A study on pitting behaviour of AA2024/SiC p composites using the double cycle polarization technique.

Abstract

A double cycle polarization technique was used to estimate the pitting potential ( E pit) of AA2024/SiC particle composites. The volume fraction of SiC particles was 8, 14, 19 and 24%. The composites and the unreinforced alloy were potentiodynamically polarized in aerated 3.5 wt.% NaCl aqueous solution. Pitting potential was clearly visible in the first anodic part of the 2nd cycle and varied between −640 and −665 mV for all the composites, as well as for the monolithic alloy. However, significant differences were observed in the residual passivation current density ( I r) (extracted from the 2nd polarization cycle) which decreased successively as the SiC p volume fraction of the composites increased. Metallographic examination of the composites and the unreinforced alloy polarized at 50 mV above E pit showed that pits were fewer as the SiC p content increased, whereas for the AA2024/19 vol.% SiC p, no pits were observed. Line scanning acquisition showed that Cu microsegregation in the a-phase fluctuates to a greater extent in composites than in the plain alloy and is more intense as the volume fraction of SiC p increases. As a result, more pitting initiation sites are created, in accordance with the protection potential values ( E prot), which are shifted in the electronegative direction as the SiC p content increases.