Enhancing the insulating and dielectric properties of barrier anodic Al2O3 on high purity aluminum

[Display omitted] •The Al microstructure and grain orientations affect barrier anodic oxide structure.•Enhanced dielectric properties (ε = 14.5 and Z = 109Ω) measured for anodic Al-oxides.•Electro-Chemical Etching is applied on high-purity Al to reveal grain structure.•Oxide defects at different scales are assessed by EIS and SKPFM.•Grain boundaries and mismatch are identified as the corrosion sites in Al2O3. This work describes the influence of 99.99% Al sheet surface microstructure on the properties (electrical, ionic defects) of high-field grown barrier anodic oxides and how Electro-Chemical Etching (ECE) can be used to analyze and optimize them. As a surface treatment, ECE process provides smooth surfaces with revealed crystallographic planes. In terms of surface roughness, grain size and crystal orientation, the different Al substrate microstructures presented as a function of ECE process time (depth in the rolled Al sheet), generate strong variations on the passive and anodic oxide Volta Potential Difference (VPD) and electrical barrier properties of anodized layers investigated at different length scales. Optimal ECE times related to the removal of surface deformation and detrimental (1 1 1) orientation allow the growth of electric barrier-like anodic oxides with enhanced dielectric constants of about 14.5. The high VPD of ~6 V, measured by AFM/SKPFM for highly capacitive anodic oxides, are associated to the formation of dense and homogeneous ionic migration-hindering oxides with minimized defect concentration. Concerning corrosion initiation, no direct correlation between the bulk crystallographic orientation and localized initiation susceptibility was found. Instead, grain boundary mismatch between differently oriented grains was identified as affecting local oxide structure and the corrosion resistance of barrier Al2O3.