Comparative analysis of explosion characteristics in fresh vs aged aluminum and magnesium powders

Aluminum and magnesium powders are crucial in aerospace and automotive metallurgy; However, their extreme reactivity presents severe dust explosion hazards. Industrial process safety models predominantly rely on data from fresh powders, ignoring surface kinetic variations caused by long-term environmental aging. This study systematically evaluates the surface chemistry evolution and explosion dynamics of micron-sized aluminum and magnesium powders after six years of natural aging (25°C, 50%RH). Particle size interference was successfully isolated by confirming minimal geometric variation (D50 difference< 1.3µm). Explosion parameters were evaluated using a 20-L spherical chamber and standard ignition sensitivity apparatus, while phase transitions were elucidated via HRXRD. HRXRD revealed contrasting aging behaviors dictated by metal oxide characteristics. Aluminum formed a protective, dense Al(OH)3 crystalline layer (0.9 mass%), acting as a physical barrier and an endothermic heat sink. Conversely, magnesium developed a porous Mg(OH)2 layer (3.2 mass%) that allowed deep hydration. At elevated explosion temperatures, rapid decomposition of the magnesium hydrate layer triggered a violent metal–water reaction, generating in-situ H2 gas that initiated a hybrid gas–solid explosion. These microscopic mechanisms drove divergent macroscopic outcomes: Aged aluminum exhibited significant thermal passivation, where its Minimum Ignition Temperature of Cloud (MITC) rose from 690 to 850 °C, Pmax dropped from 5.2 to 3.4 barg, (dP/dt)max plummeted from 403 to 166 barg/s, and tm was delayed from 134.4 to 150.4 ms$. In sharp contrast, aged magnesium displayed anomalous latent sensitization and strengthening, with its minimum ignition energy dropping sharply from 21 to 6 mJ, Pmax surging from 7.9 to 9.4 barg, and (dP/dt)max increasing from 553 to 607 barg/s. Although its tm was also delayed from 140.0 to 153.6 ms, this brief latency served as a critical preparation period for hydrogen-assisted combustion. These findings disprove the traditional industrial assumption that aging consistently mitigates hazards, highlighting the necessity of incorporating moisture-induced combustion effects into process safety management.

Yu-Chi Cheng is an assistant professor in the Department of Occupational Safety and Health at Chung Shan Medical University, Taiwan. He earned his Ph.D. in Engineering Science and Technology from National Yunlin University of Science and Technology and served as a visiting scholar at Dalhousie University, Canada. His primary research focuses on dust combustion and explosion hazard dynamics, Li-ion battery thermal runaway, and quantitative risk assessment. Cheng has published numerous high-impact peer-reviewed journal articles in chemical safety and active research projects supported by the National Science and Technology Council (NSTC).