Abstract
It is generally accepted that Alternating Current (AC) corrosion on cathodic-protected underground structures is the result of the combined action of induced AC voltage, local cathodic protection conditions, chemical and physical conditions of the soil and protected coatings. Indeed, NACE standards do not exist related to AC voltages and current density thresholds that are established to protect both personnel and underground structures, such as pipelines. As a result, the industry is currently developing an understanding of how to best mitigate and monitor such risks. One of the most widely accepted monitoring methods is tracking the current density rather than just AC voltages through the use of coupon test stations installed adjacent to the structures requiring protection. Coupons and test stations are typically installed to measure and monitor AC potentials and current densities that are compared to the levels known to produce corrosive effects, when exceeded. Such data are combined with other pipeline design/installation parameters and high voltage AC performance parameters in order to yield information for mitigating and preventing AC corrosion. The industry has depended on the use of these analytical and indirect methods and tools for measuring the effectiveness of mitigation efforts. However, there is a need for more direct methods of such measurement and mitigation efforts. This paper presents an alternative method for directly measuring the effectiveness of AC-induced corrosion control and mitigation efforts. This method takes the form of traditional coupons imbedded with ultrasound testing (UT) transducers designed to directly measure and track coupon metal loss and related corrosion-induced anomalies. Rather than managing some of the challenges of electrical resistance or electrochemical measurement techniques, ultrasound is used to directly measure coupon thickness on any required schedule – manually or remotely. UT-based coupons are integrated into traditional cathodic protection test stations, the results of which are combined with traditional AC and DC current density measurements for more accurate and repeatable tracking of the integrity of underground structures as well as impressed current system performance. Early testing results and the performance of a field installation is presented
