# Effect of CeO2@GNPs on the Corrosion Properties of 2024 Alloy
## 3 Results
### Corrosion Resistance  
The morphologies of the 2024 Al alloy, 0.5 wt%GNPs/2024 Al and 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al soaked in a 3.5 wt% NaCl solution for seven days are given in Figure 3. It can be observed that the surfaces of the samples were all covered by white flaky corrosion products or bright corrosion particles, which were presumed to be Al\({}_{2}\)O\({}_{3}\), Al(OH)\({}_{3}\) and Mg(OH)\({}_{2}\) particles when using the EDS energy spectrum analysis and the characteristic peaks in the XRD patterns, as shown in Figure 3f [17]. However, the XRD patterns collected from those composites reinforced with the GNPs specimens did not reveal any peaks of GNPs. The reason for this may have been the low concentration of GNPs in the matrix. Additionally, the surface of the 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al specimen showed fewer deposits compared to the 0.5 wt%GNPs/2024 Al sample. Figure 4 shows the mapping images of the different elements in the 0.5 wt% GNPs/2024 Al composite sample. It can be observed that the pitting crater with a diameter of approx. 40 \(\upmu\)m at P1 was accompanied by the generation of surface corrosion products as well as an oxide film in the process of pitting corrosion generation. As the corrosion continued, the passivation film on the surface was broken through and corrosion cracks sprouted and extended continuously. Therefore, it could be noticed that the Cu elements and Cl elements had only a small distribution \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Ys/MPa** & **UTs/MPa** & **EI (\%)** & **Microhardness (HV)** \\ \hline Matrix & \(152\pm 5\) & \(218\pm 6\) & \(2.77\pm 0.19\) & \(101.2\pm 3.2\) \\ \hline GNPs/2024 Al nanocomposite & \(170.6\pm 5\) & \(257.4\pm 6.4\) & \(2.53\pm 0.23\) & \(111\pm 4.8\) \\ \hline CeO\({}_{2}\)@GNPs/2024 Al nanocomposite & \(184.1\pm 4.7\) & \(271.8\pm 3.8\) & \(2.38\pm 0.34\) & \(128\pm 5.9\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of the fabricated composites. on the corrosion surface. On the other hand, a small number of new corrosion cracks were generated under the corrosion cracks, indicating that the generation of pitting corrosion entered a new phase and continued to expand. In addition, it can be clearly observed from Figure 4 that the Al elements and O elements were widely distributed on the surface-covered corrosion products, while the Mg elements were mainly distributed along the corrosion product particles. This implies that one of the main components of the corrosion product was the Mg element. Figure 4: Distribution of the scanning elements in the area shown in Figure 3 (**a**). (**b**) Al; (**c**) Mg; (**d**) O; (**e**) Cu; (**f**) Cl. Figure 3: SEM maps and energy spectra of the corrosion morphology of the 2024 aluminum alloy and its nanocomposites: (**a**) SEM map of the 2024 aluminum alloy, (**b**) SEM map of the 0.5 wt%GNPs/2024 Al nanocomposites, (**c**) SEM map of the 0.5 wt%CeO2%GNPs/2024 Al nanocomposites; (**d**) P1 point energy spectrum; (**e**) P2 point energy spectrum; (**f**) XRD patterns of the nanocomposites. Figure 5a displays the electrochemical parameters obtained using Tafel curve fitting. The OCP curves of three different samples exhibited the same trend as could been seen from Figure 5a. All the curves showed only positive fluctuations at first, gradually decreased and tended to stabilize around a certain value. This phenomenon mainly indicated that the sample surface passivation layers were rapidly destroyed. Meanwhile, due to the continuous formation of oxide, a dynamic equilibrium between local corrosion and oxide film formation was promoted, which led to the gradual stabilization of the potential curve. It is worth noting that the OCP profile of the fabricated 0.5 wt%CeO\({}_{2}\)@GNPs/2024A1 nanocomposite took more time to reach a stable state compared to the others and that this specimen had the highest open-circuit potential after stabilization.