A thermogravimetric system is described for the determination of the total oxygen consumption, sample weight‐change, and vaporization rate during the oxidation of ZrBr2 . The results show an increasing deviation from parabolic kinetics with increasing temperature at 250 mm oxygen pressure. The following equation is used to describe the total oxygen consumption in the temperature range of 800°–1500°C normalΔnO2A)(normalmoles O2cm2=5.41×10−4exp)(−25,0002RTt½+2.86exp)(−47,000RTt Because ZrBr2 is oxidized stoichiometrically, this result can be converted to the recession of ZrBr2 as follows δ)(normalcm=4.06×10−3exp)(−25,0002RT t½+2.10×101exp)(−47,000RT t

The active oxidation of Si and normalSiC was investigated in the viscous gas‐flow regime. Kinetics were measured as a function of temperature and oxygen partial pressure using thermogravimetric techniques. Active oxidation was initiated by exposing oxide‐free samples to low oxygen partial pressures in O2/normalAr gas mixtures. Oxidation rates were found to be controlled by oxygen transport through the gaseous boundary layer. Active oxidation of Si was found to occur in two distinct stages rather than in only one as expected. Linear sample weight losses were observed during the first stage; however, in the second stage unusual and destructive behavior was observed in the form of extremely rapid sample weight gains. This phenomenon was interpreted in terms of a simultaneous normalSiO vaporization and SiO2 condensation process. No such behavior was observed during normalSiC active oxidation.

The weight loss of Cr2O3 in oxidizing environments (Po2= 1 to 10−3 atm) at 1200°C was measured. Both hot‐pressed and sintered Cr2O3 pellets were investigated in O2/Ar gas mixtures, and the dependence of the weight loss on the O2 partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O2 partial pressure dependence (rate ∝ PO23/4) corresponds to that expected for the reaction Cr2O3(s)+3/2O2⇌2CrO3(g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate‐controlling step in the oxidation/vaporization of Cr2O3. Evaporation coefficients for the loss of CrO3(g) under the experimental conditions were <0.01.

The high-temperature oxidation behavior of a HfB2 ~-20 v/o SiC composite was investigated in the temperature range 1200~176 and in oxygen partial pressures at 1400~ ranging from 2.6 X 10 -9 to 3.3 X 10 -1 atm. Oxidation kinetics in pure oxygen were parabolic, with the rate constants exhibiting linear Arrhenius behavior. The rate of oxygen consumption is described by kp ----1.62 X 107 exp(--lO6,0OO/RT) g2-cm-4-hr -1 in the range 1350~176The oxide scale formed in this range consisted of an inner HfO2 layer in series with an outer SiO2 layer. The oxidation rate was found to be controlled by initial short-circuit oxygen diffusion through the HfO2, followed by volume diffusion through the SiO2 scaling layer. Below 1350~ rapid oxidation kinetics were observed, HfO~ being the primary scale constituent.Interest in diboride composites stems from their potential use in high-temperature oxidizing environments. These materials hold promise due to their chemical inertness, thermal-shock resistance, retained strength at high temperature, and oxidation resistance.Early studies by Kaufman et al. (1) on the oxidation of undoped diborides showed the oxidation resistance of HfB2 to be superior to that of its Zr, Ti, Ta, and Nb counterparts based on a comparison of the total diboride recession after a 1 hr exposure to oxygen at temperatures ranging from 1300~176Later, Kaufman et al.(2) and Berkowitz-Mattuck (3) studied the oxidation rate of HfB2 and ZrB2 as a function of temperature and oxygen partial pressure in order to determine the mechanism of oxidation. The oxidation kinetics were observed to obey a parabolic rate law, and oxygen diffusion through the scaling layer was found to be rate determining. More recently, Clougherty et at. (4) investigated the oxidation behavior of HfB2 and ZrB2 containing additions of SiC as a function of temperature only, and total diboride recession was measured after 1 hr of oxidation. The SiC addition was found to increase the oxidation resistance; however, kinetic data were not obtained. The oxidation of ZrB2 and ZrB~ -}-20 v/o SiC has been investigated by Tripp et aL (5-7) as a function of temperature and oxygen partial pressure using thermogravimetric techniques. The oxidation reaction was found to be controlled by diffusion through a SiO2 scahng layer. In the present investigation, the oxidation behavior of HfB2 -}-20 v/o SiC was studied as a function of temperature and oxygen partial pressure. The oxidation resistance of the HfB2 -9 SiC composite was compared to that of its constituent components as well as its zirconium counterpart in order to define the rate-determining oxidation mechanism. sure of 3000 psi for 90 rain, and the resulting billet was 88% dense. Coupons (--1 >< 0.5 X 0.1 cm) were 9 Electrochemical Society Active Member. cut from the billet, ground with 600-grit SiC paper, and ultrasonically cleaned in acetone and methanol.Oxidation kinetics were determined using an oxygen-consumption technique (5-7) which is very useful for studying oxidation where one or more of the reaction ...