**2. Materials and Methods**

This study used cast commercial A356 (Si—7 wt.%, Mg—0.149 wt.%, Fe—0.126 wt.%, Cu—0.01 wt.%, Mn—0.002 wt.%, Zn—0.006 wt.%, Cr—0.001 wt.%, Ti—0.178 wt.%) alloy casted in ingot, with initial dimensions of 80 mm × 40 mm × 140 mm (width × thickness × length). Using a graphite crucible, the as-cast was melted at a temperature of 750 ◦C. For the cooling slope casting process, the cooling slope of stainless steel, with a slope length of 250 mm as well as a tilt angle of 60◦, was used. The reason for selecting these conditions was based on our work [31]. The apparatus used for this process is shown in Figure 1a. This study specifically selected 620 ◦C as the pouring temperature to limit the superheating of the melt [31]. The molten metal was poured downward, on a stainless steel slope, into a mold with a vertical surface, before quenching in water. In line with the T6 procedure, this study performed a heat treatment process to the as-cast and cooling slope-cast samples, which involved the following processes in this order: (1) an eight-hour sequence of solution treatment at 540 ◦C, (2) water quenching, and (3) three-hour aging process at a temperature of 180 ◦C [32]. Following that, as illustrated in Figure 1b, as-cast and cooling slope-cast samples were machined into a rod shape, with a diameter of 9.8 mm. The samples were ECAPed in a die with a channel angle of 120◦, following route A (where the sample is not rotated between each pass). It should be noted that molybdenum disulfide (MoS2) grease was used as a lubricant in this study. The pressing of as-cast and cooling slope-cast A356 alloy samples was carried out using a 50-ton hydraulic press. As-cast samples were pressed up to four passes, without any cracks on the surface while the cooling slope casting samples were successfully pressed to six passes.

**Figure 1.** Schematic of (**a**) cooling slope casting and (**b**) ECAP mold.

Both ECAPed samples were subsequently examined under a field emission scanning electron microscope (FESEM, Zeiss, Oberkochen, Germany) and optical microscope (OM, Olympus corporation, Tokyo, Japan). Additionally, an energy dispersive x-ray (EDX) (equipped to FESEM) was used for elemental analysis. A Vickers hardness tester (micro Vickers hardness tester, Zwick, Germany; ZHVμ) was used to measure the hardness of the average of three samples per case. These samples were also prepared for microstructure analysis using silicon carbide (SiC) papers of grit between 180 and 2000, followed by a polishing process using 3 μm and 1 μm of diamond paste (Al2O3). Meanwhile, an etching process was performed using Keller's reagent (1% HF, 1.5% HCl, 2.5% HNO3, H2O solution) as an etchant. This study performed quantitative metallography analysis to measure the grain size, according to the ASTM E112 standard. The size of Si particles (the width and length of particles) was measured using the Smart Tiffv2 software, considering at least 200 particles in each case. Following that, an electrochemical experiment was performed in naturally aerated 3.5% NaCl solution at room temperature, with pH 6.5. A potentiostat GAMRY 3.2 was used to measure the rate of corrosion (characterized by *i*corr) of these samples. For this, a three-electrode cell was used, which comprised of (1) test material (as a working electrode), (2) graphite (as a counter electrode), and (3) a silver or silver chloride (Ag or AgCl) electrode (as a reference electrode). The potential dynamic polarization tests were performed at a scanning rate of 1 mV s−<sup>1</sup> with a range from −250 mV versus open circuit potential (OCP) to the final potential of 250 mV versus OCP. The potentiodynamic tests were started after about 15 min of immersion in 3.5% NaCl. Immersion tests were performed during 14 days in 3.5 wt.% NaCl naturally aerated solution to study the surface appearance. Basically, each sample was mounted in epoxy that was aired for 24 hours. Finally, these samples were smoothened using up to 1200-grit SiC before each corrosion test.
