| Summary: | This article presents the influence of the anodizing parameters and thermo-chemical treatment of Al<sub>2</sub>O<sub>3</sub> coatings made on aluminum alloy EN AW-5251 on the surface free energy. The oxide coating was produced by DC (Direct Current) anodizing in a ternary electrolyte. The thermo-chemical treatment of the oxide coatings was carried out using distilled water, sodium dichromate and sodium sulphate. Micrographs of the surface of the Al<sub>2</sub>O<sub>3</sub> coatings were characterized using a scanning microscope (SEM). The chemical composition of the oxide coatings was identified using EDS (Energy Dispersive X-ray Spectroscopy) microanalysis. Surface free energy (SFE) calculations were performed by the Owens–Wendt method, based on wetting angle measurements made using the sessile drop technique. The highest value of surface free energy for the only anodized coatings was 46.57 mJ/m<sup>2</sup>, and the lowest was 37.66 mJ/m<sup>2</sup>. The contact angle measurement with glycerine was 98.06° ± 2.62°, suggesting a hydrophobic surface. The thermo-chemical treatment of the oxide coatings for most samples contributed to a significant increase in SFE, while reducing the contact angle with water. The highest value of surface free energy for the coatings after thermo-chemical treatment was 77.94 mJ/m<sup>2</sup>, while the lowest was 34.98 mJ/m<sup>2</sup>. Taking into account the contact angle measurement with glycerine, it was possible to obtain hydrophobic layers with the highest angle of 109.82° ± 4.79° for the sample after thermal treatment in sodium sulphate.
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