Corrosion Mechanism of 5083 Aluminum Alloy in Seawater Containing Phosphate.

As a material with good corrosion resistance, 5083 aluminum alloy has a great application prospect in marine environment. In this work, the corrosion characteristics of 5083 aluminum alloy in seawater containing phosphate were investigated with Potentiodynamic Polarization, Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy Analysis (EDSA), X-ray Photoelectron Spectroscopy (XPS) and Laser Confocal Microscope. The results indicated that the effects of phosphate in seawater were two-fold. Firstly, phosphate slightly accelerated the corrosion of 5083 in seawater in the early stage of corrosion. HPO₄²⁻ competed with OH⁻ in the adsorption process on the alloy surface, which weakened the contact between OH⁻ and Al³⁺ near the interface of the alloy, and inhibited the formation as well as the self-repair of the passive film, thus accelerating the activation dissolution process. Compared with the natural seawater, the charge transfer resistance of 5083 in the seawater containing phosphate decreased faster during the early stage of corrosion, and the corrosion current density i_corr was higher in seawater containing phosphate. On the other hand, the addition of phosphate would not affect the cluster distribution of the second phase of 5083 in seawater, but it changed the composition of the corrosion product layer and had an obvious inhibitory effect on the local corrosion of 5083 in seawater. After 16-day exposure, shallower and more sparsely distributed pits could be observed on the derusted surface of 5083 in the seawater containing phosphate, and the pitting coefficient in the seawater containing phosphate was significantly lower than that in natural seawater. The reduction of pitting tendency could be realized mainly through two ways. First, the HPO₄²⁻ adsorbed on the surface of the passive film in the early stage of corrosion and repeled the corrosive anions such as Cl⁻. Second, phosphate participated in the construction of the CaHPO₄ precipitation film, which acted as a barrier and protection. [ABSTRACT FROM AUTHOR]