Nano- and Micro-Scale Characterization of Anodic Oxide Layer Coating on 2017A-T4 Aluminum Alloy

Abstract

In this paper, a multiscale characterization, down to nano, of the oxide layer coating obtained by sulfuric anodizing of 2017A-T4 aluminum alloy is proposed, under different processing conditions related to reaction/anodization time (RT) and applied electrical current (J). The multiscale characterization is carried out using scanning electron microscopy, energy-dispersive X-rays, and ImageJ post-processing software. It reveals that the oxide layer coating is a porous medium wherein the larger amount of porosity is micrometric, and that increasing RT and/or J leads to shallower and larger porosity and consequently thicker porous coating and oxide layer. Moreover, the percentage of the coating surface area covered by micrometer-scaled cavities decreases as the applied current increases. It also comes out from the multiscale analysis that the growth of the coating layer follows three main stages: the first stage is the germination of flaky and friable metastable nanoparticle oxides made of O, Al, and Si; the second stage is the growth of the oxide nanoparticles to become spherical microparticles; the third stage is the coalescence of the spherical particles to form clusters and then continuous stable layer that is rich of Si element.