# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 3 Results and Discussion
### Potential dynamic Polarization Curves  
Figure 13 shows the cyclic potentiodynamic polarization curves obtained according to the type of grain for all the compositions of Al-Ni alloys studied. In Figure 13a,b, the results obtained for both columnar and equiaxed grains zones are observed. The shape of curves is similar for all compositions and grain zones. The direct dissolution of the material is evident after reaching the corrosion potential. Other authors [16; 17] have already reported the beginning of the pitting phenomenon when studying Al-Ni alloys, with a content of around 5%Ni in solutions with much lower NaCl content than that used in this research. In Table 2, the measured corrosion potentials, E\({}_{\text{corr}}\), are presented. Figure 12: (**a**) Variation in microhardness, HV, as a function of (**a**) \(\lambda_{2}\) and (**b**) S\({}_{2}\). Figure 13: _Cont._  
From Figure 13, it is observed that the anodic Tafel slopes of the curves are practically zero. That is, when reaching the E\({}_{\text{corr}}\), the Al-Ni alloys studied are pitted. Consequently, it is possible to establish that the E\({}_{\text{corr}}\) values presented in Table 2 correspond to the E\({}_{\text{pit}}\) of the Al-Ni alloys. A similar behavior was reported by [PERSON] et al. [33] for the commercial alloy AA6061 in acid solutions containing chlorides. The coincidence of the E\({}_{\text{pit}}\) with the E\({}_{\text{corr}}\) can also be seen in the potentiodynamic curves for other Al base alloy systems: Al-Mg [18], Al-Cu [19], Al-Co [34; 35], Al-Pd [36] in solutions containing chlorides. The susceptibility to pitting of the Al-Ni alloys studied is evident by observing the evolution of the open circuit potential over time (Figure 14). The oscillatory aspect of the E\({}_{\text{co}}\) values observed in Figure 14 is very close to the potentials in Table 2, which we have assimilated with Epit. This would indicate that the Al-Ni alloys would be suffering pitting corrosion during the E\({}_{\text{co}}\) measurements. According to [PERSON] et al. [37], the oscillatory appearance of the curves, characteristic of the pitting process, is a consequence of the differences in current densities inside the pit and on the surrounding surface. Because sufficiently long linear regions could not be found in the anodic branches of the potentiodynamic curves, the corrosion current density values, icorr, were calculated by Tafel extrapolation of the cathodic polarization branch, according to the procedure proposed by [PERSON] et al. [35]. The values obtained are shown in Table 2. The icorr values obtained are in the same order of magnitude as the icorr presented for other aluminum-based binary alloy systems, such as Al-Co and Al-Pa [37]. In the interpretation of these results, it is important to keep in mind that, for alloys that are under the pitting process, the active area of the corrosion phenomenon is unknown, that is, the area inside the pits [38]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**E\({}_{\text{corr}}\)**} & \multicolumn{1}{c}{**i\({}_{\text{corr}}\)**} \\ \hline  & & **(mV)** & **mA/cm\({}^{2}\)** \\ \hline \multirow{2}{*}{Al-1 wt. \%Ni} & Columnar zone & \(-722\) & \(0.0144\) \\ \cline{2-4}  & Equiaxed zone & \(-726\) & \(0.0068\) \\ \hline \multirow{2}{*}{Al-3 wt. \%Ni} & Columnar zone & \(-721\) & \(0.0029\) \\ \cline{2-4}  & Equiaxed zone & \(-725\) & \(0.0014\) \\ \hline \multirow{2}{*}{Al-5.7 wt. \%Ni} & Columnar zone & \(-698\) & \(0.0083\) \\ \cline{2-4}  & Equiaxed zone & \(-701\) & \(0.0242\) \\ \hline \multirow{2}{*}{Al-8 wt. \%Ni} & Columnar zone & \(-707\) & \(0.0026\) \\ \cline{2-4}  & Equiaxed zone & \(-710\) & \(0.0046\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrochemical parameters of Al-Ni alloys in 0.5 M NaCl. Figure 13: Cyclic potentiodynamic curves of Al-Ni alloys in 0.5 M NaCl solution: **(a)** columnar and **(b)** equiaxed zones. In Table 2, it can be seen that the Al-5.7 wt.