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  • Revision:2007 Edition, August 2007
  • Published Date:August 2007
  • Status:Active, Most Current
  • Document Language:English
  • Published By:NACE International (NACE)
  • Page Count:21
  • ANSI Approved:No
  • DoD Adopted:No

  • General

    Structure-to-soil potential measurements to determine theeffectiveness of CP is sometimes prone to error due to unstablereference cell contact or high soil IR drops. In certain soilenvironments, effective CP is achieved even at potentials lessnegative than the applicable criteria (on or off potentials, 100-mVpolarization, etc.) because of the low corrosiveness of the soiland its chemistry.

    CP potential coupons are used to assess the adequacy of CP onpipelines through coupon potential measurements. 1,2,3These measurements include coupon off and depolarization potentialmeasurements used for alternative criteria mentioned in NACESP0169,4 such as –850-mV off or 100-mV cathodicpolarization. CP potential coupons are also used in situations inwhich it is not possible to interrupt multiple CP sources or onstructures with direct connected galvanic anodes.

    As in the case of pipeline internal corrosion rate measurements,metal weight loss coupons have been used for soil-side applicationsin the past to determine the effectiveness of CP. More recently, ERcorrosion measurement probes,5,6 typically used forinternal corrosion monitoring, have been manufactured specificallyfor use in soil-side applications.

    ER probes may be a better indicator of CP effectiveness in areasaffected by dynamic stray traction current, telluriccurrent-affected pipelines, and also in areas of alternatingcurrent (AC) induction and AC-induced corrosion (ACIC).

    LPR-type probes measure corrosion rates of metals, but onlywithout CP. The fundamental theory of the validity of LPR requiresthat the measurements be made at the freecorroding potential of themetal. The measurement technique is invalidated as soon as CPcurrent is applied. In addition, because these probes makeelectrochemical measurements, they typically need a sufficientlyconductive medium in which to make the measurements.

    (1) Photo courtesy of Naeem Khan, Saudi Aramco,Dhahran 31311, Saudi Arabia.

    (2) Photo courtesy of Schiff Associates, 431 W.Baseline Rd., Claremont, CA 91711.

    (3) Photo courtesy of Rohrback Cosasco Systems, 11841E. Smith Ave., Santa Fe Springs, CA 90670.

    (4) Photo courtesy of Naeem Khan, Saudi Aramco,Dhahran 31311, Saudi Arabia.

    (5) Drawing courtesy of Rohrback Cosasco Systems,11841 E. Smith Ave., Santa Fe Springs, CA 90670, and Naeem Khan,Saudi Aramco, Dhahran 31311, Saudi Arabia.

    (6) Graph courtesy of Rohrback Cosasco Systems, 11841E. Smith Ave., Santa Fe Springs, CA 90670.