| Oxide films, electrochemically formed on carbon steel in aerated, unbuffered sodium nitrite solutions, were studied in the presence and absence of chloride ion. Since the nitrite ion is a strong oxidizing agent and is known as corrosion inhibitor, this medium was selected for oxide film formation studies. The cyclic voltammetric results suggest that the growth of the oxide film in the absence of chloride commences with the deposition of a monolayer of Fe(OH) 2. Initially a Langmuir isotherm was assumed and it was found that a good fit between the theoretical and experimental results was obtained only when allowance was made for strong repulsive lateral forces to exist within the surface monolayer and when the first electron transfer step is rate limiting. At potentials positive of this, the oxide film is suggested to thicken by the high field growth law, converting to Fe2O3. Ellipsometric results, together with impedance measurements performed in the passive potential range, yielded the anodizing ratio and oxide film resistivity, found to be ca. 2 nm/V and 8.7 × 1012 Ωcm, respectively. Notably, the oxide film could be reduced at sufficiently high negative potentials (−1.2 V vs. SCE), providing an oxide-free surface.; In-situ optical imaging showed that, in the absence of Cl−, no surface damage occurs, even at oxygen evolution potentials (1.4 V vs. SCE). In the presence of chloride ion and at sufficiently positive potentials, pitting corrosion was observed on the surface. The hysteresis behavior observed in the CV experiments, as well as the inductive behavior seen in the impedance data, are both characteristics of pit formation. Also, biased electrochemical noise measurements, together with in-situ optical imaging, indicated that a sudden increase in current with a distinct noise pattern is seen as pits form on the surface. |
