| The excellent passivation and corrosion resistance of titanium and titanium alloys are attributed to an ultrathin passive film(～nm)spontaneously formed on the metal surface.And they are extensively used in the fields of petrochemicals,marine engineering and biomedicine and so on for the high specific strength,non-magnetism and good biocompatibility etc.In addition,the passive film has good self-healing ability after being damaged or destroyed mechanically or chemically,so the passivation and repassivation of passive film is the most important guarantee for passive metals service safety.Therefore,this thesis focuses on the local structure and structure activity relationship of passive film,the structure evolution mechanism of surface/interface during passive film growth,and the quantitative analysis of passivation kinetics of titanium and titanium alloys,and in-sit and ex-situ study of the passivation behavior of titanium alloys,the structure of titanium passive film,the self-healing of titanium passive film,and passivation mechanism of titanium have been achieved by combing the synchrotron radiation technique,the advanced surface/interface spectrum characterization and electrochemical monitoring.Firstly,the passivation behaviors of 4 typical(a+(3)titanium alloys TC4,TC6,TC11 and TC18 were investigated by using self-developed new type of electrode abrading apparatus in 3.5%NaCl solution,and the dissolution/film formation model and high electric field ions conduction model were applied to reveal the andic dissolution,nucleation and 2D growth of metal surface at the initial stage of repassivation,and the 3D growth of passive film at the following transition stage.The results indicate that the passivation behaviors of titanium alloys are significantly different,the passive region with higher Ti content is larger(>1.2V),while that of TC18 is only 0.81 V.The steady-state current density of repassivation is TC11>TC18>TC4>TC6,which is consistent with the change of self-corrosion current density,so the repassivation is in good agreement with the electrochemical results.There are two key time constants at the initial stage of repassivation:net passivation time and formation time of monolayer passive film.At the open circuit potential,the nucleation rate of passive film is TC11>TC6>TC18>TC4,and the net passivation time is opposite to the nucleation rate,38 ms at the fastest time of TC11 and 94 ms at the slowest time of TC4.And then the change of formation time of monolayer passive film and the net passivation time is the same,198 ms at the fastest time of TC11 and 380 ms at the slowest time of TC4.And the thickness of monolayer passive film on titanium is less than 1.0 nm,which is related to the addition of passive alloy elements Mo,Cr and Zr.The 3D growth rates of TC4 and TC6 passive films exponentially increase with applied potential,while that of TC11 and TC18 increase linearly.Secondly,the local structure and structure activity relationship of titanium passive film were studied by AES,XPS,synchrotron radiation XANES and EXAFS in 1.0 M H2SO4 solution in terms of the thickness,composition,chemical state and local fine structure of the topmost surface(～5.0 nm thickness).The results indicate that the thickness of passive film is proportional to the reciprocal of current density in potentiodynamic polarization.The predominant oxide formed on titanium is TiO2(～90%)for less than 2.5 nm,while with the increase of depth,the suboxides TiO and Ti2O3 rapidly increase due to the insufficient oxidation of passive film.In particular,TiO2 and Ti2O3 in the passive film may lead to passivation and dissolution,respectively,and the ratio of[TiO2]/[Ti2O3]is consistent with that of passivation/dissolution in the potentiodynamic polarization.The local fine structure of titanium passive film changes greatly with the applied potential.The increase of average Ti-O coordination causes the electrochemical passivation,and the dissolution is due to the decrease of average Ti-Ti coordination.The increase of bound water in passive film promotes the structure disorder and improves the corrosion resistance,which is the best when the content of bound water is about 12.5%.Thirdly,in-situ study of the structure evolution mechanism of surface/interface during titanium passive film growth was carried out by combing the synchrotron radiation XAFS,SERS and electrochemical methods,and the time-dependent local structure and molecular structure of self-healing of titanium passive film in Hank’s physiological solution were tracked by using the self-made novel electrochemical repassivation set-up.The results indicate that in-situ structure evolution is vastly different from that of ex-situ.The passive film in air is TiO2(x<2),while a small amount of OH-and H2O appears at the solid/liquid interface,which validates the necessity of in-situ study of multiple technologies.Two time-dependent adsorbed intermediates exist during repassivation,the adsorption layer of Ti-OHads(～1.60 A)and Ti-OH(～1.65 A),and finally the interface forms a stable film of O-Ti-OH(～1.72 A)after 1000 s with good corrosion resistance,which can be used as a structure criterion of stable repassivation.The thickness of the passive film calculated from local fine structure features three stages:quick growth of passive film(0～50 s),second growth of passive film(50～300 s),and Stabilization of passive film(300～6000 s).Fourthly,the mechanism and quantitative analysis of titanium passivation were investigated to deeply insight into the passivation kinetics.The current density of long-term passivation was collected by electrochemical potentiostatic polarization.The thickness of passive film and high electric field across the film were analyzed by EIS,and the semiconducting properties of passive film were represented by Mott-Schottky technique.The local structure of passive film was characterized by synchrotron radiation EXAFS.A quantitative model of point defect diffusion at the interface of titanium/passive film/solution in 1.0 M H2SO4 solution was established under applied potentials of passive region.The diffusivity of point defect in passive film was calculated quantitatively by combining the local structure with the results of EIS and Mott-Schottky,and the accuracy of the multiscale quantitative methodology was verified by comparing with the experimental results.The results indicate that the high electric field across the film is 1.06×106 V cm-1 and the donor density of n-type semiconducting film on titanium is about 1021 cm-3.The interfacial equilibrium of passivation and dissolution of passive film is dominated by the diffusion of oxygen vacancies,and three important parameters have been firstly proposed form EXAFS theoretical calculation at Ti K-edge to quantitatively describe the diffusivity of oxygen vacancies:① The half-jump distance equals half Ti-Ti distance;② the coordination coefficient is defined as the probability of activation jump from Ti-O coordination and less than 1;③ the Debye-Waller factor of structure disorder is related to donor density of Mott-Schottky.The diffusivity of oxygen vacancies calculated based on the local structure changes greatly with applied potential from 1.84×10-17 cm2 s-1 to 4.71×10-17 cm2 s-1,which is more precise with at least 2 orders of magnitude than that based on the high field and low field models. |
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