Stress Corrosion Crackingbehaviors Of Primary Component Weld Joints Under PWR Primary Water Environment

The stress corrosion cracking of primary circuit pressure boundary main equipment welded joints bas been a critical problem in pressurized water reactor,which seriously hindered the operation security and economical efficiency of nuclear power plants.The effects of electrode potential on high-temperature corrosion behaviors of stainless steel 316 NG weld joint and 508?-52M-690 dissimilar weld joint of safe-end were researched systematically by slow strain rate tensile tests and high-temperature electrochemical corrosion tests in high-temperature high-pressure water containing 50 mg/kg chloride.The yield strength,tensile strength and stress corrosion cracking susceptibility for each weld joint were obtained for providing the theoretical basis and data support to the operation,maintenance and life extension of nuclear power plants.In addition,the inner correlation between high-temperature electrochemical corrosion and stress corrosion cracking mechanism was discussed.The results revealed that the electrode corrosion potential increases with the increasing of dissolved oxygen,the corrosion potential of low alloy steel,stainless steel,and nickel base alloy are similar.In low corrosive environments,the weld joint cracking is plastic fracture determined by the mechanical properties,which related to the hardness distribution along the joint.In high corrosive environments(high chloride and high dissolved oxygen concentration),the weld joint cracking is a brittle fracture with large area intergranular and transgranular stress corrosion cracks,which closely related to the corrosion resistance,like corrosion current density,electrochemical impedance,and pitting potential.Apparently,the weld and heating affected zone of 316 NG weld joint exhibit higher stress corrosion cracking susceptibility than base metal,and the 508? with the lowest corrosion resistance are the most venerable sites in 508 ?-52M-690 dissimilar weld joint to crack.Therefor,the measurements of electrochemical parameters and the hardness is a rapid and effective method to estimate the stress corrosion cracking sensitivity of each part of the welding joints.The stress corrosion cracking susceptibilities of both two weld joint materials in chloride containing environments increase as the electrode potential or dissolved oxygen concentration increased.Besides,a critical potential or critical dissolved oxygen concentration is defined where the stress corrosion cracking susceptibility increases dramatically with the potential above this critical potential or the oxygen above this critical concentration.The critical potential is highly related to the electrochemical pitting potential.For austenitic stainless steel,the critical potential is nearly equal to the pitting potential.While for the low alloy steel,the oxide film fracture resistance is reduced by the assistance of stress,and the critical potential is much lower than the pitting potential obtained by electrochemical test,which means the stress corrosion cracking tolerance of dissolved oxygen and chloride for low alloy steel is further reduced.So,the strict controlling of dissolved oxygen to guarantee the electrode potential below this critical potential is of great significance in nuclear power plants.The results in this paper can provide the theoretical basis and data support for the material slection,operation and maintenance in nuclear power plants.And electrochemical parameters like corrosion current density and pitting potential is important parameters for crack growth rate model correction.