steel and progresses.

As used herein, the term "occurrence of local corrosion" means that the passive film is locally damaged on the surface of stainless steel that has come into contact with moisture, and the state changes from no local corrosion to a state in which local corrosion is observed. Point to.

Further, in the present specification, "progress of local corrosion" means that the range of local corrosion generated in stainless steel is expanded. “Expanding the range of local corrosion” includes both the expansion of local corrosion in the surface direction of the stainless steel surface and the expansion of local corrosion in the depth direction from the surface of the stainless steel.

The occurrence and progression of local corrosion is promoted in an environment where stainless steel is exposed to radiation. The “environment to be irradiated with radiation” includes both the case where the stainless steel is irradiated from the outside and the case where the radiation is emitted from a radionuclide existing in the water in contact with the stainless steel.

When irradiated with radiation, radiolysis of water in contact with stainless steel proceeds. When water is radiolyzed, the concentration of hydrogen peroxide contained in the water that comes into contact with stainless steel increases. Since hydrogen peroxide is highly corrosive (oxidizing), corrosion of stainless steel is promoted. The increased corrosiveness of water in contact with stainless steel due to the formation of hydrogen peroxide appears in the form of an increase in the natural immersion potential of stainless steel.

When a radionuclide is present in the water in contact with the stainless steel, the water evaporates due to the decay heat of the nuclide, and the chloride ion concentration in the water in contact with the stainless steel increases. Local corrosion of stainless steel is more likely to occur as the chloride ion concentration increases. Therefore, even if the chloride ion concentration is increased, the corrosion of stainless steel is promoted.

The irradiated radiation includes any of various radiations such as alpha rays, beta rays, and gamma rays. In particular, gamma rays are electromagnetic waves and are more transparent than particles such as alpha rays and beta rays. Therefore, it is difficult to shield gamma rays with a shield, and the effect on radiolysis of water is large.

In the method for suppressing local corrosion of stainless steel of the present embodiment, the pH of water in contact with stainless steel is set to 12.5 or higher. By setting the pH of the water in contact with the stainless steel to a predetermined amount or higher, it is possible to suppress the occurrence of local corrosion in the stainless steel even in an environment irradiated with radiation.

There are two possible reasons for suppressing the occurrence of local corrosion.
The first reason is that the hydrogen generation reaction is suppressed by increasing the pH of the water that comes into contact with the stainless steel. The second reason is that by setting the pH of the water in contact with the stainless steel to 12.5 or higher, the concentration of hydrogen peroxide generated by the radiolysis of water in the water is reduced.

The first reason will be specifically described.
Stainless steel is hardly corroded because the protective film (passivation film) spontaneously formed on the surface significantly suppresses the dissolution of the base metal. This protective film exists stably in a wide pH range, but dissolves in a strong acidity (eg, pH <2) and loses the underlying protective property.

When chloride ions act on stainless steel on which a protective film is formed in an aqueous solution having a neutral pH, the protective film is locally destroyed and the underlying metal is exposed. Then, a local battery is formed in which the exposed base metal portion is an anode (anode) and the other sound protective film portion is a cathode (cathode).

At the anode part, the main reaction is that the metal becomes ions and dissolves, so the metal ion concentration increases. Further, a part of the concentrated metal ions is hydrolyzed to generate and accumulate hydrogen ions, so that the pH of the anode portion is remarkably lowered and becomes strongly acidic. As a result, the protective film cannot exist stably, and local corrosion progresses at the anode portion. That is, the anode portion becomes a corroded portion.

As described above, local corrosion of stainless steel is mainly caused by a decrease in pH (acidification) at the anode portion. Therefore, when the pH of water is made alkaline, the decrease in pH (acidification) is suppressed, resulting in local corrosion. Is considered to be suppressed.

Next, the second reason will be specifically described.
Local corrosion of stainless steel is more likely to occur as the oxidizing property of water is stronger. The oxidative strength of water depends on the type of oxidant in the water. In a non-radiation environment, dissolved oxygen in water generally acts as an oxidant on corrosion.

On the other hand, in an environment irradiated with radiation, water is decomposed by radiation to generate hydrogen peroxide. Hydrogen peroxide has a stronger oxidizing power than dissolved oxygen. Therefore, it is known that local corrosion is more likely to occur in an environment irradiated with radiation than in a non-irradiated environment.

On the other hand, in the present embodiment, by setting the pH of water to 12.5 or higher, the concentration of hydrogen peroxide generated by radiolysis of water in water is remarkably reduced. It was experimentally confirmed that the amount of hydrogen peroxide was reduced by setting the pH to 12.5 or higher. That is, setting the pH of water to 12.5 or higher also has the effect of reducing the corrosion promotion phenomenon peculiar to the irradiation environment. This was confirmed for the first time by examining local corrosion in an environment irradiated with radiation.

The method of raising the pH of the water in contact with the stainless steel to 12.5 or higher is not particularly limited. For example, by adding a hydroxide as a drug to water, the pH of the water can be adjusted to 12.5 or higher.

The hydroxide added as a drug may be of any type as long as it is a hydroxide composed of hydroxide ions and cations. For example, sodium hydroxide (NaOH), calcium hydroxide (Ca (OH) ₂ ), potassium hydroxide (KOH), lithium hydroxide (LiOH), cesium hydroxide (CsOH), barium hydroxide (RbOH), hydroxide. Strontium (Sr (OH) ₂ ), barium hydroxide (Ba (OH) ₂ ), europium hydroxide (Eu (OH) ₂ ), talium hydroxide (TlOH) and the like can be used.

Among the above-mentioned hydroxides, easily available sodium hydroxide and calcium hydroxide are particularly preferable. When a saturated concentration of calcium hydroxide is added to the water, the pH of the water becomes 12.5 or more. Therefore, the pH can be easily adjusted to 12.5 or higher simply by adding excess calcium hydroxide to water without weighing it. Further, by irradiating the calcium hydroxide remaining due to supersaturation with radiation, there is no adverse effect on the suppression of local corrosion.

As the stainless steel, commonly known stainless steel can be used. In order to further suppress local corrosion, it is preferable that the stainless steel itself has high corrosion resistance. For example, it is preferable to use stainless steel having a pitting corrosion resistance index (PRE) of 18 or more, more preferably 23 or more, and 25 or more, which is obtained by the following formula (1). It is more preferred to use some stainless steel.

[Number 1]
(Pitting corrosion resistance index) = [% Cr] + 3.3 x [% Mo] + n x [% N] ... (1)
In the formula, [% Cr] is the proportion of chromium contained in the entire stainless steel (mass%), [% Mo] is the proportion of molybdenum contained in the entire stainless steel (% by mass), and [% N. ] Is the ratio (mass%) of nitrogen contained in the entire stainless steel.

The coefficient n of [% N] in the above formula (1) is a value of about 10 to 30, although it varies depending on the researcher.

Specifically, stainless steel having corrosion resistance equal to or higher than that of SUS304 (PRE = 18), which is a stainless steel defined in JIS G4303 "Stainless steel rod", is preferable. Further, SUS316L grade (PRE = 23) stainless steel, which is superior in corrosion resistance to chloride ions, is particularly preferable to general-purpose stainless steel of SUS304 class. For example, in the case of SUS316L steel specified in JIS G4303, since chromium is 16% or more and molybdenum is 2% or more, the pitting corrosion resistance index is 22.6 from the equation (1). The SUS316L grade has a range of chromium concentration (16 to 18%) and molybdenum concentration (2 to 3%), and the pitting corrosion resistance index can be set to a higher value (for example, 25 or more).

In the method for suppressing local corrosion of stainless steel of the present embodiment, the chloride ion concentration contained in the water is preferably 20000 ppm or less, more preferably 6000 ppm or less. The chloride ion concentration contained in seawater is about 20000 ppm. It is usually difficult to imagine that stainless steel is brought into contact with an aqueous solution having a chloride ion concentration higher than that of seawater, but if the chloride ion concentration is too high, the possibility of corrosion progressing increases.

In addition, each reaction constituting the occurrence of local corrosion and the progress of local corrosion tends to proceed kinetically when the temperature of water is high. Therefore, the lower the temperature of the moisture in contact with the stainless steel, the more preferable. For example, by controlling the temperature at room temperature, the reaction rate of the reaction related to local corrosion can be reduced, and the effect of the method for suppressing local corrosion of stainless steel of the present embodiment can be enhanced.

The higher the radiation dose rate, the higher the concentration of hydrogen peroxide produced, which promotes local corrosion. Therefore, the lower the irradiation dose rate, the more preferable. For example, if it is 10 kGy / h or less, the effect of the method for suppressing local corrosion of stainless steel of the present embodiment can be enhanced.

In the method for suppressing local corrosion of stainless steel of the present embodiment, a substance containing a hydroxide is added to the water content based on the step of measuring the pH of the water content and the measured value of the measured pH, and the pH of the water content is adjusted. It is preferable to have a step of making it 12.5 or more. By using such a method, the pH of the water in contact with the stainless steel can be surely set to 12.5 or higher.

If the pH of the water is known, the amount of hydroxide to be added may be determined using these values. For example, when seawater is assumed as water, it is known that the pH of seawater is around 8, and the amount of hydroxide to be added may be determined based on this value. When calcium hydroxide or the like is added excessively with respect to water, it is not necessary to measure the pH or the like.

According to the above-mentioned method for suppressing local corrosion of stainless steel, local corrosion can be easily and effectively suppressed even in a radiation environment.

(Second Embodiment: Storage method of metal container)
In the storage method of the metal container according to the second embodiment of the present invention, the pH of the water stored or retained in the stainless steel metal container irradiated with radiation is set to 12.5 or more, and the metal container is stably stored. It is a method of storage.

The stainless steel metal container in the present embodiment corresponds to the stainless steel in the first embodiment. Therefore, the configuration in the first embodiment can also be used in the second embodiment.

The shape and use of the metal container are not particularly limited. As a stainless steel metal container placed in an environment exposed to radiation, a tank or adsorption tower for storing or purifying contaminated water containing radionuclides, and a pool for storing spent fuel that emits radiation. and so on. Hereinafter, the case of the adsorption tower will be described as an example, but the metal container is not limited to the adsorption tower.

The adsorption tower is a metal container filled with a substance (radionuclide adsorbent) having an action of adsorbing radionuclides. Contaminated water containing radionuclides is passed through the adsorption tower, and the radionuclides are adsorbed in the adsorption tower to purify the contaminated water.

Contaminated water may contain a large amount of chloride ions. For example, at the Fukushima Daiichi Nuclear Power Station, seawater had to be used to cool the reactor, so contaminated water containing chloride ions and radionuclides was generated.

The used adsorption tower emits stronger radiation due to the adsorbed radionuclides, so it cannot be easily approached by humans. Therefore, it is stored for a predetermined period of time with the adsorbent contained therein, and then disposed of. Since it contains a large amount of radioactive substances, it is necessary to be careful not to damage the adsorption tower, which is a metal container, due to aging deterioration such as corrosion.

In order to prevent aging deterioration such as corrosion, water drainage or fresh water may be used as a measure for removing water and chloride ions, which are the causative substances of corrosion. However, sufficient drainage may be difficult due to strong radiation or structural problems. In this case, water containing chloride ions stays in the adsorption tower as residual water.

Radionuclides adsorbed on the adsorbent emit radiation in the adsorption tower. That is, the stainless steel constituting the adsorption tower is in an environment where radiation is irradiated.

That is, the portion of the adsorption tower that comes into contact with the residual water becomes a "wet environment in which radiation is irradiated". As described above, since hydrogen peroxide, which is highly corrosive, is generated by radiation, the corrosive environment becomes more severe than usual. Therefore, depending on the chloride ion concentration in the residual water, local corrosion may occur in the adsorption tower, and the residual water containing radioactive substances may leak. In addition, for example, in a water storage tank or the like, there is a case of "retention" instead of "retention", but the case of "retention" will be described below as an example.

In this embodiment, the pH of the water retained in the metal container is set to 12.5 or higher. The pH can be adjusted by adding a hydroxide or the like to the water retained in the metal container. By adjusting the pH of the water retained in the metal container to 12.5 or higher, corrosion of the metal container is suppressed as in the method for suppressing local corrosion of stainless steel according to the first embodiment. As a result, the metal container can be stably stored.

Hereinafter, the storage method of the metal container will be specifically described by taking the case of the adsorption tower as an example.
The storage method of the adsorption tower according to the present embodiment is a method of setting the pH of water staying in the metal container made of stainless steel to 12.5 or more.

The method for adjusting the pH is not particularly limited, but for example, the first method of adding a substance containing a hydroxide to the water in the adsorption tower and the first method of passing an adjusting liquid having a predetermined pH or higher into the adsorption tower. There are two methods.

The first method is a method of increasing the pH of the residual water by adding a substance containing a hydroxide to the residual water remaining in the adsorption tower. The required amount of hydroxide to be added may be calculated by measuring the pH of the residual water in advance, or if the substance is not decomposed by radiation, an excessive amount exceeding the saturated dissolved amount may be added. Good.

In the first method, it is preferable to pass fresh water through the adsorption tower before adding the substance containing the hydroxide. By passing fresh water, the chloride ion concentration of the residual water can be reduced. If the chloride ion concentration of the residual water is low, the possibility of corrosion of the metal container can be further reduced.

When fresh water is passed through, the chloride ion concentration of the residual water is sufficiently reduced. Therefore, stainless steel that comes into contact with residual water rarely corrodes. However, if the residual water is concentrated and the chloride ion concentration is increased, local corrosion may occur. Leakage of radioactive materials is a major problem, and it is important to provide sufficient preventive measures at the stage where local corrosion can occur.

According to the first method, the pH of the residual water becomes equal to or higher than a predetermined value. Therefore, it is possible to prevent the stainless steel constituting the adsorption tower from corroding. That is, it is possible to prevent the adsorption tower from being damaged due to aged deterioration such as corrosion, and to store the adsorption tower safely and stably.

The first method is particularly useful for improving the storage stability of the adsorption tower that has already started storage. It is only necessary to add a substance containing a hydroxide into the adsorption tower during storage, and the storage stability of the adsorption tower can be easily improved without complicated work.

The second method is a method in which a adjusting liquid having a pH adjusted to 12.5 or higher is passed through the adsorption tower. In the case of total replacement of residual water, the pH may be adjusted to 12.5 or higher, and in the case of partial replacement, the pH of the adjusting liquid can be adjusted from the amount of residual water and the amount of the adjusting liquid to be passed. preferable.

When the adjusting liquid is passed through the adsorption tower, a part of the adjusting liquid remains as residual water in the adsorption tower. Since the pH of the adjusting liquid is adjusted in advance, it is possible to prevent the stainless steel constituting the adsorption tower from corroding. That is, it is possible to prevent the adsorption tower from being damaged due to aged deterioration such as corrosion, and to store the adsorption tower safely and stably.

The second method is particularly useful for improving the storage stability of the adsorption tower, which is about to start storage. It is not necessary to provide special equipment or the like, and it can be carried out simply by replacing the freshwater water flow process that is usually performed in order to reduce the chloride ion concentration.

The stainless steel used for the adsorption tower is preferably SUS304 grade or higher stainless steel, and more preferably SUS316L grade stainless steel. As the radionuclide adsorbent, a known one can be used. For example, zeolite or the like can be used.

The radiation nuclide adsorbed in the adsorption tower is not particularly limited. For example, cesium-137, cobalt-60, strontium-90, iodine-131 and the like can be mentioned.

As described above, by using the method for storing the metal container according to the present embodiment, it is possible to prevent the pH of the residual water from becoming higher than a predetermined value and the stainless steel constituting the adsorption tower from being corroded. That is, it is possible to prevent the adsorption tower from being damaged due to aged deterioration such as corrosion, and to store the adsorption tower safely and stably.

Although preferred embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to such examples. The various shapes and combinations of the constituent members shown in the above-mentioned examples are examples, and can be variously changed based on design requirements and the like within a range not deviating from the gist of the present invention.

[Example]
Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples.

[1. Verification of the effect on suppressing the occurrence of local corrosion (crevice corrosion)]
(Test pieces)
FIG. 1 is a schematic view showing a test piece of a crevice corrosion occurrence test, FIG. 1 (a) is a side view, and FIG. 1 (b) is a front view.

As shown in the figure, in this embodiment, a flat plate-shaped test piece 1 made of stainless steel having a size of 50 mm × 24 mm × 3 mm was used. As the stainless steel, a commercially available SUS304 steel sheet was used and manufactured by machining. The chemical composition (mass%) of SUS304 is carbon (C): 0.060%, silicon (Si): 0.44%, manganese (Mn): 1.09%, and phosphorus (P) with respect to the entire stainless steel. ): 0.028%, Sulfur (S): 0.005%, Chromium (Cr): 18.30%, Nickel (Ni): 8.16%. The pitting corrosion resistance index of SUS304 is 18.3.

A bolt hole having a diameter of 6 mm was formed in the test piece 1 at a position 12 mm from one end in the longitudinal direction, and the surface was wet-polished using 600 mesh water-resistant abrasive paper.

Such a test piece 1 is sandwiched between a circular washer 2 having a diameter of 16 mm and a thickness of 1 mm, a bolt 3 is further inserted into a bolt hole of the test piece 1, and then a nut 4 is used to apply a torque of 2.0 Nm. I screwed it. The washers 2, bolts 3 and nuts 4 were made of industrial pure titanium.

(Example 1)
FIG. 2 is a schematic view showing a test method for a crevice corrosion occurrence test.
An aqueous sodium chloride (NaCl) solution 6 prepared from ion-exchanged water and a special grade reagent was injected into the glass beaker 5. The volume of the glass beaker 5 was 200 ml, and 100 ml of the NaCl aqueous solution 6 was injected. The chloride ion concentration of the NaCl aqueous solution 6 was 6000 ppm.

Then, sodium hydroxide was added to the NaCl aqueous solution 6 to adjust the pH to 13.2. Then, the test piece 1 was allowed to stand in the pH-adjusted NaCl aqueous solution 6. A lid 8 made of aluminum foil was placed on the upper surface of the glass beaker 5 in order to prevent evaporation of the aqueous NaCl solution 6.

Then, the test piece 1 immersed in the NaCl aqueous solution 6 was irradiated with gamma rays having an irradiation dose rate of 10 kGy / h for 160 hours using the gamma ray irradiation radiation source 9. As the gamma ray irradiation source 9, a Co-60 gamma ray irradiation facility (owned by the Radiation Utilization Promotion Association) was used.

After the irradiation was completed, the test piece 1 was taken out from the NaCl aqueous solution 6 and the presence or absence of crevice corrosion was confirmed. The presence or absence of crevice corrosion was visually confirmed by visually checking the gap between the test piece 1 and the washer 2. In addition, the pH and hydrogen peroxide concentration after the test were measured. The pH after the test was 13.0, and the hydrogen peroxide concentration was 0.5 ppm or less, which is the measurement limit. The results are summarized in Table 1.

(Comparative Example 1)
Comparative Example 1 is different from Example 1 in that the amount of sodium hydroxide added was changed and the pH was adjusted to 10.2. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Example 1. In addition, the pH and hydrogen peroxide concentration after the test were measured. The pH after the test was 8.2 and the hydrogen peroxide concentration was 4 ppm. The results are summarized in Table 1.

(Comparative Example 2)
Comparative Example 2 is different from Example 1 in that the amount of sodium hydroxide added was changed and the pH was adjusted to 6.6. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Example 1. In addition, the pH and hydrogen peroxide concentration after the test were measured. The pH after the test was 5.6 and the hydrogen peroxide concentration was 4 ppm. The results are summarized in Table 1.

(Example 2)
Example 2 is different from Example 1 in that the hydroxide to be added is changed to calcium hydroxide. The amount of calcium hydroxide added was supersaturated above the saturation concentration, and the pH before the test was 12.7. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Example 1. The pH after the test was also measured. The pH after the test was 12.5. Hydrogen peroxide concentration was not measured. The results are summarized in Table 1.

(Comparative Example 3)
Comparative Example 3 is different from Example 2 in that the amount of calcium hydroxide added was changed and the pH was adjusted to 9.5. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Example 2. The pH after the test was also measured. The pH after the test was 8.3. Hydrogen peroxide concentration was not measured. The results are summarized in Table 1.

(Comparative Example 4)
Comparative Example 4 is different from Example 2 in that the amount of calcium hydroxide added was changed and the pH was adjusted to 6.6. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Example 2. The pH after the test was also measured. The pH after the test was 6.4. Hydrogen peroxide concentration was not measured. The results are summarized in Table 1.

(Comparative Example 5)
Comparative Example 5 is different from Examples 1 and 2 in that no hydroxide was added. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Examples 1 and 2. In addition, the pH and hydrogen peroxide concentration before and after the test were measured. The pH before the test was 5.5, the pH after the test was 4.9, and the hydrogen peroxide concentration after the test was 5 ppm. The results are summarized in Table 1.

(Comparative Example 6)
Comparative Example 6 is different from Comparative Example 5 in that the irradiation dose rate of the irradiated radiation was changed to 1 kGy / h. As for other conditions, the presence or absence of crevice corrosion was confirmed in the same manner as in Comparative Example 5. In addition, the pH and hydrogen peroxide concentration before and after the test were measured. The pH before the test was 5.5, the pH after the test was 6.0, and the hydrogen peroxide concentration after the test was 3 ppm. The results are summarized in Table 1.

In Table 1, "○" means that crevice corrosion did not occur, and "x" means that crevice corrosion did occur. Whether or not crevice corrosion occurred was determined by visually confirming the occurrence of red rust and the presence or absence of erosion inside the crevice.

As shown in Table 1, in Example 1 and Example 2 in which the pH of the aqueous solution (pH after the test) in contact with the test piece 1 at the end of the test was 12.5 or more, no crevice corrosion occurred. Moreover, the hydrogen peroxide concentration after the test in Example 1 was remarkably small. It is considered that this is due to the combined effect of suppressing the pH decrease (acidification) in the corroded part (anode part) and suppressing the production of hydrogen peroxide, which is a corrosion promoting substance generated by irradiation. Be done. It has been newly found that the production of hydrogen peroxide, which is a corrosion-promoting substance, is suppressed by setting the pH of the aqueous solution in contact with the test piece 1 to 12.5 or higher in an environment irradiated with radiation. It is a finding.

Among local corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking, crevice corrosion occurs under the mildest conditions. Therefore, if the occurrence of crevice corrosion can be prevented, the occurrence of other local corrosion can also be prevented.

From these results, it was confirmed that the occurrence of local corrosion can be prevented even in an environment exposed to strong radiation by setting the pH of the water in contact with the stainless steel to 12.5 or higher.

1 ... test piece, 2 ... washer, 3 ... bolt, 4 ... nut, 5 ... glass beaker, 6 ... NaCl aqueous solution, 8 ... lid, 9 ... gamma ray irradiation source

Claims (7)

A method for suppressing local corrosion of stainless steel in a moist environment where water is decomposed by radiation to become hydrogen peroxide.
A method for suppressing local corrosion of stainless steel so that the pH of water in contact with the stainless steel is 12.5 or higher. The step of measuring the pH of the water and
The local part of stainless steel according to claim 1, further comprising a step of adding a substance containing a hydroxide to the water content to bring the pH of the water content to 12.5 or higher based on the measured value of the measured pH. Corrosion control method. The method for suppressing local corrosion of stainless steel according to claim 1 or 2, wherein the concentration of chloride ions contained in the water is 6000 ppm or less. The method for suppressing local corrosion of stainless steel according to any one of claims 1 to 3, wherein the irradiation dose rate of the radiation is 10 kGy / h or less. The pH of the water is set to 12.5 or higher.
While suppressing the decrease in pH at the corroded part,
The method for suppressing local corrosion of stainless steel according to any one of claims 1 to 4, wherein the concentration of hydrogen peroxide, which is a corrosion promoting substance generated by irradiation with radiation, in the water is reduced. A method for storing a metal container in which the pH of water stored or retained in a stainless steel metal container irradiated with radiation is set to 12.5 or higher. The method for storing a metal container according to claim 6, wherein the metal container is an adsorption tower filled with a radionuclide adsorbent.

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