After Less than 4 Years of Service, the Duplex Heat Exchanger Tubes Which Had Replaced Austenitic Stainless-Steel Tubes Were Leaking. What Happened?

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The top images show various pits on the tubes.  The middle images show a transverse cross-section of a pitted area; the image on the left does not have etch whereas that on the right has a 20% KOH etch.  The bottom left image shows an attacked longitudinal tube section (also etched with 20% KOH etch).  The bottom right image is a Scanning Electron Microscope (SEM) image of a pitted area.

The tubes developed pits due to selective delta ferrite attack.  Selective damage to one of the duplex phases can occur for various services (see Nickel Institute duplex publications).  The selection of alloys for this water service is challenging, and trials with alloys of various Pitting Resistance Equivalent Numbers (PREN) will be required to optimize service life.