The Criteria for the Transitions Between the Various Oxidation Modes of Binary Solid-Solution Alloys Forming Immiscible Oxides at High Oxidant Pressures

The possible high-temperature corrosion modes ofbinary solid-solution alloys forming two immisciblecompounds by a single oxidant include (1) the exclusivegrowth of external scales of the most-noble component, which may or may not be associated with theinternal oxidation of the most-reactive component, (2)the formation of composite external scales containing amixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound asan external scale. The conditions for the stability ofeach scale structure depend on a number of thermodynamicand kinetics parameters, whose effects are examined quantitatively in this paper. Theconditions for the stability of the various structuresand the criteria for the transitions among them are alsoexamined. The maximum number of possible scale structures is four, but it can reduce to threeand, in some cases, only to two. In particular, theinternal oxidation of the most-reactive component maynot occur if the stabilities of the two oxides are not sufficiently different from eachother.     

The Influence of Erosion-Induced Roughness on the Oxidation Kinetics of Ni and Ni-20Cr Alloys

Surface roughness plays a dominant role inincreasing the oxidation rate of metals and alloysduring erosion compared to the oxidation rate in staticair. Ni and Ni-20Cr were eroded at two different impact velocities (35 and 65 m/s) and for twodifferent impact angles (90° and 30°). Theeroded samples were subsequently isothermally oxidizedin static air at three different test temperatures. Theincreased oxidation kinetics in the case of Ni could beexplained on the basis of increased roughness caused byerosion prior to oxidation. In the case of Ni-20Cr, theeffect of increased roughness on oxidation was largely offset by the fact that the number ofgrain-boundary diffusion paths decreased due tocoarsening of the grains of the oxide scale.     

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BO oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.     

Composition Dependence of the Kinetics and Mechanisms of Thermal Oxidation of Titanium-Tantalum Alloys

The oxidation behavior of titanium-tantalumalloys was investigated with respective concentrationsof each element ranging from 0 to 100 wt.%. Alloys wereexposed to argon-20% oxygen at 800 to 1400°C. Theslowest oxidation rates were observed in alloys with5-20% Ta. The oxidation kinetics of alloys containingless than approximately 40% Ta were approximatelyparabolic. Pure Ta exhibited nearly linear kinetics. Alloys containing 50% or more Taexhibited paralinear kinetics. The activation energiesfor oxidation ranged between 232 kJ/mole for pure Ti and119 kJ/mole for pure Ta, with the activation energies of the alloys falling between these values andgenerally decreasing with increasing Ta content. Theactivation energies for oxidation of the end members, Tiand Ta, agree well with published values for the activation energies for diffusion of oxygenin -Ti and Ta. Scale formation in the alloys wasfound to be complex exhibiting various layers of Ti-,Ta-, and TiTa-oxides. The outermost layer of the oxidized alloys was predominately rutile(TiO₂). Beneath the TiO₂ grew avariety of other oxides with the Ta content generallyincreasing with proximity to the metal-oxide interface.It was found that the most oxidation-resistant alloys hadcompositions falling between Ti₅Ta andTi-15Ta. Although Ta stabilizes the -phase of Ti,the kinetics of oxidation appeared to be rate limited byoxygen transport through the oxygen-stabilized -phase.However, the kinetics are complicated by the formationof a complex oxide, which cracks periodically. Tantalumappears to increase the compositional range ofoxygen-stabilized -phase and reduces both the solubilityof oxygen and diffusivity of Ti in the - and-phases.     

The Formation of Two Layers in the Internal Oxidation of Binary Alloys by Two Oxidants in the Absence of External Scales

An improved treatment of the simultaneousinternal oxidation of binary solid-solution alloys bytwo different oxidants (double internal oxidation)producing two subsequent layers divided by a sharpplanar interface in the absence of external scales ofthe most-noble component has been developed by removingsome of the approximations involved in previoustheories. The kinetics of internal oxidation arecalculated, based on the assumption that all the partialprocesses follow the parabolic rate law, as expected fordiffusion-controlled processes. The overall thickness ofinternal oxidation for dual oxidants is found to be equal to or smaller than the sum of thoseproduced by the two oxidants taken separately under thesame experimental conditions. The ratio between thethickness of the two regions depends critically on the ratio between the diffusion coefficientsof the two oxidants, so that under appropriate limitingconditions, one of the two layers becomes negligible.The degree of enrichment of the most-reactive component in the region of internal oxidationby two oxidants is found to be uniform over the wholezone, but lower than what would be produced by each ofthe oxidants taken singularly. The theoretical predictions are then compared to someexperimental results. It is finally shown that theexternal layer will not contain the compound most stableon an absolute scale, but that most stable with respect to the actual composition of the gas phase,while scaling structures different from that assumedhere are also possible.     

Internal-Carburizing Behavior of Ni-V,Ni-Cr,and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, andNi-3Nb alloys were carburized in 1.5 v/oC₃H₆ (bal. H₂) over therange 700-1000°C. Carburization of Ni-5V, Ni-12V,Ni-20Cr, and Ni-3Nb obeyed the parabolic rate law. Ni2V and Ni-10Cr, however, formed onlythin carbide scales upon carburizing. Carburizationrates decreased with increased vanadium content fromNi-5V to Ni-12V for all exposure conditions. V4C3 formed throughout the reaction zones of Ni-12V.Cr₃C₂ formed in the surfaceregions and Cr₇C₃ formed withinthe interior of Ni-20Cr. NbC precipitated in Ni-3Nbunder all conditions. The precipitate morphology changed with temperature and distance from thegas-metal surface. V₄C₃ andCr₃C₂ particles were generallysmall and spheroidal near the surface of Ni-12V andNi-20Cr, respectively, increasing slightly in size with distance from the surface and withincreasing temperature. The vanadium and chromiumcarbides formed intergranular networks toward thereaction fronts. The NbC precipitates were generallylarge and became Widmanstatten at increasing distancewithin the carburized zone of Ni-3Nb. Expressions forthe diffusion coefficient of carbon in nickel from themeasured permeabilities and carbon solubility data were determined. Solubility products weredetermined for all of the carbides formed and found tobe large in comparison with the product of theactivities of the precipitate elements. Wagner's theory of internal oxidation was shown to be anapproximation to the carburization kinetics attemperatures of 900°C or higher.     

Oxidation Behavior of Nanocrystalline Al Alloys Containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (50 nm) Al-Ti alloys were fasterthan those of conventional (200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of -Al₂O₃ andTiO₂ in the outer layer and-Al₂O₃ near the alloy as aprotective layer.     

On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys

A number of copper-rich Cu-Ni alloys wereoxidized from 750 to 1000^°C at differentoxygen pressures. The oxide scales formed consist of anouter copper oxide layer and an inner porous layer where internal oxide-derived NiO particles aredispersed in a copper oxide matrix. The copper oxide maybe both single-phase CuO and a two-phase(CuO+Cu₂O). At the lower part of thetemperature range, the oxidation kinetics and oxidemorphology depend strongly upon the formation of CuO.The CuO layer is nonprotective and further oxidation ofCu₂O, forming CuO, therefore changes the oxidation from being approximately parabolic tohaving a breakaway-like behavior. The relative thicknessof nonprotective CuO increases with increasing NiO andreflects that the solid-state flux of copper across the Cu₂O decreases due to abarrier effect of the NiO particles and porosity in theoxide and NiO particles in the alloy. The beneficialeffect of Ni in reducing the oxidation rate is lost due to the extensive formation ofnonprotective CuO.     

On the High-Temperature Oxidation of Cu-Rich Cu-Ni Alloys

Cu-2 wt.%Ni and Cu-5wt.%Ni were oxidized at 800to 1050°C and oxygen pressures from from 5 ×10^-4 to 1 atm. The oxidation, as measured bythermogravimetry, was approximately parabolic. The oxidescales could be divided in two main regions: An outerregion consisting of copper oxides and an inner porousregion, which consists of Cu₂O with dispersedNiO particles. NiO particles exists as internal-oxide particles. The interface between the two layersreflects the original surface, which shows that theouter part grows by outward Cu diffusion via vacancies.The inner part grows by outward diffusion of copper and inward transport of gaseous oxygen by thedissociative-transport mechanism. The amount ofporosity, the relative thickness of the inner layercompared to the total thickness of the scale, and theoxidation rate as a function of Ni content was dependenton the reaction conditions.     

纳米晶Cu-Si合金的抗氧化性能

采用电弧熔炼和机械合金化方法制备了两种晶粒尺寸差别较大的Cu-Si合金,并采用热重分析方法对其在700℃和800℃、1×105Pa流动O2中氧化24 h的抗高温氧化性能进行了研究。结果表明,不是所有合金氧化动力学都遵循抛物线规律,甚至有些合金的动力学曲线十分不规则。总的来说,机械合金化法制备的合金单位面积氧化增重小于相应的熔炼合金。合金氧化后都未形成连续的SiO2保护膜,而形成了相似的复杂的氧化膜,其中都包含由Cu2O和SiO2组成的富含SiO2的混合氧化物层,它限制了铜和氧的进一步扩散。晶粒尺寸的减小促进了活泼组元Si的扩散,加速了SiO2和富含SiO2的Cu2O+SiO2混合氧化物层的形成,提高了Cu-Si合金的抗高温氧化性能。     

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, an-Al₂O₃ layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr₂O₃ to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al₂O₃grains. The intermediate and inner layers consist of columnarAl₂O₃ grains and equiaxedCr₂O₃ grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with thedynamic segregation model to account for the effects of reactive elements on thegrowth of Al₂O₃ scales.     

The Effects of Reactive-Element,Ion-Implantation-Induced Amorphous Layers on the Oxidation of Co-12Cr and Ni-12Cr Alloys

Nickel-chromium (Ni-12Cr, wt.%) andcobalt-chromium (Co-12Cr, wt.%) alloys were ionimplanted with 150 keV yttrium to fluences that rangedbetween 2 × 10¹⁶ and 1 ×10¹⁷ ions/cm². The influence ofthe implantation on the microstructure of the alloy wasdetermined. The effect of the highest dose implantationon the alloys' oxidation response at 1000°C, 48 hrwas measured. Both alloys contained an amorphous surface phase as a result of this fluence and one ofthe effects of oxidation was to recrystallize theamorphized alloy in the first few minutes of oxidation.The lower doses of 2 × 10¹⁶ions/cm² were sufficient to cause amorphization of both the Ni-12Cr andthe Co-12Cr. The implantation reduced the isothermalmass gain by a factor of 25% for the Ni-12Cr, but had anegligible effect on the Co-12Cr alloy. Short-term oxidation experiments at 600°C showed viatransmission electron microscopy that, in the absence ofthe yttrium implant, the Ni-12Cr alloy forms NiO in thefirst minute of oxidation and the Co-12Cr alloy forms CoO and CoCr₂O₄.The implanted Ni-12Cr, on the other hand (1 ×10¹⁷Y⁺/cm²), formsrecrystallized Ni-Cr, Y₂O₃, andNiO in the near-surface region, while the implantedCo-12Cr alloy forms CoO, CoCr₂O₄, and a recrystallized intermetallic alloy fromthe amorphized region.     

The Use of Oxygen Isotopic Labeling to Understand Oxidation Mechanisms

Oxidation of Metals - Isotopic labeling is a powerful tool to evaluatetransport and reaction mechanisms of oxidation. Theevaluation can answer the essential question inhigh-temperature oxidation:...     

The Long-Term,Cyclic-Oxidation Behavior of Selected Chromia-Forming Alloys

Long-term, cyclic-oxidation testing in still airfor about 2 years (720 days) at 982°C and 1 year(360 days) at 1093, 1149, and 1204°C has beenconducted on the commercial, high-temperaturechromia-forming HR-120®, HR160®, and 230® alloys(all trademarks of Haynes International, Inc.). Eachthermal cycle consisted of 30 days at temperaturefollowed by about 4 hr at ambient. The resultsdemonstrated the significant effects of alloy composition on long-term,cyclic-oxidation resistance. Each of the alloysexhibited scale spallation; however, the manner by whichspallation occurred varied between the alloys. The 230 alloy, which contains 0.02 wt.% La, exhibitedpartial scale spallation, thus allowing for the easierformation of a protective or semiprotectiveCr₂O₃-rich scale during subsequentoxidation. The HR-160 alloy exhibited complete spallation owinglargely to its relatively high silicon content (2.75wt.%). However, the silicon was also beneficial inpromoting protective or semiprotective scale formationwhen the exposed alloy was subsequently oxidized.The HR-120 alloy showed the poorest cyclic-oxidationresistance, due in part to poor scale adhesion and thetendency of the iron in this alloy (33 wt.%) toeventually oxidize and result in the formation of aless-protective scale. All of the alloys underwentinternal attack in the form of internal oxidation andvoid formation. In most cases, the extent of internalattack was significantly greater than that of metalloss.     

Arc-Sprayed Iron-Based Coatings for Erosion-Corrosion Protection of Boiler Tubes at Elevated Temperatures

Wire arc spraying is an economically attractive thermal spray process that is especially interesting for the protection of large-scale parts or constructions. This study presents the results of the development and investigation of a number of cored wires based on the Fe-Cr-Al system with minor addition of alloying elements (B, Mn, Ni, Si, Ti, Mg, etc.). The microstructure of the coatings, their electrochemical behavior, and isothermal oxidation performance over a temperature range of 20-700 °C were investigated. Erosion resistance at elevated temperatures was determined with a laboratory test unit under test conditions that have simulated the work conditions in fossil-fuel-fired boilers. It was established that the oxidation resistance and the gas-abrasive wear resistance of arc-sprayed coatings depend mostly on the coating microstructure and homogeneity of element distribution rather than on the general alloying level and microhardness of the coating. A new parameter for coating characterization, namely, the coefficient of chemical microheterogeneity, K _CMH, is introduced to quantify this influence. Formation of the coating microstructure at elevated temperatures and its influence on the protection ability of the coating are discussed.     

Effect of Hf Additions on the Isothermal-Oxidation Behavior of TiAl at High Temperatures

The isothermal-oxidation behavior of TiAlcoupons containing Hf of up to 5.2 mass % has beenstudied in the temperature range 1100-1400 K in a flowof purified oxygen under atmospheric pressure. Theaddition of 0.2% Hf is very effective to decrease theoxidation rate at 1200 and 1300 K. Metallographicexamination using conventional methods revealed that theinitially-formed Al₂O₃ scale ismaintained very sound by the addition. However, further additions ofHf result in a slight enhancement of oxidation at 1200K and a gradual decrease of the effect at 1300 K.Finally, there is almost no effect by the addition of 5.2% Hf at 1300 K. Excess amounts of Hf leadto the formation of oxide mounds on theAl₂O₃ scale. They grow in size andnumber during subsequent oxidation until the wholespecimen surface is covered with a thick scale. Such a scale is notprotective having a structure often reported in theliterature. The effect of the addition of 0.2% Hfbecomes small at 1350 K and at 1400 K it is inverted.Possible mechanisms for the improvement attained by thesuitable addition are discussed.     

The Internal-Nitriding Behavior of Co-Fe-Al Alloys

Co-10Fe, Co-20Fe, and Co-40Fe alloys containing3 at.% Al were internally nitrided inNH₃/H₂ mixtures over the range700-1000°C. The kinetics of thickening of theinternal-reaction zone followed the parabolic rate law, suggesting thatsolid-state diffusion was rate controlling. Nitrogenpermeabilities were obtained for each alloy. AlN was theonly nitride to form for all materials and at alltemperatures. At high temperature, the nitride precipitatesformed hexagonal plates near the surface, theprecipitates becoming more blocky near the reactionfront. Precipitate size increased with increasing depthin the alloy and increasing temperature, becauseof competition between nucleation and growth processes.Increasing iron content increased the reacion kineticsdue to increased nitrogen solubility with increasing iron content.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Internal-Nitriding Behavior of Ni-V and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, and Ni-3Nb alloys (w/o)were nitrided in 10 v/o NH₃(bal.H₂) over the range of 700-1000°C. Thegrowth rates of the reaction zones followed parabolicbehavior for all of the alloys from 700 to 900°C. At 1000°C, Ni-2V andNi-3Nb formed nitride scales, whereas Ni-5V and Ni12Vformed internal-nitride zones. Nitridation ratesdecreased with increasing vanadium content for the Ni-Valloys. VN precipitated in the Ni-V alloys and NbNprecipitated in Ni-3Nb for all exposure conditions inwhich internal nitridation occurred. The precipitatemorphology changed with temperature and distance from the gas-metal surface. The VN and NbNprecipitates were generally small and spheroidal nearthe surface, increasing in size with distance andtemperature. The NbN precipitates became Widmanstattenat higher temperatures and/or increasing distance withinthe nitrided zone. The solubility of nitrogen in pure Niwas determined and found to decrease with increasingtemperature from 700 to 1000°C. Expressions for the diffusion coefficient of nitrogen in nickelwere determined from the measured permeabilities of eachalloy and the nitrogen solubilities in nickel.     

STEM Analysis of the Transient Oxidation of a Ni-20Cr Alloy at High Temperature

High-spatial scanning transmission electronmicroscopy (STEM) has been used to study the developmentof transient scales on a commercially available Ni-20Cralloy. The samples were examined usingelectron-transparent cross sections of the metal and oxide foroxidation times between 0 and 30 min in a furnace at950°C in laboratory air. The samples were polishedto 1 m diamond before oxidation, producing a recrystallized grain structure within 100 nm ofthe surface. Upon oxidation, the initial scale consistedalmost exclusively of chromia. However, at themetal-oxide interface, thin layers of silica and alumina were detected. At longer oxidation times,(>5 min), localized thickening of the silica layerwas observed. With increased oxidation time, (>25min), these regions spread along the metal-oxideinterface until an almost continuous silica layer hadformed. The silica layer was present at much shorteroxidation times than reported by other workers, however,this may be because of the thin layer being undetectable using microprobe techniques. The scale formedwas found to be adherent, although the alloy containedsulfur and did not contain reactive elements. However,sulfur was not found to segregate to the metal-oxide interface possibly because of the presence ofthe amorphous silica layer.