A surface treatment liquid and treatment method for zinc-aluminum-magnesium coated steel plates

Abstract

This invention provides a surface treatment liquid and method for zinc-aluminum-magnesium coated steel sheets, belonging to the field of surface treatment. The surface treatment liquid comprises: molybdate, oxyacid, sodium-containing surfactant, and solvent; the molybdenum concentration in the surface treatment liquid is 0.4 g / L to 7.0 g / L, the oxyacid concentration is 2 g / L to 10 g / L, and the sodium concentration is 0.1 g / L to 0.3 g / L. This application rationally designs the composition and content of the surface treatment liquid. Molybdate polymerizes in an aqueous solution to generate polymolybdenum anions. These anions react with magnesium ions on the coating surface to form magnesium polymolybdate, providing strong protection to the zinc-aluminum-magnesium coated steel sheet surface. The oxyacid reacts with magnesium hydroxide to form magnesium ions, which quickly form a magnesium polymolybdate protective film with the molybdate. The surfactant improves the uniformity of the magnesium polymolybdate protective film, and controlling the content of the above components improves the blackening phenomenon on the zinc-aluminum-magnesium coating surface.

Description

Technical Field

[0001] This application relates to the field of surface treatment technology, and in particular to a surface treatment liquid and treatment method for zinc-aluminum-magnesium coated steel sheets. Background Technology

[0002] Hot-dip galvanized aluminum-magnesium coated steel sheets possess a delicate metallic luster and excellent corrosion resistance, primarily used in the production of building decoration materials, electrical appliance housings, and agricultural and livestock machinery parts. In recent years, various types of hot-dip galvanized aluminum-magnesium coated steel sheets have emerged. Due to the addition of a certain amount of magnesium, zinc-aluminum-magnesium coated steel sheets exhibit superior surface and cut-edge corrosion resistance compared to ordinary coated products, demonstrating significant social benefits and widespread application value.

[0003] However, when water is present on the surface of the zinc-aluminum-magnesium coating, the magnesium in the coating, acting as the anode, preferentially corrodes, generating corrosion products such as magnesium hydroxide. These corrosion products are porous and easily further corrode, causing the zinc-aluminum-magnesium coated steel sheet surface to blacken, affecting the product's appearance and its widespread use. Therefore, how to efficiently improve the blackening phenomenon on the surface of zinc-aluminum-magnesium coated steel sheets while maintaining the metallic luster of the coating surface is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application provides a surface treatment liquid and method for zinc-aluminum-magnesium coated steel sheets, which improves the blackening phenomenon on the surface of zinc-aluminum-magnesium coated steel sheets while maintaining the metallic luster of the coating surface.

[0005] In a first aspect, this application provides a surface treatment liquid for zinc-aluminum-magnesium coated steel sheet. The surface treatment liquid comprises: molybdate, oxyacid, sodium-containing surfactant, and solvent. In the surface treatment liquid, the mass concentration of molybdenum is 0.4 g / L to 7.0 g / L, the mass concentration of the oxyacid is 2 g / L to 10 g / L, and the mass concentration of sodium is 0.1 g / L to 0.3 g / L.

[0006] Optionally, the surface treatment solution further includes benzotriazole, wherein the mass concentration of benzotriazole is 1.2 g / L to 3.0 g / L.

[0007] Optionally, the molybdate includes at least one of sodium molybdate and ammonium molybdate.

[0008] Optionally, the oxyacid includes at least one of oxalic acid and phosphoric acid.

[0009] Optionally, the surfactant includes at least one of sodium dodecylbenzenesulfonate and sodium dodecyl sulfate.

[0010] Secondly, this application provides a surface treatment method for zinc-aluminum-magnesium coated steel sheets, the method comprising:

[0011] The zinc-aluminum-magnesium coated steel sheet was cleaned in a neutral degreasing agent and then in deionized water, followed by drying.

[0012] The surface treatment liquid described in any one embodiment of the first aspect is diluted with a solvent;

[0013] The dried zinc-aluminum-magnesium coated steel sheet is immersed in the diluted surface treatment solution.

[0014] The zinc-aluminum-magnesium coated steel sheet was rinsed with deionized water and then dried.

[0015] Optionally, the surface treatment liquid and the solvent are diluted at a volume ratio of 1:(10-50).

[0016] Optionally, the soaking time is ≤50s.

[0017] Optionally, the pH value of the neutral degreasing cleaning agent is 6.5 to 7.5.

[0018] Optionally, the chemical composition and corresponding mass fraction of the zinc-aluminum-magnesium coating are as follows: Mg: 0.8%–1.2%, Al: 55%–56%, Si: 1.5%–1.6%, with the remainder being Zn and unavoidable impurities.

[0019] The technical solutions provided in this application have the following advantages compared with the prior art:

[0020] This application rationally designs the composition and content of the surface treatment solution. Through the polymerization of molybdate in aqueous solution to generate polymolybdate anions, these anions can react with magnesium ions on the coating surface to form magnesium polymolybdate. Magnesium polymolybdate, together with other magnesium corrosion products, can form a relatively dense protective film, providing strong protection to the surface of zinc-aluminum-magnesium coated steel plates. Furthermore, through the reaction of oxyacid with magnesium hydroxide, magnesium forms magnesium ions, which rapidly react with molybdate to form a magnesium polymolybdate protective film, further protecting the coating surface. The use of surfactants improves the uniformity of the magnesium polymolybdate protective film, and by controlling the content of the above components, the blackening phenomenon on the zinc-aluminum-magnesium coating surface can be improved. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic flowchart illustrating a surface treatment method for zinc-aluminum-magnesium coated steel sheet provided in an embodiment of this application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0026] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0027] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0028] In a first aspect, this application provides a surface treatment liquid for zinc-aluminum-magnesium coated steel sheet. The surface treatment liquid comprises: molybdate, oxyacid, sodium-containing surfactant, and solvent. In the surface treatment liquid, the mass concentration of molybdenum is 0.4 g / L to 7.0 g / L, the mass concentration of the oxyacid is 2 g / L to 10 g / L, and the mass concentration of sodium is 0.1 g / L to 0.3 g / L.

[0029] In some embodiments, the molybdate includes at least one of sodium molybdate and ammonium molybdate.

[0030] In some embodiments, the oxyacid includes at least one of oxalic acid and phosphoric acid.

[0031] In some embodiments, the surfactant includes at least one of sodium dodecylbenzenesulfonate and sodium dodecyl sulfate.

[0032] In some embodiments, the solvent is water or the like.

[0033] The molybdenum concentration ranges from 0.4 g / L to 7.0 g / L. Molybdate readily polymerizes in aqueous solution to form polymolybdate anions. These anions react with magnesium ions on the coating surface to form magnesium polymolybdate. Magnesium polymolybdate, along with other magnesium corrosion products, forms a relatively dense protective film, providing strong protection to the zinc-aluminum-magnesium coated steel sheet surface and thus improving the blackening phenomenon. The surface treatment solution is diluted before use. If the molybdate concentration is too low, the film is not dense enough, limiting its protective effect on the coating; if the molybdate concentration is too high, the film is too dark, affecting the metallic luster of the zinc-aluminum-magnesium coated steel sheet. The molybdate content can be 0.4 g / L, 0.8 g / L, 1.2 g / L, 2.5 g / L, 3.5 g / L, 5.5 g / L, 6.5 g / L, 7.0 g / L, etc.

[0034] The positive effects of controlling the mass concentration of oxyacid to 2 g / L–10 g / L: Magnesium in zinc-aluminum-magnesium coatings is highly susceptible to corrosion, forming magnesium hydroxide. However, the magnesium hydroxide film is not dense enough, offering very limited protection to the coating, leading to corrosion and blackening. Oxalic acid or phosphoric acid can react with magnesium hydroxide, forming magnesium ions, which quickly combine with molybdate to form a magnesium polymolybdate protective film, protecting the coating surface and improving the blackening phenomenon. Oxalic acid or phosphoric acid is not very acidic and does not easily corrode the coating surface. Surface treatment solutions are diluted before use; if the concentration of oxalic acid or phosphoric acid is too low, it cannot react fully with magnesium hydroxide; if the concentration is too high, it easily corrodes the coating surface. The mass concentration of this oxyacid can be 2 g / L, 4 g / L, 6 g / L, 8 g / L, 10 g / L, etc.

[0035] The positive effects of controlling the sodium concentration to 0.1 g / L–0.3 g / L: Sodium dodecylbenzenesulfonate or sodium dodecyl sulfate, as an anionic surfactant, can improve the uniformity of the magnesium polymolybdate protective film, thereby improving the blackening phenomenon on the zinc-aluminum-magnesium coating surface. The surface treatment solution is diluted before use. If the concentration of sodium dodecylbenzenesulfonate or sodium dodecyl sulfate is too low, it is insufficient to improve the uniformity of the magnesium polymolybdate protective film; if the concentration is too high, it will coat the surface of the magnesium polymolybdate protective film, resulting in poor film formation. The surfactant content can be 0.1 g / L, 0.15 g / L, 0.20 g / L, 0.25 g / L, 0.3 g / L, etc.

[0036] In some embodiments, the surface treatment liquid further includes benzotriazole, wherein the mass concentration of benzotriazole is 1.2 g / L to 3.0 g / L.

[0037] The positive effects of controlling the mass concentration of benzotriazole to 1.2 g / L–3.0 g / L: Benzotriazole can synergistically improve the integrity of the magnesium polymolybdate protective film through its interaction with molybdate, thereby improving the blackening phenomenon on the zinc-aluminum-magnesium plating surface. The surface treatment solution is diluted before use; if the benzotriazole content is too low, it is insufficient to improve the integrity of the magnesium polymolybdate protective film; if the benzotriazole content is too high, its adsorption on the plating surface may affect the formation of the magnesium polymolybdate protective film. The mass concentration of benzotriazole can be 1.2 g / L, 1.6 g / L, 2.0 g / L, 2.4 g / L, 2.8 g / L, 3.0 g / L, etc.

[0038] Secondly, this application provides a surface treatment method for zinc-aluminum-magnesium coated steel sheets, please refer to [link to relevant documentation]. Figure 1 The method includes:

[0039] S1. The zinc-aluminum-magnesium coated steel sheet is cleaned in sequence with a neutral degreasing cleaner and deionized water, and then dried.

[0040] In some embodiments, the chemical composition and corresponding mass fraction of the zinc-aluminum-magnesium coating are as follows: Mg: 0.8%–1.2%, Al: 55%–56%, Si: 1.5%–1.6%, with the remainder being Zn and unavoidable impurities.

[0041] The positive effects of controlling the Mg mass fraction to be between 0.8% and 1.2% are as follows: Within this range, it ensures that Mg and Si form the Mg₂Si phase, Mg and Zn form the MgZn₂ phase, and Mg, Zn, and Al form a ternary eutectic phase, thus guaranteeing excellent planar corrosion resistance and cut-edge corrosion resistance of the zinc-aluminum-magnesium coated steel sheet. The negative effects of a mass fraction greater than or less than the extreme values ​​of this range are: if the value is too high, the formed Mg₂Si phase will be large, affecting the coating's formability; the MgZn₂ phase formed by Mg and Zn will be abundant on the surface, affecting the coating's resistance to blackening; if the value is too low, the Mg₂Si phase, MgZn₂ phase, and the ternary eutectic phase formed by Mg, Zn, and Al in the coating will be insufficient, affecting the coating's corrosion resistance. The Mg mass fraction can be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, etc.

[0042] The positive effects of controlling the Al mass fraction to 55%–56% are: within this range, Al can form a ternary eutectic phase with Mg and Zn, thus ensuring excellent planar corrosion resistance and cut corrosion resistance of the zinc-aluminum-magnesium coated steel sheet. The negative effects of a mass fraction greater than or less than the extreme values ​​of this range are: if the value is too high, the proportion of the Al phase will be too high, affecting the uniformity and corrosion resistance of the coating; if the value is too low, the ternary eutectic phase cannot be formed, affecting the corrosion resistance of the coating. The Al mass fraction can be 55%, 55.5%, 56%, etc.

[0043] The positive effects of controlling the Si mass fraction to be 1.5%–1.6% are: within this range, it ensures that Si and Mg form the Mg₂Si phase, while maintaining the alloy layer thickness between the coating and the substrate within a reasonable range. The negative effects of a mass fraction greater than or less than the extreme values ​​of this range are: if the value is too high, the formed Mg₂Si phase will be coarse, affecting the coating's formability; if the value is too low, the alloy layer will be too thick, also affecting the coating's formability. The Si mass fraction can be 1.5%, 1.55%, 1.6%, etc.

[0044] In some embodiments, the pH value of the neutral degreasing cleaner is 6.5 to 7.5.

[0045] Cleaning zinc-aluminum-magnesium coated steel sheets with degreasing agents can remove grease and other impurities from the surface, improving the adhesion between the sheet and the substances in the treatment solution. A neutral degreasing agent with a pH of 6.5–7.5 is recommended. Too low a pH can corrode the coating surface; too high a pH can cause the formation of large amounts of hydroxides on the coating surface, resulting in blackening. The pH of this neutral degreasing agent can be 6.5, 6.7, 7.0, 7.2, or 7.5.

[0046] Using deionized water to rinse and dry zinc-aluminum-magnesium coated steel sheets that have been soaked in neutral degreasing cleaning agents can prevent the neutral degreasing cleaning agents from affecting the subsequent surface treatment effect.

[0047] S2. Dilute the surface treatment liquid described in any one embodiment of the first aspect using a solvent;

[0048] In some embodiments, the surface treatment liquid is diluted with the solvent at a volume ratio of 1:(10-50).

[0049] The positive effects of controlling the volume ratio of surface treatment solution to solvent to be 1:(10-50): Diluting the surface treatment solution with deionized water at a volume ratio of 1:(10-50) allows the surface treatment solution to form a magnesium polymolybdate protective film on the coating surface at a suitable concentration. Too high a concentration can easily cause corrosion and other adverse effects on the coating surface; too low a concentration makes it difficult to form a magnesium polymolybdate protective film on the coating surface in a short time, thus hindering the improvement of blackening phenomena on the zinc-aluminum-magnesium coating surface. The volume ratio of surface treatment solution to solvent can be 1:10, 1:20, 1:30, 1:40, 1:50, etc.

[0050] S3. Immerse the dried zinc-aluminum-magnesium coated steel sheet in the diluted surface treatment solution.

[0051] In some embodiments, the soaking time is ≤50s.

[0052] The positive effects of controlling the immersion time to ≤50s: Immersing zinc-aluminum-magnesium coated steel sheets in diluted surface treatment solution for less than 50 seconds allows the solution to quickly form a protective film on the coating surface, resulting in short surface treatment time and high production efficiency. Excessive immersion time negatively impacts production efficiency and can easily affect the metallic luster of the coating surface. Suitable immersion times include 10s, 20s, 30s, 40s, and 50s.

[0053] S4. Rinse the soaked zinc-aluminum-magnesium coated steel sheet with deionized water and then dry it.

[0054] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0055] The chemical compositions of the zinc-aluminum-magnesium coatings of Examples 1-6 and Comparative Examples 1-2 of this application are shown in Table 1.

[0056] Table 1 shows the chemical composition (wt%) of zinc-aluminum-magnesium coatings; the remainder consists of unavoidable impurities.

[0057] Example 1 1.2 55 1.6 Example 2 1.1 56 1.5 Example 3 0.8 55 1.6 Example 4 1.1 56 1.5 Example 5 0.8 55 1.6 Example 6 0.8 55 1.6 Comparative Example 1 1.2 55 1.6 Comparative Example 2 1.2 55 1.6

[0058] Embodiments 1-6 of this application provide a surface treatment method for zinc-aluminum-magnesium coated steel sheet, the method comprising the following steps:

[0059] S11. Clean the zinc-aluminum-magnesium coated steel sheet in a neutral degreasing cleaning agent;

[0060] S21. Rinse the cleaned zinc-aluminum-magnesium coated steel sheet with deionized water.

[0061] S31. Dry the rinsed zinc-aluminum-magnesium coated steel sheet.

[0062] S41. Dilute the surface treatment solution with deionized water;

[0063] S51. Immerse the zinc-aluminum-magnesium coated steel sheet in the diluted surface treatment solution. The composition and mass fraction of the surface treatment solution are shown in Table 2.

[0064] S61. Rinse the zinc-aluminum-magnesium coated steel sheet after soaking with deionized water.

[0065] S71. Dry the cleaned zinc-aluminum-magnesium coated steel sheet. The main process parameters are shown in Table 3.

[0066] The zinc-aluminum-magnesium coated steel sheet used in Comparative Example 1 of this application is not surface treated, and the surface is passivated without chromium.

[0067] The surface treatment method for zinc-aluminum-magnesium coated steel sheet used in Comparative Example 2 of this application involves immersing the zinc-aluminum-magnesium coated steel sheet in an aqueous solution containing chloride and sulfate ions to obtain the zinc-aluminum-magnesium coated steel sheet, followed by chromium-free passivation.

[0068] Table 2. Composition and mass fraction of the surface treatment solutions in Examples 1-6

[0069]

[0070] In Table 2, Mo-g / L represents the content of molybdate in units of molybdenum elemental concentration; Na-g / L represents the content of surfactant in units of sodium elemental concentration.

[0071] Table 3. Main process parameters of the surface treatment methods for zinc-aluminum-magnesium coated steel sheets in Examples 1-6.

[0072]

[0073] Samples of zinc-aluminum-magnesium coated steel sheets obtained in Examples 1-6 and Comparative Examples 1-2 were taken respectively. The surface brightness L value of the steel sheet was measured using an X-rite SP60 spectrophotometer. Each sample was subjected to a 120-hour experiment at 50°C and 95% relative humidity in an ESPEC-SETH-Z-02R damp heat test chamber. The color difference ΔE of the steel sheet surface before and after the experiment was measured using a spectrophotometer. The results are shown in Table 4.

[0074] Table 4. Surface brightness (L value) and color difference (ΔE) of zinc-aluminum-magnesium coated steel sheets.

[0075]

[0076]

[0077] Detailed analysis in Table 4:

[0078] The L-value of steel plate surface brightness refers to the brightness of the steel plate surface. The larger the L-value, the brighter and whiter the steel plate surface is, and the more metallic it is.

[0079] The color difference ΔE on the surface of a steel plate refers to the color change of the steel plate surface after wet heat treatment. The smaller the color difference, the stronger the ability of the steel plate coating to resist blackening, and the more stable the metallic luster of the steel plate surface.

[0080] From the data in Examples 1-6, it can be seen that:

[0081] Using the method of this application, the surface blackening resistance of zinc-aluminum-magnesium coated steel sheet can be improved without passivation, while maintaining the metallic luster of the zinc-aluminum-magnesium coated steel sheet. The surface brightness L value of the steel sheet is above 90, and experiments show that the surface color difference ΔE of the steel sheet is below 2.0.

[0082] From the data in Comparative Examples 1-2, we can see that:

[0083] If the method of this application is not adopted, and the zinc-aluminum-magnesium coated steel sheet is directly subjected to chromium-free passivation treatment, the resulting steel sheet has a higher surface color difference ΔE before and after the experiment, indicating that it is not resistant to blackening; or if it is immersed in an aqueous solution containing chloride ions and sulfate ions and then subjected to chromium-free passivation treatment, the resulting steel sheet also has a higher ΔE, indicating that it has poor resistance to blackening, and its surface brightness L value is lower, indicating that its surface is dull and has poor metallic luster.

[0084] One or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0085] (1) The surface treatment liquid and its treatment method provided in this application form a magnesium molybdate composite film on the surface of zinc-aluminum-magnesium coated steel plate, which provides protection for the coating. It can improve the blackening resistance of the zinc-aluminum-magnesium coated surface under humid heat conditions without passivation. The surface color difference ΔE in the humid heat test is below 2.0.

[0086] (2) The surface treatment liquid and treatment method provided in this application embodiment will not affect the metallic luster of the zinc-aluminum-magnesium coated steel plate, thereby maintaining the metallic luster of the zinc-aluminum-magnesium coated steel plate, and the surface brightness L value of the steel plate is above 90.

[0087] (3) The surface treatment liquid and treatment method provided in the embodiments of this application have a short surface treatment time, which improves production efficiency.

[0088] (4) The aqueous solution used in the embodiments of this application does not contain chromium and is environmentally friendly.

[0089] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A surface treatment method for zinc-aluminum-magnesium coated steel sheet, characterized in that, The method includes: The zinc-aluminum-magnesium coated steel sheet was cleaned in a neutral degreasing agent and then in deionized water, and then dried. The chemical composition and corresponding mass fraction of the zinc-aluminum-magnesium coating were as follows: Mg: 0.8%~1.2%, Al: 55%~56%, Si: 1.5%~1.6%, with the remainder being Zn and unavoidable impurities. The surface treatment solution is diluted with a solvent at a volume ratio of 1:(10~50). The surface treatment solution consists of the following components: molybdate, oxyacid, sodium-containing surfactant, and solvent; wherein the mass concentration of molybdenum is 0.4 g / L~7.0 g / L, the mass concentration of the oxyacid is 2 g / L~10 g / L, and the mass concentration of sodium is 0.1 g / L~0.3 g / L; the oxyacid includes at least one of oxalic acid and phosphoric acid, and the surfactant includes at least one of sodium dodecylbenzenesulfonate and sodium dodecyl sulfate. The dried zinc-aluminum-magnesium coated steel sheet is immersed in the diluted surface treatment solution for ≤50s. The zinc-aluminum-magnesium coated steel sheet was rinsed with deionized water and then dried. The surface brightness L value of the dried steel plate is above 90, and the surface color difference ΔE in the damp heat test is below 2.

0.

2. The method according to claim 1, characterized in that, The molybdate includes at least one of sodium molybdate and ammonium molybdate.

3. The method according to claim 1, characterized in that, The neutral degreasing cleaning agent has a pH value of 6.5~7.5.

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