Abstract The effect of chloride concentration on early stages of pitting for Type 304 stainless steel has been studied by using an AC impedance method. The Warburg impedance coefficient, which is calculated from Bode plots, increases with increasing chloride concentration at low potentials in the passive region when the diffusion process begins to occur at the surface. For a pit which is nucleated under a given potential, there exists a minimum chloride concentration above which the pit on the surface of the steel can be activated into metastable propagation, and below which it cannot. It is also found the effect of chloride concentration is reflected qualitatively in the potential ( E m ) at which the metastable pit or pits start to grow on the surface of Type 304 stainless steel. The values of E m in millivolts linearly increases with the logarithm of chloride concentration of solution, following the equation, E m = −37 log( Cl −)−212.