Effect of Post-Processing Heat Treatments on the Microstructure and Corrosion Resistance of AZ91 Magnesium Alloy Produced by Laser Powder Bed Fusion

The influence of post-build heat treatments on the microstructure and corrosion behavior of laser powder bed fusion (LPBF)-processed AZ91 magnesium alloy was investigated. Solution treatment induced pronounced grain coarsening, with grain sizes exceeding 200 µm in some regions, and reduced the β-Mg 17 Al 12 fraction to 1.7 wt.% (X-ray diffraction) and 8 ± 2% (image analysis), whereas over-aging preserved a fine-grained microstructure with an average grain size of 2.5 ± 1.4 µm and increased the β-phase fraction to 16.6 wt.% and 23 ± 2%, respectively. Corrosion behavior in 0.1 M NaCl was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy, hydrogen evolution measurements, and surface characterization. The over-aged condition exhibited enhanced anodic kinetics, extensive filiform corrosion, and a steady-state corrosion rate of 0.27 mA cm −2 . In contrast, the solution-treated alloy developed a narrow pseudo-passive region of approximately 20 mV, showed predominantly localized pitting, and reached a steady-state corrosion rate of 0.08 mA cm −2 . After 2 h of immersion, X-ray photoelectron spectroscopy revealed a higher normalized Al fraction in the surface film formed on the solution-treated alloy (~ 15%) than on the over-aged condition (~ 8%). Overall, the results demonstrate that, in LPBF AZ91, similar global corrosion rates can arise from fundamentally different corrosion mechanisms, highlighting the critical role of microstructure-controlled surface film stability under AM-specific conditions, and indicating that corrosion behavior cannot be directly inferred from conventionally processed AZ91 alloys. • Post-build heat treatments strongly modify the corrosion behavior of LPBF AZ91 • Over-aging increases corrosion rates due to Al depletion and β-phase coarsening • Solution treatment promotes Al-rich surface films and improves corrosion resistance • Localized pitting dominates solution-treated AZ91 despite high impedance values • Corrosion performance depends on microstructure-controlled surface film stability