3.6.3. Pitting Corrosion Appearance

In the eutectic mixture phase, the initial corrosion stages of the as-cast sample revealed localized corrosion, whereas the silicon particles and α-Al interfaces were unaffected [8,32,33,49]. Figure 9 reveals a side view of the pitting corrosion of A356 alloy. Figure 9a,b displays the pitting corrosion of (a) the as-cast and (b) cooling slope-cast of A356 alloy samples. It should be noted that the presence of the enlarged area in the eutectic mixture phase in Figure 9a,b would eventually lead to a wider area of corrosion on the surface of alloy.

This implied that the microgalvanic corrosion between impurities of high Si and Al matrix could contribute to the occurrence of pitting corrosion within the area that contains impurities of high Si. Meanwhile, Figure 9c,d displays the pitting corrosion, after a potentiodynamic test for the heat-treated (c) as-cast sample (after four passes of ECAP process) and (d) cooling slope-cast sample (after six passes of ECAP process). The difference in the area of the α-Al separated grains and depth of corrosion in the eutectic mixture phase, between both the as-cast and cooling slope-cast samples, before and after the ECAP process, was evident, as revealed in Figure 9a,b. The reduction in the size of Si particles, as previously shown in Figure 5 and Table 1, the refined grain in an elongated shape, as well as the microstructure homogeneity of eutectic mixture phase surrounding the elongated α-Al phase led to the reduced rate of corrosion for the ECAPed A356-T6 cooling slope-cast alloy sample, compared to that of the as-cast alloy sample, as displayed in Figure 9c,d.

**Figure 9.** Pitting corrosion of (**a**) as-cast, (**b**) cooling slope, (**c**) as-cast-T6,4 passes and (**d**) cooling slope-T6, 6 passes route A of A356 alloy.

In summary, the corrosion resistance of ECAPed A356-T6 alloy sample was significantly improved due to the reduction in the galvanic potential difference, which the reduced area ratio of the noble phase to less-noble eutectic mixture phase contributed to.

The shape of Si particles influences the formation of a firm oxide film. In particular, coarsened Si particles impede the development of a protective oxide layer in the aqueous solutions and weakens the protective passive film [8]. The reduction in the area ratio of noble Si particles to less-noble eutectic Al phase around these Si particles significantly improves the resistance of pitting corrosion [5]. Furthermore, localized mechanical damage or the chemical changes of the environment may damage the protective oxide film. The increase of the applied voltage on the side of the cathodic curve, as shown in Figure 8, contributes to the rapid reduction of current to the extent that its value remains unchanged before achieving the value of *I*corr.

The steep increment in the anodic current, with the increment in the applied potential in the positive direction, breaks the developed oxide layer and causes pitting corrosion. Here, the chloride ion attacks and dissolves the aluminum surface to form an aluminum chloride compound. The SEM image and EDX of the Al2O3 oxide layer of the ECAPed A356 alloy after the corrosion test, are shown in Figure 10. The highest corrosion resistance with the lowest corrosion rate were obtained for the aluminum A356 alloy, which was subjected to the ECAP process, as tabulated in Table 2.

**Figure 10.** (**a**) Oxide layer of ECAPed A356 alloy and (**b**–**d**) EDX of oxide layer.
