Al<sub>0.3</sub>Cr<sub>0.5</sub>Fe<sub>2</sub>Mn<sub>x</sub>Mo<sub>0.15</sub>Ni<sub>1.5</sub>Ti<sub>0.3</sub> (x = 0, 0.25, 0.5, 1) compositionally complex alloys are synthesized and annealed at 1070 °C to form two-phase alloys with an FCC matrix and a second phases enriched in Al, Ti and Ni with slightly reduced density, raw element costs, and passivating elements distributed across both phases. The global corrosion resistance is evaluated in 0.01 and 0.1 M NaCl at both natural pH and pH 4. Overall corrosion resistance is suggested to be optimized at Mn concentrations of 5.0 at. %, indicated by pitting potentials comparable to or exceeding those of 316L stainless steel. Improvements in corrosion resistance and optimization of Mn concentration are further assessed by polarization, impedance, and gravimetric analysis after extended aqueous exposure. The fate of individual elements during the dissolution and passivation processes is evaluated with in-situ atomic emission spectroelectrochemistry and ex-situ x-ray photoelectron spectroscopy. Passivity was derived from combinations of Ti<sup>4+</sup>, Cr<sup>3+</sup>, and Al<sup>3+</sup> oxides in an undetermined solid solution or complex oxide. Enhanced corrosion resistance is attributed to the improved chemical homogeneity of passivating elements within the two-phase microstructure while the decreased corrosion resistance of alloys with higher Mn concentrations is attributed to high Mn dissolution rates and/or destabilization of the passive films. The underlying determinants of the role of Mn in the design of corrosion resistant lightweight compositionally complex alloys are discussed.

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