Micro-arc oxidation (MAO) is an effective surface treatment for improving the corrosion resistance of aluminum alloys. However, the protective performance of MAO coatings is commonly evaluated using surface morphology or total porosity, while the role of three-dimensional pore connectivity and substrate-dependent interfacial electrochemistry remains insufficiently understood. In this work, silicate MAO coatings were prepared on 5083, 6061 and 7075 aluminum alloys under systematically varied Na₂SiO₃ concentrations. In situ discharge observation, X-ray microscopy (XRM), grazing-incidence X-ray diffraction, transmission electron microscopy and time-dependent electrochemical impedance spectroscopy (EIS) were combined to establish a processing-structure-transport-electrochemistry relationship. XRM was used to distinguish through-thickness connected pores from isolated pores and to quantify tortuosity, effective diffusivity and a pore-transport index, P_tr, describing coating-related electrolyte accessibility. Within each alloy series, the overall electrochemical resistance after 48 h immersion generally decreased with increasing P_tr; however, no universal relationship was obtained across the three alloys. In particular, 6061- and 7075-based coatings with comparable through-thickness connected porosity exhibited markedly different long-term impedance responses. Time-dependent EIS further revealed transient electrochemical stabilization for several 5083- and 6061-based coatings, whereas the 7075-based coatings showed predominantly continuous degradation. These results demonstrate that long-term corrosion protection is controlled by the coupling of pore-network-mediated electrolyte transport and alloy-dependent coating/substrate interfacial electrochemical response. The findings highlight that durable MAO coating design requires not only suppression of through-thickness transport pathways but also stabilization of the coating/substrate interface.
 

Micro-arc oxidation,Aluminum alloy,X-ray microscopy,Pore-network transport,Electrochemical impedance spectroscopy,Corrosion protection