Preparation And Properties Of High-performance Coating On Aluminum Alloy

Aluminum alloys are widely used in aerospace and transportation industries because of its desirable properties such as high strength-to-weight ratio and good formability.However,Al alloys are susceptible to corrosion especially in aggressive media such as that in the presence of a high concentration of Cl^-,despite the presence of a natural passivating film on the surface.In order to solve the corrosion problem of Al alloys,this work used micro-arc oxidation?MAO?technology to fabricate a dense metal oxide ceramic film in-situ on the surface of Al alloys.As a result,the corrosion resistance of MAO coatings in high humidity and high salinity environments was greatly improved through various doping methods.Firstly,in order to optimize the fusion problem of the doped particles and the formed oxides films,the zinc oxide?ZnO?particles are selected to dope the alumina coatings,on account of zinc has a similar and slightly lower activity compared to aluminum.The results show that ZnO particles can achieve defect-free fusion with alumina at the high temperature of arc discharge by forming nanocrystals distributed in the films,which greatly improves the compactness of the MAO coatings.The doping of ZnO brings in more ions in the MAO electrolyte to synthsize phosphate and silicate compounds on the coatings with a better corrosion resistance.The corrosion current density(i_corr)of the ZnO modified MAO coating is 4 orders of magnitude less than that of the substrate.After immersion in 3.5 wt.%sodium chloride for 14 days,the undoped coating is obviously damaged with sponge-like loose structure and a large area of cracks,while the surface of the ZnO-doped samples remain dense structure without any obvious corrosion evidences.In order to improve the uniformity of doped coatings,a new type of electrolyte is designed by adding zinc acetate to the phosphate electrolyte to obtain an uniform MAO coating with zinc phosphate doped,because of the ultra-low solubility of zinc phosphate.The results show that the zinc phosphate-doped films not only have 4 orders of magnitude i_corr less than that of the substrate,but also have an increase of the corrosion potential by approximately 0.5 V,resulting in a better corrosion resistance.The neutral salt spray experiments show that the zinc phosphate doped coatings reach to a lifetime over 11,000 hours.The results of salt spray corrosion at different time indicate that zinc phosphate on the surface of the film will spontaneously follow the corrosion droplets into the pores under the effect of surface energy,which can effectively seal the corrosive pores and prevent the corrosion droplets from entering the aluminum matrix to form corrosive cracks.The effect of self-healing achieves an extraordinary life of aluminum alloy against salt spray corrosion.This anti-corrosion concept of self-sealing effect can be extended to other materials with certain guiding significance.