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The purpose of this paper was to characterize the surface of steel reinforcement of concrete under cathodic protection (CP), submerged in seawater, to understand the surface changes due to the application of CP and their consequences on cathodic current requirements.

Reinforced concrete specimens with applied CP were immersed in natural seawater. The experimental methodology included monitoring of corrosion potential (E_corr); measurement of galvanic current (Igalv), protection potential (E_protection) and the depolarization potential of steel during the time of exposure; and electrochemical impedance spectroscopy (EIS). The chemical composition of the steel surface was assessed using X-ray diffraction (XRD).

The application of CP leads to the formation of a deposit on the steel surface that according to XRD results, Pourbaix diagram and physical characteristics, is a protective oxide: magnetite (Fe₃O₄). This oxide causes a decrease in the corrosion rate and requires application of the protection current. It was found that the surface remained protected even after eight days when the CP system was interrupted.

It is necessary to carry out analysis of the chemical composition of deposits formed on the steel surface, perhaps using X-ray photoelectron spectroscopy, Mössbauer, to verify the presence of the magnetite.

Determination of the main cause of the decrease in current required for protection and deposit formation conditions will enable the design of a CP system to be optimized and economized. At present, the CP design considers only a constant current value for the duration of the protection time.

CP is a technique that has proven effective for the protection of metal structures. However, little attention has been devoted to the surface changes that occur under applied CP and their impact on the electrochemical behavior of the system. This paper describes the phenomena produced at the metal surface and determines kinetic parameters and their consequences on the CP behavior.