Erosion-Corrosion Failure of a Carbon Steel Pipe Elbow – A Case Study

An elbow leak was encountered after 3 years of operation of a carbon steel flow line from an artificial lift (rod pump) well. Examination of the interior surface of the elbow revealed that the surface was entirely protected by a black corrosion scale with no active corrosion except at the small area localized near the down stream girth weld where the leak occurred. Calculations showed that the flow rates were an order of magnitude below the critical flow rates for erosion/corrosion per API RP14E, suggesting erosion was not the sole reason for the observed leak. Based on the macroscopic and XRD analyses, a two-step mechanism is proposed: (a) impingement of sand-containing fluid flow that destructs protective film and scale and (b) a destructed scale exposing the base metal (localized anode) and attacked by galvanic corrosion where reformation of the protective scale is impeded due to constant impingement by localized fluid/particle flow. This self-facilitated process can result in a rate of localized corrosion orders of magnitude greater than those caused by pure erosion or corrosion and explains the observed leak. Methods for prevention of similar failure are discussed in terms of design and inhibition.