Study On Intergranular Corrosion Sensitivity Of Super304H Stainless Steel Welded Joints

Super304H austenitic heat-resistant steel is widely used in super(super)critical unit superheater and reheater materials.In the high temperature and high pressure environment of the boiler,Super304H steel and welded joints are prone to intergranular corrosion,resulting in deterioration of mechanical properties,boiler explosions and other accidents.In addition,the aggressive chloride ions and pH in the working fluid will also affect the intergranular corrosion properties of the stainless steel material.In this thesis electrochemical impedance spectroscopy(EIS)and electrochemical potential reactivation curve(EPR)are employed to study the influence of aging treatment,Cl-concentration and pH on the intergranular corrosion of Super304H steel and welded joints.The following conclusions are obtained(1)Electrochemical potentiodynamic reactivation curves show that the intergranular corrosion sensitivity of the weld,heat affected zone and Super304H without aging treatment is low,and no obvious intergranular corrosion is observed.With the increase of aging time,the intergranular corrosion sensitivity of the weld is gradually increased.However,the heat-affected zone and Super304H show a decrease in intergranular corrosion sensitivity after 30 hours of aging.The intergranular corrosion sensitivity of Super304H increased at the initial stage and then decreased The intergranular corrosion sensitivity was the must severe at aging 15h,and then began to decrease.The fitting results of electrochemical impedance spectroscopy are consistent with EPR results(2)After corrosion for 30 days in 1%and 3%NaCl solution,severe pitting occurred on the surface of the welding.The EPR results show that it is difficult to form a complete passivation film on the weld surface in 3%NaCl solution;Pitting corrosion occurs on the heat affected zone in 3%NaCl solution,and the intergranular corrosion sensitivity is significantly improved.In the heat affected zone,the Ra value is only 0.0144 in the 1%NaCl solution,and the intergranular corrosion sensitivity is low.The surface morphology of Super304H indicates that the sensitivity of intergranular corrosion is reduced.(3)Corroded in 1%NaCl solution with pH of 1 and 5 for 10 d,the impedance spectra of the weld and the heat affected zone showed shrinkage phenomenon in the low-frequency zone,indicating pitting corrosion occurs.The EPR results show that it is difficult to form a complete passivation film after the weld is corroded in the solution of pH 1 for 10 days.After 10 days of corrosion in a solution of pH 5,the Ra value is 0.065,indicating that the weld is prone to intergranular corrosion;The heat affected zone shows a high intergranular corrosion sensitivity in the two solutions;Super304H exhibit a low intergranular corrosion sensitivity in both solution.