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Selective leaching

Selective leaching, also known as dealloying, demetalification, or selective corrosion, is a type of corrosion process in which one or more constituent elements of a solid solution alloy are preferentially dissolved and removed, resulting in a porous, brittle residue composed primarily of the more noble metal.^[1] This phenomenon typically occurs in alloys exposed to corrosive environments, such as aqueous solutions, where the less noble (more reactive) component is selectively attacked, leading to significant degradation of the material's mechanical integrity without substantial change to its external appearance.^[2] The mechanism of selective leaching involves the anodic dissolution of the active element, often accompanied by the cathodic deposition or retention of the noble element, creating a spongy microstructure that compromises strength and ductility. Common examples include dezincification in brass alloys, where zinc is leached out, leaving a weakened copper-rich structure; dealuminification in aluminum bronzes, involving the removal of aluminum; and graphitic corrosion in gray cast iron, where iron is selectively dissolved, leaving a graphite skeleton that retains the component's shape but renders it friable and prone to crumbling under stress.^[1] These processes are driven by factors such as alloy composition, environmental conditions (e.g., pH, chloride presence, temperature), and exposure duration, with chloride ions often accelerating the reaction in brasses and bronzes.^[3] Selective leaching poses notable risks in industrial applications, particularly in piping, pumps, valves, and heat exchangers used in water systems, marine environments, and chemical processing, where it can lead to premature failure and safety hazards.^[4] Prevention strategies include using stabilized alloys (e.g., arsenic- or antimony-inhibited brasses), applying protective coatings, selecting more resistant materials like stainless steel or cupronickel, and controlling environmental factors through inhibitors or cathodic protection.^[2] Despite its destructive nature, controlled selective leaching has been explored in materials science for fabricating nanoporous metals with applications in catalysis, sensors, and energy storage, though such uses remain experimental.

Introduction

Definition and Process Overview

Selective leaching, also known as dealloying, demetalification, or parting, is a form of corrosion that involves the preferential removal of one or more elements from a solid solution alloy, resulting in a porous residue enriched with the more noble component.^[5] This process typically affects alloys where the elements have significantly different electrochemical potentials, leading to the selective dissolution of the less noble metal while the more noble one remains largely intact. A common example is the loss of zinc from brass alloys.^[6] The basic process of selective leaching initiates at the alloy surface upon exposure to an electrolyte, where the less noble metal undergoes anodic dissolution according to the general reaction:

\text{M} \rightarrow \text{M}^{n+} + n\text{e}^-

where M represents the less noble metal and n is the number of electrons transferred.^[7] This selective dissolution is followed by surface diffusion of the remaining more noble atoms, which reorganizes the structure and leads to the formation of pores or voids. The process then propagates into the bulk material, creating a layered or uniform porous morphology depending on the conditions.^[6] Key characteristics of selective leaching include its occurrence in aqueous electrolytes or aggressive environments such as acidic solutions, chloride-containing media, or high-temperature conditions. The rate and extent of the process are influenced by factors including alloy composition (e.g., higher content of the active element increases susceptibility), pH, temperature, and applied or natural electrochemical potential. The resulting structure is often a bicontinuous nanoporous network with ligament and pore sizes on the order of nanometers to micrometers, providing high surface area but compromising material integrity.^[6]^[8]

Historical Development

The earliest observations of selective leaching, specifically dezincification in brass pipes, emerged during the Industrial Revolution in the mid-19th century, when brass alloys were widely adopted for plumbing and condenser tubes in emerging water supply and steam engine systems. The phenomenon was first formally reported in 1866, highlighting the selective removal of zinc from copper-zinc alloys exposed to aggressive environments like seawater or chlorinated water, leading to material weakening and failures in industrial infrastructure.^[9] By the 1920s, selective leaching gained recognition as a distinct corrosion mechanism through the pioneering work of metallurgists such as Ulick Richardson Evans, who formalized its electrochemical principles in his seminal 1924 book The Corrosion of Metals.^[10] Evans' research emphasized the role of anodic dissolution of less noble elements in alloys, providing a theoretical framework that distinguished selective leaching from uniform corrosion and influenced subsequent studies on alloy durability. In the 1960s and 1970s, attention shifted to graphitic corrosion—a form of selective leaching in cast iron infrastructure—amid widespread concerns over aging urban water mains. Studies documented the preferential removal of iron, leaving behind a porous graphite matrix that compromised structural integrity in buried pipes, prompting advancements in corrosion-resistant coatings and alloy modifications.^[11] From the 1980s onward, research transitioned from viewing selective leaching solely as a destructive process to harnessing it intentionally for materials synthesis via dealloying. A key milestone was the work of Jonah Erlebacher in the 2000s, whose 2001 paper on the evolution of nanoporosity in dealloying Ag-Au alloys demonstrated controlled formation of nanoporous gold with tunable structures, opening avenues for applications in catalysis and sensing.^[12] In the 2020s, selective leaching has integrated with sustainable metallurgy, exemplified by 2024 studies on reactive vapor-phase dealloying-alloying processes that convert metal oxides into CO2-free nanoporous alloys using ammonia, achieving high porosity and hardness while eliminating carbon emissions in production.^[13]

Types and Examples

Dezincification in Copper Alloys

Dezincification is the selective removal of zinc from copper-zinc alloys, particularly alpha and beta brasses, leaving behind a porous, copper-rich matrix that compromises the material's integrity while preserving its external shape. This process transforms the alloy's microstructure, often resulting in a reddish appearance due to the exposed copper. Copper-zinc alloys containing more than 15% zinc are especially susceptible, as the electrochemical potential difference drives preferential zinc dissolution.^[14]^[15] The phenomenon typically arises in environments such as soft waters with low mineral content, solutions high in chlorides, or those containing ammonia. It is exacerbated by stagnant conditions, differential aeration from deposits, and temperatures exceeding 50°C, with acceleration also noted at pH levels below 7 or in neutral to alkaline settings with elevated salt concentrations. Two distinct microstructural morphologies occur: layer-type dezincification, which uniformly depletes zinc across a broad surface layer, and plug-type dezincification, which manifests as localized, plug-like penetrations into the alloy.^[14]^[16]^[17] Industrially, dezincification has led to significant failures in plumbing systems, where porous brass fittings develop leaks, and in heat exchangers, such as air compressor cooler tubes that failed after 17 years of service due to tube wall thinning. Historical cases include corrosion in 19th-century maritime applications, like brass sheathing and fittings on wooden ships exposed to seawater, where dezincification contributed to structural degradation in salty environments. A notable example is Admiralty brass (approximately 70% Cu, 30% Zn, 1% Sn), which exhibits layer-type dezincification in seawater heat exchanger environments at temperatures of 31–49°C and pH around 8, underscoring its vulnerability despite alloying additions intended for marine use.^[14]^[18]^[19]

Graphitic Corrosion in Cast Iron

Graphitic corrosion, also known as graphitization, is a selective leaching process in gray cast iron where the ferritic iron matrix dissolves preferentially, leaving behind an interconnected network of graphite flakes embedded in corrosion products such as iron oxides.^[20] This results in a conductive graphite skeleton that maintains the original external dimensions of the material but severely compromises its structural integrity.^[21] The process occurs because graphite acts as a noble cathode in the galvanic couple with the anodic iron, facilitating the electrochemical dissolution of iron without affecting the graphite volume.^[22] This form of corrosion is prevalent in environments such as soils or waters with high oxygen availability and low pH, which accelerate the iron dissolution, particularly in low-resistivity soils like clays containing elevated chlorides or sulfates.^[20] It commonly affects underground pipelines and marine structures, where intermittent moisture and electrolyte presence exacerbate the reaction.^[21] Unlike passive oxide layers that might protect other metals, the graphite's nobility prevents any protective passivation, allowing continued leaching and further erosion of the residual structure.^[22] The morphology of graphitically corroded cast iron appears as a soft, spongy black residue with localized pits or thin layers (1-2 mm thick), forming a macro-porous network that retains the component's shape but exhibits drastically reduced mechanical strength.^[20] This spongy residue is prone to mechanical failure under load or pressure, as the graphite flakes provide minimal load-bearing capacity compared to the original iron matrix.^[21] Industrial examples include the failure of cast iron gas mains due to graphitic corrosion, such as 12-inch mains in Philadelphia (installed 1942, failed 2011) and Allentown (installed 1928, failed 2011), which burst after decades of burial.^[20] More recent incidents, like the 2012 St. Louis gas pipeline incident, highlight ongoing risks in aging infrastructure.^[20] These failures contribute to substantial economic impacts, with water main breaks in North America—many attributable to corrosion in cast iron pipes—costing an estimated $2.6 billion annually in repairs and disruptions.^[23] In distinction from uniform corrosion, graphitic corrosion's selective nature preserves the graphite volume, resulting in macro-porosity and hidden strength loss without significant external thinning, often leading to sudden brittle fractures.^[22] Historical observations of this phenomenon in early cast iron castings date back to the 19th century, underscoring its long-recognized role in material degradation.^[21]

Selective Leaching of Other Elements

Dealuminification in aluminum bronzes (copper-aluminum alloys) involves the selective dissolution of aluminum, leaving a weakened, porous copper-rich structure. This form of dealloying commonly affects marine components such as propellers, pumps, and valves exposed to seawater or chloride-rich environments, where the more active aluminum is preferentially removed due to its lower nobility compared to copper. Factors like alloy composition (typically 5-12% Al) and microstructure influence susceptibility, with heat-treated alloys showing improved resistance.^[24] In aluminum-silicon (Al-Si) alloys, commonly used in castings such as engine pistons, selective leaching of aluminum, often termed dealuminification-like dissolution, occurs preferentially in alkaline environments due to the amphoteric nature of aluminum, which dissolves as aluminate ions while silicon remains relatively inert.^[25] This process leads to a silicon-enriched surface layer that can compromise structural integrity, particularly in applications exposed to basic coolants or processing solutions, with corrosion rates increasing at pH levels above 12.^[26] Studies on hypoeutectic Al-Si alloys have shown that the eutectic silicon particles act as cathodic sites, accelerating the anodic dissolution of the aluminum matrix in sodium hydroxide solutions.^[27] In nickel-copper alloys like Monel 400, selective leaching of nickel predominates in acidic conditions, such as those encountered in sulfuric acid plants or acetate solutions, where nickel forms soluble complexes and is preferentially removed, leaving a copper-enriched, porous structure prone to cracking.^[28] This dealloying mechanism is exacerbated by the higher nobility of copper compared to nickel, resulting in intergranular propagation and reduced mechanical strength after prolonged exposure to dilute sulfuric acid at concentrations around 5-10 g/L.^[29] Corrosion tests in 50 g/L CuSO4 and 5 g/L NiSO4 solutions simulating nickel refining leachates have demonstrated weight losses up to 0.5 mm/year for unprotected Monel in such environments.^[30] For aluminum-zinc-magnesium (Al-Zn-Mg) alloys prevalent in aerospace components, selective leaching of aluminum can initiate pitting corrosion through zinc enrichment on the surface, as aluminum's lower nobility drives its dissolution in chloride-containing electrolytes, forming a galvanically active Zn-rich layer that sustains localized attack.^[31] In 7xxx series alloys like 7050-T74, variations in Zn/Mg ratios influence this process, with higher zinc content promoting faster pitting propagation depths exceeding 100 μm after immersion in 3.5% NaCl, due to the cathodic nature of the enriched phase.^[32] This phenomenon is particularly critical in aircraft structures, where exfoliation-like pitting reduces fatigue life by up to 30% in humid, saline atmospheres.^[33] Recent studies from 2023 to 2025 on lead-bismuth eutectic (LBE) coolants for nuclear reactors have highlighted phase-selective leaching in stainless steels, where ferrite phases exhibit greater dissolution of iron and chromium compared to austenite, driven by differences in atomic diffusion and oxide formation at 600°C.^[34] In 316L stainless steel exposed to oxygen-controlled LBE, selective removal of nickel from austenitic regions induces ferritization, transforming face-centered cubic structures to body-centered cubic via a discontinuous reaction at grain boundaries, with leaching rates of 10-50 μm/year observed.^[35] These findings underscore compatibility challenges in lead-cooled fast reactors, where even oxygenated conditions fail to fully suppress dealloying, leading to embrittlement.^[36] The underlying factors in these selective leaching processes stem from thermodynamic nobility differences, where elements with more negative standard reduction potentials (e.g., Al at -1.66 V vs. Zn at -0.76 V) are preferentially dissolved, as quantified by Pourbaix diagrams and galvanic series rankings.^[29] In high-entropy alloys (HEAs), this nobility hierarchy extends to multi-principal components, enabling controlled dealloying for porous structures; for instance, in FeCoNiMoZn HEAs, selective removal of less noble Fe and Co via chemical etching yields layered nanoporous architectures with surface areas over 50 m²/g, illustrating how configurational entropy modulates leaching selectivity.^[37] Such examples from refractory HEAs highlight the role of elemental electronegativity differences (Δχ > 0.1) in dictating phase stability during exposure to aggressive media.^[38]

Mechanisms

General Electrochemical Principles

Selective leaching, also known as dealloying, is fundamentally driven by galvanic coupling within the alloy, where the less noble element behaves as the anode and undergoes preferential oxidation and dissolution, while the more noble element serves as the cathode supporting reduction reactions such as hydrogen evolution or oxygen reduction.^[6] This electrochemical disparity arises from differences in standard electrode potentials between the alloy components, leading to localized corrosion at the anodic sites. Mixed potential theory explains this process by positing that the alloy surface establishes a steady-state mixed potential where the total anodic current from the less noble metal's dissolution equals the total cathodic current on the noble metal framework, preventing net charge accumulation and enabling sustained selective leaching.^[6]^[39] Surface diffusion plays a pivotal role in the reorganization of the remaining noble metal atoms, which exhibit adatom mobility across the emerging surface to form interconnected ligaments that maintain structural integrity and prevent catastrophic bulk dissolution of the alloy.^[6] This diffusion-mediated process allows noble metal adatoms to migrate and coalesce, minimizing surface energy and creating a bicontinuous nanoporous morphology.^[40] Without sufficient adatom mobility, the structure would collapse into discrete particles rather than a coherent network, underscoring surface diffusion's critical function in propagating the dealloying front while preserving the noble phase.^[41] The propagation of selective leaching can follow volume-conserving models, where the external dimensions of the material remain largely unchanged due to the comparable atomic volumes of the alloy elements and efficient rearrangement via surface diffusion, or shrinkage models, characterized by volumetric contraction (typically 2-25%) from the removal of the less noble element, leading to densification and potential cracking.^[42] Underpotential deposition (UPD) of the noble metal influences these models by stabilizing the surface layer at potentials below the bulk dissolution threshold, promoting selective propagation over uniform corrosion and enhancing ligament formation in systems like Ag-Au alloys.^[42] The potential difference driving this process is governed by the Nernst equation:

E = E^0 - \frac{RT}{nF} \ln Q

applied to the alloy's dissolution, where E is the electrode potential, E^0 the standard potential, R the gas constant, T temperature, n electrons transferred, F Faraday's constant, and Q the reaction quotient reflecting local concentrations during selective leaching.^[43] This equation quantifies how compositional changes at the interface shift the equilibrium potential, sustaining the galvanic drive.^[44] Key factors modulating the rate and morphology of selective leaching include electrolyte composition, which alters ion availability and pH to influence dissolution kinetics; flow rate, which affects mass transport and removal of dissolved species to prevent passivation; and overpotential, which determines the onset and extent of anodic dissolution relative to the critical potential for bulk dealloying.^[43]^[45] Higher overpotentials accelerate propagation but may induce shrinkage, while optimized electrolyte flow minimizes stagnation and promotes uniform leaching.^[6]

Advanced Dealloying Techniques

Advanced dealloying techniques extend beyond traditional aqueous electrochemical processes by employing alternative media and conditions to achieve precise control over selective leaching, enabling the fabrication of complex nanostructures in materials such as intermetallics and high-entropy alloys (HEAs). Liquid metal dealloying (LMD) utilizes molten metals such as magnesium or bismuth to selectively dissolve more reactive components, such as iron, from intermetallic precursors at elevated temperatures. This method leverages differences in solubility driven by mixing enthalpies to form three-dimensional bicontinuous porous structures with interconnected ligaments. In the 2020s, advancements in LMD have focused on scaling to bulk 3D architectures, such as heterostructured composites, by optimizing dealloying temperatures (e.g., 500–700°C) and precursor compositions to minimize phase separation and enhance uniformity. For instance, partial LMD using magnesium- or bismuth-based systems has produced nickel-containing porous ferrous alloys with ligament sizes tunable to 50–200 nm, demonstrating improved mechanical integrity over aqueous counterparts.^[46] Non-aqueous methods further innovate by avoiding water-related limitations like hydrogen evolution, using solvents such as ionic liquids or deep eutectic solvents (DES) for electrochemical dealloying. In ChCl-urea DES, HEAs like FeCoNiAlMo undergo selective dissolution of the less noble face-centered cubic (FCC) phase, preferentially corroding aluminum and nickel to yield 3D porous structures with enhanced electrocatalytic activity. These processes operate at room temperature or mildly elevated conditions, reducing energy input while achieving overpotentials as low as 312 mV at 10 mA/cm² for oxygen evolution reaction (OER). Thermal shock techniques, involving rapid heating-cooling cycles (e.g., 2000–3000 K in milliseconds), have been adapted for HEAs to induce localized dealloying in multi-principal element systems, promoting uniform porosity without prolonged exposure. This approach exploits transient thermal gradients to accelerate diffusion-limited leaching in alloys like PtFeCoNiCu, yielding sub-10 nm features suitable for catalysis.^[47]^[38] Porosity development in these techniques proceeds through initial void nucleation at alloy surfaces, followed by ligament formation via surface diffusion of the nobler metal, and subsequent coarsening that governs final morphology. Ligaments evolve from atomic-scale clustering to interconnected networks (10–100 nm scales), where pore size is controlled by dealloying potential and time; lower potentials (e.g., 0.4 V) yield finer pores (~5 nm) due to restrained dissolution kinetics. Coarsening follows a surface diffusion model, with ligament diameter l scaling as l \propto t^{1/4}, where t is time, reflecting atom migration to minimize surface energy. In situ grazing-incidence small-angle X-ray scattering (GISAXS) studies of systems like AgAu and CoPd confirm this, showing retarded coarsening in higher-binding-energy alloys (e.g., CoPd with ~3 nm ligaments) compared to AgAu (~10 nm). Models incorporating activation energies (e.g., 0.5–1.0 eV for surface diffusion) enable predictive control of pore sizes in the 10–100 nm range for targeted applications.^[48]^[49] Recent innovations from 2023–2025 highlight hybrid approaches for specialized alloys. Ammonia-assisted vapor-phase dealloying converts oxide precursors (e.g., Fe₂O₃-NiO) into nanostructured porous Fe-Ni-N martensitic alloys at 700°C, using NH₃ to simultaneously reduce oxides and dope nitrogen (up to 3 at% at defects), achieving 28.9% interconnected porosity with 1.12 GPa hardness. Solvothermal dealloying, involving precursor synthesis in high-pressure solvents followed by selective leaching, has been applied to high-entropy alloys for electrocatalysts with enhanced hydrogen evolution reaction (HER) performance. As of September 2025, advances include dealloying of Ti-based alloys for biomedical nanoporous structures and electrochemical/chemical dealloying for nanoporous anodes in lithium-ion batteries, expanding non-aqueous and thermal shock mechanisms.^[13]^[50] The dealloying rate v is often expressed as v = k \cdot i_a, where i_a is the anodic current density and k incorporates Faraday constants, molar mass, density, and diffusion coefficients (e.g., k \propto D^{1/2} in diffusion-limited regimes), allowing kinetic tuning via electrolyte or temperature adjustments.^[51]

Effects on Materials

Impact on Mechanical Properties

Selective leaching, through the formation of a porous network, significantly reduces the load-bearing cross-section of affected materials, leading to a loss of cohesion and overall embrittlement. In copper-zinc alloys like brass, dezincification selectively removes zinc, leaving behind a spongy copper structure that weakens interatomic bonds and promotes brittle failure under load. This process diminishes the material's ability to distribute stress evenly, resulting in reduced tensile strength and increased susceptibility to cracking. The mechanical strength of dealloyed materials can be modeled using a modified Hall-Petch relation, where yield strength \sigma_y = \sigma_0 + k / \sqrt{d} accounts for nanoporosity, with d representing the ligament size between pores; smaller ligaments enhance strength due to grain boundary-like strengthening effects at the nanoscale. This relation highlights how the bicontinuous nanoporous architecture confines dislocation activity, elevating yield stresses to near-theoretical limits in materials like nanoporous gold. However, this strengthening comes at the cost of reduced ductility, as the porous structure limits plastic deformation.^[52] Stiffness in dealloyed materials decreases markedly due to density loss from pore formation, following scaling laws such as the Gibson-Ashby relation where the elastic modulus E scales with relative density \phi as E \propto \phi^n (with n \approx 2). For instance, nanoporous gold typically exhibits an elastic modulus of 10-30 GPa, compared to the bulk value of approximately 80 GPa, reflecting the diminished solid fraction and altered load paths through the ligament network.^[53] Pores in dealloyed structures obstruct dislocation motion, akin to Orowan strengthening mechanisms at the nanoscale, where dislocations bow around obstacles, increasing yield strength but severely limiting ductility by promoting early strain localization. This blockage confines slip to narrow ligaments, reducing the material's capacity for uniform elongation and fostering brittle fracture modes.^[6] Specific detrimental effects include fatigue crack initiation at pore bases, where stress concentrations amplify cyclic loading to nucleate microcracks that propagate through the weakened network. In biomedical applications, recent studies on dealloyed titanium alloys reveal increased brittleness, with nanoporous surfaces showing yield strengths around 35 MPa and moduli of 3-24 GPa, compromising implant durability despite improved osseointegration.^[50]

Detection and Testing Methods

Selective leaching in metals can often be initially detected through visual and macroscopic examinations, which reveal characteristic changes in appearance and texture. For instance, dezincification of red brass alloys typically manifests as a shift in color from the original yellow to a pink or reddish hue due to the preferential removal of zinc, leaving behind a copper-rich matrix.^[19]^[14] Simple macroscopic tests, such as probing the material's surface for increased softness or brittleness, can indicate underlying porosity and loss of integrity, as the leached structure becomes more friable compared to the intact alloy.^[54] Microscopic techniques provide detailed insights into the elemental composition and structural alterations caused by selective leaching. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) enables elemental mapping, revealing zinc-depleted zones and the formation of porous copper networks in dezincified brasses, with EDX confirming reduced zinc concentrations in affected areas.^[55] Transmission electron microscopy (TEM) is particularly effective for visualizing nanopores at the atomic scale, capturing the evolution of bicontinuous ligament-pore structures in dealloyed materials like nanoporous gold or tin.^[56] These methods quantify the extent of leaching by measuring pore sizes and distribution, often showing ligaments in the range of 10-50 nm.^[57] Electrochemical methods offer quantitative assessment of selective leaching by analyzing shifts in material behavior. Potentiodynamic polarization curves can detect nobility changes, where the corrosion potential shifts positively as less noble elements like zinc are selectively dissolved, indicating dealloying progression in alloys such as Cu-Zn.^[58] Electrochemical impedance spectroscopy (EIS) evaluates porosity development in corroded surfaces, with impedance spectra showing increased low-frequency resistance due to porous layers that hinder charge transfer, allowing estimation of pore fraction and corrosion product coverage.^[59] These techniques are sensitive to early-stage leaching without requiring sample destruction. Non-destructive testing (NDT) methods are essential for in-situ evaluation of larger components. Ultrasonic testing measures density loss from selective leaching by detecting changes in wave propagation velocity and attenuation, which increase as material porosity rises, enabling thickness mapping of corroded layers in pipelines or structural alloys.^[60] X-ray computed tomography (CT), particularly synchrotron-based nano-tomography, reconstructs three-dimensional pore networks in dealloyed materials, quantifying interconnectivity and volume fraction of voids with resolutions down to 50 nm, as seen in molten salt dealloying of Ni-Cr alloys.^[61] Standardized protocols ensure consistent detection across industries. ISO 6509 provides guidelines for determining dezincification depth through metallographic cross-sections of exposed copper alloys containing zinc, measuring the maximum penetration depth of affected layers (e.g., often limited to <200 μm for acceptable performance in certain applications).^[62] For nuclear applications, 2025 U.S. Nuclear Regulatory Commission (NRC) protocols mandate one-time inspections using visual, hardness, and ultrasonic methods to assess selective leaching in gray cast iron and bronze components, with follow-up if loss exceeds 10% thickness.^[63] These standards integrate multiple techniques for comprehensive risk evaluation.

Prevention Strategies

Alloy Composition Modifications

One effective approach to mitigating selective leaching involves tailoring the alloy's elemental composition to reduce the electrochemical potential differences between constituent elements, thereby inhibiting preferential dissolution. In copper-zinc alloys prone to dezincification, the addition of minor inhibiting elements such as arsenic (As), antimony (Sb), or phosphorus (P) at levels of 0.02-0.06 wt% forms protective films or alters the dissolution kinetics of zinc, significantly enhancing resistance even in brasses with up to 40 wt% zinc.^[64] These additives promote uniform corrosion over selective attack, as demonstrated in standardized alloys like CW602N, which meet European norms for dezincification resistance.^[65] Phase composition plays a critical role in susceptibility, with single-phase alpha-brass (typically 15-40 wt% zinc) exhibiting lower dezincification risk compared to dual-phase alpha-beta brasses (higher zinc content), where the beta phase undergoes faster zinc depletion due to its more active electrochemical behavior.^[65] To further enhance nobility, increasing the copper content—limiting zinc to below 15 wt%—or incorporating more noble elements reduces the driving force for selective leaching, as seen in low-zinc admiralty brasses used in condenser tubes.^[64] In stainless steels susceptible to chromium-selective leaching, duplex grades with balanced ferrite-austenite microstructures (e.g., 2205 alloy) minimize phase-specific corrosion by equalizing nobility and preventing deleterious precipitation, offering superior performance in chloride environments.^[66] Similarly, for gray cast irons vulnerable to graphitic corrosion, using specialized alloys such as austenitic Ni-Resist cast irons (containing 13-36 wt% nickel) or high-silicon cast irons (14-17 wt% silicon), which form protective matrices or silica-rich layers that inhibit iron dissolution around graphite flakes, preserving structural integrity.^[67]^[68] Recent advancements include high-entropy alloys (HEAs), which leverage multiple principal elements with near-equal atomic ratios to achieve balanced nobility and sluggish diffusion, thereby resisting dealloying across diverse environments. However, these compositional strategies involve trade-offs, such as elevated costs from rare alloying elements—dezincification-resistant brasses (DZR) compliant with plumbing standards like ASTM B584 can cost 20-50% more than standard yellow brass—balanced against extended service life in corrosive waters, justifying their adoption in critical infrastructure.^[69]

Protective Coatings and Inhibitors

Protective coatings serve as physical barriers to prevent the ingress of corrosive agents that initiate selective leaching in alloys. For cast iron, epoxy coatings are widely applied due to their superior adhesion, chemical resistance, and ability to isolate the substrate from moisture and electrolytes, thereby mitigating graphitic corrosion.^[70] Metallic overlays such as hot-dip zinc coatings provide cathodic protection by acting as sacrificial layers that preferentially corrode over the underlying iron matrix. Barrier coatings like thin dense chrome isolate the substrate from corrosive environments.^[71] In aluminum alloys, anodizing processes, particularly sulfuric acid anodizing, thicken the natural oxide layer to form a durable, porous barrier that enhances resistance to dealloying in chloride-rich environments.^[72] Chemical inhibitors offer molecular-level protection by adsorbing onto alloy surfaces to block active sites prone to selective dissolution. Benzotriazole (BTA) is a prominent organic inhibitor for copper-based alloys, forming a stable, polymeric Cu-BTA complex film that suppresses dezincification in brass by inhibiting zinc dissolution while allowing controlled copper passivation.^[73] For zinc-containing alloys, chromate conversion coatings create a thin, adherent chromate film through chemical reaction with the zinc surface, providing temporary corrosion resistance by passivating the metal and preventing selective leaching in humid or mildly acidic conditions.^[74] Cathodic protection systems counteract the electrochemical driving force of selective leaching by shifting the alloy's potential to a more noble state. In pipeline applications, sacrificial anodes made from zinc, magnesium, or aluminum alloys are connected to the structure, preferentially corroding to supply electrons and protect the pipeline material from dealloying, particularly in soil or water environments.^[75] Environmental controls adjust system conditions to reduce the aggressiveness of the medium toward selective leaching. In cooling water systems, pH adjustment to neutral or slightly alkaline levels (typically 7.5–9.0) minimizes acid-induced dissolution of alloying elements, while deaeration removes dissolved oxygen to suppress cathodic reactions that accelerate leaching processes.^[76] Recent studies (as of 2025) have explored sustainable green inhibitors derived from plant extracts, such as those containing tannins, for corrosion control in alloys, offering eco-friendly alternatives to synthetic inhibitors.

Applications

Nanoporous Material Fabrication

Selective leaching, particularly through dealloying, enables the fabrication of nanoporous gold (NPG) by selectively dissolving silver from silver-gold (Ag-Au) precursor alloys, resulting in a bicontinuous network of gold ligaments and pores.^[42] The process involves immersing Ag-Au alloys, typically with 20-50 at.% Au, in nitric acid or electrochemical setups to control the dissolution rate, allowing tunable ligament sizes ranging from 5 to 50 nm by adjusting etchant concentration, temperature, or potential.^[77] This controlled dealloying preserves the overall alloy morphology while creating uniform nanoporosity, with the dealloying front propagating through the material to achieve a porosity fraction φ ≈ 0.7-0.9, which correlates directly with the depth of silver removal and the initial alloy composition.^[42] NPG exhibits a high specific surface area of 10-100 m²/g, depending on ligament dimensions and processing conditions, which arises from the exposure of numerous undercoordinated surface atoms that enhance reactivity.^[77] These properties stem from the self-assembly during dealloying, where surface diffusion of gold atoms reorganizes the remaining structure into interconnected nanopores, as detailed in advanced dealloying techniques. The material's conductivity and mechanical robustness, combined with its open architecture, make it suitable for functional applications. In applications, NPG serves as a platform for sensors, leveraging its high surface area for analyte adsorption and detection, and actuators, where surface stress changes induce macroscopic bending or expansion.^[78] Recent studies have demonstrated NPG's efficacy in electrocatalysis, such as 2022 research showing high Faradaic efficiency (>95%) for CO production in CO₂ reduction, attributed to optimized pore structures facilitating mass transport.^[79] Challenges in NPG fabrication include ligament coarsening during annealing or use, which reduces surface area; this is mitigated by alloying with elements like platinum to stabilize the structure through segregation effects.^[80] Scalability remains an issue for macro-scale production, as uniform dealloying across large volumes is hindered by etching inconsistencies and stress-induced cracking.^[77]

Metal Recovery in Hydrometallurgy

Selective leaching in hydrometallurgy enables the targeted extraction of valuable metals like copper, nickel, and cobalt from low-grade ores, mineral concentrates, and electronic wastes, promoting resource efficiency and circular economy principles. The process primarily employs acid leaching with sulfuric acid and oxidants, or bio-leaching with acidophilic bacteria, to dissolve specific metals while leaving gangue materials intact. For example, sulfuric acid oxidation leaching of cobalt from high-silicon white alloy wastes achieves 97.76% cobalt recovery under optimized conditions of 2 mol/L H₂SO₄, 1.5 mol/L NaClO, and 80°C.^[81] Bio-leaching variants utilize mesophilic mixed bacterial populations in air-lift bioreactors to selectively extract copper from black shale ores, enhancing recovery from refractory sources. These methods address the challenges of complex matrices in e-waste, such as printed circuit boards (PCBs), where two-stage acid leaching recovers over 95% copper and subsequent gold extraction. Selectivity is achieved through tailored lixiviants that form stable complexes with target metals. Cyanide solutions selectively dissolve gold from copper-gold ores and concentrates, forming Au(CN)₂⁻, though copper interference is mitigated by additives; recoveries reach 81.79% for gold in cyanidation of tailings. Ammonia enhances selectivity in copper leaching from sulfide ores or wastes, forming soluble ammine complexes like Cu(NH₃)₄²⁺, with efficiencies up to 98% in ammoniacal systems for metallic and oxide copper phases. Recent green alternatives, such as glycine in alkaline media (pH 8–11) with H₂O₂ oxidant, enable selective recovery of copper from mining wastes and sulfides, achieving 100% copper extraction from roasted tailings at room temperature while limiting iron to <25 ppm. For nickel and cobalt, alkaline glycinate leaching from spent lithium-ion batteries (LIBs) yields 91.56% nickel and 83.18% cobalt recovery at 90°C, with minimal dissolution of manganese (31.16%) and impurities like aluminum or iron.^[82] A practical application involves glycine leaching of copper from PCB-derived concentrates, where pre-treatment optimizes >90% efficiency for base metals prior to precious metal recovery. Leaching kinetics are modeled to predict efficiency and optimize conditions, often following the form rate = k [H⁺]ᵃ [metal]ᵇ, where k is the rate constant, and exponents reflect reaction dependencies. For cobalt extraction via sulfuric acid, the process is surface chemical reaction-controlled, with an activation energy of 34.58 kJ/mol and the integrated rate equation 1 - (1 - α)^{1/3} = k t, where α is the fractional conversion. In glycinate systems for LIBs, activation energies range from 45–61 kJ/mol for nickel, cobalt, and lithium, confirming chemical control and enabling >90% overall efficiencies in 3 hours. These models guide industrial scaling, as seen in PCB processing where Avrami kinetics predict 99.47% copper recovery under optimized agitation and temperature. Environmentally, selective hydrometallurgical leaching outperforms smelting by operating at ambient to moderate temperatures (20–90°C), reducing energy use by up to 50% and cutting CO₂ emissions significantly compared to high-temperature pyrometallurgy (800–1100°C). For copper recovery, low-acid glycine processes from 2023–2025 minimize reagent toxicity and acid consumption, producing residues suitable for phytoremediation with germination indices >100%, thus lowering overall ecological footprints. Advances in bio- and glycine-assisted leaching further decrease smelting-related emissions by 27–30% in integrated recycling flowsheets, supporting sustainable metal supply from e-waste and low-grade sources.

Emerging Uses in Biomedicine and Catalysis

Selective leaching through dealloying has enabled the development of nanoporous titanium-based alloys for biomedical scaffolds, offering enhanced osseointegration while minimizing toxicity. Dealloying of Ti6Al4V and Ti-Zr alloys produces structures with ligament sizes of 0.75–1.34 µm, facilitating improved cell adhesion and proliferation, such as increased alkaline phosphatase activity in MC3T3-E1 cells by day 14. These scaffolds match bone's mechanical properties, with Young's moduli of 3.2–5 GPa, promoting biomechanical compatibility without adverse ion release. Recent 2025 studies confirm no cytotoxicity in dealloyed Ti alloys, attributing biocompatibility to reduced aluminum and vanadium content—up to 48% lower in Ti6Al7Nb—while maintaining high surface area for tissue integration.^[83] A 2024 advancement in sustainable implant materials involves ammonia-assisted vapor-phase dealloying-alloying of iron and nickel oxides to form Fe-Ni-N porous martensitic alloys. This process yields 28.9% porosity with 1.2 µm ligaments at a density of 5.69 g/cm³, achieving a specific hardness of 2.5 GPa·cm³/g through nanostructured martensite with nano-twins.^[13] The zero-carbon-footprint method uses abundant precursors and produces only water as byproduct, positioning these alloys as eco-friendly options for load-bearing implants with potential for enhanced osseointegration due to their open porosity. In catalysis, dealloying-derived nanoporous high-entropy alloys (HEAs) have shown promise for oxygen reduction reactions (ORR), leveraging lattice strain for improved activity. Nanoporous Al-Cu-Ni-Pt-Mn HEAs with 20–30 at.% Pt exhibit a half-wave potential of 0.945 V in acidic media, surpassing commercial Pt/C catalysts through ensemble, ligand, and strain effects that optimize binding energies. These structures maintain stability in Zn-air batteries, reducing Pt dependency while enhancing mass transport via bicontinuous pores.^[84] Nanoporous gold (NPG) electrodes, fabricated by dealloying Au-Ag precursors, serve as biocompatible platforms in enzymatic biofuel cells. Flexible NPG on Kapton supports, with 15–17 nm pores and roughness factors of 6–16, enable high enzyme loading for glucose/oxygen biofuel cells, delivering power densities up to 4.4 µW cm⁻² in phosphate-buffered saline and retaining 80% cathode activity after 18 hours.^[85] Dealloying also yields nanoporous Ni-Co-containing alloys for hydrogen evolution reaction (HER), achieving low overpotentials through increased active sites. A nanoporous Cu-Zn-Ni-Co alloy (with ~30% Cu, 30% Ni, 25% Co) post-dealloying in KOH exhibits an overpotential of 67 mV at 10 mA cm⁻², outperforming np-NiFeMoP (223 mV) and approaching Pt/C (78 mV), with 100–400 nm pores enhancing alkaline HER kinetics.^[86] These applications highlight the biocompatibility and high electrocatalytic surface area of dealloyed materials, enabling tailored porosity for biomedical integration and catalysis efficiency. However, challenges persist in long-term stability, particularly against corrosion in physiological or operational environments, necessitating further optimization for clinical and industrial deployment.^[83]^[84]

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