Kinetics And Coupling Mechanism Of Local Electrode Processes At The Interface Between Droplets And Pure Iron

Local corrosion of droplets is a typical corrosion process in the early stage of marine atmospheric corrosion.However,the mechanism of local electrochemical kinetics of metals under droplets has not been clarified.In this paper,the kinetics and coupling mechanism of the local electrode process at the interface between droplets and pure iron are studied in order to find out the evolution process of local corrosion of pure iron under droplets.This will not only help to clarify the mechanism of local corrosion induction at the initial stage of marine atmospheric corrosion,but also help to deepen the development of the basic theory of local corrosion electrochemistry.Firstly,a concentric three-electrode array is designed.Based on NI-PXI platform,digital versatile,high-speed switch and potentiostat,a local electrochemical testing system is set up,and the electrochemical research method for local corrosion of droplet system is established.The interface electrochemical distribution of pure iron in different volume of 3.5%sodium chloride droplets after different coupling time was studied by using a newly developed three-electrode array local electrochemical method.The electrochemical distribution characteristics of droplets are affected by droplet size.The distribution of corrosion potential and current on CTEA is symmetrical in a circular direction under 110μL droplets,while the electrochemical distribution under 25μL droplets is characterized by adjacent peaks and valleys,while that under 340μL droplets is a number of adjacent peaks and valleys.The droplets exhibit asymmetric electrochemical distribution when the volume of droplets is 25μL and 340 μL,respectively.It can be explained by the effective influence distance between droplet size and corrosion couple.The effective influence distance of corrosion couple is determined by the corrosion potential difference between anode and cathode,current,anode polarization and cathode polarization.The trend chart of current standard deviation shows that after 60 minutes of coupling,the local corrosion tendency of110μL droplets is greater.Therefore,droplets with appropriate size and symmetrical electrochemical distribution are more likely to cause the occurrence and development of local atmospheric corrosionThe electrochemical impedance spectra and polarization curves of local electrode units under different conditions were measured by a concentric circle three-electrode system.The results show that there are significant differences in the corrosion reaction of the local electrode unit,which is closely related to the size and location of the droplets.The local anode is activated and dissolved,and the local cathode is mainly controlled by the oxygen diffusion process.As a typical inhomogeneous corrosion system,droplet geometry makes its internal oxygen concentration different.At this time,droplet and metal system form oxygen concentration cell,which produces distinguishable anode and cathode,and accelerates the corrosion localization process of pure iron under sodium chloride droplet.The electrochemical response of 3.5% sodium chloride droplets on the surface of pure iron was studied by using a new concentric three-electrode array method.Volume expansion and the newly covered electrodes of droplets play a cathodic role in corrosive batteries,which enhances the local current density of the anode,and the area and position of the anode remain unchanged.When the coverage area remains unchanged,the trend of local corrosion decreases with the decrease of thickness,which is attributed to the uniform oxygen concentration tank.Therefore,with the increase of coverage area or thickness,dynamic droplets can promote and accelerate the occurrence and expansion of atmospheric corrosion.