The localized corrosion of Al-1wt.%Si-0.5wt.%Cu alloy thin film was investigated as compared with alloy bulk material in terms of pit embryo density as a function of time of exposure to a chloride solution in the passivation potential range, by employing scanning electron microscopy (SEM). The results of SEM observation showed that pit embryo size remained nearly unchanged, regardless of exposure time and applied potential, while pit embryo density increased markedly with exposure time and slightly with applied potential. The experimental result indicated that pit embryos do not grow into Stable pits, i.e. pit embryo formation occurs, followed by repassivation at applied potentials below the pitting potential. Pit embryo density was higher for the Al alloy film than for the Al alloy bulk material. Based upon Weibull distribution, analysis of the experimental data on pit embryo density demonstrated for both the Al alloy film and bulk material that pit embryo generation rate increased with applied potential prior to a certain exposure time, while it decreased after the exposure time. The applied potential dependence of the generation rate suggested that defective sites in passive film which are responsible for pit embryo generation are broken down more rapidly with increasing applied potential in the initial stage during the exposure to the chloride solution.