Bulk Metallic Glass Corrosion Resistant Alloy: Composition Design, Electrochemical Enhancement, And Industrial Applications

Fundamental Composition Strategies For Corrosion Resistant Bulk Metallic Glass Alloys

The design of corrosion resistant bulk metallic glass alloys hinges on precise control of base metal selection, alloying element ratios, and the suppression of electrochemically active phases. Unlike crystalline alloys where grain boundaries serve as preferential corrosion initiation sites, bulk metallic glasses exhibit chemical and structural homogeneity that inherently reduces localized electrochemical activity 210. This section examines the compositional frameworks that enable both high glass-forming ability and superior corrosion resistance across multiple alloy families.

Zirconium-Titanium Based Systems: Eliminating Electronegative Elements

Zr-Ti based bulk metallic glasses have historically incorporated late transition metals (LTMs) such as Ni and Cu to enhance glass-forming ability, exemplified by commercial alloys like Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5) and Vit106a (Zr58.5Nb2.8Al10.3Ni12.8Cu15.6) with critical casting thicknesses of 18 mm and 32 mm respectively 12. However, Ni and Cu are highly electronegative elements that can compromise corrosion resistance in chloride-containing and acidic environments 2. Research by Morrison et al. demonstrated that while standard Zr-Ti BMGs perform comparably to Ti-6Al-4V in phosphate-buffered saline, systematic removal of Ni and Cu yields alloys with corrosion resistance far exceeding 316L stainless steel 2. The optimized Ni-free and Cu-free Zr-Ti compositions maintain glass-forming ability through increased Zr/Ti ratios and controlled additions of Al (0.005-0.50 wt% sol. Al) 23. For instance, a Zr-based composition expressed as Zr36-n-mNi64+nAlm (where -5≤n≤5 and 0≤m≤5) exhibits maximum corrosion resistance at n=0 and m=0, demonstrating that compositional purity directly correlates with electrochemical stability 6. These alloys are particularly suited for biomedical implants where ion release must be minimized 2.

Iron-Chromium Based Metallic Glasses: Refractory Metal Synergy

Fe-Cr based bulk metallic glasses offer cost advantages over Zr-based systems while delivering exceptional corrosion resistance in acidic aqueous solutions and molten metal environments 4912. The general composition formula (Fe,Cr)100-(a+b+c)WaTMb(C,B,P)c—where TM represents Mo, Ta, V, or Nb; a=2-20 at.%, b=0-15 at.%, c=20-30 at.%, and Fe=35-55 at.%—achieves a supercooled liquid temperature region (ΔTx) ≥30°C and glass transition temperature (Tg) exceeding molten zinc temperature by ≥20°C 4912. The synergistic effect of W (2-20 at.%) and secondary refractory metals (Mo, Ta) stabilizes the amorphous phase while forming a protective passive film rich in Cr2O3 and WO3 412. Specifically, compositions with atomic ratio W/TM ranging from 0.25 to 1.0 and (C1-xBxPy) where 0.11≤x≤0.85 and 0≤y≤0.57 demonstrate superior resistance to molten zinc corrosion compared to conventional steel and ceramic materials used in hot-dip galvanizing lines 412. The high Cr content (5-30 at.%) ensures rapid passivation, while the metalloid elements (C, B, P totaling 20-30 at.%) suppress crystallization during thermal spray coating processes 912.

Gold-Based And Specialty Alloys: Tarnish Resistance For Luxury Applications

Au-based bulk metallic glasses containing ≥45 at.% Au, combined with Ag and/or Pd, Si, and Ge, exhibit exceptional tarnish resistance and hardness exceeding twice that of conventional crystalline gold alloys with similar Au content 7. These quaternary or higher-order alloys leverage the nobility of Au while achieving critical casting thicknesses suitable for luxury goods manufacturing (watches, jewelry) through optimized Si and Ge additions that enhance glass-forming ability 7. The absence of long-range crystalline order eliminates dislocation-mediated deformation, resulting in Vickers hardness values >400 HV compared to ~200 HV for 18K gold alloys 7. For applications requiring both corrosion resistance and magnetic properties, Ni-based BMGs with high refractory metal (Mo, W) and boron content form nickel solid solution phases and hard borides upon controlled heat treatment above crystallization temperatures, enabling tailored mechanical and electrochemical properties 15.

Electrochemical Enhancement Processes And Surface Modification Techniques

While compositional optimization provides intrinsic corrosion resistance, post-processing electrochemical treatments can further enhance the passive film stability and reduce susceptibility to localized corrosion in bulk metallic glasses. Anodic oxidation and thermal spray coating methods represent two primary surface engineering strategies.

Anodic Oxidation: Controlled Passive Film Growth

Electrochemical anodic treatment of Zr-, Ti-, Hf-, Al-, Mg-, and Au-based bulk metallic glass substrates using current densities from 0.5 mA·cm⁻² to 1000 mA·cm⁻² generates dense, adherent oxide layers that significantly improve corrosion resistance 1. The process involves immersing the BMG substrate in an electrolyte (typically sulfuric acid, phosphoric acid, or alkaline solutions) and applying controlled anodic polarization to grow oxide films with thicknesses ranging from 10 nm to several micrometers depending on current density and treatment duration 1. For Zr-based BMGs, anodization at 10-50 mA·cm⁻² for 10-30 minutes produces ZrO2-rich passive films with enhanced barrier properties against chloride ion penetration 1. The amorphous nature of the substrate ensures uniform oxide growth without preferential attack at grain boundaries, resulting in defect-free passive layers 1. This technique is particularly effective for Zr-Ti BMGs used in marine environments and biomedical devices where long-term stability in saline solutions is critical 12.

Thermal Spray Coating: Dense Amorphous Phase Deposition

For Fe-Cr based metallic glasses intended for molten zinc corrosion resistance, thermal spray coating methods (plasma spray, high-velocity oxygen fuel spray) enable deposition of dense, amorphous coatings with low porosity (<2%) and absence of pinholes 912. The coating composition (Fe,Cr)100-(a+b+c)WaTMb(C,B,P)c is applied to steel or ceramic substrates at temperatures below the glass transition temperature to preserve the amorphous structure 12. Key process parameters include:

• Spray distance: 100-150 mm to balance particle velocity and temperature 12
• Powder feed rate: 30-50 g·min⁻¹ for consistent coating thickness (200-500 μm) 12
• Substrate preheating: 150-200°C to minimize thermal shock and improve adhesion 12
• Post-spray heat treatment: Optional annealing at Tg-50°C for stress relief without crystallization 12

The resulting coatings exhibit glass transition temperatures (Tg) of 550-600°C and supercooled liquid regions (ΔTx) of 30-50°C, ensuring thermal stability during exposure to molten zinc at 450-460°C 912. Electrochemical impedance spectroscopy (EIS) measurements reveal coating resistances >10⁶ Ω·cm² in 3.5 wt% NaCl solution, compared to <10⁴ Ω·cm² for uncoated steel 12.

Quantitative Corrosion Performance Metrics And Testing Protocols

Rigorous electrochemical characterization is essential to validate the corrosion resistance claims of bulk metallic glass alloys and compare their performance against conventional materials. Standard testing protocols include potentiodynamic polarization, electrochemical impedance spectroscopy, immersion testing, and accelerated environmental exposure.

Potentiodynamic Polarization: Passivation Behavior And Pitting Resistance

Potentiodynamic polarization curves obtained in 3.5 wt% NaCl solution (simulating seawater) or 0.1 M H2SO4 (acidic environment) provide critical parameters including corrosion potential (Ecorr), corrosion current density (icorr), passivation potential (Epp), and pitting potential (Epit) 24. High corrosion resistant Zr-Ti BMGs free of Ni and Cu exhibit Ecorr values of -200 to -100 mV vs. saturated calomel electrode (SCE) and icorr <0.1 μA·cm⁻², compared to Ecorr = -400 mV and icorr = 1-5 μA·cm⁻² for 316L stainless steel under identical conditions 2. The absence of pitting up to +1000 mV vs. SCE indicates superior resistance to localized corrosion 2. Fe-Cr based metallic glasses demonstrate even lower icorr values (<0.01 μA·cm⁻²) in 0.5 M H2SO4 due to rapid formation of Cr2O3/WO3 passive films 4. The passivation current density (ipass) typically remains below 1 μA·cm⁻² over a wide potential range (Epp to Epit >800 mV), confirming stable passive film behavior 4.

Immersion Testing: Long-Term Stability Assessment

Long-term immersion tests in corrosive media provide realistic assessment of bulk metallic glass durability. Zr-based BMG samples immersed in phosphate-buffered saline (PBS, pH 7.4) at 37°C for 90 days show weight loss <0.5 mg·cm⁻² and surface roughness increase <10 nm, comparable to Ti-6Al-4V and superior to CoCrMo alloys 2. Fe-Cr BMG coatings exposed to molten zinc at 460°C for 500 hours exhibit erosion depths <50 μm, whereas uncoated steel suffers >2 mm erosion under identical conditions 912. The corrosion rate calculated from weight loss measurements is typically <0.01 mm·year⁻¹ for optimized BMG compositions, meeting or exceeding requirements for marine structural components (ASTM G31 standard) 24.

Accelerated Environmental Testing: Salt Spray And Humidity Exposure

Neutral salt spray testing (ASTM B117) at 35°C with 5 wt% NaCl solution for 1000 hours reveals no visible corrosion products on anodized Zr-Ti BMG surfaces, while conventional aluminum alloys show significant pitting and white rust formation after 168 hours 1. Cyclic humidity testing (85°C, 85% RH, 1000 cycles) of Au-based BMGs demonstrates tarnish resistance with color change ΔE <1.0 (CIE Lab color space), compared to ΔE >5.0 for 18K gold alloys, confirming suitability for luxury watch cases and jewelry 7.

Industrial Applications: From Marine Engineering To Biomedical Devices

The exceptional corrosion resistance of bulk metallic glass alloys, combined with high strength (tensile strength 1500-2500 MPa), hardness (Vickers hardness 400-600 HV), and wear resistance, enables deployment in demanding industrial sectors where conventional materials fail prematurely 12710.

Marine And Offshore Structures: Seawater Corrosion Mitigation

Zr-Ti based BMGs free of Ni and Cu are increasingly specified for marine fasteners, propeller shafts, and subsea connector components operating in seawater environments 2. The combination of corrosion current density <0.1 μA·cm⁻² in 3.5 wt% NaCl and tensile strength >1800 MPa provides a specific strength advantage over titanium alloys while eliminating galvanic corrosion concerns associated with stainless steel-titanium joints 2. Anodized BMG coatings on aluminum substrates extend service life of offshore platform components from 5-10 years (uncoated) to >25 years (coated), reducing maintenance costs by 60-70% 1. The critical casting thickness of 18-32 mm for commercial Zr-Ti BMGs enables production of solid components rather than thin coatings, simplifying manufacturing and improving reliability 12.

Hot-Dip Galvanizing Equipment: Molten Zinc Resistance

Fe-Cr based metallic glass coatings applied via thermal spray to sink rolls, stabilizer rolls, and dross boxes in continuous hot-dip galvanizing lines address the chronic problem of molten zinc corrosion that limits equipment lifespan to 6-12 months with conventional steel or ceramic materials 912. The glass transition temperature (Tg) of 550-600°C, exceeding molten zinc operating temperature (450-460°C) by >90°C, ensures structural stability during prolonged exposure 912. Field trials in automotive galvanizing lines demonstrate coating erosion rates <0.1 mm·year⁻¹ compared to 2-5 mm·year⁻¹ for uncoated steel, extending equipment service life to >5 years and reducing downtime by 80% 12. The dense, pinhole-free amorphous structure prevents zinc infiltration and subsequent spalling that plagues crystalline coatings 12.

Biomedical Implants And Surgical Instruments: Biocompatibility And Ion Release Control

The elimination of Ni and Cu from Zr-Ti BMG compositions addresses biocompatibility concerns related to metallic ion release and allergic reactions in orthopedic and dental implants 2. Ni-free Zr-Ti BMGs exhibit ion release rates <0.1 μg·cm⁻²·day⁻¹ in simulated body fluid (SBF), two orders of magnitude lower than 316L stainless steel and meeting ISO 10993 cytotoxicity requirements 2. The high hardness (500-600 HV) and wear resistance of BMGs make them ideal for articulating surfaces in joint replacements, reducing polyethylene wear debris generation by 40-60% compared to CoCrMo alloys 2. Magnesium-based BMG composites incorporating TiZr alloy phases are under development for biodegradable suture anchors in rotator cuff repair, offering controlled degradation rates (0.5-1.0 mm·year⁻¹) and mechanical strength (tensile strength >300 MPa) superior to pure Mg implants 17.

Luxury Goods And Consumer Electronics: Aesthetic Durability

Au-based BMGs containing ≥45 at.% Au, Ag/Pd, Si, and Ge combine the prestige of high gold content with scratch resistance and tarnish resistance unattainable in conventional gold alloys 7. Watch cases fabricated from Au-BMG exhibit Vickers hardness >400 HV (vs. 200 HV for 18K gold) and maintain mirror finish after 10,000 cycles of ASTM G133 oscillating sand abrasion testing, compared to visible scratching after 1,000 cycles for conventional alloys 7. The low casting temperature (850-950°C) and minimal shrinkage (<0.5%) during solidification