The energy transportation network of the United States consists of over 2.5 million miles of buried pipelines. It is of prime importance the integrity of the metallic assets due to degradation in soil conditions because of their constant exposure to the aggressive, dynamic, and heterogeneous environment. This degradation process, involves a sequence of process starting with the coating damages/failures and the following electrochemical reactions. External corrosion can result in gradual and usually localized metal loss on the exterior surface of failure coating, resulting in reduction of the wall thickness of the metallic asset. Indirect technologies, such as DC basis have been able to detect and pinpoint two conditions in the pipeline, intact and holiday (active surface or coating anomaly) with good confidence. In this work we consider different levels of corrosion surface severity when a coating holiday (anomaly) exists. Different X52 metallic samples were characterized by using DC polarization and AC impedance to distinguish the differences in terms of capacitive and surface corrosion severity when the metallic samples get a coating failure. Two different coatings were used to characterize different activity surface conditions when there is a coating holiday. The performed set of laboratory experiments include the effects of the applied cathodic protection potential and the metallic surface condition in the presence of holidays (specifically active and passive state) under cathodic protection and simulated soil conditions. Functional correlations and plot results could be found with the fundamentals of electrochemistry and electrical signals by following the DC polarization, relaxation plots and Electrochemical Impedance Spectroscopy results performed in the laboratory. In light of the differences between each surface condition founded experimentally we are able to screen and distinguish the severity conditions of the surface under anomaly coating conditions and potential detection methods/tools for potential pipeline inspection practices.
External corrosion on buried pipelines can result in gradual and usually localized metal loss on the exterior surface of failure coating, resulting in reduction of the wall thickness of the metallic structure. Indirect technologies, such as DC basis (i.e. DCVG, CIPS) have been able to detect and pinpoint two conditions in the pipeline, intact and holiday (active surface or coating anomaly) with good confidence. Classic DC methodologies monitor and characterize the state of the coating and effectiveness of cathodic protection by using transfer function principle (i.e. resistance). The formation of an electrochemical cell, such as buried coated pipeline with cathodic protection (steel in electrolyte) is formed at macro scale conditions [1-2]. The expected damage evolution of the coated pipeline includes the electrolyte (soil̫water) uptake within the coating. Once the water uptakes the coating it reacts at the coating/metallic interface. When the activation of the metallic surface starts the dissolution and surface is modified. Either by third party damage or natural damage process, the surface activation is influenced by the soil conditions. The anodic or corrosion reaction when occurs due to the direct bridge or interaction between the soil and steel surface can have different dissolution rates. The conditions and contents of the soil promote and influence the electrochemical reactions and as consequence the surface conditions, also the corrosion rate is proportional to the surface modification following electrochemical reactions. DC technologies can identify the anomalies resulted due to the bridge between the soil and the metallic structure, the detection considers different resistances between coating (high impedance) and steel surface (low impedance). The surface exposed to the soil will help to resolve such gradient. CIPS and DCVG are clear examples of transfer function methods assuring the identification of such gradient due to the difference between two surfaces (metal and coating) [3-5] in contact with the electrolyte or soil.
