The kinetics and mechanisms of kamacite sulfurization were studied experimentally at tempera- tures and H2S/H2 ratios relevant to the solar nebula. Pieces of the Canyon Diablo meteorite were heated at 558 K, 613 K, and 643 K in 50 parts per million by volume (ppmv) H2S-H2 gas mixtures for up to one month. Optical microscopy and x-ray diffraction analyses show that the morphology and crystal orientation of the resulting sulfide layers vary with both time and temperature. Electron microprobe analyses reveal three distinct phases in the reaction products: monosulfide solid solution (mss), (Fe,Ni,Co)l,S, pentlandite (Fe,Ni,Co)g,Ss, and a P-rich phase. The bulk composition of the remnant metal was not significantly changed by sulfurization. Kamacite sulfurization at 558 K followed parabolic kinetics for the entire duration of the experiments. Sulfide layers that formed at 613 K grew linearly with time, while those that formed at 643 K initially grew linearly with time then switched to parabolic kinetics upon reaching a critical thickness. The experimental results suggest that a variety of thermodynamic, kinetic, and physical processes control the final composition and morphology of the sulfide layers. We combine morphological, x-ray diffraction, elec- tron microprobe, and kinetic data to produce a comprehensive model of sulfide formation in the solar nebula. Then, we present a set of criteria to assist in the identification of solar nebula condensate sulfides in primitive meteorites.

Kamacite sulfurization in the solar nebula

The kinetics and mechanism of iron sulfidation have been studied as a function of temperature (950–1200 K) and sulfur pressure (10−3-0.065 atm). It has been stated that a compact Fe1−yS scale on iron grows according to the parabolic rate law as a result of outward lattice diffusion of metal ions through cation vacancies. The activation energy of sulfidation increases with sulfur pressure and the 1/n exponent increases with temperature. This nontypical dependence of iron sulfidation kinetics on temperature and pressure results from the analogous effect of both these parameters on defect concentration in ferrous sulfide. The chemical diffusion coefficients,∼DFeS, and diffusion coefficients of defects, Dd, in ferrous sulfide have been calculated on the basis of parabolic rate contacts of iron sulfidation and deviations from stoichiometry in ferrous sulfide. It has been shown that∼DFeS is practically independent of cation vacancy concentration whereas the diffusion coefficient of defects depends strongly on that parameter. A comparison of self-diffusion coefficients of iron in Fe1−yS calculated from the kinetics of iron sulfidation to those obtained from radioisotopic studies indicates that within the range studied of temperatures and sulfur vapor pressures the outward diffusion of iron across the scale occurs preferentially along the c axis of columnar ferrous sulfide crystals.

Sulfidation of iron at high temperatures and diffusion kinetics in ferrous sulfide

The mechanism of sulfidation of Fe-Cr alloys ranging from 8 to 97 wt.% chromium was determined from studies of scale structures, surface morphologies of scale, and reaction kinetics. Although the kinetics of sulfidation were quite similar to those previously determined by Mrowec et al., the structures in the present work were different, being triplex in nature. The growth mechanism of each layer was determined, and the overall sulfidation behavior was compared to the oxidation behavior. Many similarities between the two corrosion processes were observed.

On the high-temperature corrosion of Fe-Cr alloys in sulfur vapor

Stress corrosion cracking of aged Al4%Cu alloy in NaCl solution

The sulphidation behaviour of pure Cr and several CrMo alloys, containing between 0.01 and 1.0 wt.% Mo, has been studied in the temperature range 850°–1000°C, in hydrogen/hydrogen sulphide atmospheres of different compositions, chosen to give a standard sulphur activity corresponding to p S 2 = 0.8 torr for this temperature range. The reaction kinetics observed with all of the materials in this temperature range were parabolic. The rate of sulphidation of pure Cr has been shown to be very rapid, being several orders of magnitude faster than the rate of corrosion in air, oxygen or nitrogen. Increasing additions of Mo to Cr progressively reduced the rate of attack at all temperatures, and the alloy containing 1% Mo corroded at about × 0.70 to × 0.76, the rate for pure Cr, depending upon reaction temperature. The rate constant for all materials decreased with decreasing temperature, and the activation energy for the corrosion process was about 34–38 kcal mol −1 , irrespective of composition. The morphology of the scales was complex, in general both single- and double-layered scales being observed, but a number of interesting features, which were evident, could be systematically related to the experimental variables of exposure time and composition of the alloy, at given reaction temperatures. The probable mechanism(s) of sulphidation of pure Cr and the CrMo alloys are considered.

K.N. Strafford

Papers

The corrosion of Fe and some Fe-based alloys in S vapour at 500°C

Intermediate phase precipitation during the ageing of an Al−4Cu alloy, and the dissolution behaviour of the alloy in HCl