Study On Fretting Corrosion Mechanism Of Zirconium Alloy And Its Typical Modified Layers

Since the 1950s,nuclear power has developed for 70 years.At present,it has accounted for about 10%of the world’s total power generation,and even 75%in developed countries such as France.With the increase of clean energy demand in China,nuclear power has been paid more and more attention due to the advantages of high efficiency,cleanness,and low cost.However,accidents in nuclear power is always an international problem,which seriously affect the development of nuclear power.Therefore,in order to maintain the stable development of nuclear power,it is necessary to protect the lifeline of nuclear power development（nuclear safety）.As the first and most important safety barrier of nuclear power,the structural integrity of fuel cladding is one of the important factors to restrict the safe operation of nuclear power.The fuel cladding is served in harsh environment,which is high temperature,high pressure,high velocity,and strong radiation water chemical condition.At present,grid to rod fretting（GTRF）has become the main reason to cause the damage of fuel cladding.It is very significant for the safe operation of nuclear fuel and the high-speed development of nuclear power to systematic study the GTRF.In this paper,typical surface modified layer,that is ultrasonic surface rolling processing（USRP）and micro-arc oxidation（MAO）coatings with different process parameters were prepared on the zirconium（Zr）alloy fuel cladding.The morphology,structure,and phase composition of the surface modified layer were analyzed.The self-developed fretting wear test equipment combined with the electrochemical workstation was used to simulate the primary circuit water chemical environment.The fretting corrosion behavior of Zr alloy and its typical surface modified layer was studied.The fretting damage area was analyzed and the interaction mechanism of friction and corrosion during fretting process was revealed.The damage evolution and degradation mechanism of Zr alloy with different test parameters and the typical surface modified layer with different process parameters were expounded.The research results can provide scientific basis and reference value for the application of surface modified layer on Zr alloy to improve the fretting corrosion resistance.The main conclusions of this study are as follows:1.Fretting corrosion mechanism of Zr alloy in simulated water chemical environmentAccording to the potential-pH equilibrium diagram of Zr-H₂O,the fretting corrosion behavior of Zr alloy under different pH solution and corrosion potential was studied.Under fretting corrosion conditions,the effect of experimental parameter on fretting damage of Zr alloy is followed:（1）In the pH of 4,7,and 10,the wear scar shows a large number of discontinuous grooves and furrow trace,and the oxides distributed on the surface is uniform.In the pH of 1 and 13,the granular wear debris mostly accumulates in the grooves,and the main wear mechanism is oxidation wear and abrasive wear.（2）The interaction mechanism of wear and corrosion in different pH environments was analyzed.The material loss caused by pure wear is minimum and the material loss caused by wear and corrosion interaction is highest in the pH of 1.Because the surface of Zr alloy is difficult to generate oxide film under acid condition,and severe corrosion accelerate wear.（3）The increase of corrosion potential will increase the corrosion current and wear volume of Zr alloy.When the corrosion potential is high,the corrosion current after fretting would continue to increase,forming typical crevice corrosion.As a result,fretting accelerates the corrosion of Zr alloy.2.Fretting corrosion behavior of ultrasonic surface rolling layer on Zr alloy（1）After USRP,the Zr alloy form a surface nanocrystallization layer which change along the longitudinal depth gradient,and plastic deformation trace are observed.The plastic deformation on the surface of Zr alloy leads to work hardening,which increases the surface hardness.Meanwhile,the surface of Zr alloy becomes smoother after USRP.（2）The Zr alloy after USRP has a lower wear width and depth.With the decrease of rolling speed,the grain size decrease and the wear resistance of Zr alloy increases gradually.Because the surface nanocrystallization layer formed on surface has a high hardness.（3）The Zr alloy after USRP shows a lower corrosion current density,which indicates that the Zr alloy after USRP refine grain and increase the number of grain boundary.Hence,the activity of Zr alloy increase,and it’s beneficial to form passivation film.3.Characterization and fretting corrosion behavior of MAO coating on Zr alloy（1）MAO coating shows a typical porous and volcano-like structure.The coating is mainly composed of monoclinic m-ZrO₂,tetragonal t-ZrO₂,and Zr.The MAO coating can be divided into inner dense layer and outer porous layer.（2）MAO coating can significantly improve the electrochemical corrosion behavior of Zr alloy,and has a lower corrosion current density.Because the inner dense layer can prevent the corrosion solution from entering the substrate.（3）The main wear mechanism of MAO coating is abrasive wear.The coating exhibits better wear resistance and the minimum wear volume is only 1/8 compared with Zr alloy.4.Characterization and fretting corrosion behavior of nanoparticle/MAO composite coating on Zr alloy（1）The nanoparticle/MAO composite coating has a smoother and denser structure,which is caused by the higher content of t-ZrO₂ phase in the coating.Nanoparticles can stabilize the high temperature t-ZrO₂ phase,and nanoparticles mainly concentrate on the outer surface and micro-pores.（2）The composite coating has a higher corrosion potential and lower corrosion current density.Because the nanoparticles in the coating can act as a good barrier to the diffusion channel of corrosion solution,and prevent the corrosion solution from entering the substrate.（3）The graphene oxide（GO）/MAO composite coating has the best fretting corrosion resistance due to the low interlaminar shear stress of GO,which is prone to relative slide during fretting,thus reducing the damage.5.Degradation mechanism of MAO coating on Zr alloy in high temperature water/steam condition（1）The corrosion process of MAO coating in superheated steam condition include two stages.The corrosion rate is relative low before the transition point,while the corrosion rate increase after the transition point.Because the cracks formed on the coating gradually propagates under the superheated steam environment for long period exposure.As a result,the corrosion weight increases.（2）After long period exposure,the outer surface of MAO coating becomes loose.The content of t-ZrO₂ phase in MAO coating decreases with the increase of corrosion time.This phenomenon indicates that t-ZrO₂ phase transforms to m-ZrO₂ phase under the superheated steam condition.（3）The MAO coating exposed for 45 days has the best electrochemical corrosion resistance.Because the new formed oxides during the corrosion process will repair the micro-pores and defects,thus further preventing the corrosive media from entering the substrate.After long period exposure,the coating began to generate cracks,and the electrochemical corrosion resistance began to reduce.