The Improvement Of Corrosion Resistance Of Titanium In Boiling Strong Acid Environments By Anodization And Annealing

Titanium is recognized for its excellent corrosion resistance in the miscellaneous environments due to its spontaneous fabrication of the compact and stable oxide film on its surface and often specified for industrial and biomedical applications.Titanium is also a candidate material for the fabrication of containers for disposal of nuclear wastage due to its high strength to weight ratio.However,the native oxide film is amorphous,normally thin(?10 nm),and stoichiometrically defective.With increasing temperature and decreasing pH,the oxide film is not immune to corrosion and could be susceptible especially in halide solution but also in other media such as sulfate solution.Titanium dioxide films with high corrosion resistance were fabricated by electrochemical anodization and annealing at 400 ?,500 ?and 600 ?.In this thesis,Scanning electron microscopy,X-ray photo electron spectroscopy,energy dispersive X-ray spectroscopy,X-ray diffraction and electrochemical methods were used to investigate the oxide films.The results indicate that anodization then annealing at 400 ?and 500 ?promoted the formation of pure anatase on Ti,while annealing at 600 ? favored the formation of a nano-crystalline double anatase/rutile structure with the relative phase contents of about 1:1.The crystallite size of oxide increased with the increase of temperature,but decreased obviously with the formation of anatase/rutile structure.The XPS results reveal that the sample anodized and annealed at 600 ?showed high concentration of oxygen and low concentration of titanium as compared to samples treated at 400 ?and 500 ?.The sample anodized and annealed for 3 h at 600 ?showed high Ecorr and low Icorr and passivation was promoted significantly in boiling 4 M HCl as well as 4 M H2SO4 at 105 ?.EIS analysis suggests that the sample anodized and annealed at 600 ?showed highest impedance and barrier resistance,which may be attributed to the decreased grain size of the oxide film,the double anatase/rutile structure and less defects in the surface film.The effect of enhancement in the annealing time for the anodized and annealed sample at 600 ?,the results indicate that annealing for 6 h and 9 h at 600 ?favored the formation of the different relative phase content of anatase/rutile structure.The crystallite size of oxide decreased,hardness increased and the smallest crystallite size and the highest hardness is obtained for 9 h annealing at 600 0C.The sample anodized and annealed for 9 h at 600 ?showed high Ecorr and low Icorr and passivation was promoted significantly in boiling 4 M HCl as well as 4 M H2SO4 at 105 ?as compared with the sample anodized and annealed for 3 h and 6 h at 600 ?.EIS analysis suggests that the sample anodized and annealed for 9 h at 600 ?showed highest impedance and barrier resistance,which may be attributed to the decreased grain size of the oxide film,the different relative phase content of anatase/rutile structure and less defects in the surface film.