LOCALIZED CORROSION OF 1024 MILD-STEEL IN SLIGHTLY ALKALINE BICARBONATE SOLUTION WITH CL- IONS

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 bicarbon ate solutions at pH 8.9-9.7. A rotating disc electrode with a Kel-F ho lder 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 a bout 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 m ild steel surface. The breakdown potential increases linearly with NaH CO3 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 pr ior 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 con siderably in the anodic direction due to interstitial (formation of cr evices) corrosion at the mild steel/Kel-F interface.