Corrosion mechanism and model of pulsed DC microarc oxidation treated AZ31 alloy in simulated body fluid

This paper addresses the effect of pulse frequency on the corrosion behavior of microarc oxidation (MAO) coatings on AZ31 Mg alloys in simulated body fluid (SBF). The MAO coatings were deposited by a pulsed DC mode at four different pulse frequencies of 300 Hz, 500 Hz, 1000 Hz and 3000 Hz with a constant pulse ratio. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were used for corrosion rate and electrochemical impedance evaluation. The corroded surfaces were examined by X-ray diffraction (XRD), X-ray fluorescence (XRF) and optical microscopy. All the results exhibited that the corrosion resistance of MAO coating produced at 3000 Hz is superior among the four frequencies used. The XRD spectra showed that the corrosion products contain hydroxyapatite, brucite and quintinite. A model for corrosion mechanism and corrosion process of the MAO coating on AZ31 Mg alloy in the SBF is proposed.

► Corrosion properties of MAO coated Mg alloys are improved by optimizing frequency. ► The SBF has a significant effect on the corrosion behavior of MAO coated Mg alloy. ► The corrosion damage depends on the MAO coating and the corrosion product layer. ► A corrosion mechanism model for MAO coated AZ31 alloy in the SBF is proposed. ► The corrosion mechanism animation is made to explain the corrosion process..