Anode Oxidation Treatment Of Aluminium And Investigation Of AAO Film As Diffusion Barrier

Anodic Aluminum Oxide (AAO) Film exhibits good corrosion resistance and wear resistance, and has been widely used to protect aluminum alloys. With development of industry, the AAO film by conventional process is not satisfacted, thus further adjustment of the process parameters is needed. The main phase of hcp Al2O3 is formed in the AAO film. Whether the AAO film with controlled microstructures can be used as interlayer, diffusion barrier of hcp Al2O3 is good for suppressing elements interlayer at high temperature, and on the other hand, interfacial bonding of the coating and substrate can be strengthened by moderate elements diffusion through the AAO honeycomb structure.In this paper, effect of different additives, oxidation temperature, oxidation time, current density and concentration of sulfuric acid on hardness, thickness and corrosion resistance of the AAO film, the optimal process parameters were selected by using the corrosion resistance as the main evaluation means, and the mechanism of the AAO film cracking at high temperature was discussed. Pure Al film was deposited on Nb alloy substrate by vacuum evaporation, then was treated by the optimal anodic oxidation process. The barrier ability of the AAO film to the diffusion of Nb and Ni was analysed(the outer coating is Ni). Results show that:(1) The hardness, thickness and corrosion resistance of the AAO film are all found to be improved after adding different composite additives. From the result of polarization curve and EIS test, we find that the composite additive consisting of oxalic acid, boric acid, glycerol, tartaric acid and ceric sulfate is very effective.(2) The AAO film with high thickness and smaller holes exhibits higher hardness. Moreover, it has been found that 60min Oxidation time,2.5A/dm2 current density, 11wt.% sulfuric acid concentration, and 15℃ oxidation temperature are optimal process parameters.(3) Crack density decreases with the oxidation time (13cracks/cm2 at 50 min oxidation time), the current density (11.1 cracks/cm at 3A/dm), the oxidation temperature (drops to 0 cracks/cm2 at 60℃), and sulfuric acid concentration increases (9.5cracks/cm2 at 22wt.%). It can be seen that the thermal cracking density is dependent on the AAO film thickness, hardness and the area of hole in the AAO film. The film with greater thickness and smaller hole area exhibits better cracking resistance.(4) The speed of anodic oxidation on pure Al film is much faster than that on the aluminum sheet, and it is easier to form large size pores and high porosity of the AAO film. Nb2O5 can be formed on the Al film on the Nb alloy substrate if the anodic oxidation time is too long.(5) The AAO film as diffusion barrier layer exhibits a significant barrier effect on the elements diffusion. The barrier layer is blocking type in as-deposited Ni/Al2O3/Nb samples. After vacuum annealing it became consumpted type, The diffusion coefficient of Ni and Nb in the AAO barrier layer increased with annealing temperature, but the increase extent is little. After annealing, the AAO film was transformed to be dense alumina. Due to the formation of Al3Ni2, NbNi3, AlNi phase, etc, from reaction of Ni, Nb and Al in the interlayer, the diffusion barrier can effectively hinder the element diffusion.