Criteria for the formation of protective Al2O3 scales on Fe-Al and Fe-Cr-Al alloys

The conditions for the formation of external alumina scales on binary Fe-Al alloys and the nature of the third-element effect due to chromium additions have been investigated by studying the oxidation at 1000 °C in 1 atm O₂ of a binary Fe-10 at.% Al alloy (Fe-10Al) and of two ternary Fe-Cr-10 at.% Al alloys containing 5 and 10 at.% chromium (Fe-5Cr-10Al and Fe-10Cr-10Al, respectively). An Al-rich scale developed initially on Fe-10Al was subsequently replaced by a multi-layered scale containing mixtures of Fe and Al oxides plus a large number of Fe-rich oxide nodules: internal aluminum oxidation was essentially absent from this alloy. Addition of 5 at.% chromium to Fe-10Al did not suppress the formation of nodules, but they were eventually healed by the growth of an alumina layer at their base, resulting in a significant reduction of the oxidation rate. Finally, the alloy with 10 at.% Cr formed continuous external alumina scales without any Fe-rich nodule. Thus, the addition of sufficient amounts of chromium to Fe-10Al produces a third-element effect as expected. However, the process found in this alloy system does not involve a prevention of the internal oxidation of Al. Instead, it shows a transition from the growth of mixed Fe- and Al-rich external scales directly to an external Al₂O₃ scale formation. An interpretation of this kind of mechanism involving a third-element is presented along with a prediction of the critical Al contents required to produce the various possible scaling modes on binary Fe-Al alloys.     

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.     

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 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.     

High temperature corrosion of cast heat resisting steels in CO + CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe-25Cr-35Ni, Nb modified) were exposed to CO/CO₂ gases at 982 and 1080 °C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions.     

High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element

Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the α-Al₂O₃ film which is formed on the surface provided temperature is above 900 °C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales.     

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.     

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.     

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.     

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.     

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.     

Investigation of the HCl (g) attack on pre-oxidized pure Fe, Cr, Ni and commercial 304 steel at 400 °C

The paper presents the results from an investigation studying the ability of pre-oxidized metals and alloys to withstand chlorine attack in the form of gaseous HCl. The materials under investigation were pure Fe (s), Cr (s), Ni (s), and a commercial 18Cr-10Ni-Fe (304) alloy. The samples were pre-oxidized in different well defined environments, dry 10 vol.% O₂ (g) + N₂ (bal.), 10 vol.% O₂ (g) + 5 vol.% H₂O (g) + N₂ (bal.) and 10 vol.% O₂ (g) + 250 vppm SO₂ (g) + N₂ (bal.) for 24 h at 400 °C using a horizontal tube furnace. Afterwards the oxide films were characterized by GI-XRD, FEG-SEM, XPS and ToF-SIMS. The samples were then exposed further in 10 vol.% O₂ (g) + 500 vppm HCl (g) + x (x = 5 vol.% H₂O (g), 250 vppm SO₂ (g)) + N₂ (bal.). The exposure time was 100 h and after the exposures during the cool down process the reaction chamber was flushed with dry 10 vol.% O₂ (g) + N₂ (bal.). The corroded samples were then examined by the same techniques mentioned before. HCl (g) showed mainly to be aggressive toward the Fe (s) samples that form a relatively thick and porous oxide scale consisting of layered Fe₂O₃ (s)/Fe₃O₄ (s) during pre-oxidation, and the aggressiveness did not depend on the pre-oxidation conditions. All the other materials formed thin and dense oxides (20-100 nm) during pre-oxidation, and they did not suffer accelerated oxidation caused by HCl (g) during the subsequent exposure. The only exception was Ni (s) that had been pre-oxidized in an atmosphere containing SO₂ (g), in this case Ni sulphides and sulphates were formed during pre-oxidation which in turn caused accelerated oxidation to Ni when subsequently exposed to HCl (g). HCl (g) readily reacts with NiSO₄ (s) and Ni₃S₂ (s) and forms NiCl₂ (s) and SO₃ (g).     

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.     

Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe-25Cr-35Ni-1Nb, wt.%) was oxidised and carburised in CO-CO₂ corresponding to a_C = 0.1 and p_O2 = 3 × 10⁻¹⁶ atm at 1080 °C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised, solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation, assuming the scale-metal interface to be fixed. The pores are concluded to be Kirkendall voids.     

KCl-induced high temperature corrosion of the austenitic Fe-Cr-Ni alloys 304L and Sanicro 28 at 600 °C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 °C in O₂ + H₂O environment is reported. 0.10 mg/cm² KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O₂. In O₂ + H₂O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K₂CrO₄. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared.     

Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

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.     

High-Temperature Scaling of Cu–Al and Cu–Cr–Al Alloys: An Example of a Non-Classical Third-Element Effect

The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O₂. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al₂O₃ layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al₂O₃ layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al₂O₃–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al₂O₃ and Cr₂O₃.     

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.     

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.