Influence of frequency and duty cycle on microstructure of plasma electrolytic oxidized AA7075 and the correlation to its corrosion behavior

Ceramic coatings were produced on AA7075 by plasma electrolytic oxidation (PEO) technique using a pulsed DC power source with different combinations of duty cycle (20% and 80%) and pulse frequency (50 Hz and 1000 Hz). The morphology and phase composition of these coatings were analyzed by a scanning electron microscope (SEM), optical profilometer and X-ray diffraction (XRD) technique. The corrosion behavior of the PEO coatings was evaluated using open circuit potential measurements, potentiodynamic polarization in 3.5% NaCl solution and salt spray test as per the ASTM B117 standard. The adhesion strength between the coatings and substrate was assessed through scratch testing. X-ray diffraction patterns show that the PEO coatings formed on AA7075 substrate are mainly composed of γ-Al2O3. The SEM analysis reveals more pores over the surface of the samples treated at 50 Hz frequency and micro-cracks at 80% duty cycle. The 3D-images and depth profiles as obtained through optical profilometer confirmed that the 1000 Hz frequency and 20% duty cycle produced a smoother coating with fine and shallow pore morphology. Further, the coating thickness as measured by eddy current thickness gauge and SEM coating cross section confirmed that the 1000 Hz frequency and 20% duty cycle produced a thicker coating. Among all the PEO treated samples, because of the thicker coating and fine pore morphology, the sample treated at 1000 Hz frequency and 20% duty cycle demonstrated the highest pitting potential of 440 mV, highest polarization resistance (Rp) of 31 kΩ·cm2 and the highest scratch resistance of 28.1 N.