Abstract
Underground pipelines that carry hot fuel oil from loading terminals or storage tanks in urban installations have experienced corrosion failures. Generally, the pipelines are thermally insulated to reduce heat loss during oil transmission. The pipelines are also provided with cathodic protection (CP) systems. The pipelines are of 6- to 24-in. (0.15- to 0.61-m) diameter API grade carbon steel, the fuel oil is at 120 to 200°F (52 to 93°C), and the insulation is 1- to 2-in. (2.5- to 5-cm) thick foamglass or polyurethane foam applied onto the pipeline surface at the factory. Some of these lines were installed 20 to 30 years ago, and increasing instances of corrosion at the pipe/insulation interface were found in routine smart-pig surveys. The cause of this corrosion and measures of prevention were evaluated. Various long-term electrochemical, ac-impedance, and exposure tests were performed (1) to establish the mechanism by which water reaches the corrosion sites and the reasons for failure of CP systems to prevent corrosion; (2) to explore methods for effective delivery of CP current; and (3) to identify protective coatings for field and factory installation. The results showed that the insulation materials degrade on long exposure to heat and water and absorb increasingly larger amounts of water from the ground; CP current is ineffective because the available potential difference is insufficient to overcome resistance of insulation; the -100 mV and -0.85 V criteria are applicable also at fuel-line temperatures; and many currently available coatings are excellent long-term barriers to water at these temperatures.
