The pitting potential of pure aluminum thin films in 50 mM K2SO4 was measured as a function of implanted Cl fluence. Samples were implanted with 35 keV Cl+ at room temperature using fluences from 2.25 x 10(16) to 3.25 x 10(16) ions cm(-2) in increments of 0.25 x 10(16). An empirical relationship between pitting potential and fluence was found which suggests a critical Cl concentration in the oxide is necessary for pit initiation. No correlation between pitting potential and the measured Cl concentration or distribution in the metal was found. (c) 2006 The Electrochemical Society.