Eco-friendly, cost-effective silica-based protective coating for an A6092/SiC/17.5p aluminum metal matrix composite

Aluminum-based, particulate-reinforced metal matrix composites (AMMCs) have been the most popular among the composite materials due to their low density and isotropic properties in addition to the superior mechanical, thermal and electrical properties. However, AMMCs have a more inhomogeneous structure than alloys due to presence of reinforcing particles. Such an inhomogeneous structure can enhance the pitting corrosion susceptibility, as well as cause a preferential dissolution of the interface between the matrix and the reinforcing particles. This paper discusses the possibility of improving the corrosion resistance of ALCOA peak-aged Al6092/SiC/17.5p composite using silica conversion coatings. The proposed coating is environmentally friendly and cost-effective as it based on sodium metasilicate solutions of different concentrations (10, 20, 50 and 100 g/l). Electrochemical impedance spectroscopy (EIS), linear polarization and cyclic voltammetery tests were used to investigate the corrosion behavior of the composite in 3.5% NaCl. Results showed that the film formed due to a simple immersion in a silica solution is very effective for improving the localized corrosion resistance by forming a barrier protective layer. The polarization resistance (Rp) for the uncoated composite was measured by EIS to be approximately 5.0 × 103 Ω cm2. The Rp was enhanced by about three times after silica coating with a distinct improvement in the localized corrosion resistance. The optimum silica concentration to provide the highest corrosion inhibition characteristics has been determined to be 50 g/l. Increasing the silica concentration to 100 g/l has an adverse effect on the corrosion resistance.