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34 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 2H 2 O + 2e – → H 2 + 2OH – (3) • Electrolysis of water All of the foregoing reduction reactions result in an increase in the concentration of OH – ions at the structure/electrolyte inter- face and a proportionate increase in pH. Büchler, 8 in 2013, illustrated this increase in pH on a Pourbaix diagram, as shown in Figure 4. When CP current polarizes steel in the negative direction, the steel's polarized potential normally cannot be forced more negative than 80 mV past the hydrogen line, 9 where the electrolysis of water occurs and hydrogen gas is produced. Increasing the CP CD results in an increase in pH, where an increase of a single pH unit typi- cal ly require s an ord er of m a g nitu d e increase in CP CD. When the structure's polarized potential (Ep) resides on the hydrogen evolution line, the potential and the interfacial pH are linearly related by Equation (4): Ep = –316 mV vs. CSE + (–59 mV × pH) (4) The calculated potentials for pH 9 and pH 10 are –847 and –906 mV vs. C SE, respectively. It follows then that the struc- ture's polarized potential is an indirect indication of the interfacial pH when the structure polarized potential is at the hydrogen line. In aqueous aerated solutions, the polar- ized potential of a structure may not be a linear relationship with pH because the structure/electrolyte potential does not meet the hydrogen line until the CP CD exceeds the oxygen-limiting CD (i L ). This situation was illustrated by cathodic polar- ization scans carried out by Thompson and Barlo 10 in simulated groundwater solu- tions, as shown in Figure 5. Argon saturation of the aqueous solu- tion removed DO to obtain a deaerated condition where the structure/electrolyte potential is linearly related to the loga- rithm of applied CD. This is the hydrogen line, where water electrolysis and hydrogen evolution occur. It should be noted that to change the structure/electrolyte potential by 100 mV of cathodic polarization requires approximately a tenfold increase in the CP FIGURE 3 General corrosion rate of bare steel in simulated groundwater at the free corrosion potential as influenced by pH and oxygen level. FIGURE 4 On a Pourbaix diagram, cathodically polarized potentials on steel are limited by the hydrogen line and increased CP current increases the steel/electrolyte interfacial pH. CATHODIC PROTECTION