Directionally solidified dilute Zn-Mg alloys: Correlation between microstructure and corrosion properties

Zinc–Magnesium (Zn–Mg) alloys have been addressed as potential biodegradable biomaterials. These alloys enable to overcome the main drawbacks of Mg, its high corrosion rate and hydrogen evolution. Moreover, homogeneous corrosion degradation is also an interesting issue to ensure the success of load-bearing biodegradable implants. In this work, two directionally solidified dilute alloys, Zn-0.3wt-%Mg and Zn-0.5wt-%Mg were studied. Both alloys having different microstructural morphologies (cellular and dendritic arrays) depending of the distance to the cooling bottom were evaluated to correlate their microstructural features with corrosion properties. Kinetics and corrosion mechanism have been evaluated by means of DC and AC electrochemical techniques in low chloride containing solution, 0.06 M NaCl. Coarser microstructures are shown to promote higher corrosion rate. Like Zn the corrosion mechanism occurs throughout a multistep dissolution process involving the formation of intermediate monovalent ZnI, and soluble zinc hydroxychloride compounds resulting of the reaction of zincate ions with chloride ions. [Display omitted] •Cells, dendrites and eutectic mixture typify the microstructure of Zn-Mg dilute alloys.•The lamellar eutectic spacing increases with decreasing cooling rates.•DC and AC electrochemical techniques evaluated kinetics and corrosion mechanisms.•Finer microstructures are associates with higher corrosion resistances.