A modified chromium aluminum phosphate coating and its application and application method

Abstract

The present invention discloses a modified chromium aluminum phosphate coating and its application and application method. The modified chromium aluminum phosphate coating is composed of the following components in the following weight ratios: 1 to 30 parts of Al3Cr(H2PO4) 12 , 10-50 parts of Al(H2PO4)3, 50-80 parts of H2O and 40-70 parts of Al2O3 powder. The cast aluminum alloy is dipped and pulled in the prepared modified chromium aluminum phosphate coating, and then treated with a steam-assisted curing device to obtain a ceramic coating. The present invention uses Al(H2PO4)3 and Al3Cr(H2PO4) 12 It can be cured at low temperature, which improves the micro-cracks on the coating surface, forming a more uniform and smooth coating that is tightly bonded to the substrate and has better corrosion resistance and durability. It has the characteristics of simple operation steps and short preparation cycle. At the same time, the prepared ceramic coating has better durability and corrosion resistance and has great practical value.

Description

Technical Field

[0001] The invention relates to a modified chromium aluminum phosphate coating and an application and an application method thereof, belonging to the technical field of coatings. Background Art

[0002] Aluminum alloys are the most widely used nonferrous structural materials in industry, finding extensive application in aviation, aerospace, automotive, machinery manufacturing, shipbuilding, and the chemical industry. Aluminum alloys can be divided into cast and wrought aluminum alloys based on processing methods. Cast aluminum-silicon alloys, with Si (4% to 26%) as the primary alloying component, offer advantages such as high fluidity, low shrinkage, excellent casting properties, and a wide range of applications, making them the most widely used aluminum alloy.

[0003] Aluminum and its alloys are chemically active. In the natural environment, a corrosion-resistant porous oxide film with a thickness of approximately 2 to 10 nm forms on the surface. This film is easily damaged and loses its protective effect on the alloy. In addition, this film also loses its protective effect on the substrate in humid or corrosive environments.

[0004] Improving the corrosion resistance of aluminum alloys can be achieved through two approaches: enhancing the corrosion resistance of the alloy substrate and employing surface corrosion protection treatments. Surface corrosion protection technology is a key approach to improving the corrosion resistance of aluminum alloys. By isolating the substrate from the corrosive medium, it not only enhances the corrosion resistance of the alloy but also improves other alloy properties, such as wear resistance and hydrophilicity. Currently, a variety of surface modification technologies are available for aluminum alloys, primarily categorized as surface chemical treatment and surface electrochemical treatment. Surface chemical treatment is further divided into chemical pretreatment and chemical conversion treatment. Chemical pretreatment involves chemical polishing, degreasing, and alkaline / acid cleaning of the substrate. Chemical conversion treatments can be categorized as chromate conversion coatings, phosphate conversion coatings, and rare earth conversion coatings, depending on the treatment solution. Surface electrochemical treatments primarily include electroplating, anodizing, and micro-arc oxidation. These surface treatment processes have been successfully applied to wrought aluminum alloys. However, cast aluminum alloys have a high alloying element content and a large number of second-phase particles in their microstructure, which can affect the continuity of the coating and its adhesion to the substrate. Therefore, the surface treatment processes used for wrought aluminum alloys are not suitable for cast aluminum alloys.

[0005] Extensive research has been conducted on the preparation of ceramic protective layers on aluminum alloy castings, using methods such as thermal spraying, laser cladding, and micro-arc oxidation. Ceramic coatings produced on aluminum alloy castings using these techniques exhibit excellent corrosion and wear resistance, achieving promising results. However, thermal spraying and laser cladding are linear techniques, advantageous for treating regular surfaces. However, these processes are not suitable for the complex shapes and internal cavities of cast Al-Si alloy castings. Furthermore, processes such as micro-arc oxidation, anodizing, and phosphate conversion coatings generate wastewater during the process, raising issues such as wastewater disposal. Ceramic coatings produced using traditional thermochemical reaction methods exhibit excellent chemical and physical bonding with the substrate, as well as high-temperature and corrosion resistance. However, the preparation temperature is relatively high, typically above 300°C, while the aging temperature for heat treatment of cast aluminum alloys typically does not exceed 160°C, making them unsuitable for ordinary aluminum alloy die castings.

[0006] CN113174150A discloses an aluminum-zinc phosphate coating and its application method, while CN107058992A discloses a coating and its application method for preparing a composite coating on the surface of cast aluminum alloys. Both patents utilize a steam-assisted hardening process to successfully prepare a chemically bonded ceramic coating on the surface of aluminum alloy castings. However, the steam heating efficiency is low, and the temperature at which the ceramic layer forms cannot be adjusted. To address these shortcomings, there is an urgent need to develop a production process that can improve the steam heating efficiency and adjust the temperature at which the ceramic layer forms. Summary of the Invention

[0007] Purpose of the invention: In view of the existing deficiencies, the first purpose of the present invention is to provide a modified chromium aluminum phosphate coating, the second purpose of the present invention is to provide the application of the modified chromium aluminum phosphate coating in the preparation of a composite coating on the surface of a cast aluminum alloy, and the third purpose of the present invention is to provide a method for using the modified chromium aluminum phosphate coating to prepare a composite coating on the surface of a cast aluminum alloy.

[0008] Technical solution: The modified chromium aluminum phosphate coating of the present invention comprises the following components in parts by weight: 1 to 30 parts of Al3Cr(H2PO4) 12 , 10 to 50 parts of Al(H2PO4)3, 40 to 80 parts of H2O and 30 to 70 parts of Al2O3 powder.

[0009] Furthermore, the modified chromium aluminum phosphate coating comprises the following components in parts by weight: 1 to 20 parts of Al3Cr(H2PO4) 12 , 30 to 50 parts of Al(H2PO4)3, 50 to 75 parts of H2O and 50 to 65 parts of Al2O3 powder.

[0010] Furthermore, the particle size of Al2O3 powder is 600-1000 mesh. The particle size of Al2O3 powder will affect the reaction temperature and coating density. Fine Al2O3 powder is easy to react with Al3Cr(H2PO4). 12 It reacts with Al(H2PO4)3 to form a more stable substance, thereby improving the corrosion resistance of the coating.

[0011] The modified chromium aluminum phosphate coating of the present invention is used in preparing a composite coating on the surface of a cast aluminum alloy.

[0012] The method for preparing a composite coating on the surface of a cast aluminum alloy by using the modified chromium aluminum phosphate coating of the present invention comprises the following steps:

[0013] (1) Al(H2PO4)3 and Al3Cr(H2PO4) 12 Dissolve in water, add Al2O3 powder, stir, and obtain modified chromium aluminum phosphate coating;

[0014] (2) The alkali-treated aluminum alloy casting is placed in a modified chromium aluminum phosphate coating and then dipped and pulled out. After the coating on the alloy surface stops flowing significantly, the aluminum alloy is subjected to steam-assisted curing treatment to obtain an aluminum alloy casting with a ceramic coating on the surface.

[0015] Furthermore, in step (1), the added Al2O3 includes Al3Cr(H2PO4) 12 1.1 to 1.3 times the amount of Al2O3 powder required for the entire chemical reaction and 1.0 to 1.2 times the amount of Al2O3 powder required for the entire chemical reaction with Al(H2PO4)3.

[0016] Furthermore, in step (1), the stirring speed is 500±10 r / min, and the stirring time is 30 min.

[0017] Furthermore, in step (2), the immersion time is more than 3 minutes.

[0018] Furthermore, in step (2), the temperature of the steam-assisted curing treatment is 105 to 135° C., and the time of the steam-assisted curing treatment is 60 to 90 minutes.

[0019] Furthermore, in step (2), the surface of the aluminum alloy casting coated with the modified chromium aluminum phosphate coating is placed in a steam environment, heated from the other side without the coating, and cured at 105-135° C. for 60-90 minutes.

[0020] Furthermore, in step (2), the alkali treatment refers to: placing the cast aluminum alloy in an alkaline solution for alkali washing, then ultrasonically cleaning with deionized water, and drying naturally. The purpose of the alkali treatment is to remove the oxide film on the surface of the cast aluminum alloy and activate the surface of the cast aluminum alloy. After the cast aluminum alloy is alkali-treated, the -OH generated on the surface of the cast aluminum alloy is conducive to reacting with the coating, and the surface roughness is increased, thereby improving the wettability of the coating on the cast aluminum alloy and further improving the bonding ability of the coating to the substrate.

[0021] Furthermore, the alkaline solution is a mixed solution of sodium hydroxide and sodium carbonate, wherein the concentration of sodium hydroxide in the mixed solution is 4-5wt%, the concentration of sodium carbonate is 3.5-4wt%, the mass ratio of sodium hydroxide to sodium carbonate is 5:4, the alkaline washing time is 2 minutes, and the deionized water ultrasonic cleaning time is 2 minutes.

[0022] In the above preparation method, the prepared modified chromium aluminum phosphate coating only needs to be coated once. Under the action of water vapor treatment, the Al3Cr(H2PO4) in the system 12 The added Al3Cr(H2PO4)3 reacts with the added Al2O3 powder to solidify. 12 It can improve the fluidity of the coating and promote uniform coating. The introduced Cr ions reduce the stress and strain generated by the phase change process by promoting the formation of amorphous structure, and increase the disorder of the system, thereby improving the stability of the system, reducing the crack sensitivity of the coating, and making it have better corrosion resistance and durability.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0024] (1) The present invention uses Al(H2PO4)3 and Al3Cr(H2PO4) 12 As a composite binder, it can react with Al2O3 powder at a lower temperature to solidify and prepare a continuous phosphate ceramic coating on the surface of the cast aluminum alloy substrate;

[0025] (2) The present invention uses Al3Cr(H2PO4) in the coating 12 Significantly improves the fluidity of the coating, reduces the sensitivity of coating surface cracks, forms a more uniform and smooth coating that is closely bonded to the substrate, and has better corrosion resistance and durability;

[0026] (3) The steam-assisted curing device process used in the present invention separates the heating and steam environments. The surface of the aluminum alloy casting coated with the modified chromium aluminum phosphate coating is placed in the steam environment, and the other side without the coating is heated. The casting heating and the coating steam environment are temperature-controlled separately, which significantly improves the processing efficiency. It has the characteristics of simple operation steps and short preparation cycle. At the same time, the prepared ceramic coating has better durability and corrosion resistance, and has great practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of a steam-assisted curing treatment device used in the present invention;

[0028] Figure 2 The SEM images of the surface and cross-section of the modified aluminum phosphate coating of Comparative Example 1 are shown, wherein (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0029] Figure 3 The SEM images of the surface and cross-section of the modified aluminum magnesium phosphate coating of Comparative Example 2 are shown, wherein (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0030] Figure 4 The SEM images of the surface and cross-section of the modified aluminum zinc phosphate coating of Comparative Example 3 are shown, wherein (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0031] Figure 5 The SEM images of the surface and cross-section of the modified chromium aluminum phosphate coatings of Examples 1-4, wherein (a), (c), (e) and (g) are surface morphologies of Examples 1-4, respectively; (b), (d), (f) and (h) are cross-sectional morphologies of Examples 1-4, respectively;

[0032] Figure 6 These are SEM images of the surface and cross-section of the modified aluminum magnesium phosphate coating of Example 5, where (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0033] Figure 7 The SEM images of the surface and cross-section of the modified aluminum magnesium phosphate coating of Example 6, wherein (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0034] Figure 8 The SEM images of the surface and cross-section of the modified aluminum magnesium phosphate coating of Example 7, wherein (a) is a surface morphology image; (b) is a cross-sectional morphology image;

[0035] Figure 9 This is a comparison chart of the polarization curve test results of the coating;

[0036] Figure 10is the charge transfer resistance (R) of the coating after immersion in 3.5 wt% NaCl solution for different times ct ) comparison chart. DETAILED DESCRIPTION

[0037] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0038] The present invention can be better understood based on the following examples. It will be readily understood by those skilled in the art that the specific test results described in the examples are based on the cast aluminum-silicon alloy EN AC-43000 and are only used to illustrate the present invention. The surface treatment method is also applicable to other series of cast aluminum alloys, and should not and will not limit the present invention described in the claims.

[0039] Example 1

[0040] (1) 10 parts by weight of Al3Cr(H2PO4) 12 , 50 parts of Al(H2PO4)3, 60 parts of H2O are weighed as raw materials, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated as 1.2 times the number required for all chemical reactions and 1.0 times the number required for all chemical reactions with Al(H2PO4)3, 62 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 1000 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a modified chromium aluminum phosphate coating;

[0041] (2) A sample of cast aluminum alloy EN AC-43000 was placed in a mixed solution of NaOH and Na2CO3 for 2 min, then washed with pure water for 2 min, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5 wt%, the concentration of Na2CO3 was 4 wt%, and the mass ratio of NaOH to Na2CO3 was 5:4.

[0042] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the sample surface. If not, repeat the above steps. When the coating does not flow significantly on the sample surface, place the cast aluminum alloy sample with the coating on the surface of the sample. Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment. The heating temperature was 105° C. and the treatment was carried out for 90 minutes to obtain a cast aluminum alloy sample with a modified chromium aluminum phosphate coating on the surface.

[0043] Example 2

[0044] The experimental process is the same as in Example 1, specifically as follows:

[0045] (1) 5 parts by weight of Al3Cr(H2PO4) 12 , 40 parts of Al(H2PO4)3, 50 parts of H2O were weighed, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated as 1.3 times the number required for the entire chemical reaction and 1.2 times the number required for the entire chemical reaction with Al(H2PO4)3, 54.5 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 1000 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a coating;

[0046] (2) The cast aluminum alloy sample was placed in a mixed solution of NaOH and Na2CO3 for 2 minutes, then washed with pure water for 2 minutes, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5wt%, the concentration of Na2CO3 was 4wt%, and the mass ratio of NaOH to Na2CO3 was 5:4;

[0047] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the surface of the sample. If not, repeat the above steps. When the coating does not flow significantly on the surface of the sample, place the sample with the coating on a Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment, the heating temperature was 115° C., and the treatment was carried out for 50 minutes to obtain a cast aluminum alloy sample with a modified chromium aluminum phosphate coating on the surface.

[0048] Example 3

[0049] The experimental process is the same as in Example 1, specifically as follows:

[0050] (1) 20 parts by weight of Al3Cr(H2PO4) 12 , 30 parts of Al(H2PO4)3, 50 parts of H2O are weighed, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12Calculated as 1.2 times the number required for the entire chemical reaction and 1.1 times the number required for the entire chemical reaction with Al(H2PO4)3, 57 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 1000 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a coating;

[0051] (2) The cast aluminum alloy sample was placed in a mixed solution of NaOH and Na2CO3 for 2 minutes, then washed with pure water for 2 minutes, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5wt%, the concentration of Na2CO3 was 4wt%, and the mass ratio of NaOH to Na2CO3 was 5:4;

[0052] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the surface of the sample. If not, repeat the above steps. When the coating does not flow significantly on the surface of the sample, place the sample with the coating on a Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment, the heating temperature was 135° C., and the treatment was carried out for 60 minutes to obtain a cast aluminum alloy sample with a chromium aluminum phosphate ceramic coating on the surface.

[0053] Example 4

[0054] The experimental process is the same as in Example 1, specifically as follows:

[0055] (1) 20 parts by weight of Al3Cr(H2PO4) 12 , 40 parts of Al(H2PO4)3, 55 parts of H2O were weighed, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated as 1.1 times the number required for all chemical reactions and 1.0 times the number required for all chemical reactions with Al(H2PO4)3, 62 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 1000 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a coating;

[0056] (2) The cast aluminum alloy sample was placed in a mixed solution of NaOH and Na2CO3 for 2 minutes, then washed with pure water for 2 minutes, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5wt%, the concentration of Na2CO3 was 4wt%, and the mass ratio of NaOH to Na2CO3 was 5:4;

[0057] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the surface of the sample. If not, repeat the above steps. When the coating does not flow significantly on the surface of the sample, place the sample with the coating on a Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment, the heating temperature was 120°C, and the treatment was carried out for 80 minutes to obtain a cast aluminum alloy sample with a modified chromium aluminum phosphate coating on the surface.

[0058] Example 5

[0059] The experimental process is the same as in Example 1, specifically as follows:

[0060] (1) 40 parts by weight of Al3Cr(H2PO4) 12 , 20 parts of Al(H2PO4)3, 55 parts of H2O are weighed, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated as 1.1 times the number required for the entire chemical reaction and 1.0 times the number required for the entire chemical reaction with Al(H2PO4)3, 64 parts of Al2O3 powder were added, the particle size of the Al2O3 powder being 1000 mesh, and stirred at a speed of 500 r / min for 30 minutes to obtain a coating;

[0061] (2) The cast aluminum alloy sample was placed in a mixed solution of NaOH and Na2CO3 for 2 minutes, then washed with pure water for 2 minutes, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5wt%, the concentration of Na2CO3 was 4wt%, and the mass ratio of NaOH to Na2CO3 was 5:4;

[0062] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the sample surface. If not, repeat the above steps. When the coating does not flow significantly on the sample surface, place the alloy sample with the coating on the surface of the sample. Figure 1 The coating on the surface of the cast aluminum alloy sample was treated in the steam-assisted curing treatment device shown in the figure. The coating was placed in a water vapor environment at a heating temperature of 125°C for 70 minutes to obtain a cast aluminum alloy sample with a modified chromium aluminum phosphate coating on the surface.

[0063] Example 6

[0064] The experimental process is the same as in Example 1, specifically as follows:

[0065] (1) 10 parts by weight of Al3Cr(H2PO4) 12 , 50 parts of Al(H2PO4)3, 60 parts of H2O are weighed as raw materials, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated as 1.1 times the number required for the entire chemical reaction and 1.0 times the number required for the entire chemical reaction with Al(H2PO4)3, 61 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 1000 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a coating;

[0066] (2) The cast aluminum alloy sample was placed in a mixed solution of NaOH and Na2CO3 for 2 minutes, then washed with pure water for 2 minutes, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5wt%, the concentration of Na2CO3 was 4wt%, and the mass ratio of NaOH to Na2CO3 was 5:4;

[0067] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Then use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the surface of the sample. If not, repeat the above steps. When the coating does not flow significantly on the surface of the sample, place the sample with the coating on a Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment. The heating temperature was 150° C. and the treatment was carried out for 40 minutes to obtain a cast aluminum alloy sample with a chromium aluminum phosphate ceramic coating on the surface.

[0068] Example 7

[0069] The experimental process is the same as in Example 1, specifically as follows:

[0070] (1) 10 parts by weight of Al3Cr(H2PO4) 12 , 50 parts of Al(H2PO4)3, 60 parts of H2O are weighed as raw materials, and Al3Cr(H2PO4) 12 Add Al(H2PO4)3 to H2O and stir, then follow the same method as Al3Cr(H2PO4) 12 Calculated based on 1.3 times the number required for all chemical reactions and 1.0 times the number required for all chemical reactions with Al(H2PO4)3, 63 parts of Al2O3 powder were added, the particle size of the Al2O3 powder was 200 mesh, and the mixture was stirred at a speed of 500 r / min for 30 minutes to obtain a modified chromium aluminum phosphate coating;

[0071] (2) A sample of cast aluminum alloy EN AC-43000 was placed in a mixed solution of NaOH and Na2CO3 for 2 min, then washed with pure water for 2 min, and dried naturally for later use. The concentration of NaOH in the mixed solution was 5 wt%, the concentration of Na2CO3 was 4 wt%, and the mass ratio of NaOH to Na2CO3 was 5:4.

[0072] (3) Place the dried cast aluminum alloy sample in the coating and immerse it for 3 minutes. Then use tweezers to slowly pull the sample out of the liquid at a speed of 10 cm / min along a 30° angle. Observe whether the coating is evenly spread on the surface of the sample. If not, repeat the above steps. When the coating sample surface does not flow significantly, place the sample with the coating on a Figure 1 The sample was treated in the steam-assisted curing treatment device shown in the figure. The coating on the surface of the sample was placed in a water vapor environment. The heating temperature was 105° C. and the treatment was carried out for 90 minutes to obtain a cast aluminum alloy sample with a modified chromium aluminum phosphate coating on the surface.

[0073] Comparative Example 1

[0074] The specific process is the same as in Example 1, except that Al3Cr(H2PO4) is not added to the system. 12 , and the balance is made up with water to prepare an aluminum phosphate coating.

[0075] Comparative Example 2

[0076] The specific process is the same as in Example 1, except that Mg(H2PO4)2 is used instead of Al3Cr(H2PO4) in the system. 12 , a modified aluminum magnesium phosphate coating was prepared.

[0077] Comparative Example 3

[0078] The specific process is the same as in Example 1, except that Zn(H2PO4)2 is used instead of Al3Cr(H2PO4) in the system. 12 , a modified aluminum-zinc phosphate coating was prepared.

[0079] The surface structure of Examples 1-7 and Comparative Examples 1-3 was analyzed and the results were as follows: Figure 2-8 See Figure 2 In Comparative Example 1, the aluminum phosphate coating has a rough surface, a large number of holes and penetrating microcracks, and the coating has a tendency to separate from the cast aluminum substrate; Figure 3 In Comparative Example 2, the microcracks on the surface of the aluminum magnesium phosphate coating were significantly reduced, and the coating was well bonded to the cast aluminum substrate, indicating that the addition of Mg(H2PO4)2 can improve defects such as microcracks and pores on the coating surface. However, defects such as microcracks and pores still exist; see Figure 4In Comparative Example 3, the microcracks on the surface of the aluminum-zinc phosphate coating almost disappeared, but there were many holes, which were well bonded to the cast aluminum substrate, indicating that the addition of Zn(H2PO4)2 can reduce the microcracks on the coating surface; Figure 5 The modified chromium aluminum phosphate coating prepared in Examples 1-4 is composed of Figure 5 It can be seen that the surface of the modified chromium aluminum phosphate coating of Examples 1-4 is smooth and dense, the microcracks are basically gone, there are very few holes, the coating is uniform and continuous, and it is tightly bonded to the cast aluminum substrate, which shows that Al3Cr(H2PO4) 12 The addition of can reduce the sensitivity of the coating to cracks during curing, and there are almost no micro cracks on the coating surface, and only a small amount of pores; Figure 6 It can be seen that under the binder content of Example 5, the holes on the surface of the phosphate coating are relatively large, and there are even penetrating holes, and the coating defects are relatively large; Figure 7 It can be seen that at the temperature of Example 6, the phosphate coating has many fine holes on the surface, many through cracks, and poor corrosion resistance. Figure 8 It can be seen that Example 7 uses Al2O3 powder with a larger particle size, resulting in more surface defects in the coating, incomplete reaction, and a large amount of Al2O3 agglomerates together, resulting in a larger particle size, which is not conducive to forming a dense phosphate coating. As can be seen from the above, the amount of raw materials, heating temperature and particle size of Al2O3 powder can affect the performance of the combined total coating. After experiments, it was finally concluded that 1 to 30 parts of Al3Cr(H2PO4) 12 , 10-50 parts Al(H2PO4)3, 50-80 parts H2O and 40-70 parts Al2O3 powder, the results are better, especially when 1-20 parts Al3Cr(H2PO4) 12 , 30-50 parts of Al(H2PO4)3, 50-75 parts of H2O and 50-65 parts of Al2O3 powder, and the particle size of Al2O3 powder is 600-1000 mesh, the heating temperature is 105-135 and the heat treatment is 60-90 minutes, which is better.

[0080] Polarization curves were tested for the coatings in Examples 1 to 7 and Comparative Examples 1, 2, and 3, and the polarization curves were fitted to electrochemical parameters, see Tables 1 and Figure 9 .

[0081] Table 1 Electrochemical parameters fitted by polarization curves

[0082]

[0083]

[0084] from Figure 9 As can be seen from Table 1, the self-corrosion potential of the substrate (cast aluminum alloy) is -0.741 V, and the self-corrosion current density is 3.379×10-4 A.cm -2 The self-corrosion potential of the coating of Comparative Example 1 shifted to -0.652 V, and the self-corrosion current density decreased to 4.154×10 -5 A.cm -2 , compared with the substrate, it dropped by one order of magnitude, and the corrosion inhibition efficiency η was 87.71%. The self-corrosion potential of the coating in comparative example 2 shifted to -0.642, and the self-corrosion current density decreased to 3.598×10 -5 A.cm -2 , compared with the substrate, it dropped by one order of magnitude, and the corrosion inhibition efficiency η was 89.35%. The self-corrosion potential of the coating of comparative example 3 shifted to -0.657, and the self-corrosion current density decreased to 4.897×10 -5 A.cm -2 , which is one order of magnitude lower than that of the substrate, and the corrosion inhibition efficiency η is 85.51%; the self-corrosion potential of the coating of Example 1 shifts positively to -0.610, and the self-corrosion current density decreases to 5.463×10 - 6 A.cm -2 , decreased by 87% compared with comparative example 1, decreased by 84% compared with comparative example 2, decreased by 88% compared with comparative example 3, and the corrosion inhibition efficiency η reached 98.38%; the self-corrosion potential of the coating of Example 2 shifted positively to -0.539, and the self-corrosion current density decreased to 6.748×10 -6 A.cm -2 Compared with the comparative example 1, it decreased by 84%, compared with the comparative example 2, it decreased by 81%, compared with the comparative example 2, it decreased by 86%, and the corrosion inhibition efficiency η reached 98.00%. The corrosion resistance of Examples 3 and 4 is similar to that of Examples 1 and 2, and is significantly improved compared with the substrate and comparative examples 1 to 3. However, the phosphate coatings of Examples 5 to 7 have many defects, such as penetrating cracks or holes, and poor corrosion resistance. Therefore, it is finally concluded that 1 to 30 parts of Al3Cr(H2PO4) 12 , 10-50 parts Al(H2PO4)3, 50-80 parts H2O and 40-70 parts Al2O3 powder, the results are better, especially when 1-20 parts Al3Cr(H2PO4) 12 , 30-50 parts of Al(H2PO4)3, 50-75 parts of H2O and 50-65 parts of Al2O3 powder.

[0085] The coatings prepared in Examples 1 to 7 and Comparative Examples 1 to 3 were immersed in a 3.5 wt% NaCl solution. Figure 10 As the immersion time increases, the charge transfer resistance (R ct ) gradually decreases, indicating that the corrosion resistance gradually decreases. At different immersion times, the R ctIt is much larger than the substrate and also larger than comparative examples 1 to 3, indicating that the modified chromium aluminum phosphate coating has better corrosion resistance. ct is 1.636×10 5 Ω·cm 2 , which is 32 times that of the cast aluminum substrate, significantly improving the corrosion resistance. ct There are still 4.26×10 4 Ω·cm 2 , which is 8 times that of the cast aluminum substrate, indicating that it has a longer durability. The corrosion resistance of Examples 3 and 4 is similar to that of Examples 1 and 2. ct It is much larger than the substrate and also larger than Comparative Examples 1 to 3. However, the phosphate coatings of Examples 5 to 7 have more defects. ct It is obviously lower than Examples 1 to 4. It can be concluded that 1 to 30 parts of Al3Cr(H2PO4) 12 , 10-50 parts Al(H2PO4)3, 50-80 parts H2O and 40-70 parts Al2O3 powder, the results are better, especially when 1-20 parts Al3Cr(H2PO4) 12 , 30-50 parts of Al(H2PO4)3, 50-75 parts of H2O and 50-65 parts of Al2O3 powder.

Claims

1. A modified chromium aluminum phosphate coating, characterized in that: The modified chromium aluminum phosphate coating is composed of the following components in parts by weight: 5 to 20 parts of Al3Cr(H2PO4) 12 , 30 to 50 parts of Al(H2PO4)3, 50 to 75 parts of H2O and 50 to 65 parts of Al2O3 powder, the particle size of the Al2O3 powder is 600 to 1000 mesh.

2. Use of the modified chromium aluminum phosphate coating according to claim 1 in preparing a composite coating on the surface of a cast aluminum alloy.

3. The method for preparing a composite coating on a cast aluminum alloy surface using the modified chromium aluminum phosphate coating according to claim 1, characterized in that: The following steps are involved: (1) Al(H2PO4)3 and Al3Cr(H2PO4) 12 Dissolve in water, add Al2O3 powder, stir, and obtain modified chromium aluminum phosphate coating; (2) The alkali-treated aluminum alloy casting is placed in a modified chromium aluminum phosphate coating and is dipped and pulled out. After the coating on the alloy surface stops flowing significantly, the aluminum alloy is subjected to steam-assisted curing treatment to obtain an aluminum alloy casting with a ceramic coating on the surface.

4. The preparation method according to claim 3, characterized in that In step (1), the added Al2O3 includes Al3Cr(H2PO4) 12 1.1 to 1.3 times the amount of Al2O3 powder required for the entire chemical reaction and 1.0 to 1.2 times the amount of Al2O3 powder required for the entire chemical reaction with Al(H2PO4)3.

5. The preparation method according to claim 3, characterized in that In step (1), the immersion time is 3 minutes. In step (2), the stirring speed is 500±10 r / min and the stirring time is 30 minutes.

6. The preparation method according to claim 3, characterized in that In step (2), the surface of the aluminum alloy casting coated with the modified chromium aluminum phosphate coating is placed in a steam environment, heated from the other side without the coating, and cured at 105 to 135° C. for 60 to 90 minutes.

7. The preparation method according to claim 3, characterized in that In step (2), the alkali treatment refers to: placing the cast aluminum alloy in an alkaline solution for alkali washing, then ultrasonically cleaning it with deionized water, and drying it naturally.

8. The preparation method according to claim 7, characterized in that The alkaline solution is a mixed solution of sodium hydroxide and sodium carbonate, wherein the concentration of sodium hydroxide in the mixed solution is 4-5wt%, the concentration of sodium carbonate is 3.5-4wt%, the mass ratio of sodium hydroxide to sodium carbonate is 5:4, the alkaline washing time is 2-3 minutes, and the deionized water ultrasonic cleaning time is 1-2 minutes.

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