Magnetic Field Effects On The Corrosion And Stress Corrosion Cracking Of Fe-Ga Alloy

Fe-Ga alloy is a new material whose magnetostriction coefficient is intermediate between traditional magnetostrictive alloys and rare earth giant magnetostrictive materials.It has drawn much attention due to its better performance—good mechanical properties,low-field magnetostriction and so on.Fe-Ga alloy has been used in ships as a displacement sensor,That is,Fe-Ga alloy faces not only the seawater environment but also the stress in service.Therefore,it is essential to study the corrosion and stress corrosion cracking(SCC)behaviors.In addition,as a new magnetostrictive material,Fe-Ga alloy services in magnetic field,so it is necessary to study the effect of magnetic field.However,current researches on Fe-Ga alloy are mainly focused on its magnetostrictive performance,mechanical property and microstructure,and there is few reports about its corrosion and stress corrosion behaviors.Based on the above,the stress corrosion cracking behaviors and the effect of magnetic field on the corrosion and stress corrosion of Fe83Ga17 alloy were studied systematically in this paper,to provide reference and theoretical support for the application of Fe-Ga alloy.In the first place,the stress corrosion mechanism of Fe83Ga17 alloy and the SCC threshold stress were examined by the constant load method under constant potential.In the second place,the effects of magnetic fields on the corrosion,SCC and electrochemical corrosion of Fe83Ga17 alloy were studied by immersion corrosion,slow strain rate tensile(SSRT)and potentiodynamic polarization tests.In addition,the hydrogen induced delayed cracking of Fe83Ga17 alloy and the effect of magnetic field on were discussed.The main results were follows:(1)The hot-rolled Fe83Ga17 alloy with good mechanical properties were prepared and the tensile strength and elongation were 645 MPa and 15.8%respectively.The results of polarization potential on constand load SCC tests showed that the time to fracture decreased with the increasing anodic polarization potential,on the contrary,the time to fracture increased with the increment of the absolute value of cathodic polarization potential.Therefore,the SCC mechanism is might controlled by anodic dissolution.(2)The results of immersion tests showed that in the presence of a 0.15 T magnetic field applied parallel to the sample surface,both corrosion and uniform corrosion were inhibited,and the magnetic field gradient force could be responsible for these observations.The results of SSRT tests showed that when a tensile magnetic field(B1)was applied,there is a magnetic field gradient force from outside of the crack that points toward the inside of the crack,and the SCC was promoted.The SCC was inhibited if an in-plane magnetic field(B2)was applied,the magnetic field gradient force is in the opposite direction.When a perpendicular magnetic field(B3)was applied,the SCC was slightly suppressed.Based on the results,the mechanism of crack corrosion under different directions of magnetic field was proposed,and the results of magnetic fied on the corrosion of Co further verified the effect of the magnetic field gradient force,and the results was consistent with those of Fe83Ga17 alloy.(3)The results of electrochemical corrosion tests showed that when a magnetic field was imposed parallel to the sample surface,the corrosion current density increased,and the surface morphologies were more severe than samples in the absence of a magnetic field.The Lorentz force is responsible for these observations.When applied a parallel magnetic field,the magnetic field is perpendicular to the current,and the Lorentz force was maximum.The Lorentz force can induce a forced convection in the mass transport process,then accelerated the electrochemical corrosion.(4)The pre-crack propagation under dynamic hydrogen charging condition was observed by using a notched I-shaped tensile sample through a self-designed constant displacement loading device.The results showed that no hydrogen induced delayed propagation was observed during dynamic charging when the tensile direction of the sample parallel to the rolling direction.The hydrogen induced delayed propagation occurred when the tensile direction of the sample perpendicular to the rolling direction.The threshold stress of the delayed propagation for the Fe83Ga17 alloy was 0.63σb when hydrogen charging of the smooth tensile sample at a current density of 1 mA/cm2 in H2SO4 solution.The parallel magnetic field could accelerate hydrogen desorption and reduce the hydrogen content of the sample.