The study of a Mg-rich epoxy primer for protection of AZ91D magnesium alloy

A Mg-rich epoxy primer was prepared by adding pure magnesium particles to an epoxy coating. The coating properties were studied with electrochemical impedance spectroscopy (EIS), scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The Mg-rich primer showed better protection for AZ91D magnesium alloy than the same epoxy primer without Mg addition. The open circuit potential measurements showed cathodic protection effect of the Mg-rich primer on AZ91D alloy. Cross scratch testing showed that the Mg-rich primer provided better protection for the substrate than original epoxy coating. The precipitation of Mg(OH)2 in the coating also provided some degree of barrier protection.

Research highlights
► A Mg-rich epoxy primer was prepared by adding pure magnesium particles in epoxy coating. Cross scratch testing results showed that in 3% NaCl solution the Mg-rich primer showed better protection for AZ91D magnesium alloy than the same epoxy primer without Mg addition.
► The open circuit potential of AZ91D alloy in NaCl solution decreased after coated with Mg-rich coating, suggesting that cathodic protection effect of the Mg-rich coating on AZ91D alloy was present.
► EIS studies showed that during the immersion tests of AZ91D alloy with Mg-rich coating the magnesium particles in coating dissolved with the charge-transfer resistance Rct at the magnesium particle/coating interface decreased and the double-layer capacitance Qdl increased. While the coating resistance remained stable for a long time and corrosion of the AZ91D alloy substrate was obviously delayed.