# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 3 Results and Discussion
### Potential dynamic Polarization Curves
% Ni alloy, which corresponds to the eutectic composition, presents the noblest Epit values, followed by the Al-8 wt. %Ni alloy, with a hypereulectic composition. Hypooutectic alloys have the least noble potential values that are very close to each other. [PERSON] et al. [16] found that a higher fraction of the eutectic structure is associated with a nobler pitting potential. Based on the results of Table 3, an analogy could be made to this result considering that the alloy with eutectic composition, Al-5.7 wt. % Ni, presented the highest Ecorr value, which we assimilate to Epit, due to the shape of the curves. In line with these results, [PERSON] et al. [17], studying the effect of the microstructure on the corrosion resistance of an Al-5.4 wt. %Ni alloy in NaCl, found that the more homogeneous structures presented higher pitting potential values and lower corrosion densities. \begin{table}
\begin{tabular}{c c c c c c} \hline  & \(\mathbf{R_{\Omega}}\) & \(\mathbf{R_{P}}\) & \(\mathbf{CPE}\) & \(\mathbf{n}\) & \(\mathbf{C_{eff}}\) \\ \hline \multirow{3}{*}{Al-1 wt. \% Ni} & \(\mathbf{\Omega^{*}cm^{2}}\) & \(\mathbf{\Omega^{*}cm^{2}}\) & \(\mathbf{\Omega^{-1}s}\)-\(\mathbf{n^{*}cm^{-2}}\) & & \(\mathbf{E.cm^{-2}}\) \\ \cline{2-6}  & Columnar zone & 3.6 & \(2.47\times 10^{3}\) & \(4.32\times 10^{-5}\) & 0.81 & \(3.03\times 10^{-6}\) \\ \cline{2-6}  & Equiaxed zone & 3 & \(1.21\times 10^{4}\) & \(5.45\times 10^{-5}\) & 0.79 & \(2.99\times 10^{-6}\) \\ \hline \multirow{3}{*}{Al-3 wt. \% Ni} & Columnar zone & 0.39 & \(3.08\times 10^{2}\) & \(9.40\times 10^{-4}\) & 0.79 & \(1.89\times 10^{-4}\) \\ \cline{2-6}  & Equiaxed zone & 3.56 & \(6.08\times 10^{3}\) & \(7.87\times 10^{-5}\) & 0.74 & \(1.82\times 10^{-6}\) \\ \hline \multirow{3}{*}{Al-5.7 wt. \% Ni} & Columnar zone & 2.67 & \(1.58\times 10^{2}\) & \(6.94\times 10^{-4}\) & 0.7 & \(2.03\times 10^{-5}\) \\ \cline{2-6}  & Equiaxed zone & 3.2 & \(8.78\times 10^{2}\) & \(1.55\times 10^{-4}\) & 0.77 & \(7.98\times 10^{-6}\) \\ \hline \multirow{3}{*}{Al-8 wt. \% Ni} & Columnar zone & 4.62 & \(1.28\times 10^{3}\) & \(5.43\times 10^{-5}\) & 0.85 & \(7.33\times 10^{-6}\) \\ \cline{2-6}  & Equiaxed zone & 1.26 & \(4.68\times 10^{2}\) & \(1.57\times 10^{-4}\) & 0.82 & \(2.18\times 10^{-5}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Electrochemical impedance spectroscopy fitting parameters. Figure 14: Values of \(\mathbf{E_{\mathrm{oc}}}\) of Al-Ni alloys measured over time. (**a**) Columnar and (**b**) equiaxed zones. Regarding the influence of grain structures, for the same alloy composition, the zone of columnar grains presents nobler E\({}_{\text{pit}}\) values than the zone of equiaxed grains. However, these differences between E\({}_{\text{pit}}\) values are less than 5 mV, so they are not considered significant differences. Therefore, it is possible to establish that the grain structure does not influence the E\({}_{\text{pit}}\). This result is consistent with what was reported for aluminum and other alloy systems [19; 39]. After making the potentiodynamic curves, the surface of the samples was analyzed using the SEM microscope. The micrographs obtained can be seen in Figure 15. Corrosion phenomena are evident on the \(\alpha\)-phase, rich in aluminum, of the Al-1 wt. % Ni alloy (Figure 15a). In the samples with the highest Ni content, corrosion was also located in the dendritic region, very close to areas with the presence of eutectic phase (Figure 15b,c). In the hypereuetectic alloy, the corrosion effect is visible between the Al-3 wt. %Ni precipitates (Figure 15d). The presence of pits in the matrix corresponding to the \(\alpha\)-phase was already reported by other authors when studying the electrochemical behavior of Al-Ni alloys [17]. In the EDS mapping, the constituent elements of the alloy are identified. Aluminum is present in the dendritic matrix, and Ni is distributed as thin sheets, which correspond to the intermetallic Al\({}_{3}\)Ni, a constituent of the eutectic phase.