A study of the mechanism of internal sulfidation-internal oxidation during hot corrosion of Ni-base alloys

The hot corrosion of wrought Ni-16Cr-2Nb was studied at temperatures of 910–1020°C using an autoradiography technique. The autoradiographic pictures of the deposits of Na₂SO₄ enriched with³⁵Sshow that sulfur diffuses along the grain boundaries of the alloy preferentially, where it forms metallic sulfides. The sulfides are then oxidized; sulfur atoms are released, forming new sulfides at the grain boundaries or dissolving in grains and migrating inward by volume diffusion. These results provide new evidence for the sulfidationoxidation mechanism of hot corrosion.     

On the oxidation mechanism of alumina formers

The oxidation mechanism of the intermetallic compound -NiAl and Fe-23Cr-5Al-0.2Zr alloys modified by reactive-element additions was studied in the temperature range 1273–1473 K by means of the two-stage oxidation method using the oxygen isotope¹⁸O as a tracer. It was found that outward cation diffusion predominates in the scale on -NiAl. The contribution of the inward oxygen transport increased with increased reaction temperature and oxidation time. At 1473 K, implanted yttrium suppressed inward oxygen diffusion for oxidation times less than 1 hr. In the case of Fe-Cr-Al-Zr alloys the counter-current transport of reactants was observed on the non-implanted materials. Implanted yttrium was found to alter the transport mechanism. This effect appeared to be directly related to that of yttrium on the scale morphology and microstructure. Yttrium promoted the formation of cracks which provided an additional surface for the reaction.On leave from Academy of Mining and Metallurgy, Krakow.     

Anomalous behavior during internal oxidation and nitridation

Internal oxidation, wherein species on the surface of an alloy diffuse into the material and react with a less-noble solute element, is a phenomenon that has produced many unexpected research results over the years. First studied extensively in the 1950s, internal oxidation has become better understood as work progresses. This article summarizes some recent anomalies in the work aimed at understanding the phenomenon.     

The formation of solid solution oxides during internal oxidation

The diffusion processes occurring when binary alloys react with oxygen to form an oxide that contains both alloy components in solid solution, either exclusively as internal oxide or in combination with a surface scale, have been analyzed and compared with experimental results for Fe-Mn and Ni-Co alloys. The experimental results available for the Fe-Mn system were obtained under conditions of exclusive internal oxidation, and good agreement was obtained between calculated and experimental results. In the Ni-Co system, a surface scale and a zone of internal oxidation develop. Agreement between calculated and experimental depths of internal penetration is acceptable if the diffusivity of oxygen in the alloy is 3.8×10^–6 cm²/sec at 1100°C. Agreement between calculated and experimental concentration profiles is not very good.List of Principal Symbols B alloy component with higher affinity for oxygen - BO more stable scale component - a _O activity of oxygen - D _O diffusivity of oxygen in the alloy - D _O ^eff effective diffusion coefficient of oxygen in alloy - f volume or mass fraction of internal oxide - f _max maximum volume or mass fraction of internal oxide - G _AO , G _BO free energies of formation of oxides AO and BO, respectively - N _B mole fraction of component B in the alloy - N _B ^O bulk mole fraction of component B in the alloy - N _BO mole fraction of oxide BO in oxide phase - N _O atomic fraction of oxygen dissolved in alloy - N _O ^I ,N _O ^II atomic fraction of oxygen dissolved in alloy at the internal oxide-surface scale and alloy-internal oxide interfaces, respectively - R gas constant - r ratio of number of moles of precipitated oxide to total number of moles of metallic constituents in the alloy - T temperature - t time - X ₁,X ₂ positions of internal oxide-surface scale and internal oxide-alloy interfaces, respectively - x position coordinate - defined as [–1/RTG _BO ] - ₁, ₂ dimensionless rate constants describing rate of displacement of the internal oxide-surface scale and internal oxide/alloy interfaces, respectively - _O Henry's law activity coefficient for oxygen dissolved in alloy - defined as [–1/RT(G _BO –G _AO )]     

A critique of internal oxidation in alloys during the post-wagner era

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.     

The transition from internal oxidation to continuous-film formation during oxidation of dilute Ni-Si alloys

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 (w)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.     

Evidence for the formation of substoichiometric species during internal oxidation of Ag-Mg alloys

Lattice parameters of Ag-Mg (0.5 and 1 at.%) were measured. A comparison between these results and data previously obtained by gravimetry shows that (a) a high dilatation is due mainly to excess oxygen by comparison with the amount necessary to form stoichiometric MgO oxide, and (b) the earliest stage of Mg oxidation occurs in a non-expanded layer beneath the expanded one, with the formation of substoichiometric species (O/Mg < 1). The formation of substoichiometric species is explained by taking account of strain fields close to O and Mg atoms in the silver lattice. Indeed, the strong deformation introduced by oxygen in interstitial position delays the addition of oxygen on the MgO^* and Mg₂O^* initial species and favors the formation of substoichiometric species.     

On the oxidation mechanism of Ni–Pt alloys at high temperatures

The oxidation mechanism of Ni–Pt alloys has been studied as a function of alloy composition, oxygen pressure and temperature. It has been found that the oxidation rate of all the alloys follows the parabolic rate law, being thus diffusion controlled. In agreement with Wagner’s theory, the slowest step of the overall oxidation rate of alloys with higher nickel content (?40 at.%) is determined by the outward diffusion of nickel cations in the growing NiO scale. On the other hand, the oxidation rate of alloys with a lower nickel content (<40 at.%) is governed by the solid state diffusion in the metallic phase.     

On the mechanism of chemical diffusion in solids and its application to oxidation

The participation of crystal lattice atoms in an elementary act of chemical diffusion may have a collective nature. This involves a consecutive elastic translation of activated complexes, the distorted regions possessing more mobility than an equivalent quantity of point defects. Thermodynamic analysis shows that participation of an activated complex occurs during chemical diffusion. This model is applied to the kinetics of the oxidation of metals. The apparent activation energy of the process is considerably lower in this case than in the case of self-diffusion in the lattice. It is shown that there exists a limiting thickness of the scale above which further growth occurs at the cost of point defect motion with a lower rate.     

Anodic oxidation of Al–Ag alloys

The anodic oxidation of solution-treated and quenched Al–Ag alloys containing 0.3, 0.6, 0.9 and 1.2 at.%Ag, is examined in ammonium pentaborate electrolyte, which leads to growth of barrier-type anodic films. Enrichments of silver, 3.1×10¹⁵ Ag atoms cm⁻², are developed in the alloys immediately beneath the amorphous alumina films, with the level of enrichment not depending significantly upon the composition of the bulk alloy. The enrichment is relatively low due to clustering of silver atoms in the bulk alloy, which reduces the concentration of silver that is available to enrich from solid solution. Silver species are incorporated into the anodic film, where they migrate outward faster than Al³⁺ ions.     

The internal oxidation of Nb-Hf alloys

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.     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

Some observations on the growth mechanism of haematite during the oxidation of iron at 823 K

A preliminary investigation of the growth mechanism of haematite at 823 K has been carried out using an ¹⁸O tracer technique. The results are consistent with the primary diffusing species in the haematite being iron and with fast oxygen transport taking place through cracks in the haematite scale—a mechanism similar to that previously found to occur in the oxidation of Cr and some Fe-Cr alloys at 1223 K. A considerable delay occurred in the nucleation of the haematite.Further work will be required to establish the extent to which these conclusions can be applied to the oxidation of iron under experimental conditions other than those used in this experiment.     

Influence of prior internal oxidation on the oxidation of dilute Co-Cr alloys in oxygen

The influence of an initial preinternal oxidation treatment in Co/CoO on the subsequent oxidation behavior of a series of dilute Co-Cr alloys (containing 0–1.5 wt. % Cr) in 10⁵ and 10³ Pa oxygen at 1473–1623 Khas been investigated. Particular emphasis has been placed on determining the solubility and mobility of Cr³⁺ ions in CoO. Use has been made of subsequent annealing in argon .     

Generation mechanism of microdischarges during plasma electrolytic oxidation of Al in aqueous solutions

The generation mechanism of microdischarges during plasma electrolytic oxidation (PEO) of aluminium was investigated at constant current densities in aqueous alkaline solutions. Microdischarges were generated only in weakly alkaline solutions under high applied voltages. The breakdown voltage of the anodic oxide film was not dependent on the applied anodic current density while it was strongly dependent on the OH⁻ concentration in the solution. The size and density of microdischarges became larger and lower, respectively, with increasing PEO treatment time. The mechanism of microdischarge generation could be explained based on the movement of ions in the oxide film and resistive property of the oxide film.     

Accelerated oxidation,internal oxidation,intergranular oxidation,and pesting of intermetallic compounds

The compounds MoSi₂, NiAl, and NbAl₃ all form protective oxide films, particularly at high temperatures where the diffusion of Si or Al is more rapid and, for the case of MoSi₂, the transient oxides evaporate. However, at low temperatures, all three can undergo accelerated oxidation. The mechanisms of degradation are unique to the particular compound although there are some similarities. The accelerated oxidation of MoSi₂ occurs at temperatures below 600°C by the rapid growth of Mo oxides which prevent development of a continuous silica film. Internal or intergranular oxidation does not occur. If the specimen contains cracks or pores, the rapid oxidation in these defects leads to fracture of the specimen or pesting. The accelerated oxidation of NiAl occurs at temperatures below 1000°C at reduced oxygen partial pressures as the result of internal oxidation and rapid intergranular oxidation. The intergranular oxidation does not lead to pesting. Special circumstances are required for the accelerated oxidation of NiAl as it does not appear to occur in flowing gases unless sulfur is present. The accelerated oxidation of NbAl₃ also occurs at temperatures less than 1000°C and at reduced oxygen partial pressures and takes the form of intergranular oxidation of Al. The intergranular oxidation results in pesting of NbAl₃. The phenomena of accelerated oxidation, internal oxidation, intergranular oxidation, and pesting have not been investigated in detail for most other intermetallic compounds but one or more of these phenomena seems to afflict most aluminides and silicides.     

On the mechanism of the oxidation of NiCrAl-base alloys in air and air containing sulphur dioxide

Oxidation studies have been carried out on NiCrAl-base alloys in air and air containing 1% sulphur dioxide at 1000°C. The alloys used were in the cast, forged and single-crystalline form. There was considerable difference in the oxidation behavior of the as-cast alloy and that of forged alloy, when the environment contained sulphur dioxide. In the absence of the latter, the behavior of the two alloys was not very different. The oxidation behavior of the single crystals was also very similar to that of the cast and forged alloys, when the atmosphere was oxidizing. A few single-crystalline alloys, however, underwent catastrophic oxidation when about 1% sulphur dioxide gas was present, in a manner similar to that of the cast alloy. Oxidation tests at 1000°C as well as detailed scale analysis indicated that the different microstructure, especially large difference in grain size was probably the main reason for the large difference in their oxidation behavior.     

A mathematical model for internal oxidation

A mathematical model for internal oxidation kinetics was developed using numerical methods (finite difference) and computer techniques. The flexibility of the model permitted analysis of semi-infinite and finite situations with planar, cylindrical, and spherical geometries for systems with various amounts of local solute enrichment. Graphical results are presented for subscale thickness as a function of time and local enrichment as a function of position in the subscale. The model is also applied to internal oxidation with a discontinuous change in surface oxygen concentration; a graphical solution encompassing a wide range of possible experimental conditions is presented. The use of the model in analyzing nonisothermal internal oxidation problems is demonstrated.