Magnesium-based metallographic corrosion liquid and metallographic preparation method

Abstract

The application discloses a magnesium-based metallographic corrosion liquid and a preparation method of a magnesium-based metallographic sample. The application first proposes the use of hydrofluoric acid as a single active component in the field of magnesium-based metallographic corrosion. The application further provides a magnesium-based metallographic corrosion liquid which is composed of hydrofluoric acid and water. The application also provides a preparation method of a magnesium-based metallographic sample, which comprises the following steps: after polishing a magnesium-based metal, the magnesium-based metal is soaked in the metallographic corrosion liquid. The application makes the magnesium react slowly with the hydrofluoric acid by soaking the magnesium-based metal in the metallographic corrosion liquid containing the hydrofluoric acid, so that a magnesium fluoride conversion film is formed on the surface of the magnesium-based metal; the conversion film has obvious contrast inside the magnesium grains and at the grain boundaries, so that the quality of the metallographic sample is higher, the consistency and repeatability of the metallographic sample are good, the stability of the conversion film is high, and the prepared metallographic sample can be stored in air for a long time.

Description

Technical Field

[0001] This invention belongs to the field of metallographic preparation technology, specifically relating to a magnesium-based metallographic etching solution and a method for preparing magnesium-based metallographic structures. Background Technology

[0002] Magnesium-based metals are widely used in aerospace, automotive manufacturing, electronic equipment, and implantable medical devices due to their low density and high specific strength. The microstructure of magnesium-based metals directly affects their properties; therefore, metallographic analysis is a crucial step in evaluating and improving the properties of magnesium-based metals.

[0003] Currently, most metallographic etchants used for magnesium-based metals are acidic solutions, such as oxalic acid and water mixtures (CN114371057A), nitric acid and alcohol or water mixtures (CN107976443A), picric acid and glacial acetic acid mixtures (CN106400018A), lactic acid and water mixtures (CN107976443A), phosphoric acid and alcohol mixtures (CN116337566A), citric acid and alcohol or water mixtures (CN105925983A, CN104593774A), etc. Although these etchants can reveal certain microstructures, the etching rate is relatively fast (usually completed within 20 seconds), requiring high operational precision; otherwise, over-etching can easily occur, affecting the quality of the obtained metallographic samples. Moreover, due to the short time, the consistency and reproducibility of the prepared metallographic samples are poor, and the analytical results of metallographic samples prepared in different batches are prone to inconsistencies. In addition, because magnesium-based metals are relatively reactive, the prepared metallographic samples have a short storage time in air, resulting in low utilization and increased sample preparation costs. Summary of the Invention

[0004] The present invention aims to provide a novel magnesium-based metallographic etchant and its application. This etchant can form a magnesium fluoride conversion film on the surface of magnesium-based metals immersed in it, thereby improving the metallographic quality of magnesium-based metal samples, with good consistency and reproducibility, and making them easy to preserve for a long time. It can solve the problems currently faced by magnesium-based metal etchants.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides the use of hydrofluoric acid as a single active ingredient in the field of metallographic corrosion of magnesium-based metals.

[0007] Secondly, the present invention provides a magnesium-based metallographic etching solution, which is composed of hydrofluoric acid and water; the concentration of the hydrofluoric acid is 10 mol / L-25 mol / L, preferably 15 mol / L-18 mol / L.

[0008] The magnesium-based metal includes pure magnesium metal and magnesium alloys. For example, the purity of the pure magnesium is ≥4N; the magnesium alloy may be selected from commercially available AZ31 magnesium alloy.

[0009] According to an embodiment of the present invention, the magnesium-based metal is pure magnesium metal, and the concentration of hydrofluoric acid in the metallographic etching solution is 15 mol / L-18 mol / L.

[0010] According to an embodiment of the present invention, the magnesium-based metal is a magnesium alloy, and the concentration of hydrofluoric acid in the metallographic etching solution is 15 mol / L-16 mol / L.

[0011] Secondly, the present invention further improves a method for preparing a magnesium-based metallographic structure, comprising the following steps: immersing the polished magnesium-based metal in the metallographic etching solution.

[0012] The polishing method includes mechanical polishing and electrolytic polishing; the polishing makes the magnesium-based metal surface achieve a mirror finish.

[0013] The soaking time is 0.5h-24h, preferably 2h-24h.

[0014] The preparation method further includes: cleaning and drying the soaked magnesium-based metal. The cleaning solution used for cleaning can be distilled water or ethanol.

[0015] According to an embodiment of the present invention, the magnesium-based metal is pure magnesium metal, and its metallographic preparation method is carried out according to the following steps: the polished pure magnesium metal is immersed in a hydrofluoric acid solution with a concentration of 15mol / L-18mol / L for 2h-24h, and then taken out, washed with distilled water and dried.

[0016] According to an embodiment of the present invention, the magnesium-based metal is a magnesium alloy, and the metallographic preparation method is carried out according to the following steps: the polished magnesium alloy is immersed in a hydrofluoric acid solution with a concentration of 15mol / L-16mol / L for 2h-24h, and then taken out, washed with distilled water and dried.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] This invention is the first in the field of magnesium-based metals to propose a method for preparing magnesium-based metal metallographic images using conversion films. Specifically, magnesium-based metal is immersed in a metallographic etching solution containing hydrofluoric acid, causing the magnesium to react with the hydrofluoric acid and forming a magnesium fluoride conversion film with a thickness not exceeding 3 μm on the surface of the magnesium-based metal. This conversion film exhibits significant contrast within the magnesium grains and at grain boundaries, resulting in higher quality metallographic samples. Furthermore, the very slow formation rate of the magnesium fluoride conversion film ensures the consistency and repeatability of the metallographic samples. In addition, due to the high stability of the magnesium fluoride conversion film, the prepared metallographic samples are less susceptible to environmental influences and can be stored in air for extended periods. Attached Figure Description

[0019] Figure 1 Metallographic photograph of Example 1;

[0020] Figure 2 Metallographic photograph of Example 2;

[0021] Figure 3 Metallographic photograph of Comparative Example 1;

[0022] Figure 4 Metallographic photograph of Comparative Example 2;

[0023] Figure 5 Metallographic photograph of Comparative Example 3;

[0024] Figure 6 Metallographic photograph of Comparative Example 4;

[0025] Figure 7 Metallographic photographs of the metallographic sample prepared in Example 1 after being stored in air for six months. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0027] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0028] Unless otherwise specified, all reagents, materials, instruments, etc. used in the following examples are commercially available.

[0029] Example 1

[0030] The metallographic etching solution is formulated as follows: a hydrofluoric acid solution with a concentration of 18 mol / L.

[0031] The metallographic preparation steps are as follows:

[0032] The pure magnesium sample with a purity ≥4N was electrolytically polished to achieve a mirror finish. The polished pure magnesium sample was then immersed in the obtained metallographic etching solution for 2 hours, followed by washing with distilled water and drying with a hair dryer.

[0033] Example 2

[0034] The metallographic etching solution is formulated as follows: a hydrofluoric acid solution with a concentration of 15 mol / L.

[0035] The metallographic preparation steps are as follows:

[0036] Commercial AZ31 magnesium alloy samples were electrolytically polished to achieve a mirror finish; the polished pure magnesium samples were then immersed in the prepared metallographic etching solution for 30 minutes, followed by rinsing with distilled water and drying with a hair dryer.

[0037] Comparative Example 1

[0038] The metallographic etching solution is formulated as follows: a hydrofluoric acid solution with a concentration of 26 mol / L.

[0039] The metallographic preparation steps are as follows:

[0040] The pure magnesium sample with a purity ≥4N was electrolytically polished to achieve a mirror finish. The polished pure magnesium sample was then immersed in the obtained metallographic etching solution for 4 hours, followed by washing with distilled water and drying with a hair dryer.

[0041] Comparative Example 2

[0042] The metallographic etching solution is formulated as follows: a hydrofluoric acid solution with a concentration of 9 mol / L.

[0043] The pure magnesium sample with a purity ≥4N was electrolytically polished to achieve a mirror finish. The polished pure magnesium sample was then immersed in the obtained metallographic etching solution for 4 hours, followed by washing with distilled water and drying with a hair dryer.

[0044] Comparative Example 3

[0045] The difference from Example 1 is that the soaking time is 30 hours.

[0046] Comparative Example 4

[0047] The difference from Example 1 is that the soaking time is 40 hours.

[0048] Effect verification

[0049] 1. Thickness and metallographic microstructure of magnesium fluoride conversion film

[0050] The thickness of the magnesium fluoride conversion film obtained in Example 1 was 3 μm, the thickness of the magnesium fluoride conversion film obtained in Example 2 was 2 μm, the thickness of the conversion film obtained in Comparative Example 3 was 8 μm, and the thickness of the conversion film obtained in Comparative Example 4 was 10 μm.

[0051] like Figure 1 , Figure 2 As shown, within 24 hours of soaking, the magnesium fluoride conversion films prepared in Examples 1 and 2 exhibited significant contrast within the magnesium grains and at the grain boundaries, indicating that higher quality metallographic samples were obtained.

[0052] like Figure 3 As shown, after the sample was immersed in 26 mol / L hydrofluoric acid solution for 4 hours, cracks appeared on the surface, and there were no obvious grain boundaries in the parts where no cracks were formed. This indicates that the concentration of the etchant was too high, and the contrast of the conversion film would gradually disappear inside the magnesium grains and at the grain boundaries. In addition, cracks formed in the conversion coating, which did not meet the conditions for metallographic imaging.

[0053] like Figure 4As shown, after the sample was immersed in 9 mol / L hydrofluoric acid solution for 4 hours, a stable magnesium fluoride film was not formed, and the area where the magnesium fluoride film was formed also did not have obvious grain boundaries. This indicates that the concentration of the etching solution was too low, making it difficult to form a continuous magnesium fluoride conversion film and thus not meeting the conditions for metallographic imaging.

[0054] like Figure 5 , Figure 6 As shown, after prolonged corrosion, the contrast of the conversion film gradually disappears inside the magnesium grains and at the grain boundaries, and cracks form in the conversion coating, making metallographic imaging impossible.

[0055] 2. Consistency and repeatability: Due to the relatively slow formation rate of the magnesium fluoride conversion film and the wide concentration window of hydrofluoric acid in the etching solution, the consistency and repeatability of the metallographic structure can be guaranteed.

[0056] 3. Storage stability: such as Figure 7 As shown, after being stored in air for six months, the grain boundary contrast of the metallographic sample was still obvious, and the grain interior was not corroded. This indicates that the method of preparing magnesium-based metallographic samples using conversion films proposed in this invention can significantly extend the storage time of the metallographic sample, greatly improve the utilization rate of the metallographic sample, and reduce the cost of metallographic sample preparation.

[0057] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. The use of hydrofluoric acid as a single active ingredient in the preparation of magnesium-based metallographic specimens, characterized in that: The magnesium-based metal is pure magnesium, and the uses include: The polished pure magnesium was immersed in a metallographic etching solution composed of hydrofluoric acid and water for 2 hours. The concentration of hydrofluoric acid in the metallographic etching solution was 18 mol / L, so that a magnesium fluoride conversion film with a thickness of no more than 3 μm was formed on the surface of the pure magnesium. The conversion film had obvious contrast inside the magnesium grains and at the grain boundaries.

2. A method for preparing a pure magnesium metal metallographic sample, characterized in that, Includes the following steps: The pure magnesium metal is polished to achieve a mirror finish on its surface. The polished pure magnesium metal was immersed in a metallographic etching solution composed of hydrofluoric acid and water for 2 hours. The concentration of hydrofluoric acid in the metallographic etching solution was 18 mol / L, so that a magnesium fluoride conversion film with a thickness of no more than 3 μm was formed on the surface of the pure magnesium metal. The magnesium fluoride conversion film formed obvious contrast inside the pure magnesium grains and at the grain boundaries. The soaked pure magnesium metal was removed, washed with distilled water and dried to obtain the pure magnesium metal metallographic sample.

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