Failure analysis of gas-gas heater tubes for a flue gas desulfurization system

This paper presents the failure analysis of a gas-gas heater (GGH) tube in a coal-fired flue gas desulfurization (FGD) system. The GGH was operated at a temperature below those of the hydrochloric acid dew point as well as sulfuric acid dew point. Leaks were detected in the U-bend of the failed GGH tube. Corrosion products and ashes were attached on the surface of the tube, and uniform and localized corrosion were present throughout the surface. Corrosion products were mainly shown in reddish brown, orange and black color which were identified as hematite (Fe₂O₃), magnetite (Fe₃O₄), ferric sulfate (Fe₂(SO₄)₃), ferrous sulfate heptahydrate (FeSO₄·7H₂O) and ferrous carbonate (FeCO₃). In addition, S and Cl were observed in the corrosion products. This is a clear evidence that both sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion occurred simultaneously. Further, when Cu and Cr are alloyed together, Cu segregates Cr into the grain boundary, forming a Cr depletion region, and localized corrosion occurs in this area. In the case of the fin-tube joint corrosion, the galvanic corrosion occurred at the filler metal because the filler metal acts as an anode with a middle potential.