Study On The Effect Of Mechanical Loadings On Galvanic Corrosion Behavior Of The Welded Joints In The Nuclear Steam Turbine

Galvanic corrosion would take place when the welded joint of nuclear steam turbine is immersed in the solution with low temerature and high humidity of wet steam due to non-uniform distribution of chemical elements and microstructures.Stress corrosion cracking may be induced by the mechanical stress and thermal stress in operation.These may lead the welded joint to becoming one of the most susceptive zones in the steam turbine rotor.Focusing on the effect of mechanical loadings on galvanic corrosion behavior of the NiCrMoV steel welded joint,the studies including the effect of low-cycle fatigue damage on galvanic corrosion,pit evolution around the fusion line and the fracture mechanism caused by the interaction of galvanic corrosion and stress-assisted coupling corrosion were carried out.The main conclusions are listed as follow.(1)The decrease of dislocation density and the formation of low-energy structures induced by low-cycle fatigue damage enhance the corrosion resistance of the individual zone in the welded joints.With the increase of cycles or strain amplitudes,the accumulated low-cycle fatigue damage increase further the corrosion resistance.The corrosion potential variation of base metal is larger than that of weld metal under the same low-cycle fatigue damage,which may depend on the microstructures and chemical elements.However,the galvanic corrosion susceptibility of the welded joint is increasing with the increase of cycles,which is attributed to the increasement of the difference of corrosion potential between base metal and weld metal caused by the different degrees of low-cycle fatigue damage among various regions in the welded joint during the cyclic plastic deformation.(2)As the anode of galvanic corrosion in the welded joint,coarsen-grained heat affected zone(CGHAZ)would corrode preferentially with the highest positive corrosion density according to the results of scanning vibrating electrode technique(SVET)tests.This is in consonance with the lowest corrosion potential of CGHAZ acquired by the potentiodynamic polarization curves measurements.(3)The two-parameter Weibull distribution function is used to describe the distribution of the corrosion pits formed under the action of different tensile stresses at different immersion times.The Weibull moduli of the pit depth evolution suggests that the nucleation of the corrosion pits,which depends on the distribution of defects,is statistically random at the beginning of the immersion corrosion tests.The local galvanic corrosion likely is the dominant mechanism controlling the corrosion of the CGHAZ in the welded joints for the immersion time equal to or less than 588 hours under the experimental conditions.For the immersion time equal to or larger than 1080 hours,stress-assisted corrosion becomes the dominant mechanism controlling the corrosion of the CGHAZ for the experimental conditions,especially for the tensile stresses of 0.6 and 0.9 ay,the average pit depth grows with the increase of the tensile stress.(4)With the increase of applied tensile conatant loading,both the time to fracture and the percentage of stress corrosion cracking area of the welded joint decrease.For the applied stress equal or lager than 765MPa,the fracture locates in the middle of WM due to its lowest strength.The fracture locates in the fusion zone for the applied stress equal or less than 760MPa.This is resulted from the stress corrosion cracking originating from the pits induced by the interaction of galvanic corrosion and stress-assisted corrosion.