Corrosion of some pure metals in basaltic lava and simulated magmatic gas at 1150°C

Abstract

The extraction of thermal energy from subterranean magma can be achieved by the use of a suitable heat exchanger extending into the molten rock. Although the engineering feasibility of this scheme has not been proven, engineering data, including materials compatibility information, will be required ultimately. The work summarized in this paper was designed to provide an understanding of the reaction mechanisms and modes of degradation of various metals so that the best generic types of alloys can be selected for structural components and instrumentation. Fifteen pure metals were studied. These included base metals such as iron, nickel, and cobalt; some precious metals: platinum, rhodium, and palladium (possible thermocouple or lead-wire materials); refractory metals: tungsten, molybdenum, tantalum, niobium, vanadium, and rhenium; plus other high melting metals such as titanium and zirconium. Samples were exposed to basaltic lava at 1150°C for periods of 24 and 96 hr. A cover gas was used to produce oxygen and sulfur fugacities corresponding to those of the gases dissolved in basaltic melts. The corrosion behavior can be classified into five categories: (A) “no” attack (Pt and Re); (B) slight oxidation (Cr and Mo); (C) heavy oxidation (W, Ta, Nb); (D) sulfidation (Fe, Ni, Co, Pd, Rh); and (E) reaction with lava constituents (V, Ti, Zr). Group (A) metals were inert for all practical purposes. Group (B) metals formed thin adherent oxides initially, under which sulfides eventually formed in the substrate. Attack was minimal. Group (C) metals exhibited extensive oxide formation and virtually no sulfidation. Some reaction between the base-metal oxides and those in the lava took place. Group (D) metals all formed liquid sulfides which penetrated the substrate grain boundaries. All of these metals except cobalt were completely degraded. Cobalt was only partially penetrated by the liquid sulfide formed. Group (E) metals formed silicates, oxides, mixed oxides, and dissolved oxygen in the metal which completely embrittled the metal substrate. A small amount of sulfidation occurred, but sulfidation played virtually no role in the corrosion of these metals. Extensive analyses of the reaction products by scanning electron microscopy, X-ray energy dispersive analysis, electron microprobe analysis, and metallography are presented for each metal. The products formed are discussed with reference to thermodynamic stability diagrams, and the reaction path concept is used to explain some of the corrosion product morphologies.