Corrosion protection of aluminum-matrix composites

Aluminum-matrix composites are attractive in their combination of low CTE and high thermal conductivity. Anodizing is an effective surface treatment for improving the corrosion resistance of aluminum-matrix composites. For SiC filled aluminum, anodizing was performed successfully in an acid electrolyte, as usual. However, for AIN filled aluminum, anodizing needed to be performed in an alkaline (NaOH) electrolyte instead of an acid electrolyte, because NaOH reduced the reaction between AIN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration of NaOH caused the dissolution of the anodizing product (Al₂O₃) by the NaOH, whereas too low a concentration of NaOH did not provide enough ions for the electrochemical process.