Corrosion of Feedwater Heater Tubing Alloys in Peaking Service

A pilot-sized heat exchanger, containing a variety of alloy tubes, has been operated under peaking conditions for almost three years in parallel with a feedwater heater in a generating station. This test had three objectives: (1) To produce “exfoliation” scaling on commercial 70-30 copper-nickel tubes such as had occurred in commercial feedwater heaters at this and other stations under peaking conditions, (2) to evaluate modifications of the 70-30 copper-nickel alloy, and (3) to evaluate a number of other alloys. Under the test conditions, the formation of exfoliation scaling was characteristic of and limited to the 70-30 copper-nickel alloy. Exfoliation scaling was not prevented by the addition of 1.5 percent aluminum to the alloy but the addition of 5 percent iron prevented it. Other resistant alloys are 90-10, 80-20, and 60-40 copper nickels, MONEL¹ alloy 400, Nickel 200, INCONEL¹ alloy 600, stainless steels, and titanium. Carbon steel and a low-alloy steel developed adherent iron oxide coatings with only minor pitting attack. Condensate side surfaces (inside of tubes) also were examined after testing and their condition is described.