Localized corrosion of 1024 mild steel in slightly alkaline bicarbonate solution with Cl− ions

Abstract

The passive film breakdown of 1024 mild steel induced by the presence of 0.05m chloride ions had been investigated in 0.075–0.75m bicarbonate solutions at pH8.9–9.7. 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 (Eox) applied in the absence of Cl- ions. For Eox below about 0.2–0.3V vs SCE, the resistance to localized attack provided by the passive film is independent of Eox; 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 Eox below about 0.2–0.3V vs SCE most likely begins with a surface film dissolution prior to the penetration of the aggressive anions through the film. For Eox above about 0.3V 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.