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Influence of Zinc Content and Heat Treatment on Microstructures and Corrosion Behavior of Mg-Zn Alloys

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In this study, Mg-Zn alloys were prepared with different Zn concentration (1,3,5 and 7 wt.%) by a simple and clean casting process followed by a solution heat treatment (T4) at 400 oC for 12 hours. The influence of Zn content and heat treatment on microstructure and corrosion behavior of Mg-Zn alloys were investigated. The microstructure and the chemical composition of the phases were characterized using scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results revealed the appearance of MgZn intermetallic as second phase in microstructure of all as-cast alloys. Additionally, the second phase amounts and its sizes increased with increasing Zn content. The influence of T4 heat treatment on microstructure was significant in alloys with Zn concentration up to 5 wt.% with MgZn second phases were almost reduced. While in Mg-7%Zn, the second phase size become bigger but total amounts of second phase decreased. The corrosion behavior was studied by static immersion test and electrochemical test in Hanks’ solution. The results indicated that the corrosion resistance of Mg-1%Zn alloys was the highest in as-cast alloys and decreased with increasing Zn concentration. It is due to the severe micro-galvanic corrosion occurred at the boundaries of MgZn second phases and Mg-matrix.

In T4 heat treated alloys, corrosion resistance of Mg-7%Zn was the lowest while Mg-1%Zn, Mg-3%Zn and Mg-5%Zn exhibited comparable anti-corrosion property. Consequently, the remarkable enhancement in corrosion resistance of T4 heat treated alloys was resulted from the dissolution of MgZn second phase into Mg-matrix leading to reduce corrosion potential and corrosion current.

Pham Ngoc Dinh
Tokyo Institute of Technology
Additional Authors:
  • Dr. Sachiko Hiromoto
    National Institute for Materials Science
  • Prof. Equo Kobayashi
    Tokyo Institute of Technology