JP2017218656A - Local corrosion inhibition method of stainless steel and storage method of metal container - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a local corrosion inhibition method of stainless steel capable of effectively inhibiting local corrosion even under an environment in which radiation is irradiated.SOLUTION: There is provided a local corrosion inhibition method of stainless steel in a wet environment in which radiation is irradiated, which provides in the moisture which comes into contact with the stainless steel, nitric acid ions of 1 or more times the molar concentration of the molar concentration of chloride ions contained therein.SELECTED DRAWING: None

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 that do not allow corrosion. A dense protective film (passive film) is formed on the surface of the stainless steel material. Therefore, stainless steel has high corrosion resistance.

  However, corrosion may occur in stainless steel depending on the usage environment. For example, in a humid environment in which a structure made of stainless steel (hereinafter, simply referred to as “structure”) is in contact with seawater (moisture), chloride contained in seawater in contact with the surface of the structure Ions act on the passive film of stainless steel. As a result, the passive film may be locally damaged, and local corrosion may occur in the structure.

  As used herein, “local corrosion” refers to a phenomenon in which the passive film on the surface of stainless steel is locally damaged, and the new surface exposed at the site where the passive film is damaged rapidly corrodes (dissolves). Point to. Local corrosion is a phenomenon in contrast to “overall corrosion (a form in which the entire surface corrodes almost uniformly)” observed in iron and carbon steel. In the local corrosion, since the area of the corrosion site is small, the traveling speed in the depth direction is remarkably increased as compared with the general corrosion. In this specification, local corrosion includes pitting corrosion, crevice corrosion, stress corrosion cracking (SCC), and intergranular corrosion.

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

  For example, in the design stage of a structure, (1) from among various stainless steels, select stainless steel having high corrosion resistance that can sufficiently withstand the environment in which it is used, and (2) local corrosion is likely to occur. A design that does not have a gap structure is considered, and (3) a structure that suppresses deposits on the surface so as not to cause a gap is being studied.

  In addition, (4) it is also considered to install an anticorrosion facility on a structure made of stainless steel.

  Furthermore, for the aqueous solution in contact with the structure, (5) removing the aqueous solution and drying it sufficiently, (6) removing chloride ions from the aqueous solution, and (7) removing dissolved oxygen 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, (8) cooling an aqueous solution and (9) adding a chemical (corrosion inhibitor) for suppressing local corrosion to an aqueous solution in contact with a structure is being studied (for example, patents). References 1-3).

JP 2009-270131 A JP 2014-218729 A Japanese Patent Laying-Open No. 2015-67859

  However, among the above methods, the methods (1) to (3) can be reflected only at the design stage of the structure. Therefore, local corrosion cannot be suppressed for structures and equipment that are already used.

  It is known that the method (4) is a very effective method for suppressing local corrosion. However, it cannot be applied if the equipment for cathodic protection cannot be installed due to location restrictions. Moreover, when there are many structures, it will be a large-scale installation to install the anti-corrosion equipment in each structure.

  The methods (5) to (9) can be expected to be effective if applied to structures, but are often difficult to apply due to various restrictions. In addition, it is not always useful in a special environment where radiation is irradiated. Under a special environment where radiation is irradiated, an unexpected problem may occur unlike a normal environment where radiation is not irradiated. For example, when a new corrosive substance is generated by irradiation with radiation and it becomes difficult to suppress corrosion, or the added corrosion inhibitor is decomposed and altered by radiation, the corrosion inhibiting effect may be lost. In some cases, the decomposed and altered corrosion inhibitor may promote local corrosion.

  This invention is made in view of such a situation, Comprising: It aims at providing the local corrosion suppression method of the stainless steel which can suppress a local corrosion simply and effectively also in the environment irradiated with a radiation. And

The inventors of the present invention have found that local corrosion of stainless steel can be suppressed even in an environment irradiated with radiation by adjusting the concentration of nitrate ions in moisture in contact with stainless steel.
That is, this invention provides the following means in order to solve the said subject.

  The method for inhibiting local corrosion of stainless steel according to the first aspect of the present invention is a method for inhibiting local corrosion of stainless steel in a wet environment irradiated with radiation, and in the moisture that comes into contact with the stainless steel, In this method, nitrate ions having a molar concentration of 1 or more times the molar concentration of chloride ions contained are allowed to coexist.

  In the method for inhibiting local corrosion of stainless steel according to the above aspect, the step of measuring the molar concentration of chloride ions contained in the moisture, and the nitrate in the moisture based on the measured value of the molar concentration of chloride ions And a step of allowing nitrate ions having a molar concentration of 1 or more times the measured value to coexist.

  In the method for suppressing local corrosion of stainless steel according to the above aspect, the molar concentration of the nitrate ions may be 5 times or more the molar concentration of chloride ions contained in the moisture.

In the method for suppressing local corrosion of stainless steel according to the above aspect, the stainless steel may have a pitting corrosion index of 18 or more obtained by the following formula (1).
[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
In the formula, [% Cr] is a ratio (mass%) of chromium contained in the entire stainless steel, [% Mo] is a ratio (mass%) of molybdenum contained in the entire stainless steel, and [% N ] Is the ratio (mass%) of nitrogen contained in the whole stainless steel.

  In the method for inhibiting local corrosion of stainless steel according to the above aspect, the molar concentration of chloride ions contained in the moisture may be 0.6 mol / l or less.

  In the method for inhibiting local corrosion of stainless steel according to the above aspect, the temperature of the moisture may be controlled to 60 ° C. or less.

  In the method for suppressing local corrosion of stainless steel according to the above aspect, the radiation dose rate of the radiation may be 5 kGy / h or less.

  In the metal container storage method according to the second aspect of the present invention, in the stainless steel metal container irradiated with radiation, the molar concentration of chloride ions contained in the water stored or retained in the container, In this method, the molar concentration of nitrate ions is increased by a factor of 1 or more.

  In the metal container storage method of the above aspect, nitrate is added to the water stored or retained in the container, and the molar concentration of nitrate ion in the water stored or retained in the container is determined by the molar concentration of chloride ions. You may adjust 1 time or more with respect to a density | concentration.

  In the metal container storage method of the above aspect, the adjustment liquid in which nitrate ions having a molar concentration of 1 or more times the molar concentration of chloride ions coexist is passed through the metal container and stored or retained in the container The molar concentration of nitrate ions in the water to be used may be adjusted to 1 or more times the molar concentration of chloride ions.

  In the metal container storage method of the above aspect, the metal container may be an adsorption tower filled with a radionuclide adsorbent.

  ADVANTAGE OF THE INVENTION According to this invention, the local corrosion suppression method of the stainless steel which can suppress local corrosion simply and effectively also in the environment where a radiation is irradiated can be provided.

It is a schematic diagram which shows the test piece used in the constant potential crevice corrosion test. It is a cross-sectional schematic diagram of the test apparatus used for the constant potential crevice corrosion test. It is a figure which shows operation in a constant potential crevice corrosion test. It is a graph which shows the corrosion current behavior in Step1 of a constant potential crevice corrosion test. It is a graph which shows the corrosion current behavior in Step3 of the reference examples 1 and 2 at the time of simulating the environment where radiation is not irradiated. It is a graph which shows the corrosion current behavior in Step3 of Examples 1-3 and the comparative example 1 at the time of simulating the environment where a radiation is irradiated. It is a graph which shows the measurement result of the pitting corrosion potential of Examples 4-6.

(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 wet environment irradiated with radiation.

  In the method for suppressing local corrosion of stainless steel according to the present embodiment, “wet environment” refers to an environment in which moisture is constantly in contact with a structure made of stainless steel. For example, when water is flowing through 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 Refers to each environment where the steel frame is splashed with water. The method for suppressing local corrosion of stainless steel according to this embodiment can be widely applied in these environments.

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

  In this specification, “occurrence of local corrosion” means that the passive film is locally damaged on the surface of stainless steel in contact with moisture, and changes from a state without local corrosion to a state where local corrosion is observed. Point to.

  Further, in this specification, “progress of local corrosion” refers to expansion of the range of local corrosion that has occurred in stainless steel. “Expansion of the local corrosion range” includes both the expansion of the local corrosion in the surface direction of the stainless steel surface and the expansion of the 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 irradiated with radiation. The “environment in which radiation is irradiated” includes a case where radiation is irradiated from the outside to stainless steel, and a case where radiation is irradiated from a radionuclide present in water that contacts the stainless steel.

  When irradiated with radiation, the radiolysis of water in contact with the stainless steel proceeds. When water is radiolyzed, the concentration of hydrogen peroxide contained in the water that contacts the stainless steel increases. Since hydrogen peroxide is highly corrosive (oxidative), corrosion of stainless steel is promoted. The increase in the 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 radionuclides are present in the water that contacts the stainless steel, moisture evaporates due to the decay heat of the nuclides, increasing the chloride ion concentration in the water that contacts the stainless steel. Stainless steel local corrosion is more likely to occur as the chloride ion concentration increases. Therefore, corrosion of stainless steel is also promoted by increasing the chloride ion concentration.

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

  In the method for inhibiting local corrosion of stainless steel according to the present embodiment, the molar concentration of nitrate ions is set to a predetermined concentration or higher with respect to the molar concentration of chloride ions contained in the moisture in contact with the stainless steel. It is possible to suppress both the occurrence and progression of corrosion. It is thought that nitrate ion functions as a corrosion inhibitor and suppresses the occurrence and progression of local corrosion.

  In an environment where radiation is irradiated, the added corrosion inhibitor may be decomposed or altered. Therefore, a corrosion inhibitor that is widely known in an environment where radiation is not irradiated cannot be used as it is in an environment where radiation is irradiated. For example, nitrite ions, which are widely used as inhibitors against corrosion of carbon steel, have the property of being altered when irradiated with radiation.

  In contrast, nitrate ions are not decomposed or altered by irradiation with radiation. Therefore, local corrosion of stainless steel can be stably suppressed even in an environment where radiation is irradiated.

  The molar concentration of nitrate ions contained in the moisture in contact with the stainless steel is preferably at least 1 time, more preferably at least 5 times the molar concentration of chloride ions contained in the moisture. .

  If the molar concentration of nitrate ions contained in the water is 1 or more times the molar concentration of chloride ions, the chloride ion concentration of water in contact with stainless steel in the environment irradiated with radiation is equivalent to seawater. Even in this case, it is possible to prevent the occurrence of pitting corrosion, which is a kind of local corrosion. Further, the progress of crevice corrosion, which is also a kind of local corrosion, can be suppressed.

  In addition, if the molar concentration of nitrate ions contained in the water is 5 times or more than the molar concentration of chloride ions, the chloride ion concentration of water in contact with the stainless steel in the environment irradiated with radiation is seawater. Even in a considerable case, the progress of crevice corrosion, which is a kind of local corrosion, can be stopped.

  Among local corrosions such as pitting corrosion, crevice corrosion, and stress corrosion cracking, other local corrosions can be prevented if the most likely crevice corrosion can be prevented. Moreover, since it is more difficult to suppress the progress of the local corrosion than the occurrence, the measures that can stop the progress of the corrosion can also prevent the occurrence of the corrosion. Therefore, if the molar concentration of nitrate ions contained in moisture in a radiation environment is 5 times or more the molar concentration of chloride ions, it can be said that the occurrence and progression of all local corrosion can be prevented.

  Examples of stainless steel include commonly known stainless steel. In order to 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 resistance index (PRE) obtained by the following formula (1) of 18 or more, more preferably 23 or more, and more preferably 25 or more. More preferably, some stainless steel is used.

[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
In the formula, [% Cr] is a ratio (mass%) of chromium contained in the entire stainless steel, [% Mo] is a ratio (mass%) of molybdenum contained in the entire stainless steel, and [% N ] Is the ratio (mass%) of nitrogen contained in the whole stainless steel.

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

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

  The method for suppressing local corrosion of stainless steel according to this embodiment is applicable even if the molar concentration of chloride ions contained in moisture is, for example, a concentration equivalent to seawater. Specifically, if the molar concentration of chloride ion contained in the water is 0.6 mol / l or less, it can be suitably applied.

  In addition, each reaction constituting the occurrence of local corrosion and the progress of local corrosion tends to proceed kinetically when the temperature of moisture is high. Therefore, the lower the temperature of the moisture that contacts the stainless steel, the better. For example, by controlling to 60 ° C. or lower, 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 according to the present embodiment can be enhanced.

  The higher the radiation dose rate, the higher the concentration of hydrogen peroxide that is generated, which promotes local corrosion. Therefore, the lower the irradiation dose rate, the better. For example, if it is 5 kGy / h or less, the effect of the local corrosion inhibiting method for stainless steel of this embodiment can be enhanced.

  In the method for inhibiting local corrosion of stainless steel according to the present embodiment, nitrate is dissolved in moisture based on the step of measuring the molar concentration of chloride ions contained in moisture and the measured value of the molar concentration of chloride ions. And a step of coexisting nitrate ions having a molar concentration of 1 or more times the measured value. By setting it as such a method, it becomes possible to make the nitrate ion required for suppression of local corrosion coexist reliably in the water | moisture content which contacts stainless steel.

  There is no restriction | limiting in the cation used as the pair of nitrate ion added to a water | moisture content. For example, a sodium salt or potassium salt can be used.

  If the molar concentration of chloride ions contained in moisture is known, or if the molar concentration of chloride ions fluctuates, the representative value and the fluctuation range are known. It is better to use nitrate ions in water. For example, when seawater is assumed as water, the molar concentration of chloride ions contained in seawater may vary depending on the ocean region or season, but the molar concentration is more than 1 time based on the upper limit of the fluctuation range. By allowing the nitrate ions to coexist, it is possible to reliably allow the nitrate ions necessary for suppressing local corrosion to coexist with the moisture.

  According to the method for suppressing local corrosion of stainless steel as described above, local corrosion can be easily and effectively suppressed.

(Second Embodiment: Storage Method for Metal Container)
In the metal container storage method according to the second embodiment of the present invention, in the stainless steel metal container irradiated with radiation, the molar concentration of chloride ions contained in the water stored or staying in the container, In this method, the molar concentration of nitrate ions is increased by a factor of 1 or more. Further, the molar concentration of nitrate ions with respect to the molar concentration of chloride ions contained in the water stored or staying in the container is preferably 5 times or more.

  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.

  A metal container does not ask | require especially a shape, a use, etc. Stainless steel metal containers that are exposed to radiation environments include tanks and adsorption towers for storing or purifying contaminated water containing radionuclides, and pools for storing spent fuel that emits radiation. and so on. In the following, the description will proceed with an example of an adsorption tower, but the metal container is not limited to the adsorption tower.

  The adsorption tower is a metal container filled with a substance having a function of adsorbing a radionuclide (a radionuclide adsorbent). The contaminated water containing the radionuclide is passed through the adsorption tower, the radionuclide is adsorbed in the adsorption tower, and the contaminated water is purified.

  The contaminated water may contain a large amount of chloride ions. For example, at the Fukushima Daiichi Nuclear Power Station, seawater must be used to cool the reactor, and polluted water containing chloride ions and radionuclides is generated.

  Since the used adsorption tower emits strong radiation from the adsorbed radionuclide, it cannot be easily approached by humans. Therefore, it is disposed of after being stored for a predetermined period while containing the adsorbent. Because it contains a large amount of radioactive material, it is necessary to be careful not to cause damage due to aging such as corrosion in the adsorption tower, which is a metal container.

  In order to prevent aged deterioration such as corrosion, water or chloride water may be drained or passed as fresh water as a measure for removing water and chloride ions, which are 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 as residual water in the adsorption tower.

  The radionuclide adsorbed on the adsorbent emits radiation in the adsorption tower. That is, the stainless steel constituting the adsorption tower becomes an environment where radiation is irradiated.

  That is, the portion of the adsorption tower that comes into contact with the residual water becomes a “humid environment where radiation is irradiated”. As described above, hydrogen peroxide, which is highly corrosive due to radiation, is generated, resulting in a more severe corrosive environment 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. For example, in the case of a water storage tank or the like, there is a case of “retention” instead of “retention”.

  In this embodiment, in the water staying inside the metal container, the nitrate ion concentration with respect to the chloride ion concentration is adjusted to 1 or more times. Adjustment of the nitrate ion concentration can be performed by adding nitrate to the water staying in the container. Further, the adjustment of the nitrate ion concentration may be carried out by passing an adjustment solution adjusted so that nitrate ions having a molar concentration of 1 or more times the molar concentration of chloride ions coexist in the metal container. . Note that “water flow” can be performed for purposes such as “cleaning” and “replacement” regardless of the purpose.

  When the nitrate ion concentration with respect to the chloride ion concentration in the water staying in the metal container is adjusted to 1 or more times, the 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. Moreover, when the nitrate ion concentration with respect to the chloride ion concentration in the water staying in the metal container is adjusted to 5 times or more, the corrosion of the metal container is further suppressed, and the metal container can be stored more stably.

  In the metal container storage method, the first method of adjusting nitrate ion concentration by adding nitrate into the adsorption tower, and the adjustment liquid with adjusted nitrate ion concentration are passed through the adsorption tower to form nitrate ions. There is a second method for adjusting the density.

The first method is a method in which nitrate is added to the residual water remaining in the adsorption tower and the nitrate is dissolved in the residual water.
The amount of nitrate to be added is set so that the molar concentration of nitrate ions in the residual water is one or more times the molar concentration of chloride ions. Further, the amount of nitrate added is preferably such that the molar concentration of nitrate ions in the residual water is at least 5 times the molar concentration of chloride ions.

  In the first method, it is preferable to pass fresh water through the adsorption tower before adding the nitrate. By passing the water through fresh water, the chloride ion concentration of the residual water can be reduced. If the chloride ion concentration in the residual water is lowered, the amount of nitrate added is reduced.

  In addition, when fresh water is passed, the chloride ion concentration of residual water falls sufficiently. Therefore, stainless steel in contact with residual water is rarely corroded. However, if the residual water is concentrated and the chloride ion concentration is increased, local corrosion may occur. Leakage of radioactive material 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 nitrate ion concentration in the residual water is equal to or higher than the predetermined concentration. Therefore, it can suppress that the stainless steel which comprises an adsorption tower corrodes. That is, the adsorption tower can be safely and stably stored while avoiding damage to the adsorption tower due to aging and other deterioration.

  The first method is particularly useful in increasing the storage stability of an adsorption tower that has already started storage. It is only necessary to add nitrate to the adsorption tower being stored, and the storage stability of the adsorption tower can be easily improved without obtaining complicated operations.

The second method is a method of passing an adjustment liquid having a adjusted nitrate ion concentration through the adsorption tower.
The adjustment liquid is water adjusted so that the nitrate ion concentration is 1 or more times the chloride ion concentration contained in fresh water. Further, the adjustment liquid is preferably adjusted so that the nitrate ion concentration is 5 times or more the chloride ion concentration contained in the fresh water.

  When the adjustment liquid is passed through the adsorption tower, a part of the adjustment liquid remains as residual water in the adsorption tower. However, in the adjustment liquid, the chloride ion concentration and the nitrate ion concentration are already adjusted to a predetermined relationship. Therefore, it can suppress that the stainless steel which comprises an adsorption tower corrodes. That is, the adsorption tower can be safely and stably stored while avoiding damage to the adsorption tower due to aging and other deterioration.

  The second method is particularly useful in increasing the storage stability of the adsorption tower that starts storage. When the adjustment liquid is used, it is not necessary to check the chloride ion concentration in the residual water. The confirmation of the chloride ion concentration in the residual water is an operation under a radiation environment, and it is very useful that the confirmation operation is eliminated. In addition, it is not necessary to provide special equipment or the like, and it can be carried out only by replacing the fresh water flow process that is normally performed to reduce the chloride ion concentration.

  The stainless steel used for the adsorption tower is preferably SUS304 grade or higher stainless steel, more preferably SUS316L grade stainless steel. A well-known thing can be used for a radionuclide adsorption material. For example, zeolite can be used.

  The radionuclide adsorbed in the adsorption tower is not particularly limited. For example, cesium 137, cobalt 60, strontium 90, iodine 131, etc. are mentioned.

  As described above, if the metal container storage method according to the present embodiment is used, the concentration of nitrate ions in the residual water becomes equal to or higher than a predetermined concentration, and corrosion of the stainless steel constituting the adsorption tower can be suppressed. That is, the adsorption tower can be safely and stably stored while avoiding damage to the adsorption tower due to aging and other deterioration.

  In addition, the residual water may evaporate during the storage process, and the chloride ion concentration in the residual water may increase. However, even if part of the residual water evaporates, the ratio between the chloride ion concentration and the nitrate ion concentration in the residual water is kept constant, so that the effect of suppressing corrosion of the adsorption tower is not lost.

  As mentioned above, although the preferred embodiment which concerns on this invention was described, it cannot be overemphasized that this invention is not limited to the example which concerns. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

[Example]
EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[1. Verification of the effect on the progress of local corrosion (crevice corrosion)]
(Test pieces)
FIG. 1 is a schematic view showing a test piece used in this example, in which FIG. 1 (a) is a side view and FIG. 1 (b) is a front view.

  As shown in the figure, in this example, a flat test piece 1 made of stainless steel of 50 mm × 24 mm × 3 mm was used. As the stainless steel, a commercially available SUS316L solution material was used, and it was produced by machining. The chemical composition (mass%) of SUS316L is as follows: carbon (C): 0.011%, silicon (Si): 0.61%, manganese (Mn): 0.83%, phosphorus (P ): 0.024%, sulfur (S): 0.001%, chromium (Cr): 17.36%, nickel (Ni): 12.31%, molybdenum (Mo): 2.09%. The pitting corrosion index of SUS316L was 24.3.

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

  Such a test piece 1 is sandwiched between a cylindrical polysulfone gasket 2 having a diameter of 20 mm and a thickness of 5 mm and a washer 3, and a bolt 4 is inserted into a bolt hole of the test piece 1, and then a nut 5 is used. Screwed with a torque of 0 N · m. The washer 3, the bolt 4 and the nut 5 were made of industrial pure titanium, but these were insulated from the test piece 1.

  Furthermore, the lead wire 6 was electrically connected to the other end side of the test piece 1, and the lead wire 6 was covered with an insulating film 7.

(Constant potential crevice corrosion test)
FIG. 2 is a schematic cross-sectional view of a test apparatus used in the constant potential crevice corrosion test.
Ion exchange water and an aqueous NaCl solution prepared from a special grade reagent of sodium chloride (NaCl) were injected into the electrolytic cell 14. Then, the temperature of the electrolytic cell 14 was raised to 60 ° C. with the thermostatic chamber 15 and the thermometer 10. Next, argon gas was passed through the NaCl aqueous solution using the bubble tube 11 and the cooler 13 to remove oxygen in the solution.

  In addition, the lower part of the above-mentioned test piece 1 about 35 mm and the counter electrode 8 made of platinum (Pt) were immersed in an aqueous NaCl solution. The test piece 1 and the counter electrode 8 after immersion were connected to a potentio / galvanostat 16 (manufactured by Solartron, electrochemical measurement system, model number: 1280Z). The potential of the test piece 1 was measured with reference to the potential of the reference electrode 9 made of a silver / silver chloride electrode in a saturated KCl solution that was electrically conducted through the salt bridge 12. However, the potentials of the following examples are shown by values corrected by adding 0.196 V to the measured values and standard hydrogen electrode potential (SHE) standards.

  FIG. 3 is a diagram showing the following operations. FIG. 3A is a graph showing changes in applied potential, where the horizontal axis represents test time (unit: time) and the vertical axis represents potential (unit: V). FIG. 3B is a graph showing the corrosion current measured on the test piece 1 by the potential change shown in FIG. 3A, and the horizontal axis represents the test time (unit: same scale as FIG. 3A). : Time), and the vertical axis represents the measured corrosion current (unit: μA).

  First, the potentio / galvanostat 16 is set in the potential control mode, and the crevice corrosion is forcibly generated by sweeping the potential of the test piece 1 immersed in the NaCl aqueous solution at 60 ° C. in the anode direction at a sweep rate of 30 mV / min. (Step 1). Metal corrosion dissolution is an oxidation reaction in which a metal element emits electrons and ionizes. Therefore, the corrosion rate is expressed by current (strictly, current density). That is, when crevice corrosion occurs in Step 1, the current flowing through the test piece (referred to as corrosion current) increases rapidly. Therefore, it is possible to determine whether crevice corrosion has occurred by confirming the behavior of the corrosion current value.

  After the corrosion current reached 1 mA, the potentio / galvanostat 16 was switched to the current control mode, the corrosion current was kept constant at 800 μA for 24 hours, and crevice corrosion was grown (Step 2).

Next, the potentio / galvanostat 16 was switched to the potential control mode, and the potential of the test piece 1 was held at a constant holding potential (E _hold ). Four hours after the start of potential holding, a necessary amount of drug was added (Step 3). As illustrated in FIG. 3B, if a decrease tendency of the corrosion current is confirmed after the addition of the agent of Step 3, it can be determined that there is an effect of suppressing the progress of crevice corrosion under the condition of the potential E _hold .

Here, the corrosion inhibitory effect in various environmental conditions can be examined by changing the holding potential (E _hold ). For example, it is known that stainless steel immersed in neutral natural water is held at a potential of about 0.3 V by the oxidizing power of dissolved oxygen in water. Therefore, when simulating the condition where no radiation is irradiated (non-irradiation), the holding potential (E _hold ) is set to 0.3V. On the other hand, in an environment where a radiation having a dose rate of 0 to 5 kGy / h is irradiated, there is a report that the potential is maintained at a potential of about 0.5 V by the action of hydrogen peroxide having a stronger oxidizing power than dissolved oxygen. is there. Therefore, when simulating the condition of irradiation with radiation, the holding potential (E _hold ) was set to 0.5V. That is, by changing the holding potential, the condition of irradiation with radiation was simulated.

(Reference Example 1)
The concentration of the NaCl solution was 0.056 mol / l. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 0.056 mol / l, which is one time the chloride ion concentration. Assuming environment where radiation is not irradiated, holding potential was set to 0.3V.

(Reference Example 2)
The concentration of the NaCl solution was 0.56 mol / l equivalent to seawater. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 0.56 mol / l, which is one time the chloride ion concentration. Assuming environment where radiation is not irradiated, holding potential was set to 0.3V.

Example 1
The concentration of the NaCl solution was 0.056 mol / l. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 0.056 mol / l, which is one time the chloride ion concentration. Assuming an environment in which radiation is irradiated as an assumed environment, the holding potential was set to 0.5V.

(Example 2)
The concentration of the NaCl solution was 0.56 mol / l equivalent to seawater. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 0.56 mol / l, which is one time the chloride ion concentration. Assuming an environment in which radiation is irradiated as an assumed environment, the holding potential was set to 0.5V.

(Example 3)
The concentration of the NaCl solution was 0.56 mol / l equivalent to seawater. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 2.8 mol / l, five times the chloride ion concentration. Assuming an environment in which radiation is irradiated as an assumed environment, the holding potential was set to 0.5V.

(Comparative Example 1)
The concentration of the NaCl solution was 0.56 mol / l equivalent to seawater. Then, as an agent to be added in Step3, it was added sodium nitrate (NaNO _3). The nitrate ion concentration in the aqueous solution after the addition was 0.11 mol / l, which is 0.2 times the chloride ion concentration. Assuming an environment in which radiation is irradiated as an assumed environment, the holding potential was set to 0.5V.

  Table 1 shows the measurement conditions and results of the corrosion inhibition effect of Reference Examples 1 and 2, Examples 1 to 3, and Comparative Example 1. In Table 1, “◯” means that the progress of corrosion could be stopped, and “Δ” means that the progress of corrosion could be remarkably suppressed, but not stopped. Further, “x” means a case where the corrosion is hardly suppressed and continues to proceed at a high corrosion rate.

  FIG. 4 is a graph showing the corrosion current behavior in Step 1 of the above-mentioned constant potential crevice corrosion test, where the horizontal axis represents the elapsed time (Unit: hours) of Step 1 and the vertical axis represents the measured corrosion current (Unit: μA). Show. In FIG. 4, the corrosion current values measured in Examples 1 to 3 and Reference Examples 1 and 2 are shown superimposed.

  In the corrosion current behavior of FIG. 4, the stagnation current before the point at which the corrosion current rapidly increases is about 20 μA to 50 μA. According to Faraday's law, this stagnation current density is as small as 0.01 to 0.03 mm / year when converted to the corrosion rate of SUS316L steel. Therefore, it is considered that crevice corrosion has not yet occurred in the test piece 1 in a state showing such a stagnation current value, and a healthy passive state is maintained.

The lower limit value of the corrosion current I = 20 μA to 50 μA measured in Step 1 was defined as a passive state holding current I _pass . If corrosion current values measured becomes equal to or less than the passivation holding current I _pass, crevice corrosion of the test piece 1 stops the progression, it can be determined that returning to state again healthy passive.

  FIG. 5 is a graph showing the corrosion current behavior of Reference Examples 1 and 2 in Step 3 when simulating a non-irradiated environment in the above-described constant potential crevice corrosion test, corresponding to Step 3 in FIG. It is a graph to do. The horizontal axis in FIG. 5 represents the elapsed time (unit: time) of Step 3, and the vertical axis represents the measured corrosion current (unit: μA). FIG. 5 shows the corrosion current values measured in Reference Example 1 and Reference Example 2 in an overlapping manner.

As shown in the figure, in both Reference Example 1 and Reference Example 2, the corrosion current is remarkably reduced after NaNO _{3 is} added. Both were below _Ipass within 120 hours. Thereafter, the corrosion current does not exceed I _pass, and it is considered that the progress of crevice corrosion has stopped.

  From these results, in a non-irradiated environment, if the nitrate ion concentration in the aqueous solution is 1 or more times the chloride ion concentration in the aqueous solution, local corrosion of stainless steel can be stopped. It could be confirmed.

  FIG. 6 is a graph showing the corrosion current behavior of Examples 1 to 3 and Comparative Example 1 in Step 3 when simulating the environment irradiated with radiation in the above-described constant potential crevice corrosion test. It is a graph corresponding to Step3. The horizontal axis of FIG. 6 is the elapsed time (unit: time) of Step 3, and the vertical axis shows the measured corrosion current (unit: μA). In FIG. 6, the corrosion current values measured in Examples 1 to 3 and Comparative Example 1 are shown in an overlapping manner.

In Example 1 in which the chloride ion concentration condition was equivalent to one-tenth of seawater, the corrosion current value decreased markedly after addition of NaNO _{3 at the} same molar concentration as the chloride ion, and fell below _Ipass within 40 hours. It was. That is, it is considered that the progress of crevice corrosion has stopped.

In Example 2 in which the chloride ion concentration condition was equivalent to seawater, the corrosion current was significantly reduced after about 100 hours had elapsed after the addition of NaNO ₃ having the same molar concentration as the chloride ion. The addition of NaNO ₃ reduced the corrosion current by two orders of magnitude or more, but did not fall below I _pass . That is, the progress of crevice corrosion was suppressed, but it did not stop.

On the other hand, as shown in Example 3, when the nitrate ion concentration was increased to 5 times the chloride ion concentration, the corrosion current value was remarkably reduced and was less than I _pass within 30 hours. That is, it is considered that the progress of crevice corrosion has stopped.

  From these results, it is possible to suppress the progress of crevice corrosion by setting the nitrate ion concentration in the aqueous solution to 1 or more times the chloride ion concentration in the aqueous solution even in an environment irradiated with radiation. It was. It was also confirmed that the progress of local corrosion of stainless steel could be stopped by setting the nitrate ion concentration in the aqueous solution to 5 times or more the chloride ion concentration in the aqueous solution.

  On the other hand, as shown in Comparative Example 1, when the nitrate ion concentration to be added was reduced to 0.2 times the chloride ion concentration, the corrosion current gradually decreased. However, a significant decrease in the corrosion current as seen in Examples 1 to 3 was not confirmed. In the experiment of Comparative Example 1, since the corrosion progressed violently, a part of the test piece was missing, and it was suggested that this might be a factor of current reduction. That is, in order to suppress the progress of crevice corrosion in an environment irradiated with radiation, it is not sufficient to add nitrate ion 0.2 times the chloride ion concentration, and it is necessary to make it 1 or more times. I understood it.

[2. Verification of effects on suppression of occurrence of local corrosion (pitting corrosion)]
(Flat plate test piece)
In this example, a stainless steel flat plate test piece of 15 mm × 15 mm × 4 mm was used. As stainless steel, the same SUS316L steel solution material as the test piece 1 was used. Moreover, a lead wire was electrically connected to one end, and the lead wire was covered with an insulating film.

  The flat plate test piece was wet-polished using 600-mesh water-resistant abrasive paper, and then immersed in 30% nitric acid at 50 ° C. for 1 hour for passivation treatment.

(Pitting corrosion potential measurement)
In accordance with JIS G 0577 “Method for Measuring Pitting Corrosion Potential of Stainless Steel”, the pitting corrosion potential of stainless steel was measured by the following method. Here, the “pitting corrosion potential” is a lower limit potential at which pitting corrosion can occur, and the higher the value, the less pitting corrosion occurs.

First, the flat plate test piece was held in a tetrafluoroethylene jig provided with an opening having a diameter of 1.13 cm so that the surface of the flat plate test piece was in contact with the test solution by 1 cm ² .

  Immediately before the measurement, the surface of the wetted part of the flat test piece was dry-polished using 600 mesh abrasive paper and then washed with distilled water. A test solution in which NaCl and sodium nitrate were dissolved in predetermined amounts was heated to 60 ° C. and degassed by nitrogen gas ventilation for 2 hours or more, and then the flat plate test piece was immersed in the test solution.

Further, after immersing a counter electrode of platinum and measuring the natural potential of the flat plate test piece for 10 minutes, using a potentio / galvanostat (manufactured by Hokuto Denko Corporation, electrochemical measurement system, model number: HZ-5000), The potential was swept in the anode direction at 20 mV / min, and the measurement was terminated when the anode current density reached 1000 μA / cm ² .

Based on the provisions of JIS G 0577, the most noble values corresponding to the anode current density of 10 μA / cm ² in the obtained potential-current curve were determined as the pitting potential V _{C and PIT 10} . However, _{VC and PIT10} by the measurement in which pitting corrosion was not confirmed in the optical microscope observation of the flat test piece after the measurement were excluded from the results.

Example 4
Compared to 0.0056 mol / l NaCl aqueous solution, the nitrate ion concentration in the aqueous solution is 0,0 times (no sodium nitrate added), 0.1 times, 1 time the chloride ion concentration of 0.0056 mol / l. A solution in which a necessary amount of sodium nitrate was dissolved was used as a test solution, and pitting potential _VC and _PIT10 were measured, respectively.

(Example 5)
Compared to 0.056 mol / l NaCl aqueous solution, the nitrate ion concentration in the aqueous solution is 0 times (no sodium nitrate added), 0.1 times, 1 time the 0.056 mol / l chloride ion concentration. A solution in which a necessary amount of sodium nitrate was dissolved was used as a test solution, and pitting potential _VC and _PIT10 were measured, respectively.

(Example 6)
With respect to an aqueous NaCl solution having a concentration of 0.56 mol / l, the nitrate ion concentration in the aqueous solution is 0 times (no sodium nitrate added), 0.1 times and 1 times the chloride ion concentration of 0.56 mol / l. A solution in which a necessary amount of sodium nitrate was dissolved was used as a test solution, and pitting potential _VC and _PIT10 were measured, respectively.

FIG. 7 is a graph showing the results of Examples 4 to 6, in which the horizontal axis indicates the molar concentration ratio of nitrate ion to chloride ion, and the vertical axis indicates the pitting potential V _{C, PIT10} (unit: V ).

  As shown in the figure, in Examples 4 to 6, when nitrate ions having a chloride ion concentration of 0.1 times coexist, the change in pitting potential is small compared to the case where nitrate ions are not added. Therefore, the effect of suppressing pitting corrosion at this nitrate ion concentration ratio is small.

  On the other hand, in Examples 4 to 6, when nitrate ion having a concentration of 1 times the chloride ion concentration was allowed to coexist, pitting corrosion did not occur even when the potential was made noble to 1.2V. That is, the pitting potential was found to be higher than at least 1.2V. As described above, even in an environment in which intense radiation having a dose rate of 0 to 5 kGy / h is irradiated, the potential at which the material is held is about 0.5 V and does not exceed 1.2 V. Therefore, it was found that the occurrence of pitting corrosion, which is a kind of local corrosion, can be prevented by coexisting with nitrate ions having a concentration of 1 or more times the chloride ion concentration even in an environment irradiated with radiation.

  From these results, compared with the conventional local corrosion control method using a corrosion inhibitor, the present invention is effective in suppressing the occurrence and progression of local corrosion of stainless steel even in an environment irradiated with radiation. Was also found to be very effective.

DESCRIPTION OF SYMBOLS 1 ... Test piece, 2 ... Gasket, 3 ... Washer, 4 ... Bolt, 5 ... Nut, 6 ... Lead wire, 7 ... Insulating film, 8 ... Counter electrode, 9 ... Reference electrode, 10 ... Thermometer, 11 ... Bubble tube, 12 ... salt bridge, 13 ... cooler, 14 ... electrolytic cell, 15 ... constant temperature bath, 16 ... potentio / galvanostat

Claims (11)

A method for suppressing local corrosion of stainless steel in a wet environment irradiated with radiation,
A method for inhibiting local corrosion of stainless steel in which nitrate ions having a molar concentration of 1 or more times coexist with the molar concentration of chloride ions contained in the moisture in the moisture in contact with the stainless steel. Measuring the molar concentration of chloride ions contained in the moisture;
2. The step of dissolving nitrate in the water based on a measured value of the molar concentration of the chloride ion and coexisting nitrate ions having a molar concentration of 1 or more times the measured value. To prevent local corrosion of stainless steel.   The method for inhibiting local corrosion of stainless steel according to claim 1, wherein the molar concentration of the nitrate ions is 5 times or more the molar concentration of chloride ions contained in the moisture. The method for inhibiting local corrosion of stainless steel according to any one of claims 1 to 3, wherein the stainless steel has a pitting corrosion index determined by the following formula (1) of 18 or more.
[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
(In the formula, [% Cr] is the ratio (mass%) of chromium contained in the entire stainless steel, and [% Mo] is the ratio (mass%) of molybdenum contained in the entire stainless steel, [% N] is the ratio (mass%) of nitrogen contained in the entire stainless steel)   The method for inhibiting local corrosion of stainless steel according to any one of claims 1 to 4, wherein a molar concentration of chloride ions contained in the moisture is 0.6 mol / l or less.   The method for suppressing local corrosion of stainless steel according to any one of claims 1 to 5, wherein the temperature of the moisture is controlled to 60 ° C or lower.   The method for suppressing local corrosion of stainless steel according to any one of claims 1 to 6, wherein an irradiation dose rate of the radiation is 5 kGy / h or less. A method for storing a metal container using the method for inhibiting local corrosion of stainless steel according to any one of claims 1 to 7,
In a stainless steel metal container irradiated with radiation, a method for storing a metal container in which the molar concentration of nitrate ions is one or more times the molar concentration of chloride ions contained in water stored or retained in the container .   Nitrate is added to the water stored or retained in the container, and the molar concentration of the nitrate ion of water stored or retained in the container is adjusted to 1 or more times the molar concentration of chloride ion. The method for storing a metal container according to claim 8.   The adjustment solution in which nitrate ions having a molar concentration of 1 times or more with respect to the molar concentration of chloride ions coexist is passed through a metal container, and the molar concentration of nitrate ions of water stored or retained in the container is determined. The method for storing a metal container according to any one of claims 8 and 9, further comprising a step of adjusting the molar concentration of chloride ions to 1 or more times.   The method for storing a metal container according to any one of claims 8 to 10, wherein the metal container is an adsorption tower filled with a radionuclide adsorbent.

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