Al0.3Cr0.5Fe2MnxMo0.15Ni1.5Ti0.3 (x = 0, 0.25, 0.5, 1) compositionally complex alloys are synthesized and annealed at 1,070°C to form two-phase alloys with a face-centered cubic (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 M 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 Ti4+, Cr3+, and Al3+ 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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Compositionally and/or microstructurally complex alloys present multiple opportunities for achieving and optimizing desirable qualities that are not typically accessible through traditional single-principle-component alloying methodologies, without significant compromise. FBB8+Ti, a novel ferritic steel strengthened by Heusler- and B2-phase strengthening phases is one such alloy, and is investigated here across a range of titanium concentrations for its corrosion performance in a chloride environment. Pitting potentials, corrosion rates, and passivation current densities were established during full immersion in 0.01 M NaCl for alloys prepared with a (a) 0.05 m finish and (b) subsequent ion-polish, in separate experiments. Scanning electron microscopy was used to identify microstructural features that were vulnerable to metastable pitting on a sub-feature length scale, enabled by a series of potentiostatic holds in the passive region where nascent breakdown/repair occurs. No significant trend in corrosion behavior was observed with titanium content variance for specimens polished to a 0.05 m finish within the range of compositions investigated, suggesting that alloy design may be optimized in terms of mechanical and thermal performance brought about by exploring titanium content without consideration on the effects of corrosion. Preferred sites of pit initiation were sensitive to the surface finish, highlighting the delicate balance between the susceptibility of possible favored pitting sites. Results were compared to stainless steel 316L where the passive current density for the FBB8 alloys was found to be favorable, despite slightly lower pitting potentials.

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