Local pH and Its Evolution Near Mg Alloy Surfaces Exposed to Simulated Body Fluids

Abstract

Many reports are available on degradation rates and corrosion product characterization of magnesium alloys for implant application. Typically, the data are obtained ex situ, as post factum analysis of occurred degradation processes or in bulk electrolyte that does not fully reflect concentration gradients at the interface and in the diffusion layer. Meanwhile, these local conditions are essential for tissue‐implant compatibility. Only a limited number of studies employ the techniques that visualize ongoing degradation and are localized enough to observe the changes in the electrolyte layer directly adjacent to corroding magnesium. Here, local pH in Hank's solution is studied in operando, with potentiometric micro‐probes (d = 2 µm) located 10–50 µm above the surface of four Mg alloys potentially relevant for implant applications. A significant difference in local pH is observed for Mg in simple Hank's solution (near surface pH 9.9–10.5) or Hank's solution modified with 2.5 × 10−3 m Ca2+ (pH 7.8–8.5). The corresponding pH values are established during the first seconds of immersion. These findings indicate different degradation kinetics in electrolytes with or without Ca2+. The degradation rate of Mg alloys decreases by almost two times in Ca2+ containing Hank's solution. Calcium‐phosphate/carbonate protective layer stabilizes the surface pH below 8.5 controlling Mg degradation.