Relationship between Corroded Area and Corrosion Potential in the Initial Stage of Corrosion of Steel in Aqueous NaCl Solution

Yoshikazu MIYATA* and Shukuji ASAKURA

Faculty of Engineering, Yokohama National University; 79-5, Tokiwadai, Hodogaya-ku, Yokohama-shi 240-8501 Japan

In the initial stage of corrosion of steel, both corroded and bright areas were observed on a steel surface in aqueous NaCl solution under the aeration. The corroded area grew with time. The relationship between corrosion potential (Ecorr) and θ at 25 °C was studied; θ is defined as the ratio of the corroded area (Aa) to the total area (A). The electrochemical roles of bright and rusted surfaces on a steel were discussed.
In the initial stage of corrosion, the relationship between Ecorr and log{θ/(1−θ)} was linear with the slope of about −130 mV/decade, regardless of the concentration of NaCl and pH. This relation was explained on the following assumptions; (a) an anodic reaction proceeds mainly under the rust, while a cathodic reaction does on the bright surface, (b) the anodic reaction rate does not depend on the potential. The corrosion reaction could be controlled by the anodic reaction in the initial stage of corrosion.

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