Multilayer PEO/Sol-gel systems doped with inorganic inhibitors for the corrosion protection of AZ31B alloy

Plasma electrolytic oxidation (PEO) coatings offer many advantages such as improved corrosion and wear resistance. However, the main disadvantage of PEO coatings is their high energy consumption. Recently a new type of coating known as Flash-PEO has emerged as a way to obtain high performance low thicknesses (2–6 μm) coating with shorter treatment times (1–3 min). Similarly to conventional PEO, Flash-PEO requires a sealing post-treatment for long-term corrosion protection. In the present work, hybrid PEO/sol-gel systems loaded with inorganic corrosion inhibitors (Ca, Li, Zn and Ce) are used with the aim to provide self-healing properties to the AZ31 alloy, which is commonly used in transport and consumer applications. The PEO coating was produced under DC conditions in an aluminate-phosphate based electrolyte in less than ~100 s. Characterization of the coatings was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The water contact angle and the surface roughness were also evaluated. The corrosion behaviour was studied in 0.05 M NaCl aqueous solution using electrochemical impedance spectroscopy (EIS). The self-healing ability of the systems was also assessed by using scratched specimens. Findings revealed that incorporation of the inhibitors into the sol-gel layer has significant effects on the polymerization process and that Zn and Ca improve the corrosion performance attributed to self-healing mechanisms.