WO2015011743A1 - 液中電位計測技術を用いた金属の耐食性評価方法及び評価装置 - Google Patents
液中電位計測技術を用いた金属の耐食性評価方法及び評価装置 Download PDFInfo
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- WO2015011743A1 WO2015011743A1 PCT/JP2013/069728 JP2013069728W WO2015011743A1 WO 2015011743 A1 WO2015011743 A1 WO 2015011743A1 JP 2013069728 W JP2013069728 W JP 2013069728W WO 2015011743 A1 WO2015011743 A1 WO 2015011743A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N17/00—Investigating resistance of materials to the weather, to corrosion, or to light
- G01N17/006—Investigating resistance of materials to the weather, to corrosion, or to light of metals
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q30/00—Auxiliary means serving to assist or improve the scanning probe techniques or apparatus, e.g. display or data processing devices
- G01Q30/08—Means for establishing or regulating a desired environmental condition within a sample chamber
- G01Q30/12—Fluid environment
- G01Q30/14—Liquid environment
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N17/00—Investigating resistance of materials to the weather, to corrosion, or to light
- G01N17/02—Electrochemical measuring systems for weathering, corrosion or corrosion-protection measurement
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
- G01Q60/30—Scanning potential microscopy
Definitions
- the present invention relates to a metal corrosion resistance evaluation method and an evaluation apparatus using a liquid potential measurement technique.
- Metal corrosion is a phenomenon in which a metal is eluted by a chemical reaction (oxidation-reduction reaction) occurring at the interface between metal and water, or a corrosion product is deposited.
- Metals are widely used, from household items such as household items to mechanical parts and buildings. Corrosion of metals is one of the major problems that hinders the use of these artifacts in the long term and causes them to be used with peace of mind. Development is underway.
- Patent Document 1 discloses a method using polarization curve data. Since an actual polarization curve changes with time, a method of measuring an actual surface potential or current density, obtaining a polarization curve using the measured value, and estimating a polarization curve after a certain period of time has been disclosed. .
- Patent Document 2 is an apparatus for evaluating the potential of a sacrificial anticorrosive metal, has means for cooling and heating the corrosive liquid, means for injecting the corrosive liquid onto the test piece, and adjusting the clearance. It is disclosed to have a means for measuring the potential of the test strip via the means and a standard electrode.
- Patent Document 3 measures the potential and weight change of a metal surface, obtains a correlation existing between the surface potential of the metal and the amount of corrosion, and determines the measured surface potential based on the obtained correlation. Discloses a method for quantitatively evaluating the amount of corrosion.
- Patent Documents 1 to 3 The conventional techniques for predicting corrosion / corrosion prevention problems as proposed in Patent Documents 1 to 3 are based on macroscopic electrochemical measurements, and both the potential and current density are macroscopic measurements. Is not measured locally. For this reason, local corrosion, that is, specific phenomena such as pits and pitting corrosion cannot be detected, and therefore, an unpredictable corrosion mode exists, which makes life estimation difficult.
- duplex stainless steel is composed of two phases having different chemical compositions (contents of Cr, Mo, N) such as ⁇ phase (ferrite phase) and ⁇ phase (austenite phase). Corrosion resistance is often discussed by macrochemical composition, area ratio of two phases, and the like.
- the phase precipitated by the manufacturing process may include a plurality of precipitates in addition to the two phases.
- the macro chemical composition is the same, the chemical composition of the micro phase may be different. For this reason, it has been difficult to predict a microscopic corrosion depth such as pitting corrosion or crevice corrosion, that is, local corrosion, by an electrochemical method such as that found in the previous patent document.
- An object of the present invention is to provide a corrosion resistance evaluation method and an evaluation apparatus capable of estimating crevice corrosion depth and pitting corrosion depth in a short time.
- the metal corrosion resistance evaluation method of the present invention is the above-described method in which the metal to be evaluated is immersed in the liquid of the use environment.
- the surface potential distribution of the metal is determined by measuring the surface potential of the metal, the surface potential difference of the microstructure of the metal is calculated based on the surface potential distribution, and the maximum surface potential difference among the calculated surface potential differences is corroded.
- the corrosion rate of crevice corrosion of the metal or the corrosion rate of pitting corrosion is predicted.
- the present invention provides a corrosion resistance evaluation method and an evaluation apparatus that can estimate the crevice corrosion depth and pitting corrosion depth in a short time.
- Corrosion is caused by an electrochemical reaction that is oxidized at the anode (electrons are taken away) and reduced at the cathode (receives electrons). Therefore, in the conventional technology, the corrosion potential of the sample is measured by using a potentiostat, or the anode / cathode polarization curve is measured by measuring the potential by passing a current with a galvanostat. Evaluate properties and corrosion behavior.
- the potential value measured by these methods is an average value of the entire sample, and a local potential distribution cannot be measured.
- pitting corrosion or stress corrosion cracking it is necessary to know what kind of potential and what kind of potential the corrosion proceeds.
- Conventional electrochemical measurements do not measure the potential of each metallographic structure, but only measure macro potentials, that is, mixed potentials of different phases, and it is difficult to predict such local corrosion. is there.
- Corrosion is inherently closely related to the potential in the corrosive liquid of the metal structure.
- Galvanic corrosion is a typical example.
- the joints having different metal compositions have different potentials, and the potential difference serves as a driving force to cause corrosion. That is, as the potential difference is larger, a corrosion current flows and a metal dissolution reaction occurs, resulting in pitting corrosion and crevice corrosion. If the in-plane potential of the microscopic structure can be measured, it is expected that the ease of corrosion can be understood from the potential difference.
- OL-EPM open-loop potential microscope
- AFM atomic force microscope
- the present invention is to determine the surface potential distribution of the metal by measuring the surface potential of the metal in a state where the metal to be evaluated is immersed in the liquid of the use environment, and based on the surface potential distribution, the microstructure of the metal.
- This is a metal corrosion resistance evaluation method that predicts the crevice corrosion rate of the metal or the pitting corrosion rate using the maximum surface potential difference among the calculated surface potential differences as an evaluation index for corrosion evaluation. is there.
- the surface potential difference of the microstructure means the surface potential difference between the grain boundary and the grain of the crystal grains, the surface potential difference between the grains having different plane orientations of the crystal grains, the surface potential difference between the precipitate and the parent phase, the precipitate and Means the surface potential difference of the segregation layer, the surface potential difference between the segregation layer and the parent phase, the surface potential difference between the two phases, the surface potential difference between the precipitation phase and the parent phase, the surface potential difference between the precipitation phase and the segregation layer, or the surface potential difference between the precipitation phase and the precipitate.
- the surface potential difference between the grain boundary and grain of the crystal grain, which is a microstructure as described above, or between grains having different plane orientations of the crystal grain is driven by the driving force, and the corrosion proceeds.
- the greater the potential difference the easier the corrosion proceeds.
- the relationship between the corrosion rate of crevice corrosion and pitting corrosion and the potential difference of the surface potential in the plane of the microscopic structure shows that the larger the surface potential difference, the greater the crevice corrosion and pitting corrosion. There was a tendency for speed to increase. Therefore, in the present invention, the maximum surface potential difference among the calculated surface potential differences of the microstructure of the metal is used as an evaluation index for corrosion evaluation.
- the in-use liquid refers to a solution that is exposed in an environment in which the metal to be evaluated is used, such as a solution containing seawater or chemicals. Note that since the surface potential of the microstructure varies depending on the type of solution, it is important to measure the surface potential difference with the same solution that is actually exposed in the use environment.
- Prediction of crevice corrosion rate of metal or pitting corrosion rate based on surface potential difference can be performed as follows. First, using two or more metals with different compositions of known crevice corrosion rate or pitting corrosion rate in the liquid in the working environment, the maximum surface potential difference in the liquid is determined using the same method as above. I ask for it. The correlation between the obtained surface potential difference and the corrosion rate is stored as measurement data. Then, the corrosion rate is predicted based on the surface potential difference of the microstructure measured for the metal to be evaluated whose corrosion rate is unknown and the above measurement data.
- crevice corrosion rate or pitting corrosion rate of metal If there is no data on crevice corrosion rate or pitting corrosion rate of metal in a specific solution, a highly accurate method such as long-term immersion test for two or more metals with different compositions The crevice corrosion rate or pitting corrosion rate is measured, and the correlation between the surface potential difference and the corrosion rate is obtained by the same method as described above. In this case, a long-term test is required as in the conventional case, but once the correlation is obtained, a long-term test is not necessary for metal materials with different corrosion rates that have different compositions. As a result, the effect that the corrosion rate can be evaluated in a short time is obtained.
- An evaluation apparatus for executing the evaluation method of the present invention includes a probe, an AC power source for applying a bias voltage between the probe and a metal serving as a sample, and a bias between the probe and the sample.
- a capacitor provided in the middle of a closed circuit for applying a voltage
- a displacement measuring unit that outputs a voltage signal associated with an interaction force acting between the probe and the sample, and output by the displacement measuring unit
- a signal detector that detects an electrostatic force signal having a specific frequency component included in the detected voltage signal and outputs a value corresponding to the detected signal, and measures the surface potential of the sample in the liquid From the measurement device, the database storing the relationship between the surface potential difference of the microstructure of the metal of various compositions in a specific liquid and crevice corrosion, or the corrosion rate of pitting corrosion, and the surface potential measured by the potential measurement device
- the metal A prediction means for calculating a surface potential difference of the microstructure and predicting a corrosion rate at which the crevice corrosion of the metal or pitting corrosion occurs from the
- FIG. 1 shows a configuration diagram of a corrosion resistance evaluation apparatus using OL-EPM which is an electric potential measurement apparatus.
- the OL-EPM measures the surface potential of the metal that is the sample 106 placed in the solution 122.
- the OL-EPM includes a cantilever 105, an AC power source 121, an LD (Laser Diode) 109, a PD (Photodiode) 110, a preamplifier 111, a lock-in amplifier 123, and a capacitor 103.
- the AC power source 121 applies an AC bias voltage between the probe electrode provided on the tip of the cantilever 105 and the sample 106. This AC bias voltage generates an electric field between the probe electrode and the sample 106.
- the modulation frequency ⁇ m of the AC bias voltage is 10 kHz or more.
- the tip of the cantilever 105 is displaced in the vertical direction by the electrostatic force F es generated between the probe electrode and the sample 106.
- the OL-EPM 100 measures this displacement using the LD 109 and the PD 110. Specifically, the reflected light of the semiconductor laser light irradiated from the LD 109 to the tip of the cantilever 105 is received by the position detection PD 110. The light receiving position by the PD 110 changes according to the displacement of the tip of the cantilever 105 in the z-axis direction. The OL-EPM extracts this change as a voltage change amount via the preamplifier 111. Further, the OL-EPM obtains the surface potential of the sample 106 by detecting a specific frequency component included in the voltage change amount by the lock-in amplifier 123 which is a signal detection unit.
- Capacitor 103 prevents the mixing of a DC offset voltage that may occur unexpectedly when an AC bias voltage is applied. Thereby, generation
- production of an unnecessary electrochemical reaction can be prevented. More specifically, F es in OL-EPM is given by the following equation as V ts V s ⁇ V ac cos ( ⁇ m t).
- the magnitudes of the ⁇ m and 2 ⁇ m components included in F es are A 1 and A 2 , respectively.
- a 1 and A 2 are given by the following equations (2) and (3) using the transfer function G ( ⁇ ) of the cantilever 105.
- the transfer function G ( ⁇ ) of the cantilever 105 is given by the following formula (4).
- a 1 and A 2 can be measured by the lock-in amplifier 123.
- G ( ⁇ m ) and G (2 ⁇ m ) can be calculated by Equation (4) if k, ⁇ 0 , and Q are known. These parameters can be obtained by measuring the thermal vibration spectrum (n z ) of the cantilever 105 and fitting it with the following equation (5).
- k B , T, and n ds are the Boltzmann constant, the absolute temperature, and the displacement noise density of the displacement detector, respectively.
- Equation (6) the absolute value of V s can be obtained by the following Equation (6).
- V s is positive if the phase difference ⁇ with respect to the AC bias voltage of the ⁇ m component included in F es is 0 ° (in-phase), and negative if it is 180 ° (reverse phase).
- X 1 A 1 cos ⁇ 1 is detected by the lock-in amplifier 123, and the value of V s including the sign is obtained by using an expression in which A 1 in Expression (6) is replaced with X 1. be able to.
- cos ⁇ 1 can take intermediate values other than +1 and ⁇ 1. This increases the error of the measurement result. Therefore, it is more desirable to obtain the absolute value by Equation (6) and determine the sign from the sign of X 1 . That is, V s is obtained by the following equation (7).
- X 1 , A 1 , and A 2 are recorded while scanning the probe in the horizontal direction with respect to the surface of the sample 106, a surface potential image of the sample 106 can be obtained from these values.
- X 1 is necessary only to know the sign of V S. Therefore, it is not necessary to measure at all positions on the surface of the sample 106. For example, when the polarity of the potential difference between the surface of the sample 106 and the probe electrode 104 is not reversed on the entire surface of the sample 106, the signal detection unit 218 only needs to measure X 1 at any one position. If the potential calculation unit 219 determines the measured sign of X 1 only once, it can determine the sign of V S at all measurement points thereafter.
- the corrosion resistance evaluation apparatus includes a database 220 in which the relationship between the surface potential difference of the microstructures of metals of various compositions in a specific liquid and crevice corrosion or the corrosion rate of pitting corrosion is stored.
- the surface potential of the microstructure is calculated based on the surface potential distribution of the metal measured by the OL-EPM, and the maximum surface potential among the calculated surface potentials and the surface stored in the database 220 are calculated.
- the corrosion rate of the metal to be evaluated is predicted, and the result is displayed on the display device 221.
- the arithmetic processing unit 219 corresponds to the prediction means of the present invention.
- the display device 221 also displays information such as the surface potential distribution and shape image of the metal measured by OL-EPM, and the surface potential difference of the microstructure.
- the means for calculating the signal from the OL-EPM and measuring the surface potential and the means for predicting the corrosion rate are performed by one arithmetic processing unit, but these may be separated.
- the OL-EPM described with reference to FIG. 1 calculates the surface potential by using the magnitudes A 1 and A 2 of the ⁇ m component and 2 ⁇ m component of the vibration of the cantilever 105 generated by the electrostatic interaction force. This is referred to as a single frequency (SF) mode.
- SF single frequency
- G ( ⁇ ) is 1 / k on the low frequency side as shown in Equation (4), shows a peak at the resonance frequency (f 0 ) of the cantilever, and converges to 0 as the frequency increases from there. Therefore, in order to detect A 1 and A 2 as sufficient signal strength, ⁇ m and 2 ⁇ m need to be lower than f 0 .
- f 0 in a cantilever solution currently on the market is 1 MHz or less even if it is high. Higher resonance frequencies will be realized in the future by downsizing the cantilever, but there are also physical limitations.
- Fig. 2 shows a block diagram of the corrosion resistance evaluation apparatus using the DF mode OL-EPM.
- the OL-EPM configuration is the same except that the SF mode is changed from the SF mode to the DF mode, and the corrosion rate prediction means using the database 220, the arithmetic processing unit 219, and the display device 221 is the same.
- the OL-EPM using the DF mode includes a first AC power supply 101, a second AC power supply 102, a capacitor 103, a probe electrode 104, a cantilever 105, and vibration adjustment.
- the first AC power supply 101 applies a first AC voltage V 1 cos ( ⁇ 1 t) between the probe electrode 104 and the sample 106.
- the second AC power supply 102 has a second AC voltage V 2 cos having a frequency different from the frequency ⁇ 1 of the first AC voltage V 1 cos ( ⁇ 1 t) between the probe electrode 104 and the sample 106.
- ( ⁇ 2 t) is added to the first AC voltage V 1 cos ( ⁇ 1 t) and applied. That is, the OL-EPM applies a bias voltage (V 1 cos ( ⁇ 1 t) + V 2 cos ( ⁇ 2 t)) obtained by adding AC voltages of different frequencies between the probe electrode 104 and the sample 106.
- the capacitor 103 is a capacitor that removes a DC component that can be included in the voltage on which the first AC power supply 101 and the second AC power supply 102 are superimposed.
- the probe electrode 104 is made of a conductive material. Specifically, the probe electrode 104 is a probe electrode having a sharp pointed tip.
- the cantilever 105 has a probe electrode 104 at its tip. Of both ends of the cantilever 105, the end having the probe electrode 104 is a free end, and the other end is a fixed end.
- the material is, for example, silicon or silicon nitride. Note that a conductive metal coating such as gold or platinum may be used.
- the cantilever 105 is a silicon cantilever coated with gold.
- the vibration adjustment unit 210 excites the cantilever 105 by a so-called photothermal excitation method. More specifically, the vibration adjustment unit 210 includes an LD 107 and an excitation AC power supply 108 that drives the LD 107.
- the LD 107 irradiates the back surface of the silicon cantilever 105 coated with gold with intensity-modulated laser light. Since there is a difference between the thermal expansion coefficients of gold and silicon, the cantilever 105 irradiated with the intensity-modulated laser beam is excited.
- the displacement measuring unit 212 outputs a voltage corresponding to the interaction force between the probe electrode 104 and the sample 106. Specifically, the displacement measuring unit 212 measures the displacement of the tip of the cantilever 105 in the z-axis direction. The displacement measuring unit 212 measures the interaction force generated between the probe electrode 104 and the sample 106 in association with the displacement of the tip of the cantilever 105. The displacement of the tip of the cantilever 105 is also used for position control for keeping the distance between the sample 106 and the probe electrode 104 constant by a scanner unit 216 described later.
- the displacement measurement unit 212 includes an LD 109, a PD 110, and a preamplifier 111.
- the displacement measuring unit 212 receives reflected light of the semiconductor laser light irradiated from the LD 109 to the tip of the cantilever 105 by the PD 110.
- the light receiving position of the semiconductor laser light by the PD 110 changes according to the displacement in the z-axis direction of the tip of the cantilever 105.
- the displacement measurement unit 212 outputs a voltage corresponding to the interaction force between the probe electrode 104 and the sample 106 by taking out this change (shift) in the light receiving position as a voltage change amount via the preamplifier 111. .
- the position control unit 214 performs feedback control on the scanner unit 216 so as to keep the vibration amplitude of the cantilever 105 constant.
- the distance between the probe electrode 104 and the surface of the sample 106 changes due to unevenness on the surface of the sample 106, the magnitude of the interaction force acting between the probe electrode 104 and the sample 106 changes.
- the vibration amplitude of the cantilever 105 that is excited by the vibration adjustment unit 210 and vibrates with a constant amplitude changes. Therefore, by controlling the distance between the probe electrode 104 and the sample 106 so as to keep the vibration amplitude of the cantilever 105 constant, the distance between the probe electrode 104 and the sample 106 can be maintained at a constant interval.
- the position control unit 214 includes an amplitude detector 112 and a PI (Proportional-Integral) control circuit 113.
- the amplitude detector 112 acquires the displacement of the tip portion of the cantilever 105 from the displacement measurement unit 212. Thereafter, the amplitude detector 112 detects the vibration amplitude of the cantilever 105 from the acquired displacement.
- the PI control circuit 113 outputs a control signal for causing the scanner unit 216 to adjust the height (z-axis direction) of the sample holder 230 so as to keep the amplitude detected by the amplitude detector 112 constant.
- another feedback control circuit may be used.
- control signal for adjusting the z-axis direction corresponds to the unevenness of the surface of the sample 106. Therefore, by recording this value, the physical shape (information in the height direction) of the surface of the sample 106 can be measured. Therefore, the shape image of the sample surface can be obtained together with the surface potential distribution by the potential measurement.
- the position control unit 214 may control the distance between the probe electrode 104 and the sample 106 so as to keep the resonance frequency or phase of the cantilever constant instead of the vibration amplitude of the cantilever 105. .
- the position control unit 214 adjusts the distance between the surface of the sample 106 and the probe electrode 104 so that the change in the distance between the probe electrode 104 and the sample 106 is constant. Further, the amount of the adjusted distance may be output as the height information of the surface of the sample 106.
- the scanner unit 216 moves the position of the sample holder 230 by 1 nm to 1 mm in the three axial directions of x, y, and z axes orthogonal to each other.
- the movement in the z-axis direction is to keep the vibration amplitude of the cantilever 105 constant as described above.
- the movement in the x-axis and y-axis directions is for measuring the physical shape and potential distribution on the sample 106 in a planar and continuous manner.
- the amount of movement in the x-axis and y-axis directions is preferably set to 100 ⁇ m or more, and thereby, the surface potential can be measured even for a metal structure having crystal grains of 100 ⁇ m or more.
- the scanner unit 216 includes a waveform generation circuit 114, a high-voltage amplifier 115, a Z-scanner 116, an X-scanner 117, and a Y-scanner 118.
- Z-scanner 116 moves the sample holder 230 in the z-axis direction.
- the X-scanner 117 moves the sample holder 230 in the x-axis direction.
- the Y-scanner 118 moves the sample holder 230 in the y-axis direction.
- the OL-EPM includes a control signal (z-axis direction) output from the position control unit 214 as a result of feedback control, and a planar scanning signal (x-axis and y-axis directions) generated by the waveform generation circuit 114. ) Is amplified by the high-voltage amplifier 115. Thereafter, the amplified signal is output to the scanner corresponding to the axis to be moved among Z-scanner 116, X-scanner 117, and Y-scanner 118.
- the signal detection unit 218 outputs the magnitude of a specific frequency component included in the voltage output by the displacement measurement unit 212. More specifically, the signal detection unit 218 includes (1) the magnitude and phase of the frequency component of the same frequency as the frequency ⁇ 1 of the first AC voltage among the voltages output by the displacement measurement unit 212, and (2) difference between the frequency omega 2 of the frequency ⁇ 1 of the first AC voltage the second alternating voltage (i.e., omega 1 - [omega] 2) the magnitude of the frequency component of the same frequency, and outputs to the arithmetic processing unit 219.
- the signal detection unit 218 is a highly sensitive AC voltmeter.
- the signal detection unit 218, for example, a lock-in amplifier or the like can be used.
- the signal detection unit 218 according to the present embodiment includes a lock-in amplifier 119 and a lock-in amplifier 120.
- the signal detection unit 218 uses an AC voltage output from the first AC power supply 101 as a reference signal.
- the signal detection unit 218 detects X 1 , A 1 , and A L from the voltage signal output from the displacement measurement unit 212. As described later, the potential measuring apparatus 100 is not always necessary to detect the X 1.
- the arithmetic processing unit 219 calculates the surface potential of the sample 106 from the value output from the signal detection unit 218. More specifically, the potential calculation unit 219 sets V s that is the surface potential of the sample 106 to (1) a frequency having the same frequency as the frequency ⁇ 1 of the first AC voltage among the voltages output from the displacement measurement unit 212.
- a 1 which is the magnitude of the component, and (2) the cosine value of the phase difference between the frequency component having the same frequency as ⁇ 1 of the voltage output from the displacement measuring unit 212 and the first AC voltage, and A 1 the amount X 1 multiplied by preparative, (3) the frequency of the same frequency as the difference between the frequency omega 2 of the frequency omega 1 and the second alternating voltage of the first AC voltage of the voltage output by the displacement measuring unit 212 A L which is the magnitude of the component, (4) V 2 which is the amplitude of the AC voltage output from the second AC power source, and (5) transmission of the cantilever 105 in which the probe electrode 104 is attached to the free end side.
- the arithmetic processing unit 219 may output the surface potential image of the sample 106 by arranging the calculated surface potentials corresponding to the xy plane scanned by the scanner unit 216.
- a 1 , A L, G ( ⁇ 1 ), and G ( ⁇ L ) can be obtained in the same manner as the SF-mode OL-EPM described above. From these values, the arithmetic processing unit 219 can obtain the absolute value of V s by the following equation.
- V s can be determined by the same method as the SF-mode OL-EPM. Therefore, the arithmetic processing unit 219 can obtain V s by the following formula (12).
- X 1 is necessary only to know the sign of V S. Therefore, it is not necessary to measure at all positions on the surface of the sample 106. For example, when the polarity of the potential difference between the surface of the sample 106 and the probe electrode 104 is not reversed on the entire surface of the sample 106, the signal detection unit 218 only needs to measure X 1 at any one position. If the potential calculation unit 219 determines the measured sign of X 1 only once, it can determine the sign of V S at all measurement points thereafter.
- the potential distribution of the metal can be measured at the nanoscale even in the liquid environment, and the potential difference of the microstructure of the metal can be measured.
- the inventors measured the local potential distribution of duplex stainless steel in an aqueous solution using OL-EPM, and obtained the potential difference between the ferrite phase ( ⁇ phase) and the austenite phase ( ⁇ phase). From the above, it was found that the corrosion resistance of stainless steel can be evaluated. This is not limited to stainless steel, and is true in the potential difference between the precipitate and matrix in any metal structure. Such a thing becomes possible only by using OL-EPM.
- a corrosion evaluation a soaking test is often used in which a metal is immersed in a corrosive solution for a long period of six months to one year to determine the depth of corrosion.
- OL-EPM as an alternative method, This makes it possible to significantly reduce the time required for developing a new metal material.
- the precipitate in the metal structure can be qualitatively and quantified simply by measuring the surface potential in a specific liquid.
- micro-corrosion that has not been recognized in macro-corrosion evaluation that is, galvanic corrosion caused by a potential difference between precipitates formed around the grain boundary of the structure and its surroundings, and the base material
- galvanic corrosion caused by a potential difference between precipitates formed around the grain boundary of the structure and its surroundings, and the base material
- immersion tests are often used in which a metal specimen is immersed for a long period of six months to one year to determine the corrosion depth.
- the corrosion depth is determined in a short period of several hours. This makes it possible to significantly reduce the time required for developing a new metal material.
- the surface potential difference of the microstructure of the metal in a specific liquid is constant, and by applying the corrosion resistance evaluation apparatus described above, it is possible to predict the composition of the precipitate deposited in the metal structure of the metal material. It becomes possible. That is, data on the surface potential difference between the surface potential of the metal having various compositions and the surface potential of the reference metal is stored in the database 220, and the surface potential difference between the metal to be evaluated and the reference metal is determined by OL-EPM. By measuring and comparing with the data in the database, the composition of the precipitate deposited in the metal structure of the metal material can be predicted.
- Metals are known to be susceptible to corrosion when precipitates of a specific composition are generated. Therefore, qualitative and quantitative determination of precipitates is indispensable for evaluating the corrosiveness of metals.
- a method is used in which a precipitate portion is cut out from a material and analyzed using a scanning electron microscope or a transmission electron microscope. In this case, it takes time to search for a place where the precipitate exists, and it takes time to cut out only a portion where the precipitate exists.
- the composition of the precipitate can be predicted only by measuring the surface potential in a specific liquid, it becomes a very simple method for identifying and quantifying the precipitate.
- Example 1 describes an example in which the surface potential of a metal is measured using the SF-mode OL-EPM shown in FIG.
- Duplex stainless steel was used as a measurement sample, and the potential distribution was observed in a 10 mM NaCl aqueous solution.
- the sample 106 is placed in the sample holder while being immersed in the NaCl aqueous solution 122.
- the sample 106 is electrically connected and connected to the cantilever 105 via the capacitor 103 and the AC power supply 121.
- the cantilever 105 is formed by using a photothermal excitation method (a method in which the back surface of a gold-coated silicon cantilever is irradiated with intensity-modulated laser light to excite the cantilever using the difference in thermal expansion between gold and silicon).
- Excitation was performed at a frequency near the resonance frequency (1-1. 2 MHz).
- the distance between the probe 104 and the sample 106 was controlled so as to keep the vibration amplitude A of the cantilever 105 constant.
- the light reflected from the laser beam generated from the laser generator (LD) 109 on the back surface of the cantilever 105 is detected by the detector (PD) 109.
- the amount of displacement of the cantilever 105 in the Z direction from the position of the reflected light detected by the detector 109 is measured by the preamplifier 111 and input to the lock-in amplifier 123.
- a high frequency AC bias voltage of 10 kHz was applied. In the case of 10 kHz or less, the sample was confirmed to be altered. Therefore, it is desirable to select a high frequency of 10 kHz or higher.
- the tip of the cantilever 105 has a probe, and it was confirmed that the shape image and the potential image of the sample surface can be simultaneously acquired as two-dimensional data by scanning the sample surface with the probe.
- Example 2 describes an example in which the surface potential of a metal is measured using the DF mode OL-EPM shown in FIG.
- Duplex stainless steel was used as a measurement sample, and the potential distribution was observed in a 10 mM NaCl aqueous solution.
- the sample 106 is placed in the sample holder 230 while being immersed in the NaCl aqueous solution 122.
- the sample 106 is electrically connected and connected to the cantilever 6 via the capacitor 103 and the two AC power supplies 101 and 102.
- the cantilever 105 is formed by using a photothermal excitation method (a method in which the back surface of a gold-coated silicon cantilever is irradiated with intensity-modulated laser light to excite the cantilever using the difference in thermal expansion between gold and silicon). Excitation was performed at a frequency near the resonance frequency (1-1. 2 MHz). The distance between the probe 104 and the sample 106 was controlled so as to keep the vibration amplitude A of the cantilever 105 constant. The light reflected from the laser beam generated from the laser generator (LD) 109 on the back surface of the cantilever 105 is detected by the detector (PD) 109.
- LD laser generator
- PD detector
- the amount of displacement of the cantilever 105 in the Z direction from the position of the reflected light detected by the detector 109 is measured by the preamplifier 111 and input to the lock-in amplifier 123.
- a bias voltage (V 1 cos ( ⁇ 1 t) + V 2 cos ( ⁇ 2 t)) obtained by adding alternating voltages of different frequencies was applied between the sample 106 and the cantilever 105 by the AC power sources 101 and 102.
- the tip of the cantilever 105 has a probe, and it was confirmed that the shape image and the potential image of the sample surface can be simultaneously acquired as two-dimensional data by scanning the sample surface with the probe.
- the cantilever 105 is formed by using a photothermal excitation method (a method in which the back surface of a gold-coated silicon cantilever is irradiated with an intensity-modulated laser beam to excite the cantilever using the difference in thermal expansion between gold and silicon). Excitation was performed at a frequency near the resonance frequency (1-1. 2 MHz). The distance between the probe 104 and the sample 106 was controlled so as to keep the vibration amplitude A of the cantilever 105 constant.
- a photothermal excitation method a method in which the back surface of a gold-co
- LDX2101, 2205, 2507 the potential distribution of three types of duplex stainless steel. These have higher contents of Cr, Mo and N in the order of LDX 2101, 2205 and 2507, and have higher pitting corrosion resistance and strength. A surface whose surface was sufficiently polished was used for the measurement.
- FIG. 3 shows the result of observing the surface shape and potential distribution of LDX2101 at the same time.
- the DF mode was used for measurement, and the two modulation frequencies and the AC bias voltage were set to 800 kHz / 1 V and 830 kHz / 1 V, respectively.
- the austenite phase ⁇ phase
- the ferrite phase ⁇ phase
- FIG. 3 shows the result of observing the surface shape and potential distribution of LDX2101 at the same time.
- the DF mode was used for measurement, and the two modulation frequencies and the AC bias voltage were set to 800 kHz / 1 V and 830 kHz / 1 V, respectively.
- FIG. 3A shows the left side of the ⁇ phase and the right side is the ⁇ phase.
- the contrast of the potential distribution image in FIG. 3B also changes at the boundary, indicating that the potential is different between the ⁇ phase and the ⁇ phase.
- FIG. 3C shows the average value of the line distribution in the portion surrounded by the white line of the shape image and the potential image. Accordingly, the maximum potential difference between the ⁇ phase and the ⁇ phase was about +63 mV.
- FIG. 4A shows the results of measuring the surface structure and potential distribution of 2205 .
- FIG. 4B shows the average value of the line distribution of the portion surrounded by the white line of the shape image and the potential image. From this, the maximum potential difference between the ⁇ phase and the ⁇ phase was about +30 mV.
- FIG. 5 shows a surface shape image and a potential distribution image of 2507.
- the DF mode was used for measurement, and the two modulation frequencies and the AC bias voltage were set to 800 kHz / 1 V and 830 kHz / 1 V, respectively. From the difference in height, it can be seen that the left side is the ⁇ phase and the right side is the ⁇ phase.
- the contrast of the potential distribution image in FIG. 5B also changes at the boundary, indicating that the potential is different between the ⁇ phase and the ⁇ phase.
- FIG. 5C shows the average value of the line distribution in the portion surrounded by the white line of the shape image and the potential image. Accordingly, the potential difference between the ⁇ phase and the ⁇ phase was about +11 mV.
- FIG. 6 is a plot of the relationship between the crevice corrosion depth of each duplex stainless steel obtained by conducting an immersion test in seawater for one year and the potential difference between the ⁇ phase / ⁇ phase obtained as described above. As the potential difference is larger, the erosion depth tends to increase.
- Galvanic corrosion is one of the main corrosions of duplex stainless steel. Galvanic corrosion is corrosion that occurs when different kinds of metals are in contact with each other, and promotes corrosion of a metal surface having a low electrode potential. The galvanic corrosion is greatly related to the corrosion of metals having different compositions such as duplex stainless steel. Therefore, it can be seen that the potential difference between the two phases measured by OL-EPM is caused by galvanic corrosion, and the potential difference between the electrodes is related to the ease of corrosion.
- the relationship between the surface potential difference and crevice corrosion depth in the metal structures of various compositions was stored in a database.
- the maximum surface potential difference in the structure of the duplex stainless steel with an unknown degree of corrosion was measured.
- the annual crevice corrosion depth was estimated from the measured surface potential difference. From this, it is possible to estimate the period until the hole is opened in the metal pipe.
- FIG. 7 is a plot of the relationship between the pitting corrosion depth of each duplex stainless steel obtained by conducting the salt spray test for one year and the potential difference between the ⁇ phase and the ⁇ phase obtained as described above. There was a tendency for the pitting corrosion depth to increase as the potential difference increased.
- the relationship between the surface potential difference and the pitting depth in the metal structures of various compositions was stored in the database. Next, the maximum surface potential difference in the structure of the duplex stainless steel with an unknown degree of corrosion was measured. With reference to the data in the database, the annual pitting depth was estimated from the measured maximum surface potential difference. From this, it is possible to estimate the period until the hole is opened in the metal pipe.
- the surface potential was measured in the same manner as in Example 3 by collecting a metal material in use in the actual plant. The data was acquired with the result as the surface potential distribution. From this, the maximum potential difference in the surface potential distribution was determined. With reference to the data in the database, the crevice corrosion depth when immersed for a certain period was estimated from the measured maximum surface potential difference. Subsequently, the corrosion rate was estimated. Based on the thickness of the metal used in the actual plant, the period until penetration was calculated and the replacement time was predicted. All of these prediction means were implemented by the arithmetic processing unit 219.
- the surface potentials (V S1 ) of metals having various compositions were measured in the same manner as in Example 3.
- the two modulation frequencies and the AC bias voltage were measured under conditions of 800 kHz / 1V and 830 kHz / 1V, respectively.
- the surface potential (V SR ) of the reference metal was measured simultaneously.
- Data on the surface potential difference (V S1 ⁇ V SR ) with respect to the reference metal was acquired in advance and stored in a computer after making it into a database.
- the surface potential (V SX ) of the precipitate having an unknown composition was measured by the same method as described above, and at the same time, the surface potential of the reference metal was measured.
- the composition of the precipitate deposited in the metal structure was searched from the surface potential difference (V S1 ⁇ V SR ) with respect to the reference metal. All of these were controlled by a computer.
- FIG. 8 shows an example of measurement of the shape image and potential image of the precipitate of SUS304.
- a granular precipitate having a maximum potential difference of 70 mV was identified as MnS by a transmission electron microscope.
- a plate-like precipitate having a maximum potential difference of 20 mV was identified as Cr 23 C 6 by a transmission electron microscope.
- the maximum potential difference was different depending on each precipitate composition.
- an image processing means is provided.
- the area of the surface potential region of a specific precipitate is obtained based on the result of measuring the surface potential (V S1 ) in the metal liquid as an in-plane distribution. This area is calculated as an area ratio (percent) with respect to the entire area. From the area ratio, the content of precipitates precipitated in the metal structure was predicted in percentage.
Abstract
Description
Claims (13)
- 金属の耐食性評価方法であって、
評価対象の金属を使用環境の液中に浸漬した状態で前記金属の表面電位を計測して前記金属の表面電位分布を求め、
前記表面電位分布に基づいて、前記金属のミクロ組織の表面電位差を算出し、
算出した表面電位差のうち、最大の表面電位差を腐食評価の評価指標として、前記金属のすきま腐食の腐食速度、または孔食の腐食速度を予測することを特徴とする金属の耐食性評価方法。 - 請求項1に記載の金属の耐食性評価方法において、
前記金属の表面電位の計測は、先端に探針電極を備えるカンチレバーと液中に浸漬した前記金属との間に交流のバイアス電圧を印加し、前記探針電極を前記金属表面に対して水平方向に走査しながら、静電的相互作用力によって生じる前記カンチレバーの振動を利用して前記金属表面の表面電位を算出することを特徴とする金属の耐食性評価方法。 - 請求項1に記載の金属の耐食性評価方法において、
前記液中におけるすきま腐食深さ、または、孔食深さの腐食速度が既知の組成の異なる2つ以上の金属の前記液中における最大の表面電位差と腐食速度の相関関係に基づいて、前記評価対象の金属のすきま腐食の腐食速度、または孔食の腐食速度を予測することを特徴とする金属の耐食性評価方法。 - 請求項1に記載の金属の耐食性評価方法において、
プラント実機で使用中の金属材料から採取した金属、あるいはプラント設計段階の金属を評価対象とし、
予測した金属のすきま腐食の腐食速度、または孔食の腐食速度に基づいて、前記金属材料の交換時期を予測することを特徴とする金属の耐食性評価方法。 - 請求項1に記載の金属の耐食性評価方法において、
前記評価対象の金属がステンレス鋼であることを特徴とする金属の耐食性評価方法。 - 探針と、前記探針と試料となる金属の間にバイアス電圧を印加するための交流電源と、前記探針と試料との間にバイアス電圧を印加する閉回路の途中に設けられたコンデンサと、前記探針と前記試料との間に働く相互作用力に対応付けられた電圧信号を出力する変位計測部と、前記変位計測部によって出力される電圧信号に含まれる特定の周波数成分をもつ静電気力信号を検出し、検出された信号に対応する値を出力する信号検出部と、を備えた、液中の試料の表面電位を計測する電位計測装置と、
特定の液中における各種組成の金属のミクロ組織の表面電位差とすきま腐食、または、孔食の腐食速度の関係が格納されたデータベースと、
前記電位計測装置で測定された表面電位から前記金属のミクロ組織の表面電位差を算出し、前記データベースのデータを参照して前記金属のミクロ組織の表面電位差から前記金属のすきま腐食の腐食速度、または孔食の腐食速度を予測する予測手段と、を備えることを特徴とする金属材料の耐食性評価装置。 - 請求項6に記載の金属材料の耐食性評価装置において、
前記交流電源が出力する交流電圧の周波数は10kHz以上であることを特徴とする金属材料の耐食性評価装置。 - 請求項6に記載の金属材料の耐食性評価装置において、
前記探針を試料表面で走査するためのX及びY方向のスキャナと、試料の凹凸に探針を追従させるためのZ方向のスキャナと、を備え、
前記X及びY方向のスキャナの移動量が100μm以上であることを特徴とする金属材料の耐食性評価装置。 - 請求項7に記載の金属材料の耐食性評価装置において、
前記交流電源は、
前記電極と前記試料との間に、第1の交流電圧を印加する第1の交流電源と、
前記電極と前記試料との間に、前記第1の交流電圧の周波数と異なる周波数を有する第2の交流電圧を、前記第1の交流電圧に加算して印加する第2の交流電源で構成されていることを特徴とする金属材料の耐食性評価装置。 - 金属材料の組成予測方法であって、
評価対象の金属を使用環境の液中に浸漬した状態で前記金属の表面電位を計測して前記金属の表面電位分布を求め、
前記表面電位分布に基づいて、前記金属のミクロ組織の表面電位差を算出し、
算出した表面電位差に基づいて金属組織内に析出した析出物の組成を予測することを特徴とする金属材料の組成予測方法。 - 請求項9に記載の金属材料の組成予測方法において、
予め、特定の液中における各種組成の金属の表面電位を基準となる金属に対する表面電位差として計測しておき、これを用いて、1ナノメートルから100マイクロメートルまでの大きさの複数の析出物の組成を予測する金属材料の組成予測方法。 - 請求項9に記載の金属材料の組成予測方法において、
前記表面電位分布を画像処理により特定の析出物電位を有する領域の全体に対する面積率を求め、前記面積率から金属組織内に析出した析出物の含有量を予測する金属材料の組成予測方法。 - 探針と、前記探針と試料となる金属の間にバイアス電圧を印加するための交流電源と、前記探針と試料との間にバイアス電圧を印加する閉回路の途中に設けられたコンデンサと、前記探針と前記試料との間に働く相互作用力に対応付けられた電圧信号を出力する変位計測部と、前記変位計測部によって出力される電圧信号に含まれる特定の周波数成分をもつ静電気力信号を検出し、検出された信号に対応する値を出力する信号検出部と、を備え、液中の試料の表面電位を計測する電位計測装置と、
各種組成の金属の表面電位と基準となる金属の表面電位の表面電位差のデータが格納されたデータベースと、
前記電位計測装置で測定された表面電位から前記金属のミクロ組織の表面電位差を算出し、前記データベースのデータを参照して測定された表面電位差から金属組織内に析出した析出物の組成を予測する予測手段と、を備えることを特徴とする金属材料の組成予測装置。
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JP2020085556A (ja) * | 2018-11-20 | 2020-06-04 | 国立研究開発法人物質・材料研究機構 | 金属材料の表面観察方法、および該表面観察方法を用いた金属材料の化成処理性評価方法。 |
JP7141701B2 (ja) | 2018-11-20 | 2022-09-26 | 国立研究開発法人物質・材料研究機構 | 金属材料の表面観察方法を用いた金属材料の化成処理性評価方法 |
CN112417716A (zh) * | 2020-10-27 | 2021-02-26 | 河南四达电力设备股份有限公司 | 一种基于数值算法的免维护耐腐蚀接地装置设计方法 |
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US20160146719A1 (en) | 2016-05-26 |
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US10215686B2 (en) | 2019-02-26 |
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