Effect of NaOH on the Structure and Corrosion Performance of Alumina and Silica PEO Coatings on Aluminum

The effect of NaOH content of electrolyte on the properties of ceramic coatings, produced in silicate solution, was studied. Morphology, chemical analysis, phase composition, and cross-section of the ceramic coatings were investigated by SEM, EDS, XRD, and OM, respectively. The corrosion resistance and corrosion mechanism were also studied using potentiodynamic polarization and electrochemical impedance spectroscopy in a 3.5% NaCl solution. To study the surface roughness, a brightness SEM image analysis method was used. Results suggested that increasing the NaOH concentration of sediment production size causes thickness and coating roughness to decrease. The lowest corrosion rate belonged to the twofold layer coating produced in 10 g/L of NaOH. Other samples, with higher concentrations of NaOH, had reduced porosity, and thus an increase in the corrosion resistance was observed. These coatings mainly consisted of α/γ-Al₂O₃ and amorphous silica.