Study On Semiconducting Properties Of Passive Film And Corrosion Electrochemical Behaviors Of Alloy 690

Steam generator(SG)heat transfer tubes play a very important role in the safe operation of Pressurized Water Reactor(PWR)nuclear power plants,the tubes working environment is very bad as the important equipment connecting primary circuit and secondary circuit.Because of the good mechanical properties and excellent corrosion resistance properties in high temperature and high pressure environment,nickel-based Alloy 690 is widely used in pressurized water reactor nuclear power plant steam generator heat transfer tube material.Nickel-based alloys have excellent corrosion resistance in high-temperature aggressive solutions.Because the alloy will self-passivated to form a passive film with a thickness of several or hundreds of nanometers.The passivation films of alloy and metal prevent the connection of matrix and corrosion media so corrosion phenomenon can not occur.Studies show that the passive film formed on the surface of mental and alloy has a semiconductor structure,and it's semiconductor electronic properties are closely related to the corrosion resistance of passive film.In the actual working environment,the different aggressive ions(such as sulfate ion and chloride ion)and the local acid-alkali environment have different influence on the properties of passive film.Therefore,it has great theoretical and practical value to study the semiconductor properties and corrosion resistance of passivation film in corrosive environment.In this thesis,Alloy 690 was used as the working electrodes.The Cl-and SO42-were chosen as main aggressive ions.Traditional electrochemical methods like potentiodynamic polarization curves,electrochemical impedance spectroscopy(EIS)and Mott-Schottky curves were used to study the effects of ion concentration,immersion time,polarization potential and pH values on electrochemical behavior of the alloy and properties of the passive film.In addition,local electrochemical measurement techniques-scanning electrochemical microscopy(SECM)was used for further study.In feed-back mode of SECM measurements,the electrochemical activity and corrosion characteristic of the local area on alloy surface were investigated by probe approach curves and area scan imaging.The main work of this paper is as follows:1.Traditional electrochemical studies(1)In neutral NaCl,Na2SO4 solution,the potentiodynamic polarization curves,electrochemical impedance spectroscopy(EIS)and Mott-Schottky curves were used to study the influence of ion concentration,immersion time,polarization potential on electrochemical characters such as corrosion potential,corrosion current,passivation current,pitting potential.And the further analysis and fitting of the obtained data were carried out to explain the electrochemical behavior and the semiconductor properties of the passivation film formed under different conditions.(2)Influence of pH on electrochemical behavior of Alloy 690 in 0.5 M NaCl solution was studied by potentiodynamic polarization,electrochemical impedance spectroscopy and Mott-Schottky plots.The corrosion resistance and surface activity of the passive film in different pH solutions were compared.2.Local electrochemical studies(1)Cyclic voltammetry experiments were performed to test the property of Pt tip.The diffusion coefficient of FcMeOH could be calculated by using limiting diffusion current density.The calculated value was 7.37×10-6 cm2/s.(2)The feed-back mode of SECM was used with FcMeOH as redox mediator for studying the influence of immersion time,polarization potential on farady current.The local electrochemical activity distribution was correlated with the micro-electrochemical reaction of the metal/solution interface to characterize the corrosion of the alloy surface.The change of the properties of the passive film on the alloy surface was investigated by the probe approach curves,and the formation process and the change of the density of the passivation film were investigated.(3)The change of the electrochemical activity of the alloy with the immersion time and polarization potential in the acidic,neutral and alkaline 0.5 M NaCl solution was studied.