DETERMINATION OF PARABOLIC RATE CONSTANTS FROM A LOCAL ANALYSIS OF MASS-GAIN CURVES

A method is proposed to allow a more accurate evaluation of thermograv imetric data to identify diffusion or partial diffusion control of sca ling kinetics. This method is based on the fitting of mass-gain data t o a parabola over a short time interval. The translation of the time i nterval over the entire test time period provides an actual instantane ous parabolic rate constant independently of any transient stage or si multaneous reaction steps. The usefulness and limitations of this proc edure are illustrated from oxidation tests performed on several metall ic materials (pure nickel, single-crystal superalloys, and Nb-Ti-Al al loy).