Corrosion And Pre-corrositon Fatigue Properties Of TC17 Titanium Alloy

Titanium alloys are widely used in aerospace,weaponry and other fields,such as aero-engine compressors,fuselage skins,and wing structures.Although titanium alloy exhibits excellent mechanical properties and corrosion resistance,aircrafts always experience high-speed airflow and withstand the effects of corrosion media in the environment during the flight process,which would cause corrosion fatigue damage.Owing to the fact that corrosion fatigue damage can greatly weaken the bearing capacity and fatigue life of titanium alloy structure,corrosion fatigue damage has become a potential failure mode for titanium alloy structures,which may result in safety accidents.Therefore,the studies of corrosion behavior and pre-corrosion fatigue properties of titanium alloys are valuable for aircraft calendar life assessment and life extension.This paper focuses on the studies of TC17 titanium alloy used for compressor blades.Combining with immersion corrosion method and electrochemical corrosion method,the corrosion behaviors are studied,the fatigue lifes of pre-corrosion specimens are tested,the corrosion morphologies and fractures are observed by scanning electron microscope(SEM),Raman experiment,ultra-deep-field microscopy and EDS spectral scanning,corrosion and pre-corrosion fatigue failure mechanisms of titanium alloy are studied.The detailed research contents are as follows:(1)The corrosion phenomenon of TC17 titanium alloy in various conventional corrosion solutions are studied by immersion corrosion method.According to the surface morphology comparisons of the test specimens before and after immersion corrosion by microscopic observation,and the microstructure changes observation after corrosion by using scanning electron microscopy,3.5%HCl solution at 50? is determined as the corrosion accelerated test solution.The relationship of average corrosion depth with corrosion time is obtained by three-dimensional corrosion morphology characterized by ultra-deep-field 3D microscopy.It is found that the corrosion rate logarithmically varies with corrosion time within the corrosion time studied.(2)By using electrochemical corrosion method,we perform open circuit potential experiments,potentiodynamic scanning experiments and electrochemical impedance experiments for TC17 titanium alloy.The effects of hydrochloric acid solution concentration on corrosion rate,corrosion resistance and passivation film formation of TC17 are analyzed systematically.It is concluded that the increase of hydrochloric acid solution concentration not only accelerates the corrosion of titanium alloy,but also promotes the formation of the passivation film?Based on COMSOL commercial code,an electrochemical corrosion model is established to simulate the electrochemical process of corrosion for TC17 titanium alloy in 3.5%,5%and 10%HCl solution.The potential distribution and the change of titanium ion concentration in the electrolyte are obtained.(3)The fatigue properties of TC17 titanium alloy(stress concentration factor Kt=1 and Kt=3.2,respectively)after pre-corrosion for Oh,24h,120h,240h and 480h in 3.5%HCl solution at 50? are studied.The three-parameter fatigue life curves are obtained with single point testing method.The pre-corrosion fatigue limits are measured by up-and-down test method,and the law of fatigue lifes changes with immersion time is analyzed.The results show that the fatigue life of TC17 titanium alloy decreases with the increase of pre-corrosion time for the short-life data within the pre-corrosion time studied.For the data close to fatigue limits,the fatigue life of the specimens with Kt=1 decreases first and then increases with the increase of corrosion time.Yet,the fatigue life of the specimens with Kt=3.2 decreases with the increase of corrosion time.Combined with electrochemical corrosion and Raman experiments results,the mechanism of this phenomenon is revealed.(4)The pre-corrosion fatigue failure mechanism of TC 17 titanium alloy is studied by fracture morphology analysis of the specimens after fatigue tests with scanning electron microscopy.At the same time,the element contents at the fatigue source of fracture are measured by EDS spectrum scanning,and the change of Cl element is studied especially.TSic results are consistent with the test results of actual structures,which indicates that the experimental method used to simulate the structural corrosion is effective.