Role of graded microstructure in a centrifugally cast A356-SiC particle (SiCp) composite alloy disk on passivity and localized corrosion behavior in 3.5 wt.% NaCl environment has been investigated. The area fraction of SiCp has been found to decrease from the outer periphery to the core of the disk, while the apparent size and area fraction of the intermetallics and Mg-rich phase increase from the periphery to the core. As a consequence, the corrosion resistance of the as-cast disk varies with the distance from the periphery in the order 45 < 65 < 25 < 5 mm. The corrosion resistance of the centrifugally cast disk can be significantly improved through solution annealing at 540 degrees C for 2 h and solution annealing + aging at 175 degrees C for 2 h. However, between the two, the former has shown a better corrosion resistance and passivity than the latter. In the latter case, the aging treatment resulted in the precipitation of Mg2Si phase which is prone to the corrosion attack.