# Microstructural and Corrosion Properties of Burnished 6060 Aluminum Alloy
## 2 Materials and Methods  
Samples of aluminum alloy AA6060 (sheets of 1.5 mm of thickness) were used as substrate for obtaining a black coating with electroless treatments. The composition of the alloy, obtained with GDOES (Horiba Jobin Yvon RJ RF-GD profiler, Horiba, Palaiseau, France), is reported in Table 1. The initial state of the untreated aluminum sample is shown in Figure 1. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Zn** & **Ti** \\ \hline AA6060 & 0.50 & 0.20 & 0.10 & 0.10 & 0.45 & 0.05 & 0.15 & 0.10 \\ \end{tabular}
\end{table} 
Table 1: Composition of AA6060 alloy employed (wt.%). The whole process for obtaining black coatings is better summarized in Figure 2.  
Metal samples were carefully degreased with ethanol by ultrasound for 5 min in order to remove dust, grease, and oil interfering with the coloring process. Because of the passive nature of aluminum, the second step was a strong pickling in a hydrofluoric/nitric acid solution with a ratio 1:99, for a pickling time of 30 s, to activate the surface and remove the protective oxide [12]. After a tap water rinse, two different solutions were tested to perform the process (coloring step in the flow chart in Figure 2). The two solutions contained the same chemical compounds but with different amounts. Moreover, the two solutions were employed at different temperatures: solution 1 was used at 50 \({}^{\circ}\)C, in order to accelerate the reaction, whereas solution 2 was employed at ambient temperature and the reaction was accelerated, increasing the concentration of the chemical elements up to the solubility limit. Chemical composition is given in Table 2 for solution 1 and in Table 3 for solution 2 [13]. The tested immersion times were 15, 30, 50, 75, and 90 min for solution 1, and 30, 60, and 100 min for solution 2. After the coloring step, the samples were rinsed in tap water and then dried in air. Three samples were produced for each condition and then tested. The surfaces of the samples were observed with a Zeiss Stemi 2000-C stereo microscope (Carl Zeiss AG, Oberkochen, Germany) in order to obtain information on the aesthetic appearance and the surface morphology. A Cambridge Stereroscan 440 scanning electron microscope (Leica Cambridge Ltd., Cambridge, UK) was used to analyze the surface of the samples in order to identify the coating porosities and other features of the surface. X-ray diffraction (XRD) measurements were conducted to characterize the phase composition of \begin{table}
\begin{tabular}{c c} \hline \hline
**Solution 1** & **Quantity** \\ \hline H\({}_{2}\)O & 250 mL \\ KMnO\({}_{4}\) & 8 g/L \\ CuNO\({}_{3}\) & 26 g/L \\ HNO\({}_{3}\) & 4 g/L \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition of blackening solution 1. Figure 1: Stereomicroscope image of untreated aluminum surface. \begin{table}
\begin{tabular}{c c} \hline \hline
**Solution 2** & **Quantity** \\ \hline H\({}_{2}\)O & 250 mL \\ KMnO\({}_{4}\) & 16 g/L \\ CuNO\({}_{3}\) & 52 g/L \\ HNO\({}_{3}\) & 7 g/L \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition of blackening solution 2. Figure 2: Flow chart of the whole process. the obtained coating by means of a Siemens D500 X-ray diffractometer (Munich, Germany) using Cu K\(\alpha\) radiation with 20 ranging from 20\({}^{\circ}\) to 80\({}^{\circ}\) (0.05\({}^{\circ}\) step and 5 s counting time per step) operating in thin-film mode with a grazing angle of 0.5\({}^{\circ}\). Two different electrochemical methods were used to evaluate corrosion behavior of the obtained black layer: potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. In the potentiodynamic polarization and EIS tests we used as an electrolyte a commercial solution that simulates cleaning products (indicative composition: phosphonates, surfactants, phenoxyethanol, and citronello, Madel Sp.a., Spinea, Italy) for all surfaces.