Magnesium alloy calcium carbonate conversion film and method for preparing the same

Abstract

The application belongs to the technical field of metal surface treatment, and particularly relates to a magnesium alloy calcium carbonate conversion film and a preparation method thereof. The method comprises the following steps: preparing a sodium bicarbonate solution; introducing carbon dioxide gas into the sodium bicarbonate solution and adjusting the precursor solution under the monitoring of a pH meter, and before the introduction of the carbon dioxide gas is stopped, the pH value of the precursor solution is adjusted to 5.8-7.0 and kept stable; introducing a calcium salt solution into the precursor solution in a dropwise manner to form a conversion solution, and immersing a magnesium alloy sample into the conversion solution to obtain a magnesium alloy with a calcium carbonate conversion film on the surface. In addition, the application also prepares a magnesium alloy calcium carbonate conversion film by using the above preparation method. The application controls the pH value of the precursor solution to be approximately 7.0, and then introduces high-concentration calcium ions, so that the nucleation and growth behavior of calcium carbonate crystals can be significantly improved, and a calcium carbonate conversion film with finer and smaller crystal grains, a more compact structure and stronger adhesion can be obtained, and the corrosion resistance of the magnesium alloy can be significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a magnesium alloy calcium carbonate conversion film and its preparation method. Background Technology

[0002] Magnesium alloys, due to their excellent properties such as low density and high specific strength, have broad application prospects in aerospace, automotive, and other fields. However, their poor corrosion resistance severely restricts their application. Surface treatment is one of the effective means to improve the corrosion resistance of magnesium alloys.

[0003] In existing technologies, chemical conversion coatings are a commonly used surface treatment method. Calcium carbonate, as an environmentally friendly and low-cost material, has been explored for surface protection of magnesium alloys in recent years. Researchers have investigated different methods for depositing calcium carbonate conversion coatings on magnesium alloy surfaces, such as electrochemical deposition and biomimetic mineralization.

[0004] In the prior art, CN114606486A discloses a CaCO3 conversion film treatment agent and method for magnesium alloy surfaces, which directly mixes calcium chloride solution and sodium bicarbonate solution to deposit a calcium carbonate conversion film on the magnesium alloy surface at 45–85°C. This method has the following inherent defects:

[0005] Failure to recognize the crucial role of precursor solution chemical equilibrium: direct mixing leads to CO3 2- The concentration suddenly increases, and nucleation mainly occurs as homogeneous precipitation in the mixed solution bulk, rather than preferential heterogeneous nucleation on the magnesium alloy surface, resulting in poor adhesion between the calcium carbonate conversion film and the magnesium alloy substrate.

[0006] Complete lack of awareness regarding pH control: No pH control steps were involved, even though the pH of the solution directly determines the CO3 concentration. 2- Concentration and supersaturation of CaCO3 are the decisive factors affecting film formation quality;

[0007] The calcium ion concentration is too low: the concentration of the calcium chloride solution is only 0.5 to 1.5 mol / L, which cannot produce sufficient supersaturation, resulting in coarse-grained and loosely structured calcium carbonate conversion film.

[0008] Heating-driven reaction: It must be carried out under heating conditions of 45-85℃, which consumes a lot of energy and may aggravate the corrosion of magnesium alloy matrix.

[0009] A key problem in existing technologies is that when carbon dioxide gas is introduced into a sodium bicarbonate solution, the pH value of the solution initially decreases and then stabilizes. Conventional methods often add calcium salt solution when the pH value reaches its lowest point, but the resulting calcium carbonate conversion film has a loose structure and low coverage. However, controlling the pH value to approximately 7.0 yields a significantly denser calcium carbonate conversion film. This discovery has not been reported before. To address the problem of poor density in calcium carbonate conversion films on magnesium alloy surfaces, this invention provides a novel technical approach. Summary of the Invention

[0010] Therefore, the first objective of this invention is to provide a method for preparing a calcium carbonate conversion film on magnesium alloys, so as to overcome the defects of poor density and insufficient protective effect of calcium carbonate conversion films in the prior art, and to obtain a calcium carbonate conversion film with high density and high corrosion resistance by precisely controlling the key parameters in the reaction path.

[0011] The second objective of this invention is to provide a magnesium alloy calcium carbonate conversion film prepared by the above-described preparation method.

[0012] To achieve the above objectives, a method for preparing a magnesium alloy calcium carbonate conversion film includes the following steps:

[0013] S1. Prepare sodium bicarbonate solution;

[0014] S2. Carbon dioxide gas is introduced into the sodium bicarbonate solution and adjusted under pH meter monitoring to obtain a precursor solution. Before stopping the introduction of carbon dioxide gas, the pH value of the precursor solution is made to reach 5.8-7.0 and kept stable.

[0015] S3. A calcium salt solution is introduced dropwise into the precursor solution to form a conversion solution. The magnesium alloy sample is then immersed in the conversion solution to obtain a magnesium alloy with a calcium carbonate conversion film on its surface. The immersion reaction is carried out at 15–35°C.

[0016] Preferably, the specific process for preparing the sodium bicarbonate solution in step S1 is as follows: placing solid sodium bicarbonate in a beaker containing 100 mL of deionized water to obtain a sodium bicarbonate solution; wherein the concentration of the sodium bicarbonate solution is 0.02–0.05 mol / L.

[0017] Preferably, the specific process of introducing carbon dioxide gas into the sodium bicarbonate solution and adjusting it under pH meter monitoring to obtain the precursor solution in step S2, and ensuring that the pH value of the precursor solution reaches 5.8 to 7.0 and remains stable before stopping the introduction of carbon dioxide gas, is as follows: continuously introduce carbon dioxide gas into the sodium bicarbonate solution, monitor the pH value change of the sodium bicarbonate solution in real time using a pH meter, and stop introducing carbon dioxide gas when the pH value reaches and stabilizes within the range of 5.8 to 7.0 to obtain the precursor solution.

[0018] Preferably, the carbon dioxide gas is introduced at a rate of 100-300 mL / min for a duration of 10-40 min.

[0019] Preferably, in step S3, a calcium salt solution is introduced dropwise into the precursor solution to form a conversion solution, and the magnesium alloy sample is immersed in the conversion solution to obtain a magnesium alloy with a calcium carbonate conversion film on its surface. The specific process is as follows:

[0020] Under stirring conditions, calcium salt solution is slowly added dropwise to the precursor solution until a milky white precipitate appears in the precursor solution, at which point the addition of calcium salt solution is stopped to obtain a conversion solution. Subsequently, the pretreated magnesium alloy sample is immersed in the conversion solution and reacted at 15–35°C for 4–6 hours. The magnesium alloy sample is then removed, rinsed, and dried to obtain a magnesium alloy with a calcium carbonate conversion film on its surface.

[0021] More preferably, the calcium salt solution is a calcium chloride solution, the concentration of the calcium chloride solution is 2.5-3.5 mol / L, the dropping rate of the calcium chloride solution is 0.1-0.5 mL / min, and the stirring speed of the calcium chloride solution is 100-200 r / min; the rinsing operation is as follows: first rinse with deionized water for 3 min, then rinse the surface of the magnesium alloy sample with alcohol; the drying treatment is hot air drying at 60-90℃ for 5 min.

[0022] More preferably, the magnesium alloy sample undergoes the following pretreatment before the reaction: polishing with a polishing machine, ultrasonic cleaning and degreasing in an alkaline degreasing solution, acid pickling and activation, water washing, and cold air drying; the magnesium alloy is selected as AZ91 magnesium alloy, the polishing machine uses 400-2000C sandpaper, the alkaline degreasing solution contains 10-20g / L NaOH solution, the ultrasonic cleaning power is 100-300W, the frequency is 40-80kHz, the cleaning time is 5-15min, and the temperature is 50-70℃; the acid pickling and activation uses a 3-8wt% hydrochloric acid solution, and the treatment time is 20-60s.

[0023] Preferably, the molar ratio of calcium ions in the calcium chloride solution to bicarbonate ions in the sodium bicarbonate solution is 3 to 6:1.

[0024] In addition, the present invention also provides a magnesium alloy calcium carbonate conversion membrane, which is prepared by the above-described method for preparing a magnesium alloy calcium carbonate conversion membrane.

[0025] Preferably, the calcium carbonate conversion film is composed of closely packed, regular rhombohedral calcite crystals with inter-crystal gaps ≤50 nm, a film thickness of 3–5 μm, and a grain size of 0.3–0.6 μm; the magnesium alloy with the calcium carbonate conversion film on its surface exhibits a corrosion current density of 4.68 × 10⁻⁶ m / s in a 3.5 wt% NaCl solution. -6 A / cm 2 The corrosion inhibition efficiency is 96.2%, there are no interfacial pores between the calcium carbonate conversion film and the magnesium alloy substrate, and the bonding strength level is 0.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. This invention employs a revolutionary film-forming mechanism that moves from "homogeneous precipitation" to "explosive heterogeneous nucleation," whereas existing direct mixing methods primarily rely on homogeneous precipitation. This invention, through pre-adjustment of pH and the synergistic effect of high-concentration calcium ions, avoids both the difficulty of nucleation due to excessively low pH and the homogeneous precipitation of the precursor solution due to excessively high pH. The slow introduction of high-concentration calcium ions instantaneously creates high supersaturation at the interface between the magnesium alloy substrate and the conversion solution, achieving explosive heterogeneous nucleation and generating numerous fine calcite crystal nuclei, thus laying the structural foundation for the formation of a dense calcium carbonate conversion film.

[0028] 2. The high supersaturation of this invention leads to a significant increase in crystal nucleus density and restricts grain growth space, forcing crystals to pack tightly in a regular rhombohedral morphology. The resulting calcium carbonate conversion film has a grain size of 0.3–0.6 μm, representing a grain refinement of 5–10 times, with intergranular gaps ≤50 nm. Simultaneously, the rapid nucleation and growth process at the magnesium alloy substrate surface and the conversion solution interface inhibits local dissolution of the magnesium alloy substrate, eliminating interfacial porosity between the calcium carbonate conversion film and the magnesium alloy substrate, resulting in a bonding strength of 0. However, low pH or low calcium ion concentration alone cannot produce sufficient supersaturation, only forming coarse and loose crystals, significantly reducing corrosion inhibition efficiency.

[0029] 3. The magnesium alloy containing a calcium carbonate conversion film prepared by this invention exhibits a corrosion inhibition efficiency of up to 96.2% in a 3.5 wt% NaCl solution, with a corrosion current density as low as 4.68 × 10⁻⁶. -6 A / cm 2 Compared to the best existing method, the corrosion inhibition efficiency of the calcium carbonate conversion film prepared by this method is increased by 3 times, and it also shows a significant improvement compared to the single-condition optimization method. The excellent corrosion resistance is attributed to the dense calcite crystal stacking and the non-porous interface between the calcium carbonate conversion film and the magnesium alloy substrate, which effectively blocks Cl...- Penetration by corrosive media.

[0030] 4. This invention is completed at room temperature (15–35°C), significantly saving energy. Furthermore, this invention uses only sodium bicarbonate solution, carbon dioxide gas, calcium chloride solution, and deionized water as raw materials, requiring no heating or addition of toxic substances. It is simple to operate, low in cost, and has potential for industrial application. Moreover, no harmful byproducts are produced during the reaction, meeting green environmental protection requirements.

[0031] 5. This invention uses a pH meter to monitor and adjust the pH of the precursor solution in real time, locking it within a narrow window of 5.8–7.0. This eliminates the process uncertainties of the direct mixing method. Furthermore, the introduction of a high concentration of calcium ions significantly improves the nucleation and growth behavior of calcium carbonate crystals, resulting in a calcium carbonate conversion film with finer grains, a denser structure, and stronger adhesion. This method is simple, environmentally friendly, and can significantly improve the corrosion resistance of magnesium alloys, providing a novel technical approach for magnesium alloy surface treatment. Attached Figure Description

[0032] 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.

[0033] Figure 1 This is a morphological image of the magnesium alloy calcium carbonate conversion film obtained in Example 1 of the present invention.

[0034] Figure 2 This is a morphological image of the magnesium alloy calcium carbonate conversion film obtained in Example 2 of the present invention. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0036] This invention provides a method for preparing a magnesium alloy calcium carbonate conversion film, comprising the following steps:

[0037] S1. Prepare sodium bicarbonate solution.

[0038] S2. Carbon dioxide gas is introduced into the sodium bicarbonate solution and adjusted under the monitoring of a pH meter to obtain the precursor solution. Before stopping the introduction of carbon dioxide gas, the pH value of the precursor solution is made to reach 5.8-7.0 and kept stable.

[0039] S3. A calcium salt solution is introduced dropwise into the forward liquid to form a conversion liquid. The magnesium alloy sample is then immersed in the conversion liquid to obtain a magnesium alloy with a calcium carbonate conversion film on its surface. The immersion reaction is carried out at 15–35°C.

[0040] First, in step S1, sodium bicarbonate solid is placed in a beaker containing 100 mL of deionized water to prepare a sodium bicarbonate solution. The concentration of the sodium bicarbonate solution is 0.02–0.05 mol / L.

[0041] In some embodiments of the present invention, the concentration of the sodium bicarbonate solution is any value or a range formed by any combination of 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, and 0.05 mol / L. For example, the concentration of the sodium bicarbonate solution is preferably 0.05 mol / L. When the volume of a 100 mL sodium bicarbonate solution has a concentration of 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, and 0.05 mol / L, the masses of solid sodium bicarbonate weighed are 0.168 g, 0.252 g, 0.336 g, and 0.42 g, respectively.

[0042] It should be noted that the sodium bicarbonate concentration is chosen to stabilize the pH of the precursor solution within the target range. When the sodium bicarbonate solution concentration is <0.02 mol / L, the buffering capacity of the sodium bicarbonate solution is insufficient, resulting in drastic pH fluctuations when carbon dioxide gas is introduced. This makes it difficult to precisely stabilize the pH within the narrow window of 5.8–7.0, and can easily lead to excessively low pH levels causing CO3 emissions. 2- At extremely low concentrations, film formation is impossible. When the concentration of sodium bicarbonate solution is >0.05 mol / L, the buffering capacity of the sodium bicarbonate solution is too strong, resulting in a slow pH decrease after the introduction of carbon dioxide gas, requiring a longer aeration time to reach the target pH value, thus increasing process time. Simultaneously, when the introduction of carbon dioxide gas is stopped, the excessively high HCO3- concentration... - A high concentration will cause the pH to rise too quickly, easily exceeding 7.2, triggering spontaneous precipitation of the sodium bicarbonate solution and resulting in an uneven calcium carbonate conversion film. When the concentration of sodium bicarbonate solution is between 0.02 and 0.05 mol / L, the HCO3- in the sodium bicarbonate solution... - and CO3 2- The total amount is moderate. When a high-concentration calcium chloride solution is slowly added dropwise, the local calcium ion concentration at the interface between the magnesium alloy substrate and the conversion liquid is extremely high, while the CO3 concentration in the conversion liquid itself is relatively low. 2-The relatively low concentration creates a transient high supersaturation at the interface between the magnesium alloy matrix and the conversion fluid, triggering "explosive nucleation" while simultaneously inhibiting homogeneous precipitation in the bulk conversion fluid. If the concentration of the sodium bicarbonate solution is >0.05 mol / L, the CO32- in the sodium bicarbonate solution... 2- As the concentration increases accordingly, calcium ions are easily added dropwise to form CaCO3 precipitate directly in the sodium bicarbonate solution. These precipitates adhere to the magnesium alloy surface, but their adhesion is poor, resulting in a loose calcium carbonate conversion film and reduced corrosion inhibition efficiency. Controlling the supersaturation of CaCO3 at the interface between the magnesium alloy substrate and the conversion solution is crucial to prevent homogeneous precipitation of sodium bicarbonate. Ideally, the supersaturation should be highest at the interface between the magnesium alloy substrate and the conversion solution to induce heterogeneous nucleation.

[0043] In step S2, carbon dioxide gas is continuously introduced into the sodium bicarbonate solution at a rate of 100–300 mL / min for 10–40 min. The pH of the sodium bicarbonate solution is continuously monitored in real time using a pH meter to ensure that the pH remains stable within the target range of 5.8–7.0. When the pH reaches and stabilizes within the 5.8–7.0 range, the introduction of carbon dioxide gas is stopped, yielding the precursor solution. The key innovation of this step lies in the selection of the pH control point, which differs from the conventional practice of stopping the introduction of carbon dioxide gas when the pH reaches its lowest point.

[0044] In some embodiments of the present invention, the rate of carbon dioxide gas introduction is any value or a range formed by any two of the following: 100 mL / min, 110 mL / min, 120 mL / min, 130 mL / min, 140 mL / min, 150 mL / min, 160 mL / min, 170 mL / min, 180 mL / min, 190 mL / min, 200 mL / min, 210 mL / min, 220 mL / min, 230 mL / min, 240 mL / min, 250 mL / min, 260 mL / min, 270 mL / min, 280 mL / min, 290 mL / min, and 300 mL / min. The time for introducing carbon dioxide gas is any value or a range formed by any two of the following: 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, and 40 min. For example, the preferred rate of carbon dioxide gas introduction is 150 mL / min, and the preferred time is 15 min.

[0045] It should be noted that precisely adjusting the pH of the precursor solution to 5.8–7.0 within 10–40 minutes ensures a uniform and stable pH of the sodium bicarbonate solution, preventing localized over-acidity. The duration of carbon dioxide gas purging maintains the appropriate buffering capacity of the sodium bicarbonate solution and CO3 levels.2- The supply capacity is sufficient to synergize with the subsequent high-concentration calcium ion addition process to obtain a dense, highly bound calcium carbonate conversion membrane. This invention ensures the precursor solution's pH value falls precisely within the target window of 5.8–7.0, avoiding the defects of precursor solution precipitation and a loose calcium carbonate conversion membrane layer caused by excessively high pH due to insufficient time; simultaneously, it also solves the problem of CO3 accumulation due to excessively low pH caused by excessive time. 2- It avoids issues of insufficient sodium bicarbonate and film formation failure. Furthermore, it is compatible with different sodium bicarbonate concentrations and carbon dioxide gas introduction rates within a pH range of 5.8–7.0, exhibiting good process adaptability.

[0046] When carbon dioxide gas is passed into a sodium bicarbonate solution, the following reaction occurs:

[0047] The first step is for carbon dioxide gas to dissolve in water to form carbonic acid:

[0048]

[0049] The second step is the stepwise ionization of carbonic acid:

[0050]

[0051]

[0052] The introduction of carbon dioxide gas increases the amount of H2CO3 in the sodium bicarbonate solution, and the H2CO3 ionizes to release H+. + Breaking the HCO3 - The existing ionization equilibrium shifts the reaction towards the formation of H₂CO₃, causing a decrease in the pH of the precursor solution. This pH change in the precursor solution reflects the reaction towards the formation of HCO₃. - With CO3 2- To achieve concentration equilibrium, when the pH of the precursor solution drops to 6.2–6.5, the precursor solution will contain H₂CO₃ and HCO₃⁻. - Mainly CO3 2- The concentration is extremely low. However, when the pH of the precursor solution rises and stabilizes between 5.8 and 7.0, especially when the pH of the precursor solution is close to 7.0, the CO3 concentration... 2- The concentration should be within a suitable range, so that nucleation is not difficult due to excessively low pH, nor is the formation of CaCO3 precipitation too fast due to excessively high pH.

[0053] This invention, through a series of preliminary experiments, discovered that when the pH value is <5.8, the precursor solution is weakly acidic, and CO3... 2- At excessively low concentrations, CaCO3 precipitation is almost impossible, resulting in a calcium carbonate conversion film coverage of less than 30%. When the pH is between 6.2 and 6.5, CO3... 2-The concentration of CO32- is still low, resulting in a slow nucleation rate and the formation of coarse, porous calcite crystals, with a corrosion inhibition efficiency of only 82.7%. When the pH value is 6.8–7.2, CO32-... 2- With a moderate concentration, the catalyst undergoes explosive nucleation, forming a dense rhombohedral accumulation, achieving a corrosion inhibition efficiency of 96.2%. When the pH value is >7.2, a visible white precipitate begins to appear in the precursor solution, with CaCO3 spontaneously precipitating, failing to form a uniform calcium carbonate conversion film on the magnesium alloy surface. Therefore, this invention limits the pH value range to 5.8–7.0, preferably 6.8–7.0, and more preferably close to 7.0.

[0054] In step S3, the calcium salt solution is a calcium chloride solution. The specific operation process of step S3 is as follows: Under stirring conditions of 100-200 r / min, a calcium chloride solution with a concentration of 2.5-3.5 mol / L is slowly added dropwise to the precursor solution at a dropping rate of 0.1-0.5 mL / min until a milky white precipitate appears in the precursor solution. The addition of calcium chloride solution is then stopped, resulting in the conversion solution. Stirring conditions are beneficial for the uniform introduction of calcium ions and the uniform deposition of calcium carbonate. Subsequently, the pretreated magnesium alloy sample is immersed in the conversion solution and reacted at 15-35℃ for 4-6 hours. The synergistic effect of the introduction of high-concentration calcium ions and a specific pH environment is key to obtaining a dense calcium carbonate conversion film. The magnesium alloy sample is then removed, first washed with deionized water for 3 minutes, then rinsed with alcohol. After rinsing, the sample is dried with hot air at 60-90℃ for 5 minutes. After drying, a magnesium alloy with a calcium carbonate conversion film on its surface is obtained.

[0055] In some embodiments of the present invention, the stirring speed is any value or a range formed by any two of the following: 100 r / min, 110 r / min, 120 r / min, 130 r / min, 140 r / min, 150 r / min, 160 r / min, 170 r / min, 180 r / min, 190 r / min, and 200 r / min. The dropping rate is any value or a range formed by any two of the following: 0.1 mL / min, 0.2 mL / min, 0.3 mL / min, 0.4 mL / min, and 0.5 mL / min. The concentration of the calcium chloride solution is any value or a range formed by any two of the following: 2.5 mol / L, 2.6 mol / L, 2.7 mol / L, 2.8 mol / L, 2.9 mol / L, 3.0 mol / L, 3.1 mol / L, 3.2 mol / L, 3.3 mol / L, 3.4 mol / L, and 3.5 mol / L. For example, the stirring speed is preferably 150 r / min, the dropping rate is preferably 0.3 mL / min, and the concentration of the calcium chloride solution is preferably 3.0 mol / L.

[0056] It should be noted that step S3 controls the nucleation site, preferentially promoting heterogeneous nucleation on the magnesium alloy surface and preventing homogeneous precipitation of the precursor solution due to excessively high pH. This step also utilizes the supersaturation at the interface between the magnesium alloy substrate and the conversion solution to achieve explosive nucleation, resulting in fine, tightly packed calcite crystals. Furthermore, the slow addition of calcium chloride solution prevents excessive local supersaturation, ensuring a smooth film formation process. Finally, this step, by fully utilizing the high surface energy after activation, enhances the adhesion of the calcium carbonate conversion film.

[0057] A stirring speed of 100–200 r / min ensures rapid and uniform dispersion of calcium ions, avoiding localized supersaturation that could lead to calcium carbonate precipitation. This stirring speed also maintains a moderate degree of supersaturation at the interface between the magnesium alloy substrate and the conversion liquid, promoting uniform heterogeneous nucleation and further preventing excessively rapid fluid shear forces from scouring the crystal nuclei or entraining air bubbles. Combined with a dropping rate of 0.1–0.5 mL / min, this results in a uniform, dense, and highly cohesive calcium carbonate conversion film.

[0058] A dropping rate of 0.1–0.5 mL / min can maintain a suitable supersaturation at the interface between the magnesium alloy matrix and the conversion solution on the magnesium alloy surface, triggering high-density heterogeneous nucleation and obtaining fine, tightly packed calcite crystals. The selection of the dropping rate in this invention avoids defects such as precursor solution precipitation, sudden pH drop, and a loose calcium carbonate conversion film caused by excessively rapid dropping; it also avoids the occurrence of sparse crystal nuclei, coarse crystals, and an excessively thin calcium carbonate conversion film caused by excessively slow dropping.

[0059] When a calcium chloride solution with a concentration of 2.5–3.5 mol / L is slowly added dropwise to a precursor solution with a pH of approximately 7.0, an extremely high concentration of calcium ions is instantly formed at the interface between the magnesium alloy substrate and the conversion solution. The calcium chloride solution reacts with CO32- in the precursor solution. 2- The following reaction occurs:

[0060]

[0061] When the concentration of calcium ions is ≤1.5 mol / L, the supersaturation is insufficient, the nucleation rate is slow, and the calcium carbonate conversion film is loose. When the concentration of calcium ions is increased to 2.5–3.5 mol / L, the supersaturation at the interface between the magnesium alloy substrate surface and the conversion solution increases significantly, achieving explosive nucleation. When the concentration of calcium ions exceeds 4 mol / L, the viscosity of the calcium chloride solution is too high, calcium ion diffusion is restricted, and the cost increases. Therefore, this invention limits the concentration of the calcium chloride solution to 2.5–3.5 mol / L; exemplarily, the concentration of the calcium chloride solution is preferably 3.0 mol / L.

[0062] High supersaturation at the interface between the magnesium alloy substrate and the conversion fluid induces explosive nucleation, generating numerous fine calcite nuclei. These nuclei tend to grow into regular rhombohedral shapes and pack tightly together, resulting in a dense calcium carbonate conversion film. In contrast, at pH values ​​of 6.2–6.5, CO3... 2- The extremely low concentration of calcium ions results in a low nucleation rate and predominantly crystal growth, leading to large, loose crystals. Conversely, using calcium ions at concentrations ≤1.5 mol / L results in insufficient supersaturation, also preventing the formation of dense aggregates. Therefore, a significant synergistic effect exists between a specific pH environment and high concentrations of calcium ions; neither can be dispensed with.

[0063] A reaction temperature of 15–35℃ achieves a good balance between nucleation and growth rates, resulting in fine, tightly packed calcite crystals and maintaining suitable CaCO3 supersaturation. Temperatures below 15℃ lead to insufficient nucleation, while temperatures above 35℃ cause precipitation in the conversion solution. This temperature range also helps maintain the stability of the magnesium alloy matrix. Temperatures below 15℃ result in a slow reaction, while temperatures above 35℃ accelerate corrosion. A reaction time of 4–6 hours allows the calcium carbonate conversion film to grow through the induction and rapid growth phases, entering a stable phase and achieving complete and dense coverage. A reaction time less than 4 hours leads to a thin, low-coverage calcium carbonate conversion film with poor corrosion inhibition efficiency. A reaction time greater than 6 hours results in the depletion of calcium, carbonate, or bicarbonate ions in the reactants, crystal coarsening, corrosion of the magnesium alloy matrix, and decreased adhesion of the calcium carbonate conversion film.

[0064] Hot air drying within 5 minutes can thoroughly remove residual moisture and alcohol from the calcium carbonate conversion film and its pores, preventing corrosion of the magnesium alloy substrate beneath the film. At hot air drying temperatures below 60℃, incomplete drying and decreased long-term adhesion can occur; at temperatures above 90℃, thermal stress cracking, oxidation, or phase transformation of the magnesium alloy substrate may result. Therefore, hot air drying at 60–90℃ offers high efficiency and has significant industrial application potential.

[0065] Magnesium alloy samples underwent the following pretreatment before the reaction: polishing, ultrasonic cleaning in an alkaline degreasing solution, acid pickling and activation, water washing, and cold air drying. First, the magnesium alloy samples were polished on a polishing machine using 400–2000 g / L sandpaper until the surface was smooth. Then, the polished samples were placed in an alkaline degreasing solution containing 10–20 g / L NaOH and cleaned for 5–15 minutes under ultrasonic conditions at a power of 100–300 W and a frequency of 40–80 kHz to remove oil. Afterward, they were immersed in a 3–8 wt% hydrochloric acid solution for 20–60 seconds for acid pickling and activation. Finally, they were rinsed with deionized water and dried with cold air. This pretreatment helps improve the adhesion of the calcium carbonate conversion film.

[0066] For example, AZ91 magnesium alloy sheet was selected as the magnesium alloy sample and cut into samples with dimensions of 10mm × 10mm × 10mm. The sample was then polished sequentially with 400C, 800C, 1200C, and 2000C sandpaper until the surface was smooth. The polished magnesium alloy sample was then placed in an alkaline degreasing solution containing 10g / L NaOH and cleaned for 10 minutes under ultrasonic conditions at a power of 150W and a frequency of 60kHz. Afterward, it was immersed in a 5wt% hydrochloric acid solution for 30 seconds for acid pickling and activation. Finally, it was rinsed thoroughly with deionized water and dried with cold air for later use.

[0067] The molar ratio of calcium ions in calcium chloride solution to bicarbonate ions in sodium bicarbonate solution is 3–6:1.

[0068] It should be noted that a molar ratio of 3 to 6:1 can generate sufficient supersaturation at the interface between the magnesium alloy substrate and the conversion fluid, triggering explosive heterogeneous nucleation. Simultaneously, this molar ratio can suppress homogeneous precipitation in the conversion fluid, ensuring that the calcium carbonate conversion film preferentially grows on the magnesium alloy substrate surface. Furthermore, this molar ratio can also yield fine, tightly packed, and highly bonded calcium carbonate conversion films.

[0069] Meanwhile, the present invention also provides a magnesium alloy calcium carbonate conversion membrane, which is prepared by the above-mentioned method for preparing a magnesium alloy calcium carbonate conversion membrane.

[0070] In a preferred embodiment of the present invention, the calcium carbonate conversion film is composed of closely packed regular rhombohedral calcite crystals with a gap width between crystals ≤50nm, a thickness of 3-5μm, and a grain size of 0.3-0.6μm; the magnesium alloy with the calcium carbonate conversion film on its surface exhibits a corrosion current density of 4.68×10⁻⁶ in a 3.5wt% NaCl solution. -6 A / cm 2 The corrosion inhibition efficiency is 96.2%, there are no interfacial pores between the calcium carbonate conversion film and the magnesium alloy substrate, and the bonding strength level is 0.

[0071] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail with reference to the following embodiments and comparative examples.

[0072] Example 1

[0073] First step: Prepare a 0.05 mol / L sodium bicarbonate solution by weighing 0.42 g of sodium bicarbonate solid and placing it in a beaker containing 100 mL of deionized water.

[0074] The second step involves continuously introducing carbon dioxide gas at a rate of 150 mL / min into the sodium bicarbonate solution for 15 minutes. When the pH value is approximately and stabilizes at 7.0, the carbon dioxide gas introduction is stopped, yielding the precursor solution.

[0075] The third step: Under stirring at 150 r / min, a 3.0 mol / L calcium chloride solution was slowly added dropwise to the precursor solution at a dropping rate of 0.3 mL / min until a milky white precipitate appeared. The addition of calcium chloride solution was then stopped, yielding the conversion solution. Subsequently, the pretreated AZ91 magnesium alloy sample was immersed in the conversion solution and reacted at 25°C for 6 hours. The magnesium alloy sample was then removed, washed with deionized water for 3 minutes, followed by rinsing with alcohol. After rinsing, the sample was dried with hot air at 80°C for 5 minutes. After drying, a magnesium alloy with a calcium carbonate conversion film on its surface was obtained.

[0076] Example 2

[0077] First step: Prepare a 0.02 mol / L sodium bicarbonate solution by weighing 0.168 g of sodium bicarbonate solid and placing it in a beaker containing 100 mL of deionized water.

[0078] The second step involves continuously introducing carbon dioxide gas at a rate of 150 mL / min into the sodium bicarbonate solution for 20 minutes. When the pH reaches and stabilizes at 6.9, the carbon dioxide gas introduction is stopped, yielding the precursor solution.

[0079] The third step: Under stirring at 150 r / min, a 3.0 mol / L calcium chloride solution was slowly added dropwise to the precursor solution at a dropping rate of 0.3 mL / min until a milky white precipitate appeared. The addition of calcium chloride solution was then stopped, yielding the conversion solution. Subsequently, the pretreated AZ91 magnesium alloy sample was immersed in the conversion solution and reacted at 25℃ for 5 h. The magnesium alloy sample was then removed, washed with deionized water for 3 min, and then rinsed with alcohol. After rinsing, the sample was dried with hot air at 80℃ for 5 min. After drying, a magnesium alloy with a calcium carbonate conversion film on its surface was obtained.

[0080] Comparative Example 1

[0081] The procedure was strictly followed according to the best available method: a 1.0 mol / L sodium bicarbonate solution and a 1.5 mol / L calcium chloride solution were prepared and directly mixed at a volume ratio of 1:3 to obtain the treatment agent. The pretreated AZ91 magnesium alloy sample was immersed in the treatment agent and reacted in a 50°C water bath for 4 hours. After removal, it was ultrasonically cleaned with deionized water and ethanol for 5 minutes and then dried with hot air.

[0082] Comparative Example 2

[0083] This comparative example is the same as Example 1, except that in the second step, the pH value of the precursor solution is controlled and stabilized at 6.2 after carbon dioxide gas is introduced.

[0084] Comparative Example 3

[0085] This comparative example is the same as Example 1, except that in the third step, the concentration of the calcium chloride solution is 1.0 mol / L.

[0086] The magnesium alloy calcium carbonate conversion films prepared in Examples 1 and 2 and Comparative Examples 1-3 were subjected to potentiodynamic polarization curve tests using a CHI660E electrochemical workstation. The adhesion level between the calcium carbonate conversion film and the magnesium alloy substrate was evaluated using the cross-cut test method according to GB / T9286-1998. The morphology of the calcium carbonate conversion film layer was observed by SEM.

[0087] The electrochemical test used a three-electrode system: a magnesium alloy sample was used as the working electrode with an exposure area of ​​1 cm². 2 A saturated calomel electrode was used as the reference electrode, and a platinum electrode as the auxiliary electrode. The corrosive medium was a 3.5 wt% NaCl solution, and the test conditions were room temperature. Before the test, the working electrode was immersed in the 3.5 wt% NaCl solution for 30 min to achieve a stable open-circuit potential. The potentiodynamic scanning range was from -0.3 V to +0.5 V relative to the open-circuit potential, and the scan rate was 1 mV / s.

[0088] The corrosion current density was obtained by Tafel extrapolation, and the corrosion inhibition efficiency was calculated using the following formula:

[0089]

[0090] In the formula, The corrosion current density of bare magnesium alloy. To measure the corrosion current density of magnesium alloys after the conversion coating is applied, To determine corrosion inhibition efficiency, at least three parallel samples were tested for each sample, and the arithmetic mean was taken.

[0091] The bonding strength test procedure is as follows: A grid of 1mm spacing, totaling 10×10 squares, is drawn on the surface of the calcium carbonate conversion film using a cross-cutting tool. The scratch depth must penetrate the calcium carbonate conversion film layer to reach the magnesium alloy substrate. After removing debris from the calcium carbonate conversion film surface with a soft brush, 3M 600 tape is smoothly applied to the grid area. A rubber roller is used to repeatedly press the tape five times to ensure full contact between the tape and the calcium carbonate conversion film layer. After standing for 2 minutes, the tape is quickly pulled up perpendicular to the calcium carbonate conversion film surface. The extent of calcium carbonate conversion film detachment in the grid area is observed under an optical microscope, and the bonding strength grade is evaluated according to the following standards. Each sample is tested at least three different locations, and the worst grade is taken as the final result. The electrochemical test and bonding strength grade test results for bare magnesium alloy, Examples 1 and 2, and Comparative Examples 1–3 are shown in Table 1.

[0092] Table 1. Test results of bare magnesium alloys, Examples 1 and 2, and Comparative Examples 1-3

[0093]

[0094] As can be seen from Table 1, Comparative Example 1, prepared strictly according to the best existing technology, has a corrosion inhibition efficiency of only 31.3%, and the calcium carbonate conversion film layer is loose and porous with an adhesion strength of only level 3. In contrast, Example 1 of this invention has a corrosion inhibition efficiency as high as 96.2%, more than three times that of Comparative Example 1; the corrosion current density is reduced by more than one order of magnitude; and the adhesion strength is improved from level 3 to level 0. These data fully demonstrate that the technical solution of this invention represents a qualitative leap compared to existing technologies. Although Comparative Example 2 uses a high concentration of calcium ions, the pH value is controlled and stabilized at the lowest point, resulting in a corrosion inhibition efficiency of only 82.7%, significantly inferior to Example 1. Although Comparative Example 3 has a pH value close to 7.0, it uses a low concentration of calcium ions, resulting in a corrosion inhibition efficiency of only 84.0%, also inferior to Example 1. Although Example 2 falls within the scope of this invention, it belongs to the edge of the optimal conditions within the feasible range, and its performance is slightly lower than that of Example 1. This is because CO3... 2- The concentration of CO32- is extremely sensitive to pH. According to the second-order ionization equilibrium of carbonic acid, when the pH drops from 7.0 to 6.9, the concentration of CO32- increases. 2- The concentration of calcium carbonate decreases by approximately 20%. This difference leads to a reduction in supersaturation at the interface between the magnesium alloy substrate and the conversion solution when high concentrations of calcium ions are added, preventing the critical threshold for "explosive nucleation" from being reached. Nucleation density decreases, crystal growth becomes dominant, resulting in slightly coarser calcite grains and increased intergranular gaps. The density of the calcium carbonate conversion film decreases, with the bonding strength dropping from grade 0 to grade 1, and the corrosion inhibition efficiency decreasing from 96.2% to 94.1%. Therefore, only when the pH value is approximately 7.0 can the supersaturation be maximized to obtain the optimal performance of the calcium carbonate conversion film.

[0095] Only Example 1 simultaneously met both the conditions of a pH value of approximately 7.0 and a high concentration of calcium ions, achieving a corrosion inhibition efficiency as high as 96.2% and a reduction in corrosion current density by two orders of magnitude. This fully demonstrates that there is a significant synergistic effect between the pH value and calcium ion concentration specified in this invention, and neither can be dispensed with. Its core lies in the instantaneous establishment of extremely high CaCO3 supersaturation at the interface between the magnesium alloy matrix surface and the conversion fluid, triggering "explosive heterogeneous nucleation".

[0096] A pH value of approximately 7.0 allows for precise control of CO3. 2- When the concentration reaches the optimal window, the following equilibrium exists after carbon dioxide gas is passed into the sodium bicarbonate solution: pH value determines CO3 2- The concentration of CO32. According to the second-order ionization equilibrium of carbonic acid, when the pH increases from 6.5 to 7.0, the concentration of CO32... 2- The concentration increases exponentially; however, when the pH value is >7.2, CO3 concentration decreases. 2- Excessive concentration can easily lead to homogeneous precipitation of sodium bicarbonate solution. This invention has found that when the pH value is approximately 7.0, CO3... 2- The concentration is in the "critical metastable region," capable of producing high supersaturation without triggering bulk precipitation of the sodium bicarbonate solution. At this point, CO3... 2- The concentration is approximately 10 -4 The concentration is on the order of mol / L, and it mixes with the subsequently added high concentration of calcium ions at the interface. During mixing, the calcium ions react with CO3. 2- Its ion product far exceeds that of CaCO3, and its supersaturation ratio can reach 10. 3 above.

[0097] The slow dropwise addition of a high-concentration calcium chloride solution creates a localized region of extremely high concentration. When a 3.0 mol / L calcium chloride solution is added dropwise to a stirred precursor solution at a rate of 0.3 mL / min, a micrometer-scale region of high calcium ion concentration forms near the dropping point due to limited diffusion. The calcium ion concentration in this region is much higher than that in the bulk calcium chloride solution, and is comparable to the CO32- concentration provided by a pH value approximately 7.0. 2- Upon contact, an extremely high local supersaturation is instantaneously generated. This supersaturation far exceeds the critical value for homogeneous nucleation of CaCO3, but more importantly, the magnesium alloy surface provides a large number of active sites, making the energy barrier for heterogeneous nucleation much lower than that for homogeneous nucleation. Therefore, nucleation preferentially occurs on the magnesium alloy surface, rather than in the conversion fluid itself.

[0098] According to classical nucleation theory, explosive nucleation leads to high density and fine grains, specifically:

[0099]

[0100] in, For interface energy, For molecular volume, As a kinetic factor, Boltzmann's constant, Absolute temperature For nucleation rate, This indicates supersaturation. When At its maximum, the exponential term approaches 1, and the nucleation rate exhibits explosive growth. Under the conditions of this invention, up to 10 nucleation rates are generated on the magnesium alloy surface in an extremely short time. 12 ~10 14 nuclei / m 2 The nucleus density is extremely low. The spacing between these nuclei is very small, and during subsequent crystal growth, the growth space around each nucleus is limited, resulting in the final formation of submicron-sized calcite crystals, which are tightly packed into regular rhombohedral shapes due to mutual compression.

[0101] Because the calcium chloride solution is added slowly and stirred thoroughly, the high calcium ion region exists only locally, thus reducing the overall CO3 concentration in the solution. 2- The concentration remained low, and the ion product did not reach the critical value for homogeneous nucleation; therefore, no spontaneous suspended precipitate was generated in the precursor solution. All CaCO3 was deposited on the magnesium alloy surface via heterogeneous nucleation.

[0102] During the explosive nucleation process, crystal nuclei are generated directly on the surface of the magnesium alloy matrix in situ, without the pre-formation of a loose intermediate layer. At the same time, the high supersaturation promotes the formation of chemical bonds between the crystal nuclei and the magnesium alloy matrix, resulting in no nanoscale gaps between the calcium carbonate conversion film and the magnesium alloy matrix interface, and the bonding force reaches level 0.

[0103] like Figure 1 As shown, the morphological observation results of the calcium carbonate conversion film layer revealed that the calcium carbonate conversion film layer of Example 1 is continuous and complete, and the surface is composed of a large number of uniformly sized regular rhombohedral calcite crystals tightly packed together, with gaps between crystals ≤50nm, and the structure is dense. The thickness of the calcium carbonate conversion film layer is 3-5μm.

[0104] like Figure 2 As shown, the average grain size of the calcite crystals in Example 2 was 0.8–1.2 μm, while the grain size of the crystals in Example 1 was 0.3–0.6 μm, representing a grain refinement of 5–10 times. This is because the pH of the precursor solution in Example 2 was 6.9, slightly lower than the optimal value of 7.0, resulting in higher CO3 levels. 2-With a concentration decrease of approximately 20%, the supersaturation at the interface between the magnesium alloy matrix surface and the conversion solution failed to reach the critical threshold for "explosive nucleation," resulting in a lower nucleation density and relatively dominant crystal growth, thus leading to grain coarsening. In Example 2, a small number of micron-sized pores were visible between the crystals, while in Example 1, the intercrystalline gaps were ≤50 nm, with virtually no visible pores. This is because there were fewer nucleation points, and the crystal growth process could not completely fill the matrix surface, leaving residual micropores as channels for the corrosive medium to penetrate. Figure 2 In the previous example, it was observed that some areas, accounting for approximately 5-10% of the calcium carbonate conversion film area, had relatively loose crystal accumulation, even exposing local magnesium alloy substrates, appearing as dark spots. In contrast, the calcium carbonate conversion film in Example 1 was complete, continuous, and without any exposed areas. This further demonstrates that nucleation was uneven at pH 6.9, resulting in a slightly lower coverage of the calcium carbonate conversion film.

[0105] The aforementioned morphological differences directly led to a decrease in the corrosion resistance of Example 2; the loose accumulation and micropores made Cl... - It penetrates the calcium carbonate conversion film more easily to reach the magnesium alloy substrate, therefore its corrosion current density is higher than that of Example 1, and its corrosion inhibition efficiency is lower than that of Example 1. At the same time, because there are local loose areas in the calcium carbonate conversion film, pulling up the tape during the cross-cut test will cause a small amount of edge peeling off, so the adhesion grade is level 1.

[0106] The calcium carbonate conversion film in Comparative Example 1 is thin and uneven, with irregular crystal shapes, loose packing, and obvious pores. The calcium carbonate conversion film in Comparative Example 2 shows obvious localized exposed areas, indicating incomplete coverage. The crystal size is uneven, the packing is relatively loose, and there are many pores and gaps. The calcium carbonate conversion film in Comparative Example 3 has acceptable coverage, but the crystal particles are larger, the packing density is lower than in Example 1, and some porosity is present.

[0107] The comparison of Examples 1 and 2 and Comparative Examples 1 to 3 clearly shows that the magnesium alloy calcium carbonate conversion film prepared by the present invention, by precisely controlling the pH value of the precursor liquid after carbon dioxide gas treatment within the range of 5.8 to 7.0 and then reacting it with a high concentration of calcium chloride solution, is significantly superior to the existing direct mixing and single-condition optimization schemes in terms of morphology density and corrosion resistance.

[0108] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a magnesium alloy calcium carbonate conversion film, characterized in that, Includes the following steps: S1. Prepare sodium bicarbonate solution; S2. Carbon dioxide gas is introduced into the sodium bicarbonate solution and adjusted under pH meter monitoring to obtain a precursor solution. Before stopping the introduction of carbon dioxide gas, the pH value of the precursor solution is made to reach 5.8-7.0 and kept stable. S3. A calcium salt solution is introduced dropwise into the precursor solution to form a conversion solution. The magnesium alloy sample is then immersed in the conversion solution to obtain a magnesium alloy with a calcium carbonate conversion film on its surface. The immersion reaction is carried out at 15–35°C.

2. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 1, characterized in that, The specific process for preparing the sodium bicarbonate solution in step S1 is as follows: sodium bicarbonate solid is placed in a beaker containing 100 mL of deionized water to obtain a sodium bicarbonate solution; wherein, the concentration of the sodium bicarbonate solution is 0.02 to 0.05 mol / L.

3. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 1, characterized in that, In step S2, carbon dioxide gas is introduced into the sodium bicarbonate solution and adjusted under pH meter monitoring to obtain a precursor solution. Before stopping the introduction of carbon dioxide gas, the pH value of the precursor solution is brought to a stable range of 5.8 to 7.

0. The specific process is as follows: Carbon dioxide gas is continuously introduced into the sodium bicarbonate solution, and the pH value of the sodium bicarbonate solution is monitored in real time using a pH meter. When the pH value reaches and stabilizes within the range of 5.8 to 7.0, the introduction of carbon dioxide gas is stopped to obtain the precursor solution.

4. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 3, characterized in that, The carbon dioxide gas is introduced at a rate of 100–300 mL / min for a duration of 10–40 min.

5. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 1, characterized in that, In step S3, a calcium salt solution is introduced dropwise into the precursor solution to form a conversion solution. The magnesium alloy sample is then immersed in the conversion solution to obtain a magnesium alloy with a calcium carbonate conversion film on its surface. The specific process is as follows: Under stirring conditions, calcium salt solution is slowly added dropwise to the precursor solution until a milky white precipitate appears in the precursor solution, at which point the addition of calcium salt solution is stopped to obtain the conversion solution. Subsequently, the pretreated magnesium alloy sample was immersed in the conversion solution and reacted at 15–35°C for 4–6 hours. The magnesium alloy sample was then removed, rinsed, and dried to obtain a magnesium alloy with a calcium carbonate conversion film on its surface.

6. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 5, characterized in that, The calcium salt solution is a calcium chloride solution with a concentration of 2.5–3.5 mol / L, a dropping rate of 0.1–0.5 mL / min, and a stirring speed of 100–200 r / min. The rinsing operation is as follows: first, rinse with deionized water for 3 min, then rinse the surface of the magnesium alloy sample with alcohol. The drying treatment is hot air drying at 60–90 °C for 5 min.

7. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 5, characterized in that, The magnesium alloy sample underwent the following pretreatments before the reaction: polishing with a polishing machine, ultrasonic cleaning and degreasing in an alkaline degreasing solution, acid pickling and activation, water washing, and cold air drying. The magnesium alloy selected was AZ91 magnesium alloy. The polishing machine used 400-2000C sandpaper. The alkaline degreasing solution contained 10-20 g / L NaOH solution. The ultrasonic cleaning power was 100-300 W, the frequency was 40-80 kHz, the cleaning time was 5-15 min, and the temperature was 50-70℃. The acid pickling and activation used a 3-8 wt% hydrochloric acid solution for 20-60 s.

8. The method for preparing a magnesium alloy calcium carbonate conversion film according to claim 6, characterized in that, The molar ratio of calcium ions in the calcium chloride solution to bicarbonate ions in the sodium bicarbonate solution is 3 to 6:

1.

9. A magnesium alloy calcium carbonate conversion membrane, characterized in that, It is prepared by any one of the methods for preparing a magnesium alloy calcium carbonate conversion film according to claims 1 to 8.

10. A magnesium alloy calcium carbonate conversion film according to claim 9, characterized in that, The calcium carbonate conversion film is composed of tightly packed, regular rhombohedral calcite crystals with inter-crystal gaps ≤50 nm, a film thickness of 3–5 μm, and a grain size of 0.3–0.6 μm. The magnesium alloy with the calcium carbonate conversion film on its surface exhibits a corrosion current density of 4.68 × 10⁻⁶ m² in a 3.5 wt% NaCl solution. -6 A / cm 2 The corrosion inhibition efficiency is 96.2%, there are no interfacial pores between the calcium carbonate conversion film and the magnesium alloy substrate, and the bonding strength level is 0.

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