Selective removal of Fe-rich intermetallics in AA8079 by mixed HNO3/HF pretreatment: Implications for aluminum–polypropylene interfacial integrity

초록

Fe-containing intermetallic compounds (Fe-IMCs) in 8xxx aluminum alloys act as cathodic sites that promote localized corrosion and generate defects in anodic oxide films. A combined pretreatment—alkaline etching followed by mixed HNO3/HF pickling under ultrasonication (NH-U)—was applied to selectively remove Fe-rich Al13Fe4 from AA8079 before phosphoric acid anodizing. Alkaline etching exposed and partially oxidized Fe-IMCs, whereas single-acid pickling (HNO3 or HF) left residual cathodic inclusions at the surface. Mixed HNO3/HF without ultrasonication partially reduced the surface IMC population, whereas NH-U eliminated detectable surface Fe-IMCs and produced a micro-roughened surface. Anodizing on the IMC-free substrate yielded a uniform porous oxide, while untreated AA8079 formed heterogeneous films locally perturbed by Fe dissolution and incorporation. Cross-sectional elemental mapping confirmed Fe incorporation into the anodic oxide above residual IMCs, creating thinner and compositionally altered regions that can disrupt film uniformity and interfacial integrity. As an application-level indicator of film quality, lap-shear strength of anodized Al–polypropylene joints increased from ∼9 MPa to ∼21 MPa after NH-U pretreatment. Selective Fe-IMC dissolution is therefore identified as a prerequisite for mechanically robust and structurally uniform anodic films on AA8079. The pretreatment offers a scalable route to suppress cathodic-particle-driven film breakdown in battery-foil processing and joining applications. © 2026 The Author(s).

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