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To improve the corrosion resistance of 6061 Al in electric vehicle battery packs, a composite coating of nano-SiO2/Micro-Arc oxidation (MAO) ceramic structure was prepared on its surface. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (PDP) were used to evaluate the corrosion resistance of the specimens after 7 days immersion in a 3.5% NaCl solution. The corrosion resistance of the prefabricated coatings was measured via local electrochemical impedance spectroscopy (LEIS). Confocal microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure and phase composition of the specimens. An energy dispersive spectrometer (EDS) was used to detect the elemental composition of the surface of the specimen. The results showed that the specimen with nano-SiO2/MAO composite coating had the least amount of micropores and superior corrosion resistance. The global electrochemical impedance of nano-SiO2/MAO composite coating was 1.1 times higher than that of the MAO coating and 8.4 times higher than that of the 6061 Al. When the coating was defective, the local electrochemical impedance of the nano-SiO2/MAO composite coating was still two times higher than that of the MAO coating. In the presence of scratches, the nano-SiO2/MAO composite coating still showed high corrosion resistance. The collapse corrosion mechanism of the nano-SiO2/MAO composite coating was proposed. [ABSTRACT FROM AUTHOR]

[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]

The nano-scale failure behaviors of joint interfaces were investigated by in-situ straining technique to observe the crack propagations in a transmission electron microscope (TEM). The thin sections of bonded interfaces were subjected to tensile loading by using a specially designed specimen holder. The deformation at the crack tip before the initiation of crack and the initiation of the crack were observed in the nanometer level. The experiments with identical and dissimilar polymer/polymer welded specimens allowed us to see the plastic deformation ahead of the crack tip and the creation of craze, which is the typical failure mode of polymers. The failure of the interfaces between aluminum alloy (Al5052) and epoxy adhesive and between Al5052 and thermoplastic directly bonded by injection molding were investigated to visualize the failure behaviors in small scale before the macroscopic fracture of the interfaces. The in-situ straining technique under a high-resolution scanning transmission electron microscopy (STEM) mode enables the observation of micro deformation ahead of the crack, microvoids produced prior to the crack progression and the failure of the surface modified layer on Al surface. [ABSTRACT FROM AUTHOR]