JP4544635B2 - Local corrosion progress evaluation method and local corrosion progress evaluation device - Google Patents

Description

  The present invention relates to a local corrosion progress process evaluation method and a local corrosion progress process evaluation apparatus for evaluating the progress process of local corrosion of a metal material in high temperature water, and in particular, stress corrosion cracking (SCC) in high temperature water. Alternatively, the present invention relates to a local corrosion progress process evaluation method and a local corrosion progress process evaluation apparatus for evaluating the progress process of local corrosion including corrosion fatigue crack (CF) in terms of an electric circuit.

  Normally, steel materials progress with corrosion such as rust occurring in a corrosive environment, but in rust-resistant materials such as stainless steel, the surface of the steel material is covered with a very thin corrosion film, so that the corrosion progresses. It is known to be prevented. However, in a high-temperature water environment such as a nuclear power plant, stress corrosion cracking (hereinafter referred to as SCC) is generated from a sensitized grain boundary at a site where residual stress of welded stainless steel exists. And progress.

  Conventionally, as a method for suppressing the occurrence of SCC, measures have been taken to replace a sensitized member with a low-carbon stainless steel such as SUS304L or SUS316L that is resistant to corrosion, or to coat a noble metal. However, even if such measures are taken, the present situation is that the occurrence of SCC is newly found from the base material of the low-carbon stainless steel for reactors that has been surface-treated.

  Despite the fact that various countermeasures have been proposed and implemented based on a great deal of research over many years, the reason why SCC has repeatedly occurred is still fully elucidated. This is also due to the fact that an accurate evaluation method has not yet been established.

  In elucidation of the mechanism of SCC generation / progress and experiments for evaluation thereof, low-speed tensile tests, electrochemical evaluation between metal and solution interface, and fracture mechanics techniques have been used. On the other hand, as a quantitative evaluation, considering that the SCC occurrence / progress phenomenon is a phenomenon in which the three elements of material, environment and stress act simultaneously, an electric circuit method is applied to all corrosion paths, Methods for elucidating and evaluating have been studied (see, for example, Patent Document 1 to Patent Document 5).

JP 2003-279684 A JP-A-9-159601 JP-A-7-209173 JP-A-5-142139 Japanese Patent Laid-Open No. 5-11397

  However, the conventional evaluation using experimental data obtained under limited conditions has a problem that it cannot be immediately applied to evaluation in a new environment when environmental conditions change or change. In addition, the conventional evaluation method using an electric circuit method is to evaluate the progress of local corrosion in a chloride environment at room temperature, and the evaluation formula is obtained in a high-temperature water environment that hardly contains chloride ions. Because it is different, it cannot be applied as it is. Furthermore, in the evaluation method using the conventional electric circuit method, the evaluation according to the degree of sensitization of the material in a high-temperature water environment cannot be performed, and the proposed evaluation formula gives the given corrosion. There has been a problem that it is impossible to quantitatively evaluate whether or not SCC can occur and progress in the environment.

  The present invention has been made in view of such circumstances, and proposes a novel method for evaluating a local corrosion progress process including stress corrosion cracking or corrosion fatigue cracking in high-temperature water using an electric circuit method. , Local corrosion progress process evaluation method and local corrosion progress capable of quantitatively evaluating conditions where local corrosion including SCC, possibility of progress, progress rate, etc. can be evaluated for any environment, material and working stress change An object is to provide a process evaluation apparatus.

  The inventor of the present application has conducted extensive research on the evaluation method of the local corrosion progressing process, and as a result, even if an electric circuit method is used, the local corrosion progressing process can be determined only by measuring the pH value of the crack internal solution. A novel technique capable of quantitative evaluation has been found and the present invention has been achieved.

  That is, the local corrosion progress process evaluation method according to the present invention that achieves the above-described object is a local corrosion progress process evaluation that evaluates the progress process of local corrosion including stress corrosion cracking or corrosion fatigue cracking in high-temperature water in an electric circuit manner. In the corrosion path, the crack tip is an anode from which the metal elutes, the outer surface of the crack is a first cathode in which oxygen reduction is involved, and the inner wall of the crack is reduced by hydrogen ions. When a local corrosion reaction is expressed by an electric circuit configured as a second cathode in which the metal is involved, and the solution resistance inside and outside the crack is related to the eluted metal ions and hydrogen ions generated by the hydrolysis reaction A pH value measuring step of measuring the pH value of the crack internal solution using a predetermined measuring means, and the electric circuit flows based on the pH value measured in the pH value measuring step. The are crack propagation rate is represented by a metal ion concentration and the hydrogen ion concentration to calculate the current, it is characterized by comprising a calculating step of calculating the predetermined calculation means.

  In addition, the local corrosion progress process evaluation apparatus according to the present invention that achieves the above-mentioned object is a local corrosion progress process evaluation that evaluates the progress process of local corrosion including stress corrosion cracking or corrosion fatigue cracking in high-temperature water in an electric circuit manner. In the corrosion path, the crack tip is the anode from which the metal elutes, the crack outer surface is the first cathode in which oxygen reduction is involved, and the crack inner wall is the hydrogen ion reduction When a local corrosion reaction is expressed by an electric circuit configured as a second cathode in which the metal is involved, and the solution resistance inside and outside the crack is related to the eluted metal ions and hydrogen ions generated by the hydrolysis reaction An input means for inputting the pH value of the crack internal solution measured using a predetermined measuring means, and the electric circuit flows based on the pH value input through the input means. It is characterized by comprising calculating means for calculating the crack propagation rate is expressed using the metal ion concentration and hydrogen ion concentration to calculate the flow.

  In such a local corrosion progress process evaluation method and local corrosion progress process evaluation apparatus according to the present invention, a metal ion concentration having a large equilibrium constant is used instead of the total metal ion concentration that has been applied in the hydrolysis reaction. By calculating the expressed crack growth rate, the effect of the degree of sensitization on the crack growth process can be quantitatively evaluated.

  In the present invention, even when an electric circuit method is used, the local corrosion progressing process in high-temperature water can be quantitatively evaluated only by measuring the pH value of the crack internal solution.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings.

  In this embodiment, local corrosion for evaluating the progress of local corrosion including stress corrosion cracking (hereinafter referred to as SCC), corrosion fatigue cracking (CF), and the like of a metal material in high-temperature water. It is a development process evaluation device. In particular, this local corrosion progress process evaluation apparatus can quantitatively evaluate the conditions under which local corrosion including SCC, the possibility of progress, the progress rate, and the like can be quantitatively evaluated for any environment, material and working stress change. It can be done.

  The present invention is directed to local corrosion that occurs in a high-temperature water environment such as a nuclear power plant. Usually, corrosion is roughly classified into normal temperature (room temperature) corrosion and high temperature corrosion, but there is no strict definition of temperature in any of them. Speaking strongly, cold corrosion generally refers to corrosion under atmospheric temperature of about 25 ° C., for example. On the other hand, high-temperature corrosion is also called differently depending on the usage environment (water, gas, combustion ash, etc.), and high-temperature water corrosion is often referred to as corrosion in an aqueous solution of 100 ° C. or higher achieved by pressurization. Corrosion that occurs in water such as a nuclear power plant has a temperature of about 288 ° C. for a boiling water reactor (BWR) and about 310 ° C. for a pressurized water reactor (PWR). . The present invention is directed to local corrosion that occurs in a high-temperature water environment, instead of such room temperature corrosion.

  First, prior to the description of the specific configuration and operation of the local corrosion progress process evaluation apparatus, an evaluation method applied to the local corrosion progress process evaluation apparatus will be described.

  The evaluation method proposed in the present invention relates to a rate equation for obtaining a change per unit time associated with corrosion of various ion concentrations inside the crack, and has been applied as a conventional technique in the hydrolysis reaction of high-temperature water with metal ions. A metal ion concentration (chromium ion concentration) having a large equilibrium constant is used in place of the metal ion concentration, and the effect of the degree of sensitization on the crack growth process can be quantitatively evaluated. It is.

  In addition, this evaluation method uses various factors that form mathematical formulas derived from the rate equations for the metal ion and hydrogen ion concentrations in the crack internal solution, and uses various growth processes such as the propriety of crack growth and the growth rate. It is possible to quantitatively evaluate the characteristics.

FIG. 1 shows a model shape of a local corrosion crack including SCC, and ions and molecules related to corrosion inside and outside the crack. In the local corrosion phenomenon, metal elutes from the anode at the crack tip as metal ions (M ^{n +} ), and some of the metal ions hydrolyze high-temperature water to generate hydrogen ions (H ⁺ ). . The eluted metal ion (M ^{n +} ) migrates toward the outer surface of the crack. The dissolved oxygen (O ₂ ) is reduced on the outer surface of the crack, and the hydrogen ions (H ⁺ ) generated inside the crack are reduced on the inner wall surface of the crack to become hydrogen molecules (H ₂ ). . In the figure, the outer surface of the crack is shown as the first cathode, and the inner wall surface of the crack is shown as the second cathode.

Here, when such a local corrosion reaction is equivalently expressed by an electric circuit, an electric circuit model shown in FIG. 2 is obtained. In the second cathode shown in FIG. 1, when a reaction occurs in which hydrogen ions are reduced to generate hydrogen molecules, the current I ₂ flowing between BE in FIG. ₂ is indicated by an arrow in FIG. Thus, the B → E direction is obtained. On the other hand, when a reaction occurs in which hydrogen molecules are oxidized and hydrogen ions are generated at the second cathode, the current I ₂ flowing between B and E in FIG. ₂ is represented by an arrow in FIG. E → B direction. Further, in the first cathode shown in FIG. 1, even if a reaction in which hydrogen molecules are oxidized and hydrogen ions are generated by performing hydrogen injection, the electromotive force due to oxygen is large, so in FIG. The current I ₁ flowing between B and C is in the B → C direction as shown by the arrows in FIGS. 3 and 4 although the flowing current is small. In the evaluation method proposed in the present invention, when there is a change in the oxidation-reduction reaction of each electrode as described above, the circuit calculation is performed in consideration of the direction of the flowing current and the direction of the electromotive force, and the current or crack is determined. Evaluate the speed of progress.

Specifically, when the currents I ₁ , I ₂ , and I of each path are obtained based on Kirchhoff's law for the electric circuit shown in FIG. 2, it is expressed by the following equation (1).

Here, C _M ^{n +} present on the right side of the current I ₁ is the total metal ion concentration of the crack internal solution, and C _H ⁺ present on the right side of the current I ₂ is the hydrogen ion concentration of the crack internal solution. is there. Further, the factor f existing on the right side of the current I ₁ and the factor g ₂ existing on the right side of the current I ₂ are expressed by the following equations (2) and (3), respectively. Further, b ₀ at a ₀ in the equation _(2), and the following equation (3), the anode of the electric circuit shown in FIG. 2, the first cathode, and the electrical conductivity of the electromotive force and the solution of the second cathode It is a factor depending on the degree, and is expressed by the following formula (4) and the following formula (5), respectively. Furthermore, in the following expressions (2) and (3), a ₁ and b ₁ are resistance values of the anode, the first cathode, and the second cathode (electrode part) of the electric circuit shown in FIG. This factor depends on the solution resistance value. Further, ε (K _I ) is a factor that depends on the strain amount or stress intensity factor of the crack tip corresponding to the action of the stress, and the increase in the melting area of the crack tip that is estimated to depend on the acting stress. Indicates the rate. Further, in the following expressions (4) and (5), E _a , E _c1 , and E _c2 are an electromotive force of the anode, an electromotive force of the first cathode, and an electromotive force of the second cathode, respectively. . Furthermore, R _as , R _c10s , R _c1s , R _a , and R _c2s are the details of the resistance values of the electric circuit shown in FIG. 2, respectively. Specifically, R _as , R _c1s , and R _c2s are The solution resistance values inside and outside the crack _portion , and R _c10s and R _a are the electrode resistance values of the outer surface of the crack (first cathode) and the inner wall surface of the crack (second cathode), respectively. The relationship with the resistance value shown in the following equation (6). Note that R _ap shown in FIG. 2 is an electrode resistance value of the anode. In the following equations (4) and (5), Λ _M ²⁺ and Λ _H ⁺ are equivalent conductivities of metal ions and hydrogen ions, respectively, and S _M ²⁺ and S _H ⁺ are respectively It is a channel area of metal ions and hydrogen ions inside the crack, and l _M ²⁺ and l _H ⁺ are channel distances of metal ions and hydrogen ions inside the crack, respectively.

Further, the electromotive force E _C2 of the feeder裂外surface (first cathode) the electromotive force E _C1 and-out cleft side wall of the (second cathode), when assuming a high temperature water of 288 ° C., respectively, the following As represented by the equation (7) and the following equation (8), it is a function of the pH of the crack internal solution. In the following expressions (7) and (8), P _O2 and P _H2 are an oxygen partial pressure and a hydrogen partial pressure, respectively.

  Furthermore, the hydrolysis reaction by the eluted metal ions is represented by the following formula (9) to the following formula (11), and the neutralization reaction is represented by the following formula (12).

Considering these equations (1) to (12), the temporal changes in the hydrogen ion concentration C _H ⁺ and the total metal ion concentration C _M ^{n +} of the crack internal solution are expressed by the following equations (13) and (13), respectively. It is represented by the velocity equation shown in equation (14).

In the above equations (13) and (14), F is the Faraday constant, n is the valence of the eluted metal ion, and C _M ^{n +} and δ _Cr ³⁺ are The total metal ion concentration of the solution and the composition ratio (degree of sensitization) of chromium, which is a metal constituent element at the crack tip, are shown. Moreover, in the above formula (13) and the above formula (14), k ₁ and k ₂ are respectively positive direction speed constants in high temperature water, and k ₋₁ and k ₋₂ are respectively in high temperature water. Negative direction velocity constant. Here, the {−} term in the above formula (13) relates to the neutralization reaction of hydrogen ions (H ⁺ ) and hydroxide ions (OH ⁻ ), but the forward rate constant k _{2 in} high-temperature water. Can be regarded as sufficiently large, so that the neutralization reaction ends immediately. Therefore, this term can be omitted in solving the velocity equation of the above equation (13). Based on this, the following equations (15) and (16) are obtained by solving the velocity equations of the above equations (13) and (14). In the following formulas (15) and (16), δ _Cr is a composition ratio of chromium, which is a metal constituent element, and K _Cr is an equilibrium constant of a water hydrolysis reaction by chromium ions.

Therefore, currents I ₁ , I ₂ , and I flowing through the electric circuit shown in FIG. 2 are expressed by the following equation (17).

  On the right side of the total current I, the (−) term indicating the coefficient indicates the magnitude of the current or crack growth rate, the {−} term indicating the power term indicates time dependence, and the concentration of ions inside the crack This represents the factor that determines the possibility of.

Therefore, in order to evaluate whether the metal ion concentration or hydrogen ion concentration of the crack internal solution is concentrated and the corrosion rate is accelerated, constant, or whether the corrosion rate decreases with time, the following equation (18) to use the factor P _C as defined in.

This factor _{P C} is time-dependent. That is, the ion concentration or crack growth rate of the crack internal solution proceeds in a direction that increases with time when P _C > 0 at an arbitrary time, and is constant when P _C = 0. Yes, if P _C <0, it indicates that the process proceeds in a decreasing direction with time. From the above equation (2) and the above equation (3), when changing the increase rate of dissolution area of crack tip portion Ki is presumed to depend on the stress acting epsilon (K _I), factor P _C and factor f and g A curve representing ₂ × δ _Cr × K _Cr can be obtained. The patterns classified from the levels and shapes of these curves and their conditions are shown in FIG.

Figure 5 pattern - in the case of down A is parameters - data P _C becomes positive regardless of the working stresses, ion concentration or crack growth rate of the feeder fissure portion solution to increase with time Show. In the case of the pattern B, the parameters - data _{P C} is the coefficient _a 0 that is independent of the _{stress, a 1, b 0, b} 1 of the curve f and curve _{g 2} × _{δ Cr} × _{K Cr} determined by in the range to the intersection epsilon (K _I) in a negative, a positive in ε (K _I) is in the range greater than the intersection point. Furthermore, in the case of pattern C, parameters - data P _C becomes a negative or without working stress, ion concentration or crack growth rate of the feeder fissure portion solution indicates that decreases with time.

  Therefore, the conditions under which SCC occurs, that is, the conditions under which a crack propagates under the action of stress are limited to the conditions shown in the following equation (19).

On the other hand, in the above equation (17), if the metal ion concentration and the hydrogen ion concentration in the crack at an arbitrary time t are C _M ^{n +} _t and C _H ⁺ _t , respectively, the crack growth rate at time t is It is given by equation (20). In the following formula (20), M is the atomic weight of the metal, n is the valence of the eluted metal ion, F is the Faraday constant, and S is the crack tip from which the metal is eluted. Ρ is the density of the metal.

  Further, the stress dependence of the crack growth rate da / dt in the above equation (20) is the ratio S / l of the channel area S and the channel distance l shown in the above equations (4) and (5). By applying a fracture mechanics relational expression, the following equation (21) is obtained.

In the above equation (21), σ _∞ is a stress generated at a position sufficiently away from the crack tip due to an internal force or an external force such as a residual stress due to welding or processing, and σ _y is a yield stress of the metal It is changed by heat treatment or plastic working or addition of trace elements. In the above equation (21), E is the Young's modulus (longitudinal elastic modulus) of the metal, A, B, and C are constants that do not depend on stress, and D depends on the stress including “0”. Not a constant. It should be noted that the constant D is provided on the right side of the above equation (21) because, even if the stress σ _∞ is “0”, as long as a crack exists and an electromotive force exists at each electrode, the electric circuit This is because there is a case where the crack growth rate da / dt cannot be regarded as “0” because a current corresponding to the magnitude of the resistance constituting the current flows.

From the above equations (4) to (8), the factors a ₀ , b ₀ × δ _Cr × K _Cr , (1 + a ₁ ) / (1 + b ₁ ) × b ₀ × δ _Cr × K _Cr are respectively It is a function of the pH of the crack internal solution. FIG. 6 shows the relationship between each factor and the pH value at an arbitrary hot water temperature and an arbitrary material chromium content δ _Cr . In the correlation diagram shown in the figure, the region where the pH value is (1) to (3) corresponds to the state of the pattern A in FIG. 5, and the region where the pH value is (3) to (4) is that of the pattern B. Corresponding to the state, the region having a pH value of (4) or later corresponds to the state of pattern C. Therefore, when the pH value of the crack internal solution is at the level of (1) in the initial state, the pH value will naturally reach the level of (3) even if no stress is applied to the material over time. descend. Here, when there is no action of stress, the pH value becomes constant at the level of (3). On the other hand, the action of stress is required to further decrease the pH value, and the pH value decreases to an appropriate position among (3) to (4) depending on the magnitude of the stress. However, no matter how great force is applied, the pH value does not drop below the level of (4). That is, when SCC is generated from pitting corrosion that has progressed corrosion, the pH value of the initial crack internal solution may already be at the level (5). Decreases, and the pH value returns to the level where the stress between (3) and (4) is applied, and becomes constant at that position.

  Here, the short regions (3) to (4) corresponding to the state of the pattern B correspond to the generation range of SCC under high temperature water. Therefore, in the evaluation method proposed in the present invention, the level of corrosion in the pH range of the correlation diagram shown in FIG. 6 is evaluated by measuring the pH value of the crack internal solution of the actual machine. Is possible. Therefore, when the measured pH value is higher than the level of (3), the possibility of the progress of SCC is small, and when it is between (3) and (4), a measure to reduce the working stress as much as possible. If it is necessary or is below the level of (4), it becomes possible to take an evaluation and take measures such as diluting the crack internal solution or preparing for material exchange. Further, since the correlation diagram shown in FIG. 6 can be created at the stage of device design, in this evaluation method, the pH value of the crack portion of the SCC is measured in a laboratory test simulating an actual machine environment, and the safety is confirmed. Can be used for design. Furthermore, in this evaluation method, the crack growth rate at that time can also be evaluated from the above equation (20) using the pH value measured in the laboratory test. Further, the pH value at the level (4) in FIG. 6 corresponds to a sufficiently large or infinite stress, and if the plate thickness design is performed based on the crack growth rate with respect to this pH value, the fracture mechanics is obtained. It is also possible to make a safe design regardless of the specific method.

  Now, a local corrosion progress process evaluation apparatus shown as an embodiment of the present invention to which such an evaluation method is applied will be described below. This local corrosion growth process evaluation apparatus evaluates the crack growth rate with respect to the measurement of the chromium content of a metal matrix or grain boundary.

  Here, FIG. 7 shows a correlation diagram showing the relationship between the chromium content at the grain boundary of the metal material and the minimum and maximum crack growth rates with respect to the stress action.

In the figure, the solid line indicates the maximum crack growth rate relative to the chromium content of the metal on which the stress is applied, and the broken line indicates the minimum crack growth rate. The minimum crack growth rate corresponds to a pH state where the concentration is constant when stress is not applied to the crack tip at the point P _min shown in FIG. This corresponds to a pH state that is constant when an infinite stress (or a sufficiently large stress) is applied to the crack tip at the point P _max shown in FIG. The local corrosion progress evaluation apparatus obtains such an evaluation by performing correlation diagrams for various chromium contents.

  Specifically, as shown in FIG. 8, the local corrosion progress process evaluation apparatus includes a magnetic force microscope or the like that measures the chromium concentration by scanning the surface of the metal base material 1 and the welded portion 2 of the object to be measured. The sensor 11, the measurement unit 12 that calculates the chromium concentration based on the electrical signal transmitted from the sensor 11, the input unit 13 that inputs predetermined data necessary for calculating the crack growth rate, and the measurement unit 12 Based on the calculated chromium concentration and the data input via the input unit 13, a calculation unit 14 that calculates factors for calculating the crack growth rate, and the data and calculation input via the input unit 13 The display part 15 which displays the calculation result by the part 14 on a screen, and the recording part 16 which records and preserve | saves the information displayed on this display part 15 are provided.

  Note that the sensor 11 measures the chromium concentration as a point or a surface not only in a portion having a high welding residual stress in the vicinity of the welded portion 2 but also in a weld metal portion, a heat affected zone, and a base material portion. These results are displayed on the display unit 15 including the measurement position or stored in the recording unit 16. Further, the sensor 11 measures the chromium concentration even when the surface of the base material general part is processed. Furthermore, the local corrosion progress process evaluation apparatus performs processing so as not to give strain from the surface as necessary, and measures the chromium concentration in the depth direction by the sensor 11.

  Such a local corrosion growth process evaluation apparatus obtains the crack growth rate according to a series of procedures as shown in FIG.

First, in the local corrosion progress evaluation apparatus, as shown in the figure, in step S1, using the sensor 11 and the measurement unit 12, the chromium content δ _Cr of the metal including the crystal grain boundary of the object to be measured is point-measured. Alternatively, it is calculated by measuring the surface. In the local corrosion progress evaluation apparatus, predetermined data necessary for calculation of the crack growth rate is input via the input unit 13 in steps S2 to S5. Specifically, in the local corrosion progress evaluation apparatus, in step S2, as the apparatus design specification condition data, temperature T, material, dissolved oxygen concentration / hydrogen concentration, oxygen partial pressure _PO2 / hydrogen partial pressure _PH2 , cracking Enter the dimensions S _M ²⁺ , S _H ⁺ , l _M ²⁺ , l _H ⁺ . Further, in the local corrosion progress evaluation apparatus, in step S3, the initial value of the high-temperature water is the pH value of the crack internal solution measured by a predetermined sensor, the ion equivalent conductivity Λ _M ²⁺ , Λ _H ⁺ , and chromium ions. Enter the equilibrium constant K _Cr of the water hydrolysis reaction. Further, in the local corrosion progress evaluation apparatus, in step S4, electromotive force E _a , E _c1 , E _c2 , resistance values R _as , R _c10s , R _c1s , R _a , R _c2s , resistance as electric circuit constant data Factors a ₁ and b ₁ are input. Furthermore, in the local corrosion growth process evaluation apparatus, in step S5, the atomic weight M of the metal, the valence n of the metal ion, the Faraday constant F, and the density ρ of the metal are input as crack growth rate calculation constant data.

Subsequently, in the local corrosion progress process evaluation apparatus, the calculation unit 14 is based on the chromium content δ _Cr calculated by the measurement unit 12 and the data input via the input unit 13 in steps S6 to S8. Calculate the factors for calculating the crack growth rate. Specifically, in the progress of local corrosion process evaluation apparatus, in step S6, it performs the operation of the above equation (4) to calculate a factor a _0. Further, the progress of local corrosion process evaluation device, in step S7, performs calculation of the above equation (5), to calculate the factor _{b 0,} to calculate the factor _{b 0 × δ Cr × K Cr} . Further, in the local corrosion progress process evaluation apparatus, in step S8, the factor (1 + a ₁ ) / (1 + b ₁ ) × b ₀ × δ _Cr × K _Cr is calculated.

  Subsequently, in the local corrosion progress process evaluation apparatus, in step S9, using the factor calculated in steps S6 to S8 and the data input in step S5, the calculation unit 14 uses the above equation (20). To calculate the crack growth rate da / dt.

And in a local corrosion progress process evaluation apparatus, in step S10, using the calculation result so far, the calculation part 14 shows the relationship between the pH value for each chromium content and each factor as shown in FIG. In step S11, the pH values of the points P _min and P _max shown in FIG. 6 and the crack growth rate da / dt are calculated.

Further, in the local corrosion progress evaluation apparatus, in step S12, using the data calculated in steps S10 and S11, the calculation unit 14 causes the chromium content δ _{Cr and the} crack growth rate as shown in FIG. A diagram showing the relationship with da / dt is created. In step S13, the target maximum crack growth rate is calculated using the created diagram, and the series of processes is terminated.

  The local corrosion growth process evaluation apparatus obtains the crack growth rate according to such a series of procedures and displays it on the screen of the display unit 15 together with the data input via the input unit 13 or the recording unit 16. It can be recorded and stored.

  As described above, in the local corrosion growth process evaluation apparatus shown as an embodiment of the present invention, the crack growth rate of local corrosion including SCC of a metallic material in a nuclear reactor or high-temperature water is determined based on the crack internal solution. It is possible to evaluate quantitatively by measuring the pH value of the sample and inputting it as high-temperature water initial data for calculation. In addition, the local corrosion progress evaluation system measures the concentration of chromium, which is a constituent element of the crack metal, and the surface distribution of the chromium concentration even when it is difficult to measure the pH value of the microcrack internal solution. By doing so, it is possible to quantitatively evaluate the minimum crack growth rate and the maximum crack growth rate for all applied stresses at the design stage, and even if the design conditions are changed, the necessary factors Re-evaluation can be easily performed based on a quantitative formula including Furthermore, in the local corrosion progress process evaluation apparatus, since the generation range of SCC is formulated, it is possible to evaluate the range in which the generation of SCC is suppressed even when the environmental conditions change. .

  As described above, the local corrosion progress process evaluation apparatus to which the present invention is applied quantitatively evaluates the conditions under which local corrosion including SCC can occur, the possibility of progress, the progress rate, etc., while using the electrical circuit method. It is extremely innovative that various measures can be taken.

  The present invention is not limited to the embodiment described above. For example, in the above-described embodiment, the case where an oxide film is present at the crack tip is not mentioned, but the present invention can evaluate the crack growth rate even in such a case.

FIG. 10 shows the relationship between the crack growth rate da / dt at the crack tip and the factor ε (K _I ). In the figure, curve A shows the case where no film is present at the crack tip, curve B shows the case where the film resistance is medium, and curve C shows the case where the film resistance is large. As can be seen from this figure, for example, when there is a film having a large film resistance as indicated by curve C when no stress is applied, if the applied stress is increased and the factor ε (K _I ) increases, The crack growth rate da / dt increases along the curve C from the point P0 to the point P1. At this time, when a part of the film is broken and the equivalent resistance of the film decreases to the level of curve B, the crack growth rate da / dt is increased from point P2 on curve B corresponding to point P1 on curve C. Proceed along the curve B to the point P3. Further, when the damage of the film proceeds and the equivalent resistance of the film becomes zero, the crack growth rate da / dt is changed from the point P4 on the curve A corresponding to the point P3 on the curve B to the curve A. Proceed along to point P5. That is, the crack growth rate da / dt progresses in the order of P0 → P1 → P2 → P3 → P4 → P5 with the addition of stress as a whole, and the points P1 and P3 are also understood as threshold values. Therefore, in the present invention, the crack growth rate da / dt for stress below the threshold value can be evaluated based on the value from the point P0 to the point P1 on the curve C.

  Thus, in the present invention, when an oxide film is present at the crack tip, the resistance of the film, or when the oxide film is damaged by the application of stress, the oxide film Can be evaluated electrochemically by calculating the crack growth rate da / dt, assuming that the equivalent resistance of is connected in series to the electrochemical resistance of the anode corresponding to the crack tip.

In the present invention, the stress dependence of the crack growth rate da / dt can be generally evaluated by the above equation (21) as described above. However, in the case of a constant load test using a test piece made of a material having no residual stress, there is no problem even if the stresses σ _∞ and σ _y are regarded as constant. Therefore, in such a case, the stress dependence of the crack growth rate da / dt can be evaluated only as a function of the factor ε (K _I ) and the stress intensity factor K _I as in the following equation (22). .

Furthermore, in the present invention, the crack growth rate da / dt is set in consideration of the residual stress due to, for example, welding, heat treatment, or plastic working under the condition that the factor ε (K _I ) and the stress intensity factor K _I are constant. When evaluating, following Formula (23) may be used.

Thus, in the present invention, the stress dependence of the crack growth rate da / dt can be appropriately evaluated according to the situation. Particularly, in the present invention, not only the distortion and stress intensity factor K _I of crack tip section, welded, produced by internal force or external force such as the residual stress due to heat treatment or plastic working, well away from the crack tip portion It is possible to quantitatively determine the crack growth rate da / dt in consideration of the stress at the position.

  Thus, it goes without saying that the present invention can be modified as appropriate without departing from the spirit of the present invention.

It is a figure which shows the model shape of the crack part of the local corrosion containing SCC, and the ion and molecule | numerator which are related to the corrosion in the crack inside and outside. It is a figure which shows the structure of the electric circuit which modeled the local corrosion reaction shown in FIG. It is a figure for demonstrating the evaluation method proposed by this invention, and is a figure which shows the direction of the electric current which flows through the electric circuit shown in FIG. It is a figure for demonstrating the evaluation method proposed by this invention, and is a figure which shows the direction which is the direction of the electric current which flows through the electric circuit shown in FIG. 2, and is different from the direction shown in FIG. It is a figure which shows the pattern of the factor which classified the concentration state of the crack internal ion. It is a correlation diagram which shows the relationship between the pH value of a crack internal solution, and each factor. It is a correlation diagram which shows the relationship between chromium content and the minimum and maximum crack growth rate with respect to a stress action. It is a block diagram which shows the structure of the local corrosion progress process evaluation apparatus shown as embodiment of this invention. It is a flowchart which shows a series of procedures at the time of calculating | requiring a crack growth rate in the local corrosion progress process evaluation apparatus shown as embodiment of this invention. FIG. 6 is a correlation diagram showing the relationship between crack growth rate da / dt and factor ε (K _I ).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Metal base material 2 Welding part 11 Sensor 12 Measuring part 13 Input part 14 Calculation part 15 Display part 16 Recording part

Claims (8)

A local corrosion progress evaluation method for evaluating the progress of local corrosion including stress corrosion cracking or corrosion fatigue cracking in high-temperature water in terms of electrical circuit,
Of the corrosion path, the crack tip is the anode from which the metal elutes, the outer surface of the crack is the first cathode involved in the reduction of oxygen, and the inner wall of the crack is the second involved in the reduction of hydrogen ions. The local corrosion reaction is expressed by the electric circuit configured as the cathode of the crack, and when the dissolved metal ions and the hydrogen ions generated by the hydrolysis reaction are related to the solution resistance inside and outside the crack, A pH value measuring step of measuring the pH value of using a predetermined measuring means;
Based on the pH value measured in the pH value measuring step, the crack growth rate expressed using the metal ion concentration and the hydrogen ion concentration for calculating the current flowing through the electric circuit is determined by a predetermined calculation means. A method for evaluating a local corrosion progressing process, comprising: a calculating step for calculating. The local corrosion progressing process according to claim 1, wherein the composition ratio of the eluted metal constituent element is related to the hydrolysis reaction of the metal ion with respect to the hydrogen ion of the crack internal solution. Evaluation methods. In the calculation step, the crack growth rate is calculated using the factors f and g ₂ shown in the following formula (1) and the following formula (2) for calculating the current flowing in the electric circuit. The local corrosion progressing process evaluation method according to claim 1 or 2.
The local corrosion growth process evaluation method according to claim 3, wherein a condition under which a crack propagates under the action of stress is within a range represented by the following formula (3).
In the calculation step, when the crack growth rate is da / dt, the following equation (4) is used to calculate the minimum crack growth rate in a state where no stress acts on the crack tip. In addition to evaluating the hydrogen ion concentration and metal ion concentration inside the crack, the maximum crack growth rate is determined as the hydrogen inside the crack in a state where a sufficiently large or infinite stress is applied to the crack tip. The local corrosion progress process evaluation method according to any one of claims 1 to 4, wherein the evaluation is performed using an ion concentration and a metal ion concentration.
In the calculation step, when an oxide film is present at the crack tip, the resistance of the film, or the equivalent of the oxide film when the oxide film is damaged by the application of stress. 6. The crack growth rate is calculated on the assumption that a resistance is connected in series to an electrochemical resistance of an anode corresponding to the crack tip. The method for evaluating the local corrosion progressing process according to item 1. In the calculation step, the crack growth rate da / dt is calculated by using the following equation (5), and as the stress dependence factor, in addition to the factor ε (K _I ) or the stress intensity factor K _I , the crack The local corrosion progressing process according to claim 1 or 4, wherein the evaluation is performed using a stress σ _∞ generated by an internal force or an external force at a position sufficiently away from the tip portion and a yield stress σ _{y of the} metal. Evaluation methods.
A local corrosion progress evaluation device for evaluating the progress of local corrosion including stress corrosion cracking or corrosion fatigue cracking in high-temperature water in an electrical circuit,
Of the corrosion path, the crack tip is the anode from which the metal elutes, the outer surface of the crack is the first cathode involved in the reduction of oxygen, and the inner wall of the crack is the second involved in the reduction of hydrogen ions. When a local corrosion reaction is expressed by an electric circuit configured as a cathode of the electrode and the solution resistance inside and outside the crack portion is related to the eluted metal ions and hydrogen ions generated by the hydrolysis reaction, a predetermined measurement means is used. Input means for inputting the pH value of the crack internal solution measured using
Calculation means for calculating the crack growth rate expressed using the metal ion concentration and the hydrogen ion concentration for calculating the current flowing through the electric circuit based on the pH value input via the input means; An apparatus for evaluating a local corrosion progressing process.