Investigation of the long-term corrosion protection of sol-gel sealed PEO

Plasma electrolytic oxidation (PEO) is a versatile and cost-effective electrochemical surface modification technique used to obtain protective oxide coatings in valve metals, such as aluminum, titanium, and magnesium. PEO in aluminum alloys is known for increasing wear resistance, although its intrinsic porosity is a drawback for long-term corrosion resistance. Sol-gel coatings have been extensively investigated as a corrosion protection layer for aluminum substrates and might be an alternative to seal and increase the corrosion resistance of PEO coatings. In this research, the sealing ability of a TEOS/GPTMS sol-gel layer applied on porous PEO coatings on AA2024 was investigated. Two withdrawal speeds were set in order to investigate the influence of the sol-gel thickness on its sealing ability. The barrier properties and the corrosion resistance of the coatings were assessed by Electrochemical Impedance Spectroscopy (EIS) up to 28 days of immersion in 0.1 Na2SO4 and 0.1 M NaCl. Both sol-gel application conditions were able to seal the PEO surface, change its wettability, and improve its corrosion resistance. No difference was found between the sol-gel of different thickness, indicating the benefit of sol-gel relies more on the sealing of microcracks and internal pore filling than on external pores' clogging.