Production of ceramic layers on aluminum alloys by plasma electrolytic oxidation in alkaline silicate electrolytes

Plasma electrolytic oxidation (PEO) is a technology allowing obtaining hard wear- and corrosion-resistant in the form of thick and highly-adhesive oxide layer on aluminum surfaces. The process can be performed in several types of electrolyte, of which the alkaline silicate electrolytes were employed in this study. Silicate electrolytes passivate aluminum alloys, favor micro-arc discharges and contain silicate ions, which modify the technological properties of the coating. The influence of the silicate index of sodium silicates and of their concentration in the electrolyte on the composition, structure and properties of the oxide layer was studied. Electrolyte properties, electrochemical process parameters and the properties of the resulting coating were studied and compared. Optimal electrolyte compositions for the obtaining of hard and corrosion resistant ceramic layer were found. A plausible mechanism of the process was proposed.