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51316-7531-An Alternative View of the Cathodic Protection Mechanism on Buried Pipelines

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Product Number: 51316-7531-SG
ISBN: 7531 2016 CP
Author: Robert Gummow
Publication Date: 2016
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~~The conventional understanding of the cathodic protection mechanism is illustrated in the electrical 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. When this is achieved the corrosion cell driving voltage and the resulting corrosion current is reduced to negligible values. The polarization potential change at the cathodes is a result of the transfer of electrical charges across the structure/electrolyte interface via reduction reactions. Typically these reduction reactions produce hydroxyl ions that raise the pH at the interface. It is well known that 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). Measuring the interfacial pH is not as easy as measuring the polarized potential but the interfacial pH is thermodynamically related to the polarized potential when the polarized potential resides at the hydrogen line of the Pourbaix diagram. For many cathodic protection systems where the structure/electrolyte interface is unaerated the polarized potential is at or near the hydrogen line. Under these conditions 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.Keywords: Cathodic protection mechanism reduction reactions pH corrosion rate aerated environment unaerated environment polarized potential.
~~The conventional understanding of the cathodic protection mechanism is illustrated in the electrical 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. When this is achieved the corrosion cell driving voltage and the resulting corrosion current is reduced to negligible values. The polarization potential change at the cathodes is a result of the transfer of electrical charges across the structure/electrolyte interface via reduction reactions. Typically these reduction reactions produce hydroxyl ions that raise the pH at the interface. It is well known that 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). Measuring the interfacial pH is not as easy as measuring the polarized potential but the interfacial pH is thermodynamically related to the polarized potential when the polarized potential resides at the hydrogen line of the Pourbaix diagram. For many cathodic protection systems where the structure/electrolyte interface is unaerated the polarized potential is at or near the hydrogen line. Under these conditions 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.Keywords: Cathodic protection mechanism reduction reactions pH corrosion rate aerated environment unaerated environment polarized potential.
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