Electrolytic polishing and corrosion method for Fe-Cr-B alloy and application of electrolytic polishing and corrosion method

Abstract

The invention relates to the technical field of alloy sample treatment, and discloses an electrolytic polishing and corrosion method for a Fe-Cr-B alloy and application of the electrolytic polishing and corrosion method. The electrolytic polishing and corrosion method comprises the steps that a cathode and a Fe-Cr-B alloy sample serving as an anode are placed in electrolyte, and electrolysis is conducted at the temperature of 0-20 DEG C; the Fe-Cr-B alloy mainly comprises the following chemical components in percentage by mass: 15 to 23 percent of Cr and 0.8 to 1.3 percent of B, and the electrolyte comprises the following components in percentage by volume: 5-10% of perchloric acid and the balance of absolute ethyl alcohol. Due to the potential difference between the boride phase and the matrix phase in the Fe-Cr-B alloy, the anode is selectively dissolved in the electrolytic polishing and corrosion process, that is, the matrix phase is preferentially dissolved, the boride phase is reserved in the anode electrolysis process due to high electrochemical stability, a clear interface profile of the matrix and the boride phase is obtained, and OM or SEM microstructure analysis and EBSD analysis are met.

Description

Technical Field

[0001] This invention relates to the field of alloy sample processing technology, and more specifically, to an electrolytic polishing and corrosion method for Fe-Cr-B alloys and its application. Background Technology

[0002] Fe-Cr-B alloys are obtained by introducing boron (B) into Fe-Cr alloys. They possess excellent wear resistance and corrosion resistance and are widely used in surface strengthening coatings for key components in industries such as metallurgy, petrochemicals, and energy. Their service performance is significantly dependent on the microstructure formed during solidification. Due to the extremely low equilibrium solubility of boron in iron-based solid solutions, most boron atoms combine with Cr, Fe, and other elements to form high-hardness boride phases. Therefore, the microstructure of this alloy consists of a matrix phase of ferrite, austenite, or martensite, combined with phases such as M2B, M7(C,B)3, or M... 23 The boride phases, represented by (B,C)6, together constitute the multiphase microstructure. The morphology, size, distribution of the borides, and their interfacial bonding with the matrix are key structural factors controlling the material's wear resistance, fracture toughness, and corrosion behavior. Therefore, accurate characterization of the microstructure and phase composition identification of Fe-Cr-B alloys are fundamental to materials research and development, performance evaluation, and engineering application optimization.

[0003] However, existing metallographic sample preparation methods for Fe-Cr alloys have limitations in revealing the true microstructure. This needs to be understood from the essential differences between Fe-Cr and Fe-Cr-B alloys: for conventional Fe-Cr alloys with no or low B content, such as heat-resistant steel (Cr content 8.0-9.5 wt.%), martensitic precipitation-strengthened stainless steel (Cr content 15.0-17.5 wt.%), austenitic stainless steel (Cr content 16.0-20.0 wt.%), and duplex stainless steel (Cr content 21.0-25.0 wt.%), the differences in their microstructures are relatively small, and the differences in micro-region electrochemical properties are also small. In this case, although electrolysis with perchloric acid-ethanol solution can obtain a smooth surface, which is usually used for electron backscatter diffraction (EBSD) analysis, the microstructure is difficult to distinguish due to the low degree of selective solubility, making it impossible to perform microscopic morphology analysis using optical microscopy (OM) or scanning electron microscopy (SEM). Therefore, the commonly used microstructure sample preparation process for conventional Fe-Cr alloys is polishing (mechanical or electrolytic polishing) followed by chemical etching. By selecting specific chemical etchants, different phase boundaries or grain boundaries can be selectively etched, thus clearly revealing the microstructure morphology. The OM or SEM microstructure and EBSD analysis sample preparation methods for four common types of Fe-Cr alloys are summarized in the table below. Microstructure analysis requires polishing followed by chemical etching; EBSD analysis requires electrolytic polishing using a perchloric acid-ethanol solution.

[0004] Table 1. Microstructure analysis and EBSD sample preparation methods of conventional Fe-Cr alloys

[0005] Conversely, for Fe-Cr-B alloys with high boron content, the multiphase microstructure contains a Cr-rich boride phase and a Cr-poor matrix phase, with a difference in electrochemical potential between the two. If the conventional method of polishing followed by chemical etching is used, the chemical etching solution often leads to excessive erosion of the matrix. On the surface of the hard and corrosion-resistant boride framework, a layer of corrosion products composed of amorphous oxides / hydroxides will be deposited, severely obscuring the true phase interface morphology and failing to accurately present the microstructure of the boride and the bulk phase.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide an electrolytic polishing and etching method for Fe-Cr-B alloys and its applications. This method is based on the micro-potential difference between the Cr-rich boride phase and the Cr-poor matrix phase. Using a perchloric acid-ethanol solution as the electrolyte, and by controlling parameters such as electrolysis time, voltage, and current, the matrix phase undergoes micro-dissolution, and the boride phase exhibits micro-protrusion, thereby obtaining a surface with a clear interface and microstructure. This surface can meet the requirements of OM or SEM micromorphology analysis and is also suitable for EBSD analysis, accurately reflecting the microstructural characteristics of Fe-Cr-B alloys.

[0008] This invention is implemented as follows: Firstly, an electrolytic polishing and etching method for Fe-Cr-B alloys includes: The cathode and the Fe-Cr-B alloy sample used as the anode were placed in the electrolyte, and an electric current was applied to the cathode and anode to carry out electrolysis at 0~20℃. The main chemical composition of Fe-Cr-B alloy, by mass percentage, includes 15-23% Cr, 0.8-1.3% B, 0-6.5% minor elements, and the balance iron, wherein the minor elements are arbitrary elements; The electrolyte comprises 5-10% perchloric acid and the balance anhydrous ethanol by volume fraction.

[0009] In an optional implementation, the minor element is selected from at least one of C, Si, Ni, Mo, W, Cu, and Nb.

[0010] In an optional embodiment, the electrolysis temperature is 0~5℃, the electrolysis mode is constant current mode, and the current density is 0.2~0.5A / cm². 2 The electrolysis time is 2-3 minutes; Alternatively, the electrolysis temperature is 5~10℃, the electrolysis mode is constant current mode, and the current density is 0.4~0.6A / cm³. 2 The electrolysis time is 1.5~2 minutes; Alternatively, the electrolysis temperature is 10~15℃, the electrolysis mode is constant current mode, and the current density is 0.6~0.9A / cm³. 2 The electrolysis time is 1~1.5 min; Alternatively, the electrolysis temperature is 15~20℃, the electrolysis mode is constant current mode, and the current density is 0.9~1.2A / cm³. 2 The electrolysis time is 30~60s; Alternatively, the electrolysis temperature is 15~20℃, the electrolysis mode is constant voltage mode, the voltage is 20~25V, and the electrolysis time is 30~60s.

[0011] In an optional implementation, the electropolishing and etching processes are carried out in an electrolyte solution that is continuously stirred.

[0012] In an optional implementation, the stirring method is magnetic stirring.

[0013] In an optional implementation, the temperature of the electrolyte is maintained within a set range by a dry ice-ethanol cooling bath.

[0014] In an optional embodiment, the cathode is a stainless steel sheet or a copper alloy sheet.

[0015] In an optional implementation, the process further includes: Before electrolyzing the Fe-Cr-B alloy, the sample surface was first pretreated. The pretreatment methods included: grinding the sample surface step by step with metallographic sandpaper, finally grinding it to 3000 grit; then ultrasonically cleaning it in ethanol to remove residual abrasive and contaminants; and finally drying it to make the sample surface clean, flat and with residual stress basically eliminated.

[0016] Secondly, embodiments of the present invention provide a method for realizing microscopic analysis of Fe-Cr-B alloy samples, including processing the alloy sample by the electrolytic polishing and etching method provided in the embodiments of the present invention, and then performing OM or SEM microscopic morphology analysis on the electrolyzed sample.

[0017] Secondly, embodiments of the present invention provide a method for performing EBSD analysis on Fe-Cr-B alloy samples, comprising treating the alloy sample using the electrolytic polishing and etching method provided in the embodiments of the present invention, and then performing EBSD analysis on the electrolyzed sample.

[0018] The present invention has the following beneficial effects: This invention employs a perchloric acid-anhydrous ethanol solution for electrolytic polishing of Fe-Cr-B alloys. The core mechanism utilizes the potential difference between the Cr-rich boride phase and the Cr-poor matrix phase in the alloy to achieve selective dissolution during anodic electrolysis: the more electrochemically active matrix phase is preferentially dissolved, while the electrochemically stable boride phase is fully preserved and highlighted. This allows for a clear and realistic display of the microstructure morphology of both the matrix and boride phases, suitable for OM or SEM microstructure analysis. Compared to traditional methods that involve polishing followed by chemical etching, this method offers significant advantages: dissolved metal ions are promptly removed during electrolysis through solution stirring, leaving no corrosion products and thus avoiding the microstructure distortion caused by corrosion product masking.

[0019] Furthermore, by precisely controlling parameters such as electrolysis time, voltage, and current, the electrolytic polishing and etching process of this invention can selectively expose the microstructure while maintaining the smoothness of the sample surface and eliminating residual stress, thereby meeting the requirements of EBSD analysis.

[0020] Therefore, the electrolytic polishing and corrosion sample preparation method for Fe-Cr-B alloy samples provided by this invention can not only present a true and clear microstructure under OM or SEM, but also fully meet the requirements of EBSD analysis. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The microstructure of Fe-Cr-B alloy after treatment by two methods: (a) the perchloric acid-ethanol electrolytic polishing and etching method of the present invention; (b) the traditional method of polishing followed by chemical etching. Figures 2 to 4 The images are SEM images of samples 1, 2, and 3 after electrolytic polishing and etching in Example 1, respectively. Figure 5 The EBSD quality diagram of sample 1 after electrolytic polishing and etching; Figures 6 to 11 SEM images of samples from Examples 2 to 7 after electrolytic polishing and etching are shown in sequence. Figure 12 The image shows the SEM image of the sample after electropolishing in Comparative Example 1. Figure 13 The image shows the EBSD quality of the sample in Comparative Example 1 after electropolishing. Figure 14 OM image of the sample from Comparative Example 1 after further chemical etching; Figure 15 The image shows the SEM image of the sample after electropolishing in Comparative Example 2. Figure 16 The image shows the EBSD quality of the sample in Comparative Example 2 after electropolishing. Figure 17 SEM image of the sample from Comparative Example 2 after further chemical etching; Figure 18 The image shows the SEM image of the sample after electropolishing in Comparative Example 3. Figure 19 The image shows the EBSD quality of the sample in Comparative Example 3 after electropolishing. Figure 20 SEM image of the sample from Comparative Example 3 after further chemical etching; Figure 21 The image shows the SEM image of the sample in Comparative Example 4 after electropolishing. Figure 22 The image shows the EBSD quality of the sample in Comparative Example 4 after electropolishing. Figure 23 SEM image of sample 4 after further chemical etching; Figure 24 This is a photograph of the blackened surface of the sample in Comparative Example 5 (caused by improper selection of electrolysis parameters). Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0024] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0025] This invention provides an electrolytic polishing and etching method for Fe-Cr-B alloys, comprising: The cathode and the Fe-Cr-B alloy sample to be electrolyzed, which serves as the anode, are placed in the electrolyte. Electricity is passed through the cathode and anode, and electrolysis is carried out at 0~20℃. The main chemical composition of Fe-Cr-B alloy, by mass percentage, includes 15~23% Cr, 0.8~1.3% B, 0~6.5% minor elements, and the balance iron, with minor elements being arbitrary. The electrolyte comprises 5-10% perchloric acid and the balance anhydrous ethanol by volume fraction.

[0026] This invention uses an electrolyte composed of perchloric acid and anhydrous ethanol for electrolytic polishing and etching. Due to the potential difference between the boride phase and the matrix phase in the Fe-Cr-B alloy, selective dissolution occurs at the anolyte during electrolysis, meaning the matrix phase dissolves preferentially while the boride phase remains due to its high electrochemical stability. This results in a clear interface profile between the matrix and the boride phase, meeting the requirements for microscopic morphology analysis.

[0027] It should be noted that for conventional Fe-Cr alloys with no or low boron content, such as heat-resistant steel (Cr content 8.0-9.5 wt.%), martensitic precipitation-strengthened stainless steel (Cr content 15.0-17.5 wt.%), austenitic stainless steel (Cr content 16.0-20.0 wt.%), and duplex stainless steel (Cr content 21-25 wt.%), the differences in microstructure and micro-area electrochemical differences are relatively small. In this case, although electrolysis with perchloric acid-ethanol solution can obtain a smooth surface, which is commonly used for EBSD analysis, the low selective solubility and low microstructure contrast make it impossible to observe the microstructure using OM and SEM. Therefore, the commonly used microstructure sample preparation process for conventional Fe-Cr alloys is polishing followed by chemical etching. By selecting specific chemical etchants, selective etching of different phase boundaries or grain boundaries can be performed, thus clearly revealing the microstructure morphology. Therefore, for Fe-Cr alloys with no or low B content, two different sample preparation methods are required to achieve microstructure analysis and EBSD analysis.

[0028] In contrast, for Fe-Cr-B alloys with higher boron content, the multiphase microstructure contains a Cr-rich boride phase and a Cr-poor matrix phase, with a difference in electrochemical potential between the two. If the conventional method of polishing followed by chemical etching is used, the chemical etching solution often leads to excessive erosion of the matrix. On the surface of the high-hardness and corrosion-resistant boride framework, a layer of corrosion products composed of amorphous oxides / hydroxides will be deposited, severely obscuring the true microstructure and failing to accurately represent the microscopic characteristics of the boride and matrix phases.

[0029] Therefore, the method provided by this invention offers a simple approach to processing Fe-Cr-B alloy samples, requiring only one step of electrolytic polishing and etching to clearly define the boride phase-matrix boundary profile, thus meeting the requirements for microstructure analysis and EBSD analysis. It should be noted that the specific minor element is irrelevant to the implementation of this invention; it can be any element present in conventional Fe-Cr-B alloys other than iron, chromium, and boron, such as at least one of C, Si, Ni, Mo, W, Cu, and Nb.

[0030] like Figure 1As shown in (a), this is a SEM image of the Fe-Cr-B alloy sample prepared using the electrolytic polishing and etching method of this invention. The microstructure is clear and realistic, mainly composed of boride phase and matrix phase. According to the thermodynamics and kinetics of alloy solidification, the matrix phase (dark contrast area in the image) preferentially precipitates, subsequently forming a eutectic structure of boride and iron-based phases between dendrites. The boride phase (bright white contrast) is granular or needle-like, uniformly distributed within the eutectic structure, while the iron-based phase in the eutectic structure is dark-contrast eutectic ferrite. Further observation reveals that the matrix phase, affected by solid-state phase transformation during subsequent cooling, is divided into an internal matrix and an external matrix by secondary precipitation. Combined with the inventors' previous research, the internal matrix is ​​confirmed to be ferrite, and the external matrix is ​​a mixed structure of ferrite and martensite, with the ratio mainly controlled by the carbon content in the alloy.

[0031] like Figure 1 (b) shows the SEM image of the Fe-Cr-B alloy sample prepared by a method of polishing followed by chemical etching. In contrast, the traditional method of polishing combined with chemical etching results in selective preferential dissolution of the high-Cr boride phase during etching due to the low Cr content and poor corrosion resistance of the matrix phase. While the high-Cr boride phase is preserved, its surface is covered with intriguing corrosion products. This method severely obscures the true morphology of the borides, blurs and distorts the outline of the matrix phase, and makes it impossible to identify the true microstructure, thus limiting the in-depth analysis of the alloy's solidification path, phase composition, and formation mechanism. The method described in this invention fundamentally overcomes this limitation, providing a reliable technical means for accurately characterizing the microstructure of this type of complex alloy.

[0032] It is important to note that perchloric acid is a strong oxidizing inorganic acid, and its chemical activity is extremely sensitive to changes in temperature and concentration, directly determining the safety and effectiveness of the electrolytic polishing process. Therefore, relevant parameters must be strictly controlled within the ranges defined in this invention. Regarding temperature, perchloric acid-ethanol solutions are relatively stable at room temperature, but as the temperature rises, the oxidation reaction accelerates dramatically, significantly increasing thermodynamic instability and posing a risk of combustion or even explosion. If the temperature is below 0°C, the ion migration rate decreases drastically, resulting in excessively low polishing efficiency, excessively long polishing time, and difficulty in obtaining a uniformly bright surface. Regarding concentration, if the perchloric acid concentration is too high, the solution's oxidizing power is too strong, the system stability decreases, making it difficult to achieve uniform and controllable anolyte dissolution during electrolysis. Furthermore, increasing the perchloric acid concentration in the electrolyte greatly increases the risks associated with solution preparation and the electrolysis process. If the concentration is too low, the solution's oxidizing and conductive properties are insufficient, resulting in poor polishing performance. Therefore, the present invention defines the operating conditions for perchloric acid-ethanol electropolishing as follows: temperature 0~20℃ (e.g. 0℃, 5℃, 10℃, 15℃ or 20℃), perchloric acid concentration 5~10vt% (e.g. 5 vt%, 7 vt% or 10 vt%).

[0033] After polishing, the sample should be removed immediately and rinsed with a large amount of analytical grade anhydrous ethanol to thoroughly remove the electrolyte residue. Then, it can be dried with cold air or purged with an inert gas (such as nitrogen) to remove the residue and prevent oxidation by air.

[0034] Specifically, the electrolytic polishing and etching methods are as follows: S1. Provides an electrolytic system for electropolishing and etching. Specifically, the implementation of the electropolishing and etching system includes the following components: Electrolytic cell: A corrosion-resistant container used to hold the electrolyte. The material should be polytetrafluoroethylene (PTFE) or borosilicate glass to resist strong electrolyte corrosion.

[0035] DC Power Supply: A power supply device that provides DC constant current / constant voltage output. It features continuously adjustable voltage (0~30 V) and continuously adjustable current (0~5 A), and supports constant current and constant voltage operating modes as well as switching between the two.

[0036] Anode: The Fe-Cr-B alloy metallographic specimen used as the anode. Before the experiment, the sample needs to be ground to ensure that its surface reaches an initial state suitable for electrolytic polishing and corrosion.

[0037] Cathode: The conductive electrode plate serving as the cathode. Available materials include stainless steel or copper alloy sheets. The choice of these materials requires consideration of both conductivity and chemical stability. Copper alloy sheets are preferred primarily due to their superior conductivity, ensuring uniform current distribution. Furthermore, copper alloys are chemically stable in the electrolyte, resisting dissolution or passivation. Stainless steel sheets are the second choice. Although their conductivity is slightly lower than copper alloys, their advantages lie in their availability, their ability to be used as electrodes, and their lack of reduction in electrolytic polishing and corrosion effects.

[0038] Sample clamp: A device used to securely hold the working electrode (anode). The main body of the clamp is made of stainless steel. Except for the clamping parts that directly contact the sample to conduct current, all other exposed metal parts of the clamp must be tightly covered with a highly insulating and chemically resistant insulating material to prevent the formation of non-target conductive paths or short circuits in the electrolyte. The end of the clamp not holding the sample is connected to a lead wire.

[0039] Conductors: Used to connect the power source, anode, and cathode to form a closed electrolytic current loop. Conductors must have a current carrying capacity that meets the maximum current requirement (≥ 5 A), low resistivity, good flexibility, and an outer insulation layer.

[0040] The electrolyte used consists of 5%–10% (volume fraction) perchloric acid and analytical grade anhydrous ethanol. During preparation, the perchloric acid is added slowly and stirred thoroughly to ensure system stability and avoid localized thermal reactions. Each 400 mL of the prepared electrolyte can be used for the electrolytic treatment of 10–40 stainless steel metallographic samples. The specific number of uses can be flexibly adjusted according to process parameters such as sample area, current density, and electrolysis time.

[0041] To ensure the uniformity of the reaction during electrolysis, a magnetic stirring device can be used to promote solution flow. Specifically, a magnetic rotor is placed at the bottom of the electrolytic cell, and then the cell is placed on a magnetic stirring platform. The rotation of the rotor drives the solution to flow continuously, effectively eliminating air bubbles adhering to the sample surface and preventing the formation of electrolysis dead zones.

[0042] To maintain the electrolysis temperature within the set process range (0~20℃), the system needs to be equipped with a stable and controllable cooling system to ensure that the polishing reaction is in the electrochemical plateau region, ensuring the stability of the electrolysis reaction and avoiding uneven anodic dissolution caused by temperature fluctuations. A preferred method is to use a mixed cooling bath of dry ice and ethanol. Dry ice is added to anhydrous ethanol to prepare the cooling bath, which is then immersed in a beaker to rapidly lower the electrolyte to the set temperature. This method offers rapid temperature control response, strong adaptability, and is suitable for small-batch sample preparation under laboratory conditions.

[0043] S2, Preprocessing The Fe-Cr-B alloy sample to be electrolyzed was subjected to preliminary treatment. The preliminary treatment method can refer to the metallographic sample preparation process, which involves grinding with sandpaper to 3000 mesh, followed by ultrasonic cleaning with ethanol, and finally drying.

[0044] Pretreatment before electrolysis can improve the electropolishing and etching effects.

[0045] S3, Electropolishing and Etching Hold the pretreated sample in the anode fixture; pre-cool the electrolyte to the target temperature range and then inject it into the container; after completing the anode and cathode installation and power connection, turn on the power for continuous electrolysis without interruption.

[0046] Different electrolysis temperatures correspond to different optimal electrolysis parameters. To obtain the best polishing and etching effects, and to achieve better surface smoothness and microstructure of the sample, the specific polishing parameters are as follows: Optionally, the electrolysis temperature is 0~5℃ (0℃, 3℃ or 5℃), the electrolysis mode is constant current mode, and the current density is 0.2~0.5A / cm². 2 (e.g., 0.2 A / cm) 2 0.4 A / cm 2 Or 0.5 A / cm 2The electrolysis time is 2-3 minutes (e.g., 2 minutes, 2.5 minutes, or 3 minutes). Alternatively, the electrolysis temperature is 5~10℃ (e.g., 5℃, 8℃, or 10℃), the electrolysis mode is constant current mode, and the current density is 0.4~0.6A / cm². 2 (e.g., 0.4A / cm) 2 0.5A / cm 2 Or 0.6A / cm 2 The electrolysis time is 1.5 to 2 minutes (e.g., 1.5 minutes, 1.8 minutes, or 2 minutes). Alternatively, the electrolysis temperature is 10~15℃ (e.g., 10℃, 12℃, or 15℃), the electrolysis mode is constant current mode, and the current density is 0.6~0.9A / cm². 2 (e.g., 0.6A / cm) 2 0.8A / cm 2 Or 0.9A / cm 2 The electrolysis time is 1~1.5min (e.g., 1min, 1.2min or 1.5min). Alternatively, the electrolysis temperature can be 15~20℃ (e.g., 15℃, 18℃, or 20℃), the electrolysis mode can be constant current mode, and the current density can be 0.9~1.2A / cm². 2 (e.g., 0.9A / cm) 2 1A / cm 2 Or 1.2A / cm 2 The electrolysis time is 30~60s (e.g., 30s, 40s, 50s or 60s). Alternatively, the electrolysis temperature is 15~20℃ (e.g., 15℃, 18℃ or 20℃), the electrolysis mode is constant voltage mode, the voltage is 20~25V (e.g., 20 V, 22 V or 25 V), and the electrolysis time is 30~60s (e.g., 30 s, 40 s, 50 s or 60 s).

[0047] The system integration of electrolyte composition ratio, current density zone control, electrolysis time dynamic matching, low temperature regulation and solution disturbance mechanism effectively achieves polishing treatment of alloy phase samples with high consistency and good mirror effect, significantly improving the recognizability of microstructure and the reliability of microscopic observation.

[0048] The present invention provides a method for analyzing the microstructure of Fe-Cr-B alloy samples, which includes electrolytic treatment of the alloy sample as provided in the present invention, followed by microstructure analysis of the polished sample.

[0049] Because the Fe-Cr-B alloy sample was treated using the electrolytic polishing and etching method provided in this invention, the sample surface has high flatness and the microstructure outline is clear.

[0050] This invention provides a method for preparing Fe-Cr-B alloy samples for EBSD analysis, which includes treating the alloy sample using the electrolytic polishing and etching method provided in this invention, and then performing EBSD analysis on the polished sample.

[0051] Because the Fe-Cr-B alloy sample was treated using the electrolytic polishing and etching method provided by this invention, the sample surface is smooth and residual stress is eliminated, enabling EBSD analysis.

[0052] Example 1 1. Sample preparation Three Fe-Cr-B alloys (Alloy 1: Fe-17.5Cr-0.8B; Alloy 2: Fe-20.5Cr-0.9B; Alloy 3: Fe-22.5Cr-1.1B, wt.%) were used as bulk samples, with dimensions of 10 mm × 10 mm × 5 mm. The surfaces of the samples to be electrolyzed were mechanically ground to 3000 mesh, then ultrasonically cleaned with ethanol for 5 minutes and dried for later use.

[0053] 2. Electrolyte preparation Analytical grade perchloric acid and anhydrous ethanol were mixed at a volume ratio of 7:93 to prepare 400 mL of electrolyte. The operation was carried out in a fume hood. Under strict control of continuous stirring and low temperature, the perchloric acid was slowly added dropwise to the ethanol. The prepared electrolyte was then poured into a 500 mL beaker.

[0054] 3. Preparation of Electrolysis Equipment First, a 20 mm × 30 mm stainless steel sheet is placed at the bottom of a beaker containing electrolyte as the cathode, and connected to the negative terminal of a power supply via a wire. Next, the beaker containing the electrolyte is immersed in a pre-prepared dry ice-ethanol cooling bath to achieve and maintain the electrolyte temperature at a stable 5 °C. After completing these two steps, the entire apparatus (i.e., the beaker containing electrolyte, with the built-in stainless steel cathode, and placed in the cooling bath) is transferred to a magnetic stirring table. Finally, a magnetic rotor is placed at the center of the bottom of the beaker to provide and maintain adequate stirring of the electrolyte.

[0055] 4. Electrolysis process The alloy sample to be electrolyzed was securely and vertically suspended in the electrolyte solution maintained at 5°C using a stainless steel alloy clamp connected to the positive terminal of the power supply via a wire. Electrolysis was then performed in constant current mode, with a current density of 0.5 A / cm².2 The sample was immersed in the electrolyte, and a constant current was preset. The electrolysis time was 2 minutes, during which the sample could not be removed. Under these conditions, the system voltage was observed to remain stable at approximately 8–12 V. After the set 2-minute polishing time was reached, the sample was immediately removed from the electrolyte. Subsequently, the sample surface was thoroughly rinsed with a large amount of anhydrous ethanol to completely remove all residual electrolyte. After rinsing, the sample was immediately blown with cold air to dry it rapidly, thereby effectively preventing air oxidation of the sample surface during the drying process.

[0056] 5. Evaluation of Electrolysis Performance After electrolysis, the sample surface is mirror-like, without mechanical scratches or corrosion defects. Figure 2-4 As shown, Figure 2-4 The images show SEM images of samples 1-3. The bright contrast areas are borides, and the dark contrast areas are the matrix phase. The multiphase microstructure of the Fe-Cr-B alloy is clearly distinguishable. Figure 5 The image shows the EBSD quality map of sample 1. This image is a grayscale image. The clear grain boundaries in the image indicate that the sample is well prepared and the data is highly reliable. It visually reflects the microstructure of the Kikuchi diffraction pattern clarity at each test point on the sample surface, confirming that this method is suitable for preparing high-quality electrolytic surfaces and can be used for EBSD analysis.

[0057] Example 2 This embodiment is basically the same as Embodiment 1, except that: the same alloy sample as Sample 1 is used for electrolysis, the electrolysis temperature is 0℃, the constant current mode is used, and the current density is 0.2 A / cm². 2 The time is 3 minutes.

[0058] SEM images of the electrolyzed samples are shown below. Figure 6 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0059] Example 3 This embodiment is basically the same as Embodiment 1, except that: the same alloy sample as Sample 1 is used for electrolysis, the electrolysis temperature is 10℃, the constant current mode is used, and the current density is 0.6 A / cm². 2 The time is 1.5 minutes.

[0060] SEM images of the electrolyzed samples are shown below. Figure 7 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0061] Example 4 This embodiment is basically the same as Embodiment 1, except that: the same alloy sample as Sample 1 is used for electrolysis, the electrolysis temperature is 15℃, the constant current mode is used, and the current density is 0.8 A / cm². 2The time is 1 minute.

[0062] SEM images of the electrolyzed samples are shown below. Figure 8 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0063] Example 5 This embodiment is basically the same as Embodiment 1, except that: the same alloy sample as Sample 1 is used for electrolysis, the electrolysis temperature is 20℃, the constant current mode is used, and the current density is 1.2 A / cm². 2 The time is 30 seconds.

[0064] SEM images of the electrolyzed samples are shown below. Figure 9 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0065] Example 6 This embodiment is basically the same as embodiment 1, except that: the same alloy sample as sample 1 is used for electrolysis, the electrolysis temperature is 20℃, the constant voltage mode is used, the voltage is 25V, and the time is 30s.

[0066] SEM images of the electrolyzed samples are shown below. Figure 10 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0067] Example 7 This embodiment is basically the same as embodiment 1, except that: the same alloy sample as sample 1 is used for electrolysis, the electrolysis temperature is 20℃, the constant voltage mode is used, the voltage is 20V, and the time is 60s.

[0068] SEM images of the electrolyzed samples are shown below. Figure 11 As shown, the microstructure of the borides and the matrix phase is clearly distinguishable.

[0069] Comparative Example 1 This comparative example is basically the same as Example 1, except that: Sample 1 in the example is replaced with a heat-resistant steel (Cr content 8.0-9.5 wt.%) sample (sample 4).

[0070] After electrolytic polishing with perchloric acid-ethanol solution, the SEM morphology of sample 4 is as follows: Figure 12 As shown, its microstructure has low contrast and its morphology is difficult to distinguish; the EBSD quality map of sample 4 is as follows. Figure 13 As shown in the figure, clear grain boundaries indicate good sample preparation and high data reliability. The figure visually reflects the microstructure of the Kikuchi diffraction pattern clarity at each test point on the sample surface, confirming that the heat-resistant steel sample can be used for EBSD analysis after electrolytic polishing with perchloric acid-ethanol solution.

[0071] The sample 4, after electrolytic polishing, was then subjected to chemical etching using a ferric chloride hydrochloride solution. The OM image after chemical etching is shown below. Figure 14 As shown, it exhibits typical lath martensite characteristics.

[0072] This indicates that heat-resistant steel (Cr content 8.0-9.5 wt.%) can only be used for EBSD analysis if it undergoes one-step electrolytic polishing with perchloric acid-ethanol solution. It cannot clearly display the microstructure. If microscopic morphology analysis is required, chemical etching is still necessary.

[0073] Comparative Example 2 This comparative example is basically the same as Example 1, except that: Sample 1 in the example is replaced with martensitic precipitation-strengthened stainless steel (Cr content of about 15.0-17.5 wt.%) (Sample 5).

[0074] After electrolytic polishing with perchloric acid-ethanol solution, the SEM morphology of the sample surface is as follows: Figure 15 As shown, the microstructure of the martensitic deposition strengthened stainless steel has low contrast and its morphology is difficult to distinguish; the EBSD quality map of sample 5 is as follows. Figure 16 As shown in the figure, clear grain boundaries indicate good sample preparation and high data reliability. The figure visually reflects the microstructure of the Kikuchi diffraction pattern clarity at each test point on the sample surface, confirming that the martensitic precipitation-strengthened stainless steel sample can be used for EBSD analysis after electrolytic polishing with perchloric acid-ethanol solution.

[0075] The sample 5, after electrolytic polishing, was then subjected to chemical etching using a ferric chloride hydrochloride solution. The SEM image after chemical etching is shown below. Figure 17 As shown, it exhibits typical lath martensite characteristics.

[0076] This indicates that if martensitic precipitation-strengthened stainless steel (Cr content approximately 15.0-17.5 wt.%) is subjected to one-step electrolytic polishing with perchloric acid-ethanol solution, it can only be used for EBSD analysis and cannot clearly display the microstructure. If microstructure analysis is required, chemical etching is still necessary.

[0077] Comparative Example 3 This comparative example is basically the same as Example 1, except that: Sample 1 in the example is replaced with an austenitic stainless steel alloy sample (sample 6) with a Cr content of about 16.0-20.0 wt.%.

[0078] After electrolytic polishing with perchloric acid-ethanol solution, the SEM morphology of the sample surface is as follows: Figure 18 As shown, the microstructure contrast of austenitic stainless steel is low, and the austenitic grain boundaries are difficult to distinguish clearly. The EBSD mass plot of sample 6 is shown below. Figure 19 As shown in the figure, clear grain boundaries indicate good sample preparation and high data reliability. The figure visually reflects the microstructure of the Kikuchi diffraction pattern clarity at each test point on the sample surface, confirming that austenitic stainless steel samples can be used for EBSD analysis after electrolytic polishing with perchloric acid-ethanol solution.

[0079] The sample 6, after electrolytic polishing, underwent further chemical etching treatment, specifically aqua regia etching. The SEM image after chemical etching is shown below. Figure 20 As shown, it exhibits a typical irregular austenite grain morphology.

[0080] This indicates that austenitic stainless steel (Cr content approximately 16.0-20.0 wt.%) can only be used for EBSD analysis if it undergoes one-step electrolytic polishing with perchloric acid-ethanol solution. It cannot clearly display the microstructure. If micromorphological analysis is required, chemical etching is still necessary.

[0081] Comparative Example 4 This comparative example is basically the same as Example 1, except that: Sample 1 in the example is replaced with duplex stainless steel (Cr content about 21.0-25.0 wt.%) (Sample 7).

[0082] After electrolytic polishing with perchloric acid-ethanol solution, the SEM morphology of the sample surface is as follows: Figure 21 As shown, the microstructure contrast of duplex stainless steel is low, and the morphologies of the two phases are difficult to distinguish clearly; the EBSD mass profile of sample 7 is shown below. Figure 22 As shown in the figure, clear grain boundaries indicate good sample preparation and high data reliability. The figure visually reflects the microstructure of the Kikuchi diffraction pattern clarity at each test point on the sample surface, confirming that duplex stainless steel samples can be used for EBSD analysis after electrolytic polishing with perchloric acid-ethanol solution.

[0083] The sample 7, after electrolytic polishing, was then subjected to chemical etching, specifically aqua regia etching. The SEM image after chemical etching is shown below. Figure 23 As shown, the morphology, distribution, and phase interface of the ferrite and austenite phases are clearly visible at this time.

[0084] This indicates that austenitic stainless steel (Cr content approximately 21.0-25.0 wt.%) can only be used for EBSD analysis if it undergoes one-step electrolytic polishing with perchloric acid-ethanol solution. It cannot clearly display the microstructure. If micromorphological analysis is required, chemical etching is still necessary.

[0085] This demonstrates that Fe-Cr alloys with no or low B content cannot have their microstructure clearly revealed if only one electrolytic treatment is performed.

[0086] Comparative Example 5 This comparative example is basically the same as Example 1, except that the electrolysis temperature is increased to 25°C.

[0087] Macroscopic photographs of the sample surface after electrolysis are shown below. Figure 24 As shown, a black oxide film covers a localized area on the surface. This is a typical characteristic of the electrolysis process running out of control due to increased temperature. Fe and Cr in the matrix rapidly dissolve, releasing a large number of metal ions, which become supersaturated in the surface liquid layer. Perchloric acid in the electrolyte has strong oxidizing properties, and these ions are rapidly hydrolyzed, oxidized, and deposited, forming a dense black film that leads to electrolysis failure and hinders subsequent analysis.

[0088] This indicates that the electrolysis temperature should not exceed the range specified in this invention.

[0089] In summary, this invention uses an electrolytic polishing and etching solution composed of perchloric acid and anhydrous ethanol for electrolytic polishing and etching. Due to the potential difference between the boride phase and the matrix phase in the Fe-Cr-B alloy, selective dissolution occurs at the anolyte during electrolytic polishing, i.e., the matrix phase dissolves preferentially, while the boride phase is retained during anodic electrolysis due to its high electrochemical stability. This results in a clear matrix / boride phase interface profile, meeting the requirements for micro-area composition and structure characterization.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for electrolytic polishing and etching of Fe-Cr-B alloy, characterized in that, include: The cathode and the Fe-Cr-B alloy sample, which serves as the anode, are placed in an electrolyte, and an electric current is passed through the cathode and the anode to perform electrolysis at 0~20℃. The Fe-Cr-B alloy has a chemical composition of 15-23% Cr, 0.8-1.3% B, 0-6.5% minor elements and the balance iron by mass percentage, wherein the minor elements are arbitrary elements; The electrolyte comprises 5-10% perchloric acid and the balance being anhydrous ethanol by volume fraction.

2. The electrolytic polishing and etching method according to claim 1, characterized in that, The electrolysis temperature is 0~5℃, the electrolysis mode is constant current mode, and the current density is 0.2~0.5A / cm². 2 The electrolysis time is 2-3 minutes; Alternatively, the electrolysis temperature is 5~10℃, the electrolysis mode is constant current mode, and the current density is 0.4~0.6A / cm³. 2 The electrolysis time is 1.5~2 minutes; Alternatively, the electrolysis temperature is 10~15℃, the electrolysis mode is constant current mode, and the current density is 0.6~0.9A / cm³. 2 The electrolysis time is 1~1.5 min; Alternatively, the electrolysis temperature is 15~20℃, the electrolysis mode is constant current mode, and the current density is 0.9~1.2A / cm³. 2 The electrolysis time is 30~60s; Alternatively, the electrolysis temperature is 15~20℃, the electrolysis mode is constant voltage mode, the voltage is 20~25V, and the electrolysis time is 30~60s.

3. The electrolytic polishing and etching method according to claim 1, characterized in that, The electropolishing and etching processes are carried out while the electrolyte is continuously stirred.

4. The electrolytic polishing and etching method according to claim 3, characterized in that, The stirring method is magnetic stirring.

5. The electrolytic polishing and etching method according to claim 1, characterized in that, The temperature of the electrolyte is maintained within its set range by a dry ice-ethanol cooling bath.

6. The electrolytic polishing and etching method according to claim 1, characterized in that, The cathode is a stainless steel sheet or a copper alloy sheet.

7. The electrolytic polishing and etching method according to claim 1, characterized in that, Before electrolytic polishing and etching, the following steps are also included: Before electrolyzing the Fe-Cr-B alloy, the sample surface was first pretreated. The pretreatment methods included: grinding the sample surface step by step with metallographic sandpaper, finally grinding it to 3000 grit; then ultrasonically cleaning it in ethanol to remove residual abrasive and contaminants; and finally drying it to make the obtained sample surface clean, flat and free of residual stress.

8. The electrolytic polishing and etching method according to claim 1, characterized in that, The minor element is selected from at least one of C, Si, Ni, Mo, W, Cu, and Nb.

9. A method for achieving microscopic analysis of Fe-Cr-B alloy samples, characterized in that, This includes treating the alloy sample using the electrolytic polishing and etching method described in any one of claims 1 to 7, and then performing OM or SEM micromorphology analysis on the electrolyzed sample.

10. A method for EBSD analysis of Fe-Cr-B alloy samples, characterized in that, This includes treating the alloy sample using the electrolytic polishing and etching method as described in any one of claims 1 to 7, followed by EBSD analysis of the electrolyzed sample.

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