Influence of the compactness of the surface corrosion product film on the corrosion behavior of WE43 Mg alloy in different media

• Due to the acceleration of buffer ions, the corrosion rate of WE43 in SBF reached 1.84 mm/y, significantly higher than the 0.30 mm/y observed in 3.5 wt% NaCl solution. • The surface film of WE43 in 3.5 wt% NaCl solution is denser and has a higher modulus compared to SBF, exhibiting resistance to Cl ⁻ ion penetration. • DFT calculations indicate that Y increases the interfacial energy between Ca 3 (PO 4 ) 2 and Mg(OH) 2 , while Nd has the opposite effect, leading to uneven film deposition and decreased density. WE43 magnesium (Mg) alloy is a promising biomedical material extensively used in degradable medical devices such as vascular stents and bone screws. However, the corrosion behavior of WE43 in the physiological environment due to the complex ionic constituents is not well understood, as these environments differ markedly from NaCl solution used in conventional assessment in vitro . In this study, we investigate and compare the corrosion properties of WE43 in the simulated body fluid (SBF) and 3.5 wt.% NaCl solution. The corrosion rate in SBF is 1.84 mm/a and is significantly higher than the 0.30 mm/a observed in NaCl solution. Electrochemical impedance spectroscopy (EIS) reveals that low-frequency impedance in SBF peaks at 120 h and then declines, whereas the impedance in NaCl solution increases continuously, suggesting that the passivation effect of the corrosion product film in SBF is markedly weaker than that in NaCl solution. After immersion for 168 h, the corrosion product film formed in NaCl solution is denser and more homogeneous, exhibiting an elastic modulus of 26.52 ± 3.69 GPa. In contrast, the film formed in SBF has a bilayer structure consisting of a compact calcium-phosphate (Ca-P) outer layer and a porous hydroxide inner layer with a smaller elastic modulus of 21.03 ± 3.09 GPa as well as reduced compactness. First-principles calculations demonstrate that the Y-containing hydroxide in WE43 hinders the deposition of Ca-P layer, leading to its inhomogeneous growth. These findings offer new information about the distinct corrosion mechanisms of WE43 in different physiological environments.