Corrosion Fatigue of Incoloy 825 Flare Gas Line Bellows of Expansion Joints

摘要

Two out of eight expansion joints (bellows), which are part of flare gas line and made of Incoloy 825 material, failed during normal operation condition after 11 years in continuous service. The components of gas line including the expansion joint are exposed to aggressive environment where the burned gases consisting of CO_2 and H_2S pass through the 2205 duplex stainless steel piping system flared at a temperature of 60℃ and a pressure of 40 MPa. The external surfaces of the pipes were exposed to salt spray, winds, and sometimes sand storms. In addition to these, the failed expansion joints were subjected to continuous movement and/or vibration as a result of drilling operation, sea waves, etc. The received (failed) bellows showed numerous non-branched cracks over the circumference of the bellow, in addition to severe corrosion pits penetrating throughout the bellow's wall thickness. Most of the pits and cracking were observed over the most stressed part of the bellows, which is the curved area. Detailed metallurgical investigation revealed that the expansion joint failed as a result of initiation of fatigue cracks at the corrosion pits that propagated through bellow's circumference. Some of these cracks were found to extend up to 10-20 cm. Bellow's curved design "stress concentration," high chloride concentration on bellow's metal surface, and high metal surface temperature (above 60℃) in addition to the presence of bellows under continuous movement and/or vibration (fatigue type) may have led to localized corrosion and then fatigue cracking of the bellows.