Surface mechanical alloying on grain refinement and corrosion behavior of AA2024

Surface Mechanical Alloying (SMA) was applied to AA2024 aluminum alloy using two media types, steel balls and alumina balls, to evaluate the microstructural, mechanical, and electrochemical consequences of mechanically driven surface modification as a potential alternative to conventional chemical surface treatments. Steel-ball processing produced significant grain refinement to a depth of approximately 200 μm and progressively reduced the average precipitate diameter from 7.94 μm in the base alloy to 1.82 μm after 1 h of treatment, while alumina-ball processing yielded no measurable grain size reduction. Both media types facilitated incorporation of particles into the near-surface layer; however, steel-ball treatment introduced substantial iron contamination originating from the tool steel vial, reaching nearly 30 times the base alloy iron content after 0.5 h. Despite this contamination, open-circuit potential measurements showed no significant shift relative to the untreated alloy, attributable to the electrochemical similarity between low-carbon steel and AA2024. Microhardness profiling confirmed a pronounced hardness increase within the grain-refined surface layer, consistent with Hall–Petch strengthening. Potentiodynamic polarization analysis indicated that longer steel-ball treatment times improved passivation behavior and reduced corrosion current density, whereas extended alumina-ball treatment progressively degraded corrosion resistance. Accelerated exfoliation corrosion testing (ASTM G34) revealed that SMA-treated samples suffered material loss to depths of 100–250 μm after 96 h — closely corresponding to the grain-refined layer thickness — while the base alloy showed no measurable degradation. This correlation indicates that deformation-induced grain boundaries served as preferential pathways for intergranular corrosion. The findings demonstrate that surface grain refinement through SMA does not inherently improve, and may substantially impair, the corrosion resistance of AA2024 under localized attack conditions, underscoring the need for comprehensive, multi-method corrosion evaluation before mechanochemical surface treatments are considered viable substitutes for established aerospace surface protection strategies.