Study On The Corrosion And Stress Corrosion Cracking Of The Scratched Surface Of Alloy 690TT In Nuclear Power Plants

The steam generator(SG)heat-exchanger tubes in pressurized water reactor(PWR)is an important barrier between the primary and secondary loops which are full of high temperature and high pressure water.The inner and outer walls of the SG tubes are respectively served in two kinds of high temperature water with different temperatures and pressures.In the heat-transfer crevices of secondary loop,the metal ions and chemical impurities,such as sodium,sulfate,etc.,can be concentrated to a high level.The pH value of the local SG crevice environment can range from acidic to alkaline,resulting in the different types of stress corrosion cracking(SCC)failures.The stress corrosion cracking behavior is not only related to the local complicated solution environment,but also the abnormal surface defects,such as scratches.The service life of SG tubes with the scratch defects is much shorter than the design life,which indicates that the failure of SG tubes induced by scratches has become one of the major safety hazards in the Nuclear Power Plants(NPPs).Firstly,the oxidation products of pre-deformed Alloy 690TT in high temperature caustic solution was investigated systematically and comprehensively by the focused ion beam(FIB)and advanced transmission electron microscopy(TEM).Then,the corresponding corrosion mechanism was proposed.Secondly,according to the study on the oxidation products and strain distribution at the SCC crack tip of Alloy 690TT in high temperature caustic solution,the SCC model of Alloy 690TT under the coupling influence of cold work and intergranular carbide was proposed.Thirdly,the quantitative effect of scratch size on the corrosion and stress corrosion behavior of Alloy 690TT was studied.The corrosion rate kinetic equation of scratched surface in the high temperature caustic solution was established.At last,the distribution of creep cavities and creep-induced SCC behavior underneath the scratched surface of Alloy 690TT was also investigated in the simulated primary water.The main results were listed as follows:(1)The oxide film was comprised of the surface oxide layer and the inner oxide layer.The surface oxide layer exhibited a double layer structure with semi-coherent surface Ni-rich phases and subsurface Fe-rich phase.The inner oxide layer exhibited a multilayered structure:outermost large-sized NiO particle layer,transitional mixed Cr-rich spinel,NiO and Ni layer and inner Cr2O3 layer.Furthermore,the preferential oxidation channels extended along the grain boundaries and deformation bands inward respectively.(2)The SCC crack on scratched surface of Alloy 690TT in the high temperature caustic solution propagated along the oxidation zone on the grain boundaries.The oxidation zone on the crack wall was composed of mixed oxides including NiO,Ni,spinel oxide and Cr2O3,and the Cr2O3 was mainly distributed near the interface between the oxidation zone and the matrix.Due to the competitive effect of intergranular carbides and local deformation zone(LDZ)on diffused oxygen,the oxidation zone ahead of crack tip bypassed the carbides as branched oxide and then grew up to be nodular oxidation zone,which resulted in a semi-continuous nodular pattern for the oxidation zone along the crack.(3)The corrosion behavior of Alloy 690TT was aggravated by the accelerated growth of inner oxide layer on scratched surface,due to the high strain zone formed by scratching process and tension stress.The thickness distribution of inner oxide layer on scratched surface was approximately described by the Gaussian function,while that of surface oxide layer followed a linear distribution.As the ratio(?)of scratch depth(d)to scratch tip radius(p)increased,the inner oxide layer thickness at scratch tip increased linearly.The SCC at the scratch tip did not take place until? increased to a critical threshold value.The surface oxide particles near the crack mouth became sparse and the corresponding inner oxide layer became thinner,which was mainly affected by the low stress zone formed near the crack mouth.(4)With the influence of applied load,the creep cavities could be formed in the stress concentration areas on the interface between the intergranular carbides and the matrix.The distribution density of creep cavities was related to the strain level.As the strain gradient decreased,the cavities density decreased.For scratched surfaces,the cavities distribution was mainly concentrated in the high strain zones underneath the scratches.The cavities in the high strain zone underneath the scratch were mainly distributed on the grain boundaries close to the applied load direction,and the directional distribution became gradually inconspicuous as the strain gradient decreased.In addition,the creep cavities tended to form near the intergranular carbides distributed in a semi-continuous line along the grain boundaries,which was beneficial to the SCC propagation.However,the cellular carbides locally might not be conducive to the formation of creep cavities and inhibited the growth of SCC cracks.The initiation and propagation of SCC cracks could be promoted by the formation of creep cavities.Firstly,the creep cavities played the role of crack embryos in the SCC initiation process.Secondly,the formation of creep cavities would weaken the bonding force of grain boundaries,thereby accelerating the crack propagation along the grain boundaries.In the high strain zone underneath the scratch,the SCC cracks mainly propagated along or close to the direction of applied load.