JPH03153858A - Stainless steel having elution resistance in high temperature water - Google Patents

Abstract

PURPOSE:To produce a stainless steel excellent in elution resistance in high temp. water by forming an oxide film consisting of an oxide layer in which the amount of Cr-containing oxide is larger than the amount of Cr-free oxide on the surface of a base material composed of stainless steel by means of surface oxidation treatment. CONSTITUTION:A stainless steel is heated in an oxygen-containing atmosphere to undergo the surface oxydation treatment of the stainless steel, by which an oxide film consisting of an oxide layer wherein the amount of Cr-containing oxide is larger than the amount of Cr-tree oxide is formed on the stainless-steel surface. Further, it is desirable to regulate the thickness of this oxide film to 50-1000Angstrom . By this method, the stainless steel in which stainless steel components, such as Ni, Fe, Co, and Mn, are difficult to elute in high temp. water can be obtained. This stainless steel is suitable for cooling water system piping for the nuclear power plant.

Description

[Detailed Description of the Invention] (Industrial Application) The present invention relates to a stainless steel that is resistant to elution in high temperature water, and specifically, in high temperature water, Ni, Fe, Co, Mn
The present invention relates to stainless steels in which elution of stainless steel components such as stainless steel components is difficult to occur, and particularly to stainless steels resistant to elution in high-temperature water suitable for use in cooling water system piping of nuclear power plants.

(Prior Art) Stainless steel has traditionally been widely used as a constituent material for cooling water system piping and the like in nuclear power plants due to the requirement for excellent corrosion resistance.

However, in recent years, as nuclear power plants have been operating for many years, radionuclides have accumulated in the cooling water system piping, increasing the radiation dose rate emitted from the piping, which has led to serious problems such as increased radiation exposure during periodic inspection work. Problems are emerging.

Regarding the mechanism of accumulation of radionuclides, generally speaking, metal materials constituting the cooling water loop such as cooling water system piping are corroded by cooling water, and the corrosion products adhere to the fuel rod surface in the reactor core. It is thought that corrosion products that are activated by neutron irradiation and contain the radionuclides detach from the surface of the fuel rods, migrate through the cooling water loop, and adhere to and accumulate in the primary cooling water system piping. There is. Incidentally, since the cooling water is heated to a high temperature by heat exchange, the corrosion (that is, elution of metal ions) occurs in the high-temperature water.

The above radionuclides include “Co,” “Co,” “Fe+
``Mn5ICr, etc., in currently operating nuclear power plants.'' It has been reported that the main elution source of CO is stainless steel, which is the main metal material of cooling water system piping.

Therefore, in order to fundamentally solve the problem of increased radiation exposure during inspection work, it is necessary to suppress the elution of radionuclide elements from the stainless steel of nuclear power plants into the cooling water (i.e., high-temperature water). do it. Therefore, in order to suppress the elution of such radionuclide elements, a high-temperature water treatment method has been proposed in which stainless steel is brought into contact with high-temperature, high-pressure water to form a film on the surface of the stainless steel (Japanese Patent Laid-Open No. 59-089
Publication No. 775).

(Problems to be Solved by the Invention) However, the proposed high-temperature water treatment method is still insufficient in suppressing the elution of radionuclide elements, and the problem of increased radiation exposure during inspection work cannot be effectively solved. has not yet been reached. In other words, even stainless steel treated by this method has low elution resistance in the cooling water of nuclear power plants, and the rate of radioactive green use increases due to the elution and accumulation of radionuclide elements, making it difficult to use during inspection work. It is not possible to suppress the increase in radiation doses to a low level.

The present invention has been made in view of these circumstances, and its purpose is to solve the above-mentioned conventional problems, to provide excellent elution resistance in high-temperature water, and to be particularly useful for cooling nuclear power plants. The object of the present invention is to provide a stainless steel that is resistant to elution in high-temperature water and does not easily cause elution of radionuclide elements into water.

(Means for Solving the Problems) In order to achieve the above object, the stainless steel resistant to elution in high temperature water according to the present invention has the following configuration.

That is, the stainless steel according to claim 1 is a high-temperature underwater elution-resistant stainless steel having an oxide film generated by surface oxidation treatment, wherein the oxide film makes Cr-containing oxides more chromium-free than Cr-free oxides. This stainless steel is resistant to elution in high-temperature water and is characterized by being composed of a layer containing a large amount of oxides.

The stainless steel according to claim 2 is a high-temperature underwater elution-resistant stainless steel having an oxide film generated by surface oxidation treatment, wherein the oxide film is in close contact with the surface of the stainless steel base material and contains Cr-containing oxide. The Cr-containing oxide is present in close contact with a layer made of an oxide containing more Cr than the Cr-free oxide.
This stainless steel is resistant to elution in high-temperature water and is composed of a layer having a thickness of 50 Å or less and made of an oxide that contains more Cr-free oxide than Cr-containing oxide.

In the stainless steel according to claim 3, the layer made of an oxide containing more Cr-containing oxide than Cr-free oxide,
The high-temperature water elution-resistant stainless steel according to claims 1 and 2, which has a thickness of 50 Å to 1000 Å.

(Function) The present invention is directed to stainless steel that has an oxide film formed on its surface through surface oxidation treatment (hereinafter referred to as oxidized stainless steel), and stainless steel that has had part of the oxide film removed by dissolving it with acid. (Hereafter referred to as oxidized/dissolved stainless steel)
This is based on the following findings obtained by analyzing the surface film and conducting an elution resistance test in high-temperature water for non-oxidized stainless steel.

That is, the oxide film on oxidized stainless steel is thicker than the naturally occurring oxide film II* (approximately 30 or less) on non-oxidized stainless steel, and is made of oxides containing Cr, Fe, Ni, etc. As shown in FIG. 4, the amounts of Cr-containing oxides and Fe-containing oxides are overwhelmingly large, and the amounts of other oxides are extremely small. Further, the proportion of Cr-containing oxides is large near the surface of the steel base material, and the proportion of Fe-containing oxides is large near the surface of the coating. Therefore, the oxide film of oxidized stainless steel has a lower layer consisting of an oxide containing more Cr-containing oxide than Cr-free oxide (hereinafter referred to as Cr-rich oxide layer) and a lower layer consisting of oxide containing more Cr-rich oxide than Cr-free oxide. It can be said that the upper layer is composed of an oxide containing a large amount of oxide (hereinafter referred to as the Fe-rich oxide layer). Note that the above-mentioned Cr-free oxide refers to an oxide that does not substantially contain Cr, that is, Cr-free oxide.
Among the oxides other than r, Fe-containing oxides are the largest in quantity.

The oxide film on oxidized/dissolved stainless steel is a film that remains on the surface as a result of dissolving and removing a part of the oxide film, so it is thinner than the rich Cr oxide layer (lower layer) and the oxide film above. It is composed of a Fe-rich oxide layer (upper layer), or it is composed only of a Cr-rich oxide layer having a thickness equal to or less than the above-mentioned Cr-rich oxide layer. Incidentally, the oxide film having such a structure, that is, the above-mentioned surface residual film is also an oxide film generated by surface oxidation treatment.

An elution resistance test was conducted on the oxidation-treated stainless steel and the oxidation/dissolution-treated stainless steel. As a result, when the oxide film is composed only of the Cr-rich oxide layer as described above, the Cr-rich oxide layer prevents the stainless steel base material from being leached in high-temperature water, and the Cr-rich oxide layer It has been found that stainless steel itself has extremely excellent elution resistance, and therefore, it is extremely difficult for stainless steel components to elute in high-temperature water.

In addition, a Cr-rich oxide layer (lower layer) and a Fe-rich oxide layer (upper layer)
When the oxide film is made of the above, the Cr oxide layer prevents the base material from being eluted. At this time, elution of Fe from the Fe-rich oxide layer becomes a problem, but if the thickness of the Fe-rich nine compound layer is reduced to 50 Å or less, the total amount of Fe elution is very small, so radiation due to the accumulation of the elution 1.e. It was found that the increase in quantity rate hardly occurred.

As described above, stainless steel (hereinafter referred to as stainless steel S) having an oxide film formed by surface oxidation treatment and consisting only of a Cr-rich oxide layer;
Alternatively, stainless steel (hereinafter referred to as stainless steel W) having an oxide film composed of a rich CrM compound layer (lower layer) and a rich Fe oxide layer (upper layer) with a thickness of 50 Å or less is a stainless steel component in high-temperature water. It has been found that the elution of water is extremely difficult (and that the elution resistance in high-temperature water can be improved and excellent).

Therefore, as described above, the stainless steel according to claim 1 of the present invention is a stainless steel resistant to elution in high-temperature water and has an oxide film generated by surface oxidation treatment, and the oxide film is a Cr-containing oxide. The material is made of a layer consisting of an oxide containing more than Cr-free oxide, that is, the stainless steel S is made.

Further, the stainless steel according to claim 2 is a high temperature water elution resistant stainless steel having an oxide film generated by surface oxidation treatment, the oxide film is in close contact with the surface of the stainless steel base material, and the Cr A layer consisting of an oxide containing more Cr-containing oxide than a Cr-free oxide and a layer consisting of an oxide containing Cr and a core layer,
The layer is made of an oxide containing more r-free oxide than Cr-containing oxide and has a thickness of 50 Å or less. That is, the stainless steel flW is used.

Therefore, the stainless steel I (stainless steel according to claims 1 and 2) is extremely unlikely to cause elution of stainless steel components in high-temperature water, and has excellent elution resistance in high-temperature water. Therefore, if this is used in the cooling water system piping of a nuclear power plant, radionuclide elements will be leached into the cooling water.
Increase in radiation dose rate due to accumulation is less likely to occur, and increase in exposure dose during inspection work can be suppressed to a low level.

The thicker the Cr-rich oxide layer is, the greater the effect of preventing the stainless steel base metal from being leached in high-temperature water, and this effect increases particularly at 50 Å or more. However, the oxide film produced by the surface oxidation treatment becomes less dense when the number of coatings exceeds 1000, and as a result, the elution resistance deteriorates. Therefore, it is desirable that the thickness of the Cr-rich oxide layer be 50 Å to 1000 Å.

In the present invention, the type of oxide film is one generated by surface oxidation treatment as described above.

This is because when an oxide film is produced by a CVD method other than surface oxidation treatment, the adhesion between the produced film and the surface of the stainless steel base material is poor, so extrusion of the base material is likely to occur.
In addition, it is difficult to form a film because it requires surface cleaning treatment before film formation, but on the other hand, the oxide film produced by surface oxidation treatment is made when the oxide film constituent elements are stainless steel. This is because since it is directly supplied from the steel base metal, it has excellent adhesion to the base metal, and the film formation process is easy.

The method for producing stainless steel according to the present invention includes a method of subjecting stainless steel to surface oxidation treatment, or a method of subjecting stainless steel to surface oxidation treatment, and then removing a portion of the oxide film by means such as electrolytic method or acid dissolution method. However, since the latter oxidation/removal method is easier to obtain a film having the structure described above, it is preferable to adopt the oxidation/removal method. desirable.

The surface oxidation treatment may be performed by heating the stainless steel in an atmosphere containing oxygen. Gas or liquid can be used as the atmosphere, but when liquid is used, the weight of the liquid is large and the pressure inside the liquid container is high, so the container needs to have a pressure-resistant structure, which causes equipment and operational problems. There are some difficult aspects. Therefore, it is preferable to use gas as the atmosphere.

(Example) The actual stripping composition was C: 0.036. Si:0.42. Mr+:
1.80. P: 0.028°S: 0.003. Ni
:9.99. Cr:18.23. Co:0.043
Weight%, made from stainless steel plate solution treated after cold rolling, thickness: 1.5s+m, width: 30mm, length: 50
After collecting a test piece No. 11III, the entire surface was polished with #400 emery paper.

Table 1 shows the oxidation treatment conditions after the polishing of the test pieces (Nos. 6 to 11) according to Example 1, the analysis results of the oxide film, and the results of the elution resistance test. As shown in Table 1, after the above-mentioned polishing, all specimens other than No. 6 were exposed to 200.300 in the atmosphere.
．． Oxidation treatment was performed by heating at 400'C or 500°C for 1 hour. For test piece No. 10, part of the film was further dissolved in acid.

Ion microanalysis (IM) was performed on each of the above test pieces.
Perform A), investigate the component distribution in the thickness direction of the oxide film,
The thicknesses of the Cr-rich oxide layer and the Re-rich oxide layer were determined.
Figure 4 shows the IM results of the test piece (No. 9) after 400'C oxidation treatment.The oxide film consists of a Cr-rich oxide M (lower layer) and a Fe-rich oxide layer (upper layer). I understand. Furthermore, as can be seen from Table 1, the higher the oxidation treatment temperature, the greater the thickness of the Cr-rich and Fe-rich oxide layers.

Next, an elution resistance test was conducted on each of the above test pieces. The test was performed by immersing 10 specimens of the same type in 100 ml of ion-exchanged water containing 20 ppb dissolved oxygen in a static titanium autoclave with a capacity of 200 ml.
The test was carried out by holding at 15°C for 500 hours. That is, the immersion test was conducted six times in total. After the immersion test, the water in the autoclave and the precipitates and adsorbates dissolved in nitric acid were collected, and the mixed solution was analyzed by atomic absorption spectrometry and inductively coupled plasma emission spectroscopy to determine the eluted Fe+.
The amounts of Cr, Ni, Co+Mn were measured, and the amount of metal ions eluted was determined.

The elution amount is shown in Table 1, and the relationship between the elution amount and the thickness of the Cr-rich oxide layer is shown in FIG. As can be seen from these, the oxidized or acid-dissolved material has a smaller amount of metal ions eluted than the non-oxidized material (No. 6).
As the oxidation treatment temperature increases, the thickness of the Cr-rich oxide layer increases, and the amount of elution decreases accordingly. This decreasing tendency is remarkable when the thickness of the Cr-rich oxide layer in the frame is 50 Å or more.

In addition, when the relative elution ratio of Cr was calculated using the following formula 0 for the above test pieces No. 6 and No. 9, it was found that No. 6
The one is 0.0002. No. 9 is 0.000
It was 3. This shows that the amount of eluted Cr is extremely small compared to Fe, and that most of the leaching is due to Fe.

Relative elution ratio of Cr-(AC,/AP,)/(MC,/M
F,)-■ However, Acr/Ar- is the ratio of the elution amount of Cr to the elution amount of Fe, and MC-/MF- is the ratio of the Cr content to the Fe content fffi in the base material.

1 Arashi Group I Oxidation treatment of test pieces (Nos. 1 to 5) according to Example 2.

Film analysis results and elution resistance test results are shown in Table 1.
The EN test piece was prepared by performing the same polishing and oxidation treatment as in Example 1, and then dissolving the edges of the film in acid. The acid dissolution was performed by immersing the oxidized test piece in 10% sulfuric acid at room temperature for 2, 5, or 10 seconds.

After the above acid dissolution, IMA was performed in the same manner as in Example 1, and the Cr-rich
The thicknesses of the oxide layer and the Fe-rich oxide layer were determined.

Figure 3 shows the results of Group A of the N005 test piece (dipped in sulfuric acid for IO seconds after 400°C oxidation treatment). Figure 3 shows that the Fe-rich oxide layer (upper layer) was completely dissolved and removed, and the oxide film was removed. It can be seen that it consists only of a rich Cr oxide layer. Further, as can be seen from Table 1, except for the test pieces No. 5, a rich Fe oxide layer with a thickness of 50 Å or less remained.

Next, the same elution resistance test as in Example 1 and the measurement of the elution amount of metal ions were performed on the test piece after being dissolved in the acid.

The elution amount is shown in Table 1, and the relationship between the elution amount and the thickness of the Cr-rich oxide layer and the Fe-rich oxide layer is shown in FIG. 1. In FIG. It is indicated by the number (unit: +ig/m'') written nearby.The symbol ・indicates the case of Example 1.As can be seen from Table 1, the elution rate in the case of Example 2 All amounts are 9 mg/m'
As can be seen from Figure 1, the Cr-rich oxide layer Thickness: 50 Å or more, and thickness of Fe-rich oxide layer: 5
When it is 0 Å or less, the amount of elution is particularly small.

In addition, when the relative elution ratio of Cr was calculated for the case of test piece No. 5, it was 0.002. The value is the above No.
, is larger than that of specimens 6 and 9, but this is Cr
The amount of Fe eluted may have been large (because the amount of Fe eluted was smaller in the No. 5 test piece).

(Effects of the Invention) As explained above, the high-temperature underwater elution-resistant stainless steel according to the present invention has excellent elution resistance in high-temperature water. , elution of radionuclide elements into the cooling water of the plant.
Increase in radiation dose rate due to accumulation becomes less likely to occur, and therefore, increase in exposure dose during inspection work can be suppressed to a low level. In addition, regardless of whether it is a nuclear power plant, it can be suitably used as a material for equipment and equipment that require excellent corrosion resistance, and has the effect of extending the life of these equipment and equipment.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the elution amount of metal ions and the thickness of the rich Cr@ oxide layer and the rich Fe oxide layer in Examples 1 and 2, and FIG. Figure 3 is a diagram showing the relationship between the amount of ion elution and the thickness of the Cr-rich oxide layer.
FIG. 4 is a diagram showing an example of the results of ion microanalysis of the oxide film according to Example 2. FIG. 4 is a diagram showing an example of the results of ion microanalysis of the oxide film according to Example 1.

Claims (3)

[Claims] (1) A high-temperature underwater elution-resistant stainless steel having an oxide film generated by surface oxidation treatment, the oxide film consisting of a layer made of an oxide containing more Cr-containing oxides than Cr-free oxides. A stainless steel that is resistant to elution in high temperature water. (2) A high-temperature underwater elution-resistant stainless steel having an oxide film generated by surface oxidation treatment, wherein the oxide film adheres to the surface of the stainless steel base material, converting Cr-containing oxides into Cr-free oxides. A layer consisting of an oxide containing a larger amount,
A stainless steel resistant to elution in high-temperature water, comprising a layer having a thickness of 50 Å or less and comprising an oxide that is in close contact with the layer and contains more Cr-free oxide than Cr-containing oxide. (3) The stainless steel resistant to elution in high-temperature water according to Claims 1 and 2, wherein the layer made of an oxide containing more Cr-containing oxide than Cr-free oxide has a thickness of 50 Å to 1000 Å.