Preparation method of chemical conversion coating on surface of metal alloy matrix

Abstract

The invention relates to the technical field of chemical conversion films, in particular to a preparation method of a chemical conversion film on the surface of a metal alloy matrix. According to the method, different metal phosphates are mainly used as outer layer films, the metal salt with the lowest mismatch degree is obtained as the main salt according to the comparison result by calculating the mismatch degree of the preferred crystal face of the inner layer film and the preferred crystal faces of the different outer layer films, and the proper conversion film solution is prepared by adding other metal salts; the method is used for generating a high-adhesion phosphate conversion film on the surface of a metal alloy matrix. According to the preparation method, an interface between two layers of films generated on the surface of the metal alloy matrix can be eliminated, so that the adhesive force of the chemical conversion film is improved under the conditions of reducing the mismatch degree of the chemical conversion film and ensuring the corrosion resistance of the chemical conversion film, and the service life of the chemical conversion film is prolonged. The problem that the adhesive force between the two layers of the deposition type phosphate conversion film of a double-layer structure is poor is solved, and the trial and error cost is reduced by means of the design concept and the preparation method.

Description

Technical Field

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

[0002] A chemical conversion coating, also known as a metal conversion coating, is a well-adhered compound isolation layer formed on the surface of a metal or a transition layer metal by the interaction of surface atoms of the metal or the transition layer metal with ions in the medium. For example, the chemical conversion coating on the surface of a magnesium alloy.

[0003] Magnesium alloys can be used in the fields of electronic products, military, optical instruments, and aerospace due to their good specific strength, specific stiffness, electromagnetic shielding ratio, and damping and shock absorption performance. However, magnesium alloys have poor corrosion resistance, and surface treatment can effectively improve corrosion resistance. Among various surface treatments, chemical conversion coatings have become an important means of surface protection due to their simple operation and good corrosion resistance. Among them, phosphate conversion coatings with a double-layer structure have received extensive attention due to requirements such as environmental friendliness and high corrosion resistance.

[0004] In most cases, the conversion coating is generally not used alone. When used as the bottom layer of an organic coating, the adhesion of the conversion coating itself will have a great impact on the service life of the organic coating. Therefore, on the premise of ensuring corrosion resistance, it is particularly important to improve the adhesion of the conversion coating.

[0005] Regarding current research, chemical conversion coatings, such as deposited phosphate conversion coatings, need to have high corrosion resistance and high adhesion when used as the bottom layer of an organic coating, but it is difficult to achieve both at the same time. Because a conversion coating with only one layer has high adhesion, but its corrosion resistance is difficult to improve, which means that the conversion coating must have a two-layer structure to ensure its corrosion resistance. Since the film layer is a deposited film layer and the two film layers are physically adhered to each other, the interface between the two layers of the double-layer deposited phosphate conversion coating is the weak point of the adhesion of the conversion coating. Summary of the Invention

[0006] To solve the problem of poor adhesion between the two layers of the double-layer deposited phosphate conversion coating, the present invention provides a method for preparing a chemical conversion coating on the surface of a metal alloy substrate.

[0007] The double-layer deposited phosphate conversion coating prepared by the present invention has high adhesion between the two layers. The method for preparing the chemical conversion coating on the surface of the metal alloy substrate of the present invention can help reduce the trial-and-error cost and improve the preparation efficiency, while maintaining high corrosion resistance, improving the adhesion between the two layers of the film.

[0008] To achieve the above object, the technical solution of the present invention is 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: Determine the inner layer film according to the material of the metal alloy substrate, 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; construct a matching structure between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film, and obtain the atomic spacing and included angle between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film, and then use the mismatch degree calculation formula to calculate the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film; obtain the metal salt with the lowest mismatch degree as the main salt of the outer layer film; select at least one other metal salt and use it together with the main salt as the film-forming substance, and configure a conversion film solution by adding inorganic phosphate and additives; place the metal alloy substrate in the conversion film solution for film-forming reaction to generate a phosphate conversion film with high adhesion on the surface of the metal alloy substrate.

[0010] The present invention mainly uses different metal phosphates as the outer layer film. By calculating the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal planes of different outer layer films, according to the comparison results, obtain the metal salt with the lowest mismatch degree as the main salt, and configure a suitable conversion film solution by adding other metal salts for generating a phosphate conversion film with high adhesion 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. Thus, while reducing the mismatch degree of the chemical conversion film and ensuring the corrosion resistance of the chemical conversion film, the adhesion of the chemical conversion film is improved.

[0011] Preferably, the method for constructing the matching structure between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film is as follows: Obtain the preferred growth orientation and preferred crystal plane of the inner layer film, and obtain the preferred growth orientation and preferred crystal plane of the outer layer film; construct a two-dimensional lattice mismatch degree mathematical model of the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film, so as to construct the matching structure between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film.

[0012] In the present invention, according to the X-ray diffraction pattern of the metal phosphate, use the corresponding three strongest diffraction peaks on the X-ray diffraction pattern of the metal phosphate as the preferred growth orientation, and obtain the crystal plane of the three strongest diffraction peaks as the preferred crystal plane. Among them, the main components of the inner layer film and the outer layer film are both metal phosphates, and the metal elements on the inner layer film are different from the metal elements on the outer layer film.

[0013] Preferably, the method for obtaining the atomic spacing and included angle between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film is as follows: According to the matching structure of the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film, 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 film and the preferred crystal plane of the outer layer film are calculated in multiple low-index crystal direction orientations.

[0014] In the present invention, the low-index crystal direction is obtained by measuring the orientation on the preferred crystal plane through a crystal structure software.

[0015] Preferably, the mismatch degree calculation formula is: ; wherein, ( hkl ) s represents the low-index crystal plane of the nucleation phase of the inner layer film; ( hkl ) n represents the low-index crystal plane of the nucleation phase of the outer layer film; represents the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film; represents the i th ( hkl ) s low-index crystal direction on the plane; represents the i th ( hkl ) n low-index crystal direction on the plane; represents the atomic spacing between the inner layer film and the outer layer film in the orientation of the low-index crystal direction on the i th ( hkl ) s plane; represents the atomic spacing between the inner layer film and the outer layer film in the orientation of the low-index crystal direction on the i th ( hkl ) n plane; θ represents the and angle between.

[0016] Preferably, the low-index crystal plane is the preferred crystal plane obtained according to the X-ray diffraction characteristic peak; the low-index crystal direction is obtained by measuring the orientation on the preferred crystal plane through a crystal structure software.

[0017] Preferably, the preferred crystal plane is determined by determining the X-ray diffraction characteristic peak according to the X-ray diffraction pattern of the metal phosphate, and the preferred growth orientation is determined through the X-ray diffraction characteristic peak, thereby obtaining the preferred crystal plane of the metal phosphate.

[0018] Preferably, the metal alloy substrate is a magnesium alloy substrate; the main salt is calcium chloride; the other metal salts are Mn 2+ , Zn 2+ , Sr 2+ and Ba2+ Metal salts of at least one of

[0019] Preferably, the auxiliary agents are an oxidizing agent and a complexing agent. The oxidizing agent is sodium nitrate; the complexing agent is EDTA; the inorganic phosphate is ammonium dihydrogen phosphate.

[0020] Preferably, the content of each substance in each liter of the conversion film solution is as follows: The main salt is 0.1 mol / L to 0.15 mol / L, each other metal salt is 0.05 mol / L to 0.15 mol / L, the inorganic phosphate is 0.2 mol / L to 0.3 mol / L, the oxidizing agent is 0.01 mol / L to 0.04 mol / L, and the complexing agent is 0.01 mol / L to 0.05 mol / L.

[0021] 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.

[0022] Advantages of the present invention: 1. The present invention mainly uses different metal phosphates as the outer layer film. By calculating the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal plane of different outer layer films, according to the comparison results, the metal salt with the lowest mismatch degree is obtained as the main salt. By adding other metal salts, a suitable conversion film solution is formulated for generating a phosphate conversion film with high adhesion 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. Thus, while reducing the mismatch degree of the chemical conversion film and ensuring the corrosion resistance of the chemical conversion film, the adhesion of the chemical conversion film is improved, solving the problem of poor adhesion between the double-layer films of the deposited phosphate conversion film with a double-layer structure, and using the design concept and preparation method of the present invention to reduce the trial-and-error cost.

[0023] 2. The preparation method of the present invention is not only applicable to phosphate conversion films, but also applicable to all deposited conversion films that need to improve adhesion. The preparation method of the present invention can provide valuable reference and guidance for the preparation of other deposited conversion films that need to improve adhesion. Description of the drawings

[0024] Figure 1It is a schematic diagram of the atomic spacing and angle between the (-111) plane of MgHPO4 and the corresponding preferred crystal planes of various metal phosphates. Among them, (a1)-(a3) are schematic diagrams of the atomic spacing and angle between the (-111) plane of MgHPO4 and the (311), (111), and (200) planes of MnHPO4 respectively; (b1)-(b3) are schematic diagrams of the atomic spacing and angle between the (-111) plane of MgHPO4 and the (120), (-202), and (200) planes of CaHPO4 respectively; (c1)-(c3) are schematic diagrams of the atomic spacing and angle between the (-111) plane of MgHPO4 and the (220), (031), and (200) planes of ZnHPO4 respectively; (d1)-(d3) are schematic diagrams of the atomic spacing and angle between the (-111) plane of MgHPO4 and the (111), (121), and (201) planes of SrHPO4 respectively; (e1)-(e3) are schematic diagrams of the atomic spacing and angle between the (-111) plane of MgHPO4 and the (202), (111), and (022) planes of BaHPO4 respectively.

[0025] Figure 2 It is the distribution diagram of the mismatch degree of metal phosphates. Among them, the metal phosphates are MnHPO4, ZnHPO4, CaHPO4, SrHPO4, and BaHPO4 respectively.

[0026] Figure 3 It is the cross-sectional morphology diagram of the phosphate conversion coating. Among them, a is the morphology diagram of the phosphate conversion coating of control sample 1; b is the morphology diagram of the phosphate conversion coating of control sample 2; c is the morphology diagram of the phosphate conversion coating of experimental sample 1; d is the morphology diagram of the phosphate conversion coating of experimental sample 2.

[0027] Figure 4 It is the surface micro-morphology and element distribution diagram after the pull-out test of the phosphate conversion coating of control sample 1.

[0028] Figure 5 It is the adhesion - cumulative probability curve of the phosphate conversion coatings of control sample 1, control sample 2, experimental sample 1, and experimental sample 2.

[0029] Figure 6 It is the neutral salt spray pictures of different phosphate conversion coatings. Among them, a is the neutral salt spray picture of the phosphate conversion coating of control sample 1; b is the neutral salt spray picture of the phosphate conversion coating of control sample 2; c is the neutral salt spray picture of the phosphate conversion coating of experimental sample 1; d is the neutral salt spray picture of the phosphate conversion coating of experimental sample 2. Specific implementation mode

[0030] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0032] X-ray diffraction, with the full English name X-Ray Diffraction, is abbreviated as XRD.

[0033] The metal elements of the main salt and other metal salts are different. The other metal salts are metal salts other than the metal element of the main salt.

[0034] The technical solutions of the present invention will be further described below through specific embodiments.

[0035] In the following embodiments, unless otherwise specified, the methods are all conventional methods; unless otherwise specified, the reagents and materials can all be obtained in the market.

[0036] A method for preparing a chemical conversion film on the surface of a metal alloy matrix, comprising the following steps: 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.

[0037] For example, when the metal alloy matrix is a magnesium alloy matrix, for the chemical conversion film formed on the magnesium alloy matrix, the metal phosphate on the inner layer film is MgHPO4.

[0038] Use different metal phosphates as the outer layer film. For example, the metal cations are Ca 2+ , Mn 2+ , Zn 2+ , Sr 2+ and Ba 2+ metal phosphates.

[0039] The preferred crystal plane is to determine the X-ray diffraction characteristic peak according to the X-ray diffraction pattern of the metal phosphate, and determine the preferred growth orientation through the X-ray diffraction characteristic peak, so as to obtain the preferred crystal plane of the metal phosphate.

[0040] Step 2, construct the matching structure of the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film, and obtain the atomic spacing and included angle between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film.

[0041] Step 2.1: Obtain the preferred growth orientation and preferred crystal plane of the inner layer film, and obtain the preferred growth orientation and preferred crystal plane of the outer layer film.

[0042] The specific method is as follows: According to the X-ray diffraction pattern of the metal phosphate, use the corresponding three strongest diffraction peaks on the X-ray diffraction pattern of the metal phosphate as the preferred growth orientation, and obtain the crystal planes of the three strongest diffraction peaks as the preferred crystal planes. Among them, the main components of both the inner layer film and the outer layer film are metal phosphates, and the metal elements on the inner layer film are different from those on the outer layer film.

[0043] In the embodiments of the present invention, different metal phosphates are mainly used as the outer layer film. By calculating the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal planes of different outer layer films, according to the comparison results, the metal salt with the lowest mismatch degree is obtained as the main salt, and by adding other metal salts, a suitable conversion film solution is configured for generating a phosphate conversion film with high adhesion 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. Thus, while reducing the mismatch degree of the chemical conversion film and ensuring the corrosion resistance of the chemical conversion film, the adhesion of the chemical conversion film is improved.

[0044] For example, for the chemical conversion film formed on the surface of a magnesium alloy, the metal phosphate on the inner layer film is MgHPO4. Since the nucleation of the outer layer film occurs on specific crystal planes of MgHPO4 with different lattice arrangement structures, the nucleation of the outer layer film will be affected by the crystal plane structure of MgHPO4, which will affect the growth direction of the outer layer film, resulting in a mismatch degree between the inner layer film and the outer layer film, and the mismatch degree will affect the adhesion between the double-layer films.

[0045] To screen the outer layer film with a low mismatch degree with MgHPO4, the preferred growth orientation of MgHPO4 is selected for calculation during the calculation. Determine the three strongest diffraction peaks according to the X-ray diffraction pattern of MgHPO4. The preferred growth orientation is determined by the three strongest diffraction peaks on the X-ray diffraction pattern, and the crystal planes of the three strongest diffraction peaks of MgHPO4 are obtained as (-111), (220), and (113).

[0046] When the metal phosphate on the outer layer 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); the crystal planes of the three strongest diffraction peaks of BaHPO4 are (202), (111), and (022).

[0047] Step 2.2, construct a mathematical model of the two-dimensional lattice mismatch degree between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film, so as to construct a matching structure between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film.

[0048] The specific method is: according to the preferred growth orientation and preferred crystal planes of the metal phosphate on the inner layer film and the metal phosphate on the outer layer film obtained, use crystal modeling software to construct a mathematical model of the two-dimensional lattice mismatch degree between different preferred crystal planes of the inner layer film and different preferred crystal planes of the outer layer film, so as to construct a matching structure between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film.

[0049] Step 2.3, according to the constructed matching structure between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film, select multiple low-index crystal directions for each preferred crystal plane, and calculate the atomic spacing and included angle between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film in multiple low-index crystal direction orientations.

[0050] In the embodiment of the present invention, CrystalMaker crystal modeling software is mainly used to construct a matching structure between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film. By measuring the atomic spacing and included angle of the corresponding preferred crystal planes of each metal phosphate, as Figure 1 shown, it is a schematic diagram of the atomic spacing and included angle between the (-111) plane of MgHPO4 and the corresponding preferred crystal planes of each metal phosphate. Among them, the solid spheres represent the cations of the outer layer film, such as Mn, Ca, Sr, Zn or Ba. The hollow spheres represent the Mg atoms in MgHPO4.

[0051] According to the constructed matching structure between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film, select three low-index crystal directions for each preferred crystal plane, and calculate the atomic spacing and included angle between the metal phosphate on the inner layer film and the metal phosphate on the outer layer film in the three low-index crystal planes of the corresponding preferred crystal planes. In the embodiment of the present invention, the low-index crystal plane is the preferred crystal plane obtained according to the X-ray diffraction characteristic peak; the low-index crystal direction is measured by the crystal structure software for the orientation on the preferred crystal plane. The crystal structure software can be CrystalMaker crystal modeling software.

[0052] For example, the calculation method of the atomic spacing and included angle between the (220) plane or (113) plane of MgHPO4 and the corresponding preferred crystal planes of each metal phosphate is similar to the above calculation method, and the calculation process is as Figure 1 shown.

[0053] Step 3, use the mismatch degree calculation formula to calculate the mismatch degree between the preferred crystal planes of the inner layer film and the preferred crystal planes of the outer layer film; obtain the metal salt with the lowest mismatch degree as the main salt of the outer layer film.

[0054] In the embodiments of the present invention, the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film is mainly compared to obtain the metal salt with the lowest mismatch degree as the main salt of the outer layer film, so as to improve the adhesion of the generated chemical conversion film.

[0055] The mismatch degree calculation formula is: ; Wherein, ( hkl ) s represents the low-index crystal plane of the nucleation phase of the inner layer film; ( hkl ) n represents the low-index crystal plane of the nucleation phase of the outer layer film; represents the mismatch degree between the preferred crystal plane of the inner layer film and the preferred crystal plane of the outer layer film; represents the i th ( hkl ) s low-index crystal direction on the plane; represents the i th ( hkl ) n low-index crystal direction on the plane; represents the atomic spacing between the inner layer film and the outer layer film in the low-index crystal direction orientation on the i th ( hkl ) s plane; represents the atomic spacing between the inner layer film and the outer layer film in the low-index crystal direction orientation on the i th ( hkl ) n plane; θ represents the and angle between.

[0056] The low-index crystal plane is the preferred crystal plane obtained according to the X-ray diffraction characteristic peak; the low-index crystal direction is obtained by measuring the orientation on the preferred crystal plane through crystal structure software.

[0057] After testing, the atomic spacings of the three low-index crystal directions of the (-111) crystal plane of MgHPO4 are 4.1 nm, 8 nm, and 9.7 nm. The measurement data of each preferred crystal plane of MnHPO4 are shown in Table 1. Substituting the measurement data in Table 1 into the mismatch degree calculation formula, the corresponding mismatch degree is obtained. The calculation process of other metal phosphates is similar to the above calculation process. The calculation results are statistically shown in Figure 2 .

[0058] Table 1 Mismatch degree data between each preferred crystal plane of MnHPO4 and the (-111) crystal plane of MgHPO4

[0059] Note: d 1.d 2 and d 3 respectively represent the atomic spacings of the preferred crystal planes of MnHPO4 and the three low-index crystal directions of the (-111) crystal plane of MgHPO4; f 1, f 2 and f 3 respectively represent the mismatch degrees of 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 respectively represent the included angles between the preferred crystal planes of MnHPO4 and the three low-index crystal directions of the (-111) crystal plane of MgHPO4.

[0060] From Table 1 and Figure 2 the results, it can be seen that since the inner layer film and the outer layer film are heteroepitaxial nucleation, the overall mismatch degree is relatively high. Among them, when CaHPO4 is used as the outer layer film, the average mismatch degree is 40.0%, which is lower than other metal salts. Therefore, in order to improve the adhesion of the conversion film, Ca 2+ is used as the main salt of the outer layer film.

[0061] Step 4, select at least one other metal salt and use it together with the main salt as the film-forming substance, and add inorganic phosphate and additives to prepare a conversion film solution; place the metal alloy substrate in the conversion film solution for film-forming reaction to generate a phosphate conversion film with high adhesion on the surface of the metal alloy substrate.

[0062] In the embodiment of the present invention, in order to improve the corrosion resistance, it is necessary to add multiple film-forming substances. Multiple cations have different deposition abilities and multiple depositions will occur during the reaction, which will increase the film thickness and denseness. Therefore, it can be considered to add Ca 2+ as the main salt in the solution, and select Mn 2+ , Zn 2+ , Sr 2+ and Ba 2+ at least one of them as the other metal salt and use them together as the film-forming substance.

[0063] Specifically, the metal alloy substrate is a magnesium alloy substrate; the main salt is calcium chloride; the other metal salt is at least one metal salt of Mn 2+ , Zn 2+ , Sr 2+ and Ba 2+ . The additives are an oxidant and a complexing agent. The oxidant is sodium nitrate; the complexing agent is EDTA; the inorganic phosphate is ammonium dihydrogen phosphate. Thus, the content of each substance in each liter of the conversion film solution is: The main salt is 0.1 mol / L to 0.15 mol / L, each other metal salt is 0.05 mol / L to 0.15 mol / L, the inorganic phosphate is 0.2 mol / L to 0.3 mol / L, the oxidant is 0.01 mol / L to 0.04 mol / L, and the complexing agent is 0.01 mol / L to 0.05 mol / L. Among them, 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.

[0064] Taking the Mg-Gd-Y-Zr rare earth magnesium alloy as an example below, a phosphate conversion film with high adhesion is formed on the surface of the metal alloy matrix and used as a sample for performance testing.

[0065] I. Preparation of samples.

[0066] Pretreatment of the Mg-Gd-Y-Zr rare earth magnesium alloy: The rare earth magnesium alloy is cut into rare earth magnesium alloy specimens with sizes of 220 mm × 20 mm × 5 mm, 10 mm × 10 mm × 5 mm, and 10 mm × 10 mm × 12 mm by wire cutting for morphology and corrosion resistance test characterization. The rare earth magnesium alloy specimens are polished with 800#, 1000# and silica sandpaper respectively, rinsed with alcohol, and dried.

[0067] Preparation of experimental sample 1: The concentrations of the components in the conversion film solution are as follows: the concentration of calcium chloride is 0.15 mol / L, the concentration of ammonium dihydrogen phosphate is 0.25 mol / L, the concentration of sodium nitrate is 0.02 mol / L, and the concentration of EDTA is 0.02 mol / L. After mixing the components according to the set concentration ratio, the conversion treatment is carried out at a temperature of 50 °C for 20 min; then the above conversion film solution is placed in a water bath at 50 °C to obtain a conversion film solution heated at 50 °C. The rare earth magnesium alloy specimen is placed in the above conversion film solution heated at 50 °C for a film-forming reaction for 20 min. The reacted rare earth magnesium alloy specimen is taken out, rinsed with deionized water, and air-dried to obtain experimental sample 1.

[0068] Preparation of Experimental Sample 2: The concentrations of the components in the conversion film solution are as follows: the concentration of calcium chloride is 0.15 mol / L, the concentration of manganese chloride is 0.15 mol / L, the concentration of zinc chloride is 0.1 mol / L, the concentration of ammonium dihydrogen phosphate is 0.25 mol / L, the concentration of sodium nitrate is 0.02 mol / L, and the concentration of EDTA is 0.02 mol / L. After mixing the components according to the set concentration ratio, the conversion treatment is carried out at a temperature of 50 °C for 20 min; then the above conversion film solution is placed in a water bath at 50 °C to obtain a conversion film solution heated at 50 °C. The rare earth magnesium alloy sample is placed in the above conversion film solution heated at 50 °C for a film-forming reaction for 20 min. The reacted rare earth magnesium alloy sample is taken out, rinsed with deionized water, and air-dried to obtain Experimental Sample 2.

[0069] Preparation of Control Sample 1: The concentrations of the components in the conversion film solution are as follows: the concentration of manganese sulfate is 0.15 mol / L, the concentration of ammonium dihydrogen phosphate is 0.25 mol / L, the concentration of sodium nitrate is 0.02 mol / L, and the concentration of EDTA is 0.02 mol / L. After mixing the components according to the set concentration ratio, the conversion treatment is carried out at a temperature of 50 °C for 20 min; then the above conversion film solution is placed in a water bath at 50 °C to obtain a conversion film solution heated at 50 °C. The rare earth magnesium alloy sample is placed in the above conversion film solution heated at 50 °C for a film-forming reaction for 20 min. The reacted rare earth magnesium alloy sample is taken out, rinsed with deionized water, and air-dried to obtain Control Sample 1.

[0070] Preparation of Control Sample 2: The concentrations of the components in the conversion film solution are as follows: the concentration of zinc chloride is 0.1 mol / L, the concentration of ammonium dihydrogen phosphate is 0.25 mol / L, the concentration of sodium nitrate is 0.02 mol / L, and the concentration of EDTA is 0.02 mol / L. After mixing the components according to the set concentration ratio, the conversion treatment is carried out at a temperature of 50 °C for 20 min; then the above conversion film solution is placed in a water bath at 50 °C to obtain a conversion film solution heated at 50 °C. The rare earth magnesium alloy sample is placed in the above conversion film solution heated at 50 °C for a film-forming reaction for 20 min. The reacted rare earth magnesium alloy sample is taken out, rinsed with deionized water, and air-dried to obtain Control Sample 2.

[0071] II. Performance Testing of Samples.

[0072] To identify the weak points of the adhesion of the phosphate conversion film in Control Sample 1, the surface microtopography and element distribution of the phosphate conversion film after the pull-out test were observed, and the results are as Figure 4 shown.

[0073] From Figure 4It can be seen that after the drawing test, the surface elements of the phosphate conversion film in control sample 1 are Mg, P, O, and Gd elements. For the phosphate conversion film with a double-layer structure, only the inner layer film MgHPO4 remains, and the outer layer film MnHPO4 disappears. Since the inner layer film is very thin, the matrix Gd element can be observed. This shows that the weak point of the adhesion of the phosphate conversion film is between the double layers.

[0074] The morphologies of the phosphate conversion films of control sample 1, control sample 2, experimental sample 1, and experimental sample 2 were observed as Figure 3 shown.

[0075] From Figure 3 the scanning cross-sectional view, it can be seen that in control sample 1 and control sample 2, obvious transverse cracks appear in the film layer of the phosphate conversion film. These cracks indicate poor adhesion of the phosphate conversion film, which is due to the high misfit degree resulting in large interfacial stress. The transverse cracks of the phosphate conversion film of experimental sample 1 are significantly reduced, which also proves that the low misfit degree of CaHPO4 is beneficial to improving the adhesion of the film layer of the phosphate conversion film. However, one kind of cation will cause the film layer of the phosphate conversion film to be too thin, and it has little improvement on 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.

[0076] The adhesion of the phosphate conversion films of control sample 1, control sample 2, experimental sample 1, and experimental sample 2 was tested by an acoustic emission adhesion tester, and the adhesion - cumulative probability curve is as Figure 5 shown.

[0077] From Figure 5 it can be seen that the adhesion of the phosphate conversion films of control sample 1 and control sample 2 is about 1.5 N. The adhesion of the phosphate conversion film of experimental sample 1 is significantly improved, and the adhesion is increased to about 5 times that of the phosphate conversion films of control sample 1 and control sample 2. The adhesion of the phosphate conversion film of experimental sample 2 is increased to 4 times that of the phosphate conversion films of control sample 1 and control sample 2. This is because during the reaction process, ion co - deposition occurs, and it is inevitable that Mn and Zn with a high misfit degree will hinder the deposition of Ca with a low misfit degree to a certain extent, resulting in a certain degree of decrease in the adhesion of the phosphate conversion film of experimental sample 2, but it is still much higher than the adhesion of the phosphate conversion films of control sample 1 and control sample 2.

[0078] To verify the long - term corrosion resistance test of the film layer, a 96 - h neutral salt spray test was carried out, and the results are as Figure 6 shown.

[0079] From Figure 6It can be seen that after the neutral salt spray test, the phosphate conversion coatings of control sample 1 and control sample 2 showed color change on the surface and corrosion points appeared; obvious corrosion points and corrosion products existed on the surface of the phosphate conversion coating of experimental sample 1; this was caused by the too low thickness of the coating. However, for the phosphate conversion coating of experimental sample 2, due to the uniform coating and the increased coating thickness, after the 96h neutral salt spray test, only a small number of small-sized corrosion points appeared on the surface, indicating that the phosphate conversion coating of experimental sample 2 had the best corrosion resistance.

[0080] Based on the above analysis, it can be known that by using metal salts with low misfit degree as the main salts and adding other metal salts in combination, phosphate conversion coatings with high adhesion and high corrosion resistance can be prepared.

[0081] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope 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 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; Constructing a matching structure between the preferred crystal plane of the inner layer and the preferred crystal plane of the outer layer, and 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; The metal salt with the lowest mismatch degree 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 auxiliary agent are added to prepare a conversion film solution; The metal alloy substrate is placed in a conversion film solution to carry out a film-forming reaction, so as to form a phosphate conversion film with high adhesion on the surface of the metal alloy substrate.

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 method for constructing a matching structure of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film 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; 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 is constructed, thereby constructing a matching structure of the preferred crystal plane of the inner film and the preferred crystal plane of the outer film.

3. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 2, characterized in that: 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 film and the preferred crystal plane of the outer layer film, 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 film and the preferred crystal plane of the outer layer film in multiple low-index crystal directions are calculated.

4. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 1, characterized in that: 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 i indivual( hkl ) s Low-index crystal orientation on the surface; Indicates i indivual( hkl ) n Low-index crystal orientation on the surface; Indicates the inner and outer membranes in i indivual( hkl ) s The interatomic distances along the low-index crystal orientations on the plane; Indicates the inner and outer membranes in i indivual( hkl ) n The interatomic distances along the low-index crystal orientations on the plane; θ express and Angle.

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 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.

6. 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 according to the X-ray diffraction characteristic peak of the metal phosphate, and the preferred growth orientation is determined by the X-ray diffraction characteristic peak, thereby obtaining the preferred crystal plane of the metal phosphate.

7. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 1, characterized in that: The metal alloy matrix is ​​a magnesium alloy matrix; The main salt is calcium chloride; the other metal salt is Mn 2+ 、Zn 2+ , Sr 2+ and Ba 2+ At least one metal salt.

8. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 7, 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.

9. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 8, 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.

10. The method for preparing a chemical conversion film on the surface of a metal alloy substrate according to claim 1 or 9, 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.

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