| Titanium alloys are widely used in marine environments for their excellent mechanical properties and corrosion resistance.However,corrosion and hydrogen embrittlement are usually occurred in titanium alloys which served in marine environment under the complicated working conditions of corrosive medium,cathodic protection and applied stress,causing great financial losses.Therefore,it is of great theoretical and practical significance to investigate the corrosion behaviour of titanium alloy in marine environment.In this paper,microstructure characterization,corrosion tests and mechanical tests are used to investigate the corrosion behavior,hydrogen embrittlement sensitivity and mechanical behavior of the TC4 and Ti4211 titanium in simulated marine environment?3.5 wt.%NaCl?,the relationship between service properties and microstructure of titanium alloys is established,and the microscopic failure mechanism of the alloys under various test conditions is explained.Results of immersion test and corrosion electrochemical test indicate that TC4 and Ti4211 alloys have excellent corrosion resistance in 3.5 wt.%NaCl solution.After 180 days of immersion,the surfaces show no signs of any microscopic corrosion.Results of weight loss experiment show that the weight loss rate of the two alloys is similar,which is 2×10-4mg/cm2/day.In 3.5 wt.%NaCl solution,the stable corrosion potential value of Ti4211 alloy is slightly higher than that of TC4 alloy.The polarization resistance of TC4 and Ti4211alloys obtained from electrochemical impedance spectroscopy are 5.69×105?·cm2 and6.50×105?·cm2 respectively,the polarization resistance value of Ti4211 alloy is slightly larger than that of TC4 alloy.The passivation film still maintains good protective ability relative to the corrosion potential of 2 VSCE.After the potentiodynamic polarization tests,there is no pitting corrosion on the surface of the alloy which further indicates that the passivation film has a good protective ability even in high potential.It is indicated that both TC4 and Ti4211 exhibited microstructure degradation after hydrogen charging for 4 h.Hydrogen cracking,local volume expansion and microstructure loss can be observed on the surface of TC4,while there is only hydrogen induced cracking appearing on the surface of the Ti4211 alloy.The XRD analysis shows that hydrides are formed in two alloys during hydrogen charging.The longer the hydrogen charging time the greater the number of hydrides.Hydrides in the form of needle and flocculent are mainly located in the?phase and at the interface of?/?phase.After hydrogen charging for 24 h,the hydride layer of TC4 is about 60?m while the hydride layer of Ti4211 alloy can reach up to 400?m.Electrochemical corrosion tests show that compared with the uncharged samples,the polarization resistance values and the passive current densities of the TC4 and Ti4211 alloys decrease after being hydrogen charged for 1 h and 4 h.After being hydrogen charged for 24h,the pitting potential of TC4 alloy drops to 0.8 VSCE,and that of Ti4211 alloy is 1.2 VSCE.After the potentiodynamic polarization tests,the local corrosion of the two alloys mainly occurred in the?phase,and the corrosion of TC4 alloy is more serious.The yield strength,tensile strength and elongation of TC4 and Ti4211 alloy decrease with the increase of the hydrogen charging time.The elongation rate of TC4 alloy decreases from 10.8%to 1.8%after being hydrogen charged for 24 h while that of Ti4211 alloy decreases from 11.8%to 0.6%.Besides,for the sample being hydrogen charged for 4 h and8 h,both of the two alloys show the Portevin--Le Chatelier?PLC?effect during the tensile process.After being hydrogen charged for 12 h and 24 h,the TC4 alloy will still exhibit the PLC effect while the Ti4211 alloy will not show the effect after being charged for the same time. |
PDF Full Text Request