Nanoscale competition between structure and chemistry governing corrosion in pseudo high-entropy amorphous alloys

High-entropy (HE) Fe–Cr–B–based amorphous alloys are known to retain remarkable corrosion resistance even after the onset of crystallization, forming multicomponent bcc or bcc + fcc solid solutions without early boride precipitation. In the Mn-containing HE system (Fe 0.25 Co 0.25 Ni 0.25 Cr 0.125 Mn 0.125 ) 100-x B x (x = 9–13 at%), Mn enhances glass-forming ability, enabling high thermal stability and mechanically robust annealed states characterized by nanoscale mixtures of crystalline and residual amorphous phases. Here, we systematically investigate how nanoscale structure and local chemistry govern corrosion behavior across fully amorphous, partially crystallized, and fully crystalline states. Fully amorphous ribbons were produced by melt spinning for all compositions, followed by controlled annealing to induce crystallization. Electrochemical measurements in chloride-rich media (0.6 M NaCl) reveal that all amorphous alloys exhibit remarkable corrosion resistance, as demonstrated by potentiodynamic polarization and electrochemical impedance spectroscopy. X-ray photoelectron spectroscopy shows that this performance originates from a uniform, nanometric passive film composed of mixed Fe-, Co-, Ni-, Cr-, and Mn-derived oxides and hydroxides. In contrast, partial crystallization leads to a pronounced degradation in corrosion resistance, despite the absence of Cr-rich borides. This deterioration is traced to a chemistry-dominated nanoscale effect, in which Mn promotes the accumulation of deleterious Mn–O-rich species within the passive layer, while early-stage crystallization induces Cr partitioning and local depletion at crystal–matrix interfaces. These results demonstrate a transition from structure-dominated corrosion resistance in the amorphous state to chemistry-driven degradation upon crystallization in (Fe 0.25 Co 0.25 Ni 0.25 Cr 0.125 Mn 0.125 ) 100-x B x alloys, providing nanoscale insights that may guide the design of corrosion-resistant high-entropy alloys under similar conditions. • Fully amorphous Fe–Cr–Mn–B HE alloys exhibit excellent corrosion resistance in NaCl • Amorphous disorder stabilizes a uniform nanometric mixed-oxide passive film • Partial crystallization causes sharp corrosion degradation without boride formation • Mn–O enrichment and local Cr depletion destabilize passive films upon crystallization • Corrosion control shifts from structural disorder to nanoscale chemistry effects