# Microstructural and Corrosion Properties of Burnished 6060 Aluminum Alloy
## 3 Results
### Corrosion Test
#### 3.2.2 Electrochemical Impedance Spectroscopy  
In order to better understand the corrosion performance of the black coating, electrochemical impedance spectroscopy tests were also performed and the results in terms of Nyquist plot are reported in Figure 12 for solution 1 and solution 2. The employed electrolyte was the same as the one used for potentiodynamic polarization tests. Graphical evaluation of the corrosion resistance given by the coating can be considered as the real part of the impedance at low frequencies (intercept with the \(x\)-axis) in the Nyquist plots. For the blackening process at 50 \({}^{\circ}\)C with solution 1 (Figure 12a), the curves show a higher corrosion resistance for the samples immersed in the bath for 15 and 30 min. The corrosion resistance for the samples with 75 and 90 min of immersion time was quite similar. In general, a decrease of the corrosion performance of the top layer with the increasing of immersion time was observed. However, also from EIS tests, the sample with 50 min of immersion time presented the lowest corrosion resistance. This result is in accordance with the ones coming from PDP tests previously reported. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Solution 1** & **Ecorr SCE [V]** & **Icorr [A/cm\({}^{2}\)]** \\ \hline AR & \(-1.70\) & \(2.0\times 10^{-5}\) \\
15 min & \(-1.57\) & \(1.0\times 10^{-5}\) \\
30 min & \(-1.38\) & \(1.1\times 10^{-5}\) \\
50 min & \(-1.50\) & \(4.0\times 10^{-4}\) \\
75 min & \(-1.47\) & \(1.2\times 10^{-5}\) \\
90 min & \(-1.20\) & \(1.1\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Corrosion potential and corrosion current densities extrapolated from the potentiodynamic polarization plots reported in Figure 11a for the samples obtained with solution 1 at 50 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Solution 2** & **Ecorr SCE [V]** & **Icorr [A/cm\({}^{2}\)]** \\ \hline AR & \(-1.70\) & \(2.0\times 10^{-5}\) \\
30 min & \(-1.72\) & \(3.0\times 10^{-5}\) \\
60 min & \(-1.65\) & \(1.0\times 10^{-5}\) \\
100 min & \(-1.67\) & \(1.6\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Corrosion potential and corrosion current densities extrapolated from the potentiodynamic polarization plots reported in Figure 11b for the samples obtained with solution 2 at ambient temperature. The Nyquist plot for the samples obtained with solution 2 at room temperature show a remarkably higher corrosion resistance of the top layer for immersion time of 100 min. The corrosion performance of the black layer, considering the amplitude of the curve, decreased with lower immersion time in the oxidizing solution. The data were also fitted with software Z-view. The equivalent circuit used to fit the experimental data is reported in Figure 13 and is in accordance with present literature regarding protective oxide coatings [17; 18; 19]. The results of the fitting are reported in Table 6 for solution 1 and Table 7 for solution 2. From the fitting of the experimental data, it was possible to estimate the total polarization resistance of the black coating of the different samples and its evolution with the treating immersion time. Concerning the parameters of the equivalent circuit, R\({}_{1}\) represents the resistance of the electrolyte, R\({}_{2}\) is the resistance of the black layer, CPE is a constant phase element, which is used in the equivalent circuit instead of a perfect capacitor to more accurately fit the data, because often the measured capacitance is not ideal, while the coefficient n is often described as the frequency dispersion due to the surface condition, such as roughness and inhomogeneity [20; 21; 22]. Figure 12: Nyquist plots of electrochemical impedance spectroscopy (EIS) measurements performed for the samples treated in solution 1 at 50 \({}^{\circ}\)C (**a**) and for the samples treated in solution 2 at ambient temperature (**b**). Test electrolyte: commercial solution that simulates cleaning product.