Effects of pore parameters on performance of anodic film on 2024 aluminum alloy

Anodic films on aluminum alloys are prepared in sulphuric/phosphoric acid electrolyte. The relationship between the pore parameters and the mechanical properties and corrosion resistance of the film is studied. With increased film porosity, the film microhardness decreases and the bonding strength increases, while for the same film porosity, larger pores decrease microhardness but increase bonding strength. As sulfuric acid concentration increases, the number of dissolved pores increases while the interpore distance and the pore roundness decrease. All these changes would promote cracking of the anodic film after heating. With changed pore parameters, the crack density increases more quickly than the crack width. After 4800 h immersion in NaCl solution, for films with porosity of 24.3% and 28.2% the barrier layer resistance decreased slowly, but for films with porosity higher than 30%, the barrier layer resistance decreased much faster. The porous layer resistance of the films shows similar variations as the barrier layer resistance. The lower film porosity, better film continuity and more regular pore structure could improve the corrosion resistance of anodic films. From the view of corrosion protection, it is suggested that the porosity of anodic films should be controlled below 30%.