Cased pipeline crossings are segments typically located at road and railway crossings where the product pipeline (carrier pipe) is surrounded by a larger-diameter pipe (casing) for protection against mechanical damage. End seals and spacers that separate the carrier and casing pipe deteriorate over time, however, which creates an environment inside the casing—including metallic contact between the two pipes—that is conducive to external corrosion and makes the carrier pipeline vulnerable to damage, particularly if the coating system is compromised at any point.
To control external corrosion of the carrier pipe at cased crossings, a major North American pipeline operator recently implemented the use of gel-based vapor corrosion inhibitors (VCIs) in conjunction with cathodic protection (CP). A VCI fluid mixture was initially injected into the casing’s annular space at a water-like viscosity, and within a designed period of time it set to a gel-like consistency. Since the gel is electrically conductive, it enables the carrier pipe to also receive cathodic current. Although the technique is proving to be very effective based on pipe-to-soil potentials and electrical resistance probe measurements, some debate exists within industry regarding the simultaneous application of the two corrosion mitigation practices.
CORROSION 2016 paper no. 7801, “Experimental Studies to Determine Effects of Vapor Corrosion Inhibitors for Mitigating Corrosion in Casing” by L. J. Krissa, J. DeWitt, P. K. Shukla, and A. Nordquist, reviews a compatibility test of VCI gel and CP and discusses the influences, effects, and interactions between the two methodologies. The overall objective of the study was to comprehensively identify and understand any offsetting effects between the concurrent use of VCI gel and CP inside a casing. The study was to also determine whether CP current can be partially diverted to the casing from the carrier pipe within the gel-filled environment. This paper presents testing details with two commercially available VCIs, each from a major independent corrosion inhibitor supplier; the various electrochemical properties of the two VCIs; and their effects on corrosion mitigation of the casings.
The two VCIs were tested using various electrochemical methods. To test each of the VCIs, two corrosion cells were set up using a three-electrode configuration with a 1-ft (305-mm) long, 2.375 in (60-mm) diameter API 5L X52 grade carbon steel (CS) pipe as the working electrode for each corrosion cell.
The corrosion rates of CS were less than 0.01 mpy (0.25 μm) for the two VCIs, which indicate they are effective in mitigating corrosion of the carrier pipe in the cased pipeline section. The experimental data also indicate that cathodic current densities with the VCIs are somewhat higher than for a holiday in contact with soil. This suggests that more CP current would be needed to meet the 850 mV “off” potential criterion with the application of the VCI gel in a casing annulus.