3.2. X-ray diffraction (XRD)
Figure 3 presents an X-ray diffraction (XRD) pattern of the Zn coating and the Zn-Y
2O
3- Al
2O
3 coating with varying nanoparticle concentrations. Most of the diffraction lines in the X-ray diffraction pattern correspond to the hexagonal structure of Zn[
11]. The Zn-Y
2O
3-Al
2O
3 nanocomposite coating had a similar structure to the zinc coating, but the preferential orientation of Zn changed. The diffraction diagram shows six distinct peaks at 36.6°, 43.5°, 54.6°, 70.3°, 77.3°, and 82.4° on standard PDF cards, corresponding to the Zn phase (002), (101), (102), (110), (004), and (112) planes. As depicted in the graph, the addition of the Y
2O
3 and Al
2O
3 nanoparticles resulted in a significant increase in (002) crystal diffraction peaks. The intensity of most of the remaining diffraction peaks weakened, indicating that the introduction of nanoparticles further enhanced the deposition orientation of the (002) crystal surface. Low-intensity Al
2O
3 peaks were observed in all XRD profiles of the Zn-Y
2O
3-Al
2O
3 coatings at 35.02 °, 57.86 °, and 65.89 °. This proves the successful co-deposition of the Al
2O
3 nanoparticles with the zinc matrix. At the same time, the low-intensity peaks of Zn-Y
2O
3-Al
2O
3 were observed in all XRD profiles of the Y
2O
3 coatings XRD at 28.70°, 48.19°, and 57.29°. It was also proven that the Y
2O
3 nanoparticles were successfully added to the zinc matrix. In addition, the peak width of the Zn-Y
2O
3-Al
2O
3 nanocomposite coating was slightly wider than that of the Zn coating. This is because the nanoparticles inhibited the growth of the crystals, resulting in the refinement of the crystal size[
10].
3.4. Effect of particle loading on corrosion resistance
The open-circuit potential refers to the potential difference between a soluble metal electrode and an electrolyte due to the spontaneous redox reaction that occurs when the electrode is immersed in the electrolyte. The system is in a dynamic equilibrium at an open-circuit potential. This means that the redox reaction occurring on the electrode and the reaction in the electrolyte are in equilibrium, and the electrode does not undergo significant corrosion.
Figure 5a illustrates the change in the open-circuit potential of the pure Zn and Zn-Y
2O
3-Al
2O
3 composite coatings over time in a 3.5wt% NaCl solution. As shown in the figure, compared with the pure Zn coating, the composite coating exhibits a higher potential. This suggests that the composite coating performs better because it suppresses the anodic reaction and enhances the composite coating's anti-corrosion properties.
The corrosion behavior of various composite coatings was studied using linear scanning voltammetry.
Figure 5b shows a polarization curve of the Zn-Y
2O
3-Al
2O
3 composite coating in a 3.5wt% NaCl solution with various concentrations of Y
2O
3 and Al
2O
3 nanoparticles. The self-corrosion current (I
coor) and corrosion voltage (E
coor) fitted according to Tafel extrapolation are listed in
Table 3. In general, the higher the positive corrosion potential of the coating, the lower the self-corrosion current density, indicating that the coating has a lower tendency to corrode [
28]. As shown in
Table 3, the corrosion potential of the composite coatings is higher than that of the pure zinc coatings (E
coor=1.18 V). This demonstrates that the Zn coating's resistance to corrosion can be effectively increased by using nanoscale Y
2O
3 and Al
2O
3 particles. A plating solution containing 10 g/L of Y
2O
3 and Al
2O
3 nanoparticles raises the composite coating's corrosion potential to 1.02 V. Meanwhile, the self-corrosion current density of the Zn-Y
2O
3-Al
2O
3 nanocomposite coating (I
coor=6.15×10
-6) is significantly lower than that of the other coatings. From the perspective of corrosion kinetics, the corrosion rate of the nanocomposite coatings is slower than that of the pure Zn coatings.
Figure 5b shows that the current density of the Zn coating reduces as the reaction progresses in the cathode polarization region, covering a range from -2.0 V to -1.18 V. This is because electrons are attracted from the solution to the electrode surface and react with oxygen in the following manner:
When located in the anode polarization region from -1.18 V to 0.7 V, the current density of the Zn coating increases as the reaction progresses. This occurs when the coating begins to react with the electrolyte, primarily through the following reaction:
However, as the reaction progresses, the change in current density stabilizes when the scanning potential increases above 0.7 V. The stability exhibited can be attributed to the creation of an oxide-based passivation film on the surface of the coating, which impedes the rate of coating dissolution [
29]. The formation of this film primarily occurs as follows:
In addition, particle agglomeration reduces its usability, resulting in reduced co-deposition efficiency and the formation of low-quality coatings [
30]. When the concentration of these nanoparticles in the electroplating solution is increased from 0 g/L to 15 g/L, the corrosion current density of the composite coating follows a pattern of initially decreasing and then increasing. When the nanoparticles are incorporated at a concentration of 10 g/L, the nanocomposite coatings exhibit minimal corrosion current density, indicating optimal corrosion resistance. The optimal particle load for achieving this resistance is 10 g/L of nanometer Y
2O
3 and 10 g/L of nanometer Al
2O
3. The particles in the electroplating solution are dispersed at the optimum level at this concentration. Via the formation of these nanoparticles, an inert physical barrier is established on the coating’s surface, which reduces the effective contact area between the corrosive medium and the substrate metal, improving the coating’s corrosion resistance [
13].
Electrochemical impedance spectroscopy is a powerful electrochemical auxiliary technique commonly used to evaluate the corrosion resistance of prepared coating surfaces [
31]. AC impedance testing was conducted on the prepared composite coatings to investigate their corrosion resistance further in a 3.5wt% NaCl solution.
Figure 6a shows Nyquist plots of the different composite coatings obtained through EIS testing. It can be seen in the figure that a single capacitive ring with a circular arc shape is displayed from high frequency to low frequency. A relationship exists between the radius of the capacitive impedance arc and the charge transfer resistance, where a larger arc radius signifies a higher charge transfer resistance value and improved corrosion resistance performance of the coating [
32]. The impedance arc radius of the composite coating exhibits a trend of initially increasing and then decreasing as the doping concentrations of the Y
2O
3 and Al
2O
3 nanoparticles are increased. The composite coating exhibits the largest impedance arc and superior corrosion resistance at a doping concentration of 10g/L for both the Y2O3 and Al2O3 nanoparticles. This finding aligns with the polarization curve test results of the composite coating. The Randles circuit model was used for fitting to explain the EIS results further, and the equivalent circuit is shown in
Figure 6b, characterized by only one time constant [
33]. Within this equivalent circuit, R
s denotes the resistance of the electrolyte. CPE is a constant-phase angle element, and a lower CPE value signifies a higher surface quality of the composite coating [
34]. R
ct stands for charge transfer resistance, which is associated with the corrosion rate and serves as a crucial parameter reflecting the corrosion resistance of composite coatings. A higher R
ct value indicates superior corrosion resistance of the composite coating. The fitting data for the coatings are presented in
Table 4, revealing that the maximum impedance modulus of all composite coatings exceeds that of the pure Zn coatings (1385 Ω cm
2). The impedance modulus reaches a remarkable value of 10257 Ω cm
2 for the C3 coating, indicating its exceptional corrosion resistance.
Figure 6c shows a Bode modulus diagram, and | Z | represents the impedance value of the composite coating. The impedance modulus in the low-frequency zone is frequently used to assess the corrosion resistance of coatings. This is because the formation of a protective barrier, such as the corrosion product film, at the steel/coating interface effectively mitigates corrosion [
35,
36,
37]. When the doping concentrations of the Y
2O
3 and Al
2O
3 nanoparticles are both 10 g/L, the highest impedance value in
Figure 6c is 6592.84 Ω cm
2, indicating that the C3 coating has a better corrosion resistance. In addition, at higher frequencies, a larger phase angle indicates the presence of a more stable dielectric film, and the stability of the coating is also better [
38]. The maximum phase angle of the C3 coating indicates that it has a better stability and better barrier performance.
3.5. Microhardness test
In practice, coatings not only require excellent corrosion resistance but also excellent mechanical properties. Therefore, the mechanical properties of the Zn and Zn-Y2O3-Al2O3 coatings are evaluated via microhardness testing. To mitigate experimental error, ten points are randomly chosen from each sample for hardness testing. The average value of these points is considered the sample’s hardness result.
The hardness test results for the Zn and Zn-Y
2O
3-Al
2O
3 coatings are shown in
Figure 7. Based on the graph, it is observable that the composite coating exhibits a higher microhardness than the pure Zn coating. As the content of Y
2O
3 and Al
2O
3 nanoparticles increases in the plating solution, the hardness of the composite coating initially increases and then decreases. The pure zinc coating has a hardness value of 148 Hv. However, when 10 g/L of nanoscale Y
2O
3 and Al
2O
3 particles is added, the hardness of the composite coating reaches 369 Hv, indicating a 2.5-fold increase compared to the pure Zn coating. The inclusion of nanoparticles significantly enhances the microhardness of the composite coating [
39]. However, this is due to the excellent mechanical properties of the Y
2O
3 and Al
2O
3 nanoparticles, which are uniformly dispersed in the Zn matrix, playing a dispersion-strengthening role. However, nanoparticles bound to the Zn matrix increase the number of nucleation sites. In addition, they hinder crystal growth, resulting in the formation of small grain sizes and promoting a significant number of grain boundaries [
18,
40]. The coating surface is dense, and this grain refinement helps to increase the hardness value of the coating [
41]. With a 15 g/L concentration of the Y
2O
3 and Al
2O
3 nanoparticles in the plating solution, the hardness of the coating decreases. This is due to the pronounced agglomeration behavior of the nanoparticles, leading to the formation of uneven areas, such as pores and cracks, on the surface of the composite coating. As a result, the dispersion-strengthening effect of the nanoparticles within the coating diminishes, causing a reduction in the hardness value of the composite coating.