Influence of surface etching pretreatment on PEO process of eutectic Al–Si alloy

To solve the problems generally encountered during the plasma electrolytic oxidation (PEO) of Al alloys with high Si content, a pretreatment of chemical etching was applied before the process. The influence of such pretreatment was studied by SEM, EDS and XRD. The pretreatment presents a significant effect on positive voltage at the beginning stage of PEO, leading to higher voltage over the whole process. The difference between the positive voltages of non-etched and etched specimens decreases gradually with the increase of processing time. The pretreatment exhibits much less influence on the negative voltage. For the sample with surface pretreatment, the average growth rate of PEO coating is increased from 0.50 to 0.84 μm·min− 1 and the energy consumption is decreased from 6.30 to 4.36 kW·h·μm− 1·m− 2. At the same time, both mullite and amorphous SiO2 contents are decreased in the coating.

Fig. 1 is the voltage of the eutectic Al–Si alloy with and without acid etching pretreatment over the 30 min PEO treatment. During the anodic oxidation stage, the etching of the Al–Si alloy matrix accelerated its positive voltage obviously, which was close to the voltage ascent rate of pure Al. And the positive voltage of the etched alloy was always higher than that of the non-etched one at the same moment, but their gap was gradually diminished with time. But, the impact of the etching treatment on the negative voltage of the Al–Si alloy during the PEO was small.Fig. 2 is the phase constituents of the PEO layers on the non-etched and etched Al–Si alloy by 30 min treatment. Excepting the Al and Si peaks, which primarily came from the matrix, the PEO layers were mainly composed of γ-Al2O3, α-Al2O3, mullite and some SiO2 amorphous phase. And it is evident that the layer on the etched sample had a much higher Al2O3 but a lower SiO2 content than those in the layer of the non-etched one.Figure optionsDownload as PowerPoint slide