Verification of the passivity condition of cathodically protected carbon steel using in-situ electrochemical and spectroscopic techniques

Cathodic protection, in combination with coatings, is a well consolidated and widely applied technique for the protection of metallic structures from electrolytic corrosion. The protection effect is guaranteed by the circulation of a direct cathodic current between an anode and the protected structure; the current has the thermodynamic effect of decreasing the potential of the metal thus decreasing the overall corrosion rate. However, the cathodic current has a secondary effect causing a local increase of alkalinity depending on the extent of the applied cathodic current density and the diffusional motion of the involved species, all factors that strongly affect the surface state of iron. Specifically, in agreement with Pourbaix diagrams, the alkalinity should favor the passivation of the metal. In this work, results of preliminary laboratory tests with the objective of demonstrating the presence of passivity on steel in cathodic protection condition are discussed. The presence of the passive film is demonstrated by means of electrochemical impedance spectroscopy (EIS), photocurrent spectroscopy and in-situ Raman spectroscopy tests.