Plasma Electrolytic Oxidation (PEO) as pre-treatment for sol-gel coating on aluminum and magnesium alloys

Plasma Electrolytic Oxidation (PEO) is a quite new process which allows the formation of porous ceramic oxide layer on metallic substrates. In this work, hybrid organic-inorganic sol-gel hydrophobic coatings, called M7T3, were deposited on PEO layers, produced on 7020 aluminum alloy and AZ80 magnesium alloy with silicates and phosphates solutions. The hydrophobic coating was a silica film functionalized with methyl groups and it was obtained by dip coating. In detail, was studied the effect of PEO layers on the surface characteristics of the top sol-gel coatings and, consequently, on the corrosion resistance of the two alloys. The samples were characterized in terms of microstructure, morphological features, composition and corrosion properties. The top sol-gel layer resulted uniform on the various samples thank to the presence of the PEO coating. The different substrate (7020AA or AZ80) influenced the morphology of the PEO layer and, consequently, the deposition of the Sol-Gel layer. Moreover, also the durability of the sol-gel resulted increased with the possibility to use samples with the combination of PEO and sol-gel in more severe conditions than the samples with only the sol-gel layer. •PEO coating can be employed as sub-layer for M7T3 sol gel.•The presence of the PEO as sub-layer increases the corrosion resistance of the samples.•The porous morphology of PEO layer permits to enhance the durability of M7T3 sol gel.