Stainless steel electrolytic polishing solution suitable for EBSD sample preparation and operation conditions of stainless steel electrolytic polishing solution

Abstract

The invention relates to the technical field of stainless steel surface treatment. The invention relates to a stainless steel electrolytic polishing solution suitable for EBSD sample preparation and operating conditions thereof, and the polishing solution comprises the following components by volume percent: (1) 8-15% of perchloric acid (HClO4); (2) 60%-70% of ethanol (C2H5OH); (3) 15%-25% of ethylene glycol monobutyl ether (C6H14O2); (4) 2% to 4% of sodium citrate (C6H5Na3O7); and (5) 1%-3% of distilled water (H2O). According to the invention, ethylene glycol monobutyl ether and a sodium citrate reagent are creatively adopted to jointly depassivate, in the electrolytic polishing sample preparation process of the EBSD test, the use effect is good, a deformation disturbance layer caused by mechanical polishing can be effectively removed, the surface is bright and flat, the defects of corrosion pits and the like are avoided, and the EBSD test requirements of stainless steel are completely met.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel surface treatment, and in particular to a stainless steel electrolytic polishing solution suitable for EBSD sample preparation and its operating conditions. Background Art

[0002] EBSD, or electron backscatter diffraction, is an advanced material microanalysis technique based on scanning electron microscopy (SEM). EBSD has become an indispensable tool in materials science research and is widely used to reveal microscopic features such as the crystal structure and grain orientation of materials.

[0003] Sample preparation techniques used for EBSD include mechanical polishing, electrolytic polishing, vibration polishing, argon ion polishing, and focused ion beam (FIB) cutting. For EBSD sample preparation of stainless steel, electrolytic polishing is an economical and practical method that can effectively eliminate the deformation layer produced by mechanical polishing. The sample preparation effect is directly related to the formulation of the electrolytic polishing solution.

[0004] The composition of electrolytic polishing fluid is complex, and different formulations are suitable for different materials. Even for the same material, the formulation may be different when the polishing purpose is different. Therefore, there are many electrolyte formulations. Generally, the polishing electrolyte should contain three components: (1) an oxidant, which helps the anodic dissolution process and forms an oxide film on the metal surface, such as HNO3, HClO4, CrO3, H3PO4, etc.; (2) a depassivating agent, which prevents the surface film from growing too thick or too stable, so as to maintain a certain limiting current density to facilitate normal polishing, such as glycerol, butoxyethanol, etc.; (3) a solvent to facilitate the formation of a diffusion layer during the polishing process. Any component that can reduce the concentration of water molecules in the solution (such as acetic acid, acetic anhydride, ethanol, etc.) can help form a diffusion layer.

[0005] Perchloric acid (HClO4) is used as an oxidant to form a complex with metal ions. Perchlorates are soluble in organic solvents, so HClO4 is a typical high-resistance viscous polishing liquid. However, as the use of HClO4 electrolyte increases, ClO4 - Electrodecomposition releases Cl - Perchloric acid releases a lot of heat when diluted with water or solvents, so be careful when preparing it.

[0006] Phosphoric acid (H3PO4) also acts as an oxidant and is a key component in ensuring the proper polishing of the polishing solution. Excessive phosphoric acid content increases electrolyte resistance and viscosity, leading to higher voltage requirements and slower leveling. Excessive phosphoric acid content increases activation tendency and decreases passivation tendency, resulting in uneven corrosion of the stainless steel surface.

[0007] Ethanol (C2H5OH) is used as a solvent to reduce the concentration of the solution.

[0008] The effects of water are complex, primarily affecting polishing quality by forming a solid film on the surface and adjusting solution viscosity. While an appropriate amount of water can improve polishing quality, increasing the amount of water beyond a certain limit can reduce the gloss of the metal surface, and further increases can even cause wetting. Water can alter solution viscosity, affecting the diffusion rate of metal ions. Complex dissociation increases with increasing water content, hindering polishing. Therefore, the amount of water in the polishing solution must be appropriate.

[0009] Glycerol is a large, high-viscosity component. As a depassivating agent, it adsorbs onto the anode surface, forming complexes with oxidants and metal derivatives. This creates a strong barrier film on the anode surface, hindering dissolution and resulting in a bright, polished surface. Glycerol also prevents chemical corrosion of stainless steel in the electrolyte. Citric acid is also a large organic molecule, also used to form large complexes.

[0010] Sulfuric acid (H2SO4) is an activator. If its content is too high, the polished surface will be corroded and present uniform dense pitting. If its content is too low, uneven corrosion will occur.

[0011] Conventional stainless steel electrolytic polishing solutions (such as perchloric acid-ethanol and perchloric acid-methanol systems) have the following defects: (1) poor polishing uniformity, prone to pitting or oxide film residue (affecting EBSD signal acquisition); (2) high solvent volatility during low-temperature operation (methanol is flammable, and the increased viscosity of ethanol leads to decreased fluidity); (3) local enrichment of metal ions causes surface passivation, and the service life of the electrolyte is short; (4) although the addition of glycerol can improve surface finish, it will increase the viscosity of the electrolyte and reduce polishing efficiency.

[0012] The purpose of the present invention is to find an economical, efficient and stable stainless steel electrolytic polishing solution formula, which can be used in the electrolytic polishing sample preparation process of EBSD test. Summary of the Invention

[0013] The purpose of the present invention is to address the above problems and provide a stainless steel electrolytic polishing solution suitable for EBSD sample preparation and its operating conditions.

[0014] The object of the present invention is achieved as follows: a stainless steel electrolytic polishing solution suitable for EBSD sample preparation comprises: perchloric acid: 8%-15%, ethanol: 60%-70%, ethylene glycol monobutyl ether: 15%-25%, sodium citrate: 2%-4%, and distilled water: 1%-3%.

[0015] The invention discloses operating conditions of a stainless steel electrolytic polishing solution suitable for EBSD sample preparation. The operating conditions, i.e., electrolytic polishing parameters, are as follows: temperature: -15°C to 5°C, voltage: 20V to 35V, and time: 10 seconds to 60 seconds.

[0016] The beneficial effects of the present invention are as follows: the stainless steel electrolytic polishing liquid provided by the present invention has an economical and efficient formula, and has a good use effect in the electrolytic polishing sample preparation process of EBSD testing. It can effectively remove the deformation disturbance layer caused by mechanical polishing, and the surface is bright and smooth without defects such as corrosion pits, fully meeting the EBSD testing requirements of stainless steel.

[0017] Compared with traditional stainless steel electrolytic polishing solutions (such as perchloric acid-ethanol and perchloric acid-methanol systems), this solution has significant advantages: (1) Surface quality is improved, pitting and oxide film are avoided or eliminated, and the EBSD calibration rate can be further improved; it is more suitable for difficult-to-polish materials such as high-alloy steel and duplex stainless steel. (2) The electrolyte has high stability and a long service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below in conjunction with the accompanying drawings.

[0019] Figure 1 is a scanning electron microscope secondary electron image.

[0020] Figure 2 is a scanning electron microscope backscattered electron image.

[0021] Figure 3 This is the Kikuchi diffraction pattern displayed during the EBSD test process.

[0022] Figure 4 is a scanning electron microscope secondary electron image.

[0023] Figure 5 is a scanning electron microscope backscattered electron image.

[0024] Figure 6 This is the Kikuchi diffraction pattern of the ferrite phase shown during the EBSD test process.

[0025] Figure 7 This is the Kikuchi diffraction pattern of the martensite phase shown during the EBSD test process. DETAILED DESCRIPTION

[0026] The present invention provides a novel stainless steel electrolytic polishing solution, which realizes efficient and high-quality EBSD sample preparation by optimizing the solvent system and functional additives.

[0027] 1. Electrolytic polishing liquid formula: see Table 1 for the specific formula.

[0028]

[0029] This formula adopts a solution system with perchloric acid as the oxidant and ethanol as the solvent, and innovatively uses ethylene glycol monobutyl ether and sodium citrate reagents for combined depassivation, which has a very good effect.

[0030] The characteristics of this formula are: (1) perchloric acid provides strong oxidizing properties, which can quickly remove the surface material layer; (2) ethanol as a solvent reduces the viscosity of the electrolyte and improves the polishing uniformity; (3) ethylene glycol monobutyl ether can improve the fluidity of the electrolyte and reduce surface defects. A small amount of sodium citrate is used to improve the surface finish of the sample.

[0031] The ethylene glycol monobutyl ether reagent used in this formula is a colorless liquid with a medium ether smell. It plays the following main roles in the electrolytic polishing solution: (1) Depassivation: It complexes Fe³⁺ / Cr³⁺ ions, inhibits the formation of passivation film, and can prevent the surface film from growing too thick or too stable, so as to maintain a certain current density and facilitate normal polishing. (2) Solvent function: It can dissolve other components in the electrolytic polishing solution, such as acids and salts, to ensure the uniformity of the polishing solution. It cooperates with ethanol to dissolve perchloric acid oxidation products (such as CrO3) and prevent the adhesion of by-products. (3) Coupling function: It can act as a coupling agent between the water phase and the oil phase, so that the different components in the polishing solution are evenly mixed to form a transparent and stable solution. (4) Reduce volatility: Due to the high boiling point of ethylene glycol monobutyl ether (171℃), it can reduce the overall volatility of the polishing solution and extend the service life of the polishing solution. (5) Improved polishing effect: During the electrolytic polishing process, ethylene glycol monobutyl ether helps to improve the conductivity and wettability of the polishing liquid, thereby improving the polishing effect and making the stainless steel surface brighter. (6) The low freezing point of ethylene glycol monobutyl ether (-70℃) expands the applicable temperature range of the electrolyte, making it particularly suitable for low-temperature polishing of austenitic stainless steel (such as 316L).

[0032] As a macromolecular organic compound, sodium citrate has a certain chelating ability, which can stabilize the chemical composition of the polishing liquid. It also has a corrosion inhibition effect, which can prevent the chemical corrosion of stainless steel in the electrolyte.

[0033] 2. Electropolishing parameters (operating conditions): (1) Temperature: -15°C to 5°C (use an ice water bath, liquid nitrogen cooling, or a dedicated cooling device to control the temperature). (2) Voltage: 20V to 35V (DC power supply). (3) Time: 10 seconds to 60 seconds (adjust according to the surface condition of the sample).

[0034] 3. Electrolytic polishing operation steps for stainless steel samples: (1) Sample preparation: Mechanically polish the stainless steel sample until the surface is free of scratches. (2) Electrolyte preparation: Mix perchloric acid, ethanol, ethylene glycol monobutyl ether, sodium citrate and water in proportion, stir evenly and cool to room temperature. (3) Electrolytic polishing: Use the stainless steel sample as the anode and the stainless steel or platinum electrode as the cathode, immerse them in the electrolyte, turn on the power, and adjust the voltage and time. Using a professional electrolytic polishing instrument will provide better parameter control. (4) Cleaning: After the electrolytic polishing is completed, immediately clean the sample surface with ethanol or deionized water to remove residual electrolyte. (5) Drying: Use compressed air or a hair dryer to dry the sample to avoid surface oxidation.

[0035] 4. Precautions: (1) Safety: Since perchloric acid is highly oxidizing and corrosive, protective equipment must be worn during operation and the process must be performed in a fume hood. (2) Temperature control: Low temperatures help reduce surface corrosion and pitting, improving polishing quality. (3) Time control: Polishing for too long may result in excessive corrosion and the formation of corrosion pits. Example 1

[0036] Electrolytic polishing of 310S austenitic heat-resistant stainless steel hot-rolled annealed plate.

[0037] 310S stainless steel is a high-alloy austenitic stainless steel belonging to the 25Cr-20Ni series with excellent high-temperature oxidation resistance.

[0038] The electrolytic polishing liquid of the present invention was used to electrolytically polish 310S stainless steel. The specific process was as follows: 1. A 310S stainless steel hot-rolled annealed sheet was cut into a specimen measuring 15 mm in length, 10 mm in width, and 8 mm in thickness. A longitudinal section was mechanically metallographically polished to Ra ≤ 0.1 μm.

[0039] 2. Prepare the electrolyte. Electrolyte formula: 10% perchloric acid + 65% ethanol + 20% ethylene glycol monobutyl ether + 3% sodium citrate + 2% distilled water. Mix according to the appropriate proportions, stir thoroughly, and cool to room temperature.

[0040] 3. Use ElectroMet produced by Buehler Company of the United States ® 4. Electrolytic polishing was performed using an electrolytic polishing machine. Main parameter settings: (1) voltage: 32 V; (2) temperature: -10°C; (3) stirring speed: 60%; (4) time: 20 seconds.

[0041] 4. After electropolishing is completed, immediately clean the sample surface with anhydrous ethanol and then dry it with a hair dryer.

[0042] 5. Evaluate the effect of electrolytic polishing by scanning electron microscope image observation and EBSD technical test. Figure 1 This is a secondary electron image of a scanning electron microscope. It can be seen that the surface is smooth and bright, without defects such as corrosion pits. Figure 2 This is a backscattered electron image of a scanning electron microscope. It can be seen that the crystal orientation contrast is obvious, indicating that the polishing effect is very good. Figure 3 This is the Kikuchi diffraction pattern displayed during EBSD testing. The clear pattern indicates a good polishing effect. The Kikuchi pattern calibration rate during EBSD testing is ≥97% (tested using the Oxford Instruments AZtec system). Example 2

[0043] Electrolytic polishing of hot-rolled annealed stainless steel TGR-H (0.025C-0.4Si-12Cr-0.2Mn) sheets for highway guardrails.

[0044] The stainless steel TGR-H (0.025C-0.4Si-12Cr-0.2Mn) used for highway guardrails is a martensitic transformation strengthened stainless steel. Through hot rolling and annealing treatment, a two-phase structure of martensite and ferrite can be obtained.

[0045] The electrolytic polishing liquid of the present invention was used to electrolytically polish TGR-H stainless steel. The specific process was as follows: 1. A hot-rolled annealed TGR-H stainless steel sheet was cut into a specimen measuring 15 mm in length, 10 mm in width, and 4 mm in thickness. A longitudinal section was mechanically metallographically polished to Ra ≤ 0.1 μm.

[0046] 2. Prepare the electrolyte. Electrolyte formula: 10% perchloric acid + 65% ethanol + 20% ethylene glycol monobutyl ether + 3% sodium citrate + 2% distilled water. Mix according to the appropriate proportions, stir thoroughly, and cool to room temperature.

[0047] 3. Use ElectroMet produced by Buehler Company of the United States ® 4. Electrolytic polishing was performed using an electrolytic polishing machine. Main parameter settings were: (1) voltage: 30 V; (2) temperature: -8°C; (3) stirring speed: 60%; (4) time: 15 seconds.

[0048] 4. After electropolishing is completed, immediately clean the sample surface with anhydrous ethanol and then dry it with a hair dryer.

[0049] 5. Evaluate the effect of electrolytic polishing by scanning electron microscope image observation and EBSD technical test. Figure 4 This is a secondary electron image of a scanning electron microscope. It can be seen that the surface is smooth and bright, without defects such as corrosion pits. Figure 5 This is a backscattered electron image of a scanning electron microscope. The larger grains are ferrite phase, and the smaller grains are martensite phase. It can be seen that the crystal orientation contrast is obvious, indicating that the polishing effect is very good. Figure 6 and 7 This is the Kikuchi diffraction pattern of ferrite and martensite phases revealed during EBSD testing. The clear patterns indicate excellent polishing. The Kikuchi pattern calibration rate during EBSD testing is ≥95% (tested using an Oxford Instruments AZtec system).

[0050] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by any technician in this field within the scope of the present invention are included in the patent scope of the present invention.

Claims

1. A stainless steel electrolytic polishing solution suitable for EBSD sample preparation, characterized by: include: Perchloric acid: 8%-15%, ethanol: 60%-70%, ethylene glycol monobutyl ether: 15%-25%, sodium citrate: 2%-4%, distilled water: 1%-3%.

2. An operating condition for a stainless steel electrolytic polishing solution suitable for EBSD sample preparation, characterized by: The operating conditions, i.e., electrolytic polishing parameters, are: temperature: -15°C to 5°C, voltage: 20V to 35V, and time: 10 seconds to 60 seconds.

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