The S-N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages: the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.

The effect of prior corrosion on the mechanical properties of 7475-T761 aluminum alloy was investigated by immersion test, stress corrosion test, cathode charge method and electrochemical polarization test. Results show that prior corrosion in the solution with 3 wt% NaCl and 0.5 wt% H 2 O 2 leads to mechanical properties deterioration of 7475-T761 aluminum alloy. Moreover, the elongation decreases significantly. This is mainly attributed to electrochemical corrosion and hydrogen embrittlement, in which corrosion plays a major role. Tensile stress promotes the degradation of the mechanical properties by accelerating the pitting corrosion and hydrogen embrittlement.

Ceramic coatings were fabricated on 7A85 aluminum alloy by unipolar positive pulsed micro-arc oxidation (MAO) treatment for enhancing corrosion resistance. A bi-layer coating was formed with large pores between the layers. The outer layer contained several micro pores and cracks. Therefore, cerium nitrate, potash bichromate and SiO 2 sol were adopted to seal the MAO coatings. Scanning electron microscope, X-ray diffraction and electrochemistry instrument were used to investigate the effect of sealing treatments on the surface morphology, coating structure and corrosion resistance of MAO coatings. Results show that the MAO coating can not protect aluminum alloy from acid NaCl solution for the existing of defects. The defects can be sealed by precipitations produced by cerium nitrate and potash bichromate sealing treatment, and corrosion resistance of the MAO coating is improved. However, the sealed coatings also fail in the acid NaCl solution for dissolution of the precipitation. After sealing with SiO 2 sol, the MAO coating is covered with a SiO 2 gel layer has become a compact barrier layer to protect aluminum substrate from corrosion in acid NaCl solution.