Two-step anodization approach for fabricating nanoporous alumina on aluminum hollow tubes

This study reports the fabrication of nanoporous anodic aluminum oxide (AAO) layers on both the inner and outer surfaces of aluminum (Al) hollow tubes using a modified two-step anodization process. Unlike conventional AAO synthesis performed on planar or single-surface cylindrical substrates, the present method enables simultaneous anodization of concave and convex surfaces within a single system. The novelty of this work lies in the dual-surface anodization approach for hollow tubular substrates, achieving hexagonal nanoporous morphology across geometrically distinct surfaces without requiring separate processing steps. This method expands the applicability of AAO beyond traditional planar templates, offering a scalable route for complex 3D architectures. The resulting dual-surface AAO tubes hold significant potential for advanced applications, including multifunctional membranes, high-surface-area catalytic supports, and fluidic devices where inside–outside accessibility is critical. Their large surface area, high pore uniformity, and robust structure make them suitable for separation processes, sensing platforms, and energy storage devices. By enabling efficient and uniform anodization of both internal and external surfaces, this research provides a pathway for integrating AAO into next-generation tubular devices, offering enhanced performance in chemical, environmental, and biomedical applications.