JP2003050196A - Method for evaluating grain boundary corrosion sensitivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for easily evaluating grain boundary corrosion sensitivity in stainless steel in its environment, without performing long-time dipping tests. SOLUTION: An evaluation method of grain boundary corrosion sensitivity includes a surface-polishing process for polishing the measurement surface of stainless steel, a potential stabilization process for installing the stainless steel in a liquid phase cell under nearly the same conditions as specific usage environment, providing an electrode in each cell, and then stabilizing potential within 10 minutes to 5 hours, a film thickness calculation process for measuring reflected light to incidence light by microscope ellipsometry, having a resolution of up to 20 μm using a CCD camera, and calculating the film thickness in a grain boundary film and a film in a grain formed on the surface of the stainless steel, and a sensitivity evaluation process for changing potential at least two times, then repeating the film thickness calculation process for a total of at least three times, and then plotting the film thickness in the grain boundary and inside the grain with respect to the potential change.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating intergranular corrosion susceptibility, and more particularly, for example, to a stainless steel used as a fastening member or plate in a structure. The present invention relates to a method for easily evaluating the intergranular corrosion susceptibility of steel in its environment without performing an actual test. 2. Description of the Related Art Grain boundary corrosion is mainly caused by the sensitization of austenitic stainless steel, which causes a decrease in the Cr concentration in the vicinity of the grain boundaries. This is a phenomenon in which grain boundaries are corroded. If austenitic stainless steel is kept at 500-800 ° C, carbides (C
r ₂₃ C ₆ ) precipitates, the amount of Cr in the adjacent portion decreases, and a so-called Cr-depleted layer (Cr-dep1eted area) is formed. The treatment that brings this state to the steel is generally a sensitization heat treatment (Sensitiza
tion treatment). On the other hand, the corrosion resistance improves as the chromium concentration increases. Therefore, the amount of chromium in the vicinity of the grain boundary is reduced as compared with that in the grain due to the sensitization, so that the corrosion resistance at the grain boundary is reduced, and corrosion proceeds selectively in this portion. Conventionally, there is a Huey test method as a typical method for relatively evaluating the intergranular corrosion susceptibility between such materials. The Huey test method is a method of measuring the change in weight after immersing a test piece in a boiling 65% nitric acid solution.The test is performed in an area where the grain boundary part of the material is selectively dug and the sensitive material deteriorates. Is what you do. However, this test method has to be repeated 5 times every 48 hours, which requires a long test. Even when it is desired to count only the portion where chromium is reduced due to sensitization, not only erosion in the Cr-deficient layer, but also σ phase (FeCr), χ phase (Mo ₅ Cr ₅ Fe ₁₈ ),
There was a problem in that all carbides such as Laves phase (Fe ₂ Mo), amorphous Ni phosphide at grain boundary (Ni ₃ P ₂ ), TiC and Cr ₂₃ C ₆ were dissolved. In particular, there is a problem in the tightness of the test container due to the boiling condition test, and ASTEM G45-73
Is strictly regulated (condenser, etc.). [0004] In view of the above-mentioned problems in the conventional test method, the present inventors can easily evaluate the intergranular corrosion susceptibility in a short time, and reduce the cost and the evaluation time. In addition to determining whether materials can be used in a specific environment, it is necessary to determine whether or not there is intergranular corrosion susceptibility, which not only makes it possible to make comparisons between materials, but also makes it possible to evaluate whether materials can be used in a specific environment. We worked diligently to develop an evaluation method. For example, in order to maintain long-term high soundness of a nuclear power plant, a reprocessing plant, or the like, a high-level evaluation of the corrosion behavior of a metal material after a long time has elapsed is required. The corrosion resistance of stainless steel, which is a practical alloy, largely depends on the properties of the oxide film formed on the surface. Therefore, the inventors focused on the fact that the corrosion susceptibility (long-term soundness) of stainless steel can be accurately and quickly evaluated by evaluating the properties of the coating. As a result, the present inventors used a microscopic ellipsometry (microscopic ellipsometry) capable of measuring the distribution of the thickness (several Å) of the passive film as a property of the film, and found that the grain boundary becomes a practical problem. It has been found that the above problems can be solved by evaluating the relationship between the corrosion occurrence behavior and the film thickness. The present invention has been completed from such a viewpoint. [0005] That is, the present invention is a method for evaluating intergranular corrosion susceptibility of stainless steel, comprising a surface polishing step, a potential stabilizing step, a film thickness calculating step, and a susceptibility evaluating step. It is intended to provide a method for evaluating intergranular corrosion susceptibility characterized by including: In the surface polishing step, the measurement surface of the stainless steel is polished. In the potential stabilization step, the stainless steel is placed in a liquid phase cell under substantially the same conditions as a predetermined use environment, and after the electrodes are provided in the cell, the potential is stabilized in 10 minutes to 5 hours. In the film thickness calculation process,
By microscopic ellipsometry having a resolution of up to 20 μm, preferably up to 10 μm, the reflected light with respect to the incident light is measured by a CCD camera, and the film thickness of the grain boundary film and the intragranular film formed on the stainless steel surface is measured. calculate. In the sensitivity evaluation step, the potential is changed at least twice or more, and the film thickness calculation step is repeated at least three times in total. Then, the ratio of the grain boundary to the intragranular film thickness with respect to the potential change is plotted. In microscopic ellipsometry, the potential is applied and the measurement is repeated at least three times in a stable current state to measure the film thickness. In the evaluation with the CCD image, the upper part is a grain boundary, and the lower part is represented in a grain. In the liquid in the cell,
When the liquid is boiled, the liquid level is disturbed, and it is optically difficult to make the light incident and reflected. In general, the lower the corrosion resistance, the faster the growth rate at which the oxide film becomes thicker, so that the film in the weaker portion becomes thicker and the stronger portion remains intact. The coating thickness of stainless steel is such that high-Cr steel with good corrosion resistance is thin and low-Cr steel with low corrosion resistance is relatively thick.
In the present invention, the susceptibility is evaluated by utilizing the property that the corrosion resistance depends on the thickness of the film. The present invention utilizes this characteristic to detect the ratio of the grain boundary film thickness to the grain internal film thickness using a microscopic ellipsometry measuring device having a resolution of about 10 μmφ, thereby obtaining the Cr deficiency at the grain boundary ( Sensitization). In general, the higher the potential, the thicker the film thickness.However, under the condition of intergranular corrosion susceptibility, the potential dependency of the grain boundary portion film thickness is larger than the potential dependency of the inner grain film thickness. By evaluating whether or not the ratio of the thickness of the grain boundary portion coating to the thickness of the inner grain coating shows potential dependence, it is possible to evaluate the intergranular corrosion susceptibility in the environment. In the method of the present invention, it is possible to evaluate a material which has been sensitized as described below. However, the solution is not boiled at the sensitization stage, and a nitric acid thinner than 65% nitric acid is used. It has advantageous features that are different from the conventional test method.
In addition, since it is sufficient to set some potentials after immersing the material in the solution, the test can be performed in a short time, and the potential corrosion potential before corrosion can be evaluated. As an evaluation method for increasing sensitivity, an EPR measurement method (electrochemical reactivation method; el
ectrochemical potentiokinetic reactivationmethod)
and so on. Further, for the non-sensitized material, it can be determined whether or not intergranular corrosion occurs under the condition environment, for example, an environment in which intergranular corrosion occurs is, for example, a region of 950 mV or more. That is, the presence or absence of intergranular corrosion susceptibility of the material in the environment can be determined in a short time. Normally, in a mild environment, it takes a long time for corrosion to become apparent. However, according to the present invention, it is possible to make a judgment in a short time by evaluating the state of the film before the corrosion becomes apparent. Will be possible. An embodiment for carrying out the method according to the present invention will be described. Note that the present invention is not limited to the following embodiments. DESCRIPTION OF THE PREFERRED EMBODIMENTS Many stainless steels are used for fastening members such as bolts and plates used in structures. Since many grain boundaries in the stainless steel are where crystals intersect, reorganization of elements that easily accumulate and elements that do not easily accumulate occurs at these grain boundaries under various heat treatment conditions. In the case of stainless steel (a ternary system of Fe-Cr-Ni), for example, the concentration of the chromium element may decrease at the grain boundaries, and conversely, the concentration of the nickel element may increase. In other words, the composition changes at the grain boundaries of the material, and depending on the degree of the composition change, intergranular corrosion or intergranular corrosion cracking (IG
-SCC) and the like. In the material research of structures up to now, when judging the corrosion susceptibility of a new material in the environment, it is necessary to evaluate the extracted material through a long-term actual environmental test, which is a great deal of work. Effort, cost, time, etc. were required. On the other hand, as a comparative test between materials not limited to the environmental conditions of an actual machine, the Huey test method and other test methods were also evaluated under severe conditions. In the Huey test method, a boiling 65% nitric acid solution is used, which is not preferable because there is a problem that a grain boundary segregated substance is dissolved. In addition, even under such acceleration conditions, 48 hours must be repeated five times, which requires a long time. Although such a conventional method can assign the order of intergranular corrosion of the material, it does not evaluate the intergranular corrosion susceptibility of the material including environmental conditions. According to the method of the present invention, even without performing an actual machine test,
It is possible to accurately evaluate and judge the corrosion susceptibility of the material in the environment. The present invention relates to a method for evaluating intergranular corrosion susceptibility of stainless steel, which comprises a surface polishing step, a potential stabilizing step, a film thickness calculating step, and a susceptibility evaluating step. . Further, if necessary, a sensitization treatment step can be added to evaluate each stainless steel. Hereinafter, each step will be described in detail. (Surface polishing step) The surface of the stainless steel to be measured is polished. This is because the precision of the reflectance is maintained and the light is easily reflected as a pre-stage of performing the micro-ellipsometry in the film thickness calculation step described later. As a result, the film on the surface is removed, but a passivation film is newly formed as soon as the film is put into the solution, and the film is evaluated. (Potential stabilization step) Then, the stainless steel is placed in a liquid phase cell under substantially the same conditions as a predetermined use environment, and after the electrode is provided in the cell, it is usually 10 minutes to 5 hours, preferably 30 minutes. The potential (natural potential) is stabilized in minutes to 3 hours. By placing the specimen, stainless steel, in the liquid phase cell, the initial film properties in the environment are evaluated. The coating becomes thicker more slowly after a long time, but the present invention is directed to an initial coating formed after polishing. Here, in a system where a certain amount of oxidizing ions are present, in order to approach the natural potential under an actual environment, the potential is adjusted by controlling the oxidizing ion concentration in the solution. For example, in a solution of about 650 mV, a high potential environment of 850 mV or more can be obtained by adding chromium or silver as an oxidant thereto. In the system where a certain amount of oxidized ions are present, the concentration of oxidizable ions related to the potential in the solution is controlled with respect to this spontaneous potential, or an external power source (constant) is used for evaluation in the sensitivity evaluation step described below. Depending on the potential generator (potentiostat), two or more different potentials are set by shaking the potential somewhat. Thus, it is possible to assume the range of the presence or absence of sensitivity and determine whether or not use at the current natural potential is possible. (Step of calculating film thickness) Next, in the present invention,
The reflected light with respect to the incident light is measured with a CCD camera by micro-ellipsometry having a resolution of up to 10 μmφ, and the thickness of the grain boundary film and the intragranular film formed on the surface of the stainless steel is calculated. At this time, light information that is input once by irradiating light is separated and analyzed by the CCD. In the stainless steel evaluated in the present invention, the better the corrosion resistance, the thinner the film thickness. Normally, the properties of the entire surface film are measured, but the sensitivity evaluated in the present invention is an evaluation as to whether or not the corrosion resistance of the grain boundaries is inferior to the inside of the grains, and cannot be determined by macro evaluation. Must be evaluated. Therefore, in the present invention, the thickness of the film is measured by ellipsometry using microscopic ellipsometry, so that a very thin film in nm units can be measured. By imparting the microscopic function, the distribution of the film thickness can be measured up to a minute portion. According to this method, the difference between the thickness of the coating at the grain boundary and the thickness of the coating within the grain can be determined in a micro range. Ellipsometry is a technique for determining the optical constant of the sample surface from changes in the polarization state of incident light and reflected light. If the sample has a thin film such as a film on the surface of the sample, the optical constant is determined along with the optical constant. Can be determined. However, in order to evaluate the susceptibility to intergranular corrosion, which is the object of the present invention, it is necessary to provide at least a resolution of several tens of μm or less and detect the difference between the vicinity of the grain boundary and the inside of the grain (grain boundary specificity). In ellipsometry, photomar is usually used as a detector, but in the microscopic ellipsometry used in the present invention, a microscopic function is added by using a CCD camera for the detector. A CCD camera is usually composed of 512 × 512 elements, and one element having a size of 20 μm □ is used. By placing a × 10 magnifying lens in front of the detector, information of 2 μm square can be obtained. In the following, the sensitivity evaluation will be described separately for a material subjected to a sensitization treatment (sensitized material) and a material not subjected to the sensitization treatment (solution-treated material), which will be described later. For sensitizers with large intergranular corrosion, there is often a difference in the growth of the coating thickness between the grain boundaries and the grains even when the applied potential is low. In this case, the difference between the thickness of the coating at the grain boundary and the thickness of the coating within the grain becomes remarkable. In such a sensitizing material, the average of the film thickness tends to increase as the potential increases. At high potentials with high sensitivity, the growth of the intergranular coating thickness relative to the potential is greater than the growth of the intragranular coating thickness relative to the potential. This is because the corrosion resistance of the grain boundary portion is poor, and the film becomes thicker than the intragranular portion. In the present invention, in order to perform a quantitative evaluation, for example, the film thickness ratio is set to 1% average / lower 99% average, 5% average / 95% lower average, 10% upper
The average / lower 90% average is separated, and the potential dependence is evaluated as the thickness ratio at each potential (FIG. 1).
reference). Here, the upper part is the grain boundary film thickness part, and the lower part is the intragranular film thickness part. On the other hand, in the case of the solution-treated material, when the applied potential is low, there is a case where there is no difference in the growth of the film thickness between the grain boundary and the inside of the grain. There is no interfacial susceptibility. For example,
In the case as shown in FIG. 2, there is no change in the potential of the coating thickness at the grain boundary with respect to the inside of the grains up to 900 mV.
The average of the film thickness showed a tendency to increase as the potential increased. In the sensitized material shown in FIG. 1, a film grows from a region corresponding to a grain boundary portion with an increase in potential, and the film thickness near the grain boundary portion often becomes larger than the inside of the grain. The solution-hardened material shown in FIG. 2 is not susceptible to intergranular corrosion.
At 900 mV or less, the film tends to grow almost uniformly and thickly. Separately, when a long-term immersion test described later was performed, the sensitized material often had intergranular corrosion susceptibility in a region to which an electric potential was applied, and there were signs that spontaneous electric potential was observed. is there. As described above, in the nitric acid test environment, there is a possibility of intergranular corrosion susceptibility regardless of the potential load. And even in such an environment, the sensitivity to the potential tends to increase. On the other hand, the solution-treated material may not be susceptible to intergranular corrosion in a region subjected to a natural potential and a potential load such as 850,900 mV. (Sensitivity Evaluation Step) Finally, in the present invention, the potential is changed at least twice or more, and the film thickness calculating step is repeated at least three times in total. Plot the ratio of. In the sensitivity evaluation in this step, the ratio of the film thickness (grain boundary film thickness /
It can be evaluated that in the region where the thickness of the grain endothelium does not change, the intergranular corrosion susceptibility does not appear, while in the region where the ratio of the film thickness changes, the intergranular corrosion susceptibility appears. The electric potential is changed by controlling the ion concentration related to the electric potential in the solution or by using an external power supply (potentialiostat). In the former case, for example, in 3N nitric acid, the natural potential is about 650 mV
By adding oxidizing agents such as chromium and silver, a high potential such as 850 mV can be achieved. Depending on the presence or absence of a change in the ratio between the concentration (or potential) of the oxidizing agent and the film thickness, it can be classified into a sensitive region and a non-sensitive region. In a sensitive area, it cannot be used, and in a non-sensitive area, it can be used. Usually, the test is performed by moving the potential to the front or rear or higher potential side with respect to the natural potential. In this way, it is possible to assume an area where the sensitive pear can be used and determine whether the sensitive pear can be used at the natural potential. In addition, the sensitizing material can be evaluated by changing the potential in the same manner, so that even if the same material is used, it can be considered that the intergranular corrosion susceptibility differs depending on the heat treatment method. Here, for the sensitized material shown in FIG. 1, the ratio of the film thickness corresponding to the grain boundary to the average film thickness tends to increase from the corrosion potential to 950 mV. Furthermore, the sensitized material shows corrosion sensitivity at 850 mV or more. The film thickness grows faster. On the other hand, for the solution-treated material shown in FIG.
From the corrosion potential, which is unsusceptible to intergranular corrosion, to 900 mV, the ratio of the film thickness corresponding to the grain boundary to the average film thickness has not changed (the film thickness grows uniformly), but the intergranular corrosion susceptibility appears Above 900mV, the grain boundary film thickness tends to increase with respect to the grain internal film thickness (specificity of film distribution)
Is shown. From this, by microscopic ellipsometry, whether or not the grain boundary portion film thickness immediately after immersion grows faster than the grain internal film thickness, whether or not the film grows non-uniformly, is evaluated under these conditions. It is possible to evaluate whether there is susceptibility to intergranular corrosion in a short time. (Sensitizing treatment step) The stainless steel to be evaluated in the present invention can be subjected to the following heat treatment to confirm the degree of sensitization. The sample that has been heat-treated at 650 ° C x 30Hr (SUS304: sensitized material) has an EPR value of 26 to
It showed a large value of 29 C / cm ² GBA, confirming that the sensitivity was increased. For the solution-treated sample, <0.
It was 1 C / cm ² GBA. The degree of sensitization (the degree of chromium concentration deficiency at the grain boundaries) can be measured using the EPR method. In the actual evaluation test, 60 ° C. of SUS304 steel was sensitized beforehand by heat treatment is corroded grain boundaries, with 3N-HNO ₃ solution, the ratio of the grain boundary portions film thickness to particle interior coating thickness with increasing potential increase You can confirm that. The solution steel SUS304 which is not sensitized does not show the susceptibility to intergranular corrosion in a 3N-HNO ₃ solution at 60 ° C. In this case, the ratio of the thickness of the grain boundary part film to the thickness of the inner grain coating does not depend on the potential. / AgCl) indicates potential dependence. The reason why 3N nitric acid is used in the above test is to observe the difference in a severe environment where intergranular corrosion is likely to occur by setting the acid side. (Long time immersion test) The purpose of this test is to analyze the correlation between the film thickness distribution characteristics immediately after immersion and the intergranular corrosion susceptibility. Here, for example, by holding the potential constant at 3N-HN0 ₃ solution, the test can be carried out to reproduce the conditions of the transition region intergranular corrosion is expressed.
The test conditions are, for example, as follows. Specimen: SUS304 (sensitization process material and solution material) Specimen shape: polished sample solution: 3N-HN0 ₃ holding potential: 850mV (O.1g / l Cr ⁶⁺ potential during content), 900 mV, 95
0 mV (sat'dKC1 Ag / AgC1) Measurement temperature: 60 ° C., immersion time in air publishing: 120 hours. Measurement was performed at each holding potential. If the result at high potential was poor, intergranular corrosion occurred. (Measurement of Film Thickness Distribution) In order to grasp the behavior of the film thickness at the initial stage of intergranular corrosion, under the same conditions as in the long-term immersion test, the film thickness in the vicinity of the grain boundary of the sensitized material and the solution-treated material was measured. By measuring the thickness distribution, it is possible to analyze the film thickness behavior at the initial stage of the intergranular corrosion potential. At a measurement position where it is difficult to evaluate accurately by micro-ellipsometry, an indentation is made on the specimen in advance with a micro Vickers as a mark, and the measurement position is determined using the indentation as a mark. The measurement conditions are, for example, as follows. Specimen: SUS304 high carbon (sensitized material and untreated material) Specimen shape: mirror-polished (indented with micro Vickers) Optical constant measurement of metal substrate: Measurement solution: 0.5 mol / l H ₂ SO ₄ Degassing conditions Measurement current: -50 μA / cm ² 1 hour after reduction Film thickness distribution measurement: Measurement solution: 3N-HNO ₃ Measurement potential: Corrosion potential, 850 mV (potential when 0.1 g / l Cr ^{6+ is} contained) , 900mV, 950mV (sat'dKC1 Ag / AgC1) Holding potential holding time: Hold each potential for 1 hourMeasurement: Measurement of film thickness distribution after holding each potential for 1 hour Measurement equipment here is a triangular cell This is a film characteristic evaluation device that measures light thickness reflected by the specimen and analyzes the light information reflected from the specimen to measure the film thickness distribution. According to the evaluation method of the present invention, it becomes possible to evaluate the Cr deficiency at the grain boundaries in several hours including analysis. In addition, it is possible to evaluate whether or not the test material exhibits intergranular corrosion susceptibility in the environment in an extremely short time of several hours including analysis. Since the grain boundary specificity is evaluated based on the difference in the film thickness before the onset of intergranular corrosion without accelerating the environment, it is possible to evaluate the Cr deficiency without being affected by segregates at the grain boundaries.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph plotting the potential dependence of the grain boundary / intragranular coating thickness for a sensitizer. FIG. 2 is a graph plotting the potential dependence of the grain boundary / intragranular coating thickness for the solution-treated material.

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Claims (1)

Claims 1. A method for evaluating intergranular corrosion susceptibility of stainless steel, comprising: a surface polishing step of polishing a measured surface of the stainless steel; After installing the stainless steel in a liquid phase cell and providing electrodes in the cell, a potential stabilization step of stabilizing the potential for 10 minutes to 5 hours and a microscopic ellipsometry having a resolution of up to 20 μm are performed. Measuring the reflected light with respect to the light with a CCD camera, and calculating the film thicknesses of the grain boundary film and the intragranular film formed on the stainless steel surface, respectively, a film thickness calculating step, and changing the potential at least twice. And a sensitivity evaluation step of plotting the ratio of the grain boundary and the intragranular film thickness with respect to the potential change after repeating the film thickness calculation step three times or more in total. Method for evaluating intergranular corrosion susceptibility.