Corrosion Protection of Aluminum Metal-Matrix Composites

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Abstract
Corrosion protection of aluminum metal-matrix composites (MMC) by anodizing treatments was investigated. Electrochemical behavior of MMC without protection also was investigated. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were used to characterize the properties of protective surface layers. Materials studied were Al 6061/SiC (UNS A96061), alloy A356/SiC (UNS A13560), Al 2009/SiC (UNS A92009), Al 2014/Al2O3 (UNS A92014) and Al 6061/Al2O3 with various reinforcement concentrations. The MMC had similar corrosion (Ecorr) and pitting (Epit) potentials as the matrix alloy. The cathodic current density for oxygen reduction in 0.5% N sodium chloride (NaCl) increased for Al 6061/SiC MMC with reinforcement concentration, which was attributed to electrochemically active interfaces between the matrix and the reinforcement particles. Anodizing and hot-water sealing were less effective for MMC than for the matrix aluminum alloys. The reinforcement particles produced a more porous structure of the anodized layer for MMC. Improved results were noted for dichromate sealing, where chromium (Cr6+) in the pores of the outer oxide acted as an inhibitor. The effectiveness of corrosion protection methods decreased with increasing reinforcement concentration and was a function of the matrix alloy but not of the reinforcement material. The observed reduction in corrosion protection was believed to result from corrosion-susceptible interfaces formed between the reinforcement particles and the matrix.

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