Characterization and Improvement in the Corrosion Performance of a Hot-Chamber Diecast Mg Alloy Thin Plate by the Removal of Interdendritic Phases at the Die Chill Layer

This work presents results concerning the effect of the die chill layer on the corrosion performance of a hot-chamber diecast AZ91D thin plate, with particular attention to the role of interdendritic phases (primary β (Al₁₇Mg₁₂) and surrounding α phase). The primary β phase in the die chill layer was removed by Ar⁺ etching, but the other main constituent phases remained on the surface of the sample. Although previous researchers have attributed intense galvanic corrosion to the cathodic phase (primary β), this work demonstrates that the removal of the primary β phase from the diecast sample surface did not improve the corrosion performance of the sample. Transmission electron microscope (TEM) examinations were performed, to elucidate the microstructure near the die chill surface, especially focusing on the α-Mg phase that surrounds the primary β phase. The surrounding α-Mg phase (known as “Al-rich α” or eutectic α) actually did not contain a high concentration of aluminum solid solution. Instead, the Al-rich α was composed of fine Al₁₂Mg₁₇ β particles and Al-Mn-like particles (smaller than 0.5 μm) that were distributed in a low-Al-containing Mg matrix (∼4 wt pct Al). Such fine cathodic particles seem to participate strongly in the corrosion. Removing the primary β phase alone did not increase the corrosion resistance of the material, because many of the cathodic fine particles remained in the Al-rich α phase region. This work studied an HF-H₂SO₄/CaCO₃ etching method for removing interdendritic phases (both the Al-rich α and the primary β) from the surface chill layer of the diecast thin plate. Therefore, testing in 5 wt pct chloride solution demonstrated that the I _corr of the HF-H₂SO₄/CaCO₃-treated specimen was 3 to ∼16 μA/cm², which was significantly lower than the I _corr ∼ 1600 μA/cm² of the as-diecast Mg sample.