The Effect of Stress and Hot Corrosion on Nickel-Base Superalloys

This Thesis examines the effect of a molten salt environment on high temperature creep properties of Inconel 718. Sustained-load creep tests were conducted at 1472 F in laboratory air at stress levels in the range of 10 Ksi to 30 Ksi. Round bar tensile specimens were coated with a 90% Na2SO4/10% NaCl salt mixture by spraying a heated specimen with an aqueous salt solution. Creep data were collected over a 72 hour test period. Salt coated specimens were compared with uncoated specimens. Specimens were sectioned and mounted, and a metallographic analysis of the corrosion attack was conducted. Degradation of high temperature creep properties of Inconel 718 in the presence of molten salt is due primarily to oxide penetration into metal which has been depleted of alloying elements and to subsequent cracking along oxide-metal interfaces. Grain boundary sliding and void formation along the grain boundaries also occurred in the alloy-depleted zone of corrosion attack. In static hot corrosion tests on specimens of Rene 77 and Rene 80, cylindrical pins were salt coated and tested for 72 hours at 1652 F in laboratory air. Weight-change calculations and measurements of the depth of corrosion attack were made, and a metallographic analysis was done. Keywords: Microstructural changes; Corrosion-mechanical property interactions; Creep strength; Gas turbines.