Studies on the corrosion performance of an effective and novel sealing anodic oxide coating

The anodic oxide coating was formed on 2024-T3 aluminium alloy with sulfuric-boric acid anodizing. Then it was sealed with an alkaline Mn-Mo oxyanions electrolyte. This investigation revealed the surface morphology of the Mn-Mo sealed sample which showed flake-like surface cluster structure. EDS analysis results of the coating cross-section verified the relative even distribution of the oxygen, aluminium species, but Mo element existed in the outer region of the anodic coating. XPS analysis revealed the presence of hypervalent oxidation state soluble species of Mn and Mo such as MnOx, MnO2, MnO4-, MoO3/polymolybdate, MoOx and MoO2 and the hydration of Al2O3 and formation of pseudoboehmite during sealing process. The electrochemical characterization of the sealed anodic coatings with different baths was evaluated by electrochemical analysis techniques. The potentiodynamic polarization curves showed that the corrosion potential of the Mn-Mo sealed sample was the most positive. No corrosion pits were observed on the surface of Mn-Mo sample after salt spray test for 384 h. The formulated equivalent circuit is used to study the corrosion mechanism of the oxide film immersed in salt solution. The corrosion process has been summarized into four stages: general corrosion, corrosive ions penetration into porous and barrierlayer, self-healing in local region, coating damage. (C) 2019 Elsevier B.V. All rights reserved.