Alkaline Attack: How Caustic Solutions Weaken a Famous High-Entropy Alloy’s Armor

High-entropy alloys have spent two decades dazzling materials scientists with a deceptively simple idea: mix five or more metallic elements in roughly equal proportions, and the resulting solid can be stronger, tougher, and more corrosion-resistant than conventional alloys. The most celebrated of these, the equiatomic CrMnFeCoNi alloy often called the Cantor alloy, has become the field’s benchmark material, famous for retaining remarkable fracture resistance at cryogenic temperatures. But strength and toughness are only half of the engineering story. For any alloy destined for chemical plants, alkaline batteries, caustic scrubbers, or energy-storage hardware, the decisive question is what happens at the surface when the metal meets an aggressive electrolyte. A new study published in the Journal of Materials Science by Shuwen Guo, Jiahao Liu, Dianchun Ju, and colleagues at Jiangsu University of Science and Technology and their collaborators now provides one of the most detailed pictures yet of how a hot isostatically pressed version of this alloy behaves when submerged in sodium hydroxide solutions of varying concentration.

The team’s central finding is sobering for anyone hoping the Cantor alloy would shrug off caustic environments indefinitely. Using a battery of electrochemical techniques, the researchers showed that the passive film, the nanometer-thin oxide layer that acts as the alloy’s chemical armor, becomes progressively less stable and less protective as the NaOH concentration rises. In dilute alkaline solutions, the alloy exhibited lower corrosion current densities and lower passive current densities, together with higher interfacial impedance, all classic signatures of a robust, well-behaved barrier layer. As the hydroxide concentration increased, those protective metrics deteriorated in lockstep, indicating that the film thins, dissolves more readily, or conducts charge more easily under the harshest conditions tested. The work was conducted under short-term, room-temperature conditions, an important caveat that frames the results as a controlled electrochemical snapshot rather than a long-term service-life prediction.

Methodologically, the study is a tour of the modern corrosion-science toolkit. Potentiodynamic polarization swept the alloy’s potential to measure how much current it draws as it anodically dissolves or passivates, revealing the width and quality of the passive region. Electrochemical impedance spectroscopy probed the film’s resistance to charge transfer by applying small alternating-voltage perturbations across a range of frequencies, effectively fingerprinting the barrier layer’s electrical character. Cyclic polarization, in which the potential is driven upward and then reversed, was used to assess susceptibility to localized breakdown such as pitting, by looking for hysteresis between the forward and reverse scans. Finally, Mott-Schottky analysis exploited the semiconductor-like behavior of the passive film itself, measuring how its capacitance changes with applied potential to infer the density and type of electronic defects, donors or acceptors, within the oxide. Defect-rich passive films are generally more vulnerable, because point defects provide pathways for aggressive ions and metal dissolution.

Complementing the electrochemistry, the researchers examined the corroded surfaces with scanning electron microscopy and energy-dispersive X-ray spectroscopy. What they found was a spatial correlation that will resonate with anyone who has studied powder-metallurgy materials: localized surface attack clustered around microstructural heterogeneities containing sulfides and oxides that sit along prior particle boundaries, the seams where powder particles fused together during hot isostatic pressing. These boundary regions, enriched in second-phase particles and impurities inherited from the powder feedstock, appear to be the weak links in an otherwise uniform five-element matrix. The authors are careful, however, not to overclaim: their measurements establish an association between attack sites and these heterogeneities, but they do not determine the exact site or sequence in which corrosion initiates. Distinguishing whether the sulfide inclusions dissolve first and undermine the surrounding film, or whether the film breaks first and exposes the inclusions, remains an open question for future work.

That honesty extends to the chemistry of the passive film itself. The study did not directly determine the film’s composition or the valence states of its constituent elements, so the authors confine any discussion of chromium- and manganese-containing surface species to literature-informed interpretation rather than measured fact. This is a meaningful distinction in the high-entropy alloy field, where a long-standing hypothesis holds that preferential oxidation of chromium, the same element that protects stainless steels, enriches the passive film in stable chromium oxides and hydroxides. Manganese, by contrast, has often been cast as the Cantor alloy’s Achilles heel, because manganese oxides tend to be more soluble and less protective, potentially creating defects in the film. The new results are consistent with such a picture, in which film quality degrades under increasingly caustic conditions, but the authors correctly note that confirming the roles of specific elements requires direct surface spectroscopy, such as X-ray photoelectron spectroscopy, which was not part of the present measurements.

Why does alkalinity matter so much? In acidic and neutral environments, the Cantor alloy has frequently been compared, sometimes favorably, to 304 stainless steel, and prior studies have documented its passivation behavior in sulfuric acid and in carbonate-bicarbonate solutions. Strongly alkaline media change the thermodynamic and kinetic rules of the game. Hydroxide ions participate directly in the dissolution equilibria of many transition-metal oxides, and amphoteric oxides such as those of chromium can dissolve as soluble hydroxo complexes at high pH. At the same time, alkaline environments are industrially ubiquitous: caustic brines flow through chlor-alkali plants, pulp and paper mills, alumina refineries, and numerous waste-treatment streams, and concentrated hydroxide electrolytes are central to alkaline water electrolysis and several battery chemistries. A structural or functional alloy that must survive in these settings cannot be evaluated only in seawater or dilute acid, which makes the present concentration-resolved study a practical addition to the literature.

The manufacturing route adds another layer of significance. Hot isostatic pressing, or HIP, consolidates metal powders by simultaneously applying high temperature and inert-gas pressure, producing dense components with near-wrought quality and complex near-net shapes. For high-entropy alloys, whose constituent elements span a wide melting range, HIP offers a route to large, crack-free bodies that are difficult to cast homogeneously. But powder routes carry their own risks: absorbed gases, surface oxides on the powder particles, and trace sulfur contamination can survive consolidation and decorate the prior particle boundaries. The new study’s observation that localized corrosion associates with sulfide- and oxide-bearing heterogeneities along those boundaries is a direct warning that powder cleanliness and consolidation parameters are not merely mechanical concerns. They are corrosion concerns, because a single vulnerable boundary thread can seed pits that compromise an entire component.

The authors are equally candid about the limits of attribution. Because no cast, wrought, or differently consolidated reference material was examined in parallel, the measured corrosion performance cannot be ascribed to hot isostatic pressing itself. This is a point of genuine scientific discipline in a field where processing comparisons are often made across studies with different powder lots, particle sizes, and test protocols. Prior work has shown that fabrication route matters: comparisons between selective laser melting and casting, between spark plasma sintering and other routes, and between annealed and as-built conditions have all revealed measurable shifts in corrosion behavior, often traceable to grain size, residual stress, segregation, and inclusion populations. A rigorous head-to-head comparison of HIP-consolidated Cantor alloy against cast and wrought counterparts in the same NaOH solutions would be the natural next experiment, and the present study lays the electrochemical groundwork for it.

What emerges from the full picture is a nuanced portrait of a famous material under a demanding class of conditions. The hot isostatically pressed CrMnFeCoNi alloy does form a passive film in sodium hydroxide, and in dilute solutions that film performs respectably, with low dissolution currents and high impedance. But the armor is not invincible: it weakens as hydroxide concentration climbs, and it fails locally where sulfide and oxide heterogeneities interrupt the boundary network left over from powder consolidation. The study also connects to a broader design conversation in the field, where strategies such as cathodic modification to enrich chromium in the oxide layer, deliberate control of nonmetallic inclusions, and passivation pretreatments have all been explored as ways to harden the passive film of CoCrFeMnNi-family alloys. For engineers weighing high-entropy alloys against conventional stainless steels and nickel alloys for alkaline service, the message is neither a rejection nor an endorsement, but a map: the alloy’s corrosion fate in caustic media is governed jointly by solution concentration and by the microstructural fingerprints of how it was made. As high-entropy alloys move from laboratory curiosities toward real components in energy and chemical industries, studies of this kind, precise about what was measured and honest about what was not, are exactly the evidence base that responsible deployment will require.

Subject of Research: Electrochemical corrosion and passivation behavior of hot isostatically pressed CrMnFeCoNi high-entropy alloy in sodium hydroxide solutions

Article Title: Corrosion and passivation behavior of hot isostatically pressed CrMnFeCoNi high-entropy alloy in NaOH solutions

Article References: Guo, S., Liu, J., Ju, D., Chen, Z., Yue, C., Ma, H., & Wu, Z. (2026). Corrosion and passivation behavior of hot isostatically pressed CrMnFeCoNi high-entropy alloy in NaOH solutions. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13865-8

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13865-8

Keywords: high-entropy alloys, CrMnFeCoNi, Cantor alloy, corrosion, passivation, sodium hydroxide, hot isostatic pressing, electrochemical impedance spectroscopy, Mott-Schottky analysis, localized corrosion, prior particle boundaries, passive film