Text|Yanagihachihara
Editor|Yanagihachihara
Layered dihydroxides are a class of materials with special structure and multifunctional properties, which are widely used in many fields. Among them, zinc-based LDH materials have attracted much attention because of their excellent corrosion resistance and protection properties. The formation of Zn-Al-based LDH conversion coating on the surface of electrogalvanized steel sheet (EG steel) is considered to be an effective method to improve the corrosion resistance of steel plate.
Let's investigate the effect of the pH value of the solution on the formation of layered double hydroxides on electrogalvanized steel sheets.
Test samples and handlers
The test sample for this study was a 0.75 mm thick electrogalvanized mild steel sheet supplied by JFE Steel. Before processing, we cut the specimen into 50mm×70mm and degreased with ethanol to ensure a clean surface and remove any impurities.
In order to form LDH (layered double hydroxide) conversion coatings, we prepared Na2Al2O4 solutions containing 0.1MK3, 0.01MNH4OH and 0.01MZn(NO3)2·6H2O. The concentration of sodium aluminate ranges from 0.050M to 0.250M, and the aging time of the stirred solution varies from 120 minutes to 480 minutes.
We then soak the specimen in each solution for 16 hours at room temperature without stirring. The pH of the treatment solution varies from 11.5 to 13.3, depending on the concentration of sodium aluminate and the aging time. Table 1 summarizes the parameters for each solution.
It should be noted that the precipitation of aluminum hydroxide is known to occur when carbon monoxide is contained in sodium aluminate solution. Therefore, in this study, the pH of the solution may change with increasing aging time, as carbon dioxide can be dissolved into the solution from air. In order to control the pH of treatment solution 0 and study the effect of solution pH on sodium aluminate concentration and aging time, we added 9.1MNaOH solution without changing the concentration of sodium.
After rinsing with deionized water and air drying, we evaluated the corrosion resistance of each specimen and characterized the coating. By collating the above, we provide a systematic and logical approach to the formation of LDH conversion coatings.
Corrosion resistance assessment
Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion resistance of the coated specimens. After 0 h of immersion at room temperature, measurements are made in 1.1 M NaCl solution. A sinusoidal 10 mV voltage signal is used as a perturbation with a frequency range of 10,000 to 0.01 Hz. To evaluate corrosion resistance, impedance at 0.01 Hz was compared, which was demonstrated at the lowest measurement frequency.
Coating analysis
In order to determine the interlayer anion, crystal structure, microstructure, appearance and chemical composition of the resulting LDH conversion coating, the following surface observations and analyses were performed.
Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using FT-IR spectrometer (WINSPEC-100, JEOL). The measurement was performed using reflectance absorption spectroscopy with an angle of incidence of 75° and 100 accumulations.
The X-ray diffraction (XRD) pattern of the coating was determined by an X-ray diffractometer (SmartLab, Ricke). The angle of incidence is 3°, and the radiation source is a Cu Kα target. The scanning range is 5° to 45°, the scanning speed is 10°/min, and the step size is 0.01°. The tube voltage and tube current are 45 kV and 44 mA, respectively.
The depth profile of the coating was determined by glow emission spectroscopy (GDOES) (GD-Profiler 2, HORIBA). Sputtering using Ar plasma with an Ar pressure of 600 Pa, a power of 35 W, a sampling time of 10 ms, and an Ar flushing time of 30 s. The measuring area is circular with a diameter of 4 mm. By measuring the actual depth and sputtering time of the sputtering area with a 60D microscope, it was determined that the sputtering rate of mild steel plates under these conditions was approximately 3 nm/sec.
The surface of the coating is observed by a scanning electron microscope (SEM) equipped with a secondary electron (SE) detector (JSM-6060, JEOL). The accelerating voltage during observation is 5 kV.
The cross-section of the coating was observed by SEM (ULTRA PLUS, Carl Zeiss) and analyzed by energy-dispersive X-ray spectroscopy (EDX). The instrument is equipped with a backscattered electron (BSE) detector. The accelerating voltages used in the observation and analysis process are 1 kV and 5 kV, respectively. For SEM observation, we prepared 3º cross-sectional samples using the Focused Ion Beam (FIB) instrument (Quanta 45 200D, FEI).
Corrosion resistance
By performing electrochemical impedance spectroscopy (EIS) analysis on samples treated with different pH solutions, we obtained the Nyquist plot as shown. The complexity of the spectrum is shown.
Figure 2 shows impedance as a function of pH of the treatment solution at 0.01 Hz. The results show that the impedance increases significantly with the increase of the pH of the treatment solution and reaches a maximum at a pH of 12.6. When the pH of the treatment solution is above this threshold, the impedance decreases dramatically. These results show that the corrosion resistance of the coating depends on the pH of the treatment solution, and the best results can be obtained from the treatment solution with a pH of 12.6.
We performed three impedance measurements and calculated the mean and standard deviation of samples treated with treatment solution condition No. 5 (Table 1). The results show that the average impedance of the sample is 2071 Ω·cm^2, and the standard deviation is 262 Ω·cm^2.
Based on the FT-IR spectra of LDH-treated samples produced using treatment solution No. 5 (Table 1), peaks were observed at 461, 573, 874, 1365, 3296, and 3861 cm^−1 as shown. These peaks have characteristics consistent with the FT-IR spectra of solid zinc.
By analyzing all test samples, similar spectral results were obtained, which showed the successful formation of Zn_xAl_1-x(OH)_2/(CO_3)_x/2·nH_2O, an LDH structure, on the EG steel sample. This LDH structure consists of a hydroxide base layer mixed with zinc ions (Zn^2+) and aluminum ions (Al^3+) and an intermediate layer between carbon monoxide_3^2- and H_2O.
Based on the XRD spectra obtained from the samples, the spectra of each sample clearly show the presence of a zinc substrate and zinc _2Al(OH)_6(CO_3)_0.5·xH_2O(zinc oxide LDH). These results further support the previous finding that LDH conversion coatings were successfully applied to EG steel samples, and that the conversion coatings mainly consist of a mixture of Zn-Al-CO_3.
Based on XRD results, diffraction patterns of LDH consisting only of Zn-Al-CO_3 were observed at pH 12.6, while diffraction patterns of ZnO were also observed at pH above 12.6. This indicates a change in the composition of the conversion coating around pH 12.6, which is very consistent with the decrease in corrosion resistance observed at pH 12.6 in EIS experiments.
Based on a representative GDO depth profile, elements Zn, Al, O, C, and H were detected on the surface of LDH-treated specimens produced under treatment solution No. 5. This result further supports the conclusion that the LDH conversion coating successfully forms Zn-Al-CO_3. It can be inferred that the thickness of Zn-Al-CO_3 LDH is related to the Al content in it, since Al is one of the constituent components of the LDH layer, while the other layers do not contain Al.
The amount of Al is estimated by integrating the net Al intensity in the GDO profile. The background Al intensity is defined as zero and the integral Al intensity is obtained by integrating the net Al intensity in the GDO profile over the sputtering time range.
As the pH of the treated solution rises to 12.4, the integral Al intensity increases, and when the pH of the treated solution is higher than 12.4, the integral Al intensity begins to decrease. This indicates that as the pH of the treatment solution increases to 12.4, the thickness of the coating also increases, and above this pH, the thickness of the coating begins to decrease.
Converts the effect of coating thickness on corrosion resistance
The impedance shown in the figure vs. solution pH and the combined Al strength vs. solution pH indicate that coating thickness may affect corrosion. From the data, it can be seen that the impedance increases with the increase of the comprehensive Al intensity.
This means that as the thickness of the conversion coating increases, so does the corrosion resistance. However, it is worth noting that at pH 12.6, higher impedance was observed even with thinner coatings, suggesting that corrosion resistance cannot be explained by coating thickness alone.
Effect of solution pH on coating microstructure
By looking at the SEM image, it is possible to understand how the conversion coating affects corrosion resistance. After treatment at all pH values, the coated surface presents plate-like crystals, which is a typical LDH crystal shape.
However, as the pH increases from pH 12.0 to pH 12.4, the crystal size increases slightly. After the pH value exceeds 12.6, the crystal becomes significantly smaller and the size becomes thinner.
These observations further support the previous conclusion that the corrosion resistance of conversion coatings is affected by a variety of factors. The thickness, composition, and crystal shape of the coating can all play an important role in corrosion resistance. Larger crystal sizes may be associated with better corrosion resistance, while smaller crystal sizes may result in poor corrosion resistance.
By using the 9° cross-sectional image obtained by FIB, we can further understand the situation of three specimens prepared in pH 4.12, 6.12 and 12.12 solutions.
In this pH region, consistent with the previous GDOES data, the conversion coating becomes thinner as the pH increases. Although a thicker conversion coating is observed at pH 12.4, some cracks and crevices are also visible. However, at pH 12.6 and 12.9, the conversion coating was denser, more uniform than specimens prepared at pH 12.4, and no cracks and crevices were observed.
As mentioned earlier, the crystal size changes significantly around pH 12.6. This may affect whether cracks and gaps are observed, as larger crystals can cause strain and deformation, resulting in cracks and gaps in the coating.
In addition to the differences, regions of moderate brightness were observed in the lower transformation coating on the Zn substrate of the specimen prepared at pH 12.6 and 12.9. Since the contrast of the BSE image depends on the composition of the layers, this indicates that when processed at pH 12.6 or higher, the microstructure of the conversion coating is divided into two layers with different compositions.
EDX analysis was performed on the area represented by the white boxes (1-10) on the cross-section in the figure, and the results are shown in Table 2. At pH 12.4, Zn, Al, C, and O were detected in regions 1 and 2, indicating that the conversion coating consisted only of Zn-Al-CO. 3LDH。
At pH 12.6 and 12.9, the medium brightness region appears to consist of ZnO; only Zn and O are detected in regions 5, 8, and 9. The zinc oxide 3LDH layer is present on the ZnO layer. These results are consistent with the previous results obtained by XRD, again showing that the conversion coating consists of ZnO and Zn-Al-CO.
As the pH rises above 12.6, the ZnO layer tends to thicken, while the 3LDH layer composed of Zn-Al-CO tends to thin. In addition to coating thickness, microstructural differences in conversion coatings, such as the density and uniformity of LDH, and the ratio of ZnO to LDH, also have an impact on corrosion resistance.
These results further support the relationship between the microstructure of the conversion coating and corrosion resistance. We observed that in samples prepared at pH 12.6 and 12.9, the microstructure of the conversion coating was more uniform and denser, without the presence of cracks and crevices. This may help prevent corrosive media from invading the coating and reacting with the substrate.
In addition, we also noticed the thickening of the ZnO layer at high pH. Since ZnO has better corrosion resistance, this thickening may help improve the overall corrosion resistance of the coating. However, the thinning of the 3LDH layer composed of Zn-Al-CO may reduce the protective ability of the coating.
Effect of microstructure of the conversion coating on corrosion resistance
At low pH (less than 12.6), LDH crystals form directly on the surface of EG steel, resulting in cracks and crevices. Although increased coating thickness can improve corrosion resistance, cracks and gaps can cause electrolytes to enter the zinc substrate through the coating, reducing corrosion protection.
When the solution pH is about 12.6, the corrosion resistance is higher despite the thinner coating. This is due to the presence of the ZnO layer that was originally formed, which promotes the formation of a denser, more homogeneous, and finer LDH crystal structure. This change in microstructure may help improve the corrosion protection of the coating.
Above pH 12.6, the ratio of ZnO to LDH in the coating increases. This can lead to reduced corrosion resistance, as ZnO has a high solubility in NaCl solutions and does not provide effective protection.
Carbonates are identified as interlaminar anions formed in LDH crystals produced by the procedure discussed herein. Zinc 2 aluminum (Ohio) 6 (carbon monoxide 3) 0.5·xH2 When the surface is immersed in Na, O(LDH) easily forms 2 aluminum 2O4 on the surface of EG steel-based solutions.
The corrosion resistance associated with the LDH coating described above increases with increasing pH of the solution, up to pH 12.6. However, above pH 12.6, a sharp decrease in corrosion resistance is observed. This trend towards corrosion resistance can be explained in some ways by converting the coating thickness.
However, for samples treated at pH 12.6, the initial formation of ZnO results in the formation of a more homogeneous and protective LDH layer, which is more corrosive despite the thinner layer.