Scripta METALLURGICA Vol. 28, pp. 1565-1570, 1993 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved OXIDATION BEHAVIOR OF RHENIUM AT HIGH TEMPERATURES T. C. Chou,* A. Joshi, and C. M. Packer Lockheed Research & Development Division, 0/93-60, B/204 3251 Hanover St., Palo Alto, CA 94304 * Present address: The BOC Group Technical Center, 100 Mountain Ave. Murray Hill, NJ 07974 (Received January 8, 1993) (Revised April I, 1993) Rhenium (Re) metal (Tmp=3180°C, p=21 g/cm 3) possesses many attractive properties including excellent ducfifity (-35% tensile elongauon) and formability at room temperature, good high temperature strength and creep properties (creep-ruptme strength better than that of tungsten up to 2800°C), virtual immunity to thermal shock, resistance to carburizing aunospheres (does not form a stable carbide), and high eleclrical resistivity over a wide temperature range [1,2]. For these masons, Re is considered to be an outstanding material for high temperature applications. Extensive applications of Re for large structures, however, have been prohibitive because of its scarcity, high cost, and high density. Currently the major use of Re is as an "additive" to enhance properties of various materials used for aerospace, electronics, heating equipment, and catalyst applications [1]. A well-known application is the development of W-Re and Mo-Re alloys by utilizing the so-callod "rhenium effect" to improve the high-temperature strength and low-temperature ductility of tungsten and molybdenum [3]. Although Re is superior to most other refractory metals in its various high temperature properties and environmental compafibilities, its oxidation properties are rather poor. As a result, Re is normally used in oxygen- free aunospheres. When used in oxidizing environments, Re requires protection with an oxidation resistant coating, such as Ir [2,4]. Without proper protection, Re oxidizes and forms eight different kinds of oxides [5], of whom the most notable (anhydrous) ones are Re208, Re2OT, ReO3, and ReO2. Some physical and thermodynamic properties [5] of these oxides are summarized in Table I. It is noted that the formation temperatures of these Re- oxides are relatively low compared to other refractory-metal oxides. The ease of forming low-melting point Re207 is known to cause "hot shortness" failure of Re during hot-working in air [3]. Moreover, these oxides are very volatile; their volatilities follow the order of Re20"7>ReO3>ReO2 (see Fig. 1). Table I. Pro ~ies of Rhenium Oxides Rc208 - - Tf" (°C) "-" Color colorless Solubility in --- wa~r Remark least stable m'm~ee temperature; dem~'minod I Re207** 297 29O li~ht yellow soluble RcO3 34O red insoluble can be reduced to readily oxidized metallicRe at 400°C Re207; dissociates by hydrogen 400°C in vacuum R -207 and z oxidationof rheniun **volatilizes above 635 K, appears as a white smoke RcO2 = = = 4OO black insoluble to dissociates at 750°C in at vacuum to Re20 7 and to metallic Re Despite the ready availability of thermodynamic properties of various Re-oxides [6], only limite~l information is available concerning the oxidation behavior of Re at high temperatures. Most previous studies on the oxidation of Re were conducted under low pressures (10 to 10-4 Pa) and at temperatures below 1500°C [7-12]. The primary focus of these studies is on understanding the oxygen adsorption and dcsorption phenomena, and O-Re bonding at the near-surface regions. Since Re is mainly being considered for use with other materials at very high temperatures, its oxidation properties at high temperatures are crucially needed in order to realize its development as a high temperature material. In this paper, we carry out oxidation studies of Re at temperatures between 1500 and 1900°C. Oxidation rams of Re as well as activation energies of oxide formation are presented. 1565 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.