Corrosion behavior of squeeze-cast silicon carbide 2024 composites in aerated 3.5 wt.% sodium chloride

The corrosion behavior of squeeze-cast SiC particulate reinforced AA2024 composites was investigated. The specimens were processed by the stir-casting technique. The SiC particulates were added to a partially solidified matrix alloy, then the semi-solid slurry was vigorously agitated, remelted and squeeze cast in cylindrical molds. The volume fraction of the carbide particulates varied between about 8 and 24%. The microstructure of the as-cast specimens consisted mainly of alpha-dendrites of the AA2024 alloy with fine interdendritic regions. The dendritic size decreased with increasing volume fraction of carbide. The interdendritic regions consisted mainly of a binary eutectic alpha/theta-Al2Cu and a ternary eutectic alpha/Al2Cu/(CuFeMn)(3)Si2Al15. The binary eutectic al(CuFeMn),Si,Al,, was also observed and, less frequently, a binary eutectic alpha/Mg2Si and a ternary eutectic alpha/theta-Al2Cu/Mg2Si were present. The morphological and kinetic nature of corrosion was investigated metallographically and through potentiodynamic techniques in 3.5 wt.% NaCl aqueous solution. All the specimens showed almost the same behavior. No detrimental galvanic corrosion was observed between the reinforcing phase and the matrix. Instead, corrosion was described by two anodic reactions: corrosion of the alpha-phase adjacent to Al2Cu interdendritic regions, and pitting of the dendrite cores. (C) 1998 Elsevier Science S.A. All rights reserved.