Abstract
A point defect model based on the movement of cation and anion defects in an electrostatic field was carried out to explain the growth and dissolution behavior of a passivation layer on NiTi and NiTiAl thin films. The calculated value of diffusivity was in range of 10 -16 to 10 -17 cm 2 /s. The defect of oxygen vacancy revealed that the passive film was an n-type semiconductor. Mott-Schottky analysis showed that the doping level within a passive film was rather large and in the order of 10 20 to 10 21 cm -3 film, which was considered to be a highly doped structure. The high-resolution transmission electron microscopy (HRTEM) images showed that the highly doped structure consisted of amorphous and crystalline structures of TiO 2 and Al 2 O 3 , respectively. A thermodynamic evaluation for the difference between crystalline and the fully amorphous oxides was calculated to be 57.57 and 96.87 kJ/mol, respectively. In the amorphous region, the electronic level arises from the presence of an energy band gap in the ideal crystalline structure. Therefore, the smaller donor density and the lower diffusion coefficient retarded the defect movement in the passivation layer, and improved the stability of the passive film during corrosion. © 2010 ASM International.