3.2. Al-Al compound -
In many cases the oxide layer (Al2O3) acts as a natural protection against wear and corrosion since the oxidized state features excellent chemical and mechanical properties [11]. For the presented work these properties act against the desired compound. Hence the zincate treatment is applied. SEM images show the surface evolution during the process, see Figure 4.
Fig. 4 Production steps for the zincate treatment visualized by SEM images of the substrate surface; (a) Cleaned commercial Al99.5 sheet; (b) surface structure after sandblasting (SiC). The oxide layer is fractured and thus an effective dissolution with sodium hydroxide f f occurs (c); (d) Pickling deposit is dissolved by nitric acid; (e) As the substrate is dipped into the zincate solution zinc nuclei precipitate on the oxide free surface. To ensure a fully coverage the primary coating undergoes a pickling procedure; (f) a fully covered zinc surface is created by reapplication of the zincate solution (g).