# Microstructural and Corrosion Properties of Burnished 6060 Aluminum Alloy
## 3 Results  
### Characterization  
The first observation was the qualitative macroscopical investigation of the color of the obtained samples. Figures 1 and 2 underline that, besides the employed solution, samples with higher immersion time were characterized by a darker color; that is, the more desirable one during the blackening process. From a macroscopical point of view, the color intensity seemed to be related to the immersion time: by increasing blackening time, the intensity of the black color increased. This result could be observed both with solution 1 at 50 \({}^{\circ}\)C (Figure 3) and with solution 2 at ambient temperature (Figure 4). To have more information on the surface from a qualitative point of view, a stereomicroscope analysis was carried out (Figures 5 and 6). Figure 4: Image of samples treated with solution 2 with different immersion times: (**a**) 30 min, (**b**) 60 min, (**c**) 100 min. Figure 3: Image of samples treated with solution 1 with different immersion times: (**a**) 15 min, (**b**) 30 min, (**c**) 50 min, (**d**) 75 min, (**e**) 90 min. From the stereomicroscope study, the remarkable role of time in the coloring of treated metal samples was clearly observable; in fact, for both solutions, the samples blackened with higher treatment time, showing a darker color. To get a deeper analysis of the coating surface, not only in terms of intensity of color but observing the presence of defects, aluminum samples were observed with SEM and the results are reported in Figures 7 and 8. Figure 5: Stereomicroscope images of samples treated with solution 1 at 50 \({}^{\circ}\)C with different immersion times: (**a**) 15 min, (**b**) 30 min, (**c**) 50 min, (**d**) 75 min, (**e**) 90 min. Figure 6: Stereomicroscope images of the samples treated with solution 2 at ambient temperature with different immersion times: (**a**) 30 min, (**b**) 60 min, (**c**) 100 min. Figure 8: SEM images of the samples obtained with solution 2 at ambient temperature with different soaking times: (**a**) 30 min, (**b**) 60 min, (**c**) 100 min. Figure 7: SEM images of the samples obtained with solution 1 at 50 \({}^{\circ}\)C with different soaking times: (**a**) 15 min, (**b**) 30 min, (**c**) 50 min, (**d**) 75 min, (**e**) 90 min. Considering firstly the samples obtained with solution 1, Figure 7 shows very different features of the layer depending on the immersion time. It is possible to observe that the sample with immersion time of 15 min is more homogeneous than the other samples. In fact, with the increasing of immersion time it was possible to verify the presence of increasing numbers of cracks and porosities. In particular, in the sample with 50 min of treatment the surface exhibits a lot of cracks due to a non-homogeneous growth of the coating. Some cracks can be observed also for the samples with 75 and 90 immersion times. For those with 30 min of immersion time, the SEM analysis shows a little porosity. The best conditions in terms of uniformity and absence of defects in the dark layer appear to be with immersion times of 15 and 30 min for solution 1 at 50 \({}^{\circ}\)C.  
For solution 2, (Figure 8) at room temperature it is possible to verify the absence of cracks in all the samples, but the sample treated over a higher immersion time reveals a smoother and more uniform layer. For low immersion time, there are some zones partially not covered by the black film; this condition can be ascribed to the low immersion time of the treatment and the low temperature [3,14]. Figure 9 shows an example of the XRD pattern of the black samples. The reported phase analysis was performed on the sample treated with solution 2 for 100 min, which appears as the most promising from SEM observation. Only the results for this sample are reported because for the other samples the results were the same. This is in accordance with the fact that the two solutions have the same chemical composition, and so the chemical reaction between the dissolved salts in the bath and metal ions is the same.