| The operational nuclear power plants in China are mainly the second generation and the improved second generation pressurized water reactors.Z3CN20.09M stainless steel has been widely used as primary pipe in these nuclear power plants for its excellent mechanical properties and local corrosion resistance due to 12?20 vol.%ferrite contained in its austenite matrix.However,strict service environment(280?320?,?15 MPa)will cause the Z3CN20.09M stainless steel to undergo the thermal aging,decreasing its plastic toughness,corrosion resistance.In the past few decades,a large number of researchers have studied the thermal aging embrittlement and corrosion resistance,but the mechanism of deterioration of pitting resistance by thermal aging is still unclear.The main reason is the Fe-rich a phase,the Cr-rich ?' phase and the Si-rich G-phase formed during the thermal aging process are several nanometers in size.It is difficult to directly determine the role of a phase and the G-phase in the pits initiation process.Pitting is a high destructive type of corrosion that is often act as the crack source of corrosion fatigue and stress corrosion cracking.Once pitting corrosion occurs,it is easy to cause perforation damage in the primary pipe,which endangers the safe operation of nuclear power plants.In this paper,the effect of thermal aging on pitting corrosion resistance of Z3CN20.09M stainless steel is studied thoroughly.The precipitation behavior of G-phase is analyzed,the role of a phase and G-phase in pits initiation is studied,and the mechanism of thermal aging deteriorating pitting corrosion resistance of Z3CN20.09M stainless steel has been revealed.The main results are as following:(1)The electrochemical properties of samples thermally aged for different times are studied by an electrochemical workstation,and the surface morphology of samples aged for different times after polarization test is observed by SEM(Scanning Electron Microscope).The results show that the pitting corrosion resistance of Z3CN20.09M stainless steel is seriously deteriorated by thermal aging.The pitting potential of the specimens for different aging times decrease significantly with increasing aging time from 418.9 mVSCE of the un-aged to 132.8 mVSCE of the sample aged for 3000 h.Meanwhile,the stability and protection of the passive film decrease remarkably,and the defect concentration increase with decreasing the thickness of passive film.The location of pits on the surface of samples changed after the polarization test,which is transformed from the austenite phase of un-aged specimen to the ferrite phase of the samples thermally aged for 2000 hand 3000 h.(2)The structure and composition of G-phase are characterized by TEM(Transmission Electron Microscopy).The G-phase has a FCC structure(space group Fm-3m),its lattice parameter is 3 times than ferrite phase.From the result of TEM-EDS(Energy Dispersive Spectrometer)analysis,the Ni,Mn and Si elements are mainly enriched in the G-phase,in addition to a small amount of Fe and Cr elements in the G-phase.During the thermal aging process of 1000?2000 h,a large amount of G-phase precipitates until the equilibrium volume fraction reaches 8.5 vol.%.With the increase of thermal aging time,the size of the G-phase precipitates grows from 13.4 nm when aged for 1000 h to 29.5 nm when aged for 3000 h,which is in line with Ostwald ripening mechanism.(3)Meanwhile,the lattice misfit degree between G-phase and matrix increases from 0.043%when aged for 500 h to 4.579%when aged for 3000 h.The morphology of the G-phase changes from spherical shape to cubic shape due to the change in the elastic strain energy at the interface between the G-phase and matrix.The intensity change of the(400)G spot of the G-phase and the superstructure transformation have been found,mainly due to the rearrangement of the elements in the G-phase as the thermal aging time prolonged,resulting in the destruction of the original periodicity of the G-phase.(4)The preferential initiation position of pits is investigated by immersing the TEM sample thermally aged for 3000 h in 1 mol·L-1 NaCl solution.It is found that the pits formed preferentially at the interface between G-phase and matrix.No pits were found in the matrix.By 3DAPT(3D Atom Probe Tomography)analysis,there is a region where the mass fraction of Cr is less than 12.5 wt.%with a width about 23 nm in the Fe-rich a phase,however,the pits do not form here.Therefore,it is concluded that the Fe-rich a phase in the thermal aging sample is not the main reason for pits initiation.(5)From 3DAPT results,the Cr element concentration at the interface between the G-phase and the matrix is higher than 12.5 wt.%.The traditional theory of Cr-depleted cannot explain why the pits only formed at the interface between the G-phase and matrix.The strain field and strain energy at the interface between the G-phase and matrix are analyzed by GPA technique.It was found that the strain gradient at the interface increases sharply with the increase of thermal aging time,and the strain field also appears inside the G-phase.The maximum strain energy is at the interface between G-phase and matrix,where the passive film is less stable and vulnerable to damage by Cl-.After the passivation film is destroyed,the atoms with higher energy and activity located at the interface are easily to enter the solution,thereby forming the origin of corrosion pits.According to the TEM sample immersion test,3DAPT composition analysis and interface strain field analysis,it is concluded that the strain field at the interface between the G-phase and matrix in the thermal aging sample is the main reason for pits initiation. |
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