Penetration of sulfur through preformed protective oxide scales

Thermodynamic assessment of sulfur penetration through otherwise protective scales such as Cr₂O₃, Al₂O₃ has been carried out for Co-Cr- and Co-Cr-Al-base alloys. Limiting conditions for sulfide formation following gas molecular transport and solution-diffusion transport have been established and the results partially confirmed by experiments carried out on Co-10Cr, Co-25Cr, and Co-10Cr-5Al alloys in sulfurous atmospheres. The results show that molecular transport of sulfurous gas species through the growing oxide scale definitely occurs. It was not possible to confirm or disprove the solutiondiffusion mechanism.     

Breakdown of chromium oxide scales in sulfur-containing environments at elevated temperatures

Structural Fe-Cr-Ni alloys may be rapidly degraded in oxygen-sulfur mixed-gas environments at elevated temperatures unless protective oxide scales can be formed and maintained. The breakaway corrosion process was examined in model alloys of Fe-25wt.% Cr-20wt.% Ni with and without Nb and Zr additions. Oxide scales were preformed in S-free environments and subsequently exposed to oxygen-sulfur mixed-gas atmospheres. Preformed scales were found to delay the onset of breakaway corrosion. The beneficial effects of refractory metal additions were achieved via formation of a barrier layer at the Cr₂O₃ alloy interface.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

A review of the role of short-circuit diffusion in the oxidation of nickel,chromium, and nickel-chromium alloys

The evidence for short-circuit diffusion during the oxidation of nickel, chromium, and nickel-chromium alloys is reviewed. On the basis of experimental evidence for the Ni/NiO and Cr/Cr₂O₃ systems, inferences are made about the role of short-circuit diffusion through the oxide scales forming on the binary nickel-chromium alloys. The review highlights the importance of scale microstructure in governing oxidation rates.     

The Transport Properties and Defect Structure of the Oxide (Fe,Cr)2O3 formed on Fe?Cr alloys

The protective properties of a scale forming on a metal are intimately relat-ed to the way in which matter is transported through the oxide. This report re-views the oxidation behaviour the bi-nary iron-chromium alloys which form the basis of many technologically important materials and attempts to relate it to the defect structure and transport properties of (Fe, Cr)₂O₃ oxides. Existing information on this system is mainly restricted to the end members, Cr₂O₃ and Fe₂O₃. The former is a cation deficient, p-type oxide and cation transport dominates. Fe₂O₃ is n-type, probably containing iron interstitials and oxygen vacancies, with comparable transport rates for both species, but the defect structure and properties of both oxides have been only partly characterized. New measurements by thermogravimetric, electrical and high temperature deformation techniques of these properties are briefly described. Electrical measurements support the hypothesis that certain solid solutions of Cr₂O₃-Fe₂O₃have lower defect concentrations and ionic transport rates than the pure oxides. There is evidence that scales of about the required composition form on the binary alloy containing about 20% Cr and are responsible for its low oxidation rate. Possible extensions of the theory to the design of highly stoichiometric scales for the protection of tornary and more complex alloys are briefly mentioned.     

Sulfidation and sulfidation-oxidation of nickel- and cobalt-base alloys containing dispersions of stable oxides

The presence of a fine dispersion of a stable oxide is known to have a beneficial effect on the oxidation resistance of nickel- and cobalt-base heat-resisting alloys. This paper presents some preliminary experimental results relating to the hot-corrosion resistance of these alloys. Alloys forming Cr ₂O₃ scales appear to be resistant to oxidation when coated with sodium sulfate, whereas an alloy normally forming an Al ₂O₃ scale suffers accelerated attack. During sulfidation some of the alloys suffer an accelerated degradation, with sulfur penetrating rapidly along what appear to be grain boundaries. The same effect is noted in sulfidation-oxidation experiments, when the Cr ₂O₃-forming alloys suffer accelerated oxidation, the effect of the dispersoid being apparently removed. An Al ₂O₃-forming alloy resists this form of attack well. The sodium sulfate-coated test is probably a good guide to the behavior under weakly corroding conditions, whereas the sulfidation-oxidation test may give a better indication of the behavior under highly aggressive conditions.     

Effect of the possible fcc stabilizers Mn,Fe, and Ni on the high-temperature oxidation of Co-Cr alloys

Earlier studies have suggested that manganese might be attractive as an alternative fcc stabilizer to nickel in cobalt base alloys because of its beneficial effect on the sulfidation resistance. Because it has relatively high oxygen affinity, it seemed possible that it might modify the ability of the alloys to form a protective Cr ₂O₃ scale. Thus, a range of alloys having compositions close to the critical Co-20 Cr value have been investigated in the temperature range 900–1150° C for times up to 240 hr in air and oxygen. Similar, but less extensive studies, have been conducted on alloys containing iron or nickel. None of these elements has a significant effect on the oxidation behavior, or on the ability of the alloys to develop a protective Cr ₂O₃ layer. However, manganese has a significant adverse effect on the scale spallation, apparently because of its increasing the brittleness of the oxide, particularly the CoO.     

Einflüsse von HCl und N2 auf die Hochtemperaturkorrosion in Atmosphären mit niedrigem Sauerstoffdruck

Influence of HCl and N₂ on high temperature corrosion in atmospheres with low oxygen pressures The effects of HCl and N₂ on the high temperature corrosion in atmospheres with low oxygen pressures were investigated. Investigations of the alloy Fe-20Cr in He-H₂-H₂-O-HCl atmospheres at 1173 K under conditions, where no condensed chlorides could be formed, show the simultaneous growth of Cr₂O₃ and evaporation of CrCl₂. The kinetics of the reactions can be described by growth of Cr₂O₃ according to the parabolic rate law and transport of CrCl₂ in the gas flow following a linear rate law. During heating of the samples in He-H₂-H₂O-HCl condensed chlorides are formed which decrease the adherence of the oxide layers and cause oxide spallation during cooling. This can be avoided either by preoxidation in H₂-H₂O or heating in dry hydrogen. The porosity of Cr₂O₃ layers is not enhanced when HCl is added to the gas atmosphere. The oxidation and evaporation is enhanced in N₂-H₂-H₂O-HCl atmospheres compared to He-H₂-H₂O-HCl atmospheres. The Cr₂O₃ scales are coarse grained and poros. The enhanced porosity is caused by the nucleation and growth of chromium nitrides in the transient state of oxidation.     

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO₂, mixed rare earth oxides, Co₃O₄, and Cr₂O₃ powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO₂ and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO₂ powder. The presence of CeO₂ also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co₃O₄ or Cr₂O₃ powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr₂O₃.This paper is based in part on the Ph.D. thesis of G. M. Ecer (1975) and in part on the M.S. thesis of R. B. Singh (1977).     

Transition from oxidation to sulfidation of Fe-Cr alloys in gases with low oxygen and high sulfur pressures

Fe-Cr alloys with 17–30% Cr show in H₂-H₂O-H₂S mixtures at 1273 and 1073 K a transition from protective oxide scale formation to rapid sulfidation. The critical oxygen pressure to stabilize the oxide formation increases with increasing sulfur pressure of the gas and decreasing Cr content of the alloy. Cr₂O₃ with traces of Fe₂O₃ is formed under these conditions. Below the critical oxygen pressure, a primarily formed Cr₂O₃ film becomes overgrown by (Fe, Cr)S. The kinetic boundary of oxidation-sulfidation, which lies in the stability field (Fe, Cr)S + spinel Fe_1+xCr_2–xO₄ of the Fe-Cr-O-S phase diagram, is explained with the help of the Fe-Cr-O-S phase diagram and the assumption that Fe diffuses faster through the (Cr, Fe)₂O₃ solid solution than does Cr.     

Oxidation of high-chromium Ni-Cr alloys

The oxidation of binary Ni-Cr alloys containing 44 and 50 wt. % Cr has been studied over a range of oxygen partial pressures at temperatures between 800 and 1100°C. The effects of cold work, surface preparation, and distribution of the Cr-rich second phase have been studied. The oxidation behavior is complex and cannot be described by a single model. The oxide grows by short-circuit diffusion as well as bulk transport through Cr ₂ O ₃ scales. The scale-growth mechanism includes extensive metal-oxide separation requiring Cr vapor transport to the scale, compressive stresses within the oxide which result in scale bulging and cracking, and the formation of a second oxide layer which results in voids being incorporated into the scale. Any factor which reduces the oxide grain size, such as cold work, finer distribution of the Cr-rich phase or reduced oxygen pressure, results in an increased oxidation rate of binary alloys because of an increased number of grain-boundary short-circuit diffusion paths.This work is based on a portion of a thesis by G. M. Ecer submitted to the University of Pittsburgh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgical and Materials Engineering.Formerly graduate student. Department of Metallurgical and Materials Engineering. University of Pittsburgh.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

The oxidation of Ni-70 wt.%Cr in oxygen between 1073 and 1473°K

Oxidation of the relatively simple, two-phase alloy Ni-70 wt.%Cr in oxygen between 1073 and 1473°K results in the formation of a Cr₂O₃ scale containing less than O.5 wt.% Ni in solid solution. The oxidation kinetics are irreproducible for an initial period, which is brief at 1073 and 1273°K but much more pronounced at 1473°K, both in duration and degree. This behavior is associated with the failure of the protective Cr₂O₃ scale. However, after longer periods a compact layer of Cr₂O₃ becomes established under isothermal conditions and results in a change to more reproducible kinetics, especially at 1073 and 1273°K. Oxidation causes chromium depletion and the formation of a single-phase zone which separates the scale and the two-phase bulk alloy. The depth of Cr₂O₃ internal oxide coincides with this zone. The oxidation behavior is compared with that of more Ni-rich, single-phase Ni-Cr alloys, with particular reference to the effects of the constitution of the underlying alloy and the integrity of the protective oxide.     

The effect of cerium on the oxidation of Ni-50Cr alloys

The oxidation behavior of Ni-50Cr alloys with minor cerium additions was studied between 800 and 1100° Cin oxygen, air, and oxygen at reduced partial pressures. Optical and scanning electron metallography, X-ray diffraction, and electron-probe microanalysis techniques were used to characterize the changes in scale and substrate morphology and to identify the oxidation products. Platinum markers were used to determine the direction of ionic transport. The effects of cold work, initial alloy phase distribution, and cyclic oxidation were also studied. The Cr ₂ O ₃ scales on the cerium-containing alloys grew while being largely separated from the metal substrate. Oxidation rate, oxide grain growth, and the tendency of scales to spall on cooling were reduced substantially with increasing alloy cerium content. The first two effects are suggested to result from the interaction of cerium ions and cerium oxide particles with oxide grain boundaries in reducing grain-boundary diffusion and oxide-boundary mobility. The third is suggested to result from the thinner, finer-grained scales formed on the Ce-containing alloys.This work is based on a portion of a thesis by G. M. Ecer submitted to the University of Pittsburgh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgical and Materials Engineering.Formerly graduate student. Department of Metallurgical and Materials Engineering, University of Pittsburgh.     

Distribution and characterization of high temperature air corrosion products on iron-chromium alloys by Raman microscopy

Raman microscopy has been used to study the nature and distribution of corrosion products formed on iron and iron-chromium alloys in air at high temperatures. Fe and Fe-Cr alloys containing 2, 5, 14, and 18% Cr were oxidized at 400, 600, and 850°C for 2 hr, in addition samples of each alloy were oxidized for 24 hr at 400°C to obtain thicker scales at this temperature. The corroded samples showed varying distributions of the oxides Fe₂O₃, Fe₃O₄, Cr₂O₃, and FeCr₂O₄. Fe₂O₃ and Fe₃O₄ were formed exclusively on the pure iron and the 2 and 5% chromium alloys at all temperatures and on the 14% chromium alloy at 400°C. The 14 and 18% Cr alloys formed scales containing Cr₂O₃ and FeCr₂O₄ at the higher temperatures (600 and 850°C). Examples of small regions of Fe₂O₃ being formed within Cr₂O₃-FeCr₂O₄ scales are suggested as possible indications of breakaway corrosion initiation sites.     

Factors Affecting Chromium Carbide Precipitate Dissolution During Alloy Oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr₂O₃ scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr₂O₃ scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr₂O₃scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr₂O₃ growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr₂O₃ scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.     

The Effect of Pre-Oxidation Treatment on the High-Temperature Oxidation of Co–Re–Cr Model Alloys at Laboratory Air

The aim of a pre-oxidation treatment at low oxygen partial pressure is to promote the formation of a Cr₂O₃ (or Al₂O₃) scale in such a way that the oxide layer can reliably prevent the contact of oxygen with the metallic substrate also at high oxygen partial pressure. In this study, the pre-oxidation treatment was applied to the two alloys Co–17Re–23Cr and Co–17Re–30Cr (at.%). Pre-oxidation of the Co–Re–xCr alloys was found to be non-protective for the metallic substrate at high oxygen partial pressure despite the formation of a Cr₂O₃ layer. Different kinds of Cr₂O₃ scale damage were observed depending on the Cr concentration. The Cr₂O₃ scale formed on the alloy Co–17Re–23Cr loses its protective properties as a result of cation transport by lattice and grain boundary diffusion, while the Cr₂O₃ scale formed on the alloy Co–17Re–30Cr degraded as a consequence of scale cracking. In addition, the effect of alloying with Si was investigated and found to be promising.     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Development,growth, and adhesion of Al2O3 on platinum-aluminum alloys

The development, growth, and adhesion of -Al₂O₃ scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the -Al₂O₃ scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the -Al₂O₃ scales. The -Al₂O₃ scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the -Al₂O₃ may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.This work was supported by the U.S. Army Research Office, Durham, under Contract Number DAHCO 4 73 C 0021.     

Microstructure and analysis of oxide scales formed on Cr-Si-Ni compacts in air and H2O-containing atmosphere

The purpose of this study was to characterize the oxide scales formed on various Cr-Si-Ni compacts at 1273 K in air and H₂O-containing atmosphere by TEM. It was found that CrSi₂-(5-20)mass%Ni compacts form double layer scales, consisting of an outer Cr₂O₃ layer and an inner SiO₂ layer. The oxide scale changed from SiO₂- to Cr₂O₃-based scale with an increase in the Ni concentration. However, it was observed that the oxide scale formed in H₂O-containing atmospheres showed local SiO₂ growth into the substrate. This result suggests that the inward oxidant diffusion promotes the local growth of SiO₂ in the H₂O-containing atmospheres.