# Microstructural and Corrosion Properties of Burnished 6060 Aluminum Alloy
## 4 Discussion  
Corrosion behavior is closely linked to morphology and surface properties of the black layer. This makes necessary a comparison between coating morphology, studied with SEM analysis, and the results of EIS tests. For hot blackening in solution 1, EIS tests showed that the sample with 50 min of immersion was the worst in terms of corrosion resistance; this fact was confirmed by SEM observation, which showed a lot of cracks detrimental for corrosion behavior. The same situation was observed for the samples with blackening time of 75 and 90 min: in fact, they were characterized by a similar corrosion resistance and a similar morphology, in terms of presence of cracks. Stress relief cracks may be developed because of a difference in thermal expansion coefficient between the substrate and the coating. In case of blackening \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Solution 1** & **R\({}_{1}\) [\(\Omega\) cm\({}^{2}\)]** & **R\({}_{2}\) [\(\Omega\) cm\({}^{2}\)]** & **Q [F cm\({}^{-2}\) Hz\({}^{1-n}\)]** & **n** & \(\chi^{2}\) \\ \hline AR & 18 & 105 & \(1.38\times 10^{-5}\) & 0.8 & 0.03 \\
15 min & 18 & 163.4 & \(2.80\times 10^{-5}\) & 0.83 & 0.09 \\
30 min & 18 & 153 & \(2.84\times 10^{-5}\) & 0.99 & 0.01 \\
50 min & 18 & 110.2 & \(1.38\times 10^{-5}\) & 0.80 & 0.01 \\
75 min & 18 & 114.7 & \(2.41\times 10^{-6}\) & 0.81 & 0.011 \\
90 min & 18 & 129 & \(2.22\times 10^{-5}\) & 0.89 & 0.035 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of the fitting of the experimental data for the samples obtained with solution 1 at 50 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Solution 2** & **R\({}_{1}\) [\(\Omega\) cm\({}^{2}\)]** & **R\({}_{2}\) [\(\Omega\) cm\({}^{2}\)]** & **Q [F cm\({}^{-2}\) Hz\({}^{1-n}\)]** & **n** & \(\chi^{2}\) \\ \hline AR & 18 & 105 & \(1.38\times 10^{-5}\) & 0.8 & 0.03 \\
30 min & 18 & 174.2 & \(1.91\times 10^{-5}\) & 0.974 & 0.09 \\
60 min & 18 & 208.2 & \(9.83\times 10^{-6}\) & 0.966 of solution 1 with a process temperature of 50 \({}^{\circ}\)C, the cooling from this specific operating temperature and tap water rinse temperature (room temperature) provided large stress that can cause the formation of cracks on the oxide layer [24; 25]. In fact, cracks were formed on the surface of almost all the samples obtained at 50 \({}^{\circ}\)C, as can be observed in Figure 7.  
A higher corrosion resistance could be observed for lower immersion times of 15 and 30 min. The SEM images confirmed this fact, evidencing a more uniform and more homogeneous surface; only for 30 min of immersion time, it was possible to observe in a little zone some porosity. The SEM analysis confirmed that an increase of immersion time for blackening in solution 1 at 50 \({}^{\circ}\)C did not lead to an improvement in terms of corrosion resistance and surface quality of the layer. The results for room temperature blackening were quite different: EIS measurements showed higher corrosion resistance with increasing immersion time. In fact, the more corrosion resistant samples were characterized by immersion time of 60 and 100 min. SEM analysis confirmed the EIS results: increasing blackening time led to a more uniform and homogenous layer without any cracks or porosities. For solution 2, the blackening reaction was slower; this was due to the lower temperature of the process. Increasing immersion time, the growth mechanism of the layer ensured the formation of a higher quality surface and thicker layer. In this case, in fact, the reaction was not accelerated by the temperature as higher immersion times are required [26; 27; 28]. For both solutions 1 and 2, all black coatings were characterized by a higher corrosion resistance in comparison to the untreated sample. The best sample in terms of corrosion performances was the one treated with solution 2 for 100 min. This sample was characterized by a uniform and defect-free coating about one micron thick.