Grain boundary corrosive liquid and corrosion method for 5xxx and 7xxx series aluminum alloys
By using a specific composition of etching solution and polishing treatment, combined with heating and ultrasonic cleaning, the problem of difficult grain boundary display in 5xxx and 7xxx series aluminum alloys has been solved, realizing simple and efficient grain boundary observation and reducing cost and time consumption.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- TAIYUAN UNIVERSITY OF SCIENCE AND TECHNOLOGY
- Filing Date
- 2026-01-14
- Publication Date
- 2026-04-14
AI Technical Summary
In existing technologies, grain boundary display of 5xxx and 7xxx series aluminum alloys is difficult. Conventional etching methods are complex, time-consuming, and costly, making it difficult to achieve clear and complete grain boundary display.
An etching solution composed of nitric acid, hydrofluoric acid, hydrochloric acid, ferric chloride, and chromic anhydride was used, combined with polishing and heat treatment, followed by ultrasonic cleaning and constant temperature water bath etching. The metallographic structure was then observed under a microscope.
It achieves clear and complete display of grain boundaries in 5xxx and 7xxx series aluminum alloys, reducing operational complexity and economic costs, and improving observation efficiency.
Smart Images
Figure CN121852913A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of aluminum alloy metallographic corrosion technology, specifically relating to a grain boundary corrosion solution and corrosion method for 5xxx and 7xxx series aluminum alloys. Background Technology
[0002] In studying the influence of grain morphology and size on the properties of aluminum alloys, microscopic observation of grain boundaries is often required. Currently, widely used methods include electrolytic polishing combined with polarized light treatment. This process requires sample preparation followed by observation using a polarized light microscope. However, the sample preparation process is complex and cumbersome, and places high demands on the microscope equipment, significantly increasing the time and economic costs of the research. Polishing and etching are two key steps in the sample preparation process, and their effectiveness directly affects the quality of grain boundary visualization.
[0003] 5xxx series (Al-Mg series) and 7xxx series (Al-Zn-Mg-Cu series) aluminum alloys are two important types of structural aluminum alloys, widely used in shipbuilding, aerospace, and rail transportation due to their respective excellent properties. 5xxx series aluminum alloys are non-heat-treatable aluminum alloys, characterized by low density and good corrosion resistance; 7xxx series aluminum alloys, on the other hand, are heat-treatable aluminum alloys, known for their high strength and good overall performance. However, both types of alloys present challenges in displaying their metallographic structure: 5xxx series alloys, due to their excellent corrosion resistance and small difference in grain boundary and intragranular lining, are difficult to clearly display using conventional etching methods; 7xxx series alloys, due to their complex composition and high microstructure sensitivity, also make the etching process difficult to control, often resulting in incomplete display or over-etching.
[0004] Currently, metallographic observation of these two types of alloys typically employs anodized coating combined with polarized light to improve microstructure visibility. However, this method is complex, time-consuming, and costly. Therefore, developing a simple, widely applicable, and effective etching solution and corresponding etching method to achieve efficient and clear visualization of grain boundaries in 5xxx and 7xxx series aluminum alloys has become a critical issue urgently needing to be addressed in this field. Summary of the Invention
[0005] To address the shortcomings of existing technologies, this invention provides a grain boundary etching solution for 5xxx and 7xxx series aluminum alloys, which etches 5xxx and 7xxx series aluminum alloys to obtain a metallographic structure with clearly visible grain boundaries, while reducing time and economic costs.
[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:
[0007] A grain boundary etching solution for 5xxx and 7xxx series aluminum alloys is composed of the following substances: 5-10 mL of 85% nitric acid, 1-2 mL of 50% hydrofluoric acid, 10-15 mL of 85% hydrochloric acid, 2-3 g of ferric chloride, 3-6 g of chromic anhydride, and distilled water to a final volume of 100 mL.
[0008] The method for corroding 5xxx and 7xxx series aluminum alloys with the above-mentioned corrosive solution includes the following steps:
[0009] Step 1: Prepare the etching solution;
[0010] Step 2: Cut out aluminum alloy samples and embed them. Remove the embedded samples after they have cooled and solidified.
[0011] Step 3: Polish the inlaid sample with sandpaper;
[0012] Step 4: Polish the ground sample using a grinding and polishing machine;
[0013] Step 5: Place the sample in a beaker containing anhydrous ethanol, clean it with ultrasound, and then blow it dry.
[0014] Step 6: Pour 20 mL of etching solution into a container, add distilled water to dilute to 100 mL, completely immerse the sample in the diluted etching solution for pre-etching, then remove it and immediately rinse with distilled water, then clean with anhydrous ethanol and blow dry.
[0015] Step 7: Pour an appropriate amount of etching solution into a container, place it in a constant temperature water bath, heat and stabilize it at 40~60℃, completely immerse the pre-etched sample in the preheated etching solution, then take it out and rinse it with distilled water immediately, then clean it with anhydrous ethanol and blow it dry.
[0016] Step 8: Place the etched metallographic sample under a metallographic microscope to observe the metallographic structure.
[0017] Further, prepare a corrosion solution by using 5-10 mL of 85% nitric acid, 1-2 mL of 50% hydrofluoric acid, 10-15 mL of 85% hydrochloric acid, 2-3 g of ferric chloride, 3-6 g of chromic anhydride, and distilled water to make up to 100 mL.
[0018] Further, weigh out a fixed amount of ferric chloride and chromic anhydride, and then add distilled water, hydrochloric acid, nitric acid, and hydrofluoric acid in the formula proportions in sequence.
[0019] Furthermore, the inlaying in step 2 is performed by hot inlaying with phenolic resin powder at a temperature of 135°C for 5 minutes.
[0020] Furthermore, in step 3, sanding is performed using sandpaper, specifically 400 grit, 600 grit, 800 grit, 1000 grit, 1500 grit, 2000 grit, 3000 grit, and 5000 grit sandpaper in sequence.
[0021] Furthermore, in step 4, polishing specifically involves using a silk polishing cloth and a diamond polishing spray with a particle size of 1.5 μm for coarse polishing, continuously adding a small amount of water until the sample surface is free of scratches, and then using a diamond polishing paste with a particle size of 0.5 μm for fine polishing.
[0022] Furthermore, the ultrasonic cleaning time in step 5 is 60s~120s.
[0023] Furthermore, in step 6, the sample needs to be completely immersed in the preheated corrosive liquid for 10s~20s.
[0024] Furthermore, in step 7, the pre-corroded sample needs to be completely immersed in the preheated corrosion solution for 60s~120s.
[0025] Compared with the prior art, the present invention has the following advantages:
[0026] The etching solution of this invention uses chromate as an oxidation buffer to prevent over-corrosion. It exhibits good corrosion performance on 5xxx and 7xxx series aluminum alloys. Heating the etching solution and stabilizing it at 40-60°C before etching can effectively improve the corrosion effect and obtain clear and clean grain boundary contrast. It requires minimal observation equipment and the etching operation is simple and convenient. Attached Figure Description
[0027] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0028] Figure 1 The image shows the metallographic structure obtained by etching in Example 1.
[0029] Figure 2 Photographs of the metallographic structure obtained by etching in Example 2;
[0030] Figure 3 Photographs of the metallographic structure obtained by etching in Example 3;
[0031] Figure 4 A photograph of the metallographic structure obtained by etching in Comparative Example 1;
[0032] Figure 5 The image shows the metallographic structure obtained by etching in Comparative Example 2;
[0033] Figure 6 This is a photograph of the metallographic structure obtained by etching in Comparative Example 3. Detailed Implementation
[0034] To gain a deeper understanding of this invention, we will provide a comprehensive and detailed description. However, this invention has various implementations and is not limited to the specific examples listed herein. These examples are presented to enhance a full understanding of the disclosure of this invention.
[0035] Example 1
[0036] A method for intergranular corrosion of 5xxx and 7xxx series aluminum alloys, comprising the following steps:
[0037] Step 1, prepare the etching solution: the components and volume percentages are as follows: 5 mL of 85% nitric acid, 2 mL of 50% hydrofluoric acid, 10 mL of 85% hydrochloric acid, 2 g of ferric chloride, 3 g of chromic anhydride, and distilled water to make up to 100 mL.
[0038] First, weigh out a certain amount of ferric chloride and chromium trioxide, and then add distilled water, hydrochloric acid, nitric acid, and hydrofluoric acid in the formula ratio in sequence.
[0039] Step 2, metallographic sample preparation: Cut a Φ10mm×15mm 5xxx series aluminum alloy sample, and heat mount the 5xxx series aluminum alloy sample in a mounting machine using phenolic resin powder. The mounting temperature and time are 135℃ and 5min, respectively.
[0040] Step 3, Grinding: Grind with sandpaper of 400 grit, 600 grit, 800 grit, 1000 grit, 1500 grit, 2000 grit, 3000 grit and 5000 grit in sequence;
[0041] Step 4, Polishing: Polish the polished sample by using a silk polishing cloth and a diamond polishing spray with a particle size of 1.5μm for rough polishing, and continuously adding a small amount of water. Then, use a diamond polishing paste with a particle size of 0.5μm for fine polishing until there are no scratches on the sample surface.
[0042] Step 5, Cleaning and Drying: Place the sample in a beaker containing anhydrous ethanol, perform ultrasonic cleaning for 1-2 minutes, and then dry it with a hair dryer.
[0043] Step 6, Pre-etching: Pour 20 mL of etching solution into a container, add distilled water to dilute to 100 mL, immerse the sample completely in the diluted etching solution for 10 seconds, then remove it and rinse immediately with distilled water, then clean it with anhydrous ethanol and blow it dry.
[0044] Step 7, heating corrosion: Pour an appropriate amount of corrosion solution into a container, place it in a constant temperature water bath, heat and stabilize at 40°C, immerse the pre-corroded sample completely in the preheated corrosion solution for 60 seconds, then take it out and rinse it immediately with distilled water, then clean it with anhydrous ethanol, and dry it with a hair dryer.
[0045] Step 8: Observe the sample obtained by etching using the above method under a microscope. The resulting metallographic structure is as follows. Figure 1 As shown, the grain boundaries of the alloy are clear and complete, the corrosion effect is good, and it is easy to observe.
[0046] Example 2
[0047] A method for intergranular corrosion of 5xxx and 7xxx series aluminum alloys, comprising the following steps:
[0048] Step 1, prepare the etching solution: the components and volume percentages are as follows: 10 mL of 85% nitric acid, 2 mL of 50% hydrofluoric acid, 15 mL of 85% hydrochloric acid, 3 g of ferric chloride, 6 g of chromic anhydride, and distilled water to make up to 100 mL.
[0049] First, weigh out a certain amount of ferric chloride and chromium trioxide, and then add distilled water, hydrochloric acid, nitric acid, and hydrofluoric acid in the formula ratio in sequence.
[0050] Step 2, metallographic sample preparation: Cut a Φ10mm×15mm 5xxx series aluminum alloy sample, and heat mount the 5xxx series aluminum alloy sample in a mounting machine using phenolic resin powder. The mounting temperature and time are 135℃ and 5min, respectively.
[0051] Step 3, Grinding: Grind with sandpaper of 400 grit, 600 grit, 800 grit, 1000 grit, 1500 grit, 2000 grit, 3000 grit and 5000 grit in sequence;
[0052] Step 4, Polishing: Polish the polished sample by using a silk polishing cloth and a diamond polishing spray with a particle size of 1.5μm for rough polishing, and continuously adding a small amount of water. Then, use a diamond polishing paste with a particle size of 0.5μm for fine polishing until there are no scratches on the sample surface.
[0053] Step 5, Cleaning and Drying: Place the sample in a beaker containing anhydrous ethanol, perform ultrasonic cleaning for 1-2 minutes, and then dry it with a hair dryer.
[0054] Step 6, Pre-etching: Pour 20 mL of etching solution into a container, add distilled water to dilute to 100 mL, immerse the sample completely in the diluted etching solution for 15 seconds, then remove it and rinse immediately with distilled water, then clean it with anhydrous ethanol and blow it dry.
[0055] Step 7, heating corrosion: Pour an appropriate amount of corrosion solution into a container, place it in a constant temperature water bath, heat and stabilize at 50°C, immerse the pre-corroded sample completely in the preheated corrosion solution for 120 seconds, then take it out and rinse it immediately with distilled water, then clean it with anhydrous ethanol, and dry it with a hair dryer.
[0056] Step 8: Observe the sample obtained by etching using the above method under a microscope. The resulting metallographic structure is as follows. Figure 2 As shown, the grain boundaries of the alloy are clear and complete, the corrosion effect is good, and it is easy to observe.
[0057] Example 3
[0058] A method for intergranular corrosion of 5xxx and 7xxx series aluminum alloys, comprising the following steps:
[0059] Step 1, prepare the etching solution: the components and volume percentages are as follows: 8 mL of 85% nitric acid, 1.5 mL of 50% hydrofluoric acid, 13 mL of 85% hydrochloric acid, 3 g of ferric chloride, 5 g of chromic anhydride, and distilled water to make up to 100 mL.
[0060] First, weigh out a certain amount of ferric chloride and chromium trioxide, and then add distilled water, hydrochloric acid, nitric acid, and hydrofluoric acid in the formula ratio in sequence.
[0061] Step 2, metallographic sample preparation: Cut Φ10mm×15mm 7xxx series aluminum alloy samples, and heat mount 5xxx series aluminum alloy samples in a mounting machine using phenolic resin powder. The mounting temperature and time are 135℃ and 5min, respectively.
[0062] Step 3, Grinding: Grind with sandpaper of 400 grit, 600 grit, 800 grit, 1000 grit, 1500 grit, 2000 grit, 3000 grit and 5000 grit in sequence;
[0063] Step 4, Polishing: Polish the polished sample by using a silk polishing cloth and a diamond polishing spray with a particle size of 1.5μm for rough polishing, and continuously adding a small amount of water. Then, use a diamond polishing paste with a particle size of 0.5μm for fine polishing until there are no scratches on the sample surface.
[0064] Step 5, Cleaning and Drying: Place the sample in a beaker containing anhydrous ethanol, perform ultrasonic cleaning for 1-2 minutes, and then dry it with a hair dryer.
[0065] Step 6, Pre-etching: Pour 20 mL of etching solution into a container, add distilled water to dilute to 100 mL, immerse the sample completely in the diluted etching solution for 20 seconds, then remove it and rinse immediately with distilled water, then clean it with anhydrous ethanol and blow it dry.
[0066] Step 7, heating corrosion: Pour an appropriate amount of corrosion solution into a container, place it in a constant temperature water bath, heat and stabilize at 60°C, immerse the pre-corroded sample completely in the preheated corrosion solution for 90 seconds, then take it out and rinse it immediately with distilled water, then clean it with anhydrous ethanol, and dry it with a hair dryer.
[0067] Step 8: Observe the sample obtained by etching using the above method under a microscope. The resulting metallographic structure is as follows. Figure 3 As shown, the grain boundaries of the alloy are clear and complete, the corrosion effect is good, and it is easy to observe.
[0068] Comparative Example 1
[0069] Replace the etching solution with Keller's reagent. Cut a Φ10mm×15mm 5xxx series aluminum alloy sample. Heat-mount the 5xxx series aluminum alloy sample using phenolic resin powder in a mounting machine at 135℃ for 5 minutes. After heat mounting, polish the 5xxx series aluminum alloy sample sequentially with 400, 600, 800, 1000, 1500, 2000, 3000, and 5000 grit sandpaper. Then, coarsely polish with silk polishing cloth and 1.5μm diamond polishing spray, continuously adding a small amount of water. Finally, finely polish with 0.5μm diamond polishing paste until the sample surface is free of scratches. Place the sample in a beaker containing anhydrous ethanol and ultrasonically clean for 1-2 minutes, then dry with a hair dryer. Pour an appropriate amount of Keller's reagent into a container, completely immerse the sample in the etching solution for 30 seconds, then immediately rinse with distilled water, followed by cleaning with anhydrous ethanol, and finally dry with a hair dryer.
[0070] The sample obtained by etching using the above method was observed under a microscope, and the observed metallographic structure was as follows. Figure 4 As shown, although some precipitates of aluminum alloy were etched out, there was no sign of grain boundaries appearing, indicating poor corrosion performance.
[0071] Comparative Example 2
[0072] Replace the etching solution with Keller's reagent. Cut a Φ10mm×15mm 7xxx series aluminum alloy sample. Heat-mount the 7xxx series aluminum alloy sample using phenolic resin powder in a mounting machine at 135℃ for 5 minutes. After heat mounting, polish the 7xxx series aluminum alloy sample sequentially with 400, 600, 800, 1000, 1500, 2000, 3000, and 5000 grit sandpaper. Then, coarsely polish with silk polishing cloth and 1.5μm diamond polishing spray, continuously adding a small amount of water. Finally, finely polish with 0.5μm diamond polishing paste until the sample surface is free of scratches. Place the sample in a beaker containing anhydrous ethanol and ultrasonically clean for 1-2 minutes, then dry with a hair dryer. Pour an appropriate amount of Keller's reagent into a container, completely immerse the sample in the etching solution for 30 seconds, then immediately rinse with distilled water, followed by cleaning with anhydrous ethanol, and dry with a hair dryer.
[0073] The sample obtained by etching using the above method was observed under a microscope, and the observed metallographic structure was as follows. Figure 5 As shown, although some precipitates of aluminum alloy were etched out, there was no sign of grain boundaries appearing, indicating poor corrosion performance.
[0074] Comparative Example 3
[0075] The etching solution was replaced with 40% phosphoric acid reagent. A Φ10mm × 15mm 5xxx series aluminum alloy sample was cut and hot-mounted using phenolic resin powder in a mounting machine at 135℃ for 5 minutes. After hot mounting, the 5xxx series aluminum alloy sample was successively polished with 400-grit, 600-grit, 800-grit, 1000-grit, 1500-grit, 2000-grit, 3000-grit, and 5000-grit sandpaper. Finally, coarse polishing was performed using silk polishing cloth and 1.5μm diamond polishing spray. Continue to add a small amount of water, then polish with 0.5μm diamond polishing paste until the sample surface is free of scratches; place the sample in a beaker containing anhydrous ethanol, perform ultrasonic cleaning for 1-2 minutes and dry with a hair dryer; pour an appropriate amount of 40% phosphoric acid reagent into a container, place it in a constant temperature water bath, heat and stabilize at 50℃, completely immerse the sample in the preheated etching solution for 300s, then remove it and immediately rinse with distilled water, then clean with anhydrous ethanol, and dry with a hair dryer;
[0076] The sample obtained by etching using the above method was observed under a microscope, and the observed metallographic structure was as follows. Figure 6 As shown, although some precipitates of aluminum alloy were etched out, there was no sign of grain boundaries appearing, indicating poor corrosion performance.
[0077] Contents not described in detail in this specification are prior art known to those skilled in the art. Although illustrative specific embodiments of the invention have been described above to facilitate understanding by those skilled in the art, it should be understood that the invention is not limited to the scope of the specific embodiments. Various modifications are readily apparent to those skilled in the art as long as they fall within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of this invention are protected.
Claims
1. A grain boundary etching solution for 5xxx and 7xxx series aluminum alloys, characterized in that: It consists of the following substances: 5-10 mL of 85% nitric acid, 1-2 mL of 50% hydrofluoric acid, 10-15 mL of 85% hydrochloric acid, 2-3 g of ferric chloride, 3-6 g of chromic anhydride, and distilled water to make up to 100 mL.
2. A method for etching 5xxx and 7xxx series aluminum alloys using the grain boundary etching solution as described in claim 1, characterized in that: Includes the following steps: Step 1: Prepare the etching solution; Step 2: Cut out aluminum alloy samples and embed them. Remove the embedded samples after they have cooled and solidified. Step 3: Polish the inlaid sample with sandpaper; Step 4: Polish the ground sample using a grinding and polishing machine; Step 5: Place the sample in a beaker containing anhydrous ethanol, clean it with ultrasound, and then blow it dry. Step 6: Pour 20 mL of etching solution into a container, add distilled water to dilute to 100 mL, completely immerse the sample in the diluted etching solution for pre-etching, then remove it and immediately rinse with distilled water, then clean with anhydrous ethanol and blow dry. Step 7: Pour an appropriate amount of etching solution into a container, place it in a constant temperature water bath, heat and stabilize it at 40~60℃, completely immerse the pre-etched sample in the preheated etching solution, then take it out and rinse it with distilled water immediately, then clean it with anhydrous ethanol and blow it dry. Step 8: Place the etched metallographic sample under a metallographic microscope to observe the metallographic structure.
3. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: Prepare a corrosion solution by adding 5-10 mL of 85% nitric acid, 1-2 mL of 50% hydrofluoric acid, 10-15 mL of 85% hydrochloric acid, 2-3 g of ferric chloride, 3-6 g of chromic anhydride, and distilled water to a final volume of 100 mL.
4. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 3, characterized in that: After weighing out a certain amount of ferric chloride and chromic anhydride, add distilled water, hydrochloric acid, nitric acid, and hydrofluoric acid in the formula ratio in sequence.
5. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: The inlaying in step 2 is performed by hot inlaying with phenolic resin powder at a temperature of 135°C for 5 minutes.
6. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: In step 3, sandpaper is used for polishing, specifically 400 grit, 600 grit, 800 grit, 1000 grit, 1500 grit, 2000 grit, 3000 grit, and 5000 grit sandpaper are used in sequence.
7. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: In step 4, polishing is specifically performed by using silk polishing cloth and diamond polishing spray with a particle size of 1.5μm for coarse polishing, and continuously adding a small amount of water until there are no scratches on the sample surface. Then, fine polishing is performed using diamond polishing paste with a particle size of 0.5μm.
8. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: The ultrasonic cleaning time in step 5 is 60s~120s.
9. The method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: In step 6, the sample needs to be completely immersed in the preheated corrosive liquid for 10 to 20 seconds.
10. A method for etching 5xxx and 7xxx series aluminum alloys with a grain boundary etching solution according to claim 2, characterized in that: In step 7, the pre-corroded sample needs to be completely immersed in the preheated corrosion solution for 60s~120s.