| Titanium has low density,high strength and active chemical properties.In the service environment,a stable and dense oxide film is usually quickly formed on titanium surface,which can effectively block the contact between titanium and titanium alloy metal matrix and corrosive media to avoid further corrosion.The exploration of the passivation behavior and passivation mechanism of titanium is of great significance to the development of passive metal corrosion theory and industrial anti-corrosion process.Based on X-ray absorption spectroscopy(XAS),surface composition and structure characterization technology and electrochemical measurements,the difference and correlation of the oxide film properties and passivation behavior of TC4(Ti-6Al-4V)in 0.5 M sulfuric acid solution under the polarization potential in cathodic region,active region,passivae region and dissolve region were studied.The results show that the thickness of oxide film was positively related to the applied polarization potential,but the thicker oxide film doesn’t mean the better corrosion resistance,the valence state and structure of the oxide in the oxide film also have an important impact.TiO2 was the main form of titanium oxide on the surface of oxide film,and the increase of polarization potential promoted the formation of TiO2.When the dissolve region potential was applied,almost all Ti,TiO or Ti2O3 on the surface of oxide film were oxidized to TiO2,and the high content of TiO2 was conducive to improving corrosion resistance.Under the lower polarization potential,TiO2 mostly existed in the form of anatase-TiO2,and with the increase of the applied polarization potential,TiO2 transformed from anatase to rutile.Most anatase-TiO2 were transformed into rutile-TiO2 under dissolve region potential,but only when rutile-TiO2 and anatase-TiO2 coexisted,the oxide film performed the best density and corrosion resistance.Therefore,the passive film formed under higher passive region potential(1.0 and 1.5V)showed excellent stability and corrosion resistance.In order to further investigate the time evolution of TC4 passivation behavior,the polarization potential of 1.0 V was applied and the morphology and composition of passive film were characterized for samples with different polarization time.During the growth process of passive film,the current density decreased at a decreasing rate for the first 2000 s,and then reached the first dynamic equilibrium state with large fluctuation range(2000-18000 s),and with the extension of the polarization time to 18000 s,the passivation reaction reached the second dynamic equilibrium state with a smaller fluctuation range.The average growth rate of the passive film initially showed a decreasing trend,reaching a minimum value between 4 h and 8 h and the growth rate of the passive film gradually increased again between 8 and 24 h.The increase of polarization time also promoted the evolution of anatase TiO2 to rutile TiO2,while the valence of the oxide on the surface of the passive film was not affected by the polarization time,but it promoted the growth of passive film to the inside and more suboxides inside the passive film were formed.In addition,the roughness of the passive film on the sample surface was negatively correlated with the polarization time.The corrosion resistance of the passive film was enhanced with the increase of polarization time due to the increase of passive film thickness,the increase of Ti oxide valence state,the dense and smooth surface,and the increase of rutile TiO2 content.In addition to studying the passivation behavior of TC4 under different conditions by spectroscopic and electrochemical methods,the visual observation of morphology of passive film growth process needs to be further explored.Therefore,in this study,the morphological changes of the passive film growth and dissolution process of TC4 in 3.5 wt.%NaCl solution at different polarization potentials were in situ observed using electrochemical atomic force microscopy(ECAFM).When the passive region potential polarization was applied,the passivation reaction state was stable and the growth of the passive film started from the formation of small island structures.With the increase of potential,the number of small island structures increased and the size also increased,while the valence of the passive film also increased,and the roughness decreased with the growth and uniform distribution of island structures.When the dissolve region potential was applied,the larger island structures formed under the passive region potential polarization were dissolved to form the lamellar structure and the valence state of Ti oxide decreased.The passive film continued to grow on the basis of the lamellar structure formed in the dissolve region.When the secondary passive region potential was subsequently applied,the roughness of the passive film on the sample surface further decreased and the valence of the passivation film increased as the passivation reaction state in the secondary passive region proceeded steadily.In order to explore the effects of Al and V additions on passivation of titanium alloys,this study investigates the passivation behavior of TA2(Ti),TA6(Ti-5Al),TC3(Ti-5Al-4V)and TC4(Ti-6Al-4V)in 3.5 wt.%NaCl solution.The results show that with the addition of Al,the Ecorr of TA6 decreased,the icorr and ipass increased and the corrosion resistance decreased compared with that of TA2;when V was added,the Ecorr of TC3 increased slightly and the icorr and ipass decreased compared with that of TA6,and the corrosion resistance was improved;subsequently,with the addition of Al,TC4 showed a significant increase in Ecorr and a significant decreased in icorr and ipass compared to TC3,and its corrosion resistance was further improved.In addition,the passive film resistance of the four materials after 12 h immersion in 3.5 wt.%NaCl solution also reflected the same conclusion.In the passive films generated in natural environment on the sample surface,the Ti oxide valence state decreased along the depth direction,while the Al and V oxide content decreased and V was oxidized to V2O5 only in the surface of passive film.Under 1.0 V electrochemical passivation,the passive film growth process of all four materials showed transient,sub-stable and steady-state stages,and the stability relationship during the passive film growth process was TA2<TA6<TC3<TC4.In addition,only the current density amplitude of TC4 continued to decrease after the polarization time of 22000 s,which means that its passive film growth can enter the second steady-state growth stage with higher stability,and the passiv film can maintain stable growth with the highest Ti oxide valence state.Finally,based on high-throughput combinatorial material chip preparation technology,Time of flight secondary ion mass spectrometry(TOF-SIMS),X-ray diffraction(XRD),μ-X-ray fluorescence spectroscopy(μ-XRF)and the local electrochemical test system developed by our group,this study attempts to explore efficient screening methods for Ti-Al-V corrosion-resistant titanium alloys.The sample preparation and heat treatment,structure and composition characterization,and local electrochemical properties of Ti-Al-V high-throughput combined material chips were verified and analyzed,and corresponding improvements were proposed. |
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