3.6.1. Surface Morphology

Figure 7 reveals optical micrographs of the surface morphology, before and after the processes of ECAP, after immersion in 3.5% NaCl solution for ten days, for both the as-cast and cooling slope-cast samples. Overall, the size and the number of pitting corrosions, as well as large corrosion rings products around pits, before and after the process of ECAP, for the cooling slope-cast sample, were found to be lesser than those for the as-cast sample due to reduction and redistribution of cathodic phases after the ECAP process. Larger localized corrosion pits were found to be noticeable, for the as-cast sample. The surface morphology indicated the formation of stable pitting [52], which was attributed to a localized corrosion attack between the active particles and noble particles in a eutectic phase [53]. Essentially, the non-presence of corrosion products within the corrosion rings indicates that the cathodic

reaction occurs on rings, while the anodic reaction occurs inside the stable pit instead. Applied strain during the ECAP process reduces the grain size and develops crystalline defects, including dislocations of the grain boundary. An increase in both the area of grain boundaries and dislocations led to the formation of passive films and the corrosion of ultrafine grains, improved by the rapid formation of passive films at surface crystalline defects, including grain boundaries and dislocations [54,55]. In Al alloy, Al oxide film, containing eutectic Si particles, improved the pitting corrosion resistance by increasing the ECAP pass number, which was related to the decrease of the size of Si-containing impurities, because Si is the major cause of pitting corrosion. The decreases of the cathodic area led to a consequent decrease of the anodic current density [56].

**Figure 7.** Surface appearance of A356 alloy (**a**) as-cast, (**b**) cooling slope (**c**) ECAPed as-cast-T6 after 4 passes and (**d**) ECAPed cooling slope-T6 after 6 passes after immersion for 10 days.
