Corrosion protection of particulate aluminum-matrix composites by anodization

Anodization is effective for improving the corrosion resistance of aluminum-matrix composites. For SiC particle filled aluminum, anodization was performed successfully in sulfuric acid electrolyte, as usual. However, for AlN particle filled aluminum, anodization needed to be performed in an alkaline (0.7 N NaOH) electrolyte, because NaOH reduced the reaction between AlN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration caused the dissolution of the anodizing product (Al₂O₃) by the NaOH, whereas too low a concentration did not provide enough ions for the electrochemical process. The corrosion properties and anodization characteristic of pure aluminum, Al/AlN and Al/SiC were compared. Without anodization, pure Al had better corrosion resistance than the composites and Al/SiC had better corrosion resistance than Al/AlN. After anodization, the corrosion resistance of Al/AlN was better than Al/SiC and both composites were better than pure Al without anodization, but still not as good as the anodized pure Al.