Synergistic Inhibition Of Self Assembly Alkyl Phosphate And Rare Earth Composite Film On Pitting Corrosion Of Aluminum Alloy

In oceanic or polluted environment, aluminum alloys often suffer severe pittingcorrosion. Rare earth ions have good inhibition effect on pittingcorrosion, along with goodenvironmentally friendly properties. Therefore, rare earth surface modification techniqueshave attracted much attention. Meanwhile, self-assembly monolayers （SAM） technologyalso come to spotlight due toits excellent anti-corrosion ability, good environmentallyfriendly and film stability. However, SAM still have some deficiencies, such as selectiveself-assembly. Therefore, in this thesis, many researches have been done to develop a highefficient surface modification technology for aluminum alloy,which is summarized as thefollowing.First, pitting process research. Electrochemical noise was used in conjunction withSEM to monitor the position of initiation pitting and the pitting of nucleation, growth anddeath process on the aluminum alloy. The results show that heterogeneous phase particles（such as intermetallic） are prone to induce local corrosion. The local corrosion process ofaluminum alloy can be divided into two stages, first, the selective dissolution of Mg, Alactive element in the intermetallic, thus leaving "honeycomb" high purity cathode phase（Al^-Fe-Si phase）, second, high purity cathode phase promoting the corrosion solution arounditself. It is because the "autocatalytic effect" of intermetallic,the metastable pitting grow.With the depletion of Al^-Fe-Si cathode phase, the nucleation rate of metastable pitting slowdown, but the average current transient charge is increased, indicating that the metastablepitting is growing, and gradually change into a steady-state pitting.Second, the inhibition mechanism of rare earth ions on pitting corrosionElectrochemical noise together with EDX were used to research the deposition mechanismand inhibition mechanism on the growth of pitting. It turned out that rare earth ion prefer toadsorb/deposit on the intermetallic, formed infusibility to inhibit the dissolution of aluminum alloy substrate and nucleation and propagation of metastable pits.The averageelectric charge （q） of metastable pits decreased with increasing the corrosion inhibitorconcentration, but the average lifespan of noise transients remained almost constant,indicating that Ce³⁺ions did not accelerate the rehabilitation of metastable pits directly, butreduced the dissolution rate of Al inside active pits.Third, the synergistic effects among Self-assembly alkyl phosphate film and rare earthcerium deposited film on pitting corrosion of aluminum alloy.In this paper, scanningelectronic microscopy （SEM）, X-ray photoelectron spectroscopy （XPS）, DXR lasermicroscopic Raman spectrometer, contact angle meter, electrochemical impedancespectroscopy （EIS）, polarization curves and electrochemical noise were used to investigatethe structure and inhibition effects of these modified film. Results show that three kinds ofmodified film have good protection effect on aluminum alloy. Self-assembly alkylphosphate-rare earth cerium composite film have higher quality and better protectionefficiency when compared to Self-assembly alkyl phosphate film and rare earth ceriumdeposited film. After immersion in3%NaCl solution for7days, the modified film stillprotect Aluminum alloy well and Self-assembly alkyl phosphate-rare earth ceriumcomposite film performs better to inhibit corrosion from Cl－. It is attributable to synergisticeffects among Self-assembly alkyl phosphate film and rare earth cerium deposited film.Deposition a layer of rare earth conversion coating before Self-assembly alkyl phosphate isnot only improved pitting corrosion resistance of aluminum alloy, but also increasedhydrophilicity of intermetallic on aluminum alloy. It is good for self-assembly alkylphosphate. The self-assembly alkyl phosphate-rare earth cerium composite film can notonly improve general corrosion resistance but also greatly strengthen pitting resistance ofaluminum alloy.