CN116337745B - Layer-by-layer electrochemical analysis method for corrosion resistance of gradient material treated by SMAT - Google Patents
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Abstract
The invention discloses a layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material treated by adopting an SMAT, and relates to the field of analysis of the corrosion resistance of materials. The method comprises the following steps: basic corrosion parameter acquisition, layer-by-layer gradient surface treatment, layer-by-layer gradient corrosion parameter acquisition and gradient material corrosion resistance test based on passivation behavior. The invention relates to a brand new depth direction tissue and component gradient material corrosion resistance evaluation method, which can simultaneously perform gradient characterization on a gradient fine structure; the gradient surface is prepared by a simple method, so that a brand new thought is provided for characterization of a refined structure; different passivation modes are also a brand new contrast test method, and influence factors caused by passivation film formation mechanisms and gradient structures are analyzed by controlling passivation speed. The invention forms a complete set of test system from gradient surface preparation to characterization and finally to mechanism analysis, forms a brand new gradient structure material passivation method, and has great innovation and instruction significance.
Description
Technical Field
The invention relates to the field of analysis of corrosion resistance of materials, in particular to a layer-by-layer electrochemical analysis method of corrosion resistance of gradient materials treated by adopting SMAT, which is suitable for corrosion resistance analysis of stainless steel gradient materials.
Background
The austenitic heat-resistant steel is widely applied to industrial production in the application process by virtue of excellent corrosion resistance and high-temperature oxidation resistance, but the problems of low strength, low hardness and the like are limited to bearing high load and other service environments. Conventional treatments, such as heat treatment, rolling, etc., can increase the overall strength of the material, but this increase is at the expense of the plasticity of the material. Surface treatment is commonly used in the present application to obtain proper improvement of strength, and as a surface treatment, SMAT (surface mechanical grinding treatment) is widely and studied to improve the problem of matching strength with plasticity of materials.
The influence of SMAT on the material performance as a surface treatment means is not only represented on the surface, but also the research shows that the grain size of the treated material shows gradient change from the surface to the inside. Because of the gradient change of the microcosmic level, the strength of the material is greatly improved after the material passes through the SMAT, but the plasticity of the material is not severely reduced due to the plastic deformation, which is the gain brought by the nano gradient structure. However, returning to the material itself, whether stainless steel still shows excellent corrosion resistance after being subjected to SMAT treatment is the main content of our study because of its excellent corrosion resistance when applied to industrial environments. For the corrosion resistance test of materials, there are three general methods, namely a gravimetric method, a surface observation method and an electrochemical test method. However, there is no accepted method for testing the corrosion resistance of gradient materials, and in recent years, there are many different results about the exploration of the corrosion resistance of stainless steel after SMAT. Corrosion resistance studies were also performed on 316L, but two different results were presented: it is stated that the corrosion current density increases by a factor of two, but in another study the corrosion resistance decreases by a factor of 2. For such a phenomenon, there is a need to improve the existing corrosion resistance test methods for gradient materials.
Disclosure of Invention
The invention provides a layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material treated by adopting a SMAT (surface magnetic field sensor), which aims to solve the problem that a generally accepted method is lacking in the corrosion resistance test of the gradient material.
The invention is realized by the following technical scheme: a layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material treated by adopting an SMAT comprises the following steps: basic corrosion parameter acquisition, layer-by-layer gradient surface treatment, layer-by-layer gradient corrosion parameter acquisition and gradient material corrosion resistance test based on passivation behavior.
Firstly, basic corrosion parameters are obtained: adopting rolled C-HRA-5 stainless steel in a solid solution state, preparing a round sheet with the diameter of 60mm and the thickness of 4mm by linear cutting, polishing the surface, ultrasonically cleaning and drying, and then carrying out nanocrystallization tests with different parameters in an SNC2 type surface nanocrystallization tester; the processing time is selected as a specific variable in the test, and the parameters are as follows: treating with 4mm304 stainless steel balls for 0, 30min, 40min, 50min and 60 min; the surface oxide layer is removed after treatment by simple mechanical polishing, the treated sample is cut into 15mm multiplied by 15mm samples, and potentiodynamic polarization tests are carried out on the samples with different SMAT treatment times in 3.5% NaCl solution to determine basic corrosion resistance parameters, wherein the basic corrosion resistance parameters comprise self-corrosion potential Ecorr, current density Icorr, passivation interval and Viton current density Ibas, the results show that the samples after treatment for 50 or 60min show excellent corrosion resistance in a polarization curve, meanwhile EIS data show that the corrosion resistance after nanocrystallization treatment for more than 40min is stronger than the corrosion resistance after nanocrystallization treatment for less than 40min, but repeated experiments show that the promotion does not show a rule consistent with tissue change, so that layer-by-layer electrochemical tests are carried out under the basic corrosion data to find the relationship therein.
Step two, surface treatment layer by layer and basic test: selecting a sample with the treatment time of 60min at maximum, preparing a gradient surface, and using an electrochemical workstation as a main test means, and carrying out gradient depth confirmation by assistance of OM, SEM and the like to be accurate to 10 microns; on the premise of not damaging a gradient structure, preparing a sample to be tested with the thickness of 4mm, which is 15mm multiplied by 15mm, from a sample subjected to light polishing, wherein a test solution adopts a sulfuric acid solution with the thickness of 0.05mol/L, a reference electrode adopts a saturated calomel electrode, a counter electrode selects a platinum electrode, and a working electrode is the prepared sample; the preparation process of the gradient surface comprises the following steps: firstly, carrying out electrokinetic polarization test on an unground surface to determine an initial surface, wherein the initial surface is 0 mu m, then slightly polishing the surface of a sample by adopting high-mesh SiC sand paper under the wetting of absolute ethyl alcohol, wherein the Gao Mu value is > =1000 meshes, polishing is carried out for 10 times as one period, the polishing direction is the same direction, thickness characterization is carried out by using an electronic screw micrometer every three periods, polishing is carried out to about 40 mu m, and mechanical polishing is carried out by using diamond abrasive materials to 50 mu m; microcosmic characterization of OM and SEM is carried out after the required depth is reached, so that the gradient depth is ensured to be in a controllable range, and the surface treatment meets the same level as 0 mu m; then XRD test is carried out firstly, electrochemical performance test is carried out, SEM section characterization is carried out after the test is finished, and the thickness of the gradient layer is determined so as to facilitate the accuracy of polishing in the next step; determining the next grinding depth according to different gradient material depth ranges; the gradient material is divided into four parts, so that the gradient material is respectively polished to 0, 50, 150 and 500 mu m; in the process, the surface state is determined by a surface roughness tester and XRD, and meanwhile, the passivation interval and the passivation range are determined, so that a basis is provided for the next passivation test.
And a third step of: and (3) obtaining a layer-by-layer gradient corrosion parameter: the third step is combined with the second step, and the test part is analyzed; firstly, adopting a dilute solution of strong oxidation acid for electrochemical test; in the experiments, no significant passivation intervals were found to occur at depths of 0 μm and 500 μm, which is consistent with passivation behavior in the NaCl solution in the first step; determining a change in corrosion behavior is therefore based on the change in passivation behavior; meanwhile, the EIS impedance analysis can determine the type of a passivation film, and a gradient surface fitting circuit of 50 and 150 mu m is an obvious passivation film type circuit, so that the arc radius of the capacitive reactance is increased by two to three orders of magnitude; under the condition of ensuring the consistency of the roughness, carrying out factor analysis on the two aspects of residual stress and microstructure; when gradient surface preparation is carried out, XRD test is carried out, and the following gradient layer-by-layer passivation method is adopted to test the passivation behavior of the gradient material.
Fourth, gradient layer-by-layer passivation analysis: selecting two different passivation methods to conduct passivation behavior research on the prepared gradient surface, wherein the passivation behavior research comprises a concentrated acid rapid passivation method and a natural passivation method under an open circuit potential; the concentrated acid rapid passivation method provides the film forming driving force of the passivation film by a strong oxidation environment, so that the influence of microstructure change on the layer-by-layer passivation behavior can be singly studied, and the natural passivation rule under the open circuit potential can observe whether the residual stress is used as the driving force to increase the compactness and the thickness of the passivation film; samples with different depths of gradient surface and different treatment times were treated with 40% HNO 3 Passivating the solution for 30min at normal temperature, and then carrying out EIS impedance analysis and M-S curve test; along with the positionThe radius of the capacitance ring is gradually increased when the treatment time is increased, the increase rate is slow, and when the treatment time is increased to 40min, the radius is suddenly increased, and the passivation trend is presented; also from the observation of the phase angle diagram, the lower the phase angle of the low frequency band is, the worse the corrosion resistance is, the overall corrosion resistance of the stainless steel shows a lifting trend after being treated by the SMAT, and the corrosion resistance is stable after being increased along with the increase of the treatment time, which indicates that the passivation film is the most stable when the treatment time is 60 min; the grains gradually increase with increasing depth from the gradient surface data of different depths, but the stability of the passivation film gradually decreases; according to the drawn M-S curve, the semiconductor performance of the passivation film is shown, the semiconductor property is judged according to the positive and negative of the slope of the passivation film, the linear part of the curve is fitted, and judgment is carried out: when the slope is positive, the passivation film is an n-type semiconductor, the cation gap in the passivation film is more than the cation gap and the oxygen gap, the main carrier in the passivation film is the cation gap, and the value of the carrier density in the passivation film is calculated through the slope of a straight line and is called as the donor density; the value of the intersection of the straight line and the x-axis is called flat band potential; after the linear segment fitting, if the slope is gradually increasing with the increase of the processing time, the criterion is that: as the processing time increases, the corresponding donor density is continuously reduced when the slope is continuously increased, which indicates that the passivation film is more compact; meanwhile, if the flat band current is gradually increased along with the increase of the treatment time, the potential for stabilizing the growth of the passivation film after the SMAT treatment is improved, but if the potential is a negative value, the influence on the stability of the passivation film is ignored; then the same method is utilized to passivate for 2 hours by adding potential in 0.05mol/L sulfuric acid solution, then EIS impedance and M-S curve test are carried out, so that samples treated by SMAT for different time form passivation films under the open circuit potential of 2 hours, and whether the ring radius of the capacitive reactance after treatment is obviously improved is observed.
The method adopts the SPD principle, and the SMAT method prepares the gradient structure material, but the material contains Cl - A normalized corrosion resistance rule is never found in the corrosive medium, and the conventional test method is limited to: 1. external factors such as roughness, residual stress and the like cannot be eliminated; 2. cannot be provided with clear ladderThe corrosion resistance test mechanism of the material cannot be used for deeply researching the corrosion resistance of materials with different depth gradients. Therefore, the invention adopts a brand new testing system, utilizes simple and accurate gradient surface preparation processes with different depths, adopts OM, SEM, XRD, TEM, EIS and M-S curve characterization methods, and develops a comprehensive evaluation method for the corrosion resistance of the gradient material based on passivation behaviors. The influence of roughness on corrosion resistance is naturally avoided in the preparation process of the gradient material, and the same roughness principle is always maintained in the test. However, the influence of roughness cannot be avoided at the position of 0 micrometers, and the existence of roughness can be found to completely offset the gain of corrosion resistance brought by the gradient material, so that the influence of roughness can be practically illustrated by the depth as a comparison. In the process of eliminating influencing factors, variables are required to be strictly controlled, and the small variable differences bring unexplained influence to the result. Further, a series of tests will be performed after preparing the gradient surface: microcosmic characterization, potentiodynamic polarization and passivation testing. Here the potentiodynamic polarization test is a destructive test, so that two specimens are required for the test passivation and the basic corrosion data test, respectively. The surface smoothness and dryness of the sample are required to be maintained in the whole test process, and the influence of external factors such as temperature, humidity and the like on the passivation process is avoided. The method is suitable for evaluating the corrosion resistance and microstructure of the material with the tissue and the composition gradient in the depth direction, which is prepared by various methods, and can be used for testing after preparing the gradient surface by proper adjustment, but the surface state change is needed to be noted.
Compared with the prior art, the invention has the following beneficial effects: the invention provides a layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material after SMAT treatment, 1. A brand new method for evaluating the corrosion resistance of a gradient material with a depth direction tissue and components is provided, and gradient characterization can be carried out on a gradient fine structure at the same time; 2. the preparation of the gradient surface is carried out by using a simple method to carry out the preparation of the refined structure, thereby providing a brand new thought for the characterization of the refined structure; 3. the different passivation modes in the invention are also a brand new contrast test method, and the influence factors brought by the passivation film formation mechanism and the gradient structure are analyzed by controlling the passivation speed. The invention forms a complete set of test system from gradient surface preparation to characterization and finally to mechanism analysis, and even in subsequent researches, a brand new gradient structure material passivation method can be formed, so that the invention has great innovation and guiding significance.
Detailed Description
The invention is further illustrated below with reference to specific examples.
A layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material treated by adopting an SMAT comprises the following steps: basic corrosion parameter acquisition, layer-by-layer gradient surface treatment, layer-by-layer gradient corrosion parameter acquisition and gradient material corrosion resistance test based on passivation behavior;
firstly, basic corrosion parameters are obtained: adopting rolled C-HRA-5 stainless steel in a solid solution state, preparing a round sheet with the diameter of 60mm and the thickness of 4mm by linear cutting, polishing the surface, ultrasonically cleaning and drying, and then carrying out nanocrystallization tests with different parameters in an SNC2 type surface nanocrystallization tester; the processing time is selected as a specific variable in the test, and the parameters are as follows: treating with 4mm304 stainless steel balls for 0, 30min, 40min, 50min and 60 min; the surface oxide layer is removed after treatment by simple mechanical polishing, the treated sample is cut into 15mm multiplied by 15mm samples, and potentiodynamic polarization tests are carried out on the samples with different SMAT treatment times in 3.5% NaCl solution to determine basic corrosion resistance parameters, wherein the basic corrosion resistance parameters comprise self-corrosion potential Ecorr, current density Icorr, passivation interval and Viton current density Ibas, the results show that the samples after treatment for 50 or 60min show excellent corrosion resistance in a polarization curve, meanwhile EIS data show that the corrosion resistance after nanocrystallization treatment for more than 40min is stronger than the corrosion resistance after nanocrystallization treatment for less than 40min, but repeated experiments show that the promotion does not show a rule consistent with tissue change, so that layer-by-layer electrochemical tests are carried out under basic corrosion data to find the relation therein.
Step two, surface treatment layer by layer and basic test: in order to ensure accurate characterization of the corrosion resistance of the gradient structure, a sample with the longest treatment time (60 min) is selected for gradient surface preparation, an electrochemical workstation is used as a main test means, and an OM, an SEM and the like are used for gradient depth confirmation (accurate to 10 microns). Preparing a sample to be tested with the thickness of 4mm and the thickness of 15mm multiplied by 15mm from a sample subjected to light polishing (without damaging a gradient structure), wherein a test solution adopts a sulfuric acid solution with the thickness of 0.05mol/L, a reference electrode adopts a saturated calomel electrode, a counter electrode adopts a platinum electrode, and a working electrode is the prepared sample; the preparation process of the gradient surface comprises the following steps: the unpolished surface was first subjected to an electrokinetic polarization test to determine the starting surface (0 μm), then the surface of the sample was lightly polished with high-mesh (> =1000 mesh) SiC sandpaper under the wetting of absolute ethyl alcohol, 10 times polishing was performed as one cycle, the polishing direction was the same direction, thickness characterization was performed every three cycles using an electronic screw micrometer, polishing was performed to about 40 μm, and mechanical polishing was performed to 50 μm using a diamond abrasive. Microscopic characterization of OM, SEM was performed after reaching the required depth, ensuring that the gradient depth was in a controllable range and that the surface treatment met the same level as 0 μm. And then XRD testing is carried out, electrochemical performance testing is carried out, and SEM section characterization is carried out after the testing is finished to determine the thickness of the gradient layer so as to facilitate the accuracy of polishing in the next step. The next grinding depth is determined from the different gradient material depth ranges. In this example, the gradient material was ground to 0, 50, 150, 500 μm because it was divided into four parts. Different polishing depths and surfaces can be selected for subsequent testing if the materials are different. The surface state in this process was determined with a surface roughness tester and XRD. Controlling roughness and residual stress distribution is a key point of the testing technique. The electrochemical test performed in the second step mainly determines passivation interval and scope, provides basis for the next passivation test, and the following table 1 is the test result:
TABLE 1
The test results show that the surface has the minimum impedance and the worst corrosion resistance without polishing treatment. Besides the above phenomena, the passivation regions of different regions can be seen to be different, the passivation phenomena of the surface layer and the matrix are more obvious, and the passivation regions of the two regions are less stable.
And a third step of: and (3) obtaining a layer-by-layer gradient corrosion parameter: this step is combined mainly with the second step, mainly analyzing the test part. The electrochemical test is first performed with dilute solution of strong oxidizing acids such as sulfuric acid, nitric acid, chromic acid, etc., and the main reason is that this embodiment is based on the study of passivation behavior, so the corrosion resistance key data of each gradient surface is the passivation behavior. Therefore, the passivation interval and the witton current density are important concerns when performing the potentiodynamic polarization curve test. In this example, no significant passivation intervals were found to occur at depths of 0 μm and 500 μm, which is consistent with passivation behavior in the NaCl solution in the first step. It can thus be determined that the change in corrosion behavior is based on a change in passivation behavior. Meanwhile, the EIS impedance analysis can determine the type of the passivation film, and a gradient surface fitting circuit of 50 and 150 mu m is an obvious passivation film type circuit, so that the arc radius of the capacitive reactance is increased by two to three orders of magnitude. In the case of ensuring uniform roughness, the factor analysis can be performed in terms of both residual stress and microstructure. In the preparation of gradient surfaces, XRD tests were performed with the aim of: 1. determining whether a phase change has an effect on corrosion resistance during the treatment; 2. residual stress is tested using the sin2 ψ method (other residual stress testing methods can be used if higher accuracy testing is required). To explore the effect of microstructure and residual stress on passivation behavior, the following gradient layer-by-layer passivation method was developed to test the gradient material passivation behavior.
Fourth, gradient layer-by-layer passivation analysis: and selecting two different passivation methods to conduct passivation behavior research on the prepared gradient surface, and adopting a concentrated acid rapid passivation method and a natural passivation method under open circuit potential. The rapid passivation is provided by a strong oxidation environment to provide the film forming driving force of the passivation film, so that the influence of microstructure change on the layer-by-layer passivation behavior can be singly studied, and the natural passivation can observe whether the residual stress is used as the driving force to increase the compactness and the thickness of the passivation film. Samples with different depths of gradient surface and different treatment times were treated with 40% HNO 3 The solution was passivated for 30min at ambient temperature, followed by EIS impedance analysis and M-S curve testing. The capacitance ring radius is gradually increased along with the increase of the treatment time, the increase rate is slow, and the treatment time is asWhen the radius is increased to 40min, the radius is suddenly increased, and the passivation trend is presented; also from the observation of the phase angle diagram, the lower the phase angle of the low frequency band is, the worse the corrosion resistance is, the overall corrosion resistance of the stainless steel shows a lifting trend after being treated by the SMAT, and the corrosion resistance is firstly increased and then is stabilized along with the increase of the treatment time, which indicates that the passivation film is the most stable when the treatment time is 60 min; the grains gradually increase with increasing depth from the gradient surface data at different depths, but the passivation film stability gradually decreases. According to the drawn M-S curve, the semiconductor performance of the passivation film is shown, the semiconductor property is judged according to the positive and negative of the slope of the passivation film, the linear part of the curve is fitted, and judgment is carried out: when the slope is positive, the passivation film is an n-type semiconductor, the cation gap in the passivation film is more than the cation gap and the oxygen gap, the main carrier in the passivation film is the cation gap, and the value of the carrier density in the passivation film is calculated through the slope of a straight line and is called as the donor density; the straight linexThe value of the intersection of the axes is then called the flatband potential; after the linear segment fitting, if the slope is gradually increasing with the increase of the processing time, the criterion is that: as the processing time increases, the corresponding donor density is continuously reduced when the slope is continuously increased, which indicates that the passivation film is more compact; meanwhile, if the flat band current is gradually increased along with the increase of the treatment time, the potential for stabilizing the growth of the passivation film after the SMAT treatment is improved, but if the potential is a negative value, the influence on the stability of the passivation film is ignored; then the same method is utilized to passivate for 2 hours by adding potential in 0.05mol/L sulfuric acid solution, then EIS impedance and M-S curve test are carried out, so that samples treated by SMAT at different times form passivation films under the open circuit potential of 2 hours, whether the ring radius of the capacitive impedance is obviously improved after treatment is observed, and the passivation films are matched with concentrated HNO 3 Compared with the results after passivation, whether the change of the capacitance ring radius is obvious or not is observed after two passivation processes, so that the corrosion resistance after slow passivation in sulfuric acid solution is compared with the corrosion resistance after passivation in a strong oxygen medium. This step illustrates that in addition to grain reduction, residual stress also has a positive effect on the growth of the passivation film.
According to the layer-by-layer electrochemical analysis method for the corrosion resistance of the gradient material treated by the SMAT, which is designed in the embodiment, the influence of surface roughness after the SMAT is removed is determined through an electrochemical means, so that a stable passivation film is ensured to grow in a subsequent passivation experiment; the thickness of about 50 microns removed from the surface of the material after SMAT is obtained through layer-by-layer electrochemical analysis, so that the corrosion resistance is optimal.
The scope of the present invention is not limited to the above embodiments, and various modifications and alterations of the present invention will become apparent to those skilled in the art, and any modifications, improvements and equivalents within the spirit and principle of the present invention are intended to be included in the scope of the present invention.
Claims (3)
1. A layer-by-layer electrochemical analysis method for the corrosion resistance of a gradient material treated by adopting an SMAT is characterized by comprising the following steps of: the method comprises the following steps: basic corrosion parameter acquisition, layer-by-layer gradient surface treatment, layer-by-layer gradient corrosion parameter acquisition and gradient material corrosion resistance test based on passivation behavior;
firstly, basic corrosion parameters are obtained: adopting rolled C-HRA-5 stainless steel in a solid solution state, preparing a round sheet with the diameter of 60mm and the thickness of 4mm by linear cutting, polishing the surface, ultrasonically cleaning and drying, and then carrying out nanocrystallization tests with different parameters in an SNC2 type surface nanocrystallization tester; the processing time is selected as a specific variable in the test, and the parameters are as follows: treating with 4mm304 stainless steel balls for 0, 30min, 40min, 50min and 60 min; removing a surface oxide layer by simple mechanical polishing after treatment, cutting the treated sample into 15mm multiplied by 15mm samples, and performing electrokinetic polarization tests on the samples with different SMAT treatment times in a 3.5% NaCl solution to determine basic corrosion resistance parameters, wherein the basic corrosion resistance parameters comprise self-corrosion potential Ecorr, current density Icorr, passivation interval and Viton current density Ibas, the results show that the samples after treatment for 50 or 60min show excellent corrosion resistance in a polarization curve, meanwhile EIS data show that the corrosion resistance after nanocrystallization treatment for more than 40min is stronger than the corrosion resistance after nanocrystallization treatment for less than 40min, but repeated experiments show that the improvement does not show a rule consistent with tissue change, so that layer-by-layer electrochemical tests are performed under basic corrosion data to find the relation therein;
step two, surface treatment layer by layer and basic test: selecting a sample with the treatment time of 60min at maximum, preparing a gradient surface, and using an electrochemical workstation as a main test means, and carrying out gradient depth confirmation by assistance of OM and SEM to the accuracy of 10 microns; on the premise of not damaging a gradient structure, preparing a sample to be tested with the thickness of 4mm, which is 15mm multiplied by 15mm, from a sample subjected to light polishing, wherein a test solution adopts a sulfuric acid solution with the thickness of 0.05mol/L, a reference electrode adopts a saturated calomel electrode, a counter electrode selects a platinum electrode, and a working electrode is the prepared sample; the preparation process of the gradient surface comprises the following steps: firstly, carrying out electrokinetic polarization test on an unground surface to determine an initial surface, wherein the initial surface is 0 mu m, then slightly polishing the surface of a sample by adopting high-mesh SiC sand paper under the wetting of absolute ethyl alcohol, wherein the high-mesh value of the Gao Mu SiC sand paper is > =1000 meshes, 10 times of polishing is taken as a period, the polishing direction is the same direction, thickness characterization is carried out by using an electronic screw micrometer every three periods, polishing is carried out to 37-42 mu m, and mechanical polishing is carried out by using diamond abrasive to 50 mu m; microcosmic characterization of OM and SEM is carried out after the required depth is reached, so that the gradient depth is ensured to be in a controllable range, and the surface treatment meets the same level as 0 mu m; then XRD test is carried out firstly, electrochemical performance test is carried out, SEM section characterization is carried out after the test is finished, and the thickness of the gradient layer is determined so as to facilitate the accuracy of polishing in the next step; determining the next grinding depth according to different gradient material depth ranges; the gradient material is divided into four parts, so that the gradient material is respectively polished to 0, 50, 150 and 500 mu m; in the process, the surface state is determined by a surface roughness tester and XRD, and meanwhile, the passivation interval and the passivation range are determined, so that a basis is provided for the next passivation test;
and a third step of: and (3) obtaining a layer-by-layer gradient corrosion parameter: the third step is combined with the second step, and the test part is analyzed; firstly, adopting a dilute solution of strong oxidation acid for electrochemical test; in the experiments, no significant passivation intervals were found to occur at depths of 0 μm and 500 μm, which is consistent with passivation behavior in the NaCl solution in the first step; determining a change in corrosion behavior is therefore based on the change in passivation behavior; meanwhile, the EIS impedance analysis can determine the type of a passivation film, and a gradient surface fitting circuit of 50 and 150 mu m is an obvious passivation film type circuit, so that the arc radius of the capacitive reactance is increased by two to three orders of magnitude; under the condition of ensuring the consistency of the roughness, carrying out factor analysis on the two aspects of residual stress and microstructure; when gradient surface preparation is carried out, XRD test is carried out, and the following gradient layer-by-layer passivation method is adopted to test the passivation behavior of the gradient material;
fourth, gradient layer-by-layer passivation analysis: selecting two different passivation methods to conduct passivation behavior research on the prepared gradient surface, wherein the passivation behavior research comprises a concentrated acid rapid passivation method and a natural passivation method under an open circuit potential; the concentrated acid rapid passivation method provides the film forming driving force of the passivation film by a strong oxidation environment, so that the influence of microstructure change on the layer-by-layer passivation behavior can be singly studied, and the natural passivation rule under the open circuit potential can observe whether the residual stress is used as the driving force to increase the compactness and the thickness of the passivation film; samples with different depths of gradient surface and different treatment times were treated with 40% HNO 3 Passivating the solution for 30min at normal temperature, and then carrying out EIS impedance analysis and M-S curve test; the radius of the capacitive reactance ring is gradually increased along with the increase of the treatment time, the increase rate is slow, and when the treatment time is increased to 40min, the radius is suddenly increased, and the passivation trend is presented; also from the observation of the phase angle diagram, the lower the phase angle of the low frequency band is, the worse the corrosion resistance is, the overall corrosion resistance of the stainless steel shows a lifting trend after being treated by the SMAT, and the corrosion resistance is stable after being increased along with the increase of the treatment time, which indicates that the passivation film is the most stable when the treatment time is 60 min; the grains gradually increase with increasing depth from the gradient surface data of different depths, but the stability of the passivation film gradually decreases; according to the drawn M-S curve, the semiconductor performance of the passivation film is shown, the semiconductor property is judged according to the positive and negative of the slope of the passivation film, the linear part of the curve is fitted, and judgment is carried out: when the slope is positive, the passivation film is an n-type semiconductor, the cation gap in the passivation film is more than the cation gap and the oxygen gap, the main carrier in the passivation film is the cation gap, and the value of the carrier density in the passivation film is calculated through the slope of a straight line and is called as the donor density; the straight lineThe value of the intersection of the x-axis is then called flat band potential; after the linear segment fitting, if the slope is gradually increasing with the increase of the processing time, the criterion is that: as the processing time increases, the corresponding donor density is continuously reduced when the slope is continuously increased, which indicates that the passivation film is more compact; meanwhile, if the flat band current is gradually increased along with the increase of the treatment time, the potential for stabilizing the growth of the passivation film after the SMAT treatment is improved, but if the potential is a negative value, the influence on the stability of the passivation film is ignored; then the same method is utilized to passivate for 2 hours by adding potential in 0.05mol/L sulfuric acid solution, then EIS impedance and M-S curve test are carried out, so that samples treated by SMAT for different time form passivation films under the open circuit potential of 2 hours, and whether the ring radius of the capacitive reactance after treatment is obviously improved is observed.
2. A layer-by-layer electrochemical analysis method of graded material corrosion resistance after SMAT treatment according to claim 1, wherein: in the second step, in polishing of the gradient material, in the subsequent test, different polishing depths and surfaces are selected if the materials are different.
3. A layer-by-layer electrochemical analysis method of graded material corrosion resistance after SMAT treatment according to claim 1, wherein: in the third step, the dilute solution of the strong oxidizing acid adopts sulfuric acid, nitric acid or chromic acid.
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