The effect of pulse frequency on the electrochemical properties of micro arc oxidation coatings formed on magnesium alloy

Micro arc oxidation (MAO) coatings were formed on magnesium alloy AZ31B to improve the corrosion resistance using environmental friendly electrolyte solution under single-polar pulse power supply. The effect of electrical parameters of pulse frequency on the coating performance was studied at frequencies of 2.5 Hz, 25 Hz and 250 Hz. The coating performance at different frequencies was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and potentiodynamic polarization testing method. It was found that the corrosion resistance of the MAO coatings was improved when increasing the pulse frequencies from 2.5 Hz to 250 Hz. The corrosion current in 250 Hz case decreased by three orders of magnitude as compared with bare Mg alloy and the corrosion impedance also increased remarkably, means significantly enhanced corrosion resistance than bare Mg alloy. The results give us more insight in the optimization of electrical parameters to improve the MAO coating performance. The MAO technology is attractive for application on magnesium and other light alloys for surface protection in automotive and space industries.