Corrosion protection of aluminum composites by silicate/cerate conversion coating

The effect of silicate surface treatments on the corrosion inhibition characteristics of AA6061 T6-10% Al2O3 composite were studied in 3.5% NaCl. A series of specimens was carried out at different silicate concentrations using different application methods alone or combined with a sealing step in cerium solutions. The effect of surface etching and oxide thickening were also studied. Electrochemical impedance spectroscopy (EIS) was used to investigate the corrosion behavior of treated and untreated specimens. The pitting corrosion behavior and the perfect passivity domain were measured using potentiodynamic tests. The occurrence of localized corrosion due to chloride ion attack was examined by optical microscopy and SEM. Generally, silica/ceria treatments improve the corrosion resistance due to the formation of protective oxide films which act as a barrier to oxygen diffusion to the metal surface. Ceria sealing specimens showed a superior corrosion resistance indicating that cerate and not silicates plays the main part in the passivation mechanism. According to the EIS results, the etching process has a negative effect on the corrosion resistance.