EIS study of the development of passive layers on Ferrous shape memory alloys in alkaline conditions

Abstract

This work evaluates the corrosion behavior of two iron-based shape memory alloys (Fe-28Mn-6Si-5Cr and Fe-17Mn-10Cr-5Si-4Ni-1(V,C)) in simulated concrete pore environments. The study focuses on the spontaneous passivation process under different pH conditions (13, 12, and 11) and the influence of alloy composition and mechanical pre-straining. Electrochemical techniques, including Open Circuit Potential (OCP) monitoring and Electrochemical Impedance Spectroscopy (EIS), were used to characterize the formation and stability of passive films. Equivalent electrical circuits were employed to model the evolution of the oxide layer over time. Results confirm that all materials form stable passive layers at pH 13 and 12, although different kinetics were observed depending on the alloy and mechanical condition. At pH 11, lower impedance values and minimal evolution were recorded, indicating a reduced protective effect of the passive films. The influence of Cr and Mn content, as well as mechanical pre-straining, was found to significantly affect the passivation behavior of the studied alloys.