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Notes: Abstract The oxidation of Ni-2.05Si and Ni-4.45Si was studied in oxygen over the range of 600°–1000°C for 18 hr. The oxidation kinetics did not follow a parabolic rate law, bur rather a power law of the form (Δw)n=kt was followed. The value of n ranged from 2.7 to 4.9 for Ni-2.05Si and from 3.0 to 6.4 for Ni-4.45Si. The low-silicon alloy exhibited extensive internal oxidation, whereas the higher-silicon alloy did not internally oxidize. In general, NiO containing little or no silicon formed as an exterior layer on both alloys. The internal oxidation zone in Ni-2.05Si was highly irregular in thickness, and in some areas there was no internal oxidation. The higher-silicon alloy formed a continuous layer of a silicon-rich oxide. X-ray diffraction did not detect silica (amorphous), and no evidence of Ni2SiO4 was observed, although EDAX analysis suggests that small amounts of the silicate might have formed. Theaverage thickness of the internal oxidation zone was found to agree well with calculated values based on oxygen solubility and diffusivity data. No enrichment of silicon occurred in the internal oxidation zone. Calculated values, 0.033 and 0.038 (depending on the model used), of the mole fraction of silicon required for the transition from internal oxidation to continuous silica film formation agreed well with experimental data obtained in both this study and with others reported in the literature.

Notes: Abstract Ag-3a/oMg was oxidized in air over the range of 400–900°C. Internal-oxide bands of MgO formed approximately parallel to the surface, the first band appearing at some finite, but irregular depth below the surface. The region between the surface and the first band appeared to be free of precipitates, but TEM showed that very small clusters, about 50 Å in diameter, formed in the “PFZ,” causing significant hardness (greater than 300 VHN). The clusters contain more oxygen than that corresponding to stoichiometric MgO. The hardness between oxide bands was also high, but not as high as in the “PFZ.” The kinetics of thickening of the internal-band region followed the parabolic rate law between 400 and 700° C, with departures from the parabolic law occurring at higher temperatures. The activation energy for the parabolic rate constants was 19.4 Kcal/mol, a value less than the total for oxygen diffusion and oxygen dissolution. The reaction front was planar and parallel to the surface prior to band formation at temperatures of 400–600° C. Nucleation of the first band resulted in nonplanar and nonparallel oxide. Little or no correlation existed between grain boundaries and oxide formation. Nodules of virtually pure silver formed on the surface initially at grain boundaries and subsequently within grains. Nodule formation is attributed to stress-enhanced (resulting from strains associated with precipitation) diffusion of silver to the surface via dislocation pipes. Internal-band formation is discussed in terms of prior data in the literature and various models. It is thought that stress effects (induced by precipitation), nucleation, and clustering of oxygen with Mg play significant roles in causing internal-band formation.

Notes: Abstract The corrosion behavior of eight Fe-Nb-Al ternary alloys was studied over the temperature range 700–980°C in H2/H2O/H2S atmospheres. The corrosion kinetics followed the parabolic rate law for all alloys at all temperatures. The corrosion rates were reduced with increasing Nb content for Fe-x Nb -3Al alloys, the most pronounced reduction occurred as the Nb content increased from 30 to 40 wt.%. The corrosion rate of Fe-30Nb decreased by six orders of magnitude at 700°C and by five orders of magnitude at 800°C or above by the addition of 10 wt.% aluminum. The scales formed on low-Al alloys (≤3 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with Al dissolved near the outer-/inner-layer interface) and an inner complex layer of FexNb2S4(FeNb2S4 or FeNb3S6), FeS, Nb3S4 (only detected for Nb contents of 30 wt.% or higher) and uncorroded Fe2Nb. No oxides were detected on the low-Al alloys after corrosion at any temperature. Platinum markers were found to be located at the interface between the inner and outer scales for the low-Al alloys, suggesting that the outer scale grew by the outward transport of cations (Fe and Al) and the inner scale grew by the inward transport of sulfur. The scales formed on high-Al alloys (≥5 wt.% Al) were complex, consisting primarily of Nb3S4, Al2O3 and (Fe, Al)xNb2S4, and minor amounts of (Fe, Al)S and uncorroded intermetallics (FeAl and Fe2Nb). The formation of Nb3S4 and Al2O3 blocked the transport of iron through the inner scale, resulting in the significant reduction of the corrosion rates.

Notes: Abstract Co−15 at.% Nb alloys containing up to 15 at.% Al were corroded in gaseous H2−H2O−H2S mixtures over the temperature range of 600–900°C. The corrosion kinetics followed the parabolic rate law at all temperatures. Corrosion resistance improved with increasing Al content except at 900°C. Duplex scales formed on alloys consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. A small amount of Al2O3 was found only on Co−15Nb−15Al. Contrary to what formed in Co−Nb binary alloys, neither NbS2 nor NbO2 were found in the inner layer of all alloys, but Nb3S4 did form. The absence of NbS2 and NbO2 is due to the formation of stable Al2O3 and Al2S3 that effectively blocked the inward diffusion of oxygen and sulfur, respectively, and to the reduction of activity of Nb by Al additions in the alloys. Intercalation of ions in the empty hexagonal channels of Nb3S4 is associated with the blockage of the transport of cobalt. An unknown phase (possibly Al0.5NbS2) was detected. Alloys corroded at 900°C were abnormally fast and formed a scale containing CoNb3S6 and Co. Pt markers were found at the interface between the inner and outer layers.

Notes: Abstract A calculation of the parabolic rate law for internal oxidation in binary alloys expressed in terms of weight gain shows that its dependence on the concentration of the most-reactive component is different from that predicted by the classical Wagner treatment for the rate constant expressed in terms of thickness of the internal oxidation zone. It is shown that the ratio between the two rate constants for a given system is a very sensitive function of the concentration of the reactive element in the alloy.

Notes: Abstract 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 Widmanstätten 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.

Notes: Abstract The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr2O3 and an inner layer of either (or possibly both) SiO2 and Fe2SiO4. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr2O3 to the gaseous phase CrO3. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr2O3, which is a much more protective scale.

Notes: Abstract The internal oxidation of some binary Nb-Hf and several commercial Nb alloys containing Hf was studied at 1568 and 1755°C in oxygen pressures ranging from 5×10 −5 to 1×10−3 torr.The reaction kinetics were linear, suggesting that diffusion of oxygen in the substrate was not rate-controlling. The dependence of the reaction rate on oxygen pressure was linear also. Well-defined reaction fronts were observed at higher pressures and the lower temperature, whereas ill-defined fronts occurred at lower pressures and at the higher temperature. The solubility product was much higher than normally encountered in Wagnerian-type behavior and gave rise to varying solute content across the internal-reaction zone. The solute-concentration profiles (EPMA/WDS) of the matrix between particles exhibited a sigmoidal shape for well-defined reaction fronts, whereas the profiles showed a gradual decrease in solute with distance near the front for ill-defined fronts, dropping fairly abruptly at the metal/gas interface. The solute concentration never reached zero at the surface for any condition studied. In contrast to classical, Wagnerian behavior, solute continued to precipitate out after the reaction zone had passed, leading to a variation in the mole fraction of oxide in the zone. SEM/EDXA and XRD showed that precipitation occurred by the formation of “precursors” (Hf-rich regions surrounded by Hf-depleted regions), followed by precipitation of tetragonalHfO2,which in some cases transformed to monoclinicHfO2 and subsequently coarsened. The precipitate morphology varied with solute concentration, temperature, oxygen pressure, and location within the reaction zone. High temperature and high oxygen pressure favored a Widmanstätten structure, whereas low temperature and low oxygen pressure favored a spheroidal precipitate structure. Widmanstätten plates were observed to “spheroidize” at longer times, suggesting that the interfacial energy between particles and matrix was very high. The presence of a small amount of Y (0.11 w/o in C129) always resulted in spheroidal particles. It appears that Y markedly increased the particle/matrix interfacial energy. Microhardness profiles showed decreasing values with distance into the sample for some conditions and alloys but increasing values in other cases. Hardness increases in the substrate in advance of the interface showed that oxygen activity did not reach zero at the reaction front, once again contrary to classical behavior but consistent with high solubility products of the oxide. Results are analyzed in terms of oxygen-trapping by reactive solutes as noted in the literature for both lattice-parameter measurements and oxygen diffusivity studies.

Notes: Abstract The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H 2 -H 2 O-H 2 S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al 2 O 3 formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al 2 O 3 in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al 2 O 3.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al 0.55 Mo 2 S 4,and Al 2 O 3 in the inner layer.