Alloy Selection for Reheater Tubes in Flue Gas Desulfurization Systems

Abstract Service performance of reheater systems has been less than desirable in several flue gas desulfurization (FGD) units. The selection of proper alloys for optimum corrosion resistance is a key issue. Available results from high-nickel alloy evaluations suggest that optimum corrosion resistance has not been fully achieved. The conclusions state that alloy 625 had low general corrosion rate but suffered severe localized attack, while alloy C-276 did not undergo any localized attack but exhibited poor resistance to general corrosion. An alloy offering both low general corrosion rate and good resistance to localized attack has not yet been successfully identified. Recent advances in nickel-based alloy development have characterized two new improved materials to resist corrosion: HASTELLOY® alloys C-22 and H-9M. Pertinent laboratory tests were conducted to evaluate these new alloys and to assist in materials selection for reheater tubes. Data on alloy 625, alloy C-276, H-9M™ alloy and C-22™ alloy are presented and compared. The results confirm the advantage of H-9M alloy over that of alloy 625 and the better corrosion resistance of C-22 alloy compared to that of alloy C-276. The improved performance of C-22 alloy is maintained in the as finned and the as finned plus annealed tubular products.