Filamentous corrosion resistant zinc-aluminum-magnesium coated steel plate and preparation method thereof

Abstract

The invention relates to a filamentous-corrosion-resistant zinc-aluminum-magnesium coated steel plate and a preparation method thereof, and belongs to the technical field of metal surface modification. The steel plate comprises a zinc-aluminum-magnesium coating and a chemical conversion coating attached to at least part of the surface of the zinc-aluminum-magnesium coating, and the chemical conversion coating is formed by chemical reaction of chemical conversion liquid and the zinc-aluminum-magnesium coating. The components and content in the chemical conversion liquid are reasonably controlled, the chemical conversion liquid is coated on the surface of the zinc-aluminum-magnesium coating steel plate, and the chemical conversion liquid and the zinc-aluminum-magnesium coating are subjected to chemical reaction to form a compact, stable and corrosion-resistant chemical conversion film. And firm chemical bonding is formed between the chemical conversion film and the zinc-aluminum-magnesium coating steel plate, so that the whole coating system is more stable, invasion of corrosive media is effectively blocked, and the filiform corrosion resistance of the whole coating system is improved.

Description

Technical Field

[0001] This application relates to the technical field of metal surface modification, and particularly relates to a zinc-aluminum-magnesium plated steel sheet resistant to filiform corrosion and a preparation method thereof. Background Art

[0002] The zinc-based coating has both physical isolation and sacrificial anode protection effects on the steel substrate, greatly improving the corrosion resistance of the steel sheet. In the past decade, zinc-aluminum-magnesium coating materials have been widely used in the automotive industry. Compared with traditional hot-dip galvanized steel, the zinc-aluminum-magnesium coating shows more excellent corrosion resistance. This is mainly because the Mg ions formed after the eutectic phase in the zinc-aluminum-magnesium coating is corroded can buffer the increase in pH value, inhibit the alkalization of the coating surface, and mainly form insoluble colloidal corrosion products Zn5(OH)8Cl2·H2O in a low-alkaline environment. Its conductivity is low, and the reduction rate of oxygen on its surface is slow, while the corrosion products of traditional galvanized sheets are mainly ZnO, whose conductivity is higher than that of Zn5(OH)8Cl2·H2O, and the reduction rate of oxygen on its surface is faster. Therefore, the zinc-aluminum-magnesium coating materials show higher corrosion resistance in the accelerated corrosion test in a high-chlorine environment.

[0003] In order to meet the anti-corrosion requirements of no rust on the vehicle body for 5 years and no perforation for 12 years, automotive sheets are generally used in combination with surface organic coatings. Under normal working conditions, compared with pure zinc materials, zinc-aluminum-magnesium has more excellent under-film corrosion resistance. However, in the presence of some sensitive media, when the current treatment film quality is poor, there is a certain probability of filiform corrosion. This filiform corrosion will form a corrosion morphology that expands in a specific direction, affecting the appearance quality of the organic coating. Summary of the Invention

[0004] This application provides a zinc-aluminum-magnesium plated steel sheet resistant to filiform corrosion and a preparation method thereof to solve the following technical problem: how to improve the filiform corrosion resistance of the zinc-aluminum-magnesium plated steel sheet.

[0005] In the first aspect, this application provides a zinc-aluminum-magnesium plated steel sheet resistant to filiform corrosion. The steel sheet includes a zinc-aluminum-magnesium coating and a chemical conversion film attached to at least part of the surface of the zinc-aluminum-magnesium coating; the chemical conversion film is formed by a chemical conversion solution and the zinc-aluminum-magnesium coating through a chemical reaction.

[0006] Optionally, the components of the chemical conversion solution include: a fluorine-containing inorganic acid, a silane coupling agent, an inorganic salt, and a corrosion inhibitor.

[0007] Optionally, the fluorine-containing inorganic acid is at least one of fluotitanic acid and fluorozirconic acid, and the concentration content of the fluorine-containing inorganic acid is 1200 ppm to 3500 ppm.

[0008] Optionally, the silane coupling agent is at least one of KH792 and KH570, and the concentration of silicon element in the silane coupling agent is 700 ppm to 1800 ppm.

[0009] Optionally, the inorganic salt is at least one of sodium nitrate and manganese nitrate, and the concentration of the inorganic salt is 500 ppm to 4000 ppm.

[0010] Optionally, the corrosion inhibitor is at least one of sodium tripolyphosphate and mercaptobenzothiazole, and the concentration of the corrosion inhibitor is 800 ppm to 6000 ppm.

[0011] Optionally, the chemical conversion solution further includes metal ions, and the metal ions are at least one of manganese ions, zinc ions and copper ions, and the concentration of the metal element of the metal ions is 1 ppm to 50 ppm.

[0012] Optionally, the chemical composition of the zinc-aluminum-magnesium coating includes: Mg, Al and Zn; wherein, by mass, the content of Mg is 0.9 parts to 1.3 parts, and the content of Al is 1.4 parts to 2 parts.

[0013] In a second aspect, the present application provides a method for preparing a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion as described in the first aspect, and the method includes:

[0014] Hot-dip galvanize the surface of the steel substrate to obtain a zinc-aluminum-magnesium coated steel sheet;

[0015] Use the chemical conversion solution to perform surface coating on the zinc-aluminum-magnesium coated steel sheet to obtain a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion.

[0016] Optionally, the process parameters of the surface coating include: the PMT temperature is 80 °C to 100 °C, and the reaction time is 10 s to 60 s.

[0017] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0018] The present application provides a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion. The steel sheet includes a zinc-aluminum-magnesium coating and a chemical conversion film attached to at least a part of the surface of the zinc-aluminum-magnesium coating. The chemical conversion film is formed by a chemical reaction between the chemical conversion solution and the zinc-aluminum-magnesium coating. By reasonably controlling the components and contents in the chemical conversion solution and coating it on the surface of the zinc-aluminum-magnesium coated steel sheet, the chemical conversion solution reacts with the zinc-aluminum-magnesium coating to form a dense, stable and corrosion-resistant chemical conversion film. A strong chemical bonding is formed between the chemical conversion film and the zinc-aluminum-magnesium coated steel sheet, making the entire coating system more stable, thereby effectively blocking the intrusion of corrosive media and improving the filiform corrosion resistance of the entire coating system. Brief Description of the Drawings

[0019] The drawings herein are incorporated into and form a part of this specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0020] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 It is a schematic flow chart of a preparation method of a zinc-aluminum-magnesium coating steel sheet resistant to filiform corrosion provided for the embodiments of the present application. Detailed Embodiments

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0023] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and individual 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 individual numbers within that range, such as 1, 2, 3, 4, 5, and 6, regardless of the range; in addition, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0024] In this text, terms including "comprising" and the like mean "including but not limited to". Relative terms such as "first" and "second" are only used 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. "And / or", which describes the association relationship of associated objects, indicates that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "a plurality" means two or more; "at least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single item (piece) or plural items (pieces); for example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple respectively. Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in this application can be obtained through market purchase or can be prepared by existing methods.

[0025] In a first aspect, the present application provides a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion. The steel sheet includes a zinc-aluminum-magnesium coating and a chemical conversion film attached to at least a part of the surface of the zinc-aluminum-magnesium coating; the chemical conversion film is formed by a chemical conversion solution reacting with the zinc-aluminum-magnesium coating through a chemical reaction.

[0026] In some embodiments, the components of the chemical conversion solution include: a fluorine-containing inorganic acid, a silane coupling agent, an inorganic salt, and a corrosion inhibitor.

[0027] The chemical conversion film is formed by reacting the chemical conversion solution with the zinc-aluminum-magnesium coating. This conversion process generates a stable compound film, effectively blocking the direct contact between the corrosive medium and the steel sheet, thereby improving the filiform corrosion resistance of the zinc-aluminum-magnesium coated steel sheet. In the chemical conversion solution, the fluorine-containing inorganic acid helps to form a stable protective film on the surface of the zinc-aluminum-magnesium coated steel sheet, improving the corrosion resistance; the silane coupling agent can enhance the bonding force between the chemical conversion film and the zinc-aluminum-magnesium coated steel sheet, while improving the density and corrosion resistance of the film; the inorganic salt plays a catalytic role in the chemical conversion process, promoting the formation of the conversion film; the corrosion inhibitor can slow down the rate of the corrosion reaction and further extend the service life of the steel sheet.

[0028] In some embodiments, the fluorine-containing inorganic acid is at least one of fluotitanic acid and fluorozirconic acid, and the concentration of the fluorine-containing inorganic acid is 1200 ppm to 3500 ppm.

[0029] Since the film-forming reaction is realized at the latter stage of the galvanizing production line, when the content of the fluorinated inorganic acid is lower than the above range, a complete continuous film cannot be effectively and rapidly formed. When the content is higher than the above range, the compactness of the film layer decreases, affecting the corrosion resistance of the coating. Exemplarily, the concentration content of the fluorinated inorganic acid can be 1200 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, etc. In the embodiments of the present application, it is preferably 2000 - 3000 ppm.

[0030] In some embodiments, the silane coupling agent is at least one of KH792 and KH570, and the concentration content of the silicon element of the silane coupling agent is 700 ppm - 1800 ppm.

[0031] KH792, the chemical name is N-(β-aminoethyl)-γ-aminopropyltrimethoxy (ethoxy) silane; KH570, the chemical name is γ-methacryloxypropyltrimethoxysilane. KH792 and KH570 are common silane coupling agents. In the chemical conversion solution, the role of the silane coupling agent is to improve the bonding performance of the zinc-aluminum-magnesium alloy coating and shorten the film-forming time of the chemical conversion film. When the concentration content of the silicon element of the silane coupling agent is lower than the above range, the formed conversion film layer on the metal surface is thinner, affecting its bonding performance. When the content is higher than the above range, a polycondensation reaction is likely to occur, affecting its storage period. Exemplarily, the concentration content of the silicon element of the silane coupling agent can be 700 ppm, 900 ppm, 1100 ppm, 1300 ppm, 1500 ppm, 1700 ppm, 1800 ppm, etc.

[0032] In some embodiments, the inorganic salt is at least one of sodium nitrate and manganese nitrate, and the concentration content of the inorganic salt is 500 ppm - 4000 ppm.

[0033] The inorganic salt can accelerate the film-forming reaction. When the content is lower than the above range, the film-forming speed is slower; when the content is higher than the above range, it affects the appearance quality of the treated film and further affects its corrosion resistance. Exemplarily, the content of the inorganic salt can be 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, etc.

[0034] In some embodiments, the corrosion inhibitor is at least one of sodium tripolyphosphate and mercaptobenzothiazole, and the concentration content of the corrosion inhibitor is 800 ppm - 6000 ppm.

[0035] Positive effects of limiting the inhibitor content to 800 ppm to 6000 ppm: Usually, the sensitive medium for filiform corrosion of zinc-aluminum-magnesium plated materials is acetic acid. In the presence of acetic acid medium, local corrosion is likely to occur on the coating, triggering filiform corrosion. Sodium tripolyphosphate and mercaptobenzothiazole can effectively prevent the local dissolution of the coating by acetic acid. When the inhibitor content is lower than the above range, less inhibitor precipitates on the coating surface, resulting in poor inhibition effect. When the inhibitor content is higher than the above range, the welding performance of the zinc-aluminum-magnesium plated steel sheet will be affected. Exemplarily, the inhibitor content can be 800 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, etc.

[0036] In some embodiments, the chemical conversion solution further includes metal ions, and the metal ions are at least one of manganese ions, zinc ions, and copper ions. The concentration of the metal element of the metal ions is 1 ppm to 50 ppm.

[0037] Positive effects of the concentration of the metal element of the metal ions being 1 ppm to 50 ppm: The metal ions promote film formation and increase the nucleation sites of zirconium salts. If the content is higher than the above range, the displacement reaction is faster, which will affect the surface quality. If the content is lower than the above range, the number of nucleation sites is less, affecting the denseness of the film formation. Exemplarily, the concentration of the metal element of the metal ions can be 1 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, 50 ppm, etc.

[0038] In some embodiments, the zinc-aluminum-magnesium plated steel sheet includes a steel substrate and a zinc-aluminum-magnesium coating attached to at least a part of the surface of the steel substrate; the chemical composition of the zinc-aluminum-magnesium coating includes: Mg, Al, and Zn; wherein, by mass, the content of Mg is 0.9 parts to 1.3 parts, and the content of Al is 1.4 parts to 2 parts.

[0039] Mg in the coating can significantly improve the corrosion resistance of the coating, especially the corrosion resistance under high chloride ion conditions. The mechanism is that Mg in the coating will form compounds such as MgZn2, and this compound will form a binary eutectic phase with Zn in the coating, and will also form a ternary eutectic phase together with Zn, aluminum-rich phase, etc. When there is a continuous thin liquid film on the surface of the metal coating, the eutectic phase in the coating will preferentially dissolve, react with the dissolved CO2 in the thin liquid film, maintain the pH value of the liquid film stable, and is conducive to the formation of a stable protective corrosion product, basic zinc carbonate, so as to protect the coating from further corrosion. If the Mg content in the coating is less than the above range, the eutectic structure formed in the coating is less, and the atmospheric corrosion resistance of the coating is poor; if the Mg element content is greater than the above range, the coating is prone to oxidation, which will affect the surface quality of the coating, and at the same time, it is easy to form too much eutectic structure, and its filiform corrosion sensitivity will increase. Exemplarily, the content of Mg can be 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, etc.

[0040] Al in the coating can significantly improve the corrosion resistance of the coating. This is because during the corrosion process, Al can participate in the formation of a protective corrosion product, LDH (Layered Double Hydroxide) coating. LDH is layered double metal hydroxide, also known as hydrotalcite or hydrotalcite-like compound, which is a kind of inorganic functional material with a special layered structure. This coating can effectively prevent the corrosion of the substrate. If the Al content in the coating is lower than the above range, the amount of protective corrosion product formed is less, which is not conducive to long-term corrosion resistance; if the Al content in the coating is higher than the above range, the hardness of the coating increases, the elongation rate decreases, and the coating is prone to cracking during the stamping process, affecting its manufacturing performance and corrosion resistance. Exemplarily, the content of Al can be 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc.

[0041] Figure 1 It is a schematic flow chart of a preparation method of a zinc-aluminum-magnesium coated steel sheet with resistance to filiform corrosion provided by an embodiment of the present application.

[0042] Please refer to Figure 1 Second, the present application provides a preparation method of the zinc-aluminum-magnesium coated steel sheet with resistance to filiform corrosion described in the first aspect, and the method includes:

[0043] S1. Perform hot-dip galvanizing-aluminum-magnesium alloy on the surface of the steel substrate to obtain a zinc-aluminum-magnesium coated steel sheet;

[0044] S2. Use the chemical conversion solution to perform surface coating on the zinc-aluminum-magnesium coated steel sheet to obtain a zinc-aluminum-magnesium coated steel sheet with resistance to filiform corrosion.

[0045] In some embodiments, the process parameters of the surface coating include: the PMT temperature is 80°C to 100°C, and the reaction time is 10 s to 60 s.

[0046] PMT (Peak Metal Temperature) refers to the peak temperature on the surface of the strip steel. During the surface coating stage, the PMT temperature is controlled between 80°C and 100°C. Within this temperature range, the reaction rate between the chemical conversion liquid and the surface of the zinc-aluminum-magnesium coated steel sheet is moderate, which can ensure that the active components in the conversion liquid are in full contact with the coating surface and undergo chemical reactions to form a dense and uniform protective film. Temperatures higher than this range may cause some components in the chemical conversion liquid to decompose or volatilize, affecting the formation quality of the protective film.

[0047] The positive effect of limiting the reaction time of the surface coating to 10 s to 60 s: Within this time range, the chemical conversion liquid and the surface of the zinc-aluminum-magnesium coated steel sheet can undergo sufficient chemical reactions to ensure that the formed protective film reaches the expected thickness and quality. If the time is less than this range, the reaction may be incomplete, resulting in a too thin or unevenly formed protective film; while if the reaction time is greater than this range, it may waste resources and increase production costs.

[0048] The precise control of the PMT temperature and the reaction time makes the process more stable and controllable, ensuring the consistency of product quality.

[0049] In some embodiments, the surface coating methods include roll coating, spraying, full immersion, etc.

[0050] The following further elaborates on this application in combination with specific embodiments. For the experimental methods without specific conditions noted in the following embodiments, they are usually determined according to industry standards; if there are no corresponding industry standards, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0051] This application sets Examples 1 to 12 and Comparative Examples 1 to 3. For the chemical compositions of the zinc-aluminum-magnesium coatings and the chemical conversion liquid components, please refer to Table 1.

[0052] Table 1

[0053]

[0054]

[0055] The steel plates prepared according to the process parameters in the above-mentioned examples and comparative examples were evaluated for resistance to filiform corrosion. After the steel plates were pretreated with a thin film and electrophoresed, scratches were made on the surface of the electrophoretic paint film. The scratch length was 30 mm, the width was 0.2 mm, and the scratch depth reached the steel substrate. 10 μL of acetic acid solution (pH 4) was injected at the scratch position, and then it was placed in a constant temperature and humidity environment (40 °C, 85% RH) for 720 hours. Then, the average filament length of filiform corrosion on the surface of the sample was evaluated according to GB / T 30789.9. As can be seen from Table 1, the corrosion filament length of the steel plates in Examples 1 to 12 was <1 mm, and no filiform corrosion occurred; while in Comparative Examples 1 to 3, the solutions of the examples of the present application were not adopted, and filiform corrosion occurred on the prepared steel plates, and the corrosion filament length was >4 mm.

[0056] One or more technical solutions in the embodiments of the present invention at least further have the following technical effects or advantages:

[0057] 1. The combined action of the zinc-aluminum-magnesium coating and the chemical conversion film in the present invention forms a multi-layer protection system. This system not only has excellent resistance to filiform corrosion, but also can maintain high stability and durability in various corrosion environments.

[0058] 2. The chemical conversion film formed on the surface of the zinc-aluminum-magnesium alloy in the present invention greatly improves the resistance to filiform corrosion of the zinc-aluminum-magnesium material under some special coating working conditions, and at the same time does not affect the forming, welding, painting, bonding and other properties of the zinc-aluminum-magnesium material.

[0059] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features claimed in the present application.

Claims

1. A zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion, characterized in that: The steel plate includes a zinc-aluminum-magnesium coating and a chemical conversion film attached to at least a portion of the surface of the zinc-aluminum-magnesium coating; the chemical conversion film is formed by a chemical conversion solution and the zinc-aluminum-magnesium coating through a chemical reaction.

2. The steel plate according to claim 1, characterized in that: The chemical conversion liquid comprises: fluorine-containing inorganic acid, silane coupling agent, inorganic salt and corrosion inhibitor.

3. The steel plate according to claim 2, characterized in that: The fluorine-containing inorganic acid is at least one of fluorotitanic acid and fluorozirconic acid, and the concentration of the fluorine-containing inorganic acid is 1200ppm to 3500ppm.

4. The steel plate according to claim 2, characterized in that: The silane coupling agent is at least one of KH792 and KH570, and the concentration of silicon element in the silane coupling agent is 700ppm to 1800ppm.

5. The steel plate according to claim 2, characterized in that: The inorganic salt is at least one of sodium nitrate and manganese nitrate, and the concentration of the inorganic salt is 500ppm to 4000ppm.

6. The steel plate according to claim 2, characterized in that: The corrosion inhibitor is at least one of sodium tripolyphosphate and mercaptobenzothiazole, and the concentration of the corrosion inhibitor is 800ppm to 6000ppm.

7. The steel plate according to claim 1 or 2, characterized in that: The chemical conversion solution further includes metal ions, which are at least one of manganese ions, zinc ions and copper ions, and the concentration of the metal element of the metal ions is 1 ppm to 50 ppm.

8. The steel plate according to claim 1, characterized in that: The chemical composition of the zinc-aluminum-magnesium coating includes: Mg, Al and Zn; wherein, by weight, the content of Mg is 0.9 to 1.3 parts, and the content of Al is 1.4 to 2 parts.

9. A method for preparing a steel plate according to any one of claims 1 to 8, characterized in that: The method comprises: Hot-dip zinc-aluminum-magnesium alloy coating is performed on the surface of the steel substrate to obtain a zinc-aluminum-magnesium coated steel sheet; The zinc-aluminum-magnesium coated steel sheet is surface-coated using the chemical conversion solution to obtain a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion.

10. The method according to claim 9, characterized in that The process parameters of the surface coating include: PMT temperature of 80° C. to 100° C., and reaction time of 10s to 60s.

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