The failure of a wrought aluminum bronze drum (6 to 8 percent aluminum and 1.5 to 3.5 percent iron) drying a calcium carbonate paste was established as due to de-aluminization. The selective attack occurred not only along the 7'-6" longitudinal weld where two phases were present but also along the entire periphery of the 250 fins machined into the outer face of the drum. These fins were all single-phase material. The drum had been in service only a year when many fins broke off. The calcium carbonate paste was essentially neutral (pH 8.0), containing only 0.2 percent NaCl and Na2SO4 as minor impurities. Saturated steam was used inside the drum at 350 F. Associated with the failure was a hardened deposit of CaCO3 and CaSO4 acting as a crevice around the fins and in the grooves.
Laboratory tests, using a sandwich assembly of the same aluminum bronze with the various combinations of CaCO3 and liquids as the filling and with heat transmitted through one surface of the metal, established that de-aluminization occurred only with chloride present in the crevice. No attack occurred with the chemically pure CaCO3 and distilled water or CaSO4 and water. The ultimate solution was to switch to 90-10 cupro-nickel wrought plate welded with 70-30 cupro-nickel; this has given satisfactory service for two years, with no signs of attack. 6.3.6, 3.6.2
