A comparison of the oxidation behaviours of Al2O3 formers and Cr2O3 formers at 700 °C – Oxide solid solutions acting as a template for nucleation

The oxidation behaviour of a number of ferritic iron based commercial steels and model alloys containing 6 and 9 wt% Cr and 0–2.5 wt% Al have been studied at 700 °C. The oxidation time ranged from 5 min to 500 h and the atmosphere consisted of flowing dry synthetic air. The oxide layers formed were analysed by SEM, GI-XRD and ToF-SIMS. The material without Al formed a (Cr,Fe)₂O₃ film with an Fe enrichment in the outer part of the layer. The Al containing alloys showed more complex oxidation behaviour. The oxidation started initially by formation of (Cr,Fe)₂O₃ with an Cr enriched inner part. With time Al was oxidized and dissolved in the inner Cr rich part of the oxide. This process continued until it eventually was transformed into α-Al₂O₃ with minute amount of Fe in the outer and Cr in the inner part of the oxide. The thickness of all oxide films ranged from 20 to 400 nm apart from the material that contained 9% Cr and no Al, which experienced breakaway oxidation after 500 h at 700 °C. This means that materials alloyed with small amounts of Al must also be considered to be protective at 700 °C, as the thicknesses of the Al₂O₃ oxides was comparable with the ones not containing Al, and as they do not experience breakaway corrosion.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C

The oxidation behavior of two alloys containing different content of Al and Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 °C. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 °C followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 °C followed a parabolic law. The weight gain of B2 alloy was 18.9 mg/cm² after oxidation at 1200 °C for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 °C. The oxide scales of B2 alloy consisted of CrNbO₄, TiNb₂O₇ and SiO₂, which were relatively compact and protective. In addition, the oxidation mechanism of Nb-Si based alloys were also discussed.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Microstructure and oxidation behaviour of Ir-rich Ir–Al binary alloys

In the current study, alloys of Ir–11Al, Ir–23Al, Ir–30Al, Ir–41Al and Ir–45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir–Al alloys. Ir(Al)_ss and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100 °C, the Ir(Al)_ss and β-IrAl individually changed to porous and dense Al₂O₃. The microstructure of the oxide scale formed on Ir–23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir–45Al followed a parabolic law, showing the best oxidation resistance among the Ir–Al alloys.     

Improvement of the oxidation-proof property and the scale structure of Mo3Si intermetallic alloy through the addition of chromium and aluminum elements

The influences of chromium and aluminum additions to Mo₃Si intermetallic alloy on the microstructure, mechanical and oxidation properties as well as the scale evolution were investigated for alloys with the composition of Mo_75 − yCr_ySi_25 − xAl_x (y = 0, 15, 30, 45, 60, 75 mol%, x = 0, 5, 10 mol%). Cr and Al were added for the aim of not only improving the oxidation property but also changing the compound type from Daltonide to Berthollide ones. The alloys were prepared by arc-melt method and the button ingots were homogenized at 1873 K for 10 h. It was found that the homogenized (Mo,Cr)₃Si alloy consists of continuous solid solution. During oxidation test at 1173 K, the mass of Mo₃Si alloy decreased remarkably due to the evaporation of molybdenum oxides. However, the improvement of the oxidation resistance was confirmed when chromium was added up to at least 15 mol%. In particular, the (Mo,Cr)₃Si alloys with above 30 mol%Cr showed excellent oxidation resistance through the formation of the passive oxide-scale layer. Although the effect is smaller, aluminum addition also contributed to the improvement of oxidation resistance.     

High-temperature oxidation behavior of minor Hf doped NiAl alloy in dry and humid atmospheres

Oxidation behavior of NiAl and 0.05 at.% Hf doped NiAl alloys were investigated at 1100 °C in dry and humid atmospheres. Hf doping significantly improved the cyclic oxidation resistance. Water vapour promoted the formation of voids at the scale/alloy interface and accelerated scale spallation. Also, water vapour had effect on the phase transformation from θ- to α-Al₂O₃ at the initial oxidation stage. In humid atmosphere, more Hf segregated at the scale/alloy interface to form oxide pegs. Pre-oxidation process in O₂ + Ar could compromise the effect of humid atmosphere on the oxidation kinetics of the NiAl alloys.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.     

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr₂O₄ along with FeCr₂O₄ and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.     

Formation and oxidation resistance of Hf and Al modified silicide coating on Nb–Si based alloy

Codeposition of Si, Al and Hf were prepared by pack cementation at 1300 °C for 10 h. The results show that the coating is composed of a thick (Nb, X)Si₂ outer layer, a (Ti, Nb)₅Si₄ middle layer and a thin discontinuous (Cr, Al)₂(Nb, Ti) inner layer. The mass gain of the coating is only 4.12 mg/cm² after isothermal oxidation at 1250 °C for 100 h. Some “oxide pegs” form at the interface of the oxide scale and coating. The coating exhibits good cyclic oxidation resistance due to the improved adhesion between the oxide scale and coating.     

Improvement in oxidation resistance of a Ni3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni₃Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni₂Al₃ and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al₂O₃ scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.     

A thermo-gravimetric and microstructural study of the oxidation of Nbss/Nb5Si3-based in situ composites with Sn addition

The effect of Sn addition on the oxidation of the Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn (at.%) alloy (JG6) in the as cast (AC) and heat treated (HT) conditions was studied at 800 °C and 1200 °C in static air using thermo-gravimetry and microstructural analysis. The oxidation kinetics, morphology and microstructure of the oxide scale and the microstructure of the bulk of the oxidised alloy were investigated. Oxidation occurred by inward oxygen anion diffusion. The oxidation of JG6 at 800 °C and 1200 °C is compared with the oxidation of Sn-free Nb–Ti–Si–Cr–Al–Mo–Hf alloys and is found to have been improved by the addition of Sn. At 800 °C pest oxidation, which was exhibited by the heat treated Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloy (JG4-HT), was eliminated by alloying with 5 at.% Sn. The elimination of pesting at 800 °C is attributed to the nature of the Nb solid solution in the alloy which consists of Sn-rich, Si-rich and Ti lean solid solution usually surrounded by Sn-poor, Si-poor and Ti-rich solid solution. The oxide scales that formed at 1200 °C on JG6 did not separate from the base metal and consisted of Nb₂O₅, TiO₂, SiO₂, HfO₂ and TiNb₂O₇. TiN, instead of TiO₂, and the (Nb,Ti)₅(Sn_1−xSi_x)₃ phase, which is considered as a ternary phase based on Nb₅Sn₂Si, are formed in the diffusion zone of the alloys JG6-AC and JG6-HT after oxidation at 1200 °C. The formation of these phases in the oxidised alloys JG6-AC and JG6-HT controlled the penetration of oxygen into the base material. The better oxidation performance of JG6-AC compared to JG6-HT at 1200 °C is attributed to the formation of Nb₃Sn in the former. It is suggested that the presence of the Sn-poor, Si-poor and Ti-rich Nb_ss in the microstructure is a key to the formation of the Nb₃Sn phase at the scale/diffusion zone interface in the JG6-AC oxidised at 1200 °C.     

Structural and mechanical properties of Cr–Al–O–N thin films grown by cathodic arc deposition

Coatings of (Cr_xAl_1?x)_δ(O_1?yN_y)_ξ with 0.33 ? x ? 0.96, 0 ? y ? 1 and 0.63 ? δ/ξ ? 1.30 were deposited using cathodic arc evaporation in N₂/O₂ reactive gas mixtures on 50 V negatively biased WC–10 wt.% Co substrates from different Cr and Al alloys with three different Cr/Al compositional ratios. For N₂ < 63% of the total gas, ternary (Cr,Al)₂O₃ films containing <1 at.% of N forms; as determined by elastic recoil detection analysis. Increasing the N₂ fraction to 75% and above results in formation of quaternary oxynitride films. Phase analyses of the films by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy show the predominance of cubic Cr–Al–N and cubic-(Cr,Al)₂O₃ solid solutions and secondary hexagonal α-(Cr,Al)₂O₃ solid solution. High Cr and Al contents result in films with higher roughness, while high N and O contents result in smoother surfaces. Nanoindentation hardness measurements showed that Al-rich oxide or nitride films have hardness values of 24–28 GPa, whereas the oxynitride films have a hardness of ～30 GPa, regardless of the Cr and Al contents. Metal cutting performance tests showed that the good wear properties are mainly correlated to the oxygen-rich coatings, regardless of the cubic or corundum fractions.     

Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments

The mechanism of corrosion product flaking on bare copper sheet and three copper-based alloys in chloride rich environments has been explored through field and laboratory exposures. The tendency for flaking is much more pronounced on Cu and Cu–4 wt%Sn than on Cu–15 wt%Zn and Cu–5 wt%Al–5 wt%Zn. This difference is explained by the initial formation of zinc and zinc–aluminum hydroxycarbonates on Cu15Zn and Cu5Al5Zn, which delays the formation of CuCl, a precursor of Cu₂(OH)₃Cl. As a result, the observed volume expansion during transformation of CuCl to Cu₂(OH)₃Cl, and concomitant corrosion product flaking, is less severe on Cu15Zn and Cu5Al5Zn than on Cu and Cu4Sn.     

High temperature oxidation characteristics of Nb–10W–XCr alloys

The effect of Cr content on the microstructure and cyclic oxidation behavior of Nb–10W–XCr alloys with four different compositions has been investigated. Experiments were conducted in air at 900 °C and 1300 °C; the oxidation kinetics have been evaluated in terms of weight change per unit area with respect to exposure time. Alloy's microstructure consists of Nb solid solution phase regions surrounded by a network of NbCr₂ Laves phase. A trend of improvement in oxidation resistance with increase of the intermetallic phase is observed at 1300 °C and oxidation kinetics follow a parabolic behavior. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates than alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄, and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature intermetallic alloy development.     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.     

Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600 °C: A microstructural investigation

Oxidation of Fe–10Cr in dry and wet O₂ was studied at 600 °C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O₂ gives a Cr-rich protective (Fe_1−xCr_x)₂O₃ scale. The same protective oxide initially forms in O₂ + H₂O environment, but after an incubation period scale breakdown is triggered by CrO₂(OH)₂ evaporation that depletes the substrate in Cr and converts (Fe_1−xCr_x)₂O₃ to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.     

Oxidation properties of self-propagating high temperature synthesized niobium disilicide

NbSi₂ monoliths were prepared by self-propagating high temperature synthesis (SHS) and hot pressing (HP) and their oxidation behavior was investigated at various temperatures (823–1123 K) in air. The combustion mode of SHS reaction was steady state combustion, and the combustion product was single-phase NbSi₂. Oxidation studies show that the highest mass gain was 0.95675 kg m⁻² at 1023 K. In cyclic oxidation, the oxidation rate was reduced and the mass gain was only 0.15507 kg m⁻². A dense protective amorphous SiO₂ scale formed at 823 K and 923 K whereas a porous multilayer SiO₂ and α/β-Nb₂O₅ oxide scales formed at and above 1023 K and spalled off. Pest oxidation of NbSi₂ monoliths was not observed in hot pressed NbSi₂ monoliths.     

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000 °C

The effects of ZrB₂ and ZrB₂ + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB₂ + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB₂ single addition, because the oxide scale formed on C–ZrB₂–SiC is denser and thinner due to the existence of glassy SiO₂. As the oxidation temperature increases, the oxidation rate of C–ZrB₂–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.