Tiny Doses of Molybdenum Make a Corrosion-Fighting Alloy Stronger and More Resistant

High-entropy alloys have spent the better part of two decades promising a new way to think about metals. Instead of building an alloy around one dominant element with small sprinklings of others, these materials mix four, five, or more principal elements in roughly comparable proportions, producing crystalline structures that defy the conventional rules of alloy design. Now a team of researchers in China has shown that in one such alloy, a surprisingly small adjustment of a single element, molybdenum, can simultaneously reshape the material’s internal architecture, boost its strength, and change how it behaves in one of the most hostile environments metals routinely face: salty seawater.

The study, published in the Journal of Materials Science, examined a family of alloys with the chemical formula AlCrFeNi3Cu0.4Mox, where the molybdenum content x was varied from zero to 0.4. The researchers, led by Zhuoyue Chen of Qingdao University together with colleagues at Shandong University of Science and Technology, the Institute of Oceanology of the Chinese Academy of Sciences, and Qingdao Zitn Micro-electronics, tested the as-cast alloys both mechanically and electrochemically in a 3.5 weight percent sodium chloride solution, the standard laboratory stand-in for seawater. Their findings reveal a delicate trade-off between strength and corrosion resistance that hinges on the fine details of which phases form inside the alloy and how those phases interact electrically with one another.

To understand why this matters, it helps to picture what the base alloy looks like at the microscopic scale. Without molybdenum, the AlCrFeNi3Cu0.4 alloy solidifies into a dendritic structure, the tree-like branching pattern familiar to anyone who has watched frost grow on a window. The arms of these dendrites are made of a face-centered cubic phase, a crystal structure in which atoms sit at the corners and centers of each face of a cube. In the spaces between the dendrite arms, the interdendritic regions, the alloy contains an ordered body-centered cubic phase known as B2, along with nanoparticles of a body-centered cubic phase. This partitioned landscape, where different crystal structures coexist side by side, is characteristic of many high-entropy alloys and is precisely what gives designers levers to pull: each phase contributes its own mechanical properties and its own electrochemical behavior.

When the team began adding molybdenum, the first and most dramatic effect was on the alloy’s phase inventory. Molybdenum, it turns out, is a powerful promoter of the sigma phase, a hard, brittle intermetallic compound that forms in many transition-metal systems. As the molybdenum content rose, sigma phase precipitation increased throughout the microstructure. From a mechanical standpoint, this was good news. The yield strength and hardness of the alloy both climbed as the sigma phase spread, because the hard intermetallic particles impede the movement of dislocations, the tiny line defects whose motion is what allows a metal to deform plastically. For applications where a component must resist denting, scratching, or permanent bending, that extra hardness is a genuine asset.

But corrosion resistance told a more complicated story, and it is here that the study makes its most interesting contribution. When the researchers examined the corroded surfaces and mapped how the constituent elements were distributed across the microstructure, they found evidence that micro-galvanic interactions were driving the degradation. In essence, the different phases in the alloy behave like electrodes of slightly different electrical potential packed into intimate contact. Where the potentials differ, the less noble phase becomes the anode and preferentially dissolves, while the more noble phase is protected. In these alloys, the B2 phase in the interdendritic regions was the loser in this electrochemical contest, dissolving preferentially when the alloy was exposed to the chloride solution. Chloride ions are notoriously aggressive because they penetrate and destabilize the thin oxide films that normally protect metals, so any microstructural feature that concentrates attack is a serious liability in marine service.

What makes the results genuinely striking is that the corrosion behavior did not improve or worsen in a simple, linear fashion as molybdenum was added. Instead, the response was nonlinear, and the sweet spot turned out to be the intermediate composition, AlCrFeNi3Cu0.4Mo0.2. Among all the alloys investigated, this composition exhibited the lowest corrosion current density, meaning the least material loss per unit area per unit time when freely corroding, and the lowest passive current density, meaning that once its protective film formed, the least current was needed to maintain that film. In practical terms, the mid-range molybdenum alloy corroded more slowly and held its protective layer more tenaciously than either the molybdenum-free base alloy or the higher-molybdenum variants.

The explanation for this optimum lies in two complementary mechanisms. First, adding a moderate amount of molybdenum reduced the fraction of the vulnerable B2 phase in the interdendritic regions, shrinking the anodic area available for preferential dissolution. Second, and perhaps more significantly, the passive film that formed on the Mo0.2 alloy was chemically and electronically superior to the films on the other compositions. Using surface analysis, the researchers found that the protective film on this alloy contained a higher proportion of oxides containing chromium and molybdenum. Chromium oxides are the backbone of passivity in stainless steels, forming a dense, adherent barrier that slows further oxidation, and molybdenum oxides are known to enhance the stability of such films in chloride-containing environments. The film on the optimal alloy also displayed a lower point-defect density, a measure of how many vacancies and other imperfections riddle the oxide layer. According to the point-defect model of passivity developed by electrochemists over recent decades, a film with fewer defects is a better barrier, because ions and electrons must migrate through the oxide for corrosion to proceed, and defects are the highways that make that migration easy.

The nonlinear trend also carries a cautionary note for alloy designers. Pushing molybdenum beyond the optimal level apparently did not continue to improve corrosion resistance, and the study suggests that excessive additions can be counterproductive. This is consistent with a broader pattern in the high-entropy alloy literature: molybdenum tends to improve passivity in several alloy families, including the FeCoCrNiMox system studied extensively in recent years, but it also promotes the formation of brittle or anodically active secondary phases, and beyond some threshold the microstructural penalty outweighs the film-quality benefit. The present work quantifies that balance for the AlCrFeNi3Cu0.4 family and identifies where the balance tips.

The practical implications extend to any application where a structural metal must survive contact with seawater: ship hulls and propellers, offshore platforms, desalination plants, coastal infrastructure, and heat exchangers that circulate ocean water. Conventional stainless steels and nickel-based alloys serve these markets today, but high-entropy alloys offer a combination of strength, hardness, and potentially superior corrosion behavior that has kept researchers pushing to make them manufacturable and affordable at scale. The AlCrFeNi3Cu0.4Mox system is attractive in this respect because it is based on relatively inexpensive constituent elements compared with cobalt-heavy compositions, and because the as-cast material, with no need for elaborate thermomechanical processing, already delivers the improved properties. The finding that a modest 0.2 molar fraction of molybdenum delivers the best of both worlds, higher strength through sigma phase precipitation and better corrosion resistance through a leaner B2 fraction and a richer, denser passive film, gives engineers a concrete composition to target.

There is also a conceptual lesson here that reaches beyond this particular alloy family. The study demonstrates that in multiphase high-entropy alloys, corrosion is not simply a bulk chemical property that can be tuned by adjusting elemental ratios. It is governed by the interplay between phase fractions, phase chemistry, and the galvanic couples that form when dissimilar regions sit side by side in an electrolyte. A composition change that strengthens the alloy by precipitating a new phase may simultaneously create new anodic weak points, and only by examining both the mechanical and electrochemical consequences together, as this team did, can designers find compositions that genuinely improve overall performance. As high-entropy alloys move closer to real-world deployment in marine and energy infrastructure, studies of this kind, mapping the full property landscape across composition space rather than testing isolated compositions, will be the roadmap that guides the field from laboratory curiosity to engineering material.

Subject of Research: Effect of molybdenum additions on the microstructure, mechanical properties, and corrosion passivity of AlCrFeNi3Cu0.4Mox high-entropy alloys in chloride solution

Article Title: Effect of Mo addition on microstructure, mechanical properties, and passivity of AlCrFeNi3Cu0.4Mox high-entropy alloys

Article References: Effect of Mo addition on microstructure, mechanical properties, and passivity of AlCrFeNi3Cu0.4Mox high-entropy alloys. (n.d.). https://doi.org/10.1007/s10853-026-13855-w

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13855-w

Keywords: high-entropy alloys, molybdenum, corrosion resistance, passive film, sigma phase, micro-galvanic corrosion, yield strength, seawater, B2 phase, chloride, point-defect density, alloy design