Microstructure and plastic deformation effects on hydrogen generation from recycled Al–Zn chips

The influence of the scale of the dendritic original microstructure and chip compaction on hydrogen generation behavior was investigated in an Al–10 wt% Zn alloy. Directional solidification was used to produce a wide range of cooling rates, resulting in pronounced variations in primary dendrite arm spacing (λ 1 ) without significant long-range Zn macrosegregation. Machining of the alloy generated heavily deformed chips in which the original dendritic structure was fully suppressed, followed by cold compaction to form consolidated samples. Hydrogen generation experiments were conducted in aqueous NaOH at 25 °C and 45 °C using both bulk and compacted configurations, both from samples having λ 1 of 39.6 μm and 215 μm. For bulk samples, hydrogen evolution followed linear kinetics, with higher reactivity for finer microstructures at 25 °C due to enhanced micro-galvanic effects, while at 45 °C temperature dominated and the microstructural influence vanished. In contrast, compacted chips exhibited two distinct hydrogen-production regimes, reported here for the first time, with their mechanisms identified in reactive compacted systems. In Regime I, coarser and harder original microstructures generated more hydrogen due to the compaction-induced structure, which facilitates crack opening, solution penetration, and exposure of fresh metallic surfaces, reversing the bulk trend. After chip disintegration, Regime II showed linear kinetics governed mainly by surface area, Zn-oxide passivation, and temperature rather than dendritic spacing. These findings demonstrate the critical role of compaction structure and microstructural inheritance, establishing a new mechanistic understanding of hydrogen generation in reactive compacted Al–Zn alloys. • Solidification controlled dendritic scale (λ 1 ) without Zn macrosegregation. • Compaction formed deformation bands reflecting initial microstructure. • Bulk showed linear, chips showed higher H 2 generation. • Briquettes showed two-regime H 2 generation controlled by cohesion/surface.