Method for preparing chemical conversion film on surface of metal alloy substrate

Abstract

The present invention relates to the technical field of chemical conversion films, and more specifically, to a method for preparing a chemical conversion film on the surface of a metal alloy substrate. The present invention primarily utilizes different metal phosphates as outer layers, calculates the mismatch between the preferred crystal plane of the inner layer and the preferred crystal planes of different outer layers, and, based on the comparison results, obtains the metal salt with the lowest mismatch as the main salt. Other metal salts are then added to formulate a suitable conversion film solution for forming a highly adhesive phosphate conversion film on the surface of the metal alloy substrate. The preparation method of the present invention can eliminate the interface between the double-layer films formed on the surface of the metal alloy substrate, thereby improving the adhesion of the chemical conversion film while reducing the mismatch of the chemical conversion film and ensuring the corrosion resistance of the chemical conversion film. This addresses the problem of poor adhesion between the double-layer films of a double-layered deposition-type phosphate conversion film, and utilizes the design concept and preparation method of the present invention to reduce trial and error costs.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical conversion films, and in particular to a method for preparing a chemical conversion film on the surface of a metal alloy substrate. Background Art

[0002] Chemical conversion coatings, also known as metal conversion coatings, are formed by the interaction between the surface atoms of a metal or transition metal and the ions in the dielectric medium. This creates a highly adhesive compound barrier layer on the surface of the metal or transition metal. For example, a chemical conversion coating can be found on magnesium alloys.

[0003] Magnesium alloys, due to their excellent specific strength, specific stiffness, electromagnetic shielding ratio, and damping properties, are used in electronics, military applications, optical instruments, and aerospace. However, magnesium alloys have poor corrosion resistance, and surface treatment can effectively improve this resistance. Among various surface treatments, chemical conversion coatings have become an important means of surface protection due to their ease of use and excellent corrosion resistance. Among these, phosphate conversion coatings with a double-layer structure have attracted widespread attention due to their environmental friendliness and high corrosion resistance.

[0004] In most cases, conversion coatings are not used alone. When used as a base layer for organic coatings, the coating's own adhesion can significantly impact the service life of the organic coating. Therefore, while ensuring corrosion resistance, improving the coating's adhesion is crucial.

[0005] According to current research, chemical conversion coatings, such as deposited phosphate conversion coatings, when used as a base layer for organic coatings, require both high corrosion resistance and high adhesion, but achieving both is difficult. While a single-layer conversion coating may have high adhesion, its corrosion resistance is difficult to improve. This necessitates a two-layer structure to guarantee optimal corrosion resistance. Because the two layers are deposited and physically attached to each other, the weak point in adhesion between the two layers of a two-layered deposited phosphate conversion coating is the weak point in adhesion. Summary of the Invention

[0006] In order to solve the problem of poor adhesion between the double-layer films of a double-layer deposition-type phosphate conversion film, the present invention provides a method for preparing a chemical conversion film on the surface of a metal alloy substrate.

[0007] The present invention produces a double-layered, deposited phosphate conversion coating with high adhesion between the two layers. The present invention's method for preparing a chemical conversion coating on a metal alloy substrate can help reduce trial-and-error costs, improve production efficiency, and enhance adhesion between the two layers while maintaining high corrosion resistance.

[0008] To achieve the above objectives, the technical solutions of the present invention are as follows.

[0009] The present invention provides a method for preparing a chemical conversion film on the surface of a metal alloy substrate, comprising the following steps:

[0010] The inner film is determined according to the material of the metal alloy substrate, and different metal phosphates are used as the outer film; the crystal plane of the X-ray diffraction characteristic peak of the inner film is obtained as the preferred crystal plane of the inner film, and the crystal plane of the X-ray diffraction characteristic peak of the outer film is obtained as the preferred crystal plane of the outer film; the matching structure of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film is constructed, and the atomic distance and angle between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film are obtained, and then the mismatch calculation formula is used to calculate the mismatch between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film; the metal salt with the lowest mismatch is obtained as the main salt of the outer film; at least one other metal salt is selected and used together with the main salt as a film-forming substance, and an inorganic phosphate and an additive are added to prepare a conversion film solution; the metal alloy substrate is placed in the conversion film solution for a film-forming reaction to generate a phosphate conversion film with high adhesion on the surface of the metal alloy substrate.

[0011] The present invention primarily utilizes different metal phosphates as the outer layer. By calculating the mismatch between the preferred crystal planes of the inner layer and those of the different outer layers, the metal salt with the lowest mismatch is selected as the primary salt based on the comparison results. Additional metal salts are then added to formulate a suitable conversion coating solution for forming a highly adhesive phosphate conversion coating on the surface of a metal alloy substrate. This preparation method eliminates the interface between the two films formed on the surface of the metal alloy substrate, thereby improving the adhesion of the chemical conversion coating while reducing the mismatch and ensuring the corrosion resistance of the chemical conversion coating.

[0012] Preferably, the method for constructing a matching structure between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer is as follows:

[0013] Obtain the preferred growth orientation and preferred crystal plane of the inner film, as well as the preferred growth orientation and preferred crystal plane of the outer film; construct a two-dimensional lattice mismatch mathematical model of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film, thereby constructing a matching structure of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film.

[0014] In the present invention, based on the X-ray diffraction pattern of the metal phosphate, the three diffraction peaks corresponding to the metal phosphate in the X-ray diffraction pattern are used as the preferred growth orientation, and the crystal plane of the three diffraction peaks is obtained as the preferred crystal plane. The main components of both the inner and outer films are metal phosphate, and the metal element in the inner film is different from the metal element in the outer film.

[0015] Preferably, the method for obtaining the atomic distance and angle between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer is as follows:

[0016] According to the matching structure of the preferred crystal plane of the inner layer membrane and the preferred crystal plane of the outer layer membrane, multiple low-index crystal directions are selected for each preferred crystal plane, and the atomic spacing and angle between the preferred crystal plane of the inner layer membrane and the preferred crystal plane of the outer layer membrane in multiple low-index crystal directions are calculated.

[0017] In the present invention, the low-index crystal orientation is obtained by measuring the orientation on the preferred crystal plane using crystal structure software.

[0018] Preferably, the mismatch calculation formula is:

[0019] ;

[0020] in,( hkl ) s Represents the low-index crystal planes of the nucleation phase of the inner film; ( hkl ) n low-index crystal planes representing the nucleation phase of the outer film; It represents the mismatch between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film; Indicates the i indivual( hkl ) s Low-index crystal orientation on the surface; Indicates the i indivual( hkl ) n Low-index crystal orientation on the surface; Indicates the inner and outer membranes i indivual( hkl ) s The interatomic distances along the low-index crystal orientations on the face; Indicates the inner and outer membranes i indivual( hkl ) n The interatomic distances along the low-index crystal orientations on the face; θ express and Angle.

[0021] Preferably, the low-index crystal plane is a preferred crystal plane obtained based on characteristic peaks of X-ray diffraction; and the low-index crystal orientation is obtained by measuring the orientation on the preferred crystal plane using crystal structure software.

[0022] Preferably, the preferred crystal plane is determined by determining the characteristic X-ray diffraction peak based on the X-ray diffraction pattern of the metal phosphate, and the preferred growth orientation is determined by the characteristic X-ray diffraction peak, thereby obtaining the preferred crystal plane of the metal phosphate.

[0023] Preferably, the metal alloy matrix is a magnesium alloy matrix; the main salt is calcium chloride; the other metal salts are Mn 2+ 、Zn 2+、Sr 2+ and Ba 2+ At least one metal salt.

[0024] Preferably, the auxiliary agent is an oxidant and a complexing agent, the oxidant is sodium nitrate; the complexing agent is EDTA; and the inorganic phosphate is ammonium dihydrogen phosphate.

[0025] Preferably, the content of each substance in each liter of conversion coating solution is:

[0026] Main salt 0.1mol / L~0.15mol / L, each other metal salt 0.05mol / L~0.15mol / L, inorganic phosphate 0.2mol / L~0.3mol / L, oxidant 0.01mol / L~0.04mol / L and complexing agent 0.01mol / L~0.05mol / L.

[0027] Preferably, the temperature of the film-forming reaction is 45° C. to 55° C., and the time of the film-forming reaction is 15 min to 20 min.

[0028] Beneficial effects of the present invention:

[0029] 1. The present invention primarily utilizes different metal phosphates as the outer layer. By calculating the mismatch between the preferred crystal planes of the inner layer and those of the different outer layers, the metal salt with the lowest mismatch is selected as the primary salt based on the comparison results. Additional metal salts are then added to formulate a suitable conversion coating solution for forming a highly adhesive phosphate conversion coating on the surface of a metal alloy substrate. The preparation method of the present invention eliminates the interface between the two layers formed on the surface of the metal alloy substrate, thereby improving the adhesion of the chemical conversion coating while reducing the mismatch and ensuring the corrosion resistance of the chemical conversion coating. This addresses the problem of poor adhesion between the two layers of deposited phosphate conversion coatings with a double-layer structure. Furthermore, the design concept and preparation method of the present invention reduce trial-and-error costs.

[0030] 2. The preparation method of the present invention is not only applicable to phosphate conversion films, but also to all deposition-type conversion films that require improved adhesion. The preparation method of the present invention can provide valuable reference and guidance for the preparation of other deposition-type conversion films that require improved adhesion. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1Schematic diagram of the atomic distances and angles between the (-111) face of MgHPO4 and the corresponding preferred crystal faces of various metal phosphates. Among them, (a1) to (a3) are schematic diagrams of the atomic distances and angles between the (-111) face of MgHPO4 and the (311) face, (111) face and (200) face of MnHPO4; (b1) to (b3) are schematic diagrams of the atomic distances and angles between the (-111) face of MgHPO4 and the (120) face, (-202) face and (200) face of CaHPO4; (c1) to (c3) are schematic diagrams of the atomic distances and angles between the (-111) face of MgHPO4 and the (311) face, (111) face and (200) face of MnHPO4; (c1) to (c3) are schematic diagrams of the atomic distances and angles between the (-111) face of MgHPO4 and the (120) face, (-202) face and (200) face of CaHPO4; (c1) to (c3) are schematic diagrams of the atomic distances and angles between the (-111) face of MgHPO4 and the (311) face, (111) face and (200) face of MnHPO4. Schematic diagram of the atomic spacing and angles of O4 on the (220) plane, (031) plane and (200) plane; (d1) to (d3) are schematic diagrams of the atomic spacing and angles of MgHPO4 (-111) plane and SrHPO4 on the (111) plane, (121) plane and (201) plane respectively; (e1) to (e3) are schematic diagrams of the atomic spacing and angles of MgHPO4 (-111) plane and BaHPO4 on the (202) plane, (111) plane and (022) plane respectively.

[0032] Figure 2 is the mismatch distribution diagram of metal phosphates, where the metal phosphates are MnHPO4, ZnHPO4, CaHPO4, SrHPO4 and BaHPO4.

[0033] Figure 3 Figure 1 is a cross-sectional morphology of the phosphate conversion film. (a) shows the morphology of the phosphate conversion film of control sample 1; (b) shows the morphology of the phosphate conversion film of control sample 2; (c) shows the morphology of the phosphate conversion film of experimental sample 1; and (d) shows the morphology of the phosphate conversion film of experimental sample 2.

[0034] Figure 4 This is the surface micromorphology and element distribution of the phosphate conversion coating of control sample 1 after the drawing test.

[0035] Figure 5 is the adhesion-cumulative probability curve of the phosphate conversion coating of control sample 1, control sample 2, experimental sample 1 and experimental sample 2.

[0036] Figure 6 The following are neutral salt spray images of different phosphate conversion coatings. a is the neutral salt spray image of the phosphate conversion coating of control sample 1; b is the neutral salt spray image of the phosphate conversion coating of control sample 2; c is the neutral salt spray image of the phosphate conversion coating of experimental sample 1; and d is the neutral salt spray image of the phosphate conversion coating of experimental sample 2. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of the present invention.

[0039] X-ray diffraction, the full English name is X-Ray Diffraction, abbreviated as XRD.

[0040] The main salt has a different metal element from other metal salts, and other metal salts are salts of other metals except the metal element of the main salt.

[0041] The technical solution of the present invention is further described below through specific embodiments.

[0042] In the following examples, the methods described are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0043] A method for preparing a chemical conversion film on the surface of a metal alloy substrate comprises the following steps:

[0044] Step 1: Determine the inner layer film according to the material of the metal alloy matrix, and use different metal phosphates as the outer layer film; obtain the crystal plane of the X-ray diffraction characteristic peak of the inner layer film as the preferred crystal plane of the inner layer film, and obtain the crystal plane of the X-ray diffraction characteristic peak of the outer layer film as the preferred crystal plane of the outer layer film.

[0045] For example, the metal alloy substrate is a magnesium alloy substrate, and the chemical conversion film formed on the surface of the magnesium alloy substrate has the metal phosphate on the inner layer of the film being MgHPO4.

[0046] Different metal phosphates are used as the outer membrane, for example, the metal cation is Ca 2+ 、Mn 2+ 、Zn 2+ 、Sr 2+ and Ba 2+ of metal phosphates.

[0047] The preferred crystal plane is determined by determining the characteristic X-ray diffraction peak based on the X-ray diffraction pattern of the metal phosphate, and the preferred growth orientation is determined by the characteristic X-ray diffraction peak, thereby obtaining the preferred crystal plane of the metal phosphate.

[0048] Step 2: construct a matching structure between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, and obtain the atomic distance and angle between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer.

[0049] Step 2.1, obtaining the preferred growth orientation and preferred crystal plane of the inner film, and obtaining the preferred growth orientation and preferred crystal plane of the outer film.

[0050] The specific method is to select the preferred growth orientation based on the X-ray diffraction pattern of the metal phosphate, using the corresponding three diffraction peaks on the X-ray diffraction pattern of the metal phosphate as the preferred growth orientation, and obtaining the crystal plane of the three diffraction peaks as the preferred crystal plane. The main components of both the inner and outer films are metal phosphate, and the metal elements in the inner film are different from the metal elements in the outer film.

[0051] The present invention utilizes different metal phosphates as the outer coating. By calculating the mismatch between the preferred crystal planes of the inner coating and those of the different outer coatings, the metal salt with the lowest mismatch is selected as the primary salt based on the comparison results. Other metal salts are then added to create a suitable conversion coating solution for forming a highly adhesive phosphate conversion coating on the surface of a metal alloy substrate. The preparation method of the present invention eliminates the interface between the two films formed on the surface of the metal alloy substrate, thereby improving the adhesion of the chemical conversion coating while reducing the mismatch and ensuring the corrosion resistance of the chemical conversion coating.

[0052] For example, in a chemical conversion film formed on a magnesium alloy, the metal phosphate on the inner layer is MgHPO4. Because the outer layer nucleates on a specific MgHPO4 crystal plane with a different lattice arrangement, the nucleation of the outer layer is affected by the MgHPO4 crystal plane structure, which in turn affects the growth direction of the outer layer. This leads to a mismatch between the inner and outer layers, which in turn affects the adhesion between the two layers.

[0053] To select an outer film with a low mismatch with MgHPO4, the preferred growth orientation of MgHPO4 was selected for calculation. The three diffraction peaks were determined based on the X-ray diffraction pattern of MgHPO4. The preferred growth orientation was determined by the three diffraction peaks on the X-ray diffraction pattern, and the crystal planes of the three diffraction peaks of MgHPO4 were found to be (-111), (220), and (113).

[0054] When the metal phosphate on the outer film is at least one of CaHPO4, MnHPO4, ZnHPO4, SrHPO4 and BaHPO4, the same method as above is used to determine that the crystal planes of the three strongest diffraction peaks of CaHPO4 are (120), (-202) and (200); the crystal planes of the three strongest diffraction peaks of ZnHPO4 are (220), (031) and (200); the crystal planes of the three strongest diffraction peaks of SrHPO4 are (111), (121) and (201); and the crystal planes of the three strongest diffraction peaks of BaHPO4 are (202), (111) and (022).

[0055] Step 2.2, construct a two-dimensional lattice mismatch mathematical model of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, thereby constructing a matching structure of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer.

[0056] The specific method is: based on the obtained preferred growth orientation and preferred crystal plane of the metal phosphate on the inner film and the metal phosphate on the outer film, crystal modeling software is used to construct a two-dimensional lattice mismatch mathematical model of the inner film at different preferred crystal planes and the outer film at different preferred crystal planes, thereby constructing a matching structure of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film.

[0057] In step 2.3, based on the matching structure of the preferred crystal plane of the inner layer membrane and the preferred crystal plane of the outer layer membrane, multiple low-index crystal directions are selected for each preferred crystal plane, and the atomic distances and angles between the preferred crystal plane of the inner layer membrane and the preferred crystal plane of the outer layer membrane in multiple low-index crystal directions are calculated.

[0058] The embodiment of the present invention mainly uses CrystalMaker crystal modeling software to construct the matching structure of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, and measures the atomic distance and angle of the corresponding preferred crystal plane of each metal phosphate, such as Figure 1 Figure 2 shows the atomic distances and angles between the (-111) plane of MgHPO4 and the corresponding preferred crystal planes of various metal phosphates. The solid spheres represent cations in the outer membrane, such as Mn, Ca, Sr, Zn, or Ba. The hollow spheres represent Mg atoms in MgHPO4.

[0059] Based on the matching structure of the preferred crystal planes of the inner layer and the preferred crystal planes of the outer layer, three low-index crystal orientations are selected for each preferred crystal plane, and the atomic spacing and angles between the metal phosphate on the inner layer and the metal phosphate on the outer layer are calculated for the three low-index crystal planes of the corresponding preferred crystal planes. In the embodiments of the present invention, the low-index crystal planes are preferred crystal planes obtained based on characteristic peaks in X-ray diffraction; the low-index crystal orientations are obtained by measuring the orientations on the preferred crystal planes using crystal structure software. The crystal structure software can be CrystalMaker crystal modeling software.

[0060] For example, the calculation method of the atomic distance and angle between the (220) plane or (113) plane of MgHPO4 and the corresponding preferred crystal plane of each metal phosphate is similar to the above calculation method. The calculation process is as follows: Figure 1 shown.

[0061] Step 3: Calculate the mismatch between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer using the mismatch calculation formula; and obtain the metal salt with the lowest mismatch as the main salt of the outer layer.

[0062] The embodiment of the present invention mainly compares the mismatch between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film to obtain the metal salt with the lowest mismatch as the main salt of the outer film to improve the adhesion of the generated chemical conversion film.

[0063] The mismatch calculation formula is: ;

[0064] in,( hkl ) s Represents the low-index crystal planes of the nucleation phase of the inner film; ( hkl ) n low-index crystal planes representing the nucleation phase of the outer film; It represents the mismatch between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film; Indicates the i indivual( hkl ) s Low-index crystal orientation on the surface; Indicates the i indivual( hkl ) n Low-index crystal orientation on the surface; Indicates the inner and outer membranes i indivual( hkl ) s The interatomic distances along the low-index crystal orientations on the face; Indicates the inner and outer membranes i indivual( hkl ) n The interatomic distances along the low-index crystal orientations on the face; θ express and Angle.

[0065] The low-index crystal plane is a preferred crystal plane obtained based on characteristic peaks of X-ray diffraction; the low-index crystal orientation is obtained by measuring the orientation on the preferred crystal plane using crystal structure software.

[0066] After testing, the atomic spacing of the three low-index crystal directions of the (-111) crystal plane of MgHPO4 is 4.1nm, 8nm and 9.7nm. The measured data of each preferred crystal plane of MnHPO4 are shown in Table 1. Substituting the measured data in Table 1 into the mismatch calculation formula, the corresponding mismatch is obtained. The calculation process of other metal phosphates is similar to the above calculation process. The calculation results are statistically analyzed in Figure 2 .

[0067] Table 1 Mismatch data of the preferred crystal planes of MnHPO4 and the (-111) crystal plane of MgHPO4

[0068]

[0069] Note: d 1. d 2 and d 3 represents the atomic distances between the three low-index directions of the preferred crystal planes of MnHPO4 and the (-111) crystal plane of MgHPO4; f 1. f 2 and f 3 respectively represents the mismatch between the preferred crystal planes of MnHPO4 and the three low-index crystal directions of the (-111) crystal plane of MgHPO4; θ 1. θ 2 and θ 3 represent the angles between the preferred crystal planes of MnHPO4 and the three low-index crystal directions of the (-111) crystal plane of MgHPO4.

[0070] From Table 1 and Figure 2 The results show that since the inner and outer layers are heterogeneous nuclei, the overall mismatch is high. When CaHPO4 is used as the outer layer, the average mismatch is 40.0%, which is lower than that of other metal salts. Therefore, in order to improve the adhesion of the conversion film, Ca 2+ As the main salt of the outer membrane.

[0071] Step 4: Select at least one other metal salt and use it together with the main salt as a film-forming substance, and prepare a conversion coating solution by adding inorganic phosphate and additives; place the metal alloy substrate in the conversion coating solution to carry out a film-forming reaction to form a phosphate conversion coating with high adhesion on the surface of the metal alloy substrate.

[0072] In the embodiment of the present invention, in order to improve the corrosion resistance, it is necessary to add a variety of film-forming substances. A variety of cations have different deposition abilities, and multiple depositions will occur during the reaction process, which will increase the thickness and density of the film layer. Therefore, it is possible to consider adding Ca in the solution. 2+ As the main salt, and choose Mn 2+ 、Zn 2+ 、Sr 2+ and Ba 2+ At least one of the other metal salts serves as a film-forming substance.

[0073] Specifically, the metal alloy matrix is a magnesium alloy matrix; the main salt is calcium chloride; the other metal salts are Mn 2+ 、Zn 2+ 、Sr 2+ and Ba 2+ At least one metal salt. The auxiliary agents are an oxidant and a complexing agent, the oxidant is sodium nitrate; the complexing agent is EDTA; and the inorganic phosphate is ammonium dihydrogen phosphate. Therefore, the content of each substance in each liter of conversion coating solution is:

[0074] The main salt is 0.1mol / L to 0.15mol / L, each other metal salt is 0.05mol / L to 0.15mol / L, the inorganic phosphate is 0.2mol / L to 0.3mol / L, the oxidant is 0.01mol / L to 0.04mol / L, and the complexing agent is 0.01mol / L to 0.05mol / L. The film-forming reaction temperature is 45°C to 55°C, and the film-forming reaction time is 15min to 20min.

[0075] Taking Mg-Gd-Y-Zr rare earth magnesium alloy as an example, a high-adhesion phosphate conversion film is generated on the surface of the metal alloy substrate and used as a sample for performance testing.

[0076] 1. Sample preparation.

[0077] Pretreatment of the Mg-Gd-Y-Zr rare earth magnesium alloy: Wire cutting was used to cut the alloy into specimens measuring 220 mm × 20 mm × 5 mm, 10 mm × 10 mm × 5 mm, and 10 mm × 10 mm × 12 mm for morphology and corrosion resistance testing. The specimens were polished with 800#, 1000#, and silica sandpaper, rinsed with alcohol, and dried.

[0078] Preparation of Experimental Sample 1: The concentrations of the components in the conversion coating solution are: 0.15 mol / L calcium chloride, 0.25 mol / L ammonium dihydrogen phosphate, 0.02 mol / L sodium nitrate, and 0.02 mol / L EDTA. After mixing the components according to the set concentration ratio, the conversion treatment was carried out at 50°C for 20 minutes. The conversion coating solution was then placed in a 50°C water bath to obtain a 50°C heated conversion coating solution. A rare earth magnesium alloy sample was placed in the 50°C heated conversion coating solution and a film-forming reaction was carried out for 20 minutes. The rare earth magnesium alloy sample after the reaction was removed, rinsed with deionized water, and air-dried to obtain Experimental Sample 1.

[0079] Preparation of Experimental Sample 2: The concentrations of the components in the conversion coating solution are as follows: 0.15 mol / L calcium chloride, 0.15 mol / L manganese chloride, 0.1 mol / L zinc chloride, 0.25 mol / L diammonium phosphate, 0.02 mol / L sodium nitrate, and 0.02 mol / L EDTA. After mixing the components according to the prescribed concentration ratios, the conversion treatment was performed at 50°C for 20 minutes. The conversion coating solution was then placed in a 50°C water bath to obtain a 50°C heated conversion coating solution. A rare earth magnesium alloy sample was placed in the 50°C heated conversion coating solution and allowed to form a film for 20 minutes. After the reaction, the rare earth magnesium alloy sample was removed, rinsed with deionized water, and air-dried to obtain Experimental Sample 2.

[0080] Preparation of Control Sample 1: The concentrations of the components in the conversion coating solution are: 0.15 mol / L manganese sulfate, 0.25 mol / L ammonium dihydrogen phosphate, 0.02 mol / L sodium nitrate, and 0.02 mol / L EDTA. After mixing the components according to the prescribed concentration ratios, the conversion treatment was performed at 50°C for 20 minutes. The conversion coating solution was then placed in a 50°C water bath to obtain a 50°C heated conversion coating solution. A rare earth magnesium alloy sample was placed in the 50°C heated conversion coating solution and allowed to form a film for 20 minutes. After the reaction, the rare earth magnesium alloy sample was removed, rinsed with deionized water, and air-dried to obtain Control Sample 1.

[0081] Preparation of Control Sample 2: The concentrations of the components in the conversion coating solution are: 0.1 mol / L zinc chloride, 0.25 mol / L ammonium dihydrogen phosphate, 0.02 mol / L sodium nitrate, and 0.02 mol / L EDTA. After mixing the components according to the prescribed concentration ratios, the conversion treatment was performed at 50°C for 20 minutes. The conversion coating solution was then placed in a 50°C water bath to obtain a 50°C heated conversion coating solution. A rare earth magnesium alloy sample was placed in the 50°C heated conversion coating solution and allowed to form a film for 20 minutes. After the reaction, the rare earth magnesium alloy sample was removed, rinsed with deionized water, and air-dried to obtain Control Sample 2.

[0082] 2. Performance test of samples.

[0083] In order to clarify the weak point of the adhesion of the phosphate conversion coating in the control sample 1, the surface micromorphology and element distribution of the phosphate conversion coating after the pull-out test were observed. The results are as follows: Figure 4 shown.

[0084] Depend on Figure 4After the pull-out test, the surface elements of the phosphate conversion coating in control sample 1 are Mg, P, O, and Gd. In the double-layer phosphate conversion coating, only the inner layer, MgHPO4, remains, while the outer layer, MnHPO4, disappears. Because the inner layer is very thin, Gd is visible in the matrix. This suggests that the weak point in the phosphate conversion coating's adhesion lies between the two layers.

[0085] The morphology of the phosphate conversion films of control sample 1, control sample 2, experimental sample 1 and experimental sample 2 was observed. Figure 3 shown.

[0086] Depend on Figure 3 From the scanning cross-sectional diagram, it can be seen that in control samples 1 and 2, obvious transverse cracks appear in the film layer of the phosphate conversion film. These cracks indicate that the adhesion of the phosphate conversion film is poor, which is due to the high interfacial stress caused by the high mismatch degree. The transverse cracks of the phosphate conversion film of experimental sample 1 are significantly reduced, which also proves that the mismatch degree of CaHPO4 is low, which is conducive to improving the adhesion of the film layer of the phosphate conversion film. However, one cation will cause the film layer of the phosphate conversion film to be too thin, which does not significantly improve the corrosion resistance. The film layer thickness of the phosphate conversion film of experimental sample 2 is similar to that of the phosphate conversion film of control sample 2, but the transverse cracks of the film layer of experimental sample 2 are significantly reduced. It can be seen that the preparation method of experimental sample 2 can significantly improve the adhesion of the phosphate conversion film.

[0087] The adhesion of the phosphate conversion coatings of control sample 1, control sample 2, experimental sample 1 and experimental sample 2 was tested by an acoustic emission adhesion meter. The adhesion-cumulative probability curve is shown in FIG. Figure 5 shown.

[0088] Depend on Figure 5 It can be seen that the adhesion of the phosphate conversion coatings of Control Samples 1 and 2 is approximately 1.5 N. The adhesion of the phosphate conversion coating of Experimental Sample 1 is significantly improved, reaching approximately five times that of the phosphate conversion coatings of Control Samples 1 and 2. The adhesion of the phosphate conversion coating of Experimental Sample 2 is increased to four times that of the phosphate conversion coatings of Control Samples 1 and 2. This is because ions co-precipitate during the reaction, and the high mismatch of Mn and Zn inevitably hinders the deposition of the low mismatch of Ca to some extent. This results in a certain decrease in the adhesion of the phosphate conversion coating of Experimental Sample 2, but it is still much higher than the adhesion of the phosphate conversion coatings of Control Samples 1 and 2.

[0089] In order to verify the long-term corrosion resistance of the film, a 96h neutral salt spray test was carried out. The results are as follows Figure 6 shown.

[0090] Depend on Figure 6It can be seen that the phosphate conversion coatings of control samples 1 and 2 showed discoloration and corrosion spots after the neutral salt spray test. The phosphate conversion coating of experimental sample 1 also had obvious corrosion spots and corrosion products on its surface, which was caused by the film thickness being too low. However, the phosphate conversion coating of experimental sample 2 had only a few small corrosion spots on its surface after 96 hours of neutral salt spray testing due to its uniform film layer and increased film thickness, indicating that the phosphate conversion coating of experimental sample 2 had the best corrosion resistance.

[0091] From the above analysis, it can be seen that by using a metal salt with a low mismatch degree as the main salt and adding other metal salts to form a phosphate conversion film with high adhesion and high corrosion resistance, it is possible to prepare a phosphate conversion film.

[0092] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a chemical conversion film on the surface of a metal alloy substrate, characterized in that: The following steps are involved: The inner layer film is determined according to the material of the metal alloy substrate, and different metal phosphates are used as the outer layer film; the metal alloy substrate is a magnesium alloy substrate, and the inner layer film is MgHPO4; obtaining a crystal plane of a characteristic X-ray diffraction peak of the inner film as a preferred crystal plane of the inner film, and obtaining a crystal plane of a characteristic X-ray diffraction peak of the outer film as a preferred crystal plane of the outer film; Constructing a matching structure between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, obtaining the atomic distance and angle between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, and then using a mismatch calculation formula to calculate the mismatch between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer; A metal salt with the lowest mismatch degree is obtained as the main salt of the outer layer; at least one other metal salt is selected and used together with the main salt as a film-forming substance, and an inorganic phosphate and an additive are added to prepare a conversion coating solution; a metal alloy substrate is placed in the conversion coating solution to undergo a film-forming reaction, thereby forming a phosphate conversion coating with high adhesion on the surface of the metal alloy substrate; The method for constructing a matching structure between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer is as follows: Obtaining the preferred growth orientation and preferred crystal plane of the inner film, and obtaining the preferred growth orientation and preferred crystal plane of the outer film; Constructing a two-dimensional lattice mismatch mathematical model of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, thereby constructing a matching structure of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer; The method for obtaining the atomic distance and angle between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film is as follows: According to the matching structure of the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, multiple low-index crystal directions are selected for each preferred crystal plane, and the atomic distances and angles between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer in the multiple low-index crystal directions are calculated; The mismatch calculation formula is: ； in,( hkl ) s Represents the low-index crystal planes of the nucleation phase of the inner film; ( hkl ) n low-index crystal planes representing the nucleation phase of the outer film; It represents the mismatch between the preferred crystal plane of the inner film and the preferred crystal plane of the outer film; Indicates the i indivual( hkl ) s Low-index crystal orientation on the surface; Indicates the i indivual( hkl ) n Low-index crystal orientation on the surface; Indicates the inner and outer membranes i indivual( hkl ) s The interatomic distances along the low-index crystal orientations on the face; Indicates the inner and outer membranes i indivual( hkl ) n The interatomic distances along the low-index crystal orientations on the face; θ express and Angle; The low-index crystal plane is a preferred crystal plane obtained based on characteristic peaks of X-ray diffraction; and the low-index crystal orientation is obtained by measuring the orientation on the preferred crystal plane using crystal structure software.

2. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 1, characterized in that: The preferred crystal plane is determined by determining the characteristic X-ray diffraction peak based on the X-ray diffraction pattern of the metal phosphate, and the preferred growth orientation is determined by the characteristic X-ray diffraction peak, thereby obtaining the preferred crystal plane of the metal phosphate.

3. The method for preparing a chemical conversion film on a metal alloy substrate according to claim 1, wherein: The main salt is calcium chloride; the other metal salts are Mn 2+ 、Zn 2+ 、Sr 2+ and Ba 2+ At least one metal salt.

4. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 3, characterized in that: The auxiliary agents are an oxidant and a complexing agent, the oxidant is sodium nitrate; the complexing agent is EDTA; and the inorganic phosphate is ammonium dihydrogen phosphate.

5. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 4, characterized in that: The content of each substance in each liter of conversion coating solution is: Main salt 0.1mol / L~0.15mol / L, each other metal salt 0.05mol / L~0.15mol / L, inorganic phosphate 0.2mol / L~0.3mol / L, oxidant 0.01mol / L~0.04mol / L and complexing agent 0.01mol / L~0.05mol / L.

6. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 1 or 5, characterized in that: The temperature of the film-forming reaction is 45° C. to 55° C., and the time of the film-forming reaction is 15 min to 20 min.

Images