H2O2-driven corrosion of Ti-6Al-4V implants : connection between electrochemical and microstructure characteristics

Corrosion of Ti-6Al-4V implants with α+β microstructure under simulated inflammatory conditions is investigated by accelerated testing in PBS solution with up to 1 M H2O2. Within the framework of the French-Swiss project LoCoMecha, various microstructural phenomena resulting from the corrosive attack are investigated at multiple length scales. Thereby, time lapse microscopy up to 10 days reveals a selective and progressing dissolution of the β-phase. In this contribution, the connections between the observed microstructure characteristics and the electrochemical impedance spectra obtained in situ shall be established. Indeed, many microstructure features observed in the microstructure analysis can be detected in the EIS-results. Especially, a typical de Levie impedance feature is observed in the EIS-spectra, which can be directly related to the dissolution depth of the β-phase. An analytical equivalent circuit was parametrized with microstructure properties determined from FIB-SEM cross-sections (2D, time lapse) and FIB-SEM tomography. The large increase of the fitted capacitance with exposure time corresponds well to an increase of the exposed α-grain surface due to selective β-phase dissolution and allows estimating a time dependent dissolution depth of the β-phase. Correspondingly, a decrease in the polarization resistance with increasing exposure time is consistent with the additional availability of cathodic reaction sites on the α-grain surface. Moreover, the steeper evolution of capacitance and polarization resistance during the first hours of exposure are consistent with the modification of the passive film observed by imaging. Quantitative microstructure and EIS analysis can be integrated into a modelling framework for digital materials design (DMD), allowing to formulate design guidelines for optimized materials fabrication with improved corrosion resistance.