Effect Of Surface Nanocrystallization On Corrosion Resistance Of Nickel-Based Alloy 690

The steam generator of the pressurized water reactor nuclear power plant is the pressure boundary of the primary and secondary circuits in the nuclear reactor,which is served in the harsh environment of high temperature,high pressure and severe corrosion.The performance requirements of the steam generator materials are particularly high.The materials has also developed from austenitic 304 stainless steel,Inconel alloy 600,Incoloy alloy 800,etc.to the nickel-based alloy 690 currently considered to be the best alternative material.However,the nickel-based alloy 690 will also fail in corrosive environments due to prolonged service.In order to further extend the service time of nuclear power plants,improve their safety,and reduce operating costs,it is necessary to improve the corrosion resistance of nickel-based alloy 690.In this paper,the surface nanocrystallization of nickel-based alloy 690 was carried out by ultrasonic surface rolling treatment,and the corrosion resistance of alloy 690 before and after surface nanocrystallization was studied in the secondary side environment of nuclear reactor.The main research contents and related results are as follows:?1?The ultrasonic surface rolling equipment is used for surface nanocrystallization of the alloy690,and four different reduction amounts of 0 mm,0.2 mm,0.4 mm,and 0.6 mm are selected as variables.The results show that rolling treatment is beneficial for alloy 690 to flatten the surface and reduce the roughness when selecting a processing parameter with a reduction of 0.2 mm.As the reduction continues to increase,it exceeds the smoothing ability of the equipment and instead deteriorates the quality of surface.The treatment of surface nanocrystallization broadens the XRD diffraction peak of the alloy 690,and the position of the diffraction peaks also shift to the right,which phenomenon is more obvious with the amount of reduction increases.The ultrasonic surface rolling treatment improves the surface hardness of alloy 690,the original hardness is 180.2 HV_0.05,when the reduction amounts is 0.2 mm,the hardness is increased to 337.5 HV_0.05,an increase of nearly 1.9 times,and the hardness has a slight increase with the reduction increasing.Therefore,the samples with a reduction of 0.2 mm were selected as the object of later research.The microscopic morphology of nickel-based alloy 690 after surface nanocrystallization with the reduction amounts of 0.2 mm was observed,and the average grain size on the surface was found to be about 55 nm.?2?The electrochemical corrosion test of the nickel-based alloy 690 before and after surface nanocrystallization was performed in three solutions of ETA?ethanolamine?,ETA+0.5M NaCl and ETA+0.5M Na₂SO₄ at room temperature.The open circuit potential test results show that the open circuit potential of the samples after surface nanocrystallization in the three solutions is higher than the original,and the arc radius of the electrochemical impedance of alloy 690 after surface nanocrystallization in the three solutions is greater than the original.The polarization curve results also show that the self-corrosion potential of alloy 690 after nanocrystallization are higher than the original,when the corrosion current density of alloy 690 after nanocrystallization are lower than the original,which shows that the surface nanocrystallization improves the corrosion resistance of nickel-based alloy 690.?3?The nickel-based alloy 690 before and after ultrasonic surface rolling treatment was subjected to room temperature immersion experiments and high temperature and high pressure immersion experiments.The room temperature immersion experiment was carried out in ETA+1M NaCl solution for 1000h.The results showed that the surface of alloy 690 was subjected to intergranular corrosion after being immersed,and the corrosion phenomenon was very serious,which is much smaller on the surface of alloy 690 after nanocrystallization.The samples of alloy690 before and after nanocrystallization were subjected to high-temperature and high-pressure immersion corrosion for 60 minutes in two solutions of ETA and ETA+0.1M NaCl.A large amount of large-particle oxides are generated on the surface of original sample in the ETA solution,however,the corrosion phenomenon on the surface of nanocrystallization sample is much lighter.After the addition of Cl^-,corrosion pits and a large number of corrosion products appeared on the surface of both samples,however,the corrosion products aggregated on the surface of the origin sample,and the corrosion products were relatively dispersed after nanocrystallization.This shows the nickel-based alloy 690 after ultrasonic surface rolling treatment has better corrosion resistance than the original samples in the room temperature immersion experiments and high temperature immersion experiments.