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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33 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 The soil samples, which were collected from di f ferent ge og raphi cal lo cation s across the United States, were intentionally d e a e ra t e d s o t h a t t h e o n ly p o s s i b l e cathodic reaction on the steel electrode would be the reduction of hydrogen ions (i.e., H + + e – = H o ). The potential of a hydro- gen electrode is thermodynamically related to pH. This relationship is plotted on Figure 2 along with the steel electrode corrosion potential. For a steel electrode in a deaerated environment, the corrosion cell cathode potential would be the hydrogen electrode potential and the steel potential is assumed to be the anode potential of the corrosion cell. The potential difference between the steel coupon and the hydrogen electrode is c on si d ered th e c orro sion c el l driv in g potential, which gradually diminishes as the soil pH increases and reaches zero at a potential of ~ –770 mV vs. CSE, at which point th e c orrosion current w oul d b e expected to be zero. This point equates to a potential of –845 mV vs. CSE and occurs at ~pH 9. Besides providing theoretical valid- ity to the –850 mV vs. CSE criterion, the results also suggest that increasing the pH would result in corrosion reduction. The Role of pH in Controlling Steel Corrosion The effect of pH on the reduction of the steel corrosion rate was demonstrated in 1983 by Barlo, et al. 5 for aerated, oxygen- ated, and deaerated conditions, as shown in Figure 3. As the pH increases from pH 8, it is ap- parent that steel corrosion rates diminish to <1 mpy at pH 9.3 in deaerated condi- tions, at pH 10 for aerated conditions, and at pH 10.7 for oxygenated conditions. Furthermore, increasing the environ- mental pH has been shown to be effective in reducing other forms of corrosion, such as stress corrosion cracking and sulfate- reducing bacteria. 6-7 Cathodic Protection and pH When CP current is applied to a struc- ture, the current is transferred across the structure/electrolyte boundary by one or FIGURE 1 The polarization diagram illustrates the Mears and Brown CP mechanism. FIGURE 2 Steel and hydrogen electrode potentials vs. soil pH. more of the following reduction reactions, depending on the environment and the magnitude of the CD. The common reduc- tion reactions are shown in Equations (1) through (3): H + + e – → H o (1) • Hydrogen ion reduction in unaerated or low pH environments 2H 2 O + O 2 + 4e – → 4OH – (2) • Dissolved oxygen (DO) reduction in an aerated environment

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