Comparative EIS Study of AlxCoCrFeNi Alloys in Ringer’s Solution for Medical Instruments

Abstract

Depending on the properties required for the medical instruments, compared with the classical materials, the high-entropy alloys (HEAs) are a versatile option. Electrochemical Impedance Spectroscopy (EIS) measurements have been performed on AlxCoCrFeNi-type high-entropy alloys with various concentrations of Al content (x = 0.6, 0.8, and 1.0) in order to characterize their passive film and corrosion resistance at 37 ◦C under infectious simulated physiological conditions (Ringer´s solution acidulated with HCl) at pH = 3. The impedance spectra were obtained at different potential values between −0.7 and +0.7 V vs. SCE. Analysis of the impedance spectra was carried out by fitting different equivalent circuits to the experimental data. Two equivalent circuits, with one time constant and two time constants respectively, can be satisfactorily used for fitting the spectra: one time constant represents the characteristics of the compact passive film, and the second one is for the porous passive film. With the decreasing of Al content, the obtained EIS results are correlated with the evolution of the microhardness and microstructure, which is characterized by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-Ray Spectroscopy (EDAX). It can be observed for all alloys that the resistance of the passive film is very high and decreases with the potential: the very high resistance of the passive film implies a high corrosion resistance, which can be assigned to the formation of the protective oxide layer and demonstrates that the analyzed alloys fulfill the prerequisites for their use as new materials for the manufacturing of medical instruments.