Identification of Submicron-Sized Sites Causing Localized Corrosion Initiation and Growth in Die-Cast Al–Si–Cu Alloy

Optical microscopy in non-pH-buffered and pH-buffered 0.1 M NaCl solutions was performed to investigate submicron-sized sites causing localized corrosion initiation and growth in die-cast ADC12 alloy (Al–12Si–Cu). In the non-pH-buffered solution, intermetallic particle (IMP) discoloration occurred first, followed by gas bubble generation, and ring-shaped corrosion product deposition; localized corrosion initiated inside these rings. In the pH-buffered solution, IMPs discoloration and ring-shaped product deposition were suppressed, though a small gas bubble was clearly observed. When immersion was halted after bubble generation, the corrosion initiation site was identified as the final solidification region, though not all such regions acted as initiation sites. Mn, Fe, and Cu were present at initiation sites after corrosion initiation, suggesting they were not directly involved. No Mg-enriched areas were observed, indicating that the dissolution of Mg segregated in the final solidification region likely triggers localized corrosion initiation. After initiation, dissolution progressed along eutectic Al-Si boundaries. Mg segregation was also found to exist along a eutectic Al-Si boundary. This was thought to correlate with selective dissolution of eutectic Al–Si interfaces after corrosion initiation. These findings clarify the role of microstructural features, particularly Mg segregation, in controlling localized corrosion initiation and growth in die-cast Al–Si–Cu alloys.