It has been found that passive iron undergoes pitting in potentiodynamic reduction, and the pit formation is a function of scan rate, pH, chloride ion concentration, and formation potential of the passive film. Pitting potential was concluded to be a mixed potential being determined by the coupling of the cathodic reduction of the passive film and the anodic dissolution of metallic iron. However, pitting did not occur for iron without passive film on its surface. It was, therefore, established that the passive film on iron should be thinning without dissolving completely prior to pit initiation.