"CSCC"

Question:

"We are replacing a superheater U-tube bundle in a nitric acid plant, with shell side containing hot gases at 1200 deg F (Nitrogen-67.86%, Oxygen 5.91%, Nitric Oxide 9.38%, Water 16.85%) and tube side water to be superheated. The existing exchanger had CSCC, with 347 SS material for tubes and tubesheet. The suggestion was made to use 2205 as an alternate, but that material is good for only upto 600 deg F. Can you suggest any alternate materials for CSCC and at that high temperatures."

Answer:

Assuming that someone has correctly identified the correct failure mechanism of the previous tube failures as chloride SCC, one can consider any alloy that has the appropriate pitting resistance and chloride SCC resistance. Those alloys begin with alloys containing a nickel content equal to or greater than approximately ~35% and sufficient molybdenum to resist pitting corrosion. Representative alloys meeting the above chemical composition requirements would include Carpenter 20 Cb-3, alloy 825, etc.

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