Multiphase oxidation of metals
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I.
INTRODUCTION
THE scope of macroscopic kinetics is the study of chemical reactions under conditions as they occur in nature or in industry, that is, with the physical processes included.[1] The most important of these physical processes are the diffusion of the reactants/products and the evolution/propagation of thermal energy. Both of these processes are strongly influenced by the hydrodynamic conditions, that is, the kind of motion of the gas phase or the liquid phase which determines the convective transfer of thermal energy and matter. In heterogeneous systems, the presence of one or more interfaces further complicates the system description. Investigation of these complex processes has a threefold purpose: (1) to yield information pertinent to chemical processes under conditions encountered in practice; (2) to provide the extent of various rate-controlling regimes; and (3) to provide insight into the fundamental mechanisms involved. This article focuses on the first and second items for the heterogeneous burning of metals at temperatures above the melting points of the metals and their corresponding oxides. II.
NASA/ASTM PROMOTED-COMBUSTION FLAMMABILITY TEST
The NASA/ASTM promoted-combustion flammability test[2,3] is characterized by upward fire propagation on a 3.2mm-diameter cylindrical metal rod. This test configuration results in a periodic separation and dropping of the molten
THEODORE A. STEINBERG, Lecturer, is with the Department of Mechanical Engineering. The University of Queensland, 4072 Queensland, Australia. SUBHASISH SIRCAR, Scientist, is with Corporate Research, Reynolds Metals Company, Richmond, VA 23219. D. BRUCE WILSON, President, is with Quantos Consulting, Mesilla Park, NM 88047. JOEL M. STOLTZFUS, Program Manager, is with the NASA Johnson Space Center, White Sands Test Facility, Las Cruces, NM 88004. Manuscript submitted February 9, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 1—NASA/ASTM promoted-combustion flammability tests.
product. Figure 1 shows a schematic for this type of burning as well as the appearance of a quenched sample typical of several metals (Cu, Fe, Ni, etc.). Figure 2 further details the characteristic zones of reaction for a burning copper rod which has been quenched. In oxygen-solid metal reactions, the metal is initially nonporous. The reaction may or may not involve gaseous reactants, as opposed to sorbed species. The products may be a gas, solid, or mixture depending upon the level of oxidation and experimental conditions. The porosity, if present, of the solid product greatly influences the rate-controlling regime of the reaction. The chemical kinetics of this class of reactions have been extensively studied and are usually represented in terms of the Wagner theory of metal oxidation or a derivative thereof.[4,5,6] An important common feature of this type of reaction system is that the steps of mass transfer, mass transport, and chemical reaction occur in series, and any one of these steps may dictate the rate-controlling mechanism (rate-controlli
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