Synergistic Corrosion Inhibition Mechanism Of Fluoride Ion Enhanced Hybrid Inhibitor For Magnesium Alloys

Magnesium（Mg）and its alloys are highly vulnerable to corrosion attacks so as a proper corrosion protection approach is of necessity before application.Employing corrosion inhibitors is one of the most efficient methods to suppress the corrosion reactions,which the inhibitor is a kind of chemical compound that can decrease the corrosion rate of a metal merely with little addition to the medium.Currently,the efficiency of solely used inhibitors are not sufficient to meet the requirements of industry and,the selection is quite limited for application.Herein,a strategy that by combining two or more inhibitors is developed and has successfully discovered a number of hybrid inhibitors with synergistic effect in recent years.Combining halogen（F^-、Cl^-、Br^-、I^-）with other inhibitors is one of the most commonly used strategy to develop synergistic hybrid inhibitor,however,the corrosiveness of themselves cause the usage of halogen as a component of hybrid inhibitor for Mg metals to be uncertain and controversial.Therefore,the aim of this study is to figure out the feasibility of halogen ions as a component of hybrid inhibitor,and develop novel halogen ions based hybrid corrosion inhibitors for Mg alloys and reveal their synergistic inhibition mechanism.In this work,the corrosiveness of halide ions themselves to Mg was first examined.Through electrochemical tests and morphology analysis from transmission electron microscopy（TEM）,it was determined that F^-is the weakest corrosive ion to Mg,while Cl^-has the strongest corrosiveness.F^-,in the lower concentration range,is able to inhibit the corrosion of Cl^-,while Br^-and I^-have no inhibitory effect on Cl^-.Thereafter,selecting F^-as the fixed component of the hybrid combination.The presented study designed a image recognition based high-throughput screening（HTS）strategy for hybrid inhibitor and has successfully screened out two different type of combinations with significant synergistic effect,that is the precipitation-enhanced hybrid inhibitor,sodium fluoride（NaF）with malic acid（DMA）,and precipitation-adsorption co-enhanced hybrid inhibitor sodium fluoride（NaF）with 5 m M 3-amino-5-mercapto-1,2,4-triazole（ATT）,which the inhibition efficiency reached 94.1%and 95.8%,respectively.Through the cooperation of experimental techniques（electrochemical impedance spectroscopy（EIS）,X-ray photoelectron spectroscopy（XPS）,thermodynamic equilibrium phase diagram calculation）with quantum chemical calculation（QC）and molecular dynamics simulation（MD）methods,the synergism of two combinations was elucidated.The synergistic effect between NaF and DMA is resulted from the regulation effect of DMA on the nucleation and growth of NaMgF₃ particles,whereas the synergism between NaF and ATT is originated from the enhanced adsorption of ATT together with the F ion induced transformation of Mg（OH）₂ into a more stable products Mg（OH）_2-xF_x.This study proves the feasibility of F^-as the component of hybrid corrosion inhibitor for Mg alloys,and reveals two novel synergistic inhibition mechanisms from both experimental and theoretical perspective,which provides a useful guidance for the further development of more efficient corrosion inhibitors.