Preparation method of magnesium alloy conversion film and magnesium alloy conversion film

Abstract

The application relates to the technical field of material surface protection, in particular to a magnesium alloy conversion film preparation method and magnesium alloy conversion film; by adopting environment-friendly phytic acid as a main component of a conversion solution, the oxidizability of the phytic acid is used to oxidize metal elements on the surface of the magnesium alloy, metal ions after being oxidized are combined with phytic acid root ions to form a phytic acid conversion film, and the phytic acid conversion film is covered on the surface of a magnesium alloy base body and a second phase; compared with a phosphoric acid conversion film, the number of microcracks on the surface of the phytic acid conversion film is obviously less, the film layer continuously covers the surface of the magnesium alloy base body and the second phase, and the surface of the magnesium alloy base body under the covered area of the phytic acid conversion film remains polishing scratches, which indicates that the erosion of the phytic acid conversion solution to the magnesium alloy base body is weak, the phytic acid conversion film can effectively improve the corrosion resistance of the magnesium alloy base body, and can also improve the bonding force with a subsequent material layer.

Description

Technical Field

[0001] This invention relates to the field of material surface protection technology, and in particular to a method for preparing a magnesium alloy conversion film and the magnesium alloy conversion film thereof. Background Technology

[0002] Magnesium alloys are the lowest density metallic structural materials, possessing high specific stiffness / specific strength, good electromagnetic shielding properties, and biocompatibility. These advantages give magnesium alloys enormous application potential in fields such as automobiles, biomedicine, and 3C electronics. However, magnesium alloys have a standard electrode potential of only -2.37 V, are chemically very reactive, and cannot form a dense oxide film. They are easily oxidized in humid environments, and their poor corrosion resistance has become a bottleneck problem hindering the application of magnesium alloys.

[0003] Chemical conversion coating technology can effectively isolate the substrate from corrosive media by constructing a dense, stable, and firmly bonded protective coating or conversion film on the surface of magnesium alloys. Traditional chemical conversion coating processes primarily use chromate conversion films, which form amorphous films with excellent physical barrier properties and self-healing capabilities. However, the Cr6+ ions in chromate conversion films pose a significant threat to the ecological environment and human health. Therefore, to address the aforementioned problems with existing magnesium alloy surface protection technologies, there is an urgent need to find efficient and environmentally friendly chromium-free alternatives to improve the performance of magnesium alloy conversion films. Summary of the Invention

[0004] To overcome the shortcomings mentioned above, the present invention aims to provide a technical solution that can solve the above problems.

[0005] A method for preparing a magnesium alloy conversion coating includes the following steps: S1. Pretreatment: The magnesium alloy was polished sequentially with 400-grit, 600-grit, 800-grit, and 1200-grit wet sandpaper, then ultrasonically degreased in ethanol for 10 minutes, dried, and placed in a desiccator for later use. S2. Preparation of phytic acid conversion solution: Prepare a phytic acid conversion solution containing: 1–15 g / L phytic acid, 1–10 g / L calcium salt, and 0.1–1 g / L surfactant; adjust the pH to 2–5 using 0.1 mol / L nitric acid and 0.1 mol / L sodium hydroxide solution; S3. Conversion treatment: Immerse the magnesium alloy sample treated in step S1 in the phytic acid conversion solution prepared in step S2 at a temperature of 15–50℃ for 1–10 minutes. S4. Post-treatment: After conversion, wash with distilled water and dry at 120℃ for 20 minutes to obtain magnesium alloy conversion film.

[0006] Preferably, the calcium salt in step S2 is one or more of calcium dihydrogen phosphate, calcium nitrate, calcium acetate, or calcium lactate.

[0007] Preferably, the calcium salt is calcium nitrate, and the dosage is 2–5 g / L.

[0008] Preferably, the phytic acid conversion solution in step S2 further comprises one or more of manganese salt, complexing agent, and additive; the manganese salt is one or more of manganese nitrate, manganese sulfate, or manganese acetate; the complexing agent is one or more of citric acid, acetic acid, or EDTA-2Na; and the additive is ammonium metavanadate, sodium molybdate, or a mixture of both.

[0009] Preferably, in step S2, the pH is adjusted to 2.5; in step S3, the treatment temperature is 25±2℃ and the soaking time is 2–10 minutes.

[0010] Preferably, the phytic acid conversion solution in step S2 is formulated as follows: 10 g / L phytic acid, 2 g / L calcium nitrate, 0.1 g / L surfactant, and pH adjusted to 2.5; the treatment temperature in step S3 is 25±2℃, and the soaking time is 2–10 minutes.

[0011] Preferably, the magnesium alloy is AZ91D magnesium alloy, and its elemental composition and mass percentage are: Al 8.99%, Zn 0.66%, Mn 0.22%, impurities ≤0.01%, and the remainder is Mg.

[0012] Preferably, the method further includes steps of compositional and performance testing of the magnesium alloy conversion film obtained in step S4. Elemental analysis of the obtained magnesium alloy conversion film was performed using EDS. When the content of P in the magnesium matrix of the magnesium alloy conversion film is 1–5 at.%, the content of C is 10–15 at.%, the content of O is 10–20 at.%, and the content of Ca is 0.1–1.5 at.%, the obtained magnesium alloy conversion film meets the application requirements. The obtained magnesium alloy conversion film was tested using neutral salt spray and electrochemical tests. After 12 hours of neutral salt spray testing, the percentage of the corroded area was 0, and the self-corrosion current density was ≤5.94×10⁻⁶. -6 A·cm - ², and after soaking in 3.5 wt.% NaCl solution for 12 hours, the membrane resistance Rf ≥ 4.8 × 10⁻⁶. 4 When the temperature reaches Ω·cm², the resulting magnesium alloy conversion coating meets the usage requirements.

[0013] A magnesium alloy conversion film is prepared by the above-described method for preparing a magnesium alloy conversion film. The magnesium alloy conversion film is a phytic acid conversion film formed on the surface of a magnesium alloy substrate. The phytic acid conversion film continuously covers the magnesium alloy substrate and the surface of the second phase, and the surface of the magnesium alloy substrate below the area covered by the phytic acid conversion film retains grinding scratches.

[0014] Preferably, the Mg, Al, and Zn elements in the phytic acid conversion membrane originate from the magnesium alloy matrix, and the O, C, P, and Ca elements originate from the phytic acid conversion membrane, and the O and P element content in the second phase is lower than that in the magnesium matrix.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This technical solution uses environmentally friendly phytic acid as the main component of the conversion solution. The oxidizing properties of phytic acid oxidize the metal elements on the surface of the magnesium alloy, causing the oxidized metal ions to combine with phytate ions to form a phytic acid conversion film that covers the magnesium alloy substrate and the second phase surface. Compared with phosphoric acid conversion films, this phytic acid conversion film has significantly fewer microcracks on its surface, and the film layer continuously covers the magnesium alloy substrate and the second phase surface. Furthermore, the magnesium alloy substrate surface below the phytic acid conversion film area retains grinding scratches, indicating that the phytic acid conversion solution has a weaker corrosive effect on the magnesium alloy substrate. After 12 hours of neutral salt spray testing, the percentage of corrosion area on the surface of the phytic acid conversion film sample was 0, and the self-corrosion current density was only 5.94 × 10⁻⁶. -6 A·cm - The phytic acid conversion membrane (HFC) content was 23.1% of that of the AZ91D magnesium alloy substrate. After immersion in 3.5 wt.% NaCl solution for 12 hours, the membrane resistance (Rf) reached 48629 Ω·cm², significantly higher than that of the phosphate conversion membrane and the AZ91D magnesium alloy substrate. Furthermore, the membrane resistance (Rf) continued to increase with prolonged immersion time, indicating that the phytic acid conversion membrane can effectively improve the corrosion resistance of the magnesium alloy substrate and also enhance the adhesion to subsequent material layers. In addition, all reagents used in this technical solution are environmentally friendly and Cr-free. 6+ It has low preparation cost, is easy to operate, and is suitable for industrial promotion.

[0016] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a morphology diagram of the AZ91D magnesium alloy matrix of the present invention; Figure 2 This is a microscopic morphology image of the conversion membrane sample of the present invention; Figure 3These are macroscopic morphology images of AZ91D magnesium alloy, phosphate conversion membrane (PCC), and phytic acid conversion membrane (PA) samples after 12 hours of neutral salt spray testing according to this invention. Figure 4 These are the polarization curves of the phosphate conversion membrane and the phytic acid conversion membrane of the present invention; Figure 5 This is the EIS spectrum plus equivalent circuit diagram of the present invention; Figure 6 This is a graph showing the change of membrane resistance Rf of different samples of the present invention with immersion time. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020]

Example 1

[0021] Comparative Example 1 The preparation of a phosphoric acid conversion membrane, using the same magnesium alloy as in Example 1, includes the following steps: S1. Pretreatment: Same as in Example 1; S2. Preparation of phosphoric acid conversion solution: Prepare phosphoric acid conversion solution, wherein the phosphoric acid conversion solution comprises: 7.5 ml / L phosphoric acid, 20 g / L disodium hydrogen phosphate, 2 g / L calcium nitrate, and 0.1 g / L surfactant; adjust the pH to 2.5 using 0.1 mol / L nitric acid and 0.1 mol / L sodium hydroxide solution; S3. Conversion treatment: Immerse the AZ91D magnesium alloy sample treated in step S1 in the phosphoric acid conversion solution prepared in step S2. The treatment temperature is 25±2℃ and the immersion time is 2–10 minutes. S4. Post-processing: After conversion, wash with distilled water and dry at 120℃ for 20 min to obtain phosphate conversion membrane sample, labeled as PCC.

[0022]

Example 2

[0023]

Example 3

[0024]

Example 4

[0025] The magnesium alloy conversion coating samples prepared in Examples 1–4 and Comparative Example 1 were subjected to the following tests: 1. Surface Morphology and Elemental Analysis: Surface morphology and elemental analysis were performed on AZ91D magnesium alloy, the magnesium alloy conversion film (PA) prepared in Example 1, and the phosphate conversion film (PCC) prepared in Comparative Example 1 using a scanning electron microscope equipped with EDS. The results are shown in the figure below. Figure 1 , Figure 2 Tables 1 and 2.

[0026] 2. Corrosion Resistance Testing: The corrosion resistance of AZ91D magnesium alloy, the magnesium alloy conversion film (PA) prepared in Example 1, and the phosphate conversion film (PCC) prepared in Comparative Example 1 were tested using neutral salt spray and electrochemical tests. The results are shown in [Figure 1]. Figure 3 , Figure 4 , Figure 5 , Figure 6 Tables 3, 4, 5 and 6.

[0027] Specifically as follows: The surface morphology and elemental analysis of the magnesium alloy conversion film samples prepared in Examples 1–4 and Comparative Example 1 were performed: like Figure 1 The image shows the matrix morphology of AZ91D magnesium alloy, in which... Figure 1 (a) shows the morphology at low magnification. Figure 1 (b) is Figure 1 (a) High magnification morphology of region A in the middle.

[0028] Depend on Figure 1 (a) As can be seen, a large number of second phases are distributed on the surface of the AZ91D magnesium alloy. These second phases are irregularly distributed in blocky and strip-like shapes on the magnesium matrix. Figure 1 (b) High magnification morphology shows that the boundary between the second phase and the magnesium matrix is ​​clear in region A, and the size of the second phase is about a few micrometers to a dozen micrometers.

[0029] like Figure 2 The figure shows the microstructure of the conversion membrane sample, in which... Figure 2 (a) shows the low-magnification morphology of the phosphate conversion membrane sample. Figure 2 (b) is Figure 2 (a) High-magnification morphology of region B in the middle; Figure 2 (c) shows the low-magnification morphology of the phytic acid conversion membrane sample. Figure 2 (d) is Figure 2 (c) High magnification morphology of region C.

[0030] Depend on Figure 2 (a) As can be seen, after treatment with phosphate conversion solution, both the magnesium matrix and the surface of the second phase of the AZ91D magnesium alloy are covered with a phosphate conversion film, and there are no obvious scratches on the surface of the magnesium matrix. Figure 2 (b) High magnification morphology shows that there are a large number of microcracks on the surface of the phosphate conversion film and at the interface between the magnesium matrix and the second phase.

[0031] Depend on Figure 2 (c) As can be seen, after treatment with phytic acid conversion solution, the AZ91D magnesium alloy surface is covered with a phytic acid conversion film. Grinding scratches remain on the magnesium alloy substrate surface below the area covered by the phytic acid conversion film, indicating that the phytic acid conversion solution has a weak corrosive effect on the magnesium alloy substrate. Figure 2 (d) High magnification morphology shows that the number of microcracks on the surface of the phytic acid conversion film is significantly less than that of the phosphate conversion film. The phytic acid conversion film continuously covers the magnesium alloy substrate and the surface of the second phase.

[0032] Refer to Table 1: Table 1: EDS test results of AZ91D magnesium alloy matrix (at.%) As shown in Table 1, the AZ91D magnesium alloy matrix mainly contains Mg, Al, Zn and Mn elements, with Mg content ranging from 92.2 to 63.6 at.%, Al content from 7.5 to 35.1 at.%, Zn content from 0.1 to 1.2 at.%, and Mn content from 0 to 1.7 at.%.

[0033] Refer to Table 2: Table 2: EDS test results (at.%) of different regions of the conversion membrane sample Positions 1 and 2 show the EDS results of the phosphate conversion membrane sample in the magnesium matrix and the second phase, respectively, while positions 3 and 4 show the EDS results of the phytic acid conversion membrane sample in the magnesium matrix and the second phase, respectively.

[0034] As shown in Table 2, both conversion coating samples contain Mg, Al, O, C, Zn, P, and Ca elements. Among them, Mg, Al, and Zn elements come from the AZ91D magnesium alloy matrix, while O, C, P, and Ca elements come from the conversion coating.

[0035] Location 1 (phosphate conversion membrane, magnesium matrix): Mg 66.1 at.%, Al 6.1 at.%, O 18.6 at.%, C 3.1 at.%, Zn 0.3 at.%, P 4.6 at.%, Ca 1.2 at.%.

[0036] Location 2 (phosphate conversion membrane, second phase): Mg 69.9 at.%, Al 24 at.%, O 2.6 at.%, C 2.3 at.%, P 1.0 at.%, Ca 0.2 at.%.

[0037] Location 3 (phytic acid conversion membrane, magnesium matrix): Mg 68.7 at.%, Al 3.7 at.%, O 14.5 at.%, C 11.6 at.%, P 1.4 at.%, Ca 0.1 at.%.

[0038] Location 4 (phytic acid conversion membrane, second phase): Mg 50.8 at.%, Al 31.2 at.%, O 4.7 at.%, C 12.6 at.%, Zn 0.6 at.%, P 0.2 at.%.

[0039] As shown in Table 2, the O and P content in the second phase of both conversion membrane samples was significantly lower than that in the magnesium matrix. The P content in the phosphate conversion membrane sample was slightly higher than that in the phytic acid conversion membrane sample, while the C content was significantly lower. The higher C content in the phytic acid conversion membrane indicates that it contains a greater amount of organic phytate.

[0040] The corrosion resistance of the magnesium alloy conversion coating samples prepared in Examples 1–4 and Comparative Example 1 was tested. 1. Neutral salt spray test: The neutral salt spray test is conducted in a dedicated neutral salt spray chamber according to GB / T10125—2021. The solution in the chamber is a 5% NaCl solution, the pH is maintained at 6.5~7.2, the temperature in the salt spray chamber is set at 35℃, and the sample is left for 12 hours. After that, the sample is taken out, cleaned and dried, and the macroscopic morphology of the sample is recorded using a stereomicroscope.

[0041] like Figure 3 The image shows the macroscopic morphology of the AZ91D magnesium alloy, phosphate conversion coating (PCC), and phytic acid conversion coating (PA) samples after 12 hours of neutral salt spray testing. Figure 3 (a) is AZ91D magnesium alloy. Figure 3 (b) represents PCC. Figure 3(c) is PA.

[0042] Depend on Figure 3 (a) As can be seen, numerous black corrosion areas appear on the surface of the AZ91D magnesium alloy. Figure 3 (b) As can be seen, a small number of black corrosion areas appear on the surface of the PCC sample. Figure 3 (c) It can be seen that there are no obvious corrosion marks on the surface of the PA sample.

[0043] Refer to Table 3: Table 3 Percentage of corroded area on sample surface after salt spray test Table 3 shows the percentage of corrosion area on the sample surface after the salt spray test. As can be seen from Table 3, the corrosion area on the surface of the AZ91D magnesium alloy substrate reached 3.57%, the corrosion area on the surface of the phosphate conversion film sample reached 1.31%, while the corrosion area on the surface of the phytic acid conversion film sample was 0.

[0044] This phenomenon indicates that the corrosion resistance of the AZ91D magnesium alloy substrate is significantly improved after conversion coating treatment, and the corrosion resistance of the phytic acid conversion coating sample is better than that of the phosphate conversion coating sample.

[0045] 2. Polarization curve testing, such as Figure 4 The figure shows the polarization curves of the phosphate conversion membrane and the phytic acid conversion membrane. Figure 4 As can be seen, compared with the AZ91D magnesium alloy sample, the polarization curves of both conversion film samples shift significantly to the lower left. Simultaneously, the current density in the anodic region increases rapidly after exceeding a certain potential value (Ept), indicating the occurrence of pitting corrosion. Since pitting corrosion results in the absence of a clear Tafel region in the anodic area, extrapolation cannot be used to obtain anodic polarization data; therefore, only the cathode region is fitted.

[0046] Refer to Table 4: Table 4. Fitting results of the cathode polarization curves of the samples As shown in Table 4, the self-corrosion current density of AZ91D magnesium alloy is 2.57 × 10⁻⁶. -5 A·cm - ², the phosphate conversion membrane sample decreased to 8.36 × 10⁻⁶. -6 A·cm - ², which is 32.5% of AZ91D magnesium alloy, and the phytic acid conversion film sample further reduced to 5.94×10. -6 A·cm - ², which is only 23.1% of that of AZ91D magnesium alloy.

[0047] After treatment with the conversion coating, the corrosion rate of all AZ91D samples decreased, with the corrosion rate of the phytic acid conversion coating sample being lower than that of the phosphate conversion coating sample.

[0048] 3. Electrochemical Impedance Spectroscopy (EIS) was performed using a standard three-electrode system. The working electrode was the sample under test, with an exposure area of ​​1 cm². A silver / silver chloride electrode (Ag / AgCl) served as the reference electrode, and a platinum electrode as the auxiliary electrode. The electrolyte was a 3.5 wt.% NaCl solution, and the electrolyte temperature was maintained at 25 ± 2 °C during the measurement using a constant-temperature water bath. EIS measurements were conducted at open-circuit potential (OCP), with an AC amplitude set at ±10 mV vs OCP, and a test frequency range of 10 Hz. 5 Hz–10 - 2Hz.

[0049] like Figure 5 As shown, Figure 5 (a) Electrochemical impedance spectroscopy of AZ91D magnesium alloy after immersion in 3.5 wt.% NaCl solution for 3–12 h. Figure 5 (b) Electrochemical impedance spectroscopy of PCC after immersion in 3.5 wt.% NaCl solution for 3–12 h. Figure 5 (c) Electrochemical impedance spectroscopy of PA after soaking in 3.5 wt.% NaCl solution for 3–12 h. Figure 5 (d), (e), and (f) are respectively Figure 5 Equivalent circuit diagrams for (a), (b), and (c).

[0050] Depend on Figure 5 As shown in (a)–(c), the electrochemical impedance spectra of AZ91D magnesium alloy samples with different immersion times all contain three time constants: capacitive arc in the high-frequency region, capacitive arc in the mid-to-low-frequency region, and inductive arc in the low-frequency region. With increasing immersion time, the diameter of the capacitive arc in the AZ91D magnesium alloy samples continuously decreases. The diameter of the capacitive arc in the PCC sample first increases and then decreases with increasing immersion time. The diameter of the capacitive arc in the PA sample continuously increases with increasing immersion time.

[0051] use Figure 5 The equivalent circuits shown in (d), (e), and (f) are used to fit the electrochemical impedance spectroscopy data of the sample.

[0052] Figure 5 (d) shows the equivalent circuit of the AZ91D magnesium alloy substrate sample, where Rs is the solution resistance, Rf is the film resistance, CPEf represents the conversion film capacitance, including two parameters Qf and nf, Rt is the ion exchange resistance, CPEdl is a constant phase angle element representing the double layer capacitance of the substrate surface, including two parameters Qdl and ndl, and RL and L are respectively the Mg + Resistance and inductance in the ion adsorption-desorption process.

[0053] Figure 5 (e) shows the equivalent circuit of PCC sample soaked for 3–6 h and PA sample, where Rs is the solution resistance, Rf is the membrane resistance, CPEf represents the conversion membrane capacitance, which includes two parameters Qf and nf, Rt is the ion exchange resistance, and CPEdl is a constant phase angle element, representing the double layer capacitance of the substrate surface, which includes two parameters Qdl and ndl.

[0054] Figure 5 (f) is the equivalent circuit of PCC sample soaking for 12h, where Rs is the solution resistance, Rt is the ion exchange resistance, and CPEdl is a constant phase angle element representing the double layer capacitance of the substrate surface, including two parameters Qdl and ndl.

[0055] Refer to Table 5: Table 5. Fitting results of electrochemical impedance spectroscopy of AZ91D magnesium alloy matrix As shown in Table 5, the film resistance Rf of AZ91D magnesium alloy at 3h, 6h and 12h are 898Ω·cm², 479Ω·cm² and 328Ω·cm², respectively, and decrease continuously with the extension of immersion time.

[0056] Refer to Table 6: Table 6. Electrochemical impedance spectroscopy fitting results of phosphate conversion membrane and phytic acid conversion membrane samples. As can be seen from Table 6: PCC-3h: Rf = 12234 Ω·cm²; PCC-6h: Rf = 30472 Ω·cm²; PCC-12h: Rf = 15136Ω·cm²; PA-3h: Rf = 15978Ω·cm²; PA-6h: Rf = 37613 Ω·cm²; PA-12h: Rf=48629Ω·cm².

[0057] As shown in Table 6, the membrane resistance Rf of the PCC sample first increased and then decreased with prolonged immersion time, while the membrane resistance Rf of the PA sample continuously increased with prolonged immersion time. The membrane resistance values ​​of both conversion membrane samples were two orders of magnitude higher than those of the AZ91D magnesium alloy sample, and the membrane resistance of the PA sample was significantly higher than that of the PCC sample. The chi-square test value of the fitting results for each sample was less than 1 × 10⁻⁶. - ³ indicates that the fitting accuracy is high.

[0058] 4. The membrane resistance changes with immersion time, such as... Figure 6The figure shows the change in membrane resistance Rf of different samples with immersion time. Figure 6 visible: The film resistance Rf of AZ91D magnesium alloy decreases continuously with the extension of immersion time: 898 Ω·cm² after 3 hours, 479 Ω·cm² after 6 hours, and 328 Ω·cm² after 12 hours.

[0059] The membrane resistance Rf of the PCC sample showed a trend of first increasing and then decreasing with the extension of immersion time: 12234 Ω·cm² at 3h, 30472 Ω·cm² at 6h, and 15136 Ω·cm² at 12h.

[0060] The membrane resistance Rf of the PA sample continued to increase with the extension of soaking time, reaching 15978 Ω·cm² after 3 hours, 37613 Ω·cm² after 6 hours, and 48629 Ω·cm² after 12 hours.

[0061] The membrane resistivity of both conversion membrane samples was two orders of magnitude higher than that of the AZ91D magnesium alloy sample, and the membrane resistivity of the PA sample was significantly higher than that of the PCC sample. This indicates that both phosphate conversion membranes and phytic acid conversion membranes provided good protection, with the phytic acid conversion membrane offering superior protection compared to the phosphate conversion membrane.

[0062] Based on the above test results, the obtained magnesium alloy conversion coating can be subjected to composition and performance testing according to the following steps: 1. Composition analysis: Elemental analysis of the obtained magnesium alloy conversion film was performed using EDS. When the content of P element in the magnesium matrix of the magnesium alloy conversion film is 1–5 at.%, the content of C element is 10–15 at.%, the content of O element is 10–20 at.%, and the content of Ca element is 0.1–1.5 at.%, the obtained magnesium alloy conversion film meets the requirements for use.

[0063] 2. Performance Testing: The obtained magnesium alloy conversion film was tested using neutral salt spray and electrochemical tests. After 12 hours of neutral salt spray testing, the percentage of the corroded area was 0, and the self-corrosion current density was ≤5.94×10⁻⁶. -6 A·cm - ², and after soaking in 3.5wt.% NaCl solution for 12 hours, the membrane resistance Rf ≥ 4.8 × 10². 4 When the temperature reaches Ω·cm², the resulting magnesium alloy conversion coating meets the usage requirements.

[0064] Therefore, in terms of surface morphology, the number of microcracks on the phytic acid conversion film (PA) is significantly less than that on the phosphate conversion film (PCC), and the magnesium alloy substrate surface beneath the phytic acid conversion film retains grinding scratches, indicating that the phytic acid conversion solution has a weaker corrosive effect on the magnesium alloy substrate. Figure 2 As shown.

[0065] In terms of elemental composition, both conversion membranes contain Mg, Al, O, C, Zn, P, and Ca elements. Among them, Mg, Al, and Zn come from the magnesium alloy matrix, while O, C, P, and Ca come from the conversion membrane. The C element content in the phytic acid conversion membrane is significantly higher than that in the phosphate conversion membrane, indicating that the phytic acid conversion membrane contains more organic phytate components, as shown in Table 2.

[0066] In terms of salt spray resistance, after 12 hours of neutral salt spray, the corrosion area of ​​AZ91D magnesium alloy was 3.57%, PCC was 1.31%, and PA was 0. Figure 3 As shown in Table 3.

[0067] On the polarization curve, the self-corrosion current density of PA is 5.94 × 10⁻⁶. -6 A·cm - ², which is only 23.1% of AZ91D magnesium alloy and lower than PCC's 32.5%, such as Figure 4 As shown in Table 4.

[0068] In the electrochemical impedance spectroscopy, the membrane resistance Rf of PA continuously increased with the extension of immersion time, reaching 48629 Ω·cm² after 12 h, which was significantly higher than that of PCC (15136 Ω·cm²) and AZ91D (328 Ω·cm²). Figure 5-6 As shown in Table 5-6.

[0069] In summary, phytic acid conversion membranes can effectively improve the corrosion resistance of AZ91D magnesium alloys, and their protective performance is superior to that of phosphate conversion membranes.

[0070] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a magnesium alloy conversion coating, characterized in that, Includes the following steps: S1. Pretreatment: The magnesium alloy was polished sequentially with 400-grit, 600-grit, 800-grit, and 1200-grit wet sandpaper, then ultrasonically degreased in ethanol for 10 minutes, dried, and placed in a desiccator for later use. S2. Preparation of phytic acid conversion solution: Prepare a phytic acid conversion solution containing: 1–15 g / L phytic acid, 1–10 g / L calcium salt, and 0.1–1 g / L surfactant; adjust the pH to 2–5 using 0.1 mol / L nitric acid and 0.1 mol / L sodium hydroxide solution; S3. Conversion treatment: Immerse the magnesium alloy sample treated in step S1 in the phytic acid conversion solution prepared in step S2 at a temperature of 15–50℃ for 1–10 minutes. S4. Post-treatment: After conversion, wash with distilled water and dry at 120℃ for 20 minutes to obtain magnesium alloy conversion film.

2. The method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, In step S2, the calcium salt is one or more of calcium dihydrogen phosphate, calcium nitrate, calcium acetate, or calcium lactate.

3. The method for preparing a magnesium alloy conversion coating according to claim 2, characterized in that, The calcium salt is calcium nitrate, and the dosage is 2–5 g / L.

4. The method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, The phytic acid conversion solution in step S2 also contains one or more of manganese salt, complexing agent, and additive; the manganese salt is one or more of manganese nitrate, manganese sulfate, or manganese acetate; the complexing agent is one or more of citric acid, acetic acid, or EDTA-2Na; and the additive is ammonium metavanadate, sodium molybdate, or a mixture of both.

5. The method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, In step S2, the pH is adjusted to 2.5; in step S3, the treatment temperature is 25±2℃ and the soaking time is 2–10 minutes.

6. A method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, The phytic acid conversion solution in step S2 is formulated as follows: 10 g / L phytic acid, 2 g / L calcium nitrate, 0.1 g / L surfactant, and pH adjusted to 2.

5. The treatment temperature in step S3 is 25±2℃, and the soaking time is 2–10 minutes.

7. A method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, The magnesium alloy is AZ91D magnesium alloy, and its elemental composition and mass percentage are: Al 8.99%, Zn 0.66%, Mn 0.22%, impurities ≤0.01%, and the remainder is Mg.

8. The method for preparing a magnesium alloy conversion coating according to claim 1, characterized in that, It also includes steps for compositional and performance testing of the magnesium alloy conversion film obtained in step S4: Elemental analysis of the obtained magnesium alloy conversion film was performed using EDS. When the content of P in the magnesium matrix of the magnesium alloy conversion film is 1–5 at.%, the content of C is 10–15 at.%, the content of O is 10–20 at.%, and the content of Ca is 0.1–1.5 at.%, the obtained magnesium alloy conversion film meets the application requirements. The obtained magnesium alloy conversion film was tested using neutral salt spray and electrochemical tests. After 12 hours of neutral salt spray testing, the percentage of the corroded area was 0, and the self-corrosion current density was ≤5.94×10⁻⁶. -6 A·cm - ², and after soaking in 3.5 wt.% NaCl solution for 12 hours, the membrane resistance Rf ≥ 4.8 × 10⁻⁶. 4 When the temperature reaches Ω·cm², the resulting magnesium alloy conversion coating meets the usage requirements.

9. A magnesium alloy conversion coating, prepared by the method for preparing a magnesium alloy conversion coating according to any one of claims 1-8, characterized in that, The magnesium alloy conversion film is a phytic acid conversion film formed on the surface of a magnesium alloy substrate. The phytic acid conversion film continuously covers the magnesium alloy substrate and the surface of the second phase, and the surface of the magnesium alloy substrate below the area covered by the phytic acid conversion film retains grinding scratches.

10. A magnesium alloy conversion coating according to claim 9, characterized in that, The Mg, Al, and Zn elements in the phytic acid conversion membrane originate from the magnesium alloy matrix, while the O, C, P, and Ca elements originate from the phytic acid conversion membrane. Furthermore, the O and P element content in the second phase is lower than that in the magnesium matrix.

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