| Since the 1940s,as-cast nickel-aluminum bronze(NAB)has emerged as a leading alternative to brass for marine propellers because of its excellent mechanical properties and resistance to marine corrosion.Its excellent corrosion resistance thanks to the nickel-aluminum bronze alloy in chlorine-containing solution,accompanied by the occurrence of corrosion,the surface can generate a protective film of corrosion products.Shot peening,as a low-cost surface-treated process that is not limited by the shape of workpiece,is widely used to improve the mechanical,corrosion and fatigue properties of metal component.In this paper,at first,the microstructure and phase composition of nickel-aluminum bronze and the morphology,composition and valence state of its corrosion product film formed in 3.5 wt.%Sodium chloride solution were studied by means of metallographic microscope,scanning electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy,and the corrosion resistance was analyzed by electrochemical tests.Secondly,the self-healing behavior of corrosion product film was observed by in-situ Scanning Vibration Electrode Technology(SVET).In addition,in-situ atomic force microscope(AFM)was used to monitor the corrosion and film formation process on each phase of NAB.Finally,the effects of shot peening on the growth and corrosion resistance of corrosion product film on NAB were discussed.Through the research,the main results and conclusions were as follow:As-cast NAB alloy consist of copper-rich coarse?phase,retained martensitic?'phase,Fe-Al-Cu rich?_?phase with globular or rosette shape,NiAl-based?_?phase which is lamellar or globular decomposition product which eutectoid with?phase,and fine?_?phase within the?grains.When the specimens are immersed 48 h,the corrosion product film is thin,and corrosion pits at the zone of retained??phase and lamellar?phases in?+?_?eutectic can be observed.When the immersion time is up to 240 h,the corrosion product film reaches a certain thickness,corrosion pits are completely covered,and a uniform honeycomb surface structure has formed.With immersion,the film gradually thickening,and its corrosion resistance increase.The self-healing ability of the film is evaluated via SVET technique in real time,which suggests that a scratch with 1.5 mm in diameter on the film which forms by 24-hour immersion can heal itself after immersion for 150min.In the self-healing process,there is a galvanic couple between the defected region and the remaining part covered by protective film.The defected region is continuously corroding as the anode,and at the meantime,the corrosion product film is repaired.As a result,the self-healing rate is in competition with the anode corrosion rate.The in-situ AFM observation suggests that there are three types of micro galvanic cells(retained?'-?,?-?_?,and?-?_?)in NAB.With immersion time growing to 150 min,the corrosion of all phases is restrained to a certain degree.After a shot penning with the 0.6mm or 2mm diameter steel shot,residual stress,grain refinement and work hardening have a combined effect on corrosion property of NAB.Residual compressive stress inhibits corrosion,while roughness and fine crystallization accelerate corrosion and corrosion product film formation.After the peened NAB is immersed for 120 h,the corrosion product film is thicker and denser for 0.6mm diameter shot peening,and its corrosion resistance is significantly higher than non-shot peening NAB;however,the corrosion product film of NAB with 2mm diameter shot peening,is loose and poorly protective,causing local corrosion and poor corrosion resistance compared with the non-shot peening NAB,on account of its extremely high roughness. |
PDF Full Text Request