The passive film breakdown of 1024 mild steel induced by the presence of 0.05 M chloride ions had been investigated in 0.075-0.75 M bicarbonate solutions at pH 8.9-9.7. A rotating disc electrode with a Kel-F holder was used in conjunction with a rotating ring-disc electrode. The resistance to localized attack is closely linked to the preanodization potential (E-ox) applied in the absence of Cl- ions. For E-ox below about 0.2-0.3 V vs SCE, the resistance to localized attack provided by the passive film is independent of E-ox; above the breakdown potential, the localized attack is manifested by the formation of pits at the mild steel surface. The breakdown potential increases linearly with NaHCO3 concentration and pH. Passive film breakdown for E-ox below about 0.2-0.3 V vs SCE most likely begins with a surface film dissolution prior to the penetration of the aggressive anions through the film. For E-ox above about 0.3 V vs SCE under the same conditions, no pitting is noticed and the potential associated with localized attack shifts considerably in the anodic direction due to interstitial (formation of crevices) corrosion at the mild steel/Kel-F interface.