Study On Corrosion Electrochemical Property And Affecting Factors For Nickel-based Alloy 800

As well known,nickel-based alloy 800 has a high content of nickel and chromium element.As a result,it has an excellent corrosion resistance property at high temperature.Therefore,it is commonly used in industry to manufacture equipment that needs to be served in harsh environments,especially as heat transfer tube material in nuclear power plant steam generator.The corrosive environment of the heat transfer tube is relatively complicated,mainly due to the coexistence of multiple aggressive ions(mainly Cl-,SO42-and S2O32-ions)in different acid-base environments.In this thesis,the electrochemical research methods such as potentiodynamic polarization curve,electrochemical impedance spectroscopy(EIS)and Mott-Schottky method were utilized and combined with real-time,in-situ local electrochemical measurement technique-scanning electrochemical microscope(SECM),to deeply investigate the effect of main factors such as aggressive ion(type,concentration,ratio,etc.),immersion time as well as solution pH etc on the corrosion electrochemical behaviors of Alloy 800 and the semiconductor characteristics of the passive film on metallic surface.The changes of local electrochemical activity on the surface of the alloy and the formation and breakdown of passive film have been discussed.Moreover,the corrosion mechanism have been further explored.The results provided a valuable theoretical and experimental basis for the practical application of Alloy 800 in harsh environments such as nuclear power plant.The main work of this thesis is summarized as follows:1.Study on the electrochemical characteristics of Alloy 800 in aggressive solutionsIn aggressive solutions containing only Cl-or SO42-ions,potentiodynamic polarization curves,electrochemical impedance spectroscopy(EIS)and Mott-Schottky curves were measured to study the effect of concentration of aggressive ions,immersion time and solution pH on the changes of corrosion electrochemical characteristics such as alloy corrosion potential,corrosion current density,passivation current density and pitting corrosion potential.Both electrochemical behaviors of the passive film formed under different conditions and semiconductor characteristic of the passive film were explained.2.Effect of mixed aggressive ions on the corrosion electrochemical behaviors of Alloy 800The traditional electrochemical methods such as Tafel curves,EIS and Mott-Schottky curves were uesd to study the corrosion behaviors of Alloy 800 in a mixed solution containing two(Cl-,SO42-)or three(Cl-,SO42-and S2O32-)aggressive ions,respectively.In the solution with Cl' and SO42-ions mixed,it can be found that the higher the concentration of Cl-ions,the lower the corrosion resistance of Alloy 800.Additionally,by increasing the concentration of S2O32-ions,the semiconductor characteristics of the passive film on alloy surface tend to alter from type of n-p to type of n in the solution containing mixed Cl-and SO42-ions,3.In-situ electrochemical scanning probe study on the corrosion characteristics of Alloy 800(1)Cyclic voltammetry was conducted to test the performance of the Pt probe to determine the working potential during the test.Ferrocene methanol(FcMeOH)was used as the redox mediator,and the current feed-back mode of scanning electrochemical microscopy(SECM)was employed to study the effect of immersion time and pH of solutions on Faraday current on Pt probe in NaCl and Na2SO4 solution.The mechanism of the formation and breakdown of passive film on the alloy surface were explored by analyzing the 3D area scan images of SECM.The results showed that the passive film was broken down with the extention of immersion time in both acidic and neutral Cl-ion solutions.(2)By means of in-situ electrochemical scanning probe technique,i.e.,SECM,the changes of the electrochemical activity of the Alloy 800 surface with the immersion time and different ion concentration ratios in the mixed solution of NaCl,Na2SO4 and Na2S2O3 were studied.It has been found that the self-passivation of the alloy was not affected with a small amount of S2O32-ion.However,the dissolution rate of the passive film increased and pitting corrosion occurred with increasing concentration of S2O32-ion.