Oxidation of two-phase Co-54.86 wt.% Cu alloy at 700–1000°C

The oxidation in 1 atm of pure oxygen of a binary two-phase Co-Cu alloy has been studied as a simple example of the oxidation behavior of a multiphase alloy. The two-phase alloy oxidizes according to a parabolic rate law to a good approximation throughout the entire exposure period over the temperature range 700–1000°C with an oxidation rate constant greater than that for pure cobalt in the whole temperature range, and greater than that for pure copper at 900–1000°C, but lower below 900°C. The scale presents essentially the same type of layered structure at all the temperatures investigated, with an outer region composed of copper oxides, while cobalt is preferentially accumulated in the inner region of the scale, mainly in the form of CoO. A subscale formed by internal oxidation of the particles of the Co-rich phase is also present. The observed increase of the oxidation rate of the alloy in comparison with pure cobalt is attributed mainly to the presence of a high concentration of copper dissolved in CoO in the form of monovalent ions, which produces a significant modification of the concentration of defects of cobalt oxide with a consequent increase of the oxidation rate constant of the alloy if a suitable model for the defect structure of pure CoO is considered, which takes into account also the presence of a small concentration of interstitial metal ions.     

Corrosion behaviour of the new gas turbine alloy 2100 GT in hot gases and combustion products

Not only excellent high temperature mechanical properties are needed to establish a new gas turbine alloy, but also a very good oxidation behaviour, together with good resistance to so‐called “hot corrosion”. This paper describes experimental studies on the corrosion behaviour in hot gases and combustion products of a new Ni‐Cr‐Ta alloy 2100 GT in comparison to the commercially established alloys 230, C‐263 and 617. Alloy 2100 GT is a newly developed cobalt, tungsten and molybdenum free Ni‐base superalloy of Krupp VDM. It contains as major alloying elements 25 wt.‐% chromium, 8 wt.‐% tantalum, 2.4–3 wt.‐% aluminium and 0.2–0.3 wt.‐% carbon. High temperature strength is achieved by the addition of tantalum, resulting in significantly increased solid solution strengthening, carbide hardening due to the formation of primary precipitated tantalum carbides, and γ′‐precipitation hardening by aluminium and tantalum. The isothermal oxidation tests showed that the parabolic rate constant of alloy 2100 GT is similar to that of alumina‐forming alloys. This is achieved by the remarkably high aluminium content for a wrought alloy. Additions of yttrium improve the spalling resistance under thermal cycling by the formation of very thin and tightly adherent oxide layers. No deleterious effect caused by the addition of tantalum could be found. In the cyclic oxidation tests performed at temperatures between 700°C and 1200°C alloy 2100 GT showed the lowest mass change of all the alloys investigated. Na₂SO₄ has been found to be a dominant component of alkali salt deposits on gas turbine components at elevated temperatures. Combustion gases contain SO₂ because of the impure nature of the fuel. To investigate the hot corrosion behaviour of alloy 2100 GT, tests were performed with salt deposits containing 0.1 mol Na₂SO₄ and a test gas comprising air and 0.1% SO₂. Test temperatures were 600°C, 700°C, 850°C and 950°C. Alloy 2100 GT exhibited the best performance at all test temperatures. It was the only alloy which did not suffer any fluxing of the oxide layer and only slight internal sulphidation was observed.     

The effect of internal stable nitride and oxide dispersion on the high temperature oxidation of FeCr alloys

The behaviour of FeCr alloys containing either a dispersed nitride or oxide phase is examined. Fe-12, 14 or 18% Cr containing 0.5 or 1 wt% Ti were used. The oxide dispersion was introduced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ mixture in a sealed quartz capsule at 1100 or 1200°C. Internal nitrides were produced by nitridation in either an 80/20 or a 90/10 H₂/N₂ mixture at 1150°C. Subsequent oxidation tests of the treated alloys were carried out isothermally at 900, 1000 and 1100°C, or by thermal cycling (3 h cycles) at 1100°C.The dispersed nitride phase is not as effective in improving the oxidation resistance as corresponding dispersed oxides. Considerable particle coarsening is observed and the nitrides tend to dissociate and are converted to oxide near to the alloy/scale interface.     

The oxidation of cobalt—tungsten alloys

The oxidation behaviour of CoCrW alloys containing from 0–25%Cr and up to 30%W in oxygen at 900–1100°C has been studied. In CoW alloys there is a slight reduction in the oxidation rate as the tungsten content is increased, hwoever this is much mor emarked in Co15CrW alloys. Tungsten has little effect in Co25CrW alloys. On the binary alloys and CoCrW alloys which do not form Cr₂O₃, the scale has two layers: an outer, tungsten-free layer of columnar-grained CoO, and an inner layer of CoO containing CoWO₄ precipitates together with CoCr₂O₄ particles in the ternary alloys. The relative thicknesses of the two layers and the distribution of the constituents in the inner layer depends in temperature and alloy composition. The CoWO₄ and CoCr₂O₄ particles appear to be responsible for the reduction in oxidation rate by a blocking mechanism in the inner layer. There is some evidence to suggest that tungsten additions to Co?25%Cr alloys assist the exclusive formation of Cr₂O₃.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.     

Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

In the present study, the applicability of thermal barrier coatings (TBCs) on γ‐TiAl alloys was investigated. Two alloys with the chemical compositions of Ti‐45Al‐8Nb‐0.2B‐0.15C and Ti‐45Al‐1Cr‐6Nb‐0.4W‐0.2B‐0.5C‐0.2Si were used. Before TBC deposition, the specimens were pre‐oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron‐beam physical vapour deposition (EB‐PVD). The oxidation behaviour of the γ‐TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 °C. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (EDS). No spallation of the TBC was observed for pre‐oxidised specimens of both alloys when exposed to air at 850 °C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO₂/Al₂O₃ protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 °C, the oxide scales on the alloy Ti‐45Al‐8Nb‐0.2B‐0.15C pre‐oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 °C.     

The hot corrosion of cobalt-base alloys in a modified Dean's rig-I. CoCr,CoCrTa and CoCrTi alloys

A series of CoCr, CoCrTa and CoCrTi alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of dry air containing vapours of, respectively, Na₂SO₄, Na₂SO₄ + Na₂O and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the salt-bearing environments were aggressive. The basic salt was more aggressive to the binary CoCr alloys, suggesting a basic-fluxing mechanism. The presence of Ta and Ti in the ternary alloys did not significantly affect the corrosion mechanism. However, when the Ta and Ti content was high enough to produce second phases in the alloys, preferential attack along the phase boundaries occurred. The presence of NaCl in the salt greatly enhanced the attack by cracking the outer oxide scale and by forming volatile compounds.     

Mapping of the oxidation products in the ternary Co-Cr-Al system

The kinetics and products of oxidation of alloys in the Co-Cr-Al system have been studied and four mechanisms of oxidation identified. For the first mechanism, the rate-controlling process is cation diffusion of cobalt cations through CoO, and the main oxidation product is CoO. In the second mechanism, cation diffusion through CoO is still rate controlling, but the oxidation is strongly inhibited by an inner discontinuous spinel layer. The major oxidation products are CoO and CoCr₂O₄. The third mechanism of oxidation consists of preferential oxidation of chromium coupled with internal oxidation of aluminum while the fourth mechanism is the preferential oxidation of aluminum with the subsequent formation of Al₂O₃ scales, providing the best oxidation resistance. From the oxidation data obtained, an oxide map from which the oxidation behavior of various alloys may be deduced is drawn for the Co-Cr-Al system at 1100°C.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

High temperature oxidation of γ/γ′-strengthened Co-base superalloys

Isothermal oxidation was carried out on new γ/γ′ Co-base superalloys of the system Co–Al–W–B. Appropriate B-contents lead to improved oxidation resistance and oxide layer adhesion. Oxidation at 800 and 900 °C results in formation of Co₃O₄, CoO, and complex oxides (containing Co, Al, and W). A continuous and protective inner alumina layer forms only at 800 °C. Furthermore, oxidation leads to phase transformation (γ/γ′ to γ/Co₃W microstructure) at the matrix/oxide layer interface due to Al-depletion. The effect of additional alloying elements such as Ta, Cr, Nb, Si, V, Mo, and Ir on the oxidation behaviour was also investigated.     

镍基单晶高温合金DD10的恒温氧化行为

采用X射线衍射(XRD),扫描电镜(SEM)及能谱(EDX)等方法研究了不同钴含量的两种镍基单晶高温合金在900,1000和1100 ℃的恒温氧化行为。研究发现,增加合金中的Co含量,会降低合金的扩散激活能,引起氧化速率的略微增加。在900和1000 ℃氧化时符合抛物线规律,氧化膜分为3层：外层主要由Cr2O3和TiO2组成;中间层是很薄的CrTaO4和Ta2O5氧化层;内层是连续的Al2O3氧化层。在1100 ℃时氧化膜的严重剥落和CrO3的挥发使增重曲线略微偏离抛物线规律,生成了NiCr2O4, CoAl2O4, CoNiO2和Co2TiO4等尖晶石相,并发生了内氮化     

Oxidation Characteristics and Oxide Layer Evolution of Alloy 617 and Haynes 230 at 900 °C and 1100 °C

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr₂O₃ double layer and isolated NiO–NiCr₂O₄–Cr₂O₃ triple layer were observed at a steady-state condition. For Haynes 230, a MnCr₂O₄ layer was formed on top of the Cr₂O₃ layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr₂O₃ and outer MnCr₂O₄ or TiO₂. The spallation of outer layer and subsequent volatilization of the Cr₂O₃ layer produced a rugged surface and interface as well as internal oxidation.     

The oxidation of Co-20% Cr base alloys containing Nb or Ta

Alloys based on Co-20% Cr containing approximately 4, 7 or 10wt% Nb or Ta were oxidized in oxygen and air at 900, 1000 and 100°C for times up to 350 h. In general, the addition of Nb accelerated the oxidation rate, although this effect was small at the lowest temperature. Futhermore, little evidence could be found for the development of a protective Cr₂O₂ layer. In contrast, the addition of Ta proved beneficial at all temperatures, promoting the development of protective oxide scales. A critical difference appeared to be the ability of the Ta-containing alloys to form a compound oxide, CrTaO₄, whereas no similar phase could be detected in the scales on the Nb-containing alloys. The Ta-rich oxides formed a layer adjacent to the metal, while a Cr-rich layer was formed outside it. It is possible that the Ta reduced the oxygen activity at the surface of the alloy, preventing the formation of cobalt-containing oxides which might otherwise disrupt the protective scale. Both elements have restricted solubility in Co-20% Cr, forming intermetallic compounds which oxidize internally. In the case of the Nb-containing alloys, a process occurs, during oxidation, which produces a change in the intermetallic deep in to the alloy, though there is no similar change in the Ta-containing alloys. This process has not yet been defined.     

New Co-based γ-γ′ high-temperature alloys

New cobalt-based alloys containing ordered L1₂ precipitates have been investigated. With additions of Cr, Mo, Ni, Re, Ta, and V to the ternary Co-Al-W system, two phase γ-γ′ microstructures have been established. Solidus and liquidus temperatures are 100°C–150°C higher than advanced nickel-based single-crystal alloys strengthened with the L1₂ phase. An anomalous rise in flow stress with temperature is observed. Single crystals have been solidified and partitioning during solidification is limited in the ternary system, suggesting a high resistance to convective instabilities. Oxidation at 900°C results in the formation of cobalt oxide. Following oxidation, an inner layer of Al₂O₃ is observed in uncoated Cr-containing alloys and Cr₂O₃ is observed in alloys subjected to chromization.     

High temperature corrosion of cracking tubes

Coils for pyrolysis cracking of hydrocarbons are made of centrifugically cast FeNiCr‐alloys and exposed to very severe conditions, high temperatures > 1050°C, oxidizing flue gases at the outside and carburizing atmosphere at the inside. Two tube sections have been investigated after some years operation. In the first case the tube wall was thoroughly carburized, the big size of the internal carbides indicated service at too high temperatures > 1100°C. At such temperatures the protective chromia scale fails by conversion to carbides, allowing ingress of carbon. At the outer wall surface obviously Cr was lost by evaporation as CrO₂(OH)₂ and CrO₃ which is significant at high pO₂ and high temperature. In the second case the tube had been operated at more moderate temperature, but showed creep cavities indicating loss of creep strength. This is due to repeated oxidation and loss of the scale by thermal cycling, this process has caused formation of a carbide denuded zone of considerable width at the outer side somewhat less wide at the inner wall. Besides oxide spallation, at the outer wall also evaporation will have played a role.     

Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide

Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm²/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

两种Co-Nb合金在60O℃～800℃1大气压纯氧中的氧化

研究了含Ｎｂ１５和３０ｗｔ％的Ｃｏ－Ｎｂ二元合金在ｌａｔｉｎ纯氧中６００～８００℃的氧化特性。它们的氧化动力学近似地遵循抛物线规律,而其瞬时氧化速率常数随时间而减低、且以６００℃氧化者尤甚。两合金的氧化速率均高于纯Ｃｏ,但其速率增量颇低。在所有的实验条件下,两合金都发生了外氧化与内氧化,外氧化膜的外侧为连续的纯氧化钴带,其下为两个二元Ｃｏ－Ｎｂ氧化物（ＣＯＮｂ２Ｏ６和ＣＯ４Ｎｂ２Ｏ９）与基金属氧化物的混合。内氧化带为氧化钴、氧化铌（Ｎｂ２Ｏ５或／和ＮｂＯ２）的混合,而在该带的最外侧还有源于富Ｎｂ合金相的二元氧化物。在合金－氧化膜界面处都没有观察到贫铌带。从合金的和所生成氧化膜的显微组织特征,尤其是从铌在钴中溶解度低的角度,对合金的氧化行为进行了讨论。