# Effect of CeO2@GNPs on the Corrosion Properties of 2024 Alloy
## 3 Results
### Corrosion Resistance
The 0.5 wt%GNPs/2024 Al specimen was subjected to more severe corrosion than the matrix, which was almost completely covered by corrosion pits. Some of the corrosion pits were connected to the presence of larger sized corrosion pits. On the other hand, the corrosion surface of the 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al nanocomposite presented \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \multirow{2}{*}{**R\({}_{\text{s}}\) (\(\Omega\)-cm\({}^{2}\))**} & \multicolumn{3}{c}{**CPE\({}_{\text{f}}\)**} & \multicolumn{3}{c}{**CPE\({}_{\text{all}}\)**} & \multirow{2}{*}{**R\({}_{\text{ct}}\) (\(\Omega\)-cm\({}^{2}\))**} \\ \cline{3-3} \cline{5-7}  & & **Y0\({}_{\text{f}}\)** & & & & **Y0** \\  & & **(\(\Omega^{-1}\).cm\({}^{-2}\).s\({}^{n}\))** & & & **(\(\Omega^{-1}\).cm\({}^{-2}\).s\({}^{n}\))** & \\ \hline Matrix & 22.18 & \(15.7\times 10^{-6}\) & 0.88 & 10,569 & \(2.3\times 10^{-4}\) & 0.97 & 10,577 \\
2024 Al-0.5 GNPs & 22.39 & \(18.6\times 10^{-6}\) & 0.85 & 7244.85 & \(8.4\times 10^{-4}\) & 0.97 & 2932.48 \\
2024 Al-0.5 GNPs@CeO\({}_{2}\) & 22.76 & \(11.8\times 10^{-6}\) & 0.89 & 9251.39 & \(4.2\times 10^{-4}\) & 0.96 & 4697.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Electrochemical impedance spectral parameters of the different specimens. Figure 6: EIS curves and the equivalent circuit diagrams for the 2024 aluminum alloy and its nanocomposites: (**a**) Nyquist diagram; (**b**) Bode diagram; (**c**) equivalent electrical circuit. fewer corrosion pits with smaller diameters compared to the former. These phenomena demonstrated that the addition of raw GNPs accelerated the corrosion behavior of aluminum alloys and further proved the corrosion inhibition ability of the CeO\({}_{2}\) nanoparticles layer. The overall corrosion mechanism was governed by the reactions below [29]. The anodic reactions can be written as follows. Mg \[\rightarrow\] Mg \[{}^{2+}+2\text{e}^{-}\] (3) Al \[\rightarrow\] Al \[{}^{3+}+3\text{e}^{-}\] (4)  
The cathodic reaction can be written as follows. O\[{}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-}\to 4\text{OH}^{-}\] (5)  
The overall reaction occurred as follows. 2 Mg + O\[{}_{2}+2\text{H}_{2}\text{O}\to 2\text{Mg}(\text{OH})_{2}\] (6)  
4 Al + 3O\[{}_{2}+6\text{H}_{2}\text{O}\to 4\text{Al}(\text{OH})_{3}\] (7)  
2 Al(OH) \[{}_{3}\rightarrow\] AlFigure 6:0 (8)  
The corrosion types of the composites were similar to those of the matrix 2024 Al alloy, dominated by pitting and intergranular corrosion. With the increase in the immersion time, the oxide films of those were gradually destroyed (Figure 8). The GNPs had a structure similar to that of unfolded carbon nanotubes and had similar physical properties. There was a large surface potential difference between the GNPs and \(\alpha\)-Al [30; 31]. Therefore, the GNPs phase acted as a cathodic phase and the micro-coupled corrosion of the \(\alpha\)-Al phase, which contributed to the depletion of the \(\alpha\)-Al phase in the vicinity of the GNPs and the generation of white corrosion products (micro-coupling corrosion). Eventually, more severe localized corrosion occurred near the added GNPs enhanced phase [24]. Compared with agglomerated GNPs, the dispersed GNPs generated more micro-couples, leading to more severe micro-coupling corrosion. Furthermore, the inhomogeneous dispersion of the GNPs may have decreased the diffusivity of O ions [32], which resulted in an insufficiently dense oxide film. The oxide film was poorly resistant to corrosive fluid erosion and was prone to rupture and collapse, thus leading to the occurrence of pitting corrosion. Figure 6:1 SEM images of the 2024 Al alloy and its nanocomposites with the removal of the surface corrosion products: (**a**) 2024 Al alloy; (**b**) 0.5 wt%GNPs/2024 Al nanocomposite; (**c**) 0.5 wt%CeO\({}_{2}\)@GNPs/2024 Al nanocomposite.