A magnesium alloy surface carbonate composite coating and a preparation method thereof

Abstract

The application provides a magnesium alloy surface carbonate composite coating and a preparation method thereof, and particularly relates to the field of magnesium alloy corrosion protection. The specific steps are as follows: (1) preparing a carbonic acid solution; (2) soaking a magnesium alloy component in the carbonic acid solution prepared in step (1) to prepare a carbonate conversion film of the magnesium alloy component; and (3) preparing an ammonium carbonate solution, and soaking the magnesium alloy component treated in step (2) in the ammonium carbonate solution to prepare a carbonate composite coating. The application is suitable for different types and shapes of magnesium alloys, and the magnesium alloy does not need to be pretreated before soaking, and no complex operation equipment or toxic and harmful substances are used, so that the difficulty and cost of waste liquid treatment are low, and the requirements of the protective performance of the coating and environmental friendliness can be met. The prepared carbonate composite coating has no cracks, good adhesion and excellent protection effect. The carbonate composite coating is prepared on the surface of the magnesium alloy by using a chemical conversion method, and the whole process is simple to operate.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy corrosion protection, and in particular provides a magnesium alloy surface carbonate composite coating and its preparation method. Background Technology

[0002] Magnesium alloys have a low density, only one-quarter that of iron or two-thirds that of aluminum, while possessing high specific strength and specific stiffness, excellent vibration damping and electromagnetic shielding properties. They have achieved remarkable application results in the aerospace, communications, and automotive industries. Especially in the automotive sector, the application of magnesium alloys is of strategic significance. It has been reported that for every 70 kg of magnesium alloy used in a car, CO2 emissions will be reduced by more than 30%, contributing to mitigating global warming. However, it is well known that a thin film of magnesium oxide easily grows on the surface of magnesium alloys in the air. The density coefficient (PB ratio) of this film is 0.81, less than 1. Therefore, the magnesium oxide film is easily damaged by tensile stress and cannot fully cover the metal surface, resulting in extremely limited protection for the magnesium alloy substrate. Furthermore, this film is easily affected by Cl... - Corrosion. Therefore, improving the corrosion resistance of magnesium alloys is extremely important.

[0003] There are numerous methods to improve the corrosion resistance of magnesium alloys, such as micro-arc oxidation, electroplating / electroplating, plasma spraying, vapor deposition, and chemical conversion coatings. Among these methods, chemical conversion coatings require neither complex and advanced manufacturing equipment nor incur high costs, making them simpler and more readily accepted. Currently, the main types of chemical conversion coatings on magnesium alloy surfaces include chromate conversion coatings, rare earth conversion coatings, phosphate conversion coatings, and permanganate conversion coatings. Chromates are highly toxic and harmful substances, posing a risk to human health and polluting the environment, increasing the cost and difficulty of wastewater treatment. Other types of conversion coating processes are not yet mature. Summary of the Invention

[0004] The purpose of this invention is to provide a magnesium alloy surface carbonate composite coating and its preparation method.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing a carbonate composite coating on a magnesium alloy surface, comprising the following specific steps:

[0007] (1) Prepare a carbonic acid solution;

[0008] (2) Immerse the magnesium alloy component in the carbonic acid solution prepared in step (1) to prepare a carbonate conversion film for the magnesium alloy component;

[0009] (3) Prepare an ammonium carbonate solution and immerse the magnesium alloy component treated in step (2) in the ammonium carbonate solution to prepare a carbonate composite coating.

[0010] Further, in step (1) preparing the carbonic acid solution: CO2 gas is used, H2O is used as the solvent, CO2 is introduced into H2O, the flow rate of CO2 is 100-300 ml / min, the gasification time is 3-6 h, and the pH of the obtained carbonic acid solution is 4-6.

[0011] Further, in step (1) preparing the carbonic acid solution: add 10-20% carbonate and 0.01-0.03% corrosion inhibitor to water, stir evenly, and then add 0.005-0.03% buffer to obtain a carbonic acid solution with a pH of 4-6.

[0012] Furthermore, in step (2), the magnesium alloy component is immersed in the carbonic acid solution prepared in step (1) for 6 to 12 hours.

[0013] Furthermore, the composition and mass percentage of the ammonium carbonate solution in step (3) are as follows: ammonium carbonate: 10-40%, buffer solvent: 0.003% to 0.01%, the remainder being H2O; soaking time: 10-12 h; solution pH: 7-9.

[0014] Furthermore, the carbonate is one or more of sodium carbonate and potassium carbonate; the corrosion inhibitor is one or more of hexamethylenetetramine, phosphorus-containing dimethylthiourea, and aldehyde-amine condensate; and the buffer is one or more of boric acid, acetic acid, and hydrochloric acid.

[0015] Furthermore, the ammonium carbonate is one or both of ammonium bicarbonate and ammonium carbonate.

[0016] Furthermore, the buffer solvent is one or more of boric acid and acetic acid.

[0017] Furthermore, the phases in the composite coating consist of (NH4)2Mg(CO3)2·4H2O and MgCO3, and these two phases are uniformly distributed throughout the coating.

[0018] Beneficial effects of this invention:

[0019] (1) This invention is applicable to magnesium alloys of different types and shapes. No pretreatment of magnesium alloys is required before immersion, no complicated operating equipment is used, and no toxic or harmful substances are used. The waste liquid treatment is less difficult and less expensive, and can meet the requirements of coating protection performance and environmental friendliness.

[0020] (2) The carbonate composite coating prepared by the present invention has no cracks, good adhesion, and excellent protective effect.

[0021] (3) The present invention uses a chemical conversion method to prepare a carbonate composite coating on the surface of magnesium alloy. The steps are closely connected and no intermediate rinsing or treatment is required. The entire process is simple to operate. Attached image description:

[0022] Figure 1 : Coating cross-sectional morphology diagram

[0023] Figure 2 Graph showing the relationship between hydrogen volume in the substrate and coating magnesium alloys in 3.5% NaCl solution and immersion time. Detailed Implementation

[0024] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0025] Example 1

[0026] (1) Preparation of carbonic acid solution. Gas used: CO2, solvent: H2O. CO2 is introduced into H2O at a flow rate of 100 mL / min for 3 h. The resulting carbonic acid solution has a pH of 6.

[0027] (2) The magnesium alloy component is immersed in the carbonate solution prepared in step (1) for 6 hours to prepare the carbonate conversion film of the magnesium alloy component.

[0028] (3) Prepare an ammonium carbonate solution and immerse the magnesium alloy component treated in step (2) in the ammonium carbonate solution to prepare a carbonate composite coating. The composition and mass percentage of the ammonium carbonate solution used are as follows: 10% NH4HCO3, 0.005% boric acid, and the remainder is water; the solution pH is 8, and the immersion time is 10h.

[0029] Example 2

[0030] (1) Preparation of carbonic acid solution. Gas used: CO2, solvent: H2O. CO2 is introduced into H2O at a flow rate of 150 mL / min for 5 h. The resulting carbonic acid solution has a pH of 5.

[0031] (2) The magnesium alloy component is immersed in the carbonate solution prepared in step (1) for 6 hours to prepare the carbonate conversion film of the magnesium alloy component.

[0032] (3) Prepare an ammonium carbonate solution and immerse the magnesium alloy component treated in step (2) in the ammonium carbonate solution to prepare a carbonate composite coating. The composition and mass percentage of the ammonium carbonate solution used are as follows: 30% NH4HCO3, 0.005% acetic acid, and the remainder is water; the solution pH is 7.5, and the immersion time is 12h.

[0033] Example 3

[0034] (1) Prepare a carbonic acid solution. Add 10% Na2CO3 and 0.01% hexamethylenetetramine by mass to water, stir well, and then add 0.005% boric acid by mass. The resulting carbonic acid solution has a pH of 5.

[0035] (2) The magnesium alloy component is immersed in the carbonate solution prepared in step (1) for 10 hours to prepare the carbonate conversion film of the magnesium alloy component.

[0036] (3) Prepare an ammonium carbonate solution and immerse the magnesium alloy component treated in step (2) in the ammonium carbonate solution to prepare a carbonate composite coating. The composition and mass percentage of the ammonium carbonate solution used are as follows: 30% (NH4)2CO3, 0.01% boric acid, and the remainder is water; the solution pH is 9, and the immersion time is 10h.

[0037] Example 4

[0038] (1) Prepare a carbonic acid solution. Add 15% K2CO and 0.01% phosphorus-containing dimethyl thiourea to water, stir well, and then add 0.01% acetic acid. The resulting carbonic acid solution has a pH of 4.

[0039] (2) The magnesium alloy component is immersed in the carbonate solution prepared in step (1) for 12 hours to prepare the carbonate conversion film of the magnesium alloy component.

[0040] (3) In the preparation of ammonium carbonate solution, the magnesium alloy component treated in step (2) is immersed in the ammonium carbonate solution to prepare a carbonate composite coating. The composition and mass percentage of the ammonium carbonate solution used are as follows: 20% (NH4)2CO3, 0.006% acetic acid, and the remainder is water; the solution pH is 8, and the immersion time is 12h.

[0041] like Figure 1 As shown, the coatings prepared in Examples 1-4 are free of cracks. The phases in the composite coating consist of (NH4)2Mg(CO3)2·4H2O and MgCO3, which are uniformly distributed throughout the coating. (NH4)2Mg(CO3)2·4H2O is derived from MgCO3. The coatings prepared in Examples 1-4 were tested using the cross-cut adhesion test in GB / T5210-85 "Determination of Coating Adhesion". The coatings showed no large-area peeling or flaking, indicating good adhesion.

[0042] like Figure 2 As shown, hydrogen evolution tests in 3.5% NaCl solution indicate that the coating effectively inhibits the hydrogen evolution rate of the magnesium alloy substrate, demonstrating excellent protective properties.

[0043] Matters not covered in this invention are common knowledge.

[0044] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a magnesium alloy surface carbonate composite coating, characterized by: The specific steps are as follows: (1) The carbonic acid solution is configured; the carbonic acid solution is configured by adding 10-20% of carbonates and 0.01-0.03% of corrosion inhibitors in mass percentage into water, stirring uniformly, and then adding 0.005-0.03% of buffer in mass percentage, so that the obtained carbonic acid solution has a pH of 4-6; the carbonates are one or more than two of sodium carbonate and potassium carbonate; the corrosion inhibitors are one or more than two of hexamethylenetetramine, phosphorus-containing dimethylthiourea and aldehyde amine condensate; the buffer is one or more than two of boric acid, acetic acid and hydrochloric acid; (2) The magnesium alloy assembly is soaked in the carbonic acid solution configured in step (1) to prepare a carbonic acid salt conversion film of the magnesium alloy assembly; the magnesium alloy assembly is soaked in the carbonic acid solution configured in step (1) for 6-12 hours; (3) The ammonium carbonic acid salt solution is configured, and the magnesium alloy assembly treated in step (2) is soaked in the ammonium carbonic acid salt solution to prepare a carbonic acid salt composite coating; the ammonium carbonic acid salt solution is composed of 10-40% of ammonium carbonic acid salt, 0.003%-0.01% of buffer solvent and the rest of H2O; the soaking time is 10-12 hours; the solution has a pH of 7-9; the composite coating is composed of (NH4)2Mg(CO3)2·4H2O and MgCO3, and the two phases are uniformly distributed in the whole coating; the buffer solvent is one or more than two of boric acid and acetic acid.

2. The method of claim 1, wherein: Step (1) of configuring the carbonic acid solution: CO2 gas is used, the solvent is H2O, CO2 is introduced into H2O, the flow rate of CO2 is 100-300 ml / min, the aeration time is 3-6 hours, and the obtained carbonic acid solution has a pH of 4-6.

3. The method of claim 1, wherein: The ammonium carbonic acid salt is one or both of ammonium bicarbonate and ammonium carbonate.

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