The oxidation of two ternary Fe-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of two ternary Fe-Cu-Al alloys containing 10 at.% Al (Fe-65Cu-10Al and Fe-30Cu-10Al) has been studied at 800-900 °C under 1 atm O₂. Under all conditions both alloys show an initial faster stage during which Fe-65Cu-10Al corrodes more rapidly at 800 °C than at 900 °C, while Fe-30Cu-10Al follows nearly identical kinetics at both temperatures. As oxidation proceeds, a continuous alumina layer is eventually established on the surface of the two alloys, thus decreasing significantly their oxidation rates. Altogether, the Fe-rich alloy Fe-30Cu-10Al oxidizes slightly faster than the Cu-rich alloy Fe-65Cu-10Al at both temperatures. The possible reasons for the decrease in the critical Al content needed to form external alumina scales for the Cu-rich alloy in comparison with binary Cu-Al alloys are examined.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

The oxidation of three Ni-6Si-xAl alloys in 1 atm O2 at 1000 °C

The oxidation of three ternary Ni-6Si-xAl alloys containing 6, 10 and 15 at.% Al and of the corresponding binary Ni-Al alloys has been studied at 1000 °C under 1 atm O₂ to examine the effect of different Al additions on the behavior of ternary Ni-Al-Si alloys containing 6 at.% Si. Of the three binary Ni-Al alloys only Ni-15Al was able to form external alumina scales. Conversely, all the three ternary alloys formed an innermost layer of alumina directly in contact with the alloy following very similar and approximately parabolic kinetics after a short faster initial stage due to transient formation of NiO. Thus, the presence of silicon is very effective to reduce the critical Al content needed to form exclusive alumina scales with respect to binary Ni-Al alloys. The third-element effect due to silicon is interpreted on the basis of an extension of Wagner’s criterion for the transition from the internal to the external oxidation of the most reactive component in binary alloys.     

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al alloy at 973–1073 K in 101 kPa O2

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al (at.%) alloy was investigated at 973–1073 K in 101 kPa O₂. The alloy is composed of two phases. One light phase with lower Cr content forms the matrix of the alloy, and the other medium gray phase richer in Cr is presented in the form of continuous islands. At 973 and 1073 K, the kinetic curves for the present alloy deviate evidently from the parabolic rate law. They show a large mass gain in initial stage, and then their oxidation rates decrease evidently with time until they become very small up to 24 h. Cross sectional morphologies show the present alloy is able to form continuous external scales of chromia over the alloy surface with a gradual decrease in the oxidation rate. However, the previous studies showed that a ternary two-phase Cu–40Ni–20Cr alloy is unable to form protective external scales of chromia over the alloy surface, but is able to form a thin and very irregularly continuous layer of chromia at the top of the mixed internal oxidation region. Therefore, substituting Cr in Cu–40Ni–20Cr alloy with 2.5 at.% Al is able to decrease the critical content required to form Cr oxide and help to form continuous external scales of chromia under lower Cr content in two-phase alloys.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

The Effect of Cr on the Oxidation of Ni–10 at% Al in 1 atm O2 at 900–1000°C

The oxidation of three Ni–xCr–10Al alloys with a constant Al content of 10 at% and containing 3, 5, and 10 at% Cr was investigated at 900–1000°C in 1 atm of pure oxygen and compared to the behavior of Ni–10Al. At both temperatures, an external NiO scale overlying a zone of internal-oxide precipitates formed on Ni–10Al and Ni–3Cr–10Al: in addition, a discontinuous Al₂O₃ layer formed at the front of the internal oxidation for Ni–3Cr–10Al. An exclusive external scale of Al₂O₃ formed at most places on Ni–5Cr–10Al at 900°C, while, at some sites, the same alloy formed an outer NiO layer overlying an internal oxidation zone. The scales formed on Ni–5Cr–10Al at 1000°C were complex, but eventually a protective Al₂O₃ layer developed either at the alloy surface or beneath a region containing a mixture of different oxides. Finally, an exclusive external Al₂O₃ layer formed on Ni–10Cr–10Al at both temperatures. Thus, the addition of sufficient chromium to Ni–10Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum under a constant Al content. A possible mechanism for the effect of chromium on the oxidation of Ni–10Al is discussed on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most-reactive component in binary alloys.     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

Porosity-dependent cyclic-oxidation resistance at 850 °C of annealed Ni–Al-based coatings via electroplating

This study was focused on the cyclic-oxidation behaviors of porous Ni–Al-based coatings, and some alternative methods such as slow annealing and alloying with oxide additives to achieve better cyclic-oxidation resistance at intermediate temperatures. It was found that excellent cyclic-oxidation resistance at 850 °C could be obtained for the coatings even with low Al content when the initial porosity was less than 7%. When porosity was in the range of 7% to 11%, the coatings exhibited inferior resistance to cyclic oxidation in the initial stage, but improved resistance at the latter stage as pores are healed by diffusion. The formation of thin and close-packed oxide film on coating surfaces reduced the pore-healing rate in the coatings. Adding reactive oxides had little effect on oxidation resistance and pore healing under the current intermediated temperature of 850 °C and the relative short testing time (1000 h).     

Oxidation of the three-phase Cu-20Ni-30Cr and Cu-20Ni-40Cr alloys at 700-800 °C in 1 atm O2

The oxidation of two ternary Cu-Ni-Cr alloys containing approximately 30 and 40 at.% Cr, but with a similar Ni content, was studied at 700-800 °C in 1 atm of pure oxygen. Both alloys contain a mixture of three phases, where the phase with the largest copper and lowest chromium content (α) forms the matrix, while the phase with an intermediate content of Ni and Cr (β) and that richest in chromium (γ) are present in the form of particles dispersed in the α matrix. The kinetics of oxidation were rather irregular and presented two approximately parabolic stages which for the alloy with 40 at.% Cr were followed by a final nearly linear stage. Generally, the corrosion rates decreased by increasing the chromium content in the alloy under constant temperature and increased with temperature for a constant alloy composition. The scales formed on the two alloys were rather complex and consisted in most cases of an outermost copper oxide layer followed by a layer containing a mixture of oxides of nickel and copper as well as Cu-Cr and Ni-Cr spinel and finally by an innermost very irregular and convoluted but continuous Cr₂O₃ layer which protruded into the alloy and contained a number of Cu-rich metal islands.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

Cycle stabilities of the La0.7Mg0.3Ni2.55−xCo0.45Mx (M = Fe, Mn, Al; x = 0, 0.1) electrode alloys prepared by casting and rapid quenching

In order to improve the cycle stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, Ni in the alloy was partly substituted by Fe, Mn and Al, and the electrode alloys La_0.7Mg_0.3Ni_2.55−xCo_0.45M_x (M = Fe, Mn, Al; x = 0, 0.1) were prepared by casting and rapid quenching. The effects of the substitution of Fe, Mn and Al for Ni and rapid quenching on the microstructures and electrochemical properties of the alloys were investigated in detail. The results obtained by XRD, SEM and TEM indicate that element substitution has no influence on the phase compositions of the alloys, but it changes the phase abundances of the alloys. Particularly, the substitution of Al and Mn obviously raises the amount of the LaNi₂ phase. The substitution of Al and Fe leads to a significant refinement of the as-quenched alloy's grains. The substitution of Al strongly restrains the formation of an amorphous in the as-quenched alloy, but the substitution of Fe is quite helpful for the formation of an amorphous phase. The effects of the substitution of Fe, Mn and Al on the cycle stabilities of the as-cast and quenched alloys are different. The positive influence of the substitution elements on the cycle stabilities of the as-cast alloys is in proper order Al > Fe > Mn, and for as-quenched alloys, the order is Fe > Al > Mn. Rapid quenching engenders an inappreciable influence on the phase composition, but it markedly enhances the cycle stabilities of the alloys.     

Effects of cold roll and heat treatments on anomalous thermal behavior in Fe100−xAlx(x = 5–30) alloys

DSC measurements were carried out for various Fe_100−xAl_x(x = 5–30 at%) alloys to clear the effects of cold roll and quenching rate from 1173 K. In the case of cold roll free specimens, an exothermic peak was observed at around 530–560 K in quenched specimens and no peaks in slowly cooled specimens. The peak temperature and its exothermic heat depended on the alloy composition. The maximum exothermic heat was obtained for a 25 at% Al alloy and its value were about 1200 J/mol. The peak in a 5 at% Al alloy was remained as a future work. The exothermic heat was affected by the quenching temperature in alloys above 15 at% Al. The peak temperature was decreased by decreasing the quenching temperature. In a 15 at% Al alloy, the peak became negligibly small by quenching from 1023 K. The activation energies in cold roll free specimens were evaluated from the Kissinger analysis and they were 134, 108, 133 and 110 kJ/mol for 15 at% Al, 20 at% Al, 25 at% Al and 30 at% Al alloys, respectively. On the other hand, cold rolled specimens showed an exothermic peak at around 470 K, independently of the cooling rate. Their exothermic heats and temperatures were comparable order to those of furnace cooled and water quenched specimens. The present results suggested that origin of exothermic peaks of all alloys were same in nature and atomic ordering may be related to the exothermic behavior at relatively low temperatures.     

Platinum-induced oxidation of chromium in O2 at 800 °C

The effects of porous Pt on the oxidation of Cr at 800 °C have been studied with the ¹⁸O-SIMS technique, gas phase analysis and XPS. In oxide areas with Pt a pronounced inward oxygen transport takes place and a substantial oxide growth near the Cr substrate is observed. In oxide grown on areas without Pt the counts of CrO ions in SIMS and the binding energy of O (1s) in XPS depend on the distance from the area with Pt. The experimental observations are believed to be a consequence of a high dissociation efficiency of O₂ on areas with Pt in combination with a high diffusivity of O in external and internal oxide surfaces on areas both with and without Pt.     

Oxidation of a novel electrodeposited Ni–Al nanocomposite film at 1050 °C

A novel Ni–28Al nanocomposite film (in weight percentage) was developed by co-electrodeposition of Ni with Al nanoparticles (mean size 75 nm). The Al nanoparticles were dispersed in the electrodeposited Ni matrix (mean grain size 40 nm). The oxidation at 1050 °C showed that the scaling rate of the as-deposited Ni–28Al nanocomposite film was two orders of magnitude lower than that of the electrodeposited pure Ni film due to the formation of a continuous alumina scale.     

Influences of the substitution of Fe for Ni on structures and electrochemical performances of the as-cast and quenched La0.7Mg0.3Co0.45Ni2.55−xFex (x = 0–0.4) electrode alloys

In order to improve the cycle stability of the La–Mg–Ni system PuNi₃-type hydrogen storage electrode alloys, Ni in the alloy was partially substituted by Fe. The La_0.7Mg_0.3Co_0.45Ni_2.55−xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of the substitution of Fe for Ni on the structures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results of the electrochemical measurement indicate that the substitution of Fe for Ni obviously decreases the discharge capacity, high rate discharge capability (HRD) and discharge potential of the as-cast and quenched alloys, but it significantly improves their cycle stabilities, and its positive impact on the cycle life of as-quenched alloy is much more significant than on that of the as-cast one. The microstructure of the alloys analyzed by XRD, SEM and TEM show that the as-cast and quenched alloys have a multiphase structure which is composed of two major phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Fe for Ni helps the formation of a like amorphous structure in the as-quenched alloy. With the increase of Fe content, the grain sizes of the as-quenched alloys significantly reduce, and the lattice constants and cell volumes of the alloys obviously increase.     

The Effect of Silicon on the Oxidation of a Ni-6 at.% Al Alloy in 1 atm of pure O2 at 900°C

The oxidation behavior of a binary Ni–6Al alloy and of three ternary Ni–xSi–6Al alloys containing 2, 4 and 6 at.% Si has been studied at 900°C under 1 atm of pure O₂. The addition of 2 at.% Si to Ni–6Al increases the short-time oxidation rate of Ni–6Al, which is subsequently reduced and becomes similar to that of the binary alloy. However, the presence of this silicon level is already able to stop after some time the coupled internal oxidation of Al+Si by forming a healing oxide layer rich of alumina at the front of internal oxidation. The addition of 4 at.% Si to the same alloy permits a more rapid inhibition of the internal oxidation and the formation of a steady-state, inner alumina-rich scale. Finally, the addition of 6 at.% Si prevents the internal oxidation completely and leads to an earlier growth of a protective oxide layer in contact with the alloy as well as to a further reduction in the scaling rate. The role of Si in promoting the formation of protective scales in comparison with the binary alloy is examined on the basis of an extension to ternary alloys of a criterion proposed by Wagner for the transition between the internal and external oxidation of the most reactive component in binary alloys.     

KCl‐induced corrosion of Ni‐based alloys containing 35–45 wt% Cr

Ni–(35–45)Cr–4Nb alloys containing different fractions of α‐Cr were exposed to potassium chloride (KCl)‐induced corrosion. The corrosion exposures were carried out for 168 hr at 600°C in a 15% (vol/vol) H₂O (g) + 5% (vol/vol) O₂ (g) + N₂ (g; balance) atmosphere using KCl‐free (reference) and predeposited KCl samples. To mimic the KCl deposition in real boilers, 24 hr exposures where KCl vapor condensed continuously onto samples were also performed. The corrosion attack of the studied materials increased significantly when KCl was present compared to the KCl‐free samples. For the KCl exposures, the corrosion attack drastically increased when a significant α‐Cr fraction was present. α‐Cr was either selectively attacked or dissolved through solid‐state diffusion and a layered build‐up of the outer external scale of K₂CrO₄ and chromia could be observed. For the in situ condensed KCl exposure, severe corrosion was observed already within the 24 hr exposure, indicating a higher corrosion rate compared with when KCl was predeposited.     

Hydrogenation of CaLi2−xMgx (0 ≤ x ≤ 2) with C14 Laves phase structure

CaLi_2−xMg_x (0 ≤ x ≤ 2) which has the C14-type Laves phase structure has been successfully synthesized and hydrogenated. The C14-type Laves phase structure was kept after hydrogenation of CaLi_2−xMg_x (x = 0.2, 0.5, 1). After hydrogenation of CaLi₂ and CaMg₂, the Laves phase disappeared. The CaH₂ and LiH phases were formed from CaLi₂ and the CaH₂ and Mg phases from CaMg₂, respectively. CaLi_2−xMg_x (0 < x < 2) ternary alloys formed stable hydride phases with the C14-type Laves phase structure in contrast to CaLi₂ and CaMg₂ binary alloys.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.