Potential Noises of Carbon Steel in Deaerated Chloride Solutions Containing Sodium Nitrite

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The potential noise (fluctuation) generated by the local film breakdown of carbon steel electrodes was measured in deaerated chloride solutions. Sodium nitrite of up to 50mmol·dm-3 was added to the solutions as a passivator. Carbon steels have a high resistance to general attacks in this solutions. However, they also develop susceptibility to localized attacks because of being passivated.The potential noise was measured under the conditions where the electrodes were naturally immersed in the solutions. The noise when the electrode's surface was scratched with a glass tube was also measured to clarify the relation between the potential fluctuation and the film breakdown. The type of the noise measured in the experiments was changed with the corrosion potentials; the RD-type appeared at the potentials higher than -0.5V (vs. SSE), and the RR-type occurred at those around or lower than -0.5V. The current transient that generated the RR-type noise was estimated by using the reverse potential setting (RPS) method. Obtained results showed the RR-type was produced by the cathodic current transient. This suggest the local current generated by he film-breakdown was a cathodic current not a anodic one.
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