Countries

The top left image shows the tubesheet channel face and a location of a removed tube. The carbon steel tubesheet also had deep losses behind the tubes. The shell side of the tubesheet also had extensive losses. The top right image shows the pits on a removed tube. The through-wall pits progressed from the OD surface. The pits are located between the tube-to-tubesheet seal weld and the first tube groove (i.e., a crevice). The OD surface has orange scales.

A cross-section of the pit is shown in the bottom left image. The pits are steep, narrow, and tend to tunnel. The orange scale observed in some areas on the tube is rich in iron and oxygen, consistent with the corrosion scale from the carbon steel tubesheet. The bottom right image is the Energy Dispersive X-ray scan of the scale showing significant iron and other process contaminants.

The CP-Ti tube failed due to the crevice conditions created between the tubesheet and the tube. Crevice conditions are particularly aggressive, and the iron corrosion products can introduce galvanic effects that promote further crevice corrosion.