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Tantalum is resistant to 80 percent sulfuric acid (H2SO4) at 150°C (302°F). However, data on fatigue of tantalum under these conditions were unavailable. To determine design criteria for a tantalum pump impeller, high frequency fatigue tests were conducted over a range from less than 106 to over 9 x 109 cycles, establishing approximate stress amplitudes which allow tantalum to achieve up to 8 × 109 cycles of loading without failure. These stresses were found to be about 11 ksi (7.7 kg/mm2) for unalloyed tantalum and about 12.4 ksi (8.7 kg/mm2) for Ta-2.5W. These two tantalum alloys were deliberately processed to create very soft materials with large grain sizes. Such materials may indicate behavior of these alloys in or near weld areas.
Case histories are presented on two Ta-2.5W pump impellers which failed in 80 percent H2SO4. The cracks were generally associated with welds. The crack surfaces show evidence of corrosion fatigue and cleavage fractures. The areas of cleavage fracture suggest an environmental effect such as hydrogen-assisted stress corrosion cracking.
tantalum, sulfuric acid, corrosion fatigue, cleavage, fracture, hydrogen-assisted stress corrosion cracking
Research associate, E. I. du Pont de Nemours and Company, Inc., Jackson Laboratory, Wilmington, Del