This work was conducted to evaluate effectiveness of the vapor corrosion inhibitors (VCIs) in the cased-crossing application. Specifically, the work was conducted to characterize electrochemical properties of two commercially available VCIs, and their effect on delivery of cathodic protection (CP) current to carrier pipe. Both experimental and modeling studies were conducted. Experimental studies were conducted with VCI gels, VCI only, and gel only solutions. Modeling was conducted to estimate the diversion of the CP current to casing when casing annuli are filled with the VCI gel solutions. It was found that the two commercially available VCIs performed well in the VCI gel formulations, and provided high level of corrosion protection to the carrier pipe inside the casing. In addition, both VCI gel formulations have low resistivity; therefore, a fraction of CP current could get diverted to the casing. This was confirmed in a large-scale testing. The diversion of the CP current needs to be analyzed on a case-by-case basis. The extent of CP current diversion was different for the two VCIs. It was noticed that one VCI formulation shifts the corrosion potential of the carrier pipe metal in the electronegative direction, as a result, the CP current demand and CP current diversion amounts are lower compared to other VCI formulation. The risk of hydrogen generation and accumulation due to diversion of the CP current through the casing was also evaluated. It was found that hydrogen generation commences even at the CP density of 1,000 mA/m2. However, hydrogen effect is mitigated due to VCI gels’ high viscosity, and their ability to buffer hydroxyl ions produced during the hydrogen evolution reaction.
A cased-pipeline segment is generally part of a longer pipeline. The carrier pipe is inside the casing in the segment, and is commonly referred as cased-crossing. The cased-crossings are used in geographical locations where the carrier pipes could be subject to variable mechanical loads, such as mechanical loadings due to vehicle passing on roadways. Casings provide mechanical protection to the carrier pipes. The cased pipe segments are generally safer compared to uncased pipelines because time-dependent threats, including third party excavation and miscellaneous external forces are largely eliminated.1 However, a carrier pipe in a cased-crossing could be susceptible to external corrosion because of scenarios in which the annular space between carrier and casing becomes partly or completely filled with electrolyte. Such scenarios, as well as others, could cause accelerated corrosion of the carrier pipe.