Study On Orientation-dependent Corrosion Behaviors And Anodic Oxidation Of Tantalum

Tantalum is a refractory metal with excellent physical and chemical properties.High purity polycrystalline tantalum is widely used in the industry.The different grain orientations of polycrystalline metals present different physical and chemical properties.In the process of chemical or electrochemical corrosion,different grain orientations cause different corrosion sensitivity,which is called corrosion orientation correlation.In this thesis,Orientation-dependent corrosion behaviors of high-purity Tantalum under recrystallization and rolled were studied by multiple characterization methods,such as the Scanning Electron Microscope?SEM?,Electron Back Scattering diffraction?EBSD?and surface profile instrument.Ta₂O₅ oxide film and Ta₂O₅ nanotube were fabricated using an electrochemical anodic oxidation method,but which isn't widely researched.The quality of surface morphology of anodic Ta₂O₅ nanotube was improved by optimizing the pretreatment of tantalum template,and orientation-dependent anodic of Ta was studied using SEM,XRD,nanoindentation and light catalytic instrument.Main conclusions can be drawn:?1?The corrosion rate and morphology were dependent on grain orientations,when high purity polycrystalline Ta?recrystallization and cold rolled?were etched in the acid etching solution of H₂SO₄/HF/HNO₃.The corrosion rate of the grain orientation of the crystalline sample was?111?>?110?>?100?,and?111?grain orientation was composed of many“bags”appearance on the surface,the surface of?110?grain orientation presented step-shaped appearance,while the surface of the?100?grain was covered with a large numbers of“shallow pit”appearance.The analysis was based on the information of the atom layer on the grain surface.The corrosion behavior was affected by the arrangement of atom layer and free energy of the grain.The?111?grain of cold rolled Ta showed a lot of small angle grain boundaries and strong dislocation density,which was higher storage energy and more easily corroded than?100?orientation.The surface of?111?grain corroded with many uneven“bags”was rougher than the recrystallization sample.The study of orientation-dependent corrosion behavior is expected to improving the corrosion resistance of tantalum.?2?Ta₂O₅ nanotube was fabricated using anodic oxidation method and the quality surface morphology was improved by optimizing the pretreatment of tantalum.The results show that high aperture ratio and smooth-wall anodic Ta₂O₅ nanotube without oxide residue was obtained by introducing an electropolishing procedure before anodic oxidation.The length and diameter of Ta₂O₅ nanotube were controlled by changing voltage and oxidation time,and the voltage was the main influence factors.The sample was not generated the nanotube film under low voltage of 5 V.?3?Analysis results of XRD showed that anodic Ta₂O₅ nanotube was amorphous and turned into crystalline state and lost the tubular structure after annealing for 30 min at 900 ^oC in filled with argon gas.The photocatalytic of anodic Ta₂O₅ nanotube,nanotube annealed and commercial 200 nm Ta₂O₅ powder were tested.The results show that the anodic Ta₂O₅ nanotube presented high photocatalytic performance of 93%,while the powder of nanotube annealed and 200 nm Ta₂O₅ particle didn't present photocatalytic performance.?4?In this paper,three kinds of single crystals with?111?,?110?and?100?orientation were oxidized under the same condition to study the orientation correlation with nanotube.The crystal plane with?111?was first generated the nanotubes,which was the longest.Nanotube films fabricated on single crystals with?110?and?100?were similar morphology,and the length of nanotube on the single crystal with?110?was longer.The single crystal with?111?orientation had higher surface free energy.Been based on the anodic oxidation kinetics,the dense oxide film was first generated on the crystal with?111?orientation with the current density dropping in the first stage.In the second stage,the nanopore structure was first generated with rising of the current density,so the length of nanotube with?111?single crystal was the longest under the same condition of anodic oxidation.Mechanical properties of nanotube films fabricated on the three kinds of crystals were tested,separately.The hardness and elastic membrane volume of nanotube film with?111?single crystal were the largest,and single crystal with?110?and?100?had similar hardness and elastic membrane.Therefore,it is also possible to obtain controllable Ta₂O₅ nanotubes by improving the crystal orientation.