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.
Issue link: http://mp.epubxp.com/i/818289
35 NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIALS PERFORMANCE MAY 2017 CSE), let alone to a protected or polarized potential (in this case –700 mV vs. CSE). To prevent corrosion on the steel it must be 100 mV more electronegative than its po- tential would be if isolated in the same en- vironment ( –700 mV vs. CSE in Figure 3). The 100-mV criterion cannot be applied under th ese circumstances unless th e actual potential of the steel can be con- firmed, possibly by isolated coupons, and the depolarized potential is more electro- negative than the isolated potential. If the mixed metal includes metals that are more active than steel, and they are to receive CP, the 100-mV criterion is applied to the potential of the most active metal in the system. If the most active metals are sacrificial anodes that are the controlling factor in the potential , this procedure would not apply. The above discussion assumes that conditions suitable for the 100-mV crite- rion exist. Note that ISO 15589-1 states that the application of 100 mV shall be avoided at hi g h er o p erati n g t emp eratu re s , i n sulfate-reducing bacteria soils, or with interfering currents, equalizing currents, and telluric currents. NACE SP0169-2013 states that polarization >100 mV may be required under such conditions. Summary Where it is impractical to confirm a polarized potential criterion of –850 mV vs. CSE for steel, the 100-mV criterion can be applied to a partial interruption of the inf luencing CP current sources, provided the voltage error remains constant and/or causes the conservative result when the 100-mV criterion is confirmed. If the amount of depolarization due to the sources interrupted exceeds 100 mV, the polarization criterion is met. As polariza- tion from the uninterrupted sources is not included, this will normally be conservative; however, it provides definitive documenta- tion that a CP criterion was satisfied. Exces- sive conservatism must be avoided to pre- vent cathodic disbondment of the coating. Should mixed metals be present in the system, the depolarized potential must be more electronegative than the potential of isolated steel in that environment. Other restrictions on the 100-mV criterion in the standards 1-2 must be observed. References 1 NACE SP0169-2013, "Control of External Corrosion on Underground or Submerged Metallic Piping Syst ems" (Houston , TX: NACE International, 2013). 2 ISO 15589-1-2015, "Petroleum and natural gas industries—Cathodic protection of pipe- line transportation systems—Part 1: On-land pipelines" (Geneva, Switzerland: ISO: 2015). 3 A.W. Peabody, Peabody's Control of Pipeline Corrosion, 2nd ed., R.L. Bianchetti, ed. (Hous- ton, TX: NACE, 2001). 4 NACE TM0497, "Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems" (Houston, TX: NACE, 2012). 5 W.B. Holtsbaum, Cathodic Protection Survey Procedures, 3rd ed . (Houston, TX: NACE, 2016). 6 P. Charlebois, W.B. Holtsbaum, "Benefit of Applying Polarization Criterion, Case Histo- ries," NACE International Northern Area Western Conference (Victoria, BC, Canada: NACE, 2009). W. BRIAN HOLTSBAUM is president of Corsult Associates (1980), Ltd., 41203 Circle 5 Est, Calgary, AB T3Z 2T4, Canada. Since 1957, he has specialized in corrosion inspection and control on submersed and underground structures in North America and other parts of the world. He has a B.Sc. degree in electrical engineering and is a Professional Engineer. A past presi- dent of NACE International, Holtsbaum is the author of Cathodic Protection Survey Procedures, Editions 1 through 3, and has authored numerous papers and presenta- tions. He is co-author of the NACE history book, The History of NACE International, the Corrosion Society, 1943-2013— 70 Years of Progress. He is currently working on the second edition of the book to honor NACE's 75th anniversary in 2018. He is a member of the MP Editorial Advisory Board and has been a NACE member since 1957. When Polarized Potentials Are Not Practical for a CP Criterion