The influences of solution temperature, pH value and chloride ion concentration on the diffusivity of point defect (D₀) in the passive film on X80 pipeline steel, were investigated by Mott-Schottky plot analysis and point defect model. The results showed that the donor density increased with increasing solution temperature, increasing chloride ion concentration and decreasing pH value. On the base of a point defect model (PDM), the diffusivity of point defect (assuming that the donors are oxygen vacancies and/or iron interstices) was calculated to be of 10^－16～10^－17cm²•S^－1. D₀ increases with increasing solution temperature, decreasing pH value and increasing chloride ion concentration.

Key words: passive film, point defect, diffusivity D₀, X80 pipeline steel, Mott-Schottky analysis