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[Display omitted] • Simple surface treatment by steam improves adhesive bonding of aluminum remarkably. • High-resolution 3D imaging of the interfaces by STEM-tomography and STEM-EELS were performed for elucidate the mechanism. • Chemical interactions at aluminum/adhesive interfaces was proposed. A simple surface treatment by steam on aluminum substrate can enhance the bonding strength with epoxy adhesives. We studied the mechanism of the effect of steam on adhesive bonding by using scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS). The observed increase of adhesion strength was achieved by diffusion of the adhesive molecules into the nanopores of the aluminum surface, which switched the failure mode from interfacial failure to cohesive failure. Investigation of electron energy loss near edge fine structure (ELNES) in EELS allows us to find chemical bonding that form via acid–base interactions between the amino moiety of the adhesive and the hydroxyl groups on the aluminum surface. This weak bonding may not contribute directly to the increase of adhesion strength, but it induces spontaneous diffusion of the adhesion molecules into the pores via thermodynamically favored acid–base interfacial interactions. [ABSTRACT FROM AUTHOR]