Initial corrosion behavior and mechanism of 7B04 aluminum alloy under acid immersion and salt spray environments

The initial corrosion behavior and mechanism of 7B04 aluminum alloy under acid immersion and salt spray environments (pH = 3.5) are studied by Scanning Electron Microscope (SEM), optical microscope, Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD), potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS), and Scanning Kelvin Probe (SKP). The results show that pitting corrosion occurs at the initial corrosion stage, and the potential difference between the second phase particles is the main cause of pitting. Pitting pits on different locations gradually expand and coalesce with the proceeding of corrosion. The main components of corrosion products are Al₂O₃, Al(OH)₃, and AlCl₃, and the generation rate of the corrosion product layer under the salt spray environment is larger than that under acid immersion environment. Under both environments, the Volta potential distribution first disperses and then concentrates, while the charge transfer resistance first decreases and then increases with the corrosion time. The Volta potential gradually shifts in a positive direction, indicating that corrosion products have an inhibitory effect on corrosion. After the same corrosion time, the corrosion product layer resistance and the expectation of the Volta potential of the salt spray sample are higher than those of the immersion sample. Comparatively, the corrosion current density for the salt spray sample is significantly lower than the immersion sample, which indicates that the thicker the corrosion product layer, the stronger the inhibition of corrosion reaction.