Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100° to 1300°C and 300- and 1500-MPa confining pressure, with the oxygen fugacity around the specimen constrained near the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10−10 m2 s−1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300-MPa confining pressure. Crystals hydrothermally treated at 1300°C, 300-MPa confining pressure, and 10−5 s−1 strain rate are a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigations suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the “wet” experiments. First-order calculations suggest a stress exponent of n ∼ 2.5 and a water fugacity exponent of m ∼ 1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.

The role of water in the deformation of olivine single crystals

(Fe,Mg)2SiO4 occurs in the polymorphic forms of olivine (orthorhombic), wadsleyite (orthorhombic) and ringwoodite (cubic). These are some of the most common minerals in the inner solar system. Olivine occurs in the mantles of the Earth, the Moon and Mars, in meteorites, in plutonic and volcanic igneous rocks as well as in some metamorphic rocks. The high pressure polymorphs wadsleyite and ringwoodite have been reported from shocked meteorites. Diffusion coefficients of different elements in these phases have been widely used in several areas of the Earth, planetary and materials-sciences (see below for examples). The relative simplicity of structure and chemistry combined with its wide range of stability (in P - T space) uninterrupted by phase transitions also makes olivine one of the easier materials to study. As a result, olivine has served as a model material for the study of defects and diffusion in silicates. In this chapter we will focus on diffusion data in olivine, wadsleyite and ringwoodite belonging to the Fe-Mg solid solution series, (Fe,Mg)2SiO4.

### Structure of olivine and types of diffusion coefficients

Olivine, with the general formula A2SiO4, is an orthosilicate made up of isolated SiO4 tetrahedra linked to each other by two different kinds of octahedral polyhedra. The octahedral polyhedra are described as M1 and M2 and are usually occupied by divalent cations such as Fe2+, Mg2+, Ni2+, Mn2+ and Ca2+. One aspect of the structure that is relevant for discussion of diffusion data is that the M1 polyhedra form a continuous chain along the [001] direction (space group: Pbnm ) whereas the M2 polyhedra are not interlinked with each other in any direction in the structure. In this chapter reference to the [001] direction or the c-axis will always be made with reference to …

Diffusion Coefficients in Olivine, Wadsleyite and Ringwoodite

Wagner's classical treatment of internal oxidation (generic name allowing for reaction with oxygen, nitrogen, carbon or sulfur) assumed ideal conditions such as uninhibited dissolution of the gas, formation of spherical particles, diffusion of the oxidant in the solvent as the rate-controlling step, equilibrium conditions, etc. However, during the 45 years since his treatment, many observations have been made to complicate the idealized situation suggested by Wagner. This paper examines the most important modifications with respect to Wagner's original analysis. The following items are discussed. (a) The role of solute concentration: The parabolic kinetics are much higher than expected for Ni−Al alloys due to rapid interfacial diffusion of oxygen along the interfaces between cylindrical rods of Al2O3 (perpendicular to the surface) and the matrix. (b) Precipitate morphology: Spherical precipitates seem almost to be the exception. A wide variety of forms have been observed, including Widmanstatten platelets, cylindrical rods, hexagonal plates, dendritic or “fishbone” products, etc. The competition between nucleation and growth is useful to explain the observed structures. (c) Intergranular internal oxidation: Rapid oxygen diffusion in grain boundaries may lead to a wide variety of intergranular-precipitate structures. (d) Internal-oxide bands: Wavy, approximately parallel bands form at a finite distance beneath the surface in certain alloys having very reactive solutes, e.g., Ag−Mg. It is postulated that high stresses generated by precipitation play a major role. (e) Surface nodules of pure solvent metal: High stresses generated during precipitation cause extrusion of solute through dislocation pipes, leading to extensive nodule formation on either grain boundaries or on the grains (or both), depending on the alloy and oxidizing conditions. (f) Nonstoichiometric precipitates: Either hypo- or hyperstoichiometric particles can form as very small clusters in certain alloys (Ag−Al). The nature of precursors and changes in stoichiometry during reaction are discussed. (g) Trapping of oxidant: Diffusion of the oxidant may be slowed appreciably by trapping with the solute, although no precipitates need to form. Lower-than-expected kinetics (based on normal diffusivities of the oxidant) result. (h) High-solubility-product precipitates: Concentration profiles of solute, oxidant and precipitate are quite different than those expected for low-solubility-product precipitates as considered by Wagner. In particular, a variable mole fraction of precipitate exists, and further precipitation occurs in the reaction zone after the front has passed by. Linear kinetics have been observed for some Nb-base alloys at very high temperatures and low oxygen pressures. The rate-controlling step is the arrival of oxygen at the surface and not oxygen diffusion in the metal. (i) Dual oxidants: Two gases may diffuse·simultaneously and each forms its own product with the solute. The thermodynamically most-stable compound forms near the surface, and the less-stable compound deeper in the alloy. The less-stable compound is subsequently converted to the more-stable compound with a concomitant release of the second oxidant. Although numerous examples have been reported of systems which do not behave as predicted by Wagner, his theory still remains as the cornerstone of our understanding and is still the starting point for virtually every study in internal oxidation.

A Critique of Internal Oxidation in Alloys During the Post-Wagner Era

We present new experimental data on Mg tracer diffusion in oriented single crystals of forsterite (Fo100) and San Carlos olivine (Fo92) between 1000–1300° C. The activation energies of diffusion are found to be 400 (±60) kJ/mol (≈96 kcal/mol) and 275 (±25) kJ/mol (≈65 kcal/ mol) in forsterite and San Carlos olivine, respectively, along [001] at a fO2 of 10−12 bars. There is no change in activation energy of Mg tracer diffusion within this temperature range. Mg tracer diffusion in a nominally pure forsterite is found to be anisotropic (D∥c > D∥a > D ∥b) and a function of fO2. This fO2 dependence is different from that in olivine containing Fe as a major element, which suggests that the diffusion mechanism of Mg in forsterite is different from that in Fe-bearing olivine at least over some range of fO2. The diffusion mechanism in nominally pure forsterites may involve impurities present below the limits of detection or alternately, Si or Fe3+ interstitial defects, Fe being present as impurity (ppm level) in forsterite. Pressure dependence of Mg tracer diffusivity in forsterite measured to 10 GPa in a multianvil apparatus yields an activation volume of approximately 1–3.5 cm3/ mol. It is found that presence of small amounts of hydrogen bearing species in the atmosphere during diffusion anneal (fH2 ≈ 0.2 bars, fH20 ≈ 0.24 bars) do not affect Mg tracer diffusion in forsterite within the resolution of our measurement at a total pressure of 1 bar. The observed diffusion process is shown to be extrinsic; hence extrapolation of the diffusion data to lower temperatures should not be plagued by uncertainties related to change of diffusion mechanism from intrinsic to extrinsic.

Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P, T and fO2

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr2O3 double layer and isolated NiO–NiCr2O4–Cr2O3 triple layer were observed at a steady-state condition. For Haynes 230, a MnCr2O4 layer was formed on top of the Cr2O3 layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr2O3 and outer MnCr2O4 or TiO2. The spallation of outer layer and subsequent volatilization of the Cr2O3 layer produced a rugged surface and interface as well as internal oxidation.

Oxidation Characteristics and Oxide Layer Evolution of Alloy 617 and Haynes 230 at 900 °C and 1100 °C

High temperature corrosion of the nickel-based alloy Inconel 617 in helium containing small amounts of impurities

Application of the technical nickel-base alloy IN 617 in the primary circuit of the high-temperature gas-cooled reactor is limited essentially by the chemical reactions with the impurities (CO, CO2, H2, CH4, H2O) in the helium coolant. The interactions of the alloy with the reactive impurities in different helium-base gases were investigated by thermogravimetry in a gas-tight microbalance and by simultaneous measurement of the changes in gas composition by a continuous sensitive mass spectrometric analysis. The results demonstrate that the set of six reaction equations deduced in part 1 can be applied to describe the corrosion of the alloy. The occurrence of the various reactions is determined essentially by temperature. For the case of a standardized helium gas (HHT-He), three temperature regions can be distinguished. Below a critical temperature (about 1105 K), the presence of CO can cause simultaneous oxidation and carburization. Above this temperature, this reaction does not reverse itself. Rather, oxidation by CO2 and H2O takes place that shows, after a transient period, the same kinetics observed in undiluted oxygen-containing gases. At temperatures above about 1205 K, decarburization of the alloy accompanied by the production of CO takes place, leading to severe destruction of the carbide microstructure and, therefore, limiting the applicability of the material.

Mechanisms of high-temperature corrosion in helium containing small amounts of impurities. II: Corrosion of the nickel-base alloy Inconel 617

Investigations of slow exchange processes at metal and oxide surfaces and interfaces using secondary ion mass spectrometry

A computer-based method is used to simulate the process of penetration of a foreign element (e.g., oxygen) into an alloy connected with internal precipitation of a compound of this element with a solute in the alloy. The method employed is based on the solution of the differential equations corresponding to this high-temperature corrosion phenomenon with the help of the finite-difference technique. To demonstrate the possibilities of the calculation method, various cases were treated which differ especially regarding the thermodynamic stability of the internal precipitates. The calculated concentration profiles and penetration depths obtained are reported and discussed. In the case of internal precipitates of high stability, the results are compared with the approaches derived in the literature for internal oxidation and are used to determine the parameter ranges in which the analytical equations apply. The results obtained for the case of a less-stable compound is used to illustrate the influence of the solubility product on the concentration profiles.

H.G. Sockel

Papers

High temperature corrosion of the nickel-based alloy Inconel 617 in helium containing small amounts of impurities

Mechanisms of high-temperature corrosion in helium containing small amounts of impurities. II: Corrosion of the nickel-base alloy Inconel 617

Investigations of slow exchange processes at metal and oxide surfaces and interfaces using secondary ion mass spectrometry

Influence of the solubility product on the concentration profiles of internal oxidation

Penetration of foreign elements connected with internal precipitation: a computer-based description and first experimental verification☆