Electrochemical Investigation On The Stress Corrosion Cracking Of High Strength Aluminum Alloy

This paper studied electrochemical and microstructure characteristics of AA7A04 high strength aluminum alloy in the process of stress corrosion cracking(SCC). A self-made C-ring electrolytic cell was used to analyze the SCC crack initiation and propagation process of AA7A04 aluminum alloy in 3%Na Cl solution, and the load stress of the C-ring sample was about 90% yield strength. Meanwhile ECN and 3D microscope were also used to real-time monitor noise peaks and corrosion morphology in different crack development stage, ECN spectra and time domain statistical were also analyzed. The results show that the cracking development can produce intensive and regular potential and current noise peaks. In addition, the beginning time and frequency of noise peaks are consistent with the initiation and growth process of cracks via online microscope observation. By shape and lifespan analysis of noise peaks, we found that the cracking development is apparently periodic other than continuous. Further tests indicate that the deepening process of crack is almost ceased due to the relaxation of stress on the C-ring specimen. Meanwhile, the dense short-time noise peaks can be observed, which is possibly induced by the metastable pit corrosion along the crack mouth.In this paper, slow strain rate test(SSRT) and ECN were used to analyze the noise characteristics during SCC process of AA7A04 aluminum alloy in 3.5%Na Cl solution, and the spectra and time domain statistical analysis of ECN were conducted. It was found that the ECN of tensile specimen in elastic deformation stage is mainly pitting noise. When the tensile stress after the yield strength, the current baseline appeared larger fluctuation, and the characteristic current peaks were also appeared, at this time the crack entered the rapid development stage. When the tensile stress after the highest tensile strength, the current baseline's fluctuation decreased, and the characteristic current peaks were also disappeared, at this time the crack entered the fast fracture stage.SSRT was used to study the SCC susceptibility of AA7A04 aluminum alloy at different polarization potential and different temperature and different oxygen concentration. The results show that cathodic polarization and anode polarization both increased the SCC susceptibility, cathodic polarization enhanced the hydrogen brittleness, and anode polarization accelerated the anodic dissolution. With the increase of temperature, the sample anodic dissolution was accelerated and the SCC susceptibility was improved. When the oxygen concentration decreased the cathode reaction of substrate surface was restrained, and the electron loosed by anodic dissolution of crack tip was not consumed in time, so the crack propagation rate decreased and the SCC susceptibility also decreased. On the other hand, the oxygen concentration increased and the SCC susceptibility increased.This paper also studied the cerium salt, 8–hydroxyquinoline and Na F's SCC inhibition of AA7A04 smooth and notched specimen. We found that cerium salt and 8–hydroxyquinoline had good inhibitive effect on smooth specimen and poor inhibitive effect on notched specimen. But Na F had poor inhibitive effect on smooth specimen and good inhibitive effect on notched specimen. It mainly because Cerium salt and 8-hydroxyquinoline inhibited pitting on the specimen substrate surface, extended the crack initiation time. But due to large radius it's difficult to spread to the crack tip to have good inhibitive effect on crack propagation. And the radius of F- is small, so it's easy to spread to the crack tip and inhibit the crack propagation.