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.     

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.     

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.     

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.     

The effect of Si additions on the high temperature oxidation of a ternary Ni-10Cr-4Al alloy in 1 atm O2 at 1100 °C

The oxidation of four Ni-10Cr-ySi-4Al alloys was studied at 1100 °C to examine the effects of Si additions (from 2 to 6 at.%) on the behavior of the alloy Ni-10Cr-4Al. Addition of 2 at.% Si prevented completely nickel oxidation, but could not form alumina scales. Larger Si additions produced alumina only over part of the alloy surface (about 20% with 4 at.% Si and 30% with 6 at.% Si), but could not prevent completely the internal oxidation of Al. The results are interpreted by extending to quaternary alloys the mechanism of the third-element effect already proposed for ternary alloys.     

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.     

Cyclic oxidation behaviour of electrodeposited Ni3Al-CeO2 base coatings at 1050 °C

This paper presents the cyclic oxidation behaviour of electrodeposited pure, nano CeO₂ (9-15 nm)- and micron CeO₂ (5 μm)-modified Ni₃Al coatings on Fe-Ni-Cr substrate at 1050 °C for periods up to 500 h. The pure Ni₃Al coating had a marginal resistance to cyclic oxidation at 1050 °C, while the CeO₂-dispersed Ni₃Al coatings showed much better cyclic oxidation resistance. This difference was attributed to many beneficial effects of CeO₂ including changing the growth mechanism of α-Al₂O₃ scale, reducing the growth rate of the scale, improving mechanical properties of the scale, and reducing void formation at the scale/coating interface and at the scale-grain boundaries.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted 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.     

Influence of grain size on the oxidation behavior of NbCr2 alloys at 950-1200 °C

The oxidation behavior of mechanically alloyed microcrystalline NbCr₂ intermetallics was investigated at 950-1200 °C in air by SEM in comparison with coarse-grain cast alloys. Results indicate that the mechanically alloyed alloys possess a better oxidation resistance and are less permeable to nitrogen than the cast alloys. At 1200 °C, the mechanically alloyed NbCr₂ alloys show a better resistance to scale spallation than the cast materials. The differences observed above are attributed to the finer grains increasing the relaxation of the oxide scale stress and improving the adhesion of the oxide layer on the matrix.     

Oxidation of iron at 400-600 °C in dry and wet O2

The oxidation of iron in dry and wet O₂ at 400-600 °C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O₂, water vapour accelerates iron oxidation at 500 and 600 °C. At 400 and 500 °C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 °C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 °C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport.     

Oxidation of a quaternary three-phase Cu-20Ni-20Cr-5Fe alloy at 700-900 °C in 1 atm of pure O2

The oxidation of a quaternary Cu-Ni-Cr-Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu-20Ni-20Cr-5Fe), was studied at 700-900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu-20Ni-20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu-20Ni-20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.     

Oxidation of Cr2AlC at 1300 °C in air

High purity, dense Cr₂AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr₂O₃ but Al₂O₃. Since Al was consumed to produce the Al₂O₃, Al-depletion and Cr-enrichment occurred underneath the Al₂O₃ layer. This led to the formation of a Cr₇C₃ layer containing voids. These grew during oxidation, eventually destroying the Cr₇C₃ layer formed on the unoxidized Cr₂AlC matrix.     

Corrosion behavior of three iron-based model alloys in reducing atmospheres containing HCl and H2S at 600 °C

The corrosion of three Fe-xCr alloys (x = 8, 12, 18 wt.%) was examined at 600 °C in reducing atmospheres containing HCl and H₂S with two different H₂S contents and compared with the behavior of the same materials in H₂S-free H₂-CO₂ and H₂-HCl-CO₂ mixtures producing similar oxygen and chlorine pressures. Exposure to the low-H₂S gas mixture had only a reduced effect on the behavior of Fe-8Cr and Fe-18Cr, but increased significantly the corrosion rate of Fe-12Cr. Increasing the H₂S level accelerated the corrosion of all the alloys, but particularly that of Fe-18Cr. In both cases only minor amounts of sulfur and chlorine were present close to the alloy/scale interface. An increase of the Cr content reduced the corrosion rate in both H₂S gas mixtures, especially in the range 8-12 wt.% Cr, due to a larger volume fraction of Cr₂O₃ in the scale. The results have been discussed on the basis of the thermodynamic stability diagrams of the Fe-O-Cl-S and Cr-O-Cl-S systems.     

The influence of microstructure on the oxidation of duplex stainless steels in simulated propane combustion products at 1000 °C

A low nickel Type S32101 duplex stainless steel has been oxidised in simulated industrial reheating conditions. The surfaces have been studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Observations show that local breakaway regions (LBRs) form on the austenitic regions whereas thinner oxides are observed on the ferritic regions of the substrate. The reason proposed for these differences is the formation of a continuous oxide layer on the ferrite region and a discontinuous layer on the austenitic region during the early stages of oxidation. The chemical composition of these LBRs have been shown to be oxide islands of iron and manganese and oxide craters of chromium rich oxides. The more protective regions consist of chromium and manganese rich oxides. A silica layer formed below the oxide which may be attributable to a slight enrichment of silicon in the ferritic regions or due to faster rates of diffusion in ferrite.     

Corrosion of Ti3AlC2 at 800–1100 °C in Ar–0.2% SO2 gas atmosphere

Ti₃AlC₂ was corroded between 800 and 1100 °C in an Ar–0.2% SO₂ gas atmosphere according to the equation: Ti₃AlC₂ + O₂ → rutile-TiO₂ + α-Al₂O₃ + (CO or CO₂). The scales that formed on the Ti₃AlC₂ were thin and rich in α-Al₂O₃, whose growth rate was exceedingly slow. The TiO₂ was present either as the outermost surface scale or a mixture inside the α-Al₂O₃-rich scale. In the Ti₃AlC₂, the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti₃AlC₂ over TiAl.     

Sulphidation/oxidation behaviour of TiAlCr and Al2Au coated Ti45Al8Nb alloy at 750 °C

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al₂Au deposited by physical vapour deposition. The coated alloy was subjected to a H₂/H₂S/H₂O yielding pS₂ - 10⁻¹ Pa and pO₂ - 10⁻¹⁸ Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb - base alloy, Al₂O₃, TiO₂ and Cr₂S₃ developed. For the Al₂Au coated Ti45Al8Nb samples an Al₂O₃ scale containing TiO₂ nodules was observed at the surface.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

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.