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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36 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 pendent of the interfacial pH, which can be high even though the polar- ized potential is significantly less negative than the –850 mV vs. CSE criterion. • In both unaerated and aerated condi- tions, the CP mechanism is arguably the development of a high pH caused by the operation of the CP system, which can be called "chemical polar- ization." References 1 R . B . Me a r s , R . H . Br ow n , "A Th e o r y of Cathodic Protection," Transactions of the Electrochemical Society 74 (1938): p. 527. 2 S.C. Dexter, L.N. Moettus, K.E. Lucas, "On the Mechanism of Cathodic Protection," Corro- sion 41, 10 (1985): p. 606. 3 R .J. Kuhn, "Cathodic Protection of Under- ground Pipe Lines from Soil Corrosion," API Proc., Vol. 14, Section 4 (1933): p. 157. 4 W.J. Schwerdtfeger, O.N. McDorman, "Poten- tial and Current Requirements for the Ca- thodic Protection of Steel in Soils," Corrosion 11 (1952): p. 392. 5 T.J. Barlo, et al., "An Assessment of the Crite- ria for Cathodic Protection of Buried Pipe- lines," AGA, Corrosion Supervisory Commit- tee, June 1983, pp. 2-5. 6 N. Prakash, et al., "Corrosion of Mild Steel by Soil Containing Sulphate Reducing Bacteria," J. Microbiological Biotechnolog y 3, 2 (1988): p. 82. 7 R .N. Parkins, R .R . Fessler, "Line Pipe Stress Corrosion Cracking—Mechanisms and Rem- edies," CORROSION/86, paper no. 320 (Hous- ton, TX: NACE International, 1986). 8 M. Büchler, "The Physical-Chemical Signifi- cance of the IR-Free Potential," International Congress and Technical Exhibition (Brussels, Belgium: CECOR, 2013), p. 2. 9 S. Glasstone, An Introduction to Electrochem- istry (Princeton, NJ: D. Van Nostrand Co., 1942), p. 443. 10 N.G. Thompson, T.J. Barlo, "Fundamental Processes of Cathodically Protecting Steel Pipelines," Gas Research Conference Proc. held June 13-16, 1983 (Oslo, Norway : IGU, 1983). 11 Z. Lewandowski, et al., "Dissolved Oxygen and pH Microelectrode Measurements at Water Immersed Metal Surfaces," CORRO- SION/88, paper no. 93 (Houston, TX: NACE, 1993). Although measuring the interfacial pH is not as easy as measuring the polarized potential, the interfacial pH is thermody- namically related to the polarized potential if the polarized potential resides at the hydrogen line of the Pourbaix diagram (e.g., when the structure/electrolyte interface is unaerated). Therefore, the polarized poten- tial is an indirect indication of the interfa- cial pH, which in turn is an indication of the corrosion rate. The increase in pH, there- fore, can be considered the predominant protection mechanism; and the polarized potential, except in aerated conditions, is simply an indication of the interfacial pH. Steel Corrosion Potential vs. pH It has been shown that the application of CP polarizes steel in the electronegative direction, while raising the pH at the inter- face at the same time. However, electroneg- ative corrosion potential s can al so be obtained in deaerated aqueous solutions simply by placing the steel in a high pH solution. This is predicted by the Pourbaix diagram for iron; but in aerated solutions, the steel corrosion potential is somewhat independent of pH, as shown in Figure 7. These corrosion potential data were produced as a result of cathodic polariza- tion tests, conducted by Perdomo and Payer 13 in 1995, on steel samples in a 100 mM sodium sulfate (Na 2 SO 4 ) solution at varying pH values. They show that the cor- rosion potentials in the deaerated solu- tions were dependent on the pH, while in the aerated solutions they were virtually independent of the pH. The CP mechanism could be inter - preted as the increase in pH at the struc- ture/electrolyte interface coupled with the consumption of DO in the oxygen reduc- tion reaction, which creates a deaerated environment. Furthermore, in high resistiv- ity, well-drained soils, both the NACE Inter- national and ISO CP standards 14-15 permit less negative potential criteria than the –850 mV vs. CSE criterion, such as –750 and –650 mV vs. CSE. The pH in very aer- ated soils, however, can be very high at the interface despite these lower potentials, in w hi ch ca se th e cath o di c p o l ari zation mechanism can be attributed to the high pH developed at the interface. Therefore, the CP mechanism can be considered as "chemical polarization." Summary • Increasing the pH at the steel/elec- trolyte interface decreases the corro- sion rate in the absence of CP. • The reduction reactions that transfer CP current from the electrolyte to the structure increase the pH and de- crease the DO concentration in the electrolyte at the structure/electro- lyte interface. • In unaerated environments, the steel polarized potential is directly depen- dent on the interfacial pH produced by CP. • In aerated environments, the steel polarized potential is generally inde- CATHODIC PROTECTION FIGURE 7 Steel corrosion potentials vs. pH in aerated and deaerated, 100 mM Na 2 SO 4 solutions.

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