Abstract
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.
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Acknowledgments
The authors thank Trysyntec Technology Cooperation Limited (Taoyuan, Taiwan) for providing the qualified diecast AZ91D panels used in this study. The authors also greatly appreciate Professor S.K. Yen, Department of Materials Science and Engineering, National Chung Hsing University, for providing some very useful discussions. Ms. L. C. Wang, National Sun Yat-Sen University, is acknowledged for her assistance with the TEM works. The authors are also grateful to the National Science Council of Taiwan for financially supporting this work (Contract No. NSC 92-2216-E-005-018).
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Manuscript submitted October 29, 2006.
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Uan, JY., Li, CF. & Yu, BL. 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. Metall Mater Trans A 39, 703–715 (2008). https://doi.org/10.1007/s11661-007-9420-2
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DOI: https://doi.org/10.1007/s11661-007-9420-2