Corrosion characteristics of AA 7075 and AA 7075/SiC/Gr hybrid composite processed through multistep hot cross rolling

Abstract

The present work focuses on the study of corrosion characteristics of AA 7075 and AA 7075 based hybrid composite fabricated through the stir casting and hot rolling routes. The AA 7075 based hybrid composite has been prepared with 2 wt% of Silicon carbide (SiC) and 0.5 wt % of graphite (Gr) as the reinforcements. Corrosion characteristics have been studied through static immersion, potentiodynamic polarization (PDP) and electrochemical impendence spectroscopy (EIS) carried out in 3.5 wt % NaCl. Results of PDP showed that the composite as well as the base metal, cross rolled at 450 °C, possesses better corrosion resistance among all the samples (3040 Ω cm2 and 2340 Ω cm2, respectively). The compressive residual stresses on the outer surface of the samples after hot cross rolling at 450 °C led to the increased corrosion resistance. The corrosion resistance of hybrid composite is always greater than the base metal for all the processing conditions, as the addition of a small amount of graphite along with SiC improves the bonding between the matrix and the reinforcements. Further, hot cross rolling of the hybrid composite enhances this bonding. This along with the possibilities of dispersion of reinforcements, after hot cross rolling, favours the formation of a strong passivation oxide layer and increased charge transfer resistance, thereby improving corrosion resistance (from 2129 Ω in as-cast condition to 7200 Ω in hot cross rolled condition at 450 °C). The results of EIS analysis confirm the formation of a double-layer that has a compact inner layer and a porous outer layer. The field emission scanning electron microscope (FESEM) images of the corroded surfaces indicated the occurrence of pitting corrosion.