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.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

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.     

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.     

Effects of yttrium and zirconium additions on the high-temperature sulfidation behavior of an Fe−10Mo−20Al−8Mn alloy

The effects of zirconium and yttrium additions on the sulfidation behavior of an Fe–10Mo–20Al–8Mn(a/o, atom percent) alloy were examined in flowing H₂/H₂S gas of 4Pa sulfur partial pressure at 900°C. Good scale protection was obtained during the initial reaction stage of the base alloy. However, after 7–8 hr, the formation of internal (Mn,Fe) Al₂S₄ platelets triggered breakdown of the protective scale. The reaction products of the zirconium-containing alloy were nonprotective. Yttrium addition resulted in an Y(Fe_1–xAl_x)₁₂ network along the alloy ferrite grain boundaries. Preferential sulfidation of this phase led to almost complete manganese depletion from the engulfed ferrite, and consequently avoided the manganese-promoted scale breakdown.After an even slower initial stage, this alloy sulfidized at a parabolic rate two orders of magnitude slower than that of pure iron. The protection during the initial and following stages was believed to be provided by an Al₂O₃-containing layer and an Al_0.55Mo₂S₄+Fe_xMo₆S_8–z layer, respectively. The formation of Al₂O₃ is thought to be due to oxygen impurities in the H₂S gas, which cannot be removed by conventional means.     

Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

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 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.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

High-temperature oxidation of directionally solidified Ni-Cr-Nb-AI (γ/γ′-δ) eutectic alloys

The oxidation of Ni-23.1Nb-4.4Al and Ni-19.7Nb-6 Cr-2.5Al alloys in air at temperatures in the range 870–1100°C has been studied for times up to 168 hr, in the as-cast, slowly cooled, and directionally solidified forms. The oxidation rate decreases with increasing temperature for the ternary alloy, and this appears to be due to the increasing tendency to establish a continuous Al₂O₃ layer at the metal surface, although at no temperature in this range is a complete layer established. At the lowest temperature the -Ni₃Nb lamellae are preferentially oxidized, with fingers of oxide extending into the metal, but at 900°C and above a continuous single-phase 8-free layer is established at the metal surface very early in the oxidation. The oxidation rate of the quaternary alloy increases with increasing temperature. At the lower temperatures a continuous Al₂O₃ layer is established at the metal surface, but at the highest temperature the aluminum oxidizes internally and a continuous layer is not established, internal oxidation penetrating down the lamellae. It appears that niobium, like chromium, is able to promote the formation of external Al₂O₃ layers; if this fact is accepted, the beneficial role of chromium in these alloys is difficult to explain.     

The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr₂O₃ layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr₂O₃ layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that the-phase has transferred to the -phase at the temperature ofthe oxidation test (1150°C). Conversely, -phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the than in the -phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.     

Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and -Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this -Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this -Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.     

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H₂/H₂S gases of 4 Pa sulfur partial pressureat 900°C. Al₂O₃ was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al₂O₃+FeAl₂O₄,and nonreacting Fe₃Mo₂, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl₂O₃ layer further decreased the alloy reaction rate.     

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti–34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al₂O₃ barrier being formed. This is probably due to the doping of those elements in TiO₂. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al₂O₃ layer as the second layer from the outer surface and the relatively massive precipitation of Al₂O₃ in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.     

Enhanced Oxygen Diffusion Along Internal Oxide–Metal Matrix Interfaces in Ni–Al Alloys During Internal Oxidation

A study of the internal oxidation of dilute Ni–Al alloys in an NiO/Ni Rhines pack was performed at 800, 1000, and 1100°C. Considerable deviations from the classical internal oxidation model have been observed. The rate of internal oxidation depends not only on the concentration of the alloying element but also on its nature, which contributes to determining the size, shape, orientation and distribution of the internal oxide precipitates. For instance, the precipitates in the Ni–Al alloys are continuous rods, arranged in a cone-shaped configuration that extends from the surface to the internal oxide front. The observed depths of internal oxidation for the various concentrations of aluminum are discussed and related to the morphologies of the internal oxide precipitates. The apparent N^(s) _oD_o values determined from internal oxide penetrations increase with increasing solute content in the alloy. It is postulated that diffusivity of oxygen is enhanced along the internal oxide–metal matrix interface compared with that in the metal matrix.     

High-Temperature Oxidation Behavior of ODS–Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe₃Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was -Al₂O₃, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe₃Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe₃Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.     

High‐temperature oxidation resistance of Al2O3‐forming heat‐resisting alloys with noble metal and rare earth additions

High‐temperature oxidation behavior of Al₂O₃‐forming heat‐resisting alloys with noble metal (palladium, platinum, gold) and rare earth (yttrium) additions was studied in oxidizing atmospheres (oxygen, oxygen‐water vapor) for 18 ks at 1473, 1573, and 1673 K, by mass gain measurements, amount of spalled oxide, observation of surface appearance, X‐ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Mass gains of all the Al₂O₃‐forming Fe‐20Cr‐4Al alloys increased with increasing oxidation temperatures in both oxidation conditions. After oxidation in oxygen, the mass gains of the alloys with noble metal were almost the same values after any oxidation temperature. The mass gain of the alloys with yttrium decreased with increase in yttrium addition up to 0.1 mass%, and then tended to increase with 0.5 mass% yttrium addition at all oxidation temperatures studied. The amount of spalled oxide from the Fe‐20Cr‐4Al (A4) alloy showed the biggest value at 1573 K‐oxidation, and then decreased in the order of 1473 K, 1673 K. On the other hand, the amount of spalled oxide from the other alloys decreased compared with the A4 alloy. No spalled oxide from 0.5Pt, 0.05Y, and 0.5Y alloys was observed at any oxidation temperature. After oxidation in an oxygen‐water vapor mixture (dew point: 353 K), the mass gain of all the alloys showed similar values to that obtained in oxygen after any oxidation temperature. The amount of spalled oxide from the A4 alloy was about the same after oxidation at 1473 and 1573 K in oxygen, but then was higher when oxidized at 1673 K. The amount of spalled oxide from the other alloys obtained in oxygen–water vapor increased compared with those obtained in oxygen. On the other hand, the amount of spalled oxide from the 0.5Y alloy was zero after any oxidation temperature, and that from the 0.5Pt alloy was also zero after 1673 K‐oxidation.