Cyclic oxidation behavior of Ni3Al-0.1B base alloys containing a Ti,Zr, or Hf addition

Ni ₃Al and Ni₃Al-0.1B, with and without additions of about 2% Ti, Zr, or Hf were subjected to a thermal cycling oxidation test in pure flowing oxygen at atmospheric pressure at temperatures cycled between 400 and 1300 K. The scales formed on Ni₃Al and Ni₃Al-0.1B spalled repeatedly, resulting in a considerable mass loss of the specimen. The Ti addition to Ni₃Al led to a repeated scale spollation, whereas Ti added to Ni₃Al-0.1B resulted in a very adherent scale, although the oxidation kinetics were linear and the formation of deeply penetrating Al₂O₃ along the alloy grain boundaries took place. The scales were very adherent on alloys containing Zr and Hf. This was attributed to the so-called keying mechanism, because uneven penetration of Al₂O₃ into the alloy took place, leading to irregularly shaped scale/alloy interfaces. ZrO₂ and HfO₂ particles were incorporated into the Al₂O₃ layer and protrusions, and some of them were formed ahead of the Al₂O₃. The shape of these particles was not stringerlike as found with other alloys. The Ti, Zr, and Hf additions tended to decrease the density of voids formed at the scale/alloy interface, but the extent of the change seems to be insufficient to support the vacancy-sink mechanism. The Hf addition was found to be most effective in forming a protective scale.     

Oxidation behavior of Ni2Al-0.1B containing 2Cr

Ni ₃ Al-0.1B containing 2% Cr was subjected to a thermal cycling oxidation test in pure flowing oxygen at atmospheric pressure at temperatures cycled between 400 and 1300 K. Scales formed on the alloy spalled repeatedly after several tens of cycles, resulting in a considerable mass loss of mass of the specimen. The isothermal oxidation behavior of the alloy was also studied at temperatures of 1300, 1400, and 1500 K in the same oxidizing atmosphere. Characteristic breakaway oxidation caused by extensive scale blistering was observed at 1300 and 1400 K after a protective period of about 50 ksec, whereas the oxidation at 1500 K followed a parabolic law without any significant blistering. Particularly at 1300 K, the alloy grain boundaries provided favorable sites for blistering. The influence of Cr may be explained by the modification of mechanical property of the scale.     

Effect of preoxidation and Ti or Hf additions on the corrosion behavior of Fe-23Cr-5Al alloys in Ar-10SO2 at 1200 K

An Fe-23Cr-5Al alloy and those containing 0.21% Ti or 0.12% Hf were cyclically corroded in flowing Ar-10SO₂ gas under atmospheric pressure and in a temperature range varying from room temperature to 1200 K. The corrosion kinetics were assessed by gravimetry, while morphological examinations were carried out using x-ray diffractometry, scanning electron microscopy, and microanalysis. Similar corrosion tests and examinations of the corrosion products were performed for the specimens preoxidized at 1200 K for 20 ksec in pure oxygen under atmospheric pressure. The main corrosion product under all the experimental conditions was -alumina, growing mainly outward in the form of thin needles. Many voids formed beneath the adherent scales. The degree of outward growth decreased by the additives. Partial spallation and formation of scales repeated on the base alloy, resulting in continuous mass losses. The addition of Ti prevented scale spallation, resulting in gradual mass gains; whereas the addition of Hf resulted in poorly adherent scales, under which grooving at alloy grain boundaries and void formation inside the grains occurred. Preoxidation resulted in the formation of adherent scales on all the alloys; however, with increasing corrosion cycles, the outward growth of the oxide became significant. Penetration of sulfur to the substrate under the adherent scale took place.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni₃Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl₂O₄, and an inner layer of Al₂O₃, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al₂O₃ layer at the grain boundaries which favor oxygen penetration through the grain interior.     

Oxidation resistance of sputtered Ni3(AlCr) nanocrystalline coating

Isothermal and cyclic oxidation resistance at 1000°C in air were investigated for a cast Cr-containing Ni₃Al-base alloy and its sputtered nanocrystalline coating. The results indicated that both the cast Ni₃Al alloy and its sputtered coating exhibit excellent isothermal oxidation resistance as a result of the formation of Al₂O₃ scales. However, the cast alloy possesses very poor cyclic oxidation resistance because of the spallation of the initially formed Al₂O₃ scale during cooling and subsequent formation of NiO. On the contrary, the sputtered Ni₃(AlCr) nanocrystalline coating exhibits very good cyclic oxidation resistance due to the significant improvement of the adhesion of Al₂O₃.     

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.     

Effect of Cr,Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

The oxidation behavior of Ni₃Al+2.90 wt.% Cr, Ni₃Al+3.35 wt% Co, and Ni₃Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni₃Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni₃Al alloys was the existence of small -Al₂O₃ particles in the middle of the -Al₂O₃ scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

Carburization Behavior of Electrodeposited Ni3 Al–CeO2-Base Coatings on Fe–Ni–Cr Alloys

The cyclic carburization of electrodeposited pure and CeO₂-dispersed Ni₃Al intermetallic coatings on Fe–Ni–Cr alloys has been investigated at 850 and 1050°C for periods up to 500 h in a reducing 2%CH₄–H₂ atmosphere. At 850°C, all Ni₃Al-base-coating samples showed excellent carburization resistance and slow mass increases due to the formation of a thin γ-Al₂O₃ scale and a low carbon activity (a _c = 0.73). At 1050°C and a high carbon activity (a _c = 3.21), all coatings are superior to the uncoated Fe–Ni–Cr alloy in terms of carburization resistance. A thin α-Al₂O₃ scale slowly formed on all Ni₃Al coatings effectively blocked the carbon attack. The addition of CeO₂ particles in the Ni₃Al coatings significantly mitigated the cracking of the α-Al₂O₃ scale and the resultant internal oxidation and carburization. For all coatings, Ni-rich particles were found to be formed on the α-Al₂O₃ scale during oxidation, which had led to the deposition of catalytic coke.     

The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600–800°C

The corrosion behavior of Ni₃Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na₂SO₄–NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni₃Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO–Al₂O₃ scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO₄ can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni₃Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni₃Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

The effect of aluminum as an alloying addition or as an implant on the high-temperature oxidation of Ni-25Cr

The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000°C and 1100°C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr₂O₃ scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

The isothermal oxidation of Fe<Subscript>3</Subscript>Al−4%Cr−(0, 0.5, 1, 2%)Mo alloys at 1000°C

The air oxidation characteristics of Fe₃Al-4%Cr-(0, 0.5, 1, 2%)Mo alloys at 1000°C were studied using TGA, XRD, EPMA, and TEM/EDS. Molybdenum increased the oxidation resistance of Fe₃Al-4%Cr alloys. The whole Al₂O₃ grains that formed on the alloy surface contained a small amount of dissolved Fe ions. The Al₂O₃ grains next to the oxide-matrix interface additionally contained a small amount of dissolved Cr and Mo ions. Beneath the thin but non adherent Al₂O₃ layer, an Al-depleted, Fe-enriched matrix zone formed due to the consumption of Al in the scale.     

Degradation behavior of Ni<Subscript>3</Subscript>Al plasma-sprayed boiler tube steels in an energy generation system

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni₃Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 μm thick layer sprayed onto the surface before applying the 200 μm coating of Ni₃Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.     

Oxidation behavior of Ti3AlC2 at 1000-1400 °C in air

The isothermal oxidation behavior of bulk Ti₃AlC₂ has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti₃AlC₂ has excellent oxidation resistance. The oxidation of Ti₃AlC₂ generally followed a parabolic rate law with parabolic rate constants, k_p that increased from 4.1×10⁻¹¹ to 1.7×10⁻⁸ kg² m⁻⁴ s⁻¹ as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al₂O₃. The outer layer changed from rutile TiO₂ at temperatures below 1200 °C to a mixture of Al₂TiO₅ and TiO₂ at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al₂TiO₅ and, predominantly, α-Al₂O₃, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al₂O₃ layers.     

The corrosion behavior of Ni-Mo alloys in a H2/H2O/H2S gas mixture

The corrosion behavior of Ni-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range of 550–800C in a mixed gas of H₂/H₂O/ H₂S. The scales formed on all alloys contained only sulfides and were doublelayered. The outer scale was single-phase Ni₃S₂. Depending on the alloy composition and reaction conditions, the inner scale was: (1) a mixture of MoS₂ plus Ni₃S₂ with/without Ni, (2) MoS₂, or (3) MoS₂ plus intermetallic particles and/or double sulfide Ni_2.5Mo₆S_6.7. Neither internal oxidation nor internal sulfidation were observed at lower temperatures. Internal sulfidation was however observed at higher temperature when the scale apparently melted. The parabolic law was generally obeyed for the most concentrated alloys. For the two more-dilute alloys the kinetics were mostly linear. A decrease in the corrosion rate occurred with increasing Mo content of the alloy and may be attributed to the presence of increasing volume fractions of MoS₂ and/or of a double Ni-Mo sulfide in the inner region of the scale. For the two most concentrated alloys this may also be due to the presence of a number of particles of the unsulfidized intermetallic compound, which is Ni₃Mo for Ni-30Mo, but NiMo for Ni-40Mo.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.