Deposition, characterization and electrochemical properties of silica-phosphate composite coatings formed over A6092/SiC/17.5p aluminum metal matrix composite

This paper investigates the effect of 2.5 wt.% (NH4)3PO4 post-treatment after silica treatment on the formation of silica-phosphate (Si-P) composite coatings over A6092/SiC/17.5p aluminum metal matrix composite (AMMC). In previous studies, the optimum concentration, treatment time and pH of silica treatment have been determined. The aim of the current paper is to determine the optimum conditions under which Si-P composite coatings can provide a maximum localized corrosion protection to AMMC. The surface morphology of Si-P composite coatings and the ability to protect AMMC substrate from localized corrosion in Cl− media were examined as a function of phosphate treatment time (0, 2, 5 and 10 min). Results showed that phosphate post-treatment after silica coating significantly affects the microstructure and the pitting corrosion resistance of the ultimate coated composites. The coating deposited on AMMC post-treated for 2 min provides better pitting corrosion resistance than that post-treated for 5 and 10 min. It is largely because the morphologies of the coatings are remarkably different under the three post-treatment conditions.