General

Structure-to-soil potential measurements to determine the effectiveness of CP is sometimes prone to error due to unstable reference cell contact or high soil IR drops. In certain soil environments, effective CP is achieved even at potentials less negative than the applicable criteria (on or off potentials, 100-mV polarization, etc.) because of the low corrosiveness of the soil and its chemistry.

CP potential coupons are used to assess the adequacy of CP on pipelines through coupon potential measurements. ^1,2,3 These measurements include coupon off and depolarization potential measurements used for alternative criteria mentioned in NACE SP0169,⁴ such as –850-mV off or 100-mV cathodic polarization. CP potential coupons are also used in situations in which it is not possible to interrupt multiple CP sources or on structures 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 applications in the past to determine the effectiveness of CP. More recently, ER corrosion measurement probes,^5,6 typically used for internal corrosion monitoring, have been manufactured specifically for use in soil-side applications.

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

LPR-type probes measure corrosion rates of metals, but only without CP. The fundamental theory of the validity of LPR requires that the measurements be made at the freecorroding potential of the metal. The measurement technique is invalidated as soon as CP current is applied. In addition, because these probes make electrochemical measurements, they typically need a sufficiently conductive 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, 11841 E. 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, 11841 E. Smith Ave., Santa Fe Springs, CA 90670.