# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy  
The electrochemical impedance spectra were obtained for the Al-Ni alloys in order to characterize the electrochemical phenomenon that takes place on the surface of the samples. Figure 15: EDS mapping of Al-Ni alloys after corrosion tests: (**a**) Al-1 wt. %Ni, (**b**) Al-3 wt. %Ni, (**c**) Al-5.7 wt. %Ni and (**d**) Al-8 wt. %Ni. In Figure 16a,b, the Nyquist diagrams can be seen, and in Figure 16c,d, the Bode diagrams are presented (Figure 16a,c for columnar grains and Figure 16b,d for equiaxed grains). The presence of a single capacitive contribution is clearly observed. The capacitive diameter observed in the Nyquist diagrams can be associated with the corrosion resistance of the materials. A larger arc diameter indicates greater corrosion resistance [42]. It is then observed that the equiaxed grain zone has greater corrosion resistance than the columnar grain zone for hypoeutectic alloys. On the other hand, for the hypereutectic alloy, Al-8 wt. % Ni, the trend reverses. To adjust the spectra, the Randles equivalent circuit was used (Figure 17), commonly used to describe interfaces that are undergoing the initial stages of the corrosion process [38; 43]. The physical meaning of the elements that constitute the equivalent electrical circuit of Figure 17 is described below: R\({}_{\Omega}\) represents the resistance of the electrolyte, R\({}_{p}\) is the  
Figure 16: Impedance spectra for Al-Ni alloys in 0.5 M NaCl solution. (**a**) Nyquist diagram for the columnar zone, (**b**) Nyquist diagram for the equiaxal zone, (**c**) Bode diagram for the columnar zone, (**d**) Bode diagram for the equiaxial zone. Figure 17: Equivalent circuit used to fit the experimental data. resistance to polarization related to the charge transfer resistance of the oxide film, and CPE is the capacitive element associated with this oxide layer. The use of a CPE to replace a capacitor is due to the need to represent the non-ideal behavior of the surface due to the lack of uniformity caused by roughness, porosity or other factors [18, 19, 38, 42]. The parameter values obtained from the EIS fit are shown in Table 3.  
The parameter used as an indicator of non-ideal behavior is n. According to [PERSON] et al. [43], for surfaces that are suffering from pitting, this value ranges between 0.7 and 0.95. This is coincident with the values of n measured for the Al-Ni alloys. The values of effective capacitance, C\({}_{\text{eff}}\), were calculated using Equation (1) proposed by [PERSON] et al. [44]:  
\[\text{C}_{\text{eff}}=\text{CPE}^{1/n}\Big{(}\text{R}_{\text{s}}{}^{-1}+\text{ R}_{\text{p}}{}^{-1}\Big{)}^{(1-\text{n})/\text{n}} \tag{1}\]  
The higher the R\({}_{\text{p}}\) value, the higher the corrosion resistance [18, 19, 28]. Figure 18 shows the R\({}_{\text{p}}\) values depending on the composition of Al-Ni alloys for the two grain structures. It is observed that by increasing the Ni content, R\({}_{\text{p}}\) decreases for samples with equiaxed grain structure throughout the range of compositions studied. Furthermore, the equiaxed grain structure presents higher R\({}_{\text{p}}\) values than the columnar grain zone for alloys with a composition equal to or lower than the eutectic composition. It is evident that for Al-Ni alloys containing up to 5.7% Ni, the phase distribution generated by the equiaxed grain structure is more favorable than the phase distribution of the columnar structure. This may be due to the fact that the equiaxed grain structure, associated with higher values of \(\lambda_{2}\), decreases the contact areas between the \(\alpha\)-dendritic matrix and the second phase particles, Al\({}_{3}\)Ni. The effect of microstructural morphology for alloys consisting of two phases has already been reported by the authors of the Al-Cu system [19]. [PERSON] et al. [17] reported that a homogeneous distribution of the eutectic mixture is essential to improve corrosion resistance when studying an Al-5.4 wt. %Ni alloy. Similarly, [PERSON] et al.