Study Of Inhition Effects For Pitting Corrosion On An Aluminum Alloy With Three Inhibitors

Pitting corrosion occurs frequently on Al and alloys while containing aggressive ions such as chloride (Cl-) in the environment. This had caused serious damage to property. It is significantly important to look forward to the prevention and controlling of pitting corrosion by researching the mechanism of pitting. The use of inhibitors is a highly efficient and common method to prevent the uniform corrosion of metals and alloys. But there was little study for controlling pitting corrosion by using inhibitors.Potentiodynamic polarization, Mott-Schottky, scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) were applied to study the corrosion behaviors and the inhibition mechanisms of pitting corrosion for aluminum alloy2024-T3in the solutions of0.1mol/L NaCl with different amounts of inhibitors The additional inhibitors were sodium stannate (Na2SnO3), trisodium phosphate (Na3PO4) and sodium oleate (C17H33COONa). The inhibition effects of uniform corrosion and pitting corrosion were discussed and the variations of the corrosion potential Eb and the repassivation potential Ep what could characterize the pitting corrosion behaviors were summarized. In addition, the effects of inhibitors on the pitting initiation and the repassivation process were researched by the summaries of Eb and Ep with different concentrations of various inhibitors. What’s more, the initiation rates and the processes of growth and repassivation of metastable pitting corrosion were researched by statistical methods with incremental concentrations of C17H33COONa in the0.1mol/L NaCl solution. The changes of the structures and stabilities for the surface films on AA2024-T3were discussed and the mechanisms of various inhibitors for pitting corrosion were interpreted. The main conclusions are as followed:(1) The uniform corrosion behaviors on AA2024-T3are inhibited by the right concentrations of three inhibitors. All of inhibitors can promote the repassvation of AA2024-T3and reduce the susceptibility to pitting corrosion. C17H33COONa shows the best performance on the reducing and the repassvation processes among three inhibitors. With the right concentrations which can efficiently promote the stable pitting corrosion potential, Na2Sn03and Na3PO4show little effect on the repassovation process of the stable pitting corrosion. C17H33COONa has showed the efficient inhibition on the uniform corrosion and the stable pitting with a wide concentration range (0.05-0.4g/L). But it has little effect on the repassivation of the stable pitting corrosion.(2) The defect concentrations on the aluminum surface films decrease with the right concentrations of inhibitors. The stabilities of the surface films increase with inhibitors. The semiconductor structures of the surface films are changed from p-type to n-type after the additions of inhibitors.(3) One same part on the inhibition effects of inhibitors is that the combination between OH-and Al3+can form a surface film which can promote the passivation on the surface. The oxide formed by Na2SnO3covers on the surface with promoting the passivation of the surface what suppresses the pitting corrosion. A dissolution film formed by the combination Al3+and PO43-covers on the surface what can suppress the corrosion. PO43-has a strong hydrolysis in the solution. The pH increases significantly with high additions of PO43-which thinned the Al(OH)3passivation layer. So the passive current density increases and it is not conducive to the promotion of the passivation of the surface. C17H33COO-can form the protective film on the surface by the chemical adsorption of the polar portion. The non-polar portion has the hydrophobic property to prevent Cl-adsorbing on the surface to inhibit the corrosion.(4) With incremental amounts of C17H33COONa, the stable pitting corrosion potential Eb shifts to positive firstly and then remains stable even a slight increasing. The metastable pitting corrosion potential Em increases continually and the difference of (Eb-Em) increases then decreases with the increasing concentrations of C17H33COONa. The increasing difference of (Eb-Em) can promote the transformation from the metastable pitting to the stable pitting with high concentrations. But the increasing metastable pitting potential is conducive to reduce the metastable pitting by inhibiting the nucleation of metastable pitting corrosion. It remains conducive to inhibit the stable piiting.