Corrosion-resistant structure for high-temperature water system and corrosion-preventing method thereof

Abstract

The present invention provides a corrosion-resistant structure for a high-temperature water system comprising: a structural material 1; and a corrosion-resistant film 3 formed from a substance containing at least one of La and Y deposited on a surface in a side that comes in contact with a cooling water 4, of the structural material 1 which constitutes the high-temperature water system that passes a cooling water 4 of high temperature therein. Due to above construction, there can be provided the corrosion-resistant structure and a corrosion-preventing method capable of operating a plant without conducting a water chemistry control of cooling water by injecting chemicals.

Claims

1. A corrosion-preventing method for a high-temperature water system of a secondary cooling system of a pressurized-water type nuclear power plant for preventing a corrosion of a structural material constituting the high-temperature water system which passes a cooling water of high temperature therein, the method comprising: preparing a corrosion inhibitor comprising at least one substance selected from the group consisting of a La compound and a Y compound; and depositing a prepared corrosion inhibitor on a surface in a side of the structural material which comes in contact with the cooling water having a temperature of 20° C. or higher and 350° C. or lower to form a corrosion-resistant film consisting of at least one selected from the group consisting of La.sub.2(CO.sub.3).sub.3 and La.sub.2(C.sub.2O.sub.4).sub.3.

2. The corrosion-preventing method for the high-temperature water system according to claim 1, further comprising previously subjecting the surface in the side of the structural material which comes in contact with the cooling water to any one of a treatment selected from the group consisting of machining treatment, immersion treatment in high-temperature water and chemical cleaning treatment, before depositing the corrosion inhibitor.

3. The corrosion-preventing method for the high-temperature water system according to claim 1, wherein the structural material is at least one structural material selected from the group consisting of a carbon steel, a copper alloy and a Ni-based alloy.

4. The corrosion-preventing method for the high-temperature water system according to claim 1, wherein a deposition amount of the substance is 1 μg/cm.sup.2 or more and 200 μg/cm.sup.2 or less.

5. The corrosion-preventing method for the high-temperature water system according to claim 1, further comprising forming an oxide film of the structural material on a surface of the structural material, and then depositing the corrosion inhibitor on a surface of the oxide film.

6. The corrosion-preventing method for the high-temperature water system according to claim 1, wherein the corrosion-resistant film consists of La.sub.2(CO.sub.3).sub.3.

7. The corrosion-preventing method for the high-temperature water system according to claim 1, wherein the corrosion-resistant film consists of La.sub.2(C.sub.2O.sub.4).sub.3.

8. A method for preventing corrosion of a secondary cooling system of a pressurized-water type nuclear power plant high-temperature water system, comprising: depositing a corrosion inhibitor comprising at least one substance selected from the group consisting of a La compound and a Y compound on a water-contact surface of the secondary cooling system to form a corrosion resistant film on the water-contact surface of the secondary cooling system, wherein the corrosion resistant film consists of at least one selected from the group consisting of La.sub.2(CO.sub.3).sub.3 and La.sub.2(C.sub.2O.sub.4).sub.3, wherein the secondary cooling system passes cooling water having a temperature of 20° C. or higher and 350° C. or lower during operation.

9. The method according to claim 8, further comprising: before the depositing, subjecting the water-contact surface to any one treatment selected from the group consisting of a machining treatment, an immersion treatment in high-temperature water and a chemical cleaning treatment.

10. The method according to claim 8, wherein the structural material is at least one selected from the group consisting of a carbon steel, a copper alloy and a Ni-based alloy.

11. The method according to claim 8, wherein a deposition amount of the substance is 1 μg/cm.sup.2 or more and 200 μg/cm.sup.2 or less on the water-contact surface of the structural material.

12. The method according to claim 8, further comprising: forming an oxide film on the water-contact surface of the structural material, then depositing the substance on the oxide film.

13. The method according to claim 8, wherein the corrosion-resistant film consists of La.sub.2(C.sub.2O.sub.4).sub.3.

14. The method according to claim 8, wherein the depositing forms a corrosion-resistant film consisting of La.sub.2(CO.sub.3).sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are cross-sectional views illustrating examples of a corrosion-suppressing structure for conducting a corrosion-resistant structure and a corrosion-preventing method for a high-temperature water system according to the present invention; FIG. 1A is a view of an example in which a corrosion-resistant film containing La is formed on a surface of a structural material (substrate, base member) having an oxide film formed thereon; and FIG. 1B is a cross-sectional view illustrating an example in which a corrosion-resistant preventing film containing La is directly formed on a surface of a structural material from which an oxide film has been removed.

(2) FIG. 2 is a graph illustrating a relationship between a corrosion-suppressing effect and the corrosion-resistant structure, in the corrosion-resistant structures illustrated in FIGS. 1A and 1B.

(3) FIG. 3 is a graph illustrating a corrosion-suppressing effect of a corrosion-resistant structure which has a corrosion-resistant film formed from Y(OH).sub.3 formed thereon.

(4) FIG. 4 is a graph illustrating an influence of a temperature change of a corrosion-resistant structure having the corrosion-resistant film formed from Y(OH).sub.3 formed thereon.

(5) FIG. 5 is a graph illustrating a relationship between deposition amounts of corrosion-resistant films and corrosion amounts (corrosion rate) of structural members.

(6) FIG. 6 is a graph illustrating a relationship between a method for forming the corrosion-resistant film and a corrosion amount of a structural member.

(7) FIG. 7 is a graph illustrating a relationship between the type of chemical compounds (corrosion inhibitor) contained in corrosion-resistant films and the corrosion-suppressing effect.

MODE FOR CARRYING OUT THE INVENTION

(8) Examples of the corrosion-resistant structure and the corrosion-preventing method for the high-temperature water system according to the present invention will be more specifically described hereinbelow with reference to the attached drawings.

Example 1

(9) Firstly, an example of the present invention in which a corrosion-resistant film containing a La compound as a corrosion inhibitor is formed on a structural material will be concretely described below with reference to the attached FIGS. 1A and 1B and FIG. 2.

(10) A corrosion-resistant structure for a high-temperature water system according to the present example 1 includes two types of structures, as are illustrated in FIGS. 1A and 1B and FIG. 2. Specifically, FIG. 1A is a view of an example in which a corrosion-resistant film 3 formed from La.sub.2O.sub.3 has been formed on the surface of a carbon steel that is used as a structural material (substrate, base member) 1 and has a uniform oxide film 2 formed thereon; and FIG. 1B is a view illustrating an example (test piece) in which the corrosion-resistant film 3 formed from La.sub.2O.sub.3 has been directly formed on a surface of the structural material 1 from which an ununiform oxide film has been previously removed.

(11) For information, the oxide film 2 in FIG. 1A was formed by oxidizing a surface portion of the carbon steel which was used as the structural material 1, in the atmosphere of 150° C. In addition, a carbon steel 1 that was used as the structural material in FIG. 1B had a newly-formed surface exposed thereon which had a smooth and uniform surface roughness, by acid-pickling the surface.

(12) Next, a test piece was prepared as a Comparative Example (reference) which was formed only from a carbon steel and did not have an oxide film and a corrosion-resistant film formed thereon, in addition to the two types of the examples in which the corrosion-resistant film was prepared by depositing La.sub.2O.sub.3 on the carbon steel as was described above. The surface portions of these three types of the test pieces were subjected to a corrosion test under conditions of being immersed in the hot water which contained less than 5 ppb of dissolved oxygen and had a pH of 9.8 at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours. Corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation results are shown in FIG. 2.

(13) As is clear from the result illustrated in FIG. 2, it was proved that the corrosion rates were remarkably suppressed in the two types of the test pieces in the example in which the corrosion-resistant film 3 formed from La.sub.2O.sub.3 was deposited, in comparison with the test piece formed only from the carbon steel. In addition, it was also confirmed that the corrosion-suppressing effect became more remarkable when the oxide film 2 existed. Thus, it was proved that the corrosion-suppressing function for the carbon steel could be effectively shown by La.sub.2O.sub.3 which was deposited on the surface of the structural material.

(14) It is expected according to the above described experimental results that an effect of suppressing general corrosion due to a cooling water and an effect of suppressing a wall thinning phenomenon due to flow-accelerated corrosion can be exhibited by an La-containing compound which has been deposited on a surface of a carbon steel material constituting a secondary cooling system of a pressurized-water type atomic power generation facility.

(15) For information, it is confirmed by an experiment that the above described corrosion-preventing effect is not limited to the case in which La.sub.2O.sub.3 was used as the corrosion inhibitor but the similar effect can be shown also in the case in which La(OH).sub.3, La.sub.2(CO.sub.3).sub.3, La(CH.sub.3COO).sub.3 or La.sub.2(C.sub.2O.sub.4).sub.3 was used as the corrosion inhibitor to be deposited on the surface.

Example 2

(16) Next, an example of the present invention, in which a corrosion-resistant film containing a Y compound as a corrosion inhibitor has been formed on a structural material, will be described below with reference to the attached FIG. 3.

(17) A corrosion resistant structure for a high-temperature water system according to the present example has a structure as is illustrated in a schematic view FIG. 1B. Specifically, a surface of a test piece of the present example is a newly-formed surface which is exposed by removing the oxide film with chemicals. Y(OH).sub.3 was used as a corrosion inhibitor.

(18) Then, a corrosion-resistant film 3 was formed with the use of a spray coating method of spraying a chemical agent containing Y(OH).sub.3 onto the cooling water contact surface of a carbon steel together with nitrogen gas and depositing the chemical agent. As a result of having examined a state of the formed corrosion-resistant film 3 through SEM observation, it was confirmed that a spot-shaped lump of Y(OH).sub.3 of a micrometric order was formed on a surface portion of the carbon steel. It was proved from this observation result that the deposition uniformity of the corrosion-resistant film 3 was low and the deposition amount of Y(OH).sub.3 was 90 μg/cm.sup.2, but that the film thickness considerably dispersed or scattered depending on the site of the carbon steel.

(19) Next, a test piece was prepared as a Comparative Example (reference) which was formed only from a carbon steel and did not have an oxide film and a corrosion-resistant film formed thereon, in addition to the example in which the corrosion-resistant film was prepared by depositing Y(OH).sub.3 on the carbon steel as was described above. The surface portions of these two types of the test pieces were subjected to a corrosion test under conditions of being immersed in the hot water which contained less than 5 ppb of dissolved oxygen and had a pH of 9.8 at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours, in a similar way to that in Example 1. Corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation results are shown in FIG. 3.

(20) As is clear from the result illustrated in FIG. 3, it was proved that the corrosion rate was suppressed to approximately one-tenth in the test piece in Example 2 in which the corrosion-resistant film formed from Y(OH).sub.3 was deposited, and that an excellent corrosion-preventing effect could be shown, in comparison with the test piece formed only from the carbon steel. Thus, it was proved that the corrosion-suppressing function for the carbon steel could be effectively shown by Y(OH).sub.3 which was deposited on the surface of the structural material.

(21) It is expected on the basis of the above described experimental result that an effect of suppressing general corrosion of the structural material and an effect of suppressing a wall thinning phenomenon due to flow-accelerated corrosion are shown when Y(OH).sub.3 has been deposited on a surface of a structural material constituting a secondary cooling system of a pressurized-water type atomic power generation facility.

(22) In addition, it is confirmed by an experiment that the above described corrosion-preventing effect is not limited to the case in which Y(OH).sub.3 was used as a corrosion inhibitor, but that the similar effect can be shown also in the case in which Y.sub.2(CO.sub.3).sub.3, Y(CH.sub.3COO).sub.3 or Y.sub.2(C.sub.2O.sub.4).sub.3 was used as the corrosion inhibitor to be deposited on the surface of the structural material.

Example 3

(23) Next, an influence which a difference of an operation temperature (temperature of cooling water) gives on a corrosion-resistant structure will be described below with reference to the following Example 3 and FIG. 4.

(24) A corrosion-resistant structure for a high-temperature water system according to the present Example 3 has a structure as is illustrated in a schematic view FIG. 1B. Specifically, a test piece used for the test piece of the present example was in such a state that the surface of a carbon steel before a corrosion-resistant film was deposited thereon had been polished and degreased by a sandpaper with #600, and that an oxide film and a foreign substance had been removed therefrom.

(25) Then, the test piece according to Example 3 was prepared by depositing Y(OH).sub.3 onto the surface (newly-formed surface) of this carbon steel with a spray method. A deposition amount of Y(OH).sub.3 in this test piece was set at 50 μg/cm.sup.2 by adjustment of a spraying period of time. As a result of having examined a state of the formed corrosion-resistant film 3 through SEM observation, the uniformity was low similarly to that in Example 2.

(26) Next, a test piece was prepared as a Comparative Example which was formed only from a carbon steel and did not have an oxide film and a corrosion-resistant film formed thereon, in addition to the example in which the corrosion-resistant film was prepared by depositing Y(OH).sub.3 on the carbon steel as was described above.

(27) Then, the surface portions of these two types of the test pieces were subjected to a corrosion test under conditions of being immersed in the hot water which contained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at a temperature in two levels of 150° C. and 280° C. under a pressure of 4 MPa and 8 MPa, for 500 hours, in a similar way to that in Example 1. Corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation result is shown in FIG. 4.

(28) As is clear from the result illustrated in FIG. 4, the corrosion amount of the test piece formed only from the carbon steel also decreases under the condition that the temperature is as high as 280° C. This is considered to be because the formed oxide film has high stability because the temperature is high.

(29) On the other hand, it is understood that the corrosion rate becomes large when the temperature is 150° C. because the solubility of the oxide film to be formed under the condition of the present test is high, and that the corrosion-suppressing function works due to the deposition of Y(OH).sub.3. Therefore, the corrosion-resistant structure can be applied in such an environment that a cooling water is 20° C. or higher and 350° C. or lower which is an operation temperature of a secondary cooling system of a pressurized-water type atomic power generation facility, in view of the fact that Y(OH).sub.3 is resistant to high temperature.

(30) In addition, as is clear from FIG. 4, the corrosion-resistant structure according to the present example is particularly effective in a range of an operation temperature of 150° C. or higher after a deaerator, in a secondary cooling system of a pressurized-water type atomic power generation facility, and it is expected that an effect of suppressing an general corrosion of a structural material and a function of suppressing a wall thinning phenomenon due to flow-accelerated corrosion are effectively shown when a chemical agent containing Y is injected into the system and is deposited on a surface of a structural material.

Example 4

(31) Next, an influence which a difference of a deposition amount of a corrosion inhibitor to be deposited on a surface of a structural material gives on a corrosion amount will be described below with reference to the following Example 4 and FIG. 5.

(32) A corrosion-resistant structure for a high-temperature water system according to the present Example 4 has a structure as is illustrated in a schematic view FIG. 1B. Specifically, a test piece used for the test piece of the present Example 4 was in such a state that the surface of a carbon steel before a corrosion-resistant film was deposited thereon had been polished and degreased by a sandpaper with #600, and an oxide film and a foreign substance had been removed therefrom.

(33) Then, a large number of two types of test pieces according to Example 4 were prepared by depositing La.sub.2O.sub.3 or Y(OH).sub.3 onto the surface (newly-formed surface) of this carbon steel with a spray method. For information, a deposition amount of La.sub.2O.sub.3 or Y(OH).sub.3 was varied and adjusted in a range of 0 to 300 μg/cm.sup.2 by adjustment of a spraying period of time.

(34) Next, a test piece was prepared as a Comparative Example which was formed only from a carbon steel and did not have an oxide film and a corrosion-resistant film formed thereon, in addition to the example in which the corrosion-resistant film was prepared by depositing La.sub.2O.sub.3 or Y(OH).sub.3 on the surface of the carbon steel as was described above.

(35) Then, the surface portions of these test pieces were subjected to a corrosion test under conditions of being immersed in the hot water which contained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours, in a similar way to that in Example 1. Corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation result is shown in FIG. 5.

(36) As is clear from the result illustrated in FIG. 5, it was confirmed that the corrosion amount tended to decrease and the corrosion-suppressing effect tended to increase, as the deposition amount of the corrosion-resistant film increased. It was also confirmed that the corrosion-suppressing effect was saturated and the corrosion rates reached approximately a same level, in a range of a deposition amount of 20 μg/cm.sup.2 or more. Accordingly, the deposition amount of the corrosion-resistant film is necessary and sufficient to be in the range of 20 to 120 μg/cm.sup.2.

(37) Here, a deposition amount of the corrosion inhibitor remaining on a surface of the test piece of which the deposition amount had been set to approximately 50 μg/cm.sup.2 before the corrosion test was examined after the corrosion test, and as a result, it was confirmed that the deposition amount was 1 μg/cm.sup.2 or less.

(38) As a result, it was confirmed that the corrosion-preventing effect continued as long as a fixed deposition amount of an La-containing or Y-containing chemical agent was attained in an initial stage of the application, even though the deposition amount was not always kept constant or the deposition amount decreased due to an exfoliation of the deposited chemical agent during an operation period.

(39) It is technically difficult to uniformly deposit the present corrosion inhibitor on the surface of the structural material of the secondary cooling system of the pressurized-water type atomic power generation facility so that the deposition amount becomes uniform, and it is anticipated that the deposition amount of the corrosion inhibitor greatly varies according to an influence of a flow of a cooling water, and depending on a temperature of the cooling water and a structure of the high-temperature water system.

(40) However, such a technological knowledge is an important premise for the technology that an initial corrosion-preventing effect develops even when the deposition amount of the corrosion inhibitor has greatly varied depending on the site of the structural body as has been described above, and is extremely useful when the technology is applied to an actual apparatus.

Example 5

(41) Next, an influence which a difference between methods of depositing a corrosion inhibitor on a surface of a structural material gives will be described below with reference to the following Example 5 and FIG. 6.

(42) A corrosion-resistant structure for a high-temperature water system according to the present Example 5 has a structure as is illustrated in a schematic view FIG. 1B. Specifically, a test piece used for the test piece of the present Example 5 was in such a state that the surface of a carbon steel before a corrosion-resistant film was deposited thereon had been polished and degreased by a sandpaper with #600, and an oxide film and a foreign substance had been removed therefrom.

(43) Then, two types of test pieces according to Example 5 were prepared by depositing La.sub.2O.sub.3 onto the surface (newly-formed surface) of this carbon steel with a spray method or a chemical deposition method of injecting a chemical substance into a high-temperature water and depositing the chemical substance. In the above description, the deposition amount of La.sub.2O.sub.3 was adjusted to 50 μg/cm.sup.2 by adjustment of a spraying period of time or an amount of the chemical agent to be injected into the high-temperature water.

(44) Here, the above described chemical deposition method is a method of making a substance to be deposited exist in a fluid, and depositing the substance onto a surface of a structural material by a flow of the fluid.

(45) Next, the surface portions of the two types of the test pieces which were prepared by depositing La.sub.2O.sub.3 on the surface of the carbon steel with different methods as was described above were subjected to a corrosion test under conditions of being immersed in the hot water that contained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours, in a similar way to that in Example 1. Then, corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation result is shown in FIG. 6.

(46) As is clear from the result illustrated in FIG. 6, the corrosion-resistant film which had been deposited and formed with the chemical deposition method was different from and could be more uniformly deposited than the corrosion-resistant film which had been formed with the spray method, and it was confirmed that the corrosion-resistant film which had been formed with the chemical deposition method had a greater corrosion-rate-suppressing function.

(47) It is expected that the deposition of the corrosion-resistant film having high uniformity can be achieved by injecting an La-containing substance into a high-temperature cooling water during an operation of the secondary cooling system of the pressurized-water type atomic power generation facility and by depositing the substance onto the surface of the structural material, and that thereby an effect of suppressing general corrosion and an effect of suppressing a wall-thinning phenomenon due to flow-accelerated corrosion are shown. A similar effect can be shown also when a Y-containing substance has been injected into the high-temperature cooling water.

Example 6

(48) Next, an effect appearing when La(OH).sub.3 or Y.sub.2(CO.sub.3).sub.3 as other corrosion inhibitors has been deposited on a surface of a structural material will be described below with reference to the following Example 6 and FIG. 7.

(49) A corrosion-resistant structure for a high-temperature water system according to the present Example 6 has a structure as is illustrated in a schematic view FIG. 1B. Specifically, a test piece used for the test piece of the present Example 6 was in such a state that the surface of a carbon steel before a corrosion-resistant film was deposited thereon had been polished and degreased by a sandpaper with #600, and an oxide film and a foreign substance had been removed therefrom.

(50) Then, two types of test pieces according to Example 6 were prepared by depositing La(OH).sub.3 or Y.sub.2(CO.sub.3).sub.3 onto the surface (newly-formed surface) of this carbon steel with the use of a spray method. For information, a deposition amount of La(OH).sub.3 or Y.sub.2(CO.sub.3).sub.3 was adjusted to 50 μg/cm.sup.2 by adjustment of a spraying period of time.

(51) Next, the surface portions of the two types of the test pieces which were prepared by depositing La(OH).sub.3 or Y.sub.2(CO.sub.3).sub.3 on the surface of the carbon steel as was described above were subjected to a corrosion test under conditions of being immersed in the hot water that contained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours, in a similar way to that in Example 1. Then, corrosion amounts (corrosion rates) were calculated from weight changes before and after the corrosion test of each test piece. The measurement calculation result is shown in FIG. 7.

(52) As is clear from the results illustrated in FIG. 7, when the corrosion amounts of the two types of the test pieces which were prepared by depositing La(OH).sub.3 or Y.sub.2(CO.sub.3).sub.3 on the surface of the carbon steel were compared to each other, the corrosion amounts were not greatly different from each other, but it was confirmed that when the two types of the test pieces were compared to the test piece formed only from the carbon steel illustrated in Examples 1 and 2, the corrosion rates were remarkably suppressed.

(53) It was experimentally proved that a great corrosion-preventing effect was obtained by depositing and forming a hydroxide of La or a carbonate of Y on the surface of the structural material as in the above described Example 6. Accordingly, it is expected that an effect of suppressing general corrosion of the structural material and an effect of suppressing a wall thinning phenomenon due to flow-accelerated corrosion are shown also when the hydroxide and the carbonate are deposited on the surface of the structural material in the secondary cooling system of the pressurized-water type atomic power generation facility.

INDUSTRIAL APPLICABILITY

(54) According to the corrosion-resistant structure and the corrosion-preventing method for the high-temperature water system of the embodiments of the present invention, a corrosion-resistant film formed from a substance containing at least one of La and Y is deposited on the surface of the structural material, accordingly the structural material can be effectively prevented from causing corrosion, and an elution of a metal component such as iron from the cooling water contact surface of the structural material can be greatly reduced. In addition, the above described corrosion-resistant film shows an excellent corrosion-preventing effect even when the deposition amount is small, and on the other hand, can maintain the corrosion-preventing effect for a long period of time because of having high adhesion strength between the corrosion-resistant film and the structural material.

DESCRIPTION OF SYMBOLS

(55) 1 Structural material (carbon steel) 2 Oxide film (Oxide layer) 3 Corrosion-preventing film (La.sub.2O.sub.3 film, Y(OH).sub.3 film, La(OH).sub.3 film or Y.sub.2(CO.sub.3).sub.3 film) 4 Cooling water (Coolant)