Abstract
In near-neutral solutions, mild steel can be protected by two different passivating films, a ferric oxide film stable at more positive potentials and a magnetite film stable at more negative potentials. In air-saturated solutions of hydrogen carbonate ions at 25°c, as the concentration is increased in the range up to 1O−2 M, the ferric oxide film on steel becomes less stable at its Flade potential; in the range 10−2 to 10−l M, the ferric oxide film increases in stability at its Flade potential until eventually its breakdown is suppressed and at 10−1 M and above, corrosion inhibition occurs. However, inhibition may be unstable, since the stability of the magnetite film at its Flade potential decreases with increasing hydrogen carbonate concentration. In the presence of chloride ions, critical breakdown potentials are observed for both the ferric oxide and the magnetite passivaiing films, but the magnetite film is much more stable than the ferric oxide film. The inhibition of corrosion of steel in air-saturated solutions by anions is due to their stabilising effect on the ferric oxide film with respect to its breakdown at its Flade potential. In non-inhibitive solutions, the initial development of corrosion of steel depends on the stability of the magnetic film. This film also affects the corrosion behaviour of steel when coupled to more electronegaiive metals, e.g., zinc, cadmium, and when polarised for cathodic protection.
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