Preparation and analysis method of hot-dip Zn.Al.Mg series alloy coating

Abstract

The invention provides a preparation and analysis method of a hot-dip Zn-Al-Mg alloy coating, and belongs to the technical field of metal surface coating. The preparation method comprises the steps of steel strip pretreatment, hot dipping and post-plating treatment, and an alloy plating layer with a uniform structure is obtained by controlling parameters of all working procedures; the invention also provides a matched coating analysis method which comprises the following steps: performing phase analysis by adopting an X-ray diffractometer, and performing quantitative characterization on the microscopic structure of the coating by adopting a scanning electron microscope in combination with image analysis software. The corrosion resistance of the coating is remarkably improved by increasing the content of Al and cooperatively adding Mg and Si, the microstructure characteristics of the coating are revealed through analysis, a theoretical basis is provided for component optimization and process control, and industrial application prospects are achieved.

Description

Technical Field

[0001] This invention relates to the field of surface treatment technology for metallic materials, and to a method for preparing and analyzing hot-dip Zn-Al-Mg alloy coatings. Background Technology

[0002] Steel is widely used in building structures, transportation, energy facilities and home appliance manufacturing due to its excellent mechanical properties, processing performance and economy. However, steel is very susceptible to electrochemical corrosion in service environments, especially under atmospheric, high humidity, high salt spray or industrial pollution conditions, which leads to rust and a decline in mechanical properties, thus significantly shortening its service life.

[0003] To improve the corrosion resistance of steel, hot-dip galvanizing is one of the most widely used protective methods. It involves forming a layer of metallic zinc on the surface of steel, using the cathodic protection and physical shielding effects of zinc to delay the corrosion of the base material. However, traditional pure zinc coatings still have insufficient corrosion resistance in harsh corrosive environments, making it difficult to meet the protection requirements for long-term service.

[0004] In recent years, Zn-Al-Mg alloy coatings formed by adding aluminum and magnesium to a zinc bath have become a research hotspot. The addition of aluminum and magnesium can form a dense and stable composite corrosion product film (such as ZnO, Al2O3, MgO and their hydroxides or basic salts) during corrosion, significantly improving the corrosion resistance of the coating and even endowing it with a certain degree of self-healing ability. However, the aluminum content in existing Zn-Al-Mg coatings (such as typical ZAM coatings) is relatively low, and the density and long-term stability of their corrosion product film still have room for improvement in extreme environments. Studies have shown that appropriately increasing the aluminum content in the coating helps to generate more aluminum-rich dense oxide phases, thereby further improving the corrosion resistance and high-temperature oxidation resistance of the coating.

[0005] Currently, existing technologies for Zn-Al-Mg alloy coatings with high aluminum content are still incomplete. In particular, there is a lack of systematic and in-depth analysis of the phase composition, microstructure evolution, and structure-property relationship of the coatings in high aluminum composition systems. This situation leads to a lack of reliable theoretical basis for composition optimization and production process control (such as hot-dip coating temperature and cooling rate), which limits the industrial application and full realization of the performance potential of Zn-Al-Mg alloy coatings with high aluminum content.

[0006] Therefore, developing a high-aluminum-content Zn-Al-Mg hot-dip coating with a reasonable composition design, controllable structure, and excellent corrosion resistance, and clarifying its microstructure characteristics, is of great practical significance for improving the protection level of steel. Summary of the Invention

[0007] The purpose of this invention is to address the problems existing in the prior art by providing a method for preparing and analyzing hot-dip Zn-Al-Mg alloy coatings. This method solves the problems that existing Zn-Al-Mg coatings have relatively low aluminum content and lack systematic analysis of the phase composition, microstructure evolution law, and structure-property relationship of coatings with high aluminum content.

[0008] Therefore, the present invention adopts the following technical solution: A hot-dip Zn-Al-Mg alloy coating is applied to the surface of a steel substrate. The chemical composition of the coating, by mass percentage, is as follows: Al: 8.0%~30.0%, Mg: 1.0%~7.0%, Si: 0.01%~0.8%, with the balance being Zn and unavoidable impurities.

[0009] This invention provides a method for preparing the above-mentioned alloy coating, comprising the following steps: S1. Steel strip pretreatment: The steel strip is degreased, washed with water and annealed to obtain a clean surface structure suitable for hot-dip galvanizing; Specifically, the degreasing treatment uses an alkaline cleaning solution, and the process parameters are: temperature 50-70℃, alkaline solution concentration 10-40g / L, and treatment time 30-120s; The alkaline solution includes NaOH / Na2CO3 and a surfactant; The annealing process is carried out in a protective atmosphere consisting of N2 and H2, with an H2 volume fraction of 3-10%. The annealing temperature curve is as follows: rapidly heat to 720-780℃ and hold for 30-120 seconds, then control the cooling rate to reduce the temperature of the steel strip to 460-480℃ before entering the hot-dip galvanizing process.

[0010] S2. Hot-dip galvanizing: The pretreated steel strip is immersed in a molten alloy plating solution containing Zn, Al, Mg and Si elements for hot-dip galvanizing treatment, forming an alloy coating on the surface of the steel strip. Specifically, the hot-dip galvanizing temperature is 420℃-500℃, the immersion time of the steel strip in the galvanizing solution is 4-25s, and the final alloy coating thickness is 10-40μm.

[0011] S3. Post-plating treatment: The hot-dip galvanized steel strip is cooled, solidified and passivated to obtain a plate with a Zn-Al-Mg alloy coating on the surface. Specifically, the cooling and curing is carried out by air cooling or air cooling combined with atomized water cooling, with a cooling rate of 10-50℃ / s, so that the surface temperature of the coating drops rapidly to 60-120℃. In the passivation treatment, the passivation film concentration corresponds to an effective metal ion concentration of 0.5-2.0 g / L, the pH value is 1.5-5.0, the contact time is 1-5 s, the drying temperature after passivation is 80-120℃, and the drying time is 30-90 s.

[0012] Furthermore, the present invention also provides a method for analyzing the above-mentioned hot-dip Zn-Al-Mg alloy coating, comprising the following steps: S4. Sample preparation: Cut out the sample for analysis from the plated material obtained after post-plating treatment, and clean and dry the sample. Specifically, the dimensions of the cut sample are 10-11cm in length, 9-11cm in width, and 1.0-1.2mm in thickness; The cleaning process is as follows: first, ultrasonically clean in anhydrous ethanol for 600 seconds, then wipe the coating surface with a lint-free cloth soaked in acetone for 1-2 minutes, finally rinse with deionized water, and dry at 60°C for 30 minutes.

[0013] S5. Phase Analysis: The sample is tested using an X-ray diffractometer, and the test data is analyzed using analysis software to determine the types of phases in the coating. Specifically, the X-ray diffractometer's test parameters are: measurement range 10-90°, X-ray source Cu-Kα rays, and scanning speed 5-10° / min; the analysis software is Jade 9.0.

[0014] S6. Microstructure observation and analysis: The surface and cross-sectional microstructure of the coating are observed using a scanning electron microscope, and the microstructure images are analyzed using image analysis software to obtain microstructural feature information, including dendrite size and phase area fraction. Specifically, the observation parameters of the scanning electron microscope are: accelerating voltage 30kV, current 0.4nA, and backscattered electron imaging mode. The image analysis software is ImageJ, which is used to calculate the perimeter of Al dendrites on the coating surface and the area fraction of each phase in the coating cross section.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The alloy coating prepared by this invention has a significantly increased aluminum content of 8.0-30.0 wt.%, and is synergistically supplemented with 1.0-7.0 wt.% magnesium and 0.01-0.8 wt.% silicon, which optimizes the alloy system. The high aluminum content can generate more dense and stable aluminum-rich corrosion products during the corrosion process, which significantly enhances the integrity and physical shielding effect of the corrosion product layer, thereby greatly improving the corrosion resistance and oxidation resistance of the coating in high salt spray, high humidity and high temperature environments. 2. The preparation method provided by the present invention ensures the smooth formation of high aluminum content alloy coating and the uniformity of solidification structure by precisely controlling the process parameters of pretreatment, hot-dip galvanizing and post-plating treatment. The process is stable and reliable and suitable for industrial production. 3. The coating analysis method provided by this invention obtains the microstructural characteristics of the coating by quantitatively characterizing the phase composition and microstructure, such as statistically analyzing the perimeter of Al dendrites, the area fraction of each phase, and their distribution along the coating thickness. The above analysis results provide a scientific theoretical basis and data support for a deeper understanding of the corrosion resistance mechanism of the coating, optimization of alloy composition, and adjustment of production process. Attached Figure Description

[0016] Figure 1 A flowchart of the method provided by the present invention; Figure 2 The X-ray diffraction pattern of the alloy coating sample obtained in the embodiment of the present invention; Figure 3 This is a scanning electron microscope backscattered electron image of the alloy coating surface obtained in an embodiment of the present invention; Figure 4 This is a schematic diagram showing the results of phase area fraction statistics on the coating surface image in an embodiment of the present invention; Figure 5 This is a scanning electron microscope backscattered electron image of the cross-section of the alloy coating obtained in an embodiment of the present invention; Figure 6 This is a bar chart showing the statistical results of the proportion of each tissue at different thicknesses from the steel substrate according to an embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the specific embodiments of this invention are only for explaining the invention and are not intended to limit the scope of protection of this invention. Example

[0018] This embodiment provides a hot-dip Zn-Al-Mg alloy coating and its preparation and analysis method.

[0019] The Zn-Al-Mg alloy coating prepared in this embodiment comprises, by mass percentage: Al 15.0%, Mg 3.0%, Si 0.2%, with the balance being Zn and unavoidable trace impurities.

[0020] like Figure 1 As shown, the method for preparing the above-mentioned coating provided in this embodiment includes the following steps: S1. Steel strip pretreatment: The steel strip to be plated is selected and first degreased. Specifically, degreasing is performed using an alkaline cleaning solution. The process parameters are controlled as follows: temperature 60℃, alkaline solution concentration 25g / L, and treatment time 80s, to remove oil and rolling residues from the surface of the steel strip. The alkaline solution includes NaOH or Na2CO3 and surfactants.

[0021] After degreasing, the strip is washed with water and then annealed in a continuous annealing furnace under a protective atmosphere. Specifically, the annealing atmosphere is a N2-H2 mixture with a H2 volume fraction of 5%. The annealing temperature curve is as follows: the steel strip is rapidly heated to 750°C and held for 60 seconds to complete recrystallization and microstructure adjustment. Then, by controlling the cooling rate, the temperature of the steel strip is reduced to 470°C, ready for entry into the plating bath.

[0022] S2, Hot-dip galvanizing: The pretreated steel strip is introduced into a molten alloy plating bath at a temperature of 460℃. The composition of the plating bath matches the composition of the target coating. The steel strip is immersed in the plating bath for 10 seconds. The coating thickness is controlled by devices such as air knives. The final alloy coating thickness is approximately 20μm.

[0023] S3. Post-plating treatment: After hot-dip galvanizing, the strip steel is cooled and cured; specifically, air cooling is used, and the cooling rate is controlled at 30℃ / s, so that the surface temperature of the coating drops rapidly to about 80℃.

[0024] After cooling, passivation treatment is performed. Specifically, the passivation solution is an environmentally friendly passivation solution containing zirconium salt, wherein the effective metal ion concentration is 1.0 g / L, the pH value is adjusted to 3.0, the contact time is 3 s, and after passivation, it is dried at a temperature of 100℃ for 50 s, finally obtaining a plate with a Zn-Al-Mg alloy coating on the surface.

[0025] The coated sheet obtained above was sampled and analyzed. The specific steps are as follows: Sample preparation: The sample for analysis was cut from the coated sheet using an electrical discharge machining (EDM) machine. The sample dimensions were 10.5 cm in length, 10 cm in width, and 1.1 mm in thickness.

[0026] Clean the cut sample: First, place it in anhydrous ethanol and ultrasonically clean for 600 seconds; then remove it and use an ultra-fine lint-free cloth dipped in acetone to gently wipe the coating surface for 2 minutes to remove any possible organic contaminant residue; finally, rinse it with deionized water and place it in an oven to dry at 60°C for 30 minutes to ensure that the sample surface is dry and clean.

[0027] Phase analysis: Set the X-ray diffractometer parameters; specifically, use a Cu-Kα ray source, with a scanning angle range of 10-90° and a scanning speed of 8° / min.

[0028] The processed sample is placed in the instrument for testing, and the X-ray diffraction pattern of the sample is obtained, such as... Figure 2 As shown, the spectrum contains obvious characteristic diffraction peaks of Zn, MgZn2 and Al, indicating that the main phase composition of the coating surface is Zn, MgZn2 intermetallic compounds and Al-rich phase.

[0029] The XRD data were analyzed using Jade 9.0 software. By comparing the data with the standard PDF card, it was confirmed that the sample contained three main phases: Zn, MgZn2, and Al.

[0030] Microscopic observation and analysis: The cleaned and dried sample is fixed on the sample stage of the scanning electron microscope with conductive adhesive and then sent into the sample chamber for vacuuming.

[0031] The scanning electron microscope (SEM) parameters were set as follows: accelerating voltage 30 kV, current 0.4 nA, and backscattered electron imaging mode.

[0032] First, observe the surface structure of the coating; the resulting backscattered electron image is as follows: Figure 3 As shown, different microstructure regions can be clearly distinguished based on different contrasts: the dark gray dendritic region is α-Al dendrite (Al-rich phase), the light gray region is MgZn2 phase, and the black and white eutectic region is Zn+MgZn2 binary eutectic structure. At the same time, a small amount of diffusely distributed Mg2Si phase can be observed.

[0033] Using ImageJ image analysis software to analyze... Figure 3 Quantitative analysis was performed, and the area fraction of each phase was statistically analyzed. The results are as follows: Figure 4 As shown, the statistical results indicate that the area fraction of the α-Al-rich phase is 48.27%, the area fraction of the MgZn2 phase is 36.84%, and the area fraction of the Zn+MgZn2 binary eutectic structure is 14.89%.

[0034] The perimeter of Al dendrites in the image was statistically analyzed using ImageJ software. The results are shown in Table 1, with an average perimeter of 156.65 μm.

[0035] Table 1. Statistical results of Al dendrite perimeter on the coating surface Next, the cross-section of the sample was observed, and the resulting backscattered electron image is shown below. Figure 5 As shown.

[0036] Using ImageJ software, the coating thickness direction was divided into five regions according to the distance from the substrate: 0-5μm, 5-10μm, 10-15μm, 15-20μm, and 20-25μm. The area fractions of η-Zn phase, Zn-Mg eutectic phase, MgZn2 phase, oxide, and pores in each region were statistically analyzed. The statistical results are listed in Table 2.

[0037] Table 2. Statistical results of the proportion of each tissue within different thicknesses from the substrate. The trend diagram of the proportion of each phase as a function of thickness, based on the data in Table 2, is shown below. Figure 6 As shown in the figure, the analysis results reveal the gradient distribution law of the coating cross-sectional structure, providing key data for understanding the coating formation mechanism and performance.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing and analyzing hot-dip Zn-Al-Mg alloy coatings, characterized in that, in: The preparation method includes the following steps: Steel strip pretreatment: The steel strip is degreased, washed with water, and annealed; Hot-dip galvanizing: The pretreated steel strip is immersed in molten alloy plating solution for hot-dip galvanizing, forming an alloy coating on the surface of the steel strip. Post-plating treatment: The hot-dip galvanized steel strip is cooled, cured, and passivated to obtain a sheet with a Zn-Al-Mg alloy coating on the surface. The analysis method includes the following steps: Sample preparation: Cut out a sample from the coated sheet and clean and dry the sample; Phase analysis: The sample was tested using an X-ray diffractometer, and the types of phases in the coating were determined using analysis software; Microstructure observation and analysis: The surface and cross-sectional microstructure of the coating were observed using a scanning electron microscope, and the microstructural characteristics were obtained using image analysis software.

2. The method according to claim 1, characterized in that, The degreasing treatment in step one uses an alkaline cleaning solution, and the process parameters are: temperature 50-70℃, alkaline solution concentration 10-40g / L, and treatment time 30-120s. The annealing process is carried out in a protective atmosphere consisting of N2 and H2, with the volume fraction of H2 being 3-10%. The annealing temperature curve is as follows: rapidly heat to 720-780℃ and hold for 30-120 seconds, then control the cooling rate to reduce the steel strip temperature to 460-480℃ before proceeding to step two.

3. The method according to claim 1, characterized in that, In step two, the hot-dip galvanizing temperature is 420℃-500℃, the immersion time is 4-25s, and the thickness of the resulting alloy coating is 10-40μm.

4. The method according to claim 1, characterized in that, The cooling and curing in step three is carried out by air cooling or air cooling supplemented by atomized water cooling, with a cooling rate of 10-50℃ / s, so that the surface temperature of the coating is reduced to 60-120℃. The passivation treatment uses a passivation film concentration corresponding to an effective metal ion concentration of 0.5-2.0 g / L, a pH value of 1.5-5.0, a contact time of 1-5 s, a drying temperature of 80-120℃ after passivation, and a drying time of 30-90 s.

5. The method according to claim 1, characterized in that, In the sample preparation step, the dimensions of the cut sample are 10-11cm in length, 9-11cm in width, and 1.0-1.2mm in thickness. The cleaning process is as follows: first, ultrasonically clean in anhydrous ethanol for 600s, then wipe the coating surface with a lint-free cloth soaked in acetone for 1-2 minutes, then rinse with deionized water, and dry at 60℃ for 30 minutes.

6. The method according to claim 1, characterized in that, The X-ray diffractometer test parameters in the phase analysis step are as follows: measurement range 10-90°, X-ray source Cu-Kα rays, and scanning speed 5° / min-10° / min.

7. The method according to claim 1, characterized in that, In the microscopic tissue observation and analysis steps, the observation parameters of the scanning electron microscope are: accelerating voltage 30kV, current 0.4nA, and backscattered electron imaging mode.

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