Corrosion Behavior And Anodizing Sealing Process Of7A04-T6Aluminum Alloy

7A04aluminium alloy has been widely employed in aerospace and transportation industries for its excellent properties, such as low density and high strength as well as superior corrosion resistance. Their properties were enhanced by precipitation of intermetallic compounds formed by the major alloying elements during heat treatment at the expense of degration of corrosion resistance. When aluminium alloys components serve in chloride-containing environment, they would undergo pitting corrosion, exfoliation, intergranular corrosion and stress corrosion cracking, which lead to the failure of components.The research material of this study is hot extrusion rod of7A04aluminium alloy. Corrosion behavior of AA7A04-T6in3.5wt.%NaCl solution and its influence on tensile strength was studied by application of scanning laser confocal microscopy, scanning electron microscopy and energy spectrum analysis as well as electronic universal testing machine. The optimum technology parameters of developing oxidation films on7A04aluminium alloy in oxalic acid were explored. Then the very kinds of films were undergone boiling water sealed and sol-gel coated respectively. Oxidation films on7A04aluminium alloy, sealed oxidation films and oxidation films coated with sol-gel were observed by applying scanning laser confocal microscopy, scanning electron microscopy and energy spectrum analysis. Corrosion resistance of above mentioned three kinds of films were measured by corrosion tests and electrochemical tests.The results show that development of trench around the intermetallic particles periphery and dissolution of matrix is relatively slow in the first two weeks. The deposition of corrosion product on the surface, increase in the reaction area and newly exposed intermatallic particles are responsible for the higher rate of weight loss. The mechanical integrity of the tensile specimens was damaged by the connection and penetration of trenches or intergranular corrosion, leading to the disappearance of necking phase.The optimum technology parameters of developing oxidation films on7A04aluminium alloy in oxalic acid were as follows,60g/L oxalic acid, stepping voltage (60V-70V), temperature (15℃), time (2h). The thickness of film developed under this condition is18μm, while the micro-hardness is292HV0.025. The results of corrosion tests indicate that cavities caused by larger intermetallic particles in the anodic films on 7A04aluminium alloy are responsible for corrosion beneath anodic films, anodic films crack and final complete failure. Corrosion resistance of oxidation films was boosted remarkably by boiling water sealing, whereas cavities cannot be sealed. Sol-gel coating dried at300℃has high coherency. The thicker the film is, the better the corrosion resistance is. This kind of film possesses a better corrosion resistance in chloride-containing environment, even though localized corrosion appears later.