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35 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 CD. Comparing these data with Figure 4 indicates there is a direct proportional rela- tionship between pH and the logarithm of CD, which is similar to the potential/pH relationship on the hydrogen line. For the 20% aerated condition, where 100% aeration would be ~8 ppm of DO, the structure's polarized potential does not indicate the interfacial pH until the limit- ing CD for oxygen reduction is reached at a C D of ~ 1 0 – 4 A / c m 2 . T h e l i m i t i n g C D increases as the DO concentration in the electrolyte increases. Nevertheless, in an aerated solution, the charge transfer reaction is the reduction of DO and the production of OH – ions, even though the polarized potential is not lin- early related to the pH. Even in an aqueous solution purged with air, the DO concentra- tion at the structure/electrolyte interface is reduced to negligible concentrations as the structure is polarized to more negative val- ues. This was demonstrated by Lewan- dowski, et al. 11 in 1988 when they conducted cathodic polarization tests on stainless steel in a solution purged with air. At poten- tials more negative than –800 mV vs. CSE, the interfacial DO concentration was negli- gible. Where the interfacial environment is either naturally unaerated or deaerated because of the CP reduction reactions, a pH >9.5 is sufficient to reduce the corrosion rate to <25 µm/y (~1 mpy). When the inter- facial environment is aerated, a pH >10.5 is sufficient to reduce the corrosion rate to <25 µm (~1 mpy). The Interfacial pH Gradient The highest pH occurs at the interface but diminishes rapidly as the distance from the interface increases, as shown in Figure 6. Measurements of pH at distances from the interface were taken by Kobayashi 12 for various bulk solution pHs in aqueous solu- tions at a relatively high CD of 38 µA/cm 2 . The solution pH generally drops to the bulk solution pH within a millimeter of the interface. This illustrates the difficulty in achieving protection on structures in a low pH solution. Even at a bulk solution pH of 4, the pH at the interface is only a little more than 8. To raise the interfacial pH by one unit would require increasing the CP CD by a factor of 10. FIGURE 5 Cathodic polarization scans on steel in a deaerated and a 20% aerated aqueous solution at pH 7. An Alternative View of the Cathodic Protection Mechanism on Buried Pipelines FIGURE 6 Variation in pH with distance from the steel interface when cathodically polarized at a CD of 38 µA/cm 2 in a 3% sodium chloride (NaCl) solution at different bulk solution pH.