| This paper prepared anodic oxide film of aluminum alloy on 7075 aluminum alloy matrix in mixed acid electrolyte of sulphuric acid and oxalic acid. The influence of oxidation temperature (5-20 ℃) and oxidation time (30-90min), current density (1-4A/dm2), concentration of sulfuric acid (90-180g/L) and the concentration of oxalic acid (5～20g/L) on the thickness, hardness, corrosion resistance, and capability of stress-corrosion resistance of aluminum alloy oxidation film was explored by the method of orthogonal experiment. The electrochemical impedance spectroscopy(EIS)and exfoliation corrosion test was used to study the influence of various process parameters and the sealing method with boiling water on the oxide film corrosion resistant performance. And the stress corrosion behavior of the oxide film of 7075 aluminum alloy in the NaCl solution of different pH(pH=2, pH=3, pH=4) and denudation solution were discussed.In the process of 7075 aluminum alloy hard anodic oxidation, the main influence factor of the oxidation film thickness is the current density and oxidation time. The oxidation film gets thicker with the current density increasing and the oxidation treating time lasting in a certain time range (0-120min).Under this experimental condition, the maximum thickness process for 7075 aluminum alloy anodic oxidation film is the temperature of 10℃, the current density of 4A/dm2, sulfuric acid concentration of 90g/L, oxalic acid concentration of 10g/L, oxidation time of 90min, and the thickness of the film is 107μm in this process.In the process of 7075 aluminum alloy hard anodic oxidation, the most important affecting factors of oxidation film hardness is oxidation temperature and oxidation time. When the anodic temperature is low the hardness of the oxidation film is high. With the growing of the oxidation temperature the oxidation film hardness decreases; in the same way, with the prolonging of the oxidation time, the hardness of the oxidation film falls sharply. Under this experimental condition, the highest hardness process of the anodic oxidation is for T=5℃, J=2A/dm2, c(H2SO4)=90g/L, c(oxalicacid)=10g/L, t=30min, and in this process we get the film which its hardness is 405HV0.05.In the process of anodic oxidation without hole sealing of aluminum alloy 7075, the biggest affects reason of the corrosion resistance of oxide film is current density. The corrosion resistance of oxide film firstly increases and then decreases along with the increasing of current density. Sealing holes with boiling water can significantly improve the corrosion resistance of oxide film. The corrosion resistance of oxide film witch sealing hole with boiling water is mainly affected by the oxidation time and current density, but the optimal process is the same. Under this experimental condition, the greatest corrosion resistance of 7075Al alloy anodic oxidation film process is for T=15℃, J=2A/dm, c(H2SO4)=90g/L, c(oxalic acid)=10g/L, T=70min. According to the process the drab oxide film is prepared which the thickness is 42.3 microns, the micro hardness is 305HV0.05, and the low frequency impedance is 435×104D-cm2. After sealing hole with boiling water the oxidation film turns light golden and its low frequency impedance gets to 1830×104Ω2cm2. The optimum corrosion resistance oxidation film also had the best ability to resist the exfoliation corrosion.In the process of hard anodic oxidation on 7075Al alloy, the most important factor which influence the stress corrosion resistance of 7075 aluminum alloy anodic oxidation film is the current density, and the second is oxidation time. With the increasing of the current density, the performance of stress corrosion resistance oxide film increases firstly and then decreases. Under this experiment condition, the process of optimal stress corrosion resistance of 7075 aluminum alloy oxidation film is for T=5℃, J=2A/dm2, c(H2SO4)=120g/L, c(oxalic acid)=10g/L, T=90min. The SCC susceptibility of 7075Al alloy increased with reduction of the pH of NaCl solution, and the anode oxidation film was proved to significantly improve the property of stress corrosion resistance of 7075A1. The 7075 aluminum alloy matrix broke to denudation and stress corrosion in denudation solution. When the oxide film is thin, the tensile stress promotes the occurrence of intergranular corrosion of aluminum alloy. The thick oxidation film can improve the resistance to erosion and stress corrosion resistance ability of 7075 aluminum alloy in the denudation solution greatly. |
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