Formation process and characteristics of an amorphous SiO2 ceramic coating on 6061 aluminum alloy by plasma electrolytic oxidation in organosilicon electrolyte

Abstract

An amorphous silica ceramic coating with thicknesses exceeding 100 μm (adjustable thickness) was prepared on 6061 aluminum alloy by plasma electrolytic oxidation in organosilicon electrolyte. To investigate its formation process and structural characteristics, the digital microscope system, SEM, X-ray diffractometer and microhardness tester were used to characterize morphology, structure and hardness at different voltages and times. The results show that it thickens rapidly after the voltage is raised to 480–500 V, and its surface begins to roughen and the micropores appear to enlarge. In organosilicon electrolyte, unlike conventional electrolytes, the coarse Al–Fe–Si second phase particles in the matrix do not negatively affect the PEO treatment. The produced coatings increase the surface hardness of 6061 aluminum alloy by more than 6 times and are stable in common acid solutions such as HCl and HNO3. Furthermore, the energy consumption rate of PEO treatment can be significantly reduced to 0.24–0.31 kJ cm−2 μm−1. The potentiodynamic polarization curves measured in 1 mol L−1 HCl solution show that the ceramic coating can reduce the corrosion current density of 6061 aluminum alloy from 3.74 × 10−2 A cm−2 to 2.24 × 10−6 A cm−2. And its corrosion resistance can be further improved by adding a 2nd low-voltage PEO treatment.