A chromium-free phosphate coating and its preparation method and use method

Abstract

The present invention provides a chromium-free phosphate coating and a preparation method and a use method thereof, belonging to the technical field of phosphate coatings. The chromium-free phosphate coating provided by the present invention includes aluminum hydroxide and phosphoric acid, so that the paint film has good heat resistance and adhesion; the present invention utilizes Al powder to react with phosphate to form an aluminum phosphate complex, which can enhance the chemical stability, high temperature resistance, oxidation resistance and adhesion of the coating; the present invention utilizes rare earth to improve the corrosion resistance and high and low temperature cycle impact resistance of the coating, and the addition of Si is beneficial to improve the high temperature wear resistance of the coating; the present invention utilizes zinc oxide, phosphomolybdate and hexamethylenetetramine as a compound passivating agent, which can play a role of slow release; the present invention utilizes magnesium oxide to reduce the curing temperature of the chromium-free phosphate coating; the present invention adds fiber as a filler to improve the comprehensive mechanical properties of the coating.

Description

Technical Field

[0001] The invention relates to the technical field of phosphate coatings, and in particular to a chromium-free phosphate coating and a preparation method and a use method thereof. Background Art

[0002] Inorganic phosphate metal aluminum powder coating is a coating formed by curing divalent and trivalent metal phosphates, chromates and aluminum powder. It has good high temperature resistance, oxidation resistance, salt spray resistance and radiation resistance, and is widely used in the fields of aerospace, steam turbines, and industrial compressor parts corrosion protection. However, inorganic phosphate coatings have the following problems: First, the existing commercially available phosphate coatings use hexavalent chromium, which is harmful, as a corrosion inhibitor, which does not meet the requirements of modern industrial development for environmental protection. Second, after the curing temperature of the inorganic phosphate coating exceeds 310°C, the dehydration reaction between molecules will be thorough and the coating will be completely cured. Otherwise, the coating will have moisture reversion, resulting in reduced bonding strength, conductivity and corrosion resistance of the coating; and various types of low-temperature or even room-temperature curing inorganic phosphate composite coating materials have been prepared in the relevant prior art, but the density of the cured coating deteriorates and the bonding strength with the substrate weakens, resulting in poor comprehensive mechanical properties of the coating.

[0003] Therefore, there is an urgent need to develop a chromium-free phosphate coating with a low curing temperature and high comprehensive mechanical properties of the cured coating. Summary of the invention

[0004] The object of the present invention is to provide a chromium-free phosphate coating with low curing temperature and high comprehensive mechanical properties of the cured coating, as well as a preparation method and a use method thereof.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The invention provides a chromium-free phosphate coating, which is prepared from raw materials including the following components by mass content: 2-5% aluminum hydroxide, 5-15% phosphoric acid, 1-5% rare earth oxide, 1-5% compound passivating agent, 1-3% boric acid, 30-50% Al powder, 5-10% Si powder, 1-5% magnesium oxide, 0.5-5% fiber and the balance of water;

[0007] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4-0.5:1;

[0008] The composite passivating agent comprises zinc oxide, phosphomolybdate and hexamethylenetetramine;

[0009] The fibers include glass fibers and / or silica fibers.

[0010] Preferably, the rare earth oxide comprises cerium oxide and / or lanthanum oxide.

[0011] Preferably, the phosphomolybdate comprises zinc phosphomolybdate and / or sodium phosphomolybdate.

[0012] Preferably, the mass ratio of the zinc oxide, phosphomolybdate and hexamethylenetetramine is (1-2): (3-8): (0.5-2).

[0013] Preferably, the particle size of the Al powder is 0.1-10 μm.

[0014] Preferably, the particle size of the Si powder is 0.1-10 μm.

[0015] Preferably, the diameter of the fiber is 5-20 μm; the length of the fiber is 50-100 μm.

[0016] The present invention also provides a method for preparing the chromium-free phosphate coating according to the above technical solution, comprising the following steps:

[0017] (1) mixing aluminum hydroxide and phosphoric acid, performing a neutralization reaction, and obtaining a base material solution;

[0018] (2) mixing the base material solution obtained in step (1) with Al powder to obtain an Al-containing slurry;

[0019] (3) mixing the Al-containing slurry obtained in step (2) with rare earth oxide, a compound passivating agent, magnesium borate oxide and water to obtain a binder;

[0020] (4) The binder obtained in step (3) is mixed with Si powder and fiber to obtain a chromium-free phosphate coating.

[0021] Preferably, the neutralization reaction temperature in step (1) is 90-120° C., and the neutralization reaction time is 1-2 h.

[0022] The present invention also provides a method for using the chromium-free phosphate coating described in the above technical solution or the chromium-free phosphate coating prepared by the preparation method described in the above technical solution, comprising: applying the chromium-free phosphate coating to a substrate, and sequentially performing low-temperature heat treatment and high-temperature heat treatment to obtain a chromium-free phosphate coating;

[0023] The temperature of the low-temperature heat treatment is 60-80°C, and the insulation time of the low-temperature heat treatment is 30-60 minutes; the temperature of the high-temperature heat treatment is 150-220°C, and the insulation time of the high-temperature heat treatment is 30-120 minutes.

[0024] The invention provides a chromium-free phosphate coating, which is prepared from raw materials comprising the following components by mass content: 2-5% aluminum hydroxide, 5-15% phosphoric acid, 1-5% rare earth oxide, 1-5% compound passivator, 1-3% boric acid, 30-50% Al powder, 5-10% Si powder, 1-5% magnesium oxide, 0.5-5% fiber and the balance of water; the molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4-0.5:1; the compound passivator comprises zinc oxide, phosphomolybdate and hexamethylenetetramine; and the fiber comprises glass fiber and / or silicon dioxide fiber. The chromium-free phosphate coating provided by the present invention comprises aluminum hydroxide and phosphoric acid. When the molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4-0.5:1, the paint film can have good heat resistance and adhesion. Al powder is added to the coating provided by the present invention, and the aluminum phosphate complex is formed by reacting the Al powder with phosphate. The complex can enhance the chemical stability, high temperature resistance, oxidation resistance and adhesion of the coating. Rare earth oxide and Si powder are added to the coating provided by the present invention. Rare earth can improve the corrosion resistance and high and low temperature cycle impact resistance of the coating. The addition of i is beneficial to improve the high-temperature wear resistance of the coating; the present invention uses zinc oxide, phosphomolybdate and hexamethylenetetramine as compound passivators. After the phosphate is cured, free phosphoric acid will be formed, which is easy to absorb water and cause the coating to be sticky. Zinc oxide can neutralize the phosphoric acid; the compound use of phosphomolybdate and hexamethylenetetramine can play a role of slow release; the present invention uses magnesium oxide to reduce the curing temperature of chromium-free phosphate coating; the present invention adds fibers as fillers. When the coating is cured, the binder will spread along the fibers, improve the density of the coating, and then improve the comprehensive mechanical properties of the coating. The results of the embodiment show that the curing temperature of the coating provided by the present invention is as low as 160°C, and the coating has comprehensive mechanical properties comparable to those of traditional high-temperature cured phosphate coatings. DETAILED DESCRIPTION

[0025] The invention provides a chromium-free phosphate coating, which is prepared from raw materials including the following components by mass content: 2-5% aluminum hydroxide, 5-15% phosphoric acid, 1-5% rare earth oxide, 1-5% compound passivating agent, 1-3% boric acid, 30-50% Al powder, 5-10% Si powder, 1-5% magnesium oxide, 0.5-5% fiber and the balance of water;

[0026] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4-0.5:1;

[0027] The composite passivating agent comprises zinc oxide, phosphomolybdate and hexamethylenetetramine;

[0028] The fibers include glass fibers and / or silica fibers.

[0029] In the present invention, unless otherwise specified, the raw materials used are commercially available products well known to those skilled in the art.

[0030] In the present invention, the raw materials for preparing the chromium-free phosphate coating include aluminum hydroxide with a mass content of 2-5%, preferably 3-4%. The addition of aluminum hydroxide in the present invention can form an aluminum phosphate compound as the main component of the coating.

[0031] In the present invention, the raw material for preparing the chromium-free phosphate coating includes phosphoric acid with a mass content of 5-15%, preferably 6-12%, and more preferably 8-10%. The phosphoric acid added in the present invention can form an aluminum phosphate compound with aluminum hydroxide as the main component of the coating. The present invention does not specifically limit the concentration of the phosphoric acid, and conventional commercially available phosphoric acid can be used. In an embodiment of the present invention, the phosphoric acid can be industrial phosphoric acid with a mass concentration of 85%.

[0032] In the present invention, the molar ratio of Al in the aluminum hydroxide and P in the phosphoric acid is 0.4 to 0.5: 1, preferably 0.4: 1 or 0.5: 1. The present invention controls the molar ratio of the two within the above range, so that the obtained aluminum phosphate compound is mainly aluminum dihydrogen phosphate, thereby improving the heat resistance and stability of the coating.

[0033] In the present invention, the raw materials for preparing the chromium-free phosphate coating include rare earth oxides with a mass content of 1 to 5%, preferably 2 to 4%. In the present invention, the rare earth oxides preferably include cerium oxide and / or lanthanum oxide. The addition of rare earth oxides in the present invention can improve the conductivity of the coating, thereby improving the corrosion resistance and high and low temperature cycle impact resistance of the coating.

[0034] In the present invention, the raw materials for preparing the chromium-free phosphate coating include a compound passivating agent with a mass content of 1-5%, preferably 2-4%.

[0035] In the present invention, the compound passivating agent includes zinc oxide, phosphomolybdate and hexamethylenetetramine. In the present invention, the phosphomolybdate preferably includes zinc phosphomolybdate and / or sodium phosphomolybdate. In the present invention, the phosphate forms free phosphoric acid after solidification, which easily absorbs water and causes the coating to become sticky. Zinc oxide can neutralize phosphoric acid and improve the adhesion of the coating; the compound use of phosphomolybdate and hexamethylenetetramine can play a role of slow release. The compound passivating agent used in the present invention can replace chromium and solve the problem of high toxicity of chromium.

[0036] In the present invention, the mass ratio of zinc oxide, phosphomolybdate and hexamethylenetetramine is preferably (1-2): (3-8): (0.5-2), and more preferably (1-2): (4-6): (1-2). In the present invention, the mass ratio of zinc oxide, phosphomolybdate and hexamethylenetetramine is controlled within the above range, which is more conducive to improving the compactness of the coating.

[0037] In the present invention, the raw materials for preparing the chromium-free phosphate coating include boric acid with a mass content of 1-3%, preferably 2-3%. The present invention can improve the heat resistance of the coating by adding boric acid.

[0038] In the present invention, the raw materials for preparing the chromium-free phosphate coating include Al powder with a mass content of 30-50%, preferably 40-50%, and more preferably 45-50%. The present invention utilizes aluminum powder to react with phosphate to form an aluminum phosphate complex, which can enhance the chemical stability and adhesion of the coating. In the present invention, the particle size of the Al powder is preferably 0.1-10 μm, and more preferably 1-10 μm.

[0039] In the present invention, the raw materials for preparing the chromium-free phosphate coating include Si powder with a mass content of 5-10%, preferably 6-8%. The addition of Si powder in the present invention is beneficial to improving the wear resistance of the coating. In the present invention, the particle size of the Si powder is preferably 0.1-10 μm, more preferably 1-10 μm.

[0040] In the present invention, the raw material for preparing the chromium-free phosphate coating includes 1-5% by mass of magnesium oxide, preferably 2-4%. The present invention uses magnesium oxide as a curing agent, which can reduce the curing temperature of the chromium-free phosphate coating.

[0041] In the present invention, the raw materials for preparing the chromium-free phosphate coating include fibers with a mass content of 0.5-5%, preferably 1-4%. In the present invention, the diameter of the fibers is preferably 5-20 μm, more preferably 10-20 μm; the length of the fibers is preferably 50-100 μm, more preferably 60-80 μm. In the present invention, the fibers include glass fibers and / or silica fibers, preferably glass fibers. The present invention uses fibers as fillers, and when the coating is cured, the binder will spread along the fibers to improve the density of the coating.

[0042] In the present invention, the preparation of the chromium-free phosphate coating includes a balance of water. The present invention utilizes water as a dispersion medium for the coating.

[0043] The chromium-free phosphate coating provided by the invention can cure the chromium-free phosphate coating at low temperature by improving the coating components, and at the same time, the cured coating can maintain good comprehensive mechanical properties.

[0044] The present invention also provides a method for preparing the chromium-free phosphate coating according to the above technical solution, comprising the following steps:

[0045] (1) mixing aluminum hydroxide and phosphoric acid, performing a neutralization reaction, and obtaining a base material solution;

[0046] (2) mixing the base material solution obtained in step (1) with Al powder to obtain an Al-containing slurry;

[0047] (3) mixing the Al-containing slurry obtained in step (2) with rare earth oxide, a compound passivating agent, magnesium borate oxide and water to obtain a binder;

[0048] (4) The binder obtained in step (3) is mixed with Si powder and fiber to obtain a chromium-free phosphate coating.

[0049] The invention mixes aluminum hydroxide and phosphoric acid, performs a neutralization reaction, and obtains a base material solution.

[0050] In the present invention, the phosphoric acid is preferably industrial phosphoric acid with a mass concentration of 85%.

[0051] The present invention has no special limitation on the method of mixing the aluminum hydroxide and phosphoric acid. Conventional mixing methods can be used to uniformly mix the two.

[0052] In the present invention, the temperature of the neutralization reaction is preferably 90-120°C, more preferably 100-110°C; the time of the neutralization reaction is preferably 1-2h, more preferably 1.5-2h. Under the above conditions, the present invention is more conducive to fully reacting aluminum hydroxide and phosphoric acid to obtain a base material solution, in which the main component of the phosphate compound is aluminum dihydrogen phosphate.

[0053] After obtaining the base material solution, the present invention mixes the base material solution with Al powder to obtain Al-containing slurry.

[0054] In the present invention, the method for mixing the base material solution with the Al powder is preferably mechanical ball milling, the rotation speed of the mechanical ball milling is preferably 150-500 r / min, more preferably 200-300 r / min; the time of the mechanical ball milling is preferably 30-60 min, more preferably 40-60 min. The present invention can make the phosphate compound in the base material solution wrap the Al powder through mechanical ball milling, passivate the Al powder, improve the room temperature storage stability of the coating, and further improve the uniformity of the coating after the coating is cured.

[0055] After obtaining the Al-containing slurry, the present invention mixes the Al-containing slurry with rare earth oxide, a compound passivating agent, magnesium borate oxide and water to obtain a binder.

[0056] The present invention has no particular limitation on the method of mixing the Al-containing slurry with the rare earth oxide, the compound passivating agent, the boric acid magnesium oxide and water, and a conventional mixing method is used to uniformly mix the components. In the present invention, the mixing method is preferably stirring, and the stirring time is preferably 30 to 60 minutes, more preferably 40 to 60 minutes.

[0057] After obtaining the binder, the present invention mixes the binder with Si powder and fiber to obtain a chromium-free phosphate coating.

[0058] In the present invention, the fibers include glass fibers and / or silica fibers.

[0059] In the present invention, the method of mixing the binder with the Si powder and the fiber is preferably stirring, and the stirring time is preferably 30 to 120 minutes, more preferably 60 to 120 minutes. The present invention can fully mix the Si powder and the fiber with the binder by stirring.

[0060] The preparation method provided by the invention is simple to operate, and a dispersed and stable chromium-free phosphate coating can be obtained by mixing various materials for reaction.

[0061] The present invention also provides a method for using the chromium-free phosphate coating described in the above technical solution or the chromium-free phosphate coating prepared by the preparation method described in the above technical solution, comprising: coating the chromium-free phosphate coating on a substrate, and sequentially performing low-temperature heat treatment and high-temperature heat treatment to obtain a chromium-free phosphate coating.

[0062] In the present invention, the substrate preferably includes carbon steel, stainless steel, titanium, aluminum, aluminum alloy or magnesium alloy. In the present invention, since the coating has a strong bonding force, it can be applied to a variety of substrates.

[0063] The present invention has no particular limitation on the coating method, and any conventional coating method can be used. In the present invention, the coating method preferably includes dipping, brushing or spraying. The present invention has no particular limitation on the parameters of the dipping, brushing or spraying, and the parameters can be adjusted as needed to form a coating layer with uniform thickness on the substrate.

[0064] In the present invention, the coating layer formed by coating preferably has a thickness of 5 to 80 μm, more preferably 10 to 70 μm. The present invention controls the thickness of the coating layer within the above range, and after the coating layer is cured, a coating with uniform thickness and strong bonding ability with the substrate can be obtained.

[0065] In the present invention, the temperature of the low-temperature heat treatment is preferably 60-80°C, more preferably 65-70°C; the holding time of the low-temperature heat treatment is preferably 30-60min, more preferably 40-60min; the temperature of the high-temperature heat treatment is preferably 150-220°C, more preferably 160-180°C; the holding time of the high-temperature heat treatment is preferably 30-120min, more preferably 60-120min. The present invention first helps to improve the density of the coating and reduce the microporous structure of the coating, increase the film density of the coating, and promote the curing of the coating through high-temperature heat treatment to form a dense coating.

[0066] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0067] Example 1

[0068] Raw materials: phosphoric acid with a mass concentration of 85%; Al powder: particle size of 5~8μm; Si powder: particle size of 5~10μm; fiber: glass fiber with a length of 50~60μm and a diameter of 5~10μm;

[0069] A chromium-free phosphate coating is prepared from the following raw materials by weight: 2% aluminum hydroxide, 7% phosphoric acid, 3% rare earth oxide lanthanum oxide, 4% compound passivator (the mass ratio of zinc oxide, zinc phosphomolybdate and hexamethylenetetramine in the compound passivator is 1:4:1), 2% boric acid, 40% Al powder, 6% Si powder, 3% magnesium oxide, 2% fiber and the balance water;

[0070] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4:1;

[0071] The preparation method of the above-mentioned chromium-free phosphate coating comprises the following steps:

[0072] (1) Aluminum hydroxide and phosphoric acid are mixed and reacted at 100° C. for 1 hour to obtain a base material solution;

[0073] (2) ball milling the base material solution obtained in step (1) and Al powder at 200 r / min for 60 min to obtain an Al-containing slurry;

[0074] (3) stirring the Al-containing slurry obtained in step (2) with rare earth oxide, a compound passivating agent, magnesium borate oxide and water for 60 minutes to obtain a binder;

[0075] (4) The binder obtained in step (3) is stirred with Si powder and fiber for 60 minutes to obtain a chromium-free phosphate coating.

[0076] Example 2

[0077] The difference from Example 1 is that the chromium-free phosphate coating is prepared from the following raw materials by mass content: 2% aluminum hydroxide, 6% phosphoric acid, 3% lanthanum oxide, 4% compound passivator (the mass ratio of zinc oxide, zinc phosphomolybdate and hexamethylenetetramine in the compound passivator is 1:4:1), 2% boric acid, 40% Al powder, 6% Si powder, 3% magnesium oxide, 2% fiber and the balance water;

[0078] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.5:1;

[0079] The remaining parameters are the same as those in Example 1.

[0080] Example 3

[0081] The difference from Example 1 is that the chromium-free phosphate coating is prepared from the following raw materials, calculated by mass content: 2% aluminum hydroxide, 6% phosphoric acid, 3% cerium oxide, 4% compound passivator (the mass ratio of zinc oxide, sodium phosphomolybdate and hexamethylenetetramine in the compound passivator is 1:5:1), 2% boric acid, 40% Al powder, 6% Si powder, 5% magnesium oxide, 2% fiber and the balance water;

[0082] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.5:1;

[0083] The remaining parameters are the same as those in Example 1.

[0084] Comparative Example 1

[0085] The difference from Example 1 is that the chromium-free phosphate coating is prepared from the following raw materials by mass content: 2% aluminum hydroxide, 20% phosphoric acid, 3% cerium oxide, 4% compound passivator (the mass ratio of zinc oxide, sodium phosphomolybdate and hexamethylenetetramine in the compound passivator is 1:4:1), 2% boric acid, 40% Al powder, 6% Si powder, 5% magnesium oxide, 2% fiber and the balance water;

[0086] The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.15:1;

[0087] The remaining parameters are the same as those in Example 1.

[0088] Comparative Example 2

[0089] The difference from Example 1 is that the chromium-free phosphate coating is prepared from the following raw materials by mass content: 2% aluminum hydroxide, 7% phosphoric acid, 3% cerium oxide, 2% boric acid, 40% Al powder, 6% Si powder, 5% magnesium oxide, 2% fiber and the balance water;

[0090] The remaining parameters are the same as those in Example 1.

[0091] Comparative Example 3

[0092] The difference from Example 1 is that the chromium-free phosphate coating is prepared from the following raw materials by mass content: 2% aluminum hydroxide, 7% phosphoric acid, 3% cerium oxide, 4% compound passivator (the mass ratio of zinc oxide, sodium phosphomolybdate and hexamethylenetetramine in the compound passivator is 1:4:1), 2% boric acid, 40% Al powder, 6% Si powder, 2% fiber and the balance water;

[0093] The remaining parameters are the same as those in Example 1.

[0094] Comparative Example 4

[0095] The difference from Example 1 is that the phosphate coating is prepared from the following raw materials by mass content: 2% aluminum hydroxide, 7% phosphoric acid, 3% cerium oxide, 4% chromic anhydride, 2% boric acid, 40% Al powder, 6% Si powder, 2% fiber and the balance water;

[0096] The remaining parameters are the same as those in Example 1.

[0097] Comparative Example 5

[0098] The difference from Example 1 is that the phosphate coating is prepared from the following raw materials, calculated by mass content: 2% aluminum hydroxide, 6% phosphoric acid, 3% cerium oxide, 4% compound passivator (the mass ratio of sodium phosphomolybdate and hexamethylenetetramine in the compound passivator is 4:1), 2% boric acid, 40% Al powder, 6% Si powder, 5% magnesium oxide, 2% fiber and the balance water;

[0099] The remaining parameters are the same as those in Example 1.

[0100] Test Case

[0101] The phosphate coatings prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were respectively sprayed onto a stainless steel substrate with an average thickness of 60 μm, and then subjected to low-temperature heat treatment by keeping the temperature at 70° C. for 60 min, and then subjected to high-temperature heat treatment by keeping the temperature at 160° C. for 120 min, and then cooled to room temperature with the furnace to obtain a phosphate coating.

[0102] The coatings prepared in the examples and comparative examples were tested for performance, and the results are shown in Table 1 below.

[0103] Table 1 Test results of coating properties obtained by curing the coatings prepared in the examples and comparative examples

[0104]

[0105] In Table 1, the test method for high and low temperature thermal shock resistance is: the coated stainless steel substrate is kept at 550°C for 30 minutes, cooled in cold water, and then kept at 550°C for 30 minutes, and the test is repeated 100 times. Observe whether the coating has fallen off or cracked.

[0106] It can be seen from Table 1 that the adhesion and neutral salt spray resistance of Comparative Example 1 are lower than those of Example 1. This is because the content of phosphoric acid in Comparative Example 1 is too high, which causes the coating to be too acidic and easy to absorb moisture, resulting in poor quality of the coating, poor adhesion, resistance to neutral salt spray, and resistance to high and low temperature thermal shock. Therefore, the adhesion and neutral salt spray resistance of Comparative Example 1 are lower than those of Example 1.

[0107] Compared with Example 1, Comparative Example 2 does not add a passivating agent, which results in the coating's adhesion, neutral salt spray resistance, and high and low temperature thermal shock resistance being worse than those of Example 1.

[0108] Compared with Example 1, Comparative Example 3 did not add magnesium oxide and could not achieve low-temperature curing.

[0109] Comparative Example 4 is a common phosphate coating, which contains chromic anhydride. The low melting point of chromic anhydride helps to form a gel-like film-like substance, which helps to improve the density of the coating and reduce the microporous structure of the coating. However, the addition of chromic anhydride will make the coating less environmentally friendly.

[0110] Comparative Example 5 does not contain zinc oxide, and the coating has poor adhesion and neutral salt spray resistance. This is because phosphate forms free phosphoric acid after solidification, which easily absorbs water and causes the coating to become sticky. Zinc oxide can neutralize phosphoric acid and improve the adhesion of the coating.

[0111] It can be seen from the results of the above examples that the phosphate coating provided by the present invention does not contain chromium and has a lower curing temperature. Moreover, the coating obtained under the premise of achieving low-temperature curing has comprehensive mechanical properties comparable to those of conventional high-temperature cured phosphate coatings, solving the problem of decreased comprehensive mechanical properties of conventional low-temperature cured phosphate coatings after low-temperature curing.

[0112] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A chromium-free phosphate coating, characterized in that: The material is prepared from the following raw materials, calculated by mass content: 2-5% aluminum hydroxide, 5-15% phosphoric acid, 1-5% rare earth oxide, 1-5% compound passivating agent, 1-3% boric acid, 30-50% Al powder, 5-10% Si powder, 1-5% magnesium oxide, 0.5-5% fiber and the balance water; The molar ratio of Al in the aluminum hydroxide to P in the phosphoric acid is 0.4-0.5:1; The composite passivating agent comprises zinc oxide, phosphomolybdate and hexamethylenetetramine; The fibers include glass fibers and / or silica fibers; The mass ratio of zinc oxide, phosphomolybdate and hexamethylenetetramine is (1-2): (3-8): (0.5-2); The particle size of the Si powder is 0.1-10 μm.

2. The chromium-free phosphate coating according to claim 1, characterized in that: The rare earth oxides include cerium oxide and / or lanthanum oxide.

3. The chromium-free phosphate coating according to claim 1, characterized in that: The phosphomolybdate includes zinc phosphomolybdate and / or sodium phosphomolybdate.

4. The chromium-free phosphate coating according to claim 1, characterized in that: The particle size of the Al powder is 0.1-10 μm.

5. The chromium-free phosphate coating according to claim 1, characterized in that: The diameter of the fiber is 5-20 μm; the length of the fiber is 50-100 μm.

6. The method for preparing the chromium-free phosphate coating according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) mixing aluminum hydroxide and phosphoric acid, performing a neutralization reaction, and obtaining a base material solution; (2) mixing the base material solution obtained in step (1) with Al powder to obtain an Al-containing slurry; (3) mixing the Al-containing slurry obtained in step (2) with rare earth oxide, a compound passivating agent, magnesium borate oxide and water to obtain a binder; (4) The binder obtained in step (3) is mixed with Si powder and fiber to obtain a chromium-free phosphate coating.

7. The preparation method according to claim 6, characterized in that: The temperature of the neutralization reaction in step (1) is 90-120° C.; and the time of the neutralization reaction is 1-2 h.

8. A method for using the chromium-free phosphate coating according to any one of claims 1 to 5 or the chromium-free phosphate coating prepared by the preparation method according to any one of claims 6 to 7, comprising: The chromium-free phosphate coating is applied to the substrate, and low-temperature heat treatment and high-temperature heat treatment are performed in sequence to obtain the chromium-free phosphate coating; The temperature of the low-temperature heat treatment is 60-80°C, and the insulation time of the low-temperature heat treatment is 30-60 minutes; the temperature of the high-temperature heat treatment is 150-220°C, and the insulation time of the high-temperature heat treatment is 30-120 minutes.

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