純鈦與Ti-6Al-4V合金於水溶液及離子液體中之腐蝕行為研究

Abstract

Selective dissolution behavior of Ti-6Al-4V alloy in aqueous solution and ionic liquids (ILs) were investigated in this study. Selective dissolution of the Ti-6Al-4V alloy in aqueous solution was investigated by means of conducting in-situ corrosion monitoring experiment in 0.5 M H2SO4 + 1 M HCl mixed reducing acid by using electrochemical atomic force microscope (ECAFM). On the other hand, the corrosion behavior of Ti metal in ILs was first focused on the electrochemical behaviors of commercial pure Ti in four different kinds of ILs, namely N-butyl-N-methylpyrrolidinium dicyanamide (BMP-DCA), choline chloride-urea (ChCl-urea), choline chloride-chlorozincate (ChCl-ZnCl2), and 1-ethyl-3-methylimidazolium chloride aluminum chloride (EMIC-AlCl3) for the reason of simplification of the study of the corrosion or passivation mechanism. For solution annealed followed by furnace-cooled Ti-6Al-4V titanium alloy, selective dissolution of α phase and non-uniform corrosion at α/β interface caused by galvanic effect resulted from significant chemical composition divergence was clearly observed. However, no selective dissolution was found in the air-cooled alloy due to the similarity in chemical composition of the passive film formed on α and β phases with less compositional divergence. The effect of potential on the corrosion behavior was also explored. Selective dissolution of α phase with respect to β phase occurred when the potential was controlled at -0.9 VPt. However, for the air-cooling treated Ti-6Al-4V alloy, also considerable dissolution was also found during potentiostatic etching at -0.9 VPt, but no selective dissolution behavior was observed due to less compositional divergence of α and β phases. The potentiodynamic polarization measurement results show that CP Ti could be passivated in BMP-DCA, ChCl-urea and ChCl-ZnCl2 ILs. However, in EMIC-AlCl3 ionic liquid, the polarization curve only exhibited active dissolution behavior, without the presence of a passive region. The passivation behavior of CP Ti in ILs was also characterized by means of X-ray photoelectron spectroscopy (XPS) to examine the roles of ILs with different chemical structures. However, XPS analyses revealed that the passive films formed in BMP-DCA, ChCl-urea and ChCl-ZnCl2 ILs consisted as least two layers. Beside an IL-adsorbed surface layer, nitrogen-containing compounds (including titanium nitrides) were formed in the inner layer of the passive films, which were responsible for passivation providing effective corrosion resistance for CP Ti. Electrochemical behavior and selective dissolution of Ti-6Al-4V alloy in 1-ethyl-3-methylimidazolium chloride aluminum chloride (EMIC-AlCl3) ionic liquid (IL) system where the composition ranged from Lewis base to acid was investigated. The results show that Ti-6Al-4V alloy has the highest corrosion susceptibility when heptachlorodialuminate ion (Al2Cl7-) was the anion in IL. Selective dissolution of Ti-6Al-4V alloy was found when Al2Cl7- presented in EMIC-AlCl3 IL while AlCl4- resulted in β phase preferential dissolution.