Study On Seawater Corrosion Of New Ti6321 Alloy And Its Welded Joint

Titanium alloy is widely used in marine environment due to its high specific strength and good corrosion resistance.Ti6321 alloy is a new kind of nearlyαtype titanium alloy developed independently in China.It has high aluminum content and containsβphase stable elements Nb、Mo and neutral element Zr.It has excellent weldability and can be used in the components of deep submersibles and surface ships.The complex marine environment poses a challenge to the service performance of Ti6321 alloy,and the influence of some environmental factors and variables on the service performance of Ti6321 alloy is not clear.In the deep and shallow marine environment,the changes of temperature and dissolved oxygen concentration may affect Ti6321 alloy passivation behavior,and the change of stress can lead to crack of welding joint,in waters that are rich in Cl^-,prone to autocatalytic reaction,forming a high Cl^-low p H occluded environment,do great harm to the performance of the welded joint.Therefore,this topic takes Ti6321 alloy base metal and the alloy welding joint obtained by TIG welding as materials,selects two environmental factors,temperature and dissolved oxygen concentration,to study their influence on the passivation behavior of Ti6321 alloy base metal in artificial seawater.By simulating crack tip solution,the dissolution behavior of different structures at the welded joint is studied,so as to enrich the corrosion theoretical system of Ti6321 alloy and establish the relationship between environmental variables,organizational structure and service performance of Ti6321 alloy.The main research contents are as follows:（1）The effects of temperature and dissolved oxygen on the passivation behavior of Ti6321 alloy were systematically studied.Electrochemical experiments showed that the reduction of temperature inhibited the reactivity,resulting in a decrease in anodic and cathodic current densities,while the removal of dissolved oxygen inhibited the conversion of non-stoichiometric titanium oxides to titanium dioxide at 1.5 V_SCE,resulting in the disappearance of anodic peaks.The removal of dissolved oxygen is detrimental to passivation,resulting in higher passive current density.The parameters related to the passive film were calculated by EIS.The removal of dissolved oxygen reduced the electric field intensity in the passive film and increased the film forming ratio.The increase of temperature increases the electric field intensity of passive film and decreases the film forming ratio.The lower dissolved oxygen concentration increases the passive film growth rate.The passive film of Ti6321 alloy is n-type semiconductor,and oxygen vacancy is the main defect type.XPS was used to characterize the composition of the passivation film,in which Ti O₂ was the main component,and trace amounts of Al、Nb and Zr oxides were detected.The removal of dissolved oxygen reduces the O^2-content.（2）The corrosion behavior of the base metal with dual-state structure and the weld metal with Widmanstatten structure in the simulated crack tip solution of 5 M HCl and the control group of 1 M HCl was compared.The results of electrochemical test showed that the base metal and weld metal exhibited spontaneous passivation behavior at 1 M HCl,with similar electrochemical behavior.In 5 M HCl,it shows active dissolution,and the base metal and weld metal are severely corroded.The base metal exhibits better corrosion resistance compared to weld metal.Through the morphology observation and composition analysis,it is believed that there are preferentially corroded areas in the base metal and the weld metal.Because of the different elements inαphase andβphase of base metal and the weld metal,the electrochemical activities of the two phases are different.Theαphase andβphase suffer different degrees of corrosion,and the corrosion ofαphase is more serious.According to EIS analysis,with the increase of immersion time in 1 M HCl,the thickness of the base metal surface film increases slowly,while the thickness of the weld metal surface film changes little with the increase of time.In the two-step reaction mechanism,the charge transfer rate on the weld metal surface is accelerated and the surface coverage rate increases.The difference in corrosion resistance between the base metal and the weld metal is mainly due to the difference in element and microstructure,and the tendency of forming micro galvanic cells in the weld metal is greater,which leads to the weld is more susceptible to corrosion.