Protective silica-based coating for aluminum 6092/SiCp metal matrix composite in chloride media

Aluminum alloys are used widely in automotive and aerospace industries. The demand for materials of superior mechanical, thermal and electrical properties has focused attention on aluminum metal matrix composites, (AMMCs). 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 6092/SiC 17.5p T6 AMMCs using silica conversion coatings. The proposed silica coating is environmentally-friendly and is based on low cost 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 analyze the corrosion of the AMMCs in 3.5% NaCl. The polarization resistance (Rp) for the uncoated AMMCs was measured by EIS to be approximately 3.0×10³ Ω.cm2. The Rp improved about one order of magnitude for the substrate coated with silica conversion coating with a distinct improvement in the localized (pitting, crevice and intergranular) corrosion resistance. The corrosion rate decreased for the coated specimens, indicating that silica coating behaves as barrier to prevent the chloride attack and hence, decreases the susceptibility of the AMMCs to localized corrosion. The optimum silica concentration to provide the highest corrosion inhibition characteristics for AMMCs was 50 g/l. Increasing the silica concentration to 100 g/l was found to have an adverse effect on the corrosion resistance. Results showed that a simple immersion in a silica solution is very effective for improving the corrosion resistance by forming a barrier protective layer over AMMCs substrate.