Preparation and corrosion resistance of hybrid coatings formed by PEN/C plus PEO on AZ91D magnesium alloys

Abstract

A novel surface modification technique is developed to further improve the corrosion resistance of AZ91D magnesium alloys. Hybrid coatings were fabricated successfully by a combination of plasma electrolytic nitrocarburizing (PEN/C) and plasma electrolytic oxidation (PEO) on magnesium alloys. The microstructures, element distribution and phase compositions of the coatings were analyzed by SEM, EPMA and XRD, respectively. The potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) were employed to evaluate the corrosion resistance of the coatings. The results show that PEN/C process predominantly depends on the adsorption and diffusion effects of the active atoms of [N] and [C], and it creates a good substrate surface with predesigned chemical composition to get newly born phases, such as SiC and Si3N4 with excellent properties, in the following hybrid coatings. PEN/C pretreatment also provide a favorable microstructure to enlarge the coating thickness and to make the coating more compact for the hybrid coatings followed. There exists an overlapping phenomenon regarding coating thickness rather than piling up layer by layer during the formation of the hybrid coatings. The hybrid coatings exhibit more excellent corrosion resistance than both single PEN/C modified layers and single PEO coatings. Particularly, the hybrid coatings can provide relatively superior long-term corrosion protection and suppress the occurrence of the localized corrosion for AZ91D substrates. It is worth noting that the key of this investigation is keeping the PEO process unchanged for both PEO treatment only and PEN/C + PEO combining procedure.