Materials Performance

MAR 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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32 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 CATHODIC PROTECTION T The conventional understanding of the cathodic protection (CP) mecha- nism is illustrated in the definition of CP arising from the research of Mears and Brown in 1938. That is, CP is com- plete when the corrosion cell cath- odes are polarized electronegatively to the open circuit potential of the most electronegative anode site on the structure. Typically the reduction reactions, which transfer CP current across the structure/electrolyte inter- faces, produce hydroxyl ions and raise the pH at the interface. For steel, an increase in pH reduces the corro- sion rate, depending on the degree of aeration of the environment. The conventional understanding of the cathodic protection (CP) mechanism is founded in th e results of exp erim ents carried out by Mears and Brown in 1938. 1 They stated that, "For CP to be entirely effective, the local cathodes on the corrod- ing specimen must be polarized to the potential of the unpolarized local anodes." At that point, as illustrated in Figure 1, the corrosion current density (CD) is reduced to the exchange CD (i o ) at which point the corrosion CD is zero. This theory was supported by Dexter, et al., 2 who conducted polarization tests on steel in seawater and concluded that, "CP was achieved by polarizing to, or at least An Alternative View of the Cathodic Protection Mechanism on Buried Pipelines R.A. Gummow, S. Se GA ll, A nd d . Fin GAS , Corrosion Service Co., Ltd., Markham, Ontario, Canada toward, the potential of the local action anodes in agreement with the Mears and Brown and Hoar theories." For complete CP protection, therefore, the true criterion for any corrosion cell is the open circuit potential of the corrosion cell anode (E a,oc ). Extending this electrical analogy to a structure having many corro- sion cells means that for complete CP, all cathode sites need to be polarized electro- negatively to the most electronegative anode site on the structure. However, this presents a serious problem because of the impracticality of determining the potential of the most electronegative open circuit anode on the structure. Kuhn 3 addressed this problem based on empirical data that were obtained on the C P of c a st i r o n w a t e r m a i n s i n Ne w Orleans, Louisiana, USA. He proposed a potential of –850 mV vs. copper/copper sulfate (Cu/CuSO 4 ) electrode (C SE) "to which a pipe must be lowered in order to stop corrosion ." Although there was no verification on the effectiveness of this cri- terion at the time, it was assumed that this potential criterion was more electronega- tive than any open circuit anode potential on a ferrous structure. The ef fectiveness of CP was further investigated by Schwerdtfeger and McDor- man at the National Bureau of Standards 4 in the early 1950s. The potential of steel electrodes, placed in soil samples of vary- ing pH, were measured and plotted against pH, as illustrated in Figure 2.

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