Hastelloy alloy metallographic corrosion method, composite corrosive agent and application of Hastelloy alloy metallographic corrosion method

Abstract

The invention relates to the technical field of nickel-based corrosion-resistant alloy metallographic phase sample preparation, in particular to a Hastelloy alloy metallographic phase corrosion method, a composite corrosive agent and application of the Hastelloy alloy metallographic phase corrosion agent. The method comprises the steps that the surface of a hastelloy sample is sequentially ground, polished and wiped with absolute ethyl alcohol, and a pretreated sample is obtained; the pretreated sample is statically soaked in a composite corrosive agent at the temperature of 20-25 DEG C for 6-10 s; and washing the hastelloy alloy sample subjected to static soaking with flowing clear water for at least 30 seconds, then soaking the hastelloy alloy sample with absolute ethyl alcohol for 5-10 seconds, and blow-drying the hastelloy alloy sample with cold air to obtain the corrosion sample which can be directly used for metallographic microscope observation. According to the method, traditional electrolytic corrosion is replaced by normal-temperature and short-time chemical soaking, and the grain boundary and the second phase can be displayed at the normal temperature of 20-25 DEG C only in the ultra-short time of 6-10 s.

Description

Technical Field

[0001] This application relates to the field of metallographic sample preparation technology for nickel-based corrosion-resistant alloys, and in particular to a metallographic etching method for Hastelloy alloys, a composite etchant and its application. Background Technology

[0002] Hastelloy C-276 and C-22, due to their high nickel, high molybdenum, and high chromium content, form a dense Cr2O3-MoO3 composite passivation film on their surface. Conventional aqua regia or oxalic acid electrolysis systems struggle to establish a sufficient potential difference between the grain boundaries and the matrix, resulting in unclear grain boundaries, phase boundaries, and nanoscale second phases. The existing patent CN202010193349.7's two-step "aqua regia + oxalic acid electrolysis" method relies on a DC power supply and an electrolytic cell, which is cumbersome, has a narrow window, low repeatability, and functional redundancy, making it unsuitable for single Hastelloy alloys. Furthermore, it contains highly toxic components such as chromic acid and hydrofluoric acid, leading to high waste liquid treatment costs and failing to meet the requirements for green and efficient detection. Summary of the Invention

[0003] This application provides a metallographic etching method for Hastelloy, a composite etchant, and its application to solve the following technical problem: how to clearly visualize the grain boundaries and second phase of Hastelloy without electrolysis within a short time at room temperature.

[0004] In a first aspect, embodiments of this application provide a method for metallographic etching of Hastelloy alloys, including: The surface of the Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite corrosive agent at 20℃–25℃ for 6s–10s. The Hastelloy sample that has been statically immersed is rinsed with running water for at least 30 seconds, then immersed in anhydrous ethanol for 5-10 seconds and dried with cold air to obtain a corrosion sample that can be directly observed under a metallographic microscope.

[0005] Optionally, the composite corrosive agent is composed of 36wt% concentrated hydrochloric acid, 68wt% concentrated nitric acid, 30wt% hydrogen peroxide, and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70).

[0006] Optionally, the volume ratio of the composite etchant is 20:8:10:62.

[0007] Optionally, the Hastelloy sample is a C-276, C-22, or C-2000 alloy.

[0008] Optionally, the static soaking time is 6s–8s.

[0009] Optionally, the grinding steps involve progressively wet grinding with 180 mesh → 400 mesh → 600 mesh → 800 mesh → 1000 mesh → 1200 mesh metallographic wet sandpaper, with the grinding debris rinsed off with clean water after each grinding step.

[0010] Optionally, the polishing step involves cross-polishing with 1.5μm–2.5μm diamond spray abrasive on a 4000r / min polishing machine, followed by rinsing with clean water for 1–2 minutes after polishing.

[0011] Secondly, this application provides a composite etchant composed of 36wt% concentrated hydrochloric acid, 68wt% concentrated nitric acid, 30wt% hydrogen peroxide, and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70), used to statically immerse Hastelloy samples at 20℃–25℃ for 6s–10s to reveal the metallographic structure.

[0012] Optionally, the volume ratio is 20:8:10:62.

[0013] Thirdly, embodiments of this application provide the use of the composite etchant described in the second aspect in metallographic analysis, defect detection, or process optimization of Hastelloy alloys.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a metallographic etching method for Hastelloy alloys. This method replaces traditional electrolytic etching with "room temperature + short-time chemical immersion," enabling grain boundaries and the second phase to appear simultaneously in an ultra-short time of only 6–10 seconds at room temperature (20–25 °C). The core idea is: The Hastelloy surface is subjected to "static immersion" using a composite etchant (containing oxide-complex components). Within seconds, it selectively dissolves Cr- and Mo-depleted regions at grain boundaries, as well as second phases such as γ′ and carbides, forming nanoscale pits. The pits and the substrate exhibit a significant reflection difference, with grain boundaries appearing as continuous black lines and the second phases as bright white particles. These can be directly distinguished under an optical microscope without electrolysis. The process is simplified to "grinding-polishing-wiping with anhydrous ethanol-composite agent for 6–10 seconds-rinsing with water-bleaching with ethanol-cooling with cold air," with the entire process taking ≤1 minute. This avoids electrolytic equipment, heating, and toxic strong acids, achieving the four major goals of room temperature, no electricity, a few seconds, and clear imaging. Attached Figure Description

[0015] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0017] Figure 1 Figure 1 shows a metallographic microscopic observation of a corrosion sample provided in an embodiment of this application. Figure 2 Two metallographic microscopic images of the corrosion samples provided in the embodiments of this application; Figure 3 Three metallographic microscopic images of the corrosion samples provided in the embodiments of this application; Figure 4 Four images show metallographic microscopic observations of the corrosion samples provided in the embodiments of this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0020] In a first aspect, embodiments of this application provide a method for metallographic etching of Hastelloy alloys, including: The surface of the Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite corrosive agent at 20℃–25℃ for 6s–10s. The Hastelloy sample that has been statically immersed is rinsed with running water for at least 30 seconds, then immersed in anhydrous ethanol for 5-10 seconds and dried with cold air to obtain a corrosion sample that can be directly observed under a metallographic microscope.

[0021] Grinding removes the cutting deformation layer thickness ≥30 µm, polishing reduces the surface roughness Ra to ≤0.05 µm, and wiping with anhydrous ethanol removes residual grease and moisture, thereby exposing the original metal surface that is free from distortion, oxidation, and contamination. This allows the composite etchant to make instantaneous and uniform contact with the substrate within the subsequent 6–10 s, avoiding "corrosion blind spots" caused by local passivation film residue.

[0022] Maintaining the corrosive agent viscosity at 20℃–25℃ to 1.0–1.1 mPa·s and the hydrogen peroxide decomposition rate at 0.2–0.3% / s ensures that Cr... 3+ →Cr 6+ The oxidation reaction is completed within 6 s–10 s, thus precisely penetrating the Cr2O3-MoO3 composite passivation film without penetrating the substrate; static immersion eliminates the concentration gradient caused by flow, thereby allowing the grain boundaries and the second phase to be selectively dissolved by 5–15 nm in the same time, forming visible optical contrast.

[0023] Run clean water for at least 30 seconds to remove residual Cl from the surface. - NO3 - The concentration is diluted to <1 ppm to terminate the corrosion reaction; anhydrous ethanol is used to replace the water for 5 s–10 s, and the water stains are dried with cold air to prevent the water stains from being re-oxidized, thereby fixing the nanoscale relief morphology of the grain boundary / second phase. This ensures that the grain boundary appears as a continuous black line and the second phase appears as bright white particles in the 200×–500× field of view under a metallographic microscope, achieving "clear visualization within a few seconds at room temperature without electrolysis".

[0024] In some embodiments, the composite corrosive agent consists of 36 wt% concentrated hydrochloric acid, 68 wt% concentrated nitric acid, 30 wt% hydrogen peroxide, and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70).

[0025] This quaternary ratio maintains the solution pH at [value missing]. 0.8 to With a redox potential of 0.5, ranging from +1.35 V to +1.42 V, the potential difference between the grain boundary and the substrate is amplified to 20–30 mV while ensuring that the passivation film is oxidized and broken. This results in a grain boundary dissolution rate that is 3–5 times faster than that within the grain, enabling the formation of trenches with a depth of 5–15 nm within "a few seconds," which directly contributes to the "clear visualization of grain boundaries and the second phase."

[0026] Concentrated hydrochloric acid volume ratio (15–25): 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25; concentrated nitric acid volume ratio (5–10): 5, 6, 7, 8, 9, 10; hydrogen peroxide volume ratio (8–12): 8, 9, 10, 11, 12; distilled water volume ratio (55–70): 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70.

[0027] In some embodiments, the volume ratio of the composite etchant is 20:8:10:62.

[0028] At this single-point ratio, the solution pH is = With a redox potential of 0.68 and a hydrogen peroxide decomposition rate of 0.25% / s, the error in the grain boundary / second phase dissolution depth is controlled within ±2 nm, thereby ensuring that the coefficient of variation of grain boundary width in metallographic images of the same batch of samples is ≤5%, directly solving the sub-problem of "poor repeatability".

[0029] In some embodiments, the Hastelloy sample is a C-276, C-22, or C-2000 alloy.

[0030] All three alloys contain 55–59% Ni, 14–16% Cr, and 15–17% Mo, and their surface passivation films are mainly composed of Cr2O3-MoO3. This ensures that the composite etchant has the same rupture potential requirement, thus guaranteeing that the "no electrolysis in a few seconds at room temperature" solution can be applied directly without adjusting the formula, solving the efficiency problem of "different grades requiring different etchants".

[0031] In some implementations, the static soaking time is 6s–8s.

[0032] The grain boundary dissolution depth is 5–10 nm in the 6 s–8 s interval, and the second phase detachment diameter is ≤50 nm. This avoids the "over-corrosion" artifact caused by the widening of grain boundary trenches when the time is >10 s, thus ensuring that the grain boundary lines are fine and continuous under the metallographic microscope, directly improving the "clarity".

[0033] In some embodiments, the grinding steps involve progressively wet grinding with 180-grit → 400-grit → 600-grit → 800-grit → 1000-grit → 1200-grit metallographic wet sandpaper, with the grinding debris rinsed off with clean water after each grinding pass.

[0034] The 180-mesh etchant removes the hardened layer to a depth of ≥50 µm, and the 1200-mesh etchant reduces the surface scratch width to ≤3 µm. Each pass is followed by a water rinse to prevent coarse abrasive particles from embedding, resulting in a smooth surface without deep distortion. This reduces subsequent polishing time by 30%, ensuring uniform action of the etchant within 6–10 seconds, directly supporting the achievement of the "a few seconds" target.

[0035] In some embodiments, the polishing step involves cross-polishing with 1.5μm–2.5μm diamond spray abrasive on a 4000r / min polishing machine, followed by rinsing with clean water for 1–2 minutes after polishing.

[0036] The 1.5 µm–2.5 µm particle size reduces the surface roughness Ra to 0.02–0.05 µm, the cross-shaped pattern eliminates directional scratches, and the 1 min–2 min water rinsing removes residual diamond particles, thereby preventing polishing defects from being amplified into pseudo-grain boundaries within 6 s–10 s of etching, thus ensuring the authenticity of grain boundary appearance and solving the sub-problem of "illusion interference".

[0037] Secondly, this application provides a composite etchant composed of 36wt% concentrated hydrochloric acid, 68wt% concentrated nitric acid, 30wt% hydrogen peroxide, and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70), used to statically immerse Hastelloy samples at 20℃–25℃ for 6s–10s to reveal the metallographic structure.

[0038] This composite etchant itself provides a closed loop of "film breaking-selective dissolution-complexation", thus eliminating the need for an external power source and directly contributing to the core requirement of "electrolysis-free".

[0039] In some embodiments, the volume ratio is 20:8:10:62.

[0040] This single-point ratio ensures that users can obtain clear contrast of grain boundaries / second phase within 6 s–10 s by preparing the solution according to this ratio, thus saving experimental exploration time and achieving standardization of "a few seconds at room temperature".

[0041] Thirdly, embodiments of this application provide the use of the composite etchant described in the second aspect in metallographic analysis, defect detection, or process optimization of Hastelloy alloys.

[0042] This single-point ratio, as an independent product claim, ensures that users can obtain clear contrast of grain boundaries / second phase within 6 s–10 s by preparing the product according to this ratio, thereby saving experimental exploration time and achieving standardization of "a few seconds at room temperature".

[0043] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If no corresponding national standard exists, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0044] I. Implementation Examples Example 1 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 °C for 6 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62.

[0045] Example 2 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 25 °C for 10 s. The statically immersed Hastelloy sample was rinsed with running water for 40 s, then immersed in anhydrous ethanol for 10 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62.

[0046] Example 3 The surface of the C-22 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 22 °C for 8 s. The statically immersed Hastelloy sample was rinsed with running water for 35 s, then immersed in anhydrous ethanol for 7 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62.

[0047] Example 4 The surface of the C-2000 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 23 °C for 7 s. The statically immersed Hastelloy sample was rinsed with running water for 32 s, then immersed in anhydrous ethanol for 6 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62.

[0048] Example 5 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 24 °C for 6 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 15:5:8:55.

[0049] Example 6 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 21 °C for 9 s. The statically immersed Hastelloy sample was rinsed with running water for 38 s, then immersed in anhydrous ethanol for 9 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 25:10:12:70.

[0050] Example 7 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 °C for 6 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 18:7:9:60.

[0051] Example 8 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 25 °C for 10 s. The statically immersed Hastelloy sample was rinsed with running water for 40 s, then immersed in anhydrous ethanol for 10 s and dried with cold air to obtain a corrosion sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 22:9:11:65.

[0052] Example 9 The surface of the C-276 Hastelloy sample was sequentially ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 ℃ for 6 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62. The grinding steps were performed by wet grinding with 180 mesh → 400 mesh → 600 mesh → 800 mesh → 1000 mesh → 1200 mesh metallographic wet sandpaper, and the grinding debris was rinsed off with water after each grinding step. The polishing steps were performed by cross-polishing with 1.5µm diamond spray abrasive on a polishing machine at 4000 r / min. After polishing, the sample was rinsed with water for 1 min.

[0053] Example 10 The surface of the C-276 Hastelloy sample was sequentially ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 25 ℃ for 10 s. The statically immersed Hastelloy sample was rinsed with running water for 40 s, then immersed in anhydrous ethanol for 10 s and dried with cold air to obtain a corroded sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62. The grinding steps were performed by wet grinding with 180 mesh → 400 mesh → 600 mesh → 800 mesh → 1000 mesh → 1200 mesh metallographic wet sandpaper, and the grinding debris was rinsed off with water after each grinding step. The polishing steps were performed by cross-polishing with 2.5 µm diamond spray abrasive on a polishing machine at 4000 r / min. After polishing, the sample was rinsed with water for 2 min.

[0054] Example 11 The surface of the C-276 Hastelloy sample was sequentially ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 22 ℃ for 8 s. The statically immersed Hastelloy sample was rinsed with running water for 35 s, then immersed in anhydrous ethanol for 7 s and dried with cold air to obtain a corroded sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62. The grinding steps were performed by wet grinding with 180 mesh → 400 mesh → 600 mesh → 800 mesh → 1000 mesh → 1200 mesh metallographic wet sandpaper, and the grinding debris was rinsed off with water after each grinding step. The polishing steps were performed by cross-polishing with 2.0µm diamond spray abrasive on a polishing machine at 4000 r / min. After polishing, the sample was rinsed with water for 1.5 min to achieve a polish.

[0055] Example 12 The surface of the C-22 Hastelloy sample was sequentially ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 23 °C for 7 s. The statically immersed Hastelloy sample was rinsed with running water for 32 s, then immersed in anhydrous ethanol for 6 s and dried with cold air to obtain a corroded sample that could be directly observed under a metallographic microscope. The volume ratio of the composite etchant was 20:8:10:62. The grinding steps were performed by wet grinding with 180 mesh → 400 mesh → 600 mesh → 800 mesh → 1000 mesh → 1200 mesh metallographic wet sandpaper, and the grinding debris was rinsed off with water after each grinding step. The polishing steps were performed by cross-polishing with 1.8 µm diamond spray abrasive on a polishing machine at 4000 r / min. After polishing, the sample was rinsed with water for 1.2 min to achieve a polished finish.

[0056] II. Comparative Example Comparative Example 1 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in aqua regia at 20 °C for 30 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample.

[0057] Comparative Example 2 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was electrolyzed in a saturated oxalic acid solution at 20 °C with a DC voltage of 8 V for 20 s. The electrolyzed Hastelloy sample was rinsed with running water for 30 s, then soaked in anhydrous ethanol for 5 s and dried with cold air to obtain a corrosion sample.

[0058] Comparative Example 3 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 °C for 4 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample. The volume ratio of the composite etchant was 20:8:10:62.

[0059] Comparative Example 4 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 °C for 15 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample. The volume ratio of the composite etchant was 20:8:10:62.

[0060] Comparative Example 5 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 20 °C for 8 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample. The volume ratio of the composite etchant was 10:3:5:82.

[0061] Comparative Example 6 The surface of the C-276 Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite etchant at 35 °C for 8 s. The statically immersed Hastelloy sample was rinsed with running water for 30 s, then immersed in anhydrous ethanol for 5 s and dried with cold air to obtain a corroded sample. The volume ratio of the composite etchant was 20:8:10:62.

[0062] III. Results Data Experimental methods for evaluating results: Grain boundary visibility: The corroded sample was placed under a metallographic microscope with a field of view of 500× and 10 frames were randomly photographed. The proportion of the continuous length of the grain boundary to the total theoretical grain boundary length was measured using image analysis software, and the average value was taken. The result is expressed as a percentage.

[0063] Second phase display sharpness: In the same 500× field of view, the proportion of the actual number of second phase particles with a diameter ≥ 0.5 µm to the theoretical total number is statistically analyzed, and the average is taken over 10 frames. The result is expressed as a percentage.

[0064] Over-corrosion area: The proportion of the area of ​​corrosion pits in the field of view to the total area of ​​the field of view is measured using image analysis software. The average of 10 frames is taken, and the result is expressed as a percentage.

[0065] Total sample preparation time: Record the total time from the completion of cutting to the end of sample drying, accurate to 1 minute.

[0066] Repeatability pass rate: For 10 parallel samples processed in the same batch, if the grain boundary visibility is ≥90% and the over-corrosion area is ≤1%, it is considered as qualified. The percentage of qualified samples is calculated and expressed as a percentage.

[0067] Table 1. Results data for both the examples and comparative examples.

[0068] As can be seen from Table 1, the technological advancements of this application include: 1. The clarity of grain boundary visibility is improved from a maximum of 70% in the comparative example to a maximum of 99% in the embodiment, an increase of 29% in absolute value, thereby directly solving the technical problem of "clearly displaying Hastelloy grain boundaries within a few seconds at room temperature without electrolysis".

[0069] 2. The clarity of the second phase is improved from a maximum of 73% in the comparative example to a maximum of 98% in the embodiment, an increase of 25% in absolute value, thereby ensuring that the nanoscale Laves phase can be counted under a 200×–500× metallographic microscope, meeting the requirements for quantitative analysis of defects.

[0070] 3. The over-corrosion area was reduced from a minimum of 2% in the comparative example to 0% in the general example, with a maximum reduction of 20%, thereby eliminating the industry pain point of "over-corrosion after a slight extension" and widening the operating window from ±2 s / ±2 ℃ to ±10 s / ±5 ℃.

[0071] 4. The total sample preparation time was further stabilized at 27 min (Examples 9–12) from the shortest comparative example of 28 min (Comparative Example 3), and no DC power supply, electrolytic cell, or saturated oxalic acid solution was required, thereby reducing the equipment investment to zero and increasing the single-piece inspection efficiency by more than 50%.

[0072] 5. The repeatability pass rate has been increased from a maximum of 80% for comparative samples to a maximum of 100% for example samples. All 10 parallel samples within a batch meet the standards, thereby achieving seamless integration between laboratory single-piece testing and factory batch inspection, and completely solving the R&D misjudgment caused by "large differences within the same batch".

[0073] Explanation of the attached diagram: Figure 1 Figure 1 shows a metallographic microscopic observation of a corrosion sample provided in an embodiment of this application. Figure 2 Two metallographic microscopic images of the corrosion samples provided in the embodiments of this application; Figure 3 Three metallographic microscopic images of the corrosion samples provided in the embodiments of this application; Figure 4 Four images show metallographic microscopic observations of the corrosion samples provided in the embodiments of this application.

[0074] Depend on Figures 1 to 4 We can obtain: Figure 1 (Scale bar 100 μm) shows that the austenite grain boundaries are continuous black lines with complete grain outlines and no over-corrosion grooves. The grain boundary visibility is ≥99%, directly verifying that the 6 s–10 s room temperature static immersion described in claim 1 can break through the Cr2O3-MoO3 composite passivation film and selectively dissolve the grain boundaries.

[0075] Figure 2 (Scale bar 20 μm) shows white granular second phase with a size of 0.5–2 μm distributed at the grain boundaries. The particles have sharp edges and a contrast with the matrix of ≥30%, proving that the volume ratio of the composite etchant described in claim 2 (15–25):(5–10):(8–12):(55–70) has the ability to simultaneously reveal the second phase of C-276, C-22 and C-2000 alloys at 20 ℃–25 ℃.

[0076] Figure 3 (Scale bar 20 μm) shows that there are no additional corrosion pits inside the crystal, the surface roughness Ra≤0.05 μm, and the over-corrosion area is 0%, thus quantitatively supporting the claim 5 that the 6 s–8 s immersion time window can avoid the industry pain point of "over-corrosion if slightly extended".

[0077] Figure 4 (Scale bar 20 μm) shows that the coefficient of variation of grain boundary width in 5 parallel fields of view in the same batch is ≤5%, and the repeatability pass rate is 100%. This image evidence solidifies the batch consistency improvement effect brought about by the optimal volume ratio of 20:8:10:62 in claim 3.

[0078] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method of metallographic etching of a Hastelloy alloy, characterized in that, include: The surface of the Hastelloy sample was successively ground, polished, and wiped with anhydrous ethanol to obtain a pretreated sample. The pretreated sample was statically immersed in a composite corrosive agent at 20℃–25℃ for 6s–10s; The Hastelloy sample that has been statically immersed is rinsed with running water for at least 30 seconds, then immersed in anhydrous ethanol for 5-10 seconds and dried with cold air to obtain a corrosion sample that can be directly observed under a metallographic microscope.

2. The metallographic etching method for Hastelloy alloys according to claim 1, characterized in that, The composite corrosive agent is composed of 36wt% concentrated hydrochloric acid, 68wt% concentrated nitric acid, 30wt% hydrogen peroxide, and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70).

3. The metallographic etching method for Hastelloy alloys according to claim 2, characterized in that, The volume ratio of the composite etchant is 20:8:10:

62.

4. The metallographic etching method for Hastelloy alloys according to claim 1, characterized in that, The Hastelloy alloy samples were C-276, C-22, or C-2000 alloys.

5. The metallographic etching method for Hastelloy alloys according to claim 1, characterized in that, The static soaking time is 6s–8s.

6. The metallographic etching method for Hastelloy alloys according to claim 1, characterized in that, The grinding process involves progressively wet grinding with 180-grit → 400-grit → 600-grit → 800-grit → 1000-grit → 1200-grit metallographic wet sandpaper, with the grinding debris rinsed off with clean water after each grinding step.

7. The metallographic etching method for Hastelloy alloys according to claim 1, characterized in that, The polishing step involves cross-polishing with 1.5μm–2.5μm diamond spray abrasive on a 4000r / min polishing machine, followed by rinsing with clean water for 1–2 minutes after polishing.

8. A composite corrosive agent, characterized in that, Composed of 36wt% concentrated hydrochloric acid, 68wt% concentrated nitric acid, 30wt% hydrogen peroxide and distilled water in a volume ratio of (15–25):(5–10):(8–12):(55–70), it is used to statically immerse Hastelloy samples at 20℃–25℃ for 6s–10s to achieve metallographic microstructure visualization.

9. The composite corrosive agent according to claim 8, characterized in that, The volume ratio is 20:8:10:

62.

10. The use of the composite etchant of claim 8 in metallographic analysis, defect detection or process optimization of Hastelloy alloys.

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