Research on the difference of characteristics at Steel/electrolyte interface under cathodic protection and in High-pH alkaline solution

Abstract

• In situ spectroelectrochemistry is used to compare steel/electrolyte interfaces under CP to those at high-pH. • No protective passive film with high impedance formed under CP conditions. • Anodic excursions can induce unstable passivition on steel surfaces under high CP. In situ spectroelectrochemistry measurements, including electrochemical impedance spectroscopy (EIS) and UV–vis light reflecting spectroscopy, were conducted simultaneously to compare the characteristics at the steel/electrolyte interface under cathodic protection (CP) to that in high-pH alkaline solution. The results show that the electrochemical characteristics and optical light reflectance of the steel/electrolyte interface under CP are significantly different from those with a passive film formed in high-pH alkaline solution. These data indicate that no protective passive film is formed under the CP potential of −1.2V SSC , presumably because of the continuous supply of CP current to the cathodic interface and subsequent cathodic reduction reactions. The essence of the cathodic protection mechanism is to eliminate the anodic and cathodic zones by cathodic polarization, not to produce passive film on the steel surface. Another interesting phenomenon observed in the experiments was for the steel specimen under CP potentials equal to or more negative than −1.2 V SSC , for which the interruption of CP current and anodic excursions could induce passivation on the steel surface for a certain time. Analyses indicate that the occurrence of passivation in this case is related to the high pH environment produced by previous cathodic reactions and the positive potential shift caused by the anodic excursion; whereas the disappearance of passivation correlates with a decreasing pH and the subsequent electrochemical reactions, which could provide some insights for understanding the effect of dynamic stray current on cathodically protected steel.