Bibliothek

Notizen: 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.

Notizen: Abstract Fe-30Nb-Al alloys containing up to 9.1 wt.% Al were sulfidized at 0.01 atm sulfur vapor over the temperature range of 600–900°C. The sulfidation kinetics followed the parabolic rate law for all the alloys at all temperatures. The parabolic rate constants decreased with increasing Al content. Extremely slow sulfidation rates, even slower than that of pure Nb at low temperatures, were observed for alloys containing high Al (〉4.8 wt.%). Duplex sulfide scales formed on alloys containing small amounts of Al. The outer layers were compact FeS, while the inner layers were a double sulfide, Fe xNb2S4,containing partially sulfidized intermetallic islands. Very thin scales formed on the alloys containing high Al, but the nature of the scales is unknown. The intercalation of Al into the Nb-sulfides and the associated charge transfer induced a blockage of the transport of iron through the sulfide as well as a greater incorporation of Nb into the scale.

Notizen: 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.

Notizen: Abstract A commercial superalloy, MP35N, which was modified by adding various amounts of Al up to 10 wt.%, has been studied over the temperature range of 600–900°C in 0.01 atm sulfur vapor. The Al-modified alloys contained a second phase whose amount increased with increasing Al. The sulfidation rate followed the parabolic rate law and decreased with increasing Al content. An activation energy of 46 kcal/mol was obtained for MP35N, MP35N-5Al, and MP35N-7Al, while a value of 56 kcal/mol was found for MP35N-10Al at 700–800°C. Triplex sulfide scales formed on MP35N, consisting of an outer layer of solid solution (Co, Ni) 3S4,an intermediate layer of Cr 2S3,and a complex, heterophasic inner layer of mostly MoS 2.The Al-modified alloys also formed an outer scale of (Co, Ni) 3S4,and an inner layer containing a mixture of CoCr 2S4 and Al 2S3.An internal-sulfidation zone containing an Al-rich sulfide was formed in the substrate. Only a moderate beneficial effect of Al additions to MP35N was observed at 600°C. The observation of the location of platinum markers suggests that cobalt, nickel, chromium, and aluminum diffuse outward to form the major part of scale, and sulfur diffuses inward to produce an internal-sulfidation zone. Transport processes in the internal-sulfidation zone governed the sulfidation rate of the alloys.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: Abstract The sulfidation behavior of Ni-Mo alloys containing up to 40 wt.% Mo was studied at $$P_{s_2 } $$ =0.01 atm. over the temperature range of 550–800°C. The alloys included two solid solutions (Ni-10Mo and Ni-20Mo), the single-phase intermetallic compound Ni4Mo(Ni-29Mo), and two alloys which were two-phase, Ni-30Mo and Ni-40Mo (Ni4Mo+Ni3Mo). The sulfidation of all alloys followed the parabolic rate law. The rate of sulfidation decreased with increasing amounts of Mo. Activation energies for sulfidation gave values of 39.1±1.0 kcal/mol. The sulfide scales were bilayered, consisting of an outer layer nickel sulfide (NiS1+x and Ni3S2) and an inner, complex layer of MoS2 plus intermetallic particles. The rate-controlling step of the sulfidation for the alloys was inward sulfur diffusion and/or outward nickel diffusion through the inner MoS2 layer. Neither selective sulfidation nor internal sulfidation were observed. No significant difference in the sulfidation kinetics, sulfide structure, and scale constitution could be noted between single-phase alloys and two-phase alloys. The location of the markers was the interface between the inner and outer layers, indicating that the inner layer formed by inward diffusion of sulfur, and the outer layer grew by outward nickel diffusion. The inability to form a continuous protective molybdenum sulfide layer is discussed in terms of the structure of MoS2 and changes caused by intercalation of Ni into the layered crystal structure. The decrease in sulfidation rate with increasing Mo was attributed to increasing amounts of the intermetallic compound. The increasing volume fraction of particles decreased the available diffusion area in the inner layer and provided a “blocking” effect.