Caustic stress corrosion cracking occurs in carbon steels under tensile stress and exposure to caustic solutions. In this paper, we present metallurgical analyses performed on a pipe-to-tee weld and a tee-to-flange weld that leaked during operation. The pipe was in treated LPG service in a saturated gas plant. Wet fluorescent magnetic particle inspection indicated the presence of crack-like indications parallel to the welds on the outside and inside surfaces of the pipe, with more crack branching being observed on the inside surface. SEM fractography revealed that the fracture surface of these cracks was predominantly intergranular, which was confirmed during metallographic examination as well. Several temperature spikes above the acceptable range for caustic SCC were also reported by the client. Based on these observations, it was concluded that the cracks occurred as a result of caustic SCC and initiated on the inside surface of the welds.
Caustic stress corrosion cracking (SCC) is known to occur in carbon steels under tensile stress and exposure to caustic solutions from 115°F to boiling temperatures.1,2 Alternating wet and dry conditions tend to increase SCC susceptibility. Localized overheating of the metal, such as solar radiation, heat tracing, steam outs and excursions should also be considered.3 Caustic SCC was first reported in 1980 when the top of a continuous kraft digester vessel blew off in Pine Hill, Alabama.4,5 It was found that the tensile residual stresses present in non-stress relieved carbon steel weld seams and the corrosive environment (caustic) were responsible for the cracking. It was postulated that selective dissolution of the metal coupled with dislocation interaction led to caustic SCC. This damage mechanism has also been reported in superheated steam piping in heat recovery steam generators.6,7
This case study adds to the body of literature on this topic with real examples of caustic SCC occurring on non-post weld heat treated carbon steel pipe fittings. SES was contacted by a client to perform a metallurgical analysis to determine why a pipe-to-tee weld and a tee-to-flange weld leaked during operation. The pipe was in treated LPG service in the sats gas plant, with some caustic carryover. As reported by the client's process engineer, the LPG is routed from the debutanizer bottoms/depropanizer feed heat exchanger (shell side) to the depropanizer tower (LPG Merox unit) which uses caustic for extraction and about 1 ppm of carryover was expected. It was believed that caustic was concentrated within the exchanger bypass piping resulting in the cracking of these welds. Temperature monitoring at the exchanger outlet over the past 1 year indicated temperature spikes (up to 300°F from the average of 140°F), which likely increased cracking susceptibility. Leaks were found at three locations downstream of the heat exchanger during a Snoop® leak detection test of the welds, as shown in Figure 1.
