Monitoring Electrochemical Signals from Sacrificial Coatings for Pipeline Materials with Membrane-Based Sensors

Abstract

Monitoring internal corrosion within pipelines is essential to the safe management of gas transportation infrastructures. Membrane-based electrochemical sensors are a promising new method of monitoring corrosion risk, as they can operate in humidified gas environments which were previously inaccessible to conventional electrochemical monitoring methods. In parallel, new sacrificial coatings and coating application methods are being explored to extend the lifetime of existing pipeline structures and minimize repair costs. Herein, we extend the use of membrane-based electrochemical sensors to examine the response of electrodes with sacrificial coatings for a series of fluids with varying extents of corrosivity applicable to natural gas pipelines. Multiple sensors with X65 steel working electrodes were coated then exposed to humidified gases with differing relative humidities (RH) (5 , 50 , and 95 %) and condensed phases with differing salt content (deionized water and 0.1 mol kg-1 NaCl solution). Three electrochemical techniques (potentiometry, impedance spectroscopy and potentiodynamic scans) were used to monitor the corrosive environment and probe the extent of coating coverage. For the conditions studied, impedance values were extremely sensitive to changes in water content through changes in the membrane resistance (from ~1 kΩ in deionize water to ~10 MΩ with 5 % RH). Polarization resistances (Rp) consistently decreased with increasing corrosivity (~10 kΩ in deionized water to 1 MΩ with 5 % RH). Of the methods used, open circuit potential (OCP) values were the most sensitive to the presence of a sacrificial coating. Fully coated samples typically had OCP values 0.2 V lower than uncoated and partially coated samples in humidified environments, suggesting this sensing method made provide a means to monitor coating lifetime.