Carbon steel exchanger tubes in a rich/lean exchanger failed due to circumferential cracking near the tubesheet.

Carbon steel exchanger tubes experienced circumferential cracking and leaking near the tubesheet of a Rich/Lean Exchanger in a natural gas sweetening unit. Carbon steel tubes in the top half of the exchanger had been replaced with stainless steel tubes due to failures of an unknown type. The fracture face of the failed tube samples appeared blocky and brittle appearing. It was heavily scaled and the tube I.D. was corroded. After chemically removing the scale/deposits from the fracture, SEM examination showed that the fracture was intergranular. The polished and etched microstructure revealed surface cracking, an intergranular crack path in a matrix of annealed ferrite grains. Based on the pearlite content, the carbon content of the tube was low. the intergranular fracture, plus knowledge of the MEA service suggested failure of the tubes due to amine stress corrosion cracking, or alkaline stress corrosion cracking. Amine SCC in a exchanger tube was considered unusual in that amine cracking of carbon steel components usually occurs in non stress-relieved welded components. It was hypothesized that differential thermal stresses created the high tube stresses responsible for cracking.