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An Alternative View of the Cathodic Protection Mechanism on Buried Pipelines

Published: 20160306 by NACE International
Author(s): Robert Gummow
Co-Author(s): Sorin Segall - Corrosion Service, Daniel Fingas - Corrosion Service

The conventional understanding of the cathodic protection mechanism is illustrated in the definition of cathodic protection arising from the research of Mears and Brown in 1938. That is, cathodic protection is complete when the corrosion cell cathodes are polarized electronegatively to the open circuit potential of the most electronegative anode site on the structure. Typically, the reduction reactions, that transfer cathodic protection current across the structure/electrolyte interfaces produce hydroxyl ions, raise the pH at the interface. For steel, an increase in pH reduces the corrosion rate, depending on the degree of aeration of the environment. Where the interfacial environment is either naturally unaerated or deaerated, because of the cathodic protection reduction reactions, a pH > 9.5 is enough to reduce the corrosion rate to less than 25μm/y (~ 1mpy). When the interfacial environment is aerated, a pH>10.5 is sufficient to reduce the corrosion rate to less than 25μm (~ 1mpy). Although measuring the interfacial pH is not as easy as measuring the polarized potential, the interfacial pH is thermodynamically related to the polarized potential, if the polarized potential resides at the hydrogen line of the Pourbaix diagram, such as when the structure/electrolyte interface is unaerated. Therefore, the polarized potential is an indirect indication of the interfacial pH, which in turn is an indication of the corrosion rate. It can therefore be considered that the increase in pH is the predominant protection mechanism and the polarized potential, except in aerated conditions, is simply an indication of the interfacial pH.