The pitting behavior of carbon steel in chloride-containing solutions was studied by the electrochemical noise technique. The semiconducting nature of the passive film formed on carbon steel was revealed as well. The initiation of metastable pits is generally indicated by a typical current and potential transient with the shape of a quick current rise and potential drop followed by a slow recovery. The potential fluctuations mainly come from the response of the electrode capacitance to pit growth charge. Only the current transients directly reflect the metastable pitting process. The potential dependence of the pit initiation rate is well illustrated by the point defect model, which assumes that pitting initiation is due to the anion-catalyzed cation vacancy condensation at the film/metal interface. Pit growth kinetics are controlled by the ohmic potential drop across the cover over the pits. The repassivation time of metastable pits is affected by the potential drop across the pit cover. A pit stabilization criterion of the ratio of peak pit current to pit radius indicates that the critical condition to maintain the stable pit growth must exceed 2 x 10⁻² A cm⁻¹ to avoid repassivation. The main role of chloride ions in pitting is to increase the chance of the breakdown of a passive film, rather than to inhibit surface repassivation. The initiation o f a metastable pit will have a certain influence on subsequent pitting events in the case of high pitting activity. When the pitting activity decreases, the metastable pitting events will follow the Poisson distribution.

Spectral analysis of noise data indicates that any transient having a sudden birth or a sudden death generates f⁻² noise, while that without sudden change shows f⁻⁴ A behavior. The roll-off frequency reflects the repassivation or growth rate of metastable pits.

The noise resistance coincides with the polarization resistance only in passivity or general corrosion. For pitting, the noise resistance cannot track quantitatively the change of the corrosivity of the system.

The passive film on carbon steel is a highly disordered n-type semiconductor. There are two donor levels in the space charge layer of the passive film. The divalent iron cation vacancies caused by the ionization of deep donor levels and then the complexity of iron cation with chloride ions are mainly responsible for pitting. The diffusivity of oxygen vacancies within the passive film is estimated to be in the range of 10⁻¹⁶ – 10⁻⁻¹⁵ cm² s⁻¹.