Tailoring microstructure and corrosion behavior of Mg-10Gd-1Eu-1Zn-0.2Zr alloy for biomedical use

The effect of heat treatment on the microstructure and corrosion behavior of the Mg-10Gd-1Eu-1Zn-0.2Zr alloy (hereafter referred to as Mg-Gd-Eu-Zn-Zr) was investigated in simulated body fluid (SBF), using the commercial WE43 alloy as reference. The as-cast Mg-Gd-Eu-Zn-Zr alloy exhibits a microstructure composed of α-Mg dendritic matrix with intermetallic phases distributed along interdendritic regions. After heat treatment, the microstructure transforms into a coarser dendritic α-Mg matrix accompanied by a reduced fraction of intermetallics. Similar microstructural features have been observed for the cast WE43 alloy. However, heat treatment results in the complete dissolution of the intermetallic phases. Regarding electrochemical response, both alloys show enhanced corrosion resistance after heat treatment. The corrosion response of magnesium is governed by the simultaneous presence of Mg⁺, Mg(OH) 2 , and MgO at the corroding surface, while relaxation processes associated with Mg⁺ and MgO exhibit capacitive behavior, Mg(OH) 2 produces an inductive response. Linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) data reveal an initial increase in polarization resistance ( R p ) for the Mg-Gd-Eu-Zn-Zr alloy during the early stages of immersion, followed by a pronounced decrease. This collapse in R p is attributed mainly to the presence of intermetallics in the eutectic constituents, which compromise the integrity of the corrosion product layer, and the Mg-rich matrix that preferentially dissolves due to galvanic interactions with intermetallics acting as cathodes. The heat-treated Mg-Gd-Eu-Zn-Zr alloy samples exhibit a delayed decrease in R p , resulting from the reduced intermetallic fraction and a more homogeneous distribution of rare-earth elements in solution in the Mg matrix. The reference WE43 alloy samples show a more pronounced improvement in corrosion resistance after heat treatment, owing to the complete dissolution of intermetallic phases and the formation of a more stable and protective surface film. • Microstructure and corrosion behavior of Mg–10Gd–1Eu–1Zn–0.2Zr (wt%) alloy in simulated body fluid (SBF). • Microstructural evolution comparable to that of the commercial WE43 alloy. • Novel approach to the corroding surface of Mg alloys, showing that the surface is mainly composed of three species: Mg⁺, Mg(OH)₂, and MgO. • From a kinetic standpoint, heat treatment enhances corrosion resistance due to a reduced fraction of intermetallics and an increased rare-earth content in the matrix.