Research On The Performance Of Ag/AgCl And Ag/AgX Reference Electrode Prepared By Thermal-dip Coating Method

In the impressed current cathodic protection(ICCP) system for marine metallic structures,it is necessary to use reference electrodes to make measurement of the potential in order to monitor the protection status of the structure.At the same time, reference electrode can also provide control signal for the power source of autorectifier (or potentiostat).Being used in cathodic protection engineering,reference electrode should usually have good potential stability and reproducibility;favorable anti-polarization ability and polarization potential stability;long service life and easily prepared,etc.The reference electrode of Ag/AgCl has long been considered as a suitable reference electrode for the cathodic protection in seawater.Because of the existence of Br in seawater,AgCl and AgBr will become solid solution.So Ag/AgX electrode is also used as reference electrode in ocean engineering.At present,the Ag/AgCl reference electrodes used in cathodic protection engineering are mostly prepared with powder-pressed method.But using this kind of approach,the reference electrodes will easily suffer from gradual loosing and loss of the powder in the course of service and impact the service life of the reference electrodes.In this paper,in order to deal with the problems of reference electrode such as short service life and less mechanical strength,Ag/AgCl and Ag/AgX reference electrodes were prepared with thermal dip-coating method,and the performance of the electrodes was tested. The results show that the reference electrodes studied are characteristic of good potential stability,smaller polarization resistance and good anti-polarization ability.Ag/AgCl and Ag/AgX reference electrodes were prepared by thermal dip-coating with hydrogen reduction and electrochemically reductive methods.The microstructure of the reference electrodes were analysed with scanning electron microscopy(SEM),energy dispersed X-ray spectroscopy(EDX) and X-ray diffraction(XRD).And the performance of the electrodes by different reductive methods was tested by electrode potential monitoring,polarization under constant current,linear polarization testing,electrochemistry impedance spectroscopy(EIS).It shows that the surface of the electrodes prepared by thermal dip coating-hydrogen reduction method is not uniform with island-like areas containing reduced Ag particles preferentially on the grain boundary.The electrode prepared by electrochemical reduction has smaller internal resistance,larger active surface area and better anti-polarization ability than the electrode with hydrogen reduction.Because the environment is complicated in engineering project,the influence to reference electrode potential by various factors are studied,such as chlorine ion concentration,temperature,light illumination,speed of water flow,pH value,Br^- and so on.The result showed that the Ag/AgCl and the Ag/AgX reference electrode prepared by thermal dip-coating have good Nernst response and temperature response characteristic.If dissolved oxygen existed in the solution,the electrode surface will turn into Ag₂O under sunshine and the electrode potential will drift.The flow rate of seawater has great effect on electrode's electrode potential,however,the change of pH value has no effect on the electrode potential.The Br^- can make the Ag/AgCl electrode potential decrease.And the decrease of Ag/AgX electrode potential is more obvious.The failure mechanism of electrode is studied by acceleration method.The microstructure of the reference electrode were analysed with kinds of microscopic analysis method and EIS in electrode failure process.The life of the new reference electrode was evaluated through the polarized capacity test.The result indicates the electrode surface transform into Ag completely at the end.The failed electrode can regenerate through the reverse electrical current,but its life will reduce greatly. Through the polarized capacity test,the electrode was estimated to be used for more than 20 years.The course of dissolution of AgCl at the Ag/AgCl electrode surface is studied with EQCM.The microstructure and component of electrode surface is studied using SEM,EDX,and the relation between AgCl content at the Ag/AgCl reference electrode surface and the electrode potential is discussed.The result indicated that the electrode potential is quite stable and closed to standard electrode potential when the mass percent of AgCl is 8%～97%.