Effect of different post-treatments on the corrosion resistance and tribological properties of AZ91D magnesium alloy coated PEO

• The performance of sealed PEO Mg alloys can be underestimated by SEM.
• Electrochemical tests have provided complementary to surface evaluation by SEM.
• Salt spray results were in agreement with impedance results.
• Open micro-pores on the surface were effectively sealed by PEO + TEOS:MPTES:Dithiol.
• Thin PA/PE-Si coating with nanoparticles improves PEO protection.

Plasma electrolytic oxidation (PEO) is widely used to improve the corrosion resistance and protection of magnesium and its alloys by the formation of ceramic oxide layers. However, this anodization processes generate porous and rough layers with poor tribological properties. In order to improve corrosion protection and tribological capabilities, three different post-treatments (novel sol–gel coating, polytetrafluoroethylene and acrylate-ethylene copolymer with nanoparticles) have been used as sealing agents on PEO anodized AZ91D magnesium alloys. Characterization of unsealed and sealed systems was made by Fourier transform IR (FTIR), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDX) in order to know the chemical structure, morphology and composition of PEO/coating systems. Corrosion resistance was evaluated by electrochemical measurements (open circuit potential and electrochemical impedance spectroscopy) in 5 wt.% NaCl solution at room temperature and by accelerated neutral salt spray test. Tribological properties were obtained using a ball-on-disk test. According to results, post-treatments have improved the corrosion protection performance of PEO systems. However, while internal physical interlocking of pores seemed to be crucial to enhance corrosion protection using polytetrafluoroethylene and sol–gel post-treatments, the formation of an external top layer was the main cause of the improvement of the barrier properties for acrylate–ethylene copolymer post-treatment. On the other hand, despite that tribological performance was mainly governed by the PEO coating, the formation of a top layer on the surface and its chemical composition can significantly reduce the friction coefficient.