Corrosion characteristics of rigid polyurethane thermally insulated pipeline with insulation defects

Electrochemical tests were conducted to study the external corrosion of a steel pipeline beneath the disbonded insulation with a holiday in synthetic groundwater. The corrosion rate Of Pipeline Steel under the insulated condition Was much lower than that under the uninsulated condition. The better corrosion property of the insulated steel was attributable to the formation of a protective rust layer under the insulation. This is mainly because the mass transfer or the thickness loss rate through the insulation was small compared to the convective mass transfer directly to the electrolyte without insulation. Breaks or holidays in insulation exposed the pipeline steel to possible galvanic corrosion. Galvanic corrosion occurred on the insulated and uninsulated steels. Galvanic corrosion at the cracked insulation developed because the uninsulated steel (cracked) was active to the insulated steel (crack-free) and galvanic attack is concentrated in a small cracked area by a large cathode (crack-free)/anode (cracked) area ratio, Pipeline-insulation crevices resulting from insulation disbondments on the pipeline did not induce crevice corrosion because the solution and oxygen could permeate through the insulation, Thus, corrosion at the pipeline-insulation interface was anodic dissolution rather than crevice corrosion.