Corrosion Resistant Multi-Principal Ni60Cr15Co15Ti5Al2.5Nb2.5 Alloy: Loss of Passivation (LOP) and Recovery of Passivation (ROP) upon Heat Treatment

This study explores the corrosion behavior of a nonequiatomic Ni60Cr15Co15Ti5Al2.5Nb2.5 high-entropy alloy (HEA) subjected to thermal aging. Electrochemical testing in 3.5 wt% NaCl solution revealed that short-term aging (≤1 h) induces a loss of passivation (LOP), marked by a corrosion potential drop to −260 mVSCE and an increase in corrosion current density icorr to 2.7 μA/cm2. In contrast, prolonged aging (≥10 h) promotes a recovery of passivation (ROP), driven by microstructural elemental redistribution. This recovery is evidenced by a positive shift in corrosion potential to −125 mVSCE, a significant reduction in icorr to 7.1 × 10–2 μA/cm2, and an increase in pitting potential (Epit) from +413 mVSCE to +495 mVSCE. Electrochemical impedance spectroscopy (EIS) further confirmed enhanced passivity through a reduction in effective capacitance (from 3.2 to 2.6 μF/cm2) and an increase in polarization resistance (from 960 to 1084 kΩ·cm2). However, excessive aging (>50 h) may trigger a secondary LOP. An optimal aging window (30–50 h) yields a stable and protective passive film.