NASA TR R-232: Theoretical and Experimental Investigation of the Oxidation of Molybdenum at Temperatures at Which Its Trioxide is Volatile
by David R. Schryer, Gerald D. Walberg, United States. National Aeronautics and Space Administration
Category: Molybdenum

A theoretical and experimental investigation of the oxidation of molybdenum to molybdenum trioxide at temperatures at which the trioxide is volatile has been carried out. A rate equation has been derived which includes the effects of mass-transfer parameters as well chemical parameters and is applicable to any situation for which a mass-transfer coefficient can be obtained. The derived equation has been applied to previously reported rate data taken under laboratory conditions using air and oxygen-helium mix-reported rate data taken under laboratory conditions using air and oxygen-helium mixtures flowing at low subsonic velocities, and a set of values for the activation energy of the oxidation of molybdenum has been obtained. The average of these values is 8.51 x 107 J-(kg-mole)−1 (20.3 kcal-(g-mole)−1) with a standard deviation of 2.1 percent. In addition, 32 molybdenum hemisphere-cylinder specimens have been oxidized in a Mach 2.1 heated airstream at a free-stream stagnation pressure of 1.07 x 106 N-m-−2 (10.5 atm) and free-stream stagnation temperatures from 1765°K to 2120°K. The stagnation-point surface temperature and recession rate were determined for each of the specimens. The observed recession rates were compared with the corresponding values predicted theoretically by the derived rate equation and agreement within a factor of about 2 was obtained for most specimens. However, a few specimens ignited, exhibiting recession rates much greater than those predicted theoretically. The high recession rates associated with ignition were found to be the result of a sharp rise in surface temperature which caused some melting of the specimens in addition to their normal oxidation.