In-situ Observation Of Adsorption Behavior Of Corrosion Inhibitor On Magnesium Alloy Surface And Study On Its Corrosion Inhibition Mechanism

Magnesium and its alloys are known as"one of the star materials in the future metal industry"because of their advantages such as low density,high specific strength,large elastic modulus,strong electromagnetic shielding and good biocompatibility.However,magnesium alloys are easy to be corroded due to its low electrode potential and active chemical properties.Corrosion inhibitors are widely used to inhibit the corrosion of magnesium alloys due to their advantages of environmental protection,economic efficiency,etc.The corrosion inhibition mechanism is mainly attributed to the formation of a protective film on the anode（cathode）phases of the alloy,which blocks the charge or material transfer between the corrosive medium and the anode（cathode）phases,thus inhibiting the anode dissolution or cathode activity of the alloy.However,the corrosion inhibition mechanism of existing corrosion inhibitors on magnesium alloys is basically explained by the electrochemical action of corrosion inhibitors,and the lack of in-situ observation of the adsorption behavior of corrosion inhibitors on the anode（cathode）phases of magnesium alloys leads to the lack of evidence for the development of efficient corrosion inhibitors.In this paper,a fluorescence inhibitor and its in-situ observation device were designed and developed to realize fluorescence visualization of the adsorption behavior of the inhibitor on the surface of magnesium alloy.The corrosion kinetics of the alloy under the action of the inhibitor and the corrosion inhibition mechanism of the inhibitor molecules were revealed by combining the micro-electrochemical in-situ observation technique and theoretical calculation.The main research contents of this paper are as follows:1.The adsorption behavior and long-term protection mechanism of sodium dodecyl benzene sulfonate（SDBS）on AZ31 magnesium alloy surface in 3.5 wt.%Na Cl solution were studied.The corrosion inhibition and adsorption behavior of the alloy before and after the addition of the inhibitor were characterized by electrochemical and surface analysis.The electrochemical results show that the corrosion inhibition efficiency reaches the maximum value of 93.9%after 72 h of immersion in the solutions containing 0.008M SDBS.The improvement of corrosion inhibition performance can be attributed to SDBS forming a uniform hydrophobic protective layer on the alloy surface through physical and chemical adsorption,which isolates the alloy surface from the electrolyte.Combined with molecular dynamics simulation,the long-term corrosion inhibition of SDBS molecules on Mg（0 0 0 1）surface was revealed.2.The evolution of the local corrosion area of WE43 magnesium alloy before and after the addition of sodium silicate（SS）was observed in real time.In situ observation showed that the surface of the alloy was covered with a transparent protective film after adding SS inhibitor,and the local corrosion was obviously inhibited.Electron Probe Micro Analysis（EPMA）further verified that a uniform protective layer of magnesium silicate with a thickness of～2μm was formed in the uncorroded area of the alloy surface.The electrochemical data showed that the corrosion inhibition efficiency of the alloy was obviously improved after SS inhibitor was added.Quantum chemical calculations indicated that4)₃^2-can be adsorbed on the surface of Mg O（1 0 0）in parallel direction through the coordination bond between O and Mg atoms,and the adsorption/interaction mechanism of4)₃^2-on the Mg O（1 0 0）surface was analyzed by charge density distribution and atomic density.In the local corrosion area,the free4)₃^2-reacts with Mg²⁺and OH^-to form insoluble magnesium silicate compounds,which hinders the corrosion expansion.3.A cationic fluorescent dye was designed and synthesized to label sodium dodecyl sulfate（SDS）inhibitor and visualized the adsorption behavior of SDS on theα-Mg andβ-Li phases of Mg-Li alloy.In situ observation found that obvious fluorescence adsorption was preferentially observed on theα-Mg phase,whileβ-Li phase had no obvious adsorption.In addition,stronger fluorescence intensity was observed at the local corrosion site.The corrosion inhibition mechanism was explained from the perspective of corrosion kinetics by micro-scale in situ observation under electrochemical control.The potential of the adsorbed film is only18 m V,which can effectively inhibit the micro-galvanic corrosion.Through experimental and theoretical calculations,it was confirmed that a SDS protective layer was formed on theα-Mg phase through physical and chemical adsorption.At the local corrosion point,SDS would chemically react with Mg²⁺,which precipitated at the corrosion point and inhibited the further expansion of corrosion.4.Based on the ideas of Chapter 3,a new fluorescent dye（SN）with lipophilic was designed and synthesized based on the principle of cell membrane imaging,and the hydrophobic part containing N was selected as the probe,and the long alkyl chain of sodium monododecyl phosphate（SDP）was embedded through hydrophobic interaction.The adsorption behavior of SDP on the alloy surface was visualized by in situ light microscopy and confocal laser microscopy.In situ Raman was used to confirm the chemical composition changes of the alloy surface before and after adding SDP inhibitor,which further demonstrated the adsorption behavior of SDP on the alloy surface.The electrochemical test showed that the corrosion inhibition properties of the alloy were obviously improved after the addition of SDP.SDP is more inclined to adsorb on theα-Mg phase to form a membrane-like layer,which would protect the alloy from contacting the corrosive electrolyte and play a protective role.Theoretical calculation indicated that the phosphoric acid head and long alkyl chain of the SDP inhibitor molecule are parallel to the surface of Mg（0 0 0 1）,thus providing a comprehensive and compact protection.