Pyrophoric Iron Nanoparticles Formed by Selective Extraction from Fe–Al Alloys: A Mössbauer Study

Pyrophoric iron nanoparticles were unexpectedly formed during the preparation of Fe–Al alloys for resonance Mössbauer detectors. Selective dissolution of aluminium in dilute NaOH produced aggregates of ultrafine α-Fe particles that self-heated and ignited in air. The precursor, fresh pyrophoric product and aged oxidation product were characterised by 57Fe Mössbauer spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. The fresh material contains crystalline α-Fe grains, a broadened disordered surface/intercrystalline component and a minor superparamagnetic or residual Fe–Al fraction. A model-based particle diameter of 7–8 nm was obtained. Paraffin-coated samples stored for more than 20 years retained substantial metallic iron, while slow oxidation produced magnetite. The results relate nanoscale disorder to pyrophoricity and highlight a persistent industrial hazard.