Materials Performance for Residential High Efficiency Condensing Furnaces

In high efficiency furnaces, the condensate that is produced from the flue gas contains trace amounts of such contaminants as $SOx=$⁠, Cl⁻ (chloride ions), $NO3−$ (nitrate ion), $NO2−$ (nitrate ion), F⁻ (fluoride ions), and organic acids. The condensate after concentration by reevaporation becomes very corrosive to the heat exchanger materials. An electrochemical investigation has been conducted using 316L as a reference alloy and synthetic condensates to identify the environmental factors most significant in corrosion. The condensates were formulated using a statistical experimental design approach. Through a regression analysis of the results, chloride and temperature were identified as the most damaging factors. Investigation of several other alloys in an aggressive condensate showed that their corrosion performance was dependent upon their Cr and Mo contents. Subsequently, a large number of alloys were evaluated in a gas-fired furnace by exposing them to chloride-spiked condensates under alternate wetting and drying conditions for 90 days. The alloys included austenitic, ferritic, and duplex stainless steels, and also a few aluminum, copper, and nickel-based alloys. Most of the alloys performed well in the condensate having 26 ppm Cl⁻; a few of the alloys did well in 200 ppm Cl⁻, but at 1000 ppm Cl⁻ only those alloys which had high Cr and Mo contents survived.