Oxidation and Sulfidation-oxidation of Fe-based Alloys in H2-h2s-co2 Gas

Introduction: A primary goal of meteoritics is to determine the physical and chemical environments in the early solar system that led to the properties of planets and other solid bodies. Since metal is highly reac-tive in oxidizing and sulfidizing atmospheres, it and its corrosion scales can record the composition and conditions of any alteration fluid that interacted with it. Metal-troilite-oxide assemblages are major components of primitive chondritic meteorites. In previous surveys of these assemblages [1], we noted that some metal grains exposed at chondrule boundaries are highly corroded. The corrosion product is dominantly troilite with minor phosphate, chromite, magnetite, and fayalite. The troilite is euhedral and compact and occurs in intimate contact with the metal. Together, the metal and troilite have nearly circular, 'droplet' morphologies , suggesting that metal sulfidation occurred above the Fe-FeS eutectic (988 °C). This requires pressures greater than 10-4 bars and (O+S)/H ratios between 100 and 1000× solar [2]. Our prior studies used thermodynamic calculations to constrain the temperature, pressure, and composition of the gas phase. However, such an approach does not take into account reaction kinetics, transport mechanisms, and chemical communication between phases. Experimental simulation of metal corrosion under conditions relevant to the early solar system is necessary. Here we present the results of an experimental study of corrosion at 700-1000 °C in gases with enhanced O/H and at 1000 °C in gases with enhanced (O+S)/H ratios, compared to solar abundances. Experimental Procedure: Experiments were performed using an Fe-based alloy containing 4.75 wt.% Ni, 0.99 wt.% Co, 0.89 wt.% Cr, and 0.66 wt.% P. Oxidation experiments were done at 700 – 1000 °C in 17.2% CO 2-H 2 gax mixtures. Upon heating this mixture reacted to form an H 2 O-CO-CO 2-H 2 gas mixture. The sulfidation-oxidation experiments were performed at 1000˚C in gas mixtures with a constant H 2 S/H 2 ratio , but increasing CO 2 (Table 1). Previous studies have shown the dependence of this reaction on temperature and on different H 2 S/H 2 ratios [3]. The sulfidation-oxidation experiments were performed using a 5x5 mm square foil. The foil was suspended in a horizontal Lindbergh furnace using an alumina rod and Pt wire. After the experiment, the samples were weighed to determine weight gain (Table 1). The duration of each experiment was 1 hr. The