In this study, the passivity and localized corrosion of single- and multi-phase Ru-free Ni-Fe-Cr-Mo-W-X (X= Mn, Al, and Cu) multi-principal element alloys (MPEAs) were studied using AC and DC electrochemical methods. While each alloy was found to resist localized pitting corrosion at ambient temperature, X-ray photoelectron spectroscopy of passivated alloy specimen surfaces revealed constituent elements to dissolve in a non-congruent manner. Heat treatment of the single-phase alloys at 800 °C to promote precipitation of hard, strength-enhancing secondary phases resulted in the formation of Cr,Mo-rich σ-type phases in varying volume fractions. Copious in-grain precipitation and several metastable phases were observed in the Al-containing MPEA. This phase transformation increased the hardness of the alloy from 180 HV to 340 HV. After the aging heat treatment, the MPEAs containing Al, Mn and Cu became susceptible to breakdown at the matrix/precipitate interface at ambient temperatures.