3.6.2. Potentiodynamic Test

The electrochemical behavior of A356 alloy was evaluated through the exposure of these samples to a corrosive environment, based on a simulation of sea water, using 3.5% NaCl electrolyte solution at room temperature [57]. The rate of corrosion was measured using the linear polarization technique through the Tafel extrapolation method to identify the corrosion resistance of both as-cast and cooling slope-cast samples. To compare these samples, Figure 8a–c depicts the polarization curves for the as-cast and cooling slope-cast samples, before and after the ECAP process. The corrosion performance of A356 alloy in 3.5 wt.% NaCl solution are tabulated in Table 2.

Based on the curves presented, the estimated average corrosion potentials were found to be approximately similar, with trivial differences. The results further revealed that the reduction in the corrosion rate and increment of polarization resistance, after the T6 heat treatment process for both as-cast and cooling slope-cast samples, could be attributed to the modification in the shape of certain Si particles, where these particles became substantially finer after the processes of ECAP and cooling slope casting [40]. Therefore, this could be associated with a reduction in the area ratio of cathodic to anodic phases. The corrosion rate of the as-cast alloy was 0.042 mmpy, reduced to 0.0015 mmpy after T6-4 passes route of A. Additionally, the corrosion rate of the cooling slope casting alloy was 0.019 mmpy, reduced to 0.0014 and 0.00125 mmpy after 4 and 6 passes, respectively.

**Figure 8.** Polarization curves of A356 alloy before and after ECAP in 3.5 wt.% NaCl. (**a**) as-cast before and after T6; (**b**) cooling slope before and after T6; (**c**) heat-treated T6, as-cast and cooling slope after 4 and 6 passes.

Essentially, the polarization resistance depends on the microstructural state. After the ECAP process, the polarization resistance for both samples increased with more ECAP passes. Nevertheless, after the ECAP process, the polarization resistance for the as-cast sample was found to be inferior to that of cooling slope-cast sample, which positively affected the reconstruction of the metal protective layer, as shown in Table 2. However, the rate of corrosion for the heat-treated as-cast and cooling slope-cast samples decreased with more ECAP passes.

The cooling slope-cast sample mainly demonstrated an exceptional corrosion resistance, which was greater than that of the as-cast sample. The fine-grained structure, with higher grain boundaries, reduces the concentration of chloride per grain boundary, which reduces the current density [58,59], and this provides the advantage of forming more a stable and intact passivation film, improving the corrosion resistance. The obtained result in this study is in line with [60–62].


**Table 2.** Average of corrosion rate (CR), polarization resistance (*Rp*) and current density (*Icorr*).
