New insights on the influence of low frequency pulsed current on the characteristics of PEO coatings formed on AZ31B

In this work, anodic oxide layers on the surface of an AZ31 magnesium alloy were obtained by plasma electrolytic oxidation (PEO) process under low frequency pulsed current. For this, electrolytical solutions containing hexamethylenetetramine and sodium fluoride were used. The morphology and chemical composition of formed coatings were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Also, salt spray test, hydrogen evolution and electrochemical tests (potentiodynamic polarization and electrochemical impedance spectroscopy) were conducted in order to study the corrosion behavior of the coated samples. It was found that the use of low frequency pulsed current for the PEO process reduces the film porosity and increases its thickness, compared with PEO films obtained by continuous anodization. The effect of the pulsed current signal was also analyzed for a two steps PEO process, observing changes in the morphological characteristics of the coatings which allow a better corrosion according electrochemical tests (short term corrosion measurements). However, long term tests results as hydrogen evolution and salt spray tests, indicated the opposite. Both the film porosity and thickness were affected by either the pulsing of the current or the use of a two-step process.