An additively manufactured magnesium-aluminium alloy withstands seawater corrosion

The paper addresses whether laser powder bed fusion (LPBF) can be leveraged to create magnesium alloys with compositions and microstructures that deliver inherently superior corrosion resistance—overcoming the longstanding rapid degradation of Mg alloys in aqueous environments—and whether LPBF can enable unique alloy states not achievable by conventional processing. Magnesium’s low density makes it attractive for lightweight applications, and LPBF further reduces component mass via topology optimization. While LPBF has produced Mg alloys with excellent mechanical properties, most Mg alloys, including many LPBF counterparts, typically corrode rapidly due to soluble surface oxides/hydroxides. The authors explore an LPBF-fabricated Mg-10.6Al-0.6Zn-0.3Mn (AZ111) alloy with higher Al than AZ91 to test if LPBF-induced supersaturation and microstructural states can lead to dramatically improved passivation and corrosion resistance. They report the lowest corrosion rate for any Mg alloy tested to date, including in seawater, and examine the time-dependent passivation mechanism.

Materials preparation: The bulk alloy was chill cast and then pulverized via gas atomization. The powder (D10 34.8 µm, D50 54.5 µm, D90 88.4 µm) was processed by LPBF on a modified AconityMINI (max 400 W). Processing parameters: 50 W laser power, 400 mm/s scan speed, 40 µm hatch spacing, 30 µm layer thickness, zig-zag pattern with 90° rotation between layers. Build plate was pure Mg; argon atmosphere with O2 < 200 ppm. Composition by ICP-OES (wt.%): Mg bal., Al 10.6, Zn 0.54, Mn 0.28, Fe 0.004, Cu 0.002, Ni 0.003, Si 0.02.

Electrochemical testing: Three-electrode flat cell with SCE reference and Pt mesh counter; working electrode specimens mounted in epoxy and ground to 2000 grit (ethanol). Electrolyte: 200 mL of 0.1 M NaCl (pH 6.5–7). Potentiodynamic polarization at 1 mV/s; Tafel-type analysis (EC-Lab v11.27) using a linear approximation of the cathodic branch ~50–150 mV below Ecorr to estimate icorr. EIS: 100 kHz–10 mHz, 10 mV perturbation. Tests performed in triplicate; additional replicates up to 96 h on samples from different build heights.

Immersion and hydrogen collection: Specimens immersed in 200 mL of quiescent solutions at 23 °C: 0.1 M and 0.6 M NaCl (pH 6.5), natural seawater (pH 8), up to 14 days; orthophosphoric acid (pH 2.5) up to 72 h. Prior to immersion: 4000 grit grind, colloidal silica polish, ultrasonic ethanol clean, air-dry, precise weighing. Hydrogen evolution measured using inverted funnel and burette during immersion; mass loss measured after removing corrosion products in CrO3/AgNO3/Ba(NO3)2 solution (60 s). SEM used to inspect morphology pre-cleaning.

Microscopy: SEM (Zeiss UltraPlus) with EBSD (Oxford Symmetry), 20 kV, 12 nA, 0.4 µm step; sample prep: 4000-grit grinding, 50 nm colloidal silica polish, Leica TIC 3X ion polish (8 kV, 3 mA, 6 min, 7.5°). STEM and STEM-EDXS (JEOL 2100, 200 kV); foils prepared by mechanical thinning to ~50 µm and cryo ion polishing.

XPS: Thermo Scientific Nexsa with monochromatic Al Kα (1486.6 eV), spot 400×800 µm², 72 W; survey at 150 eV pass energy, HR at 50 eV; base pressure <5×10⁻⁸ mbar; charge neutralization with dual-beam flood gun; Avantage v5.9921 processing, calibrated to C1s at 284.8 eV. Depth profiles collected to several hundred nanometers.

XRD: PANalytical X’Pert PRO MRD, Cu Kα, 40 kV/40 mA, 2θ = 20–80°, 0.01° step; calibrated to NIST LaB6. Sample area 10×10 mm² ground to 4000 grit.

Mechanical testing: Compression on as-built rod (Ø 7.5 mm, height 11 mm) at 0.6 mm/min (Instron 4505). Vickers hardness (Matsuzawa MHT-1) at 1 kgf, seven indents per orientation; surfaces ground to 4000 grit.

• The LPBF Mg-10.6Al-0.6Zn-0.3Mn (AZ111) alloy exhibits record-low degradation among Mg alloys in relevant electrolytes and withstands seawater corrosion over extended immersion.
• Time-dependent passivation: In 0.1 M NaCl, as-ground icorr ≈ 2.8 µA·cm⁻² with Ecorr ≈ −1.33 V vs SCE; after 48 h immersion (open-circuit conditioning), icorr decreases to ≈ 0.63 µA·cm⁻², with strongly expressed passivation in anodic scans and decreased cathodic kinetics.
• EIS corroborates passivation: Low-frequency charge transfer resistance increases from ~5600 to >15,000 Ω·cm² over 48 h; Nyquist plots at 24–48 h show a diffusional feature (positive −Z(ω)), consistent with protective surface layers.
• Extremely low hydrogen evolution rates after 14 days: ≈ 4×10⁻⁴ ml·cm⁻²·h⁻¹ (0.1 M NaCl), ≈ 8×10⁻⁴ ml·cm⁻²·h⁻¹ (0.6 M NaCl), and ≈ 5×10⁻⁴ ml·cm⁻²·h⁻¹ (seawater), lower than high-purity Mg and previously reported corrosion-resistant Mg alloys (e.g., Mg-0.3Ge, BCC Mg-Li-Al, nanotwinned AZ80).
• Corrosion current measured under the same conditions is ~10× lower than extruded AZ91; visual inspection after 14 days shows largely pristine surfaces with limited filiform-like corrosion confined near edges.
• XPS depth profiling reveals, after NaCl immersion, a ~225 nm bilayer surface film: outer hydroxide-rich layer (0–75 nm) and inner oxide-rich layer (75–225 nm). Al/Mg ratio increases from ~0.2 at the surface to ~0.4 by ~150 nm; Al₂O₃ fraction ~12–20% within the film. Beneath the film (to ~364 nm), the substrate is Al-enriched (elemental Al up to ~30% of metallic species).
• Even without immersion, air exposure produces near-surface Al enrichment to ~180 nm depth; O and C decrease with depth; OH⁻ dominates near the surface transitioning to O²⁻ deeper.
• In phosphoric acid (pH 2.5), a thicker oxide layer forms (<3 µm at 24 h, <4 µm at 72 h) with pronounced Al enrichment at the oxide/substrate interface and no severe localized attack, demonstrating exceptional durability even in acidic media.
• Microstructure from LPBF: equiaxed grains (~10 µm) along build direction and elongated grains perpendicular (equivalent diameter ~28 µm); melt pools 100 µm wide and 20–50 µm tall; high density of ultra-fine particles—Al/Zn-rich Mg₁₇(Al,Zn)₁₂ (100–300 nm) and Al–Mn nanoparticles (≤20 nm). Matrix is supersaturated (~7.5 wt.% Al, ~0.5 wt.% Zn). Texture is weak/random (≈2.0 mrd along BD; ≈3.6 mrd perpendicular). Mechanical properties include compressive yield strength ~300 MPa and Vickers hardness ~99–100 HV.

The findings directly address the research question by demonstrating that LPBF can create a Mg–Al–Zn–Mn alloy that undergoes dynamic, self-improving passivation in chloride-containing electrolytes, achieving unprecedentedly low corrosion rates for Mg and even resisting seawater corrosion. The passivation mechanism is attributed not primarily to grain or intermetallic refinement but to a critical, time-dependent aluminium enrichment at and beneath the surface—enabled by LPBF-induced supersaturation of Al in the Mg matrix and high microstrain. XPS reveals a bilayer film (hydroxide-rich outer and oxide-rich inner), with significant Al₂O₃ content and an Al-rich substrate beneath, which suppresses Mg dissolution and hydrogen evolution over time, consistent with increased impedance and anodic passivation in polarization. Comparisons to cast Mg–Al alloys with similar Al contents, which corrode much faster, indicate that composition alone is insufficient; the LPBF process state (rapid solidification, supersaturation) is essential. Air-exposed surfaces also show Al enrichment (albeit weaker), and acid immersion produces thicker Al-rich oxides without severe localized attack, further supporting the role of Al enrichment in durability. Given slow Al diffusion at room temperature, preferential Mg dissolution and surface segregation under immersion likely drive the enrichment, while residual stress/microstrain and supersaturation increase the thermodynamic drive for segregation. The results are significant for enabling ultra-lightweight Mg structures with corrosion resistance suitable for marine and other aggressive environments.

LPBF processing of an Mg-10.6Al-0.6Zn-0.3Mn alloy produced a supersaturated, microstrained microstructure that facilitates dynamic Al enrichment at the surface, forming an Al-containing passive film that strengthens over time in chloride environments. The alloy exhibits record-low corrosion rates for Mg, including in seawater, corroborated by decreasing icorr, increasing impedance, and ultra-low hydrogen evolution over extended immersion. Surface analyses show a ~225 nm bilayer passivation film with meaningful Al₂O₃ content and an Al-rich substrate. The material also exhibits attractive mechanical properties (compressive yield strength ~300 MPa; hardness ~99–100 HV). This work demonstrates a viable path to corrosion-resistant, ultralight Mg components via LPBF-enabled alloy states. Future work could explore optimization of Al supersaturation levels, effects of other alloying additions, long-term field exposures in diverse environments, post-processing heat treatments to tune surface enrichment kinetics, and scale-up/geometry effects on corrosion behavior.