Thermal Parameters and Microstructural Development in Directionally Solidified Zn-Rich Zn-Mg Alloys
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THE addition of Mg to improve corrosion stability of Zn coatings is a technologically common practice for corrosion protection of products of use in the civil and automotive industries.[1–8] Despite the fact that Mg is less noble than Zn, the role of Zn-Mg intermetallics on the corrosion performance has been the subject of a number of recent studies in the literature with a view to investigating specific mechanisms, which have not yet been explained.[9–14] Vlot et al. examined the corrosion behavior of Zn-Mg alloys in the range of compositions 1.5–2.0 wt-pct Mg, characterized by microstructures formed by Zn grains surrounded by a eutectic mixture of Zn/MgZn2 and reported a period of roast formation up to 20 times higher than that of a reference hot dip galvanized coating.[1] Dutta et al. carried out hot dip experiments with Zn-0.5 wt-pct Mg and Zn-2.5 wt-pct Mg alloys and reported that both hardness of the top surface of the TALITA A. VIDA, Ph.D. Student, EMMANUELLE S. FREITAS, Researcher, and NOE´ CHEUNG and AMAURI GARCIA, Professors, are with the Department of Manufacturing and Materials Engineering, University of Campinas, Mendeleyev Street 200, Campinas, SP 13083-860, Brazil. Contact email: cheung@ fem.unicamp.br CRYSTOPHER BRITO, Professor, is with the Marine Institute, Federal University of Sa˜o Paulo, Alm. Saldanha da Gama Avenue 88, Santos, SP 11030-400, Brazil. MARIA A. ARENAS and ANA CONDE, Researchers, and JUAN DE DAMBORENEA, Professor, are with the National Center for Metallurgical Research (CENIM-CSIC), Avda Gregorio del Amo 8, 28040 Madrid, Spain. Manuscript submitted July 22, 2015. Article published online April 8, 2016 3052—VOLUME 47A, JUNE 2016
coatings and the resulting corrosion resistance were higher than those attained in a pure Zn bath.[4] Yao et al. conducted an experimental study with Zn-Mg coatings having from 1 to 4 wt-pct Mg and reported improved anticorrosion properties of the Zn-3 wt-pct Mg coatings when compared with a pure Zn coating, as well as with a significant increase in hardness.[7] Zn and Mg are also considered potential elements regarding biodegradable bone-implant applications. Mg alloys have mechanical properties close to those of bone; however, corrosion rates accompanied by hydrogen release are conducive to high degradation rates. Zn is a biologically tolerable element even when alloyed to Mg (up to 50 pct Mg), which makes Zn-Mg alloys potential alternatives as biodegradable materials. Furthermore, Zn has a much lower corrosion rate in physiological solutions when compared with Mg. Despite the lack of systematic studies concerning the behavior of Zn-rich Zn-Mg alloys in biological environments, it is known that such alloys have other important characteristics such as lower melting points and better machinability when compared with Mg.[15–24] It is well known that microstructural features play a significant role on mechanical and corrosion properties of metallic alloys.[25–34] Despite the potential of applications of Zn-Mg alloys, the literature is scarce on studies of mi
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