## 3 Results and Discussion
### Electrochemical Characteristic of Crept HPDC AA365 Alloy  
Figure 3 showed the variation of the open circuit potential (OCP) of the crept AA365 alloys at various temperatures as a function of immersion time in 3.5% wt.% NaCl. It can be shown that continuous fluctuations were found in all samples during an hour of immersion. This would imply that passivation film on the surface would be removed with the chemical reaction in the solution at first. However, the surface would be covered with an oxide film by reacting to the solution later. This phenomenon explained why continuous fluctuations were observed [17]. The corrosion characteristics of crept AA365 alloys were indicated after the cyclic potentiodynamic polarization in 3.5 wt.% NaCl, according to  
Figure 1: SEM and EDS analysis of microstructure of crept HPDC AA365 alloys: (**a**) crept AA365 alloy at 373 K; (**b**) crept AA365 alloy at 473 K; (**c**) crept AA365 alloy at 573 K; (**d**) A higher magnification micrograph and EDS analysis of crept AA365 alloy at 473 K.  
Figure 2: EPMA micrograph and chemical element distribution maps of crept AA365 alloy at 373 K: Al, Mg, Fe, Si, Mn. Figure 4. The corrosion potential (E\({}_{\rm corr}\)) and the corrosion current density (I\({}_{\rm corr}\)) were calculated by using Tafel extrapolation from these curves, and the values of these results were listed in Table 2. All crept HPDC AA365 alloys showed that the corrosion potentials were comparable to the pitting potential (E\({}_{\rm pit}\)) with the rapid increase in current density on the anodic branch. Few passivation behaviors were observed in the anodic branch, and the trend of all AA365 alloys brought the pitting attack without any limitations at the corrosion potential. Moreover, the corrosion potentials indicated \(-\)687, \(-\)684, and \(-\)673 mV\({}_{\rm SCE}\) of crept AA365 alloys at 373 K, 473 K, and 573 K, respectively. It meant that the corrosion occurred rapidly on the crept AA365 alloy at 473 K rather than other alloys. Above the pitting potential, new pits on the matrix were expected to nucleate, grow, and propagate, causing the pitting corrosion on the crept alloy. After reversing the scan in these curves, the pit's growth declines incrementally until reaching the repassivation potential. Especially, the corrosion current density (I\({}_{\rm corr}\)) was investigated to evaluate the corrosion resistance of the alloys in general. The values of I\({}_{\rm corr}\) were determined as 8.02, 13.3, and 0.51 \(\times\) 10\({}^{-6}\) Acm\({}^{-2}\), respectively. The crept HPDC AA365 alloy at 473 K showed the highest corrosion current density, rather than others. It could be argued that the high amount of intermetallic compounds led the crept alloy at 473 K to severe corrosion, as proved in Figure 4. In attempting to understand the corrosion attack on the crept HPDC AA365 alloys after the electrochemical test in 3.5 wt.% NaCl, the corroded surface morphology of these alloys was exhibited in Figure 5. As expected about the micro-galvanic corrosion between the intermetallic precipitates and the Al matrix, the localized corrosion on the Al matrix in the vicinity of the intermetallic compounds was formed owing to the high potential difference between the Al matrix and the intermetallic compounds. Especially, the corrosion of crept HPDC AA365 alloy at 473 K severely occurred owing to the highest density of the intermetallic precipitates. Based on the results, it can be concluded that the intermetallic compounds on the Al matrix gave rise to the harmful micro-galvanic corrosion of crept HPDC AA365 alloys rather than the point defects, such as micro-voids, shrinkages, and trapped gas. Figure 3: Variation of the open circuit potential (OCP) of crept AA365 alloys in 3.5 wt.% NaCl during a hour of immersion: (**a**) crept AA365 alloy at 373 K; (**b**) crept AA365 alloy at 473 K; (**c**) crept AA365 alloy at 573 K.  
Figure 4: Cyclic potentiodynamic polarization (CPDP) curves of crept HPDC AA365 alloys in 3.5 wt.% NaCl.