Incorporation of nano-particles in plasma electrolytic oxidation (PEO) coatings produced on aluminum alloys

The corrosion resistance of aluminum and its alloys could be improved by performing plasma electrolytic oxidation (PEO), a surface treatment aimed at producing protective oxide coatings on the metal surface. The structure and properties of PEO coatings could be modified by properly tuning the operational electrical parameters and the chemical composition of the electrolytic bath. An example is the addition of corrosion-resistant particles in the electrolytic solution; such particles would be incorporated into the final coatings further improving their protective effect against corrosion. The present research is focused on the comparison of the effects of particle incorporation within PEO coatings on two aluminum alloys, the AA2024 and the AA6082. The selected particles are titanium dioxide nanoparticles (TiO2, average diameter of about 21 nm). The PEO treatment is performed working in alkaline solutions based on NaOH and KOH with the addition of Na2SiO3. Both continuous (DC) and alternated (AC) electrical regimes at different frequencies are applied to evaluate the influence of the electrical parameters on the particle uptake. PEO coatings are analysed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses. In addition, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in NaCl 3.5 wt% are carried out. Different trends are observed depending on the treated alloy. First of all, it is noticed that the use of nanoparticles causes a darkening of the oxide layers produced on AA6082 substrate with respect to ceramic coatings produced in the absence of TiO2 particles, while lighter coatings are produced on disks of the other aluminum alloy. Working with AA2024 the particle uptake increases for longer anodic periods, while the opposite behaviour is observed for the AA6082 for which the Ti content is larger in the AC-produced coatings. On the other hand, the surface morphology of the oxide layers is quite similar for the two types of aluminum; the samples treated in the alternated electrical regime are characterized by the presence of rounded pores, while DC coatings presented elongated pores alternated at flatter regions. Even the electrochemical trends present some analogies. The coatings produced in AC are characterized by higher impedance values with respect to DC samples that in the case of both the alloys show the lowest polarization resistances. Thus, AC coatings exhibit a better barrier effect against the penetration of corrosive species, which results in improved corrosion resistance.