Effect of thickness and sealing treatments on the corrosion protection properties of anodic oxide coatings on AA5005

Abstract

This paper aims to investigate the potential of anodic oxidation to protect AA5005 against corrosion. Anodic layers of diverse thickness (1.6–6.0 μm) have been developed at a potential of 6 V in 175 g/L H2SO4 solution at 40 °C. The effect of two sealing methods in autoclave has been assessed: (i) a standard de-mineralized water sealing (to promote the formation of pseudo-boehmite within the pores), and (ii) a sealing performed in a zinc nitrate solution (to promote the formation of layered double hydroxides within the pores and on the surface). SEM investigation revealed that the sealing promotes the formation of cracks that the electrochemical tests showed not to be detrimental for the corrosion protection properties. Furthermore, it has been found that the presence of the anodic oxide strongly enhances the pitting resistance, in particular if the porous layer is hot water sealed. Moreover, polarization curves showed that when a hot water sealing is performed, the anodic current density is shifted from 10−5 Acm−2 to 10−7 Acm−2 regardless of the thickness of the anodic oxide (as far as the investigated thickness range is concerned, 1.6–6.0 μm). Accordingly, the Electrochemical Impedance Spectroscopy characterization highlighted the improved corrosion resistance of the barrier oxide when a hot water sealing is performed on the anodic layer: the corresponding resistance has been found to be >108 Ωcm2. The exposure of the different anodic oxides to the wet/dry cycles in the Prohesion test chamber for 1000 h revealed that all performed treatments protect the substrate against pitting corrosion, thus suggesting that a protective anodic oxide has been developed.