Study of passivation behavior for titanium aluminides

The general corrosion behavior of titanium aluminides in acidic, neutral and alkaline solutions was investigated using electrochemical techniques. Results showed that titanium aluminides passivated in all experimental solutions. In solutions containing NO{dollar}sb3sp-{dollar} ions, titanium aluminides showed susceptibility to pitting corrosion. The pitting potential increased as Ti content increases in the investigated intermetallics.; Passive film formation and dissolution behavior of titanium aluminides were studied using electrochemical and surface analysis techniques. The passive film dissolution rates were found to be pH dependent. The lowest dissolution rate was found in neutral solutions. In acidic solutions, the dissolution rate increased with the increase of Al content in the materials. In alkaline solutions, the dissolution rate was not sensitive to the solution pH. The results of X-ray Photoelectron Spectroscopy (XPS) showed that the compositions of the passive films formed on the surface of the investigated materials differed in acidic and alkaline solutions. In the acidic solutions, the ratios of Ti vs. Al in the passive films were similar to that of the bulk materials. In the alkaline solution, no detectable Al was present in the topmost layer of the passive films.; The electronic properties of passive films formed on titanium aluminides were examined using Electrochemical Impedance Spectroscopy (EIS). The passive films were found to have n-type semiconductor properties and the donor density of the passive films were calculated. It was found that the passive films on titanium aluminides had non-homogenous donor distribution. A modified Mott-Schottky relation was developed based on a linearly-graded donor distribution model.; Based on both the electrochemical measurements and surface analysis results, it was found that the passive behavior of titanium aluminides was closely related to their passive film composition and the semiconductor properties of the passive films. A modified point defect model was developed to explain the passivation behavior of titanium aluminides in acidic and alkaline solutions.