# Effect of CeO2@GNPs on the Corrosion Properties of 2024 Alloy
## 3 Results
### Corrosion Resistance
This illustrates that the 0.5 wt%CeO\({}_{2}\)@GNPs-2024A1 nanocomposite had the lowest tendency to be corroded compared to the other two materials [22; 23]. The polarization curves of the different samples in a corrosive solution of 3.5 wt% NaCl prepared from a plating bath are shown in Figure 5b. Correspondingly, the extracted data of the entire Tafel curve are shown in Table 3. It can be seen that the matrix of the 2024 Al alloy had the highest corrosion potential (Ecorr) and the lowest corrosion current density (Icorr). The specimens of the 0.5 wt%GNPs-2024A1 nanocomposite exhibited a lower Ecorr and a higher Icorr compared to the matrix, indicating that the addition of GNPs weakened the corrosion resistance of the fabricated nanocomposite. This was mainly attributed to agglomerated GNPs in the matrix, which became cathodic sites and resulted in the galvanic corrosion of the nanocomposites [24; 25]. Meanwhile, it can also be seen from Figure 5b that the Ecorr increased and the Icorr decreased after adding 0.5 wt%CeO\({}_{2}\)@GNPs, indicating that the existence of the CeO\({}_{2}\) layer improved the corrosion resistance to some extent compared to the 0.5 wt%GNPs/2024A1 [26]. \begin{table}
\begin{tabular}{c c c} \hline
**Specimen** & **E\({}_{\text{corr}}\) (V\({}_{\text{SCE}}\))** & **I\({}_{\text{corr}}\) (A-cm\({}^{-2}\))** \\ \hline Matrix & \(-0.665\) & \(2.757\times 10^{-3}\) \\ \hline GNPs/2024 Al nanocomposite & \(-1.03\) & \(3.540\times 10^{-3}\) \\ \hline CeO\({}_{2}\)@GNPs/2024 Al nanocomposite & \(-0.772\) & \(2.776\times 10^{-3}\) \\ \end{tabular}
\end{table} 
Table 3: Tafel curve parameters for the different materials. Figure 5: Open-circuit potential curves and potentiodynamic polarization curves for the 2024 aluminum alloys and their nanocomposites: (**a**) OCP curves; (**b**) Tafel curves. The Nyquist and Bode plots obtained through fitting using the ZsimpWin 3.6 software are shown in Figure 6. It was evident that the specimens were composed of two capacitive arcs, a high frequency and a low and medium frequency. The Nyquist plots of the 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al and 0.5 wt%GNPs/2024 Al nanocomposites within the investigated range of frequencies had a similar trajectory. Additionally, a larger diameter of the capacitive loop in the 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al compared with the 0.5 wt%GNPs/2024 Al nanocomposite signified a higher resistance to corrosion [27]. Table 4 exhibits the related EC fitted parameters. The polarization resistance (R\({}_{\text{p}}\)) could be calculated using Equation (2) as follows [28]. \[\text{R}_{\text{p}}=\text{R}_{\text{s}}+\text{R}_{\text{f}}+\text{R}_{\text{ct}} \tag{2}\]  
where Rs is the electrolyte resistance, which is related to the conductivity of the electrolyte. R\({}_{\text{f}}\) is the oxide layer resistance. R\({}_{\text{ct}}\) represents the oxide charge transfer resistance. CPE\({}_{\text{f}}\) and CPE\({}_{\text{all}}\) are the capacitances of the oxide film and the double layer, respectively. They are usually associated with cracks, surface oxide films, impurities and inhomogeneous distribution. The calculation results according to Table 4 demonstrated that the R\({}_{\text{p}}\) of those fabricated nanocomposites were significantly lower than that of the matrix 2024 Al alloy (21,168 \(\Omega\)). The matrix 2024 Al alloy suffered a decrease in the corrosion resistance of the surface oxide layer as well as the internal matrix due to the addition of GNPs. Additionally, the surface modification of GNPs using the attached CeO\({}_{2}\) nanoparticles improved the corrosion resistance of the material and partially eliminated the effect of GNPs on the corrosion resistance damage of aluminum-based materials. The SEM images of the matrix and the fabricated nanocomposite after the removal of the corrosion products are shown in Figure 7. It can be clearly observed that the smooth surfaces of the specimens were characterized by a large number of corrosion pits and corrosion micropores.