Electrolytic polishing method for duplex stainless steel

Abstract

To provide an electropolishing method capable of smoothing the surface of two phase stainless steel.SOLUTION: An electropolishing method for a duplex stainless steel including a ferrite phase and an austenite phase includes a preparation step of immersing an object to be polished, which is the duplex stainless steel, and a counter electrode in an electrolytic solution, a high-potential-side electropolishing step of applying, to the object to be polished, a first potential at which the ferrite phase dissolves more preferentially than the austenite phase, and a low-potential-side electropolishing step of applying, to the object to be polished, a second potential that is lower than the first potential and at which the austenite phase dissolves more preferentially than the ferrite phase.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for electropolishing duplex stainless steel. [Background technology]

[0002] For example, as disclosed in Patent Document 1, stainless steel containers whose inner surfaces are electrolytically polished to a mirror finish are known. A mirror-finished inner surface has the advantage of being smooth and less susceptible to the adhesion of particles such as foreign matter. Austenitic stainless steel is widely used as the material for stainless steel containers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-233254 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to improve the strength and corrosion resistance of stainless steel containers and reduce costs, it has been considered to use duplex stainless steel, which contains a ferrite phase and an austenite phase, instead of austenitic stainless steel. However, the applicant has confirmed that when duplex stainless steel is electropolished in the same manner as austenitic stainless steel, a haze remains on the surface and the surface irregularities are greater than those of austenitic stainless steel.

[0005] In view of the above background, an object of the present invention is to provide an electrolytic polishing method capable of smoothing the surface of duplex stainless steel. [Means for solving the problem]

[0006] In order to solve the above problems, one aspect of the present invention is a method for electropolishing duplex stainless steel containing a ferrite phase and an austenite phase, comprising: a preparation step of immersing a workpiece, which is the duplex stainless steel, and a counter electrode in an electrolyte; a high-potential-side electropolishing step of applying a first potential to the workpiece, which dissolves the ferrite phase preferentially over the austenite phase; and a low-potential-side electropolishing step of applying a second potential to the workpiece, which is lower than the first potential and dissolves the austenite phase preferentially over the ferrite phase.

[0007] This embodiment provides an electropolishing method capable of smoothing the surface of duplex stainless steel. The ferrite and austenite phases constituting duplex stainless steel have different preferential dissolution potentials. Therefore, when electropolishing is performed at a single potential, one of the ferrite and austenite phases dissolves preferentially over the other. The ferrite and austenite phases are dispersed and mixed as fine crystal grains on the surface of the duplex stainless steel, and the preferential dissolution of one of the ferrite and austenite phases results in the formation of irregularities on the surface of the duplex stainless steel. The high-potential-side electropolishing step applies a first potential to the workpiece at which the ferrite phase dissolves preferentially over the austenite phase, and the low-potential-side electropolishing step applies a second potential to the workpiece that is lower than the first potential and at which the austenite phase dissolves preferentially over the ferrite phase, thereby reducing the irregularities on the surface of the duplex stainless steel and smoothing it.

[0008] In the above embodiment, the second potential may be 0.17 to 0.30 V (vs. SHE).

[0009] According to this embodiment, the austenite phase can be dissolved preferentially over the ferrite phase in the low potential side electropolishing step.

[0010] In the above aspect, the first potential may be 2.5 to 10 V (vs. SHE).

[0011] According to this embodiment, the ferrite phase can be dissolved preferentially over the austenite phase in the high-potential-side electrolytic polishing step.

[0012] In the above aspect, in the high-potential-side electropolishing step, the first potential may be controlled within a range of 2.5 to 10 V (vs. SHE) so that the current flowing between the object to be polished and the counter electrode becomes a target current value.

[0013] According to this embodiment, it becomes easier to control the amount of ferrite phase polished in the high-potential-side electrolytic polishing step.

[0014] In the above aspect, the electrolytic solution may be a solution containing phosphoric acid and sulfuric acid.

[0015] According to this embodiment, the ferrite phase and the austenite phase can be efficiently electropolished.

[0016] In the above embodiment, the pH of the electrolytic solution may be 0 or less.

[0017] According to this embodiment, in the low-potential-side electrolytic polishing step, the austenite phase can be electrolytically polished preferentially over the ferrite phase.

[0018] In the above aspect, the high-potential-side electrolytic polishing step may be performed after the low-potential-side electrolytic polishing step.

[0019] According to this embodiment, the surface of the duplex stainless steel can be efficiently smoothed. When the high-potential-side electropolishing step is performed, a passive film is formed on the surface of the duplex stainless steel. Therefore, when the low-potential-side electropolishing step is performed after the high-potential-side electropolishing step, the austenite phase is less likely to dissolve.

[0020] In the above aspect, the proportion of the ferrite phase relative to the entire duplex stainless steel may be 30 wt % or more and 70 wt % or less.

[0021] According to this embodiment, duplex stainless steels of various compositions can be electropolished. [Effects of the Invention]

[0022] According to the above aspect, it is possible to provide an electrolytic polishing method that can smooth the surface of duplex stainless steel. [Brief explanation of the drawings]

[0023] [Figure 1] An explanatory diagram showing the configuration of an electrolytic polishing apparatus [Figure 2] Diagram showing the polarization curve of duplex stainless steel [Figure 3] (A) Photographs of the main surface of the workpiece before electrolytic polishing, (B) after the first stage of electrolytic polishing, and (C) after the second stage of electrolytic polishing. [Figure 4] 1A and 1B are scanning electron microscope photographs of a polished object after a first stage of electrolytic polishing, showing (A) the boundary between the first cut surface and the main surface, and (B) the boundary between the first cut surface and the second cut surface. [Figure 5] Graph showing the natural electrode potential of each substance [Figure 6] Graph showing the relationship between potential and current density in the low-potential side electrolytic polishing process [Figure 7] Graph showing the relationship between the pH of the electrolyte and the first active dissolution current peak in the low potential side electrolytic polishing process [Figure 8] (A) Photographs of the main surface of SUS329J1 before electrolytic polishing, (B) after the first stage of electrolytic polishing, and (C) after the second stage of electrolytic polishing. [Figure 9] (A) Photographs of the main surface of SUS329J3L before electrolytic polishing, (B) after the first stage of electrolytic polishing, and (C) after the second stage of electrolytic polishing. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, an embodiment of the electrolytic polishing method for duplex stainless steel according to the present invention will be described. In the following description, when a numerical value is written as 1 to 10, it means that the value is 1 or more and 10 or less.

[0025] Duplex stainless steel contains a ferrite phase (α phase) and an austenite phase (γ phase). The proportion of the ferrite phase in the entire duplex stainless steel is preferably 30 wt% or more and 70 wt% or less. The remainder is preferably austenite phase. The ferrite phase and austenite phase are dispersed and mixed in the structure as fine crystal grains. The average crystal grain size of the ferrite phase and austenite phase is 5 to 20 μm. When the duplex stainless steel is a rolled steel plate, the crystal grains of the ferrite phase and austenite phase are elongated in the rolling direction.

[0026] Duplex stainless steel has a composition, in mass%, of 0.080% or less C, 1.00% or less Si, 4.0% or less Mn, 0.04% or less P, 0.03% or less S, 1.0 to 9.0% Ni, 18 to 33% Cr, 5.0% or less Mo, 3.0% or less Cu, and 0.6% or less N, with the balance being Fe and small amounts of unavoidable impurities (unavoidable impurities). Duplex stainless steel may also contain W, etc. Duplex stainless steel preferably has a yield stress (YS) of 390 MPa or more and a tensile strength (TS) of 590 MPa or more. Examples of duplex stainless steel include SUS329J1 and SUS329J3L. SUS329J1 has a composition, by mass%, of 0.016% C, 0.39% Si, 1.24% Mn, 0.026% P, 0.001% or less S, 5.12% Ni, 23.65% Cr, 1.29% Mo, 0.34% Cu, 0.165% N, with the balance being Fe and unavoidable impurities. SUS329J3L has a composition, by mass%, of 0.017% C, 0.57% Si, 1.82% Mn, 0.027% P, less than 0.0010% S, 5.70% Ni, 22.50% Cr, 3.10% Mo, 0.17% N, with the balance being Fe and unavoidable impurities.

[0027] As shown in FIG. 1, the electrolytic polishing apparatus 1 includes an electrolytic cell 2, a power supply 3, and a counter electrode 4. The electrolytic cell 2 contains an electrolyte 5. A workpiece 6 made of duplex stainless steel is connected to the positive electrode of the power supply 3. The counter electrode 4 is connected to the negative electrode of the power supply 3. The counter electrode 4 is not particularly limited, but may be a known electrode such as Au, Pt, Cu, Ni, or C. The workpiece 6 and the counter electrode 4 are immersed in the electrolyte 5. The power supply 3 is a DC power supply. The power supply 3 is connected to a commercial power supply or a battery and includes a controller capable of constant potential (or constant voltage) control and constant current control. The power supply 3 may be a DC stabilized power supply or a potentio-galvanostat.

[0028] The electrolytic solution 5 is a solution containing phosphoric acid and sulfuric acid. The electrolytic solution 5 may be, for example, a 75 wt% H3PO4-25 wt% H2SO4 solution. The pH of the electrolytic solution 5 is preferably 0 or less, more preferably -1 to 0. The water content of the electrolytic solution 5 is preferably 20% or less. The temperature of the electrolytic solution 5 is preferably 40 to 80°C.

[0029] The method for electropolishing duplex stainless steel includes a preparation step of immersing a duplex stainless steel workpiece 6 and a counter electrode 4 in an electrolyte 5, a high-potential-side electropolishing step of applying a first potential to the workpiece 6 at which the ferrite phase dissolves preferentially over the austenite phase, and a low-potential-side electropolishing step of applying a second potential to the workpiece 6 that is lower than the first potential and at which the austenite phase dissolves preferentially over the ferrite phase. Either the high-potential-side electropolishing step or the low-potential-side electropolishing step can be performed first. The high-potential-side electropolishing step can be performed after the low-potential-side electropolishing step, or the low-potential-side electropolishing step can be performed after the high-potential-side electropolishing step.

[0030] Prior to the preparation step, the duplex stainless steel may be buffed, for example with a 400-grit buff. After buffing, the duplex stainless steel may be cleaned with a neutral or alkaline detergent and then degreased with acetone.

[0031] The first potential applied to the workpiece 6 in the high-potential-side electropolishing step is preferably 2.5 to 10 V (vs. SHE (Standard Hydrogen Electrode)), more preferably 2.5 to 7 V (vs. SHE). The first potential may be expressed, for example, as 2.48 to 9.98 V (vs. Ni) by changing the reference. The second potential applied to the workpiece 6 in the low-potential-side electropolishing step is preferably 0.17 to 0.30 V (vs. SHE). When the workpiece 6 is made of SUS329J1, the second potential is preferably 0.22 to 0.30 V (vs. SHE), more preferably 0.24 to 0.28 V (vs. SHE). When the object to be polished 6 is made of SUS329J3L, the second potential is preferably 0.17 to 0.27 V (vs. SHE), more preferably 0.19 to 0.25 V (vs. SHE). The second potential may be expressed based on a different standard, for example, as 0.15 to 0.28 V (vs. Ni).

[0032] FIG. 2 is a graph showing the polarization curve of duplex stainless steel. As shown in FIG. 2, in the high-potential region where the potential is approximately 1.23 V (vs. SHE) or higher, the current increases as the potential increases in both the ferrite phase and the austenite phase. That is, the amount of electrolytic polishing (dissolution) of both the ferrite phase and the austenite phase increases as the potential increases. At this time, the amount of electrolytic polishing of the ferrite phase is greater than that of the austenite phase, and it can be seen that the ferrite phase is electrolytically polished preferentially over the austenite phase. From the above, it can be seen that at the first potential, the ferrite phase dissolves preferentially over the austenite phase.

[0033] On the other hand, in the low potential region of 0 to 1.23 V (vs. SHE), the ferrite phase and the austenite phase have dissolution current peaks that are offset from each other. The current peak of the austenite phase is offset toward the high potential side relative to the current peak of the ferrite phase. The current peak of the austenite phase is 0.15 to 0.30 V (vs. SHE), and it can be seen that in this range, the austenite phase dissolves preferentially over the ferrite phase. Specifically, when the polished object 6 is SUS329J1, the current peak of the austenite phase is 0.22 to 0.30 V (vs. SHE), and when the polished object 6 is SUS329J3L, the current peak of the austenite phase is 0.15 to 0.25 V (vs. SHE). From the above, it can be seen that at the second potential, the austenite phase dissolves preferentially over the ferrite phase.

[0034] In the low-potential-side electropolishing step, the power supply 3 may perform constant voltage control so that the potential of the workpiece 6 becomes the second voltage. In the high-potential-side electropolishing step, the power supply 3 may perform constant current control so that the current flowing between the workpiece 6 and the counter electrode 4 becomes a predetermined target current value. That is, in the high-potential-side electropolishing step, the first potential is controlled in the range of 2.5 to 10 V (vs. SHE) so that the current flowing between the workpiece 6 and the counter electrode 4 becomes the target value. In the high-potential-side electropolishing step, the target current value is, for example, 100 to 3000 A / m 2 The constant current control has the advantage that the amount of electrolytic polishing can be easily controlled. The implementation times of the low potential side electrolytic polishing process and the high potential side electrolytic polishing process can be set arbitrarily based on the results of experiments, etc.

[0035] According to the electropolishing method of the embodiment, the surface of duplex stainless steel can be smoothed. The ferrite and austenite phases constituting duplex stainless steel have different preferential dissolution potentials. Therefore, when electropolishing is performed at a single potential, one of the ferrite and austenite phases dissolves preferentially over the other. The ferrite and austenite phases are dispersed and mixed as fine crystal grains on the surface of duplex stainless steel, and the preferential dissolution of one of the ferrite and austenite phases forms irregularities on the surface of the duplex stainless steel. Therefore, the ferrite and austenite phases are each dissolved by a high-potential-side electropolishing process in which a first potential at which the ferrite phase dissolves preferentially over the austenite phase is applied to the workpiece 6, and a low-potential-side electropolishing process in which a second potential lower than the first potential at which the austenite phase dissolves preferentially over the ferrite phase is applied to the workpiece 6, thereby smoothing the irregularities on the surface of the duplex stainless steel.

[0036] By performing the high-potential-side electrolytic polishing step after the low-potential-side electrolytic polishing step, the surface of the duplex stainless steel can be efficiently smoothed. If the low-potential-side electrolytic polishing step is performed after the high-potential-side electrolytic polishing step, a passivation film is more likely to be formed in the high-potential-side electrolytic polishing step. [Example]

[0037] (First Example) An example of electropolishing of duplex stainless steel using the electropolishing method of the above embodiment will be described. The electropolishing apparatus 1 shown in FIG. 1 was used. The workpiece 6 was a 6 mm steel plate of duplex stainless steel SUS329J1 cut to a size of 20 mm x 20 mm. The workpiece 6 had a main surface, a first cut surface along the rolling direction, and a second cut surface perpendicular to the rolling direction. Prior to electropolishing, the workpiece 6 was polished with a #400 buff, washed with a neutral detergent, and degreased with acetone. The electrolyte 5 was 500 mL of a 75 wt% H3PO4-25 wt% H2SO4 solution. The power source 3 was a DC stabilized power supply connected to a commercial power source. The counter electrode 4 was Cu. The workpiece 6 and counter electrode 4 were immersed in the electrolyte 5, and the workpiece 6 was connected to the positive terminal of the power source 3, and the counter electrode 4 was connected to the negative terminal. The temperature of the electrolyte 5 was maintained at 50°C.

[0038] The electrolytic polishing process consisted of a first-stage electrolytic polishing process and a subsequent second-stage electrolytic polishing process. The first-stage electrolytic polishing process was a high-potential side electrolytic polishing process, and the target current value was 15 A / dm 2 A constant current control with a target potential of -0.18 V (vs. Cu) (second potential) was performed for 10 minutes. The potential of the workpiece 6 at this time was about 4 V (vs. Cu) (first potential). The second electrolytic polishing step was a low potential side electrolytic polishing step, and a constant voltage control with a target potential of -0.18 V (vs. Cu) (second potential) was performed for 10 minutes.

[0039] Figure 3 shows photographs of the main surface of the object 6 (A) before electropolishing, (B) after the first stage of electropolishing, and (C) after the second stage of electropolishing. A mirror image of the surrounding image is observed on the main surface of each object 6. The mirror image observed on the main surface of the object 6 after the first stage of electropolishing (Figure 3(B)) was found to be hazy compared to the mirror image observed on the main surface of the object 6 before electropolishing (Figure 3(A)). This indicates that the first stage of electropolishing increased the roughness of the main surface of the object 6 compared to before electropolishing, resulting in increased scattering on the main surface. On the other hand, the mirror image observed on the main surface of the object 6 after the second stage of electropolishing (Figure 3(C)) has clearer contours than the mirror image observed on the main surface of the object 6 before electropolishing (Figure 3(A)). From this, it was visually confirmed that the surface roughness of the main surface of the object to be polished 6 was reduced by carrying out the second stage of electrolytic polishing, resulting in a surface closer to a mirror finish.

[0040] Figure 4 shows scanning electron microscope images of the polished object 6 after the first stage of electropolishing. Figure 4(A) shows the boundary between the first cut surface of the polished object 6 and the main surface, and Figure 4(B) shows the boundary between the first cut surface of the polished object 6 and the second cut surface. As shown in Figures 4(A) and (B), a structure was confirmed on the surface of the polished object 6 in which the austenite phase (γ phase) protruded outward relative to the ferrite phase (α phase). At the boundary between the first cut surface of the polished object 6 and the main surface, the austenite phase protruded approximately 0.6 μm relative to the ferrite phase. At the boundary between the first cut surface of the polished object 6 and the second cut surface, the austenite phase protruded approximately 2.3 μm relative to the ferrite phase. This is due to the preferential dissolution of the ferrite phase over the austenite phase during the first stage of electropolishing.

[0041] The surface roughness of the main surface of the object 6 to be polished before electropolishing, after the first stage of electropolishing, and after the second stage of electropolishing was measured using a surface roughness measuring device. The arithmetic mean roughness (Ra) of the main surface of the object 6 to be polished before electropolishing was 0.075 μm, and the ten-point mean roughness (Rz) was 0.640 μm. The arithmetic mean roughness (Ra) of the main surface of the object 6 to be polished after the first stage of electropolishing was 0.212 μm, and the ten-point mean roughness (Rz) was 1.185 μm. The arithmetic mean roughness (Ra) of the main surface of the object 6 to be polished after the second stage of electropolishing was 0.112 μm, and the ten-point mean roughness (Rz) was 0.768 μm.

[0042] (Consideration of Opposite 4) Figure 5 is a graph showing the natural electrode potentials of Cu, SUS329J1, SUS329J3L, SUS316L, and Ni. The natural electrode potentials are measured using a saturated KCl-silver / silver chloride electrode (SSE) as the reference electrode. The electrolyte 5 is a 75 wt% H3PO4-25 wt% H2SO4 solution at 50°C. Ni has a lower natural electrode potential than the duplex stainless steels SUS329J1 and SUS329J3L. Therefore, when Ni is used as the counter electrode 4, the second potential in the low-potential-side electropolishing process is positive. This allows the low-potential-side electropolishing process to be performed using a commonly used monopolar power supply 3. On the other hand, when Cu, which has a higher natural electrode potential than SUS329J1 and SUS329J3L, is used as the counter electrode 4, the second potential in the low-potential-side electropolishing process is negative.

[0043] (Consideration of the second potential) The range of the second potential in the low-potential side electropolishing process was confirmed. The workpiece 6 was SUS329J1 or SUS329J3L, and the counter electrode 4 was Ni. The electrolyte 5 was a 75 wt% H3PO4-25 wt% H2SO4 solution at 50°C. The potential of the workpiece 6 was varied from 0.08 V (vs. Ni) to 0.6 V (vs. Ni), and the current density was measured. As shown in Figure 6, for SUS329J1, a first peak P1 was observed between 0.05 and 0.2 V (vs. Ni), and a second peak P2 was observed between 0.2 and 0.28 V (vs. Ni). The first peak P1 indicates preferential dissolution of the ferrite phase over the austenite phase. The second peak P2 indicates preferential dissolution of the austenite phase over the ferrite phase. Therefore, when the object to be polished 6 is made of SUS329J1, the austenite phase can be dissolved preferentially over the ferrite phase by setting the potential at 0.2 to 0.28 V (vs. Ni) (0.22 to 0.30 V (vs. SHE)).

[0044] In the case of SUS329J3L, a first peak P1 was observed at 0.04 to 0.15 V (vs. Ni), and a second peak P2 was observed at 0.15 to 0.25 V (vs. Ni). The first peak P1 indicates that the ferrite phase dissolves preferentially over the austenite phase. The second peak P2 indicates that the austenite phase dissolves preferentially over the ferrite phase. Therefore, when the workpiece 6 is made of SUS329J3L, the austenite phase can be dissolved preferentially over the ferrite phase by setting the potential to 0.15 to 0.25 V (vs. Ni) (0.17 to 0.27 V (vs. SHE)).

[0045] (Study of electrolyte 5) The effect of the electrolyte 5 on the low-potential side electropolishing process was confirmed. The workpiece 6 was SUS329J1, and the counter electrode 4 was Ni. The electrolyte 5 was a 75 wt% H3PO4-25 wt% H2SO4 aqueous solution at 50°C, with pH values ​​of -1, 0, 1, and 2. The potential of the workpiece 6 was varied from -0.3 V (vs. SSE) to 0.6 V (vs. SSE), and the current density was measured. As shown in Figure 7, when the pH of the H3PO4-H2SO4 aqueous solution 5 was -1 and 0, a primary active dissolution peak was observed in the current, indicating that the workpiece 6 was being dissolved. However, when the pH of the electrolyte 5 was 1 and 2, the current peak disappeared. Therefore, the pH of the H3PO4-H2SO4 aqueous solution 5 should be less than 1, preferably 0 or less.

[0046] (Second Example) As a second example, an example in which a low-potential side electropolishing process was performed in the first stage and a high-potential side electropolishing process was performed in the second stage will be described. The electropolishing apparatus 1 used was the same as in the first example. The workpiece 6 was a 6 mm steel plate made of duplex stainless steel SUS329J1 or SUS329J3L, cut to a size of 20 mm x 20 mm. Prior to electropolishing, the workpiece 6 was polished with a #400 buff, washed with a neutral detergent, and degreased with acetone. The electrolyte 5 was 500 mL of a 75 wt% H3PO4-25 wt% H2SO4 solution. The power supply 3 was a DC stabilized power supply connected to a commercial power source. The counter electrode 4 was Ni. The workpiece 6 and counter electrode 4 were immersed in the electrolyte 5, and the workpiece 6 was connected to the positive terminal of the power supply 3, and the counter electrode 4 was connected to the negative terminal. The temperature of the electrolyte 5 was maintained at 50°C.

[0047] The electrolytic polishing was performed in a first stage and a second stage. The first stage was a low-potential side electrolytic polishing process, in which constant voltage control was performed for 10 minutes with a target potential of 0.2 V (vs. Ni) (second potential). The second stage was a high-potential side electrolytic polishing process, in which a target current value was 1500 A / m 2 At this time, the potential of the object to be polished 6 was about 4 V (vs. Ni) (first potential).

[0048] The surface roughness of the main surface of the workpiece 6 before electropolishing, after the first stage of electropolishing, and after the second stage of electropolishing was measured using a surface roughness measuring device, and the main surface of the workpiece 6 was photographed. Figure 8 shows photographs of the main surface of SUS329J1 (A) before electropolishing, (B) after the first stage of electropolishing, and (C) after the second stage of electropolishing, along with the arithmetic mean roughness (Ra) and ten-point mean roughness (Rz). Figure 9 shows photographs of the main surface of SUS329J3L (A) before electropolishing, (B) after the first stage of electropolishing, and (C) after the second stage of electropolishing, along with the arithmetic mean roughness (Ra) and ten-point mean roughness (Rz). In the case of SUS329J1 and SUS329J3L, it was confirmed that the ten-point mean roughness (Rz) value was larger after the first electrolytic polishing compared to before electrolytic polishing, but that the ten-point mean roughness (Rz) value was smaller after the second electrolytic polishing compared to before electrolytic polishing. In other words, it was confirmed that the ten-point mean roughness was improved by the second electrolytic polishing.

[0049] Although the description of the specific embodiment has been completed above, the present invention is not limited to the above embodiment and can be widely modified and implemented. [Explanation of symbols]

[0050] 1:Electrolytic polishing equipment 2: Electrolytic cell 3: Power supply 4: Opposite 5: Electrolyte 6: Object to be polished

Claims

1. 1. A method for electropolishing a duplex stainless steel containing a ferrite phase and an austenite phase, comprising: a preparation step of immersing the object to be polished, which is the duplex stainless steel, and a counter electrode in an electrolyte; a high-potential side electrolytic polishing step of applying a first potential to the workpiece at which the ferrite phase dissolves preferentially over the austenite phase; and a low-potential-side electropolishing step of applying to the workpiece a second potential that is lower than the first potential and that dissolves the austenite phase preferentially over the ferrite phase.

2. 2. The electrolytic polishing method according to claim 1, wherein the second potential is 0.17 to 0.30 V (vs. SHE).

3. 2. The electrolytic polishing method according to claim 1, wherein the first potential is 2.5 to 10 V (vs. SHE).

4. In the high-potential side electropolishing step, the first potential is controlled in the range of 2.5 to 10 V (vs. SHE) so that the current flowing between the object to be polished and the counter electrode becomes a target current value. The electropolishing method according to claim 3.

5. 2. The electrolytic polishing method according to claim 1, wherein the electrolytic solution is a solution containing phosphoric acid and sulfuric acid.

6. 6. The electrolytic polishing method according to claim 5, wherein the pH of the electrolytic solution is 0 or less.

7. The electrolytic polishing method according to claim 1 , wherein the high-potential-side electrolytic polishing step is performed after the low-potential-side electrolytic polishing step.

8. 2. The electrolytic polishing method according to claim 1, wherein the ratio of the ferrite phase to the entire duplex stainless steel is 30 wt % or more and 70 wt % or less.

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