Investigation of duplex stainless steel flow line failure

Abstract

In the current study, a 2205 duplex stainless-steel gas flowline with a diameter of 6-inch experienced a leak at two different joints after two years of service. The retrieved failed joints showed severe corrosion damage in the weld and heat-affected zones. This investigation revealed that the damage occurred in the flow line at the 4 to 8 o'clock position (i.e., angle positions of 120° to 240°) in contact with the high salinity water, and negligibly occurred in the top part of the pipe exposed to the dry gasses only. The high salinity broke down the passive layer of the steel. Furthermore, the high carbon dioxide level accelerated the corrosion rate at the de-passivated locations. The selective dissolution of the ferrite was partially accelerated due to the enriched-Ni filler metal. Moreover, the excessive heat input resulted in a coarse-grained heat-affected zone. The low nitrogen levels in the pipes and the flanges impaired the pitting and crevice corrosion resistance of the joints. The relatively high temperature also played a role in accelerating the selective dissolution of metal at the damaged locations.