Improvement of corrosion behavior of A3xx.x/SiCp composites in 3.5 wt % NaCl solution by Ce surface coatings

The effects of the silicon carbide particles (SiCp) proportion and the matrix composition of aluminum matrix composites (A3xx.x/ SiCp) modified by cerium-based conversion or electrolysis coating were evaluated in 3.5 wt % NaCl aerated solution. The intermetallic compounds were preferentially covered by cerium-based conversion coating obtained by immersion in 50 degrees C solution of Ce(III) salt, and the intermetallic compounds, SiCp, and aluminum matrix were covered by electrolysis treatment performed in ethylene glycol monobutyl ether solution. The kinetic of the corrosion process was studied on the basis of gravimetric tests, and the corrosion process was evaluated by electrochemical impedance spectroscopy. The nature of both Ce coating and corrosion products was analyzed before and after accelerated testing by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and low-angle X-ray diffraction to determine the influence of microstructural changes on corrosion behavior during exposure to the corrosive environment. The corrosion process is influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement. Both cerium-treated surfaces present better behavior to chloride solution corrosion than original composite surfaces without treatment; however, electrolysis affords a higher degree of protection than the conversion treatment because the coating is more extensive. (c) 2006 The Electrochemical Society.