Synthesis and electrochemical behavior of a magnesium fluoride-polydopamine-stearic acid composite coating on AZ31 magnesium alloy

To improve the corrosion resistance of AZ31 magnesium alloys, a magnesium fluoride-polydopamine-stearic acid (MgF2/PDA/SA) composite coating was synthesized on a AZ31 substrate in an easy and low cost way by successively forming a polydopamine (PDA) layer and a stearic acid (SA) layer on a HF-treated magnesium alloy surface. SEM observations show that the MgF2/PDA/SA coating presents a more compact and uniform morphology compared to the MgF2/SA coating. Impedance and polarization analyses conducted in 3.5 wt% NaCl aqueous solution show that the MgF2/PDA/SA coated Mg alloys are significantly improved in corrosion resistance, with a remarkably low corrosion current density and three orders of magnitude in corrosion resistance higher than in the case of the bare AZ31 alloy. It is found that the formation of PDA layer plays a key process in the formation of MgF2/PDA/SA composite coating. The PDA layer not only acts as an efficient anchoring layer between the Mg alloy surface and stearic acid layer, but also plays a protective role on Mg alloy substrate.