Preparation method of chlorine salt conversion film with high corrosion resistance

Abstract

The invention provides a preparation method of a chlorine salt conversion film with high corrosion resistance, which comprises the following steps: dissolving manganese chloride and calcium chloride in water, then adding ammonium dihydrogen phosphate, sodium nitrate and EDTA (Ethylene Diamine Tetraacetic Acid), and carrying out conversion treatment to obtain a conversion film solution containing free Mn < 2 + > and Ca < 2 + > and Mn < 2 + > and Ca < 2 + > complexed by EDTA; placing the conversion film solution in a water bath kettle with the temperature of 50-60 DEG C for heating and heat preservation; the magnesium alloy is put into a conversion film solution, monohydrogen phosphate of Mn < 2 + > preferentially nucleates and grows to form a high-thickness film layer, then monohydrogen phosphate of Mn < 2 + > and Ca < 2 + > complexed by EDTA preferentially performs targeted nucleation at the defect position of the film layer, and a conversion film with high thickness and density is formed on the surface of the magnesium alloy. According to the preparation method of the chlorine salt high-corrosion-resistance conversion film, the preparation cost is low, and the prepared film layer is thick and uniform.

Description

Technical Field

[0001] The present invention relates to the technical field of corrosion-resistant treatment of metal surfaces, and particularly to a method for preparing a chloride salt highly corrosion-resistant conversion film. Background Art

[0002] Due to its good specific strength, specific stiffness, electromagnetic shielding ratio and damping and shock absorption performance, magnesium alloys can be widely used in electronic products, military, optical instruments, aerospace and other fields. However, because magnesium alloys have poor corrosion resistance, surface treatment of magnesium alloys is usually an effective means to improve the corrosion resistance of magnesium alloys. Chemical conversion films have become an important means of surface protection for magnesium alloys due to their environmental friendliness, simple operation and good corrosion resistance. Especially when magnesium alloys are applied in a strong corrosion environment, it is more necessary to prepare a highly corrosion-resistant conversion film on the surface of magnesium alloys.

[0003] From the perspective of industrial production, how to form a highly corrosion-resistant conversion film on the surface of magnesium alloys at low cost is the current research focus. In the existing phosphate conversion film technology, the film-forming solution is mainly manganese-based phosphate. At present, manganese-based phosphate is mainly manganese sulfate. It is difficult to improve the corrosion resistance of manganese salt conversion films and put forward corresponding theoretical guidance. For example, some studies have shown that metal ions such as Mg 2+ , Al 3+ etc. will participate in film formation during the film-forming process, but their influence on film layer deposition has not been studied. Another study pointed out that adding a small amount of Mg 2+ to the conversion film solution can act as a heterogeneous nucleating agent, reduce the nucleation work, increase the nucleation rate, and improve the corrosion resistance of the film layer. The nucleation rate of the film layer is fast. Although it will reduce the defects of the coating to a certain extent, the defects existing in the film layer still cannot be avoided, and the problems existing in its primary nucleation itself still cannot be solved. Because the energy required for growth is much less than the energy required for nucleation, once nucleation occurs, it will tend to grow, and it is difficult to have secondary nucleation again. The defects caused by nucleation cannot be compensated during the growth process. These defects cannot be repaired only by increasing the nucleation rate. The existence of defects will lead to a decrease in the corrosion resistance of the coating. At the same time, due to the fast nucleation rate, the grain growth space is limited, and it is also difficult to increase the film layer thickness. Therefore, it is very difficult for primary nucleation to simultaneously form a conversion film with a high film thickness and a dense and defect-free film layer. However, the film layer thickness and the denseness of the film layer are two conditions that a highly corrosion-resistant conversion film must possess simultaneously.

[0004] Therefore, how to prepare a magnesium alloy conversion film with a uniform film layer and a relatively high film layer thickness at low cost is an important problem that needs to be solved urgently at present. If two nucleations can occur during the film-forming process, the first nucleation increases the film layer thickness, and the second nucleation fills the defects caused by the first nucleation, the problem of defects caused by primary nucleation can be solved, and a conversion film that takes into account both the film layer thickness and denseness can be prepared. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method of a chloride salt highly corrosion-resistant conversion film with low preparation cost and capable of forming a secondary film to make the formed film layer thick, dense and uniform.

[0006] To solve the above technical problem, the present invention provides a preparation method of a chloride salt highly corrosion-resistant conversion film, including the following steps:

[0007] Dissolve manganese chloride and calcium chloride in water, and then add ammonium dihydrogen phosphate, sodium nitrate and EDTA for conversion treatment to obtain a conversion film solution containing free Mn 2+ and Ca 2+ and Mn complexed by EDTA 2+ and Ca 2+ ;

[0008] Place the conversion film solution in a water bath at 50°C - 60°C for heating and insulation;

[0009] Put the magnesium alloy into the conversion film solution, and the monohydrogen phosphate of Mn 2+ preferentially nucleates and grows to form a film layer with high thickness, and then the monohydrogen phosphate of Mn complexed by EDTA 2+ and Ca 2+ preferentially nucleates at the film layer defects in a targeted manner to form a conversion film with high thickness and density on the surface of the magnesium alloy;

[0010] Take out the magnesium alloy, wash and dry the conversion film, and then conduct characterization.

[0011] Furthermore, the concentration of manganese chloride in the conversion film solution is 0.2 - 0.3 mol / L, the concentration of calcium chloride is 0.2 - 0.3 mol / L, the concentration of ammonium dihydrogen phosphate is 0.24 - 0.30 mol / L, the concentration of sodium nitrate is 0.02 - 0.04 mol / L, and the concentration of EDTA is 0.02 - 0.05 mol / L.

[0012] Furthermore, the temperature for dissolving manganese chloride and calcium chloride in water and then adding ammonium dihydrogen phosphate, sodium nitrate and EDTA for conversion treatment is 50 - 60°C, and the conversion treatment time is 10 - 15 min.

[0013] Furthermore, the film-forming temperature of the magnesium alloy in the conversion film solution is 50°C - 60°C, and the film-forming time is 10 - 15 min.

[0014] Furthermore, the film-forming temperature is 60°C.

[0015] Furthermore, the film-forming time is 10 min.

[0016] Further, the magnesium alloy is a magnesium alloy block sized 30mm×30mm×5mm, 10mm×10mm×5mm or 11mm×11mm×12mm cut by wire cutting.

[0017] Further, the magnesium alloy is polished with 800#, 1000#, and 1200# silica sandpaper and then rinsed with alcohol and dried.

[0018] A preparation method of a highly corrosion-resistant chloride conversion film provided by the present invention uses manganese chloride to replace traditional manganese sulfate when preparing the conversion film solution, introduces calcium chloride into the conversion film solution, and simultaneously adds a complexing agent EDTA. During the film formation process, it can solve the problem that different cations participate in film formation relying on metal dissolution or adding different contents of cations as heterogeneous nucleating agents to promote nucleation to improve the nucleation rate of primary nucleation, but cannot essentially solve the defects brought by primary nucleation through secondary nucleation.

[0019] A preparation method of a highly corrosion-resistant chloride conversion film provided by the present invention quickly screens out the cations Mn 2+ and Ca 2+ required for film formation by calculating the precipitation equilibrium phase diagram from the perspective of theoretical guidance, enabling two nucleation processes to occur during the film formation process. First, through the preferential growth of the first nucleation, a film layer with a high thickness is formed. Then, the second batch of nucleation preferentially deposits at the defects of the film layer formed by the first nucleation, achieving targeted repair of the defects of the film layer and realizing the self-densification function of the conversion film during the film formation process, thereby obtaining a conversion film with a high thickness, high density, and uniformity, and solving the problem that the defects of the film layer cannot be repaired during the current film layer deposition process.

[0020] Since the film-forming solution of the present invention will form free Mn 2+ and Ca 2+ monohydrogen phosphates with different deposition abilities and complexed Mn 2+ and Ca 2+ during the film formation process on the magnesium alloy, when forming the film, MnHPO4 with a low K sp preferentially grows through the first nucleation, forming a film layer with a thickness of up to 15 - 20 μm. Due to the intense electrochemical reaction at the defect sites, the pH value and reaction entropy increase, and when the ion concentration at the defect sites reaches the K sp required for the second batch of nucleation, it promotes the second batch of nucleation of the monohydrogen phosphates of complexed Mn 2+ and Ca 2+ , enabling the second batch of nucleation to targetedly fill the defect positions of the film layer generated by the first nucleation, realizing the self-densification of the film layer during the film formation process, and thus ensuring the thickness, uniformity, and densification of the formed conversion film layer.

[0021] The preparation method of a highly corrosion-resistant chloride conversion film provided by the present invention is not only limited to the technical field of magnesium alloy conversion films, but also applicable to the formation of all film layers involving the "dissolution-ionization-deposition" system, and can more efficiently and accurately design the film-forming solution and prepare a highly corrosion-resistant film layer with a high film layer thickness and high film layer uniformity and density.

[0022] Moreover, for the preparation method of a highly corrosion-resistant chloride conversion film provided by the present invention, manganese chloride and calcium chloride in the conversion film solution are cheap and easily available, making the preparation cost of the conversion film relatively low and the economic benefit relatively high, which is worthy of popularization and application. Description of the Drawings

[0023] Figure 1 It is a flow chart of the preparation method of a highly corrosion-resistant chloride conversion film provided by an embodiment of the present invention;

[0024] Figure 2 It is a precipitation equilibrium phase diagram in the preparation method of a highly corrosion-resistant chloride conversion film provided by an embodiment of the present invention;

[0025] Figure 3 It is a diagram of the batch nucleation mechanism in the preparation method of a highly corrosion-resistant chloride conversion film provided by an embodiment of the present invention;

[0026] Figure 4 It is an XRD analysis diagram provided by Comparative Example 1, Comparative Example 2 and Example 1 of the present invention;

[0027] Figure 5 It is a surface morphology diagram of the conversion films prepared by Comparative Example 2 and Example 1 of the present invention;

[0028] Figure 6 It is a cross-sectional morphology diagram of the conversion films prepared by Comparative Example 2 and Example 1 of the present invention;

[0029] Figure 7 It is an internal defect CT analysis diagram of the conversion films prepared by Comparative Example 2 and Example 1 of the present invention;

[0030] Figure 8 It is a Nyquist diagram and a Bode diagram of the electrochemical impedance performance of the magnesium alloy or conversion film prepared by Comparative Example 1, Comparative Example 2 and Example 1 of the present invention after soaking in 3.5% NaCl solution for 0 h;

[0031] Figure 9 It is a Nyquist diagram and a Bode diagram of the electrochemical impedance performance of the magnesium alloy or conversion film prepared by Comparative Example 1, Comparative Example 2 and Example 1 of the present invention after soaking in 3.5% NaCl solution for 24 h;

[0032] Figure 10Nyquist diagrams and Bode diagrams of the electrochemical impedance performance of the magnesium alloys or conversion coatings prepared in Comparative Example 1, Comparative Example 2 and Example 1 of the present invention after immersion in 3.5% NaCl solution for 72 h;

[0033] Figure 11 Macrocorrosion morphologies of the magnesium alloys or conversion coatings prepared in Comparative Example 1, Comparative Example 2 and Example 1 of the present invention after neutral salt spray tests for 24 h, 72 h and 120 h, respectively. Detailed implementation manners

[0034] See Figure 1 A preparation method of a chloride salt highly corrosion-resistant conversion coating provided by an embodiment of the present invention includes the following steps:

[0035] Step 1) Dissolve manganese chloride and calcium chloride in water, and then add ammonium dihydrogen phosphate, sodium nitrate and EDTA for conversion treatment to obtain a conversion coating solution containing free Mn 2+ and Ca 2+ and Mn 2+ complexed with EDTA and Ca 2+ for realizing two-stage nucleation.

[0036] Among them, the concentration of manganese chloride in the conversion coating solution is 0.2 - 0.3 mol / L, the concentration of calcium chloride is 0.2 - 0.3 mol / L, the concentration of ammonium dihydrogen phosphate is 0.24 - 0.30 mol / L, the concentration of sodium nitrate is 0.02 - 0.04 mol / L, and the concentration of EDTA is 0.02 - 0.05 mol / L.

[0037] Among them, when manganese chloride and calcium chloride are dissolved in water and then ammonium dihydrogen phosphate, sodium nitrate and EDTA are added for conversion treatment, the temperature is 50 - 60 °C and the conversion treatment time is 10 - 15 min.

[0038] In the conversion coating solution prepared by the present invention, manganese chloride and calcium chloride are relatively low in price and easy to obtain compared with manganese salts and calcium salts such as manganese sulfate, manganese nitrate and calcium nitrate. Thus, the preparation cost is relatively low and the economic benefit is relatively high.

[0039] Moreover, manganese chloride and calcium chloride can promote two-stage nucleation during the film formation process. The first-stage nucleation can increase the thickness of the film layer, and the second-stage nucleation can targetedly fill the film layer defect positions generated by the first-stage nucleation, ensuring the thickness and self-density of the formed conversion coating film layer, thereby greatly improving the corrosion resistance of the formed conversion coating.

[0040] See Table 1 for the precipitation equilibrium constants of several different film-forming substances. According to the data in Table 1, the thermodynamic precipitation equilibrium phase diagrams of several film-forming substances can be calculated using the precipitation equilibrium formula, and the obtained phase diagrams are as shown in Figure 2 shown.

[0041] Table 1. Precipitation Equilibrium Constants of Each Film-Forming Substance

[0042] substance <![CDATA[MnHPO4]]> <![CDATA[CaHPO4]]> <![CDATA[SrHPO4]]> <![CDATA[BaHPO4]]> <![CDATA[MgHPO4 <!-- 3 -->]]> <![CDATA[K sp > <![CDATA[3.6×10 -8 > <![CDATA[1.0×10 -7 > <![CDATA[2.1×10 -7 > <![CDATA[3.2×10 -7 > <![CDATA[4.3×10 -7 >

[0043] The conversion film solution of the present invention is designed from the perspective of the thermodynamic phase diagrams of different film-forming substances. It can be seen from the precipitation equilibrium constants and the corresponding precipitation equilibrium phase diagrams of several film-forming substances that the K of MnHPO4 sp is the lowest, and it is the easiest to deposit during the film-forming process. CaHPO4 comes second, while the K of BaHPO4, SrHPO4, and MgHPO4 sp is relatively large.

[0044] Due to a method for preparing a chloride salt highly corrosion-resistant conversion film provided by the present invention, Mn 2+ and Ca 2+ are selected as the main film-forming cations of the conversion film solution. After introducing these two cations of Mn 2+ and Ca 2+ , because there is a complexing agent EDTA in the solution, through the complexing action of EDTA, Mn 2+ and Ca 2+ simultaneously complexed will be formed in the solution. In this way, three forms of ions will be formed in the film-forming solution, namely, free Mn 2+ and Ca 2+ as well as Mn 2+ and Ca 2+ simultaneously complexed, and their reactions are shown in formulas (1) and (2).

[0045]

[0046]

[0047] Therefore, manganese chloride and calcium chloride are dissolved in water, and then ammonium dihydrogen phosphate, sodium nitrate, and EDTA are added for conversion treatment to obtain a conversion film solution containing free Mn 2+ , Ca 2+ as well as Mn 2+ and Ca 2+ complexed by EDTA.

[0048] Step 2) Place the conversion film solution in a water bath at 50 - 60 °C and heat it for insulation.

[0049] Step 3) Put the magnesium alloy into the conversion film solution. During film formation, the primary hydrogen phosphate of Mn 2+ preferentially nucleates and grows to form a high-thickness film layer, and then the primary hydrogen phosphate of Mn 2+ and Ca 2+ complexed by EDTA preferentially nucleates at the film layer defects in a targeted manner to form a conversion film with high thickness and density on the surface of the magnesium alloy.

[0050] When magnesium alloy is added to the conversion film solution containing free Mn 2+ and Ca 2+ and Mn complexed with EDTA 2+ and Ca 2+ , the following reactions will occur:

[0051] Mg - 2e → Mg 2+ (3)

[0052] H + + 2e → H2 (4)

[0053]

[0054]

[0055] In this way, HPO4 - will combine with Mn 2+ , Ca 2+ to form MnHPO4 and CaHPO4 respectively. At the same time, it also combines with the complexed ions Mn 2 + and Ca 2+ to form (Mn,Ca)HPO4.

[0056] Moreover, the complexed ions Mn 2+ and Ca 2+ will deposit simultaneously during the nucleation process and cause doping phenomenon, making the K sp of the complexed (Mn,Ca)HPO4 between that of MnHPO4 and CaHPO4. Among them, the batch nucleation mechanism diagrams of several substances are as Figure 3 shown.

[0057] When the magnesium alloy is placed in the conversion film solution provided by the present invention, during the film formation process, MnHPO4 with a low K sp nucleates preferentially. As the reaction proceeds, due to the intense film formation reaction at the defect sites, the hydrogen evolution reaction is accelerated and the consumed hydrogen ions increase significantly, so the pH value at the defect sites increases significantly (as shown in reaction formulas (3)-(4)). At the same time, it promotes the ionization of dihydrogen phosphate to more monohydrogen phosphate (as shown in reaction formula (5)), and the ion concentration at the defect sites reaches the K sp required for the second batch of nucleation. At this time, (Mn,Ca)HPO4 starts the second batch of nucleation (as shown in reaction formula (6), Me 2+ represents Mn 2+ or Ca 2+) Therefore, the second batch of nucleation preferentially deposits at the defects, thereby enabling targeted repair of the defects in the coating, and at the same time, since the second batch of nucleation fills the defect sites in the coating, self-compaction of the conversion coating during the film formation process is achieved.

[0058] Therefore, according to the precipitation equilibrium phase diagram of each film-forming substance, the present invention selects Mn 2+ and Ca 2+ as the film-forming cations. By controlling the batch nucleation of different substances, the first nucleation preferentially grows, increasing the coating thickness. Subsequently, the second nucleation deposits at the defects caused by the first nucleation, achieving self-compaction of the coating, and improving the coating thickness and density of the conversion coating, thereby significantly enhancing the corrosion resistance of the conversion coating.

[0059] Among them, the magnesium alloy is cut into magnesium alloy blocks with sizes of 30mm×30mm×5mm, 10mm×10mm×5mm, or 11mm×11mm×12mm by wire cutting.

[0060] And, after the magnesium alloy is cut into blocks, the surface oxide layer of the magnesium alloy is polished successively with 800#, 1000#, and 1200# silica sandpaper, then rinsed with alcohol and dried for use.

[0061] Among them, the film-forming temperature of the magnesium alloy in the conversion coating solution is controlled at 50°C - 60°C, and the film-forming time is controlled within 10 - 15 minutes.

[0062] If the film-forming temperature is lower than 50°C, the film-forming reaction rate will be slower, which easily leads to fewer nuclei in the coating and uneven coating coverage, resulting in poor corrosion resistance of the conversion coating. However, if the film-forming temperature is higher than 60°C, since the conversion coating solution itself is in a supersaturated state, too high a temperature will cause the conversion coating solution to decompose and fail, affecting film formation. Therefore, during the film formation process of the magnesium alloy, controlling the film-forming temperature at 50°C - 60°C can not only ensure the film-forming effect and film-forming reaction rate of the conversion coating solution itself, enable sufficient nucleation of the coating, and make the coating coverage uniform, but also promote sufficient film formation of the conversion coating, ensure the film-forming thickness of the conversion coating, and ensure the density of the coating, thereby enabling the coating to have high corrosion resistance.

[0063] Regarding the film-forming time, if the film-forming time is less than 10 minutes, the film-forming time is short and the thickness of the formed coating is thin. However, if the film-forming time is longer than 15 minutes, it is also difficult to further increase the coating thickness, which will affect the film-forming efficiency and the formed coating is prone to cracking. Therefore, controlling the film-forming time within a certain range can not only ensure the thickness of the coating, avoid cracking of the conversion coating, but also enable sufficient nucleation of the coating, ensure the uniformity of the coating, and ensure the density of the coating, thereby enabling the formed conversion coating to have high corrosion resistance.

[0064] Therefore, for the preparation method of a highly corrosion-resistant conversion film of chloride salt provided by the present invention, by controlling the film-forming temperature at 50°C - 60°C and the film-forming time at 10 - 15 min, not only can the film-forming effect and film-forming reaction rate of the conversion film solution itself be ensured, enabling the film layer to nucleate sufficiently and the film layer to cover evenly, but also it can promote the full film formation of the conversion film, ensure the thickness of the conversion film, and avoid cracking of the conversion film, thereby making the formed conversion film have high corrosion resistance.

[0065] As an optimal specific implementation manner of the present invention, the optimal film-forming temperature of the magnesium alloy in the conversion film solution is preferably 60°C.

[0066] Moreover, as an optimal specific implementation manner of the present invention, the optimal film-forming time of the magnesium alloy in the conversion film solution is preferably 10 min.

[0067] Step 4) After the film formation of the magnesium alloy is completed, take out the magnesium alloy and rinse the conversion film on its surface with deionized water, then air-dry it, and perform subsequent characterization after 10 - 15 h.

[0068] The following specifically illustrates the preparation method of a highly corrosion-resistant conversion film of chloride salt provided by the present invention through comparative examples and examples.

[0069] Comparative Example 1

[0070] In this comparative example, only the magnesium alloy substrate is prepared without performing conversion treatment on the magnesium alloy substrate.

[0071] (1) Use wire cutting to cut the magnesium alloy into pieces of 30 mm × 30 mm × 5 mm for morphology and corrosion resistance test characterization.

[0072] (2) Grind the specimens successively with 800#, 1000#, and 1200# silicon carbide sandpaper, and rinse and blow dry with alcohol.

[0073] The XRD phase analysis results of the magnesium alloy in Comparative Example 1 of the present invention are as shown in Figure 4 the (a) curve in.

[0074] In order to characterize the electrochemical impedance performance of the magnesium alloy conversion film in this Comparative Example 1, immerse the magnesium alloy obtained in this Comparative Example 1 in 3.5% NaCl solution, and characterize the electrochemical impedance performance of the magnesium alloy by soaking for different times. Among them, the Nyquist diagram of the magnesium alloy obtained in this Comparative Example 1 soaked in 3.5% NaCl solution for 0 h is as shown in Figure 8 (a), and the Bode diagram of the magnesium alloy obtained in this Comparative Example 1 soaked in 3.5% NaCl solution for 0 h is as shown in Figure 8 (b); the Nyquist diagram of the magnesium alloy obtained in this Comparative Example 1 soaked in 3.5% NaCl solution for 24 h is as shown in Figure 9(a) shows the Bode plot obtained by immersing in 3.5% NaCl solution for 24 h as Figure 9 (b) shows; the Nyquist plot obtained by immersing the magnesium alloy obtained in this Comparative Example 1 in 3.5% NaCl solution for 72 h is as Figure 10 (e) shows, and the Bode plot obtained by immersing in 3.5% NaCl solution for 72 h is as Figure 10 (f) shows.

[0075] In order to evaluate the long-term corrosion resistance of the magnesium alloy in this Comparative Example 1, a neutral salt spray test was carried out on the magnesium alloy in this Comparative Example 1. The macroscopic corrosion morphology of its conversion film after neutral salt spray tests at 24 h, 72 h and 120 h is as Figure 11 shown.

[0076] Comparative Example 2

[0077] In this Comparative Example, after the magnesium alloy substrate was prepared, ordinary conversion treatment was carried out on the magnesium alloy substrate.

[0078] (1) The magnesium alloy was cut into pieces of 30 mm×30 mm×5 mm by wire cutting for morphology and corrosion resistance test characterization.

[0079] (2) The specimens were successively polished with 800#, 1000#, and 1200# silicon carbide sandpaper and rinsed and dried with alcohol.

[0080] (3) Chemical conversion treatment: For the conversion treatment, 0.25 mol / L of manganese sulfate was dissolved in water, and then 0.27 mol / L of ammonium dihydrogen phosphate, 0.03 mol / L of sodium nitrate, and 0.04 mol / L of EDTA were added, and the conversion treatment was carried out at a temperature of 60 °C for 10 min.

[0081] (4) The above solution was placed in a water bath at 60 °C.

[0082] (5) The magnesium alloy was put into the above heated conversion film solution to form a film for 10 min.

[0083] (6) The specimens were taken out, rinsed with deionized water, and dried in air, and characterization was carried out after 12 h.

[0084] The XRD phase analysis results of the magnesium alloy conversion film prepared in Comparative Example 2 of the present invention are as Figure 4 shown in curve (b) in.

[0085] The surface morphology of the magnesium alloy conversion film prepared in Comparative Example 2 of the present invention is as Figure 5 (a) shows, and the cross-sectional morphology of the magnesium alloy conversion film prepared in Comparative Example 2 is as Figure 6 (a) shows.

[0086] To characterize the compactness of the magnesium alloy conversion film prepared in Comparative Example 2 of the present invention, CT analysis was performed on the internal defects of the magnesium alloy conversion film of this Comparative Example 2, and the CT analysis diagram of the internal defects of the obtained conversion film is as shown in Figure 7 (a).

[0087] To characterize the electrochemical impedance performance of the magnesium alloy conversion film of this Comparative Example 2, the magnesium alloy with surface film formation obtained in this Comparative Example 2 was immersed in 3.5% NaCl solution, and the electrochemical impedance performance of the conversion film on its surface was characterized by soaking for different times. Among them, the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Comparative Example 2 after soaking in 3.5% NaCl solution for 0 h is as shown in Figure 8 (a), and the Bode diagram obtained after soaking in 3.5% NaCl solution for 0 h is as shown in Figure 8 (b); the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Comparative Example 2 after soaking in 3.5% NaCl solution for 24 h is as shown in Figure 9 (a), and the Bode diagram obtained after soaking in 3.5% NaCl solution for 24 h is as shown in Figure 9 (b); the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Comparative Example 2 after soaking in 3.5% NaCl solution for 72 h is as shown in Figure 10 (e), and the Bode diagram obtained after soaking in 3.5% NaCl solution for 72 h is as shown in Figure 10 (f).

[0088] To evaluate the long-term corrosion resistance of the magnesium alloy conversion film of this Comparative Example 2, a neutral salt spray test was carried out on the conversion film on the surface of the magnesium alloy of this Comparative Example 2. The macroscopic corrosion morphology of its conversion film after neutral salt spray tests for 24 h, 72 h and 120 h is as shown in Figure 11 .

[0089] Example 1

[0090] A method for preparing a chloride salt highly corrosion-resistant conversion film provided in this example includes the following steps:

[0091] (1) Use wire cutting to cut the magnesium alloy into pieces with a size of 30 mm × 30 mm × 5 mm for morphology and corrosion resistance test characterization.

[0092] (2) Grind the specimen successively with 800#, 1000#, and 1200# silicon carbide sandpaper, and rinse and dry it with alcohol.

[0093] (3) Chemical conversion treatment: For the conversion treatment, 0.25 mol / L of manganese chloride is dissolved in water, 0.25 mol / L of calcium chloride is dissolved in water, and then 0.27 mol / L of ammonium dihydrogen phosphate, 0.03 mol / L of sodium nitrate, and 0.04 mol / L of EDTA are added. The conversion treatment is carried out at a temperature of 60 °C for 10 min.

[0094] (4) Put the above solution into a water bath at 60 °C.

[0095] (5) Put the magnesium alloy into the above heated conversion film solution and form a film for 10 min.

[0096] (6) Take out the specimen, rinse it with deionized water, and air-dry it. Perform characterization after 12 h.

[0097] The XRD phase analysis results of the magnesium alloy conversion film prepared in Example 1 of the present invention are as shown in the Figure 4 curve (c).

[0098] From Figure 4 it can be seen that for Comparative Example 1, there are mainly diffraction peaks of the magnesium alloy matrix and the second phase. For Comparative Example 2, in addition to the diffraction peaks of the magnesium alloy matrix and the second phase, the main components MgHPO4 and MnHPO4 of the conversion film layer also exist. In Example 1, in addition to the diffraction peak of the second phase, there is a left-shifted MnHPO4 peak, which is due to Ca 2+ being doped into the lattice of MnHPO4, replacing the site of Mn 2+ , resulting in lattice expansion, which is the main component of the second batch of nucleation. This result shows that Example 1 is batch nucleation, which can effectively increase the film layer thickness and the uniformity of the film layer, thereby improving the corrosion resistance of the conversion film.

[0099] The surface morphology of the magnesium alloy conversion film prepared in Example 1 of the present invention is as shown in Figure 5 (b), and the cross-sectional morphology of the magnesium alloy conversion film prepared in Example 1 of the present invention is as shown in Figure 6 (b).

[0100] From Figure 5 it can be seen that for Comparative Example 2, when the film-forming cation is only Mn 2+ , the number of nuclei is large and the crystal nuclei are difficult to grow, which results in a smaller grain size of the film layer structure. For Example 1, due to the low precipitation equilibrium constant of MnHPO4, it preferentially nucleates and preferentially grows. When Ca 2+ is introduced, during the film-forming process, Ca 2+ and Mn 2+Doping occurs, which results in the precipitation equilibrium constant being between Mn and Ca. With the increase of pH during the reaction, the second nucleation occurs, forming small grains, and finally forming a structure with coexisting large and small grains. The small grains fill the defects in the film layer, thus ensuring the uniformity of the conversion film layer.

[0101] From Figure 6 It can be seen that the film layer thickness of the conversion film cross-section of Comparative Example 2 is about 10 μm, and the film layer thickness is uneven, with relatively weak areas in the outer film. While the film layer thickness of the conversion film cross-section of Example 1 is uniform and the film layer thickness increases. The overall film layer thickness is about 15 - 20 μm, and the film layer thickness increases after secondary nucleation.

[0102] Therefore, in Example 1 of the present invention, by controlling batch nucleation, the film layer thickness and the uniformity of the film layer are effectively improved, thereby improving the corrosion resistance of the conversion film.

[0103] In order to characterize the compactness of the magnesium alloy conversion film prepared in Example 1 of the present invention, CT analysis was performed on the internal defects of the magnesium alloy conversion film of this Example 1, and the CT analysis diagram of the internal defects of the obtained conversion film is as Figure 7 (b) shown.

[0104] Internal defects of the conversion film are an important factor that reduces the corrosion resistance of the film layer. From Figure 7 (a) and Figure 7 (b) comparison, it can be seen that the size of the internal defects of the conversion film in this Example 1 is reduced compared to Comparative Example 2, and the maximum size defect is reduced from 22 μm to 12 μm. These defects are caused by the non-compact grain structure of the film layer, which can prove that the compactness of the conversion film in Example 1 of the present invention has been greatly improved.

[0105] In order to characterize the electrochemical impedance performance of the magnesium alloy conversion film of this Example 1, the magnesium alloy with surface film formation obtained in Example 1 of the present invention was immersed in 3.5% NaCl solution, and the electrochemical impedance performance of the conversion film on its surface was characterized by soaking for different times. Among them, the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Example 1 soaked in 3.5% NaCl solution for 0 h is as Figure 8 (a) shown, and the Bode diagram of the conversion film of the magnesium alloy obtained in this Example 1 soaked in 3.5% NaCl solution for 0 h is as Figure 8 (b) shown; the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Example 1 soaked in 3.5% NaCl solution for 24 h is as Figure 9 (a) shown, and the Bode diagram of the conversion film of the magnesium alloy obtained in this Example 1 soaked in 3.5% NaCl solution for 24 h is as Figure 9 (b) shown; the Nyquist diagram of the conversion film of the magnesium alloy obtained in this Example 1 soaked in 3.5% NaCl solution for 72 h is asFigure 10 As shown in (e), the Bode diagram obtained by soaking in 3.5% NaCl solution for 72 h is as Figure 10 shown in (f).

[0106] The magnitude of the impedance modulus represents the level of corrosion resistance. From Figure 8 (a) and Figure 8 (b), it can be seen that at the initial stage of soaking, the capacitive arcs at high frequencies of the magnesium alloys after chemical conversion treatment in Comparative Example 2 and Example 1 increased, and the capacitive arc of Example 1 was the largest. Moreover, the modulus value |Z|0.01Hz of Example 1 was significantly increased by 12942 (Ω·cm2) compared with the modulus value |Z|0.01Hz of Comparative Example 2. This result indicates that the corrosion resistance of Example 1 is higher than that of Comparative Example 2. Furthermore, referring to Figure 9 (a) and Figure 9 (b), as the soaking time prolongs, at 24 h of soaking, although the modulus value of Example 1 decreased to 11091 (Ω·cm2), it was still higher than the modulus value of Comparative Example 2, indicating that the corrosion resistance of Example 1 was still higher than that of Comparative Example 2 although it decreased slightly under long-term soaking. Referring to Figure 10 (e) and Figure 10 (f), at 72 h of soaking, the impedance modulus value of Example 1 was still three times that of Comparative Example 2, still having good corrosion resistance.

[0107] In order to evaluate the long-term corrosion resistance of the conversion film of the magnesium alloy in Example 1, a neutral salt spray test was carried out on the conversion film on the surface of the magnesium alloy in Example 1. The macroscopic corrosion morphology of the conversion film after 24 h, 72 h, and 120 h of neutral salt spray test is as Figure 11 shown.

[0108] From Figure 11 it can be seen that at 24 h of the salt spray test, only a small amount of corrosion appeared at the edge of the conversion film of Example 1, and no corrosion appeared on the surface. More corrosion points had appeared in Comparative Example 1 and Comparative Example 2. At 120 h of the salt spray test, only a small amount of corrosion points appeared on the surface of the conversion film of Example 1. The corrosion products of Comparative Example 1 almost covered the surface, and the number of corrosion points of Comparative Example 2 increased. Thus, it can be explained that the corrosion resistance of the magnesium alloy conversion film prepared in Example 1 of the present invention is much higher than that of the traditional conversion film.

[0109] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A preparation method of a highly corrosion-resistant conversion film of chloride salt, characterized in that, It includes the following steps: Dissolve manganese chloride and calcium chloride in water, and then add ammonium dihydrogen phosphate, sodium nitrate and EDTA for conversion treatment to obtain a conversion film solution containing free Mn 2+ and Ca 2+ and Mn complexed by EDTA 2+ and Ca 2+ ; Place the conversion film solution in a water bath at 50°C - 60°C for heating and insulation; Put the magnesium alloy into the conversion film solution, and the monohydrogen phosphate of Mn 2+ preferentially nucleates and grows to form a film layer with a high thickness. Then, the Mn 2+ complexed with EDTA and the monohydrogen phosphate of Ca 2+ preferentially target nucleate at the film layer defects to form a conversion film with a high thickness and density on the surface of the magnesium alloy; Take out the magnesium alloy, wash and dry the conversion film, and then conduct characterization.

2. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 1, characterized in that: In the conversion film solution, the concentration of manganese chloride is 0.2 - 0.3 mol / L, the concentration of calcium chloride is 0.2 - 0.3 mol / L, the concentration of ammonium dihydrogen phosphate is 0.24 - 0.30 mol / L, the concentration of sodium nitrate is 0.02 - 0.04 mol / L, and the concentration of EDTA is 0.02 - 0.05 mol / L.

3. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 2, characterized in that: The manganese chloride and calcium chloride are dissolved in water, and then ammonium dihydrogen phosphate, sodium nitrate and EDTA are added. The temperature of the conversion treatment is 50 - 60°C, and the conversion treatment time is 10 - 15 min.

4. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 1, characterized in that: The film formation temperature of the magnesium alloy in the conversion film solution is 50°C - 60°C, and the film formation time is 10 - 15 min.

5. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 4, characterized in that: The film formation temperature is 60°C.

6. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 4, characterized in that: The film formation time is 10 min.

7. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 1, characterized in that: The magnesium alloy is a magnesium alloy block with sizes of 30mm×30mm×5mm, 10mm×10mm×5mm or 11mm×11mm×12mm cut by wire cutting.

8. The preparation method of the chloride salt highly corrosion-resistant conversion film according to claim 7, characterized in that: The magnesium alloy is polished with 800#, 1000#, 1200# silicon carbide sandpaper and then rinsed with alcohol and dried by blowing.

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