| Ti-15V-3Cr-3Al-3Sn(commonly referred to as Ti-15-3),as an age-hardenable metastable beta titanium alloy,has been widely used in aerospace field due to its excellent cold deformability,high specific strength and good corrosion resistance.However,equipment used for service(such as launching tower)in tropical marine climates is prone to cause non-uniform local corrosion,which can cause the failure of metal structural parts in severe cases and even lead to sudden catastrophic accidents.Localized corrosion tends to be tiny,very hidden,and difficult to predict and detect.At present,the corrosion protection technology of titanium alloy components used in equipment and facilities in tropical marine environment is far from perfect,and in-depth and systematic research needs to be carried out.In this paper,the effects of different surface states,different salt spray time and different coupling methods on the pitting behavior and corrosion resistance of Ti-15-3 alloys were investigated by combining electrochemical impedance and electrochemical noise,and the salt spray corrosion protection effect of aluminized PVC film and composite coating on Ti-15-3 alloy is also studied.The main research results are as follows:(1)The salt spray corrosion characteristics and pitting resistance of pristine and polished Ti-15-3 alloys were investigated by neutral salt spray corrosion test.The microscopic morphology and corrosion product composition of Ti-15-3 alloys with different surface states were characterized by SEM,CLSM-3D and XPS.The results showed that large pits appeared on the surface of pristine Ti-15-3 alloy,and there are microcracks inside the pits,while micro-pitting,deposits and grooves appeared on the surface of polished Ti-15-3 alloy.Using electrochemical data tested in 3.5 wt.%Na Cl solution,the corrosion behaviors of Ti-15-3 alloys with different surface states after salt spray corrosion are investigated by means of electrochemical impedance experiments in combination with electrochemical noise analysis based on discrete wavelet transform(DWT)and Hilbert-Huang transform(HHT).The results showed that the smooth surface of polished Ti-15-3 alloy was not prone to the coexistence of metastable pitting and stable pitting,and can improve its pitting resistance.The interface behavior and corrosion resistance of polished Ti-15-3 alloy after different salt spray time were calculated and analyzed by using the capacitance formula and the corrosion rate formula.The results showed that the capacitance C_f values of the oxide film first decreased and then increased gradually with the salt spray time,and corrosion rate(C_R)values first decreased gradually and then increased with salt spray time.Through time domain statistical analysis,frequency domain analysis and the Hilbert spectrum analysis,it was found that three types of corrosion occurred on the polished Ti-15-3 alloy after different salt spray time,namely metastable pitting,passivation state and stable pitting.Combined with the observation of the corrosion morphology of polished Ti-15-3 alloy,it was found that its pitting resistance and corrosion form are closely related to the surface morphology of the corroded samples.(2)The corrosion pattern of Ti-15-3/CFRP coupling after different salt spray time was systematically studied.The corroded sample surfaces were tested and characterized by SEM,CLSM-3D and XPS,it was found that pitting corrosions occurred on the surface of coupled Ti-15-3 alloy,and the surface of coupled CFRP also suffered from certain corrosion.Using the electrochemical data tested in 3.5 wt.%Na Cl solution,the corrosion behavior of the coupled Ti-15-3 alloy after different salt spray times was investigated.Through the comparative analysis of electrochemical impedance spectroscopy(EIS)of uncoupled and coupled Ti-15-3 alloy after different salt spray time,it was confirmed that its galvanic effect significantly promoted the corrosion of Ti-15-3 alloy.Using the polarization resistance and polarization curve analysis,it was confirmed that the corrosion rate of coupled Ti-15-3 alloy after 312 h of salt spray is relatively small.Combined with Hilbert spectrum analysis.The results showed that the surface passivation of coupled Ti-15-3 alloy occurred after 312 h of salt spray.The microstructure study of pitting corrosion using TEM technique confirmed the inference of electrochemical data analysis that passive films were formed on the bottom surface of pitting.(3)The corrosion behavior of Ti-15-3/aluminized PVC film overlaps after different salt spray time was studied in detail.The results showed that at the initial stage of corrosion,the galvanic corrosion of Ti-15-3/aluminized PVC film coupling within the crevice occurred due to the potential difference between them,this sacrificial anode(aluminized PVC film)has a cathodic protection effect on the Ti-15-3 alloy.Compared with Ti-15-3/PTFE overlaps,using open circuit potential,polarization curve and electrochemical impedance spectroscopy analysis,the results showed that the galvanic effect of Ti-15-3/aluminized PVC film coupling significantly alleviated the corrosion of Ti-15-3 alloy within the crevice.In addition,from 2 h to 72 h of salt spray,the corrosion resistance of Ti-15-3 alloy within the crevice increased with the salt spray time due to self-passivation of Ti-15-3 alloy within the crevice.After 120 h of salt spray,the Ti-15-3 alloy within the crevice is depassivated due to the deterioration of acidizing environment inside the crevice,and the potential difference between the surface of the Ti-15-3 alloys inside and outside the crevice caused severe crevice corrosion.(4)The shot peening layer,R+(H+GO)and f-{R+(H+GO)}composite coating on the surface of Ti-15-3 alloy were prepared by shot-peening and coating technologies.The corroded sample surfaces were tested and characterized by SEM,CLSM-3D and XPS,it was found that the surface corrosion degree of the f-{R+(H+GO)}composite coating sample was relatively small.Using the calculation of impedance spectrum fitting data,the results showed that the polarization resistance of the shot peened layer,R+(H+GO)and f-{R+(H+GO)}coatings increased gradually,that is,the f-{R+(H+GO)}coating had high resistance to salt spray corrosion.Using spectral noise resistance and Hilbert spectrum analysis,it was confirmed that the f-{R+(H+GO)}coating had a large Rsn value(below 1 m Hz)and a large amount of the distribution of perturbations in high frequency.It showed that this coating had a good blocking effect on salt spray. |
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