Water Vapor Effects on the Oxidation Behavior of Fe–Cr and Ni–Cr Alloys in Atmospheres Relevant to Oxy-fuel Combustion

The oxidation behavior of a number of Fe–Cr- and Ni–Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650?°C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O₂) CO₂?+?30%H₂O and Ar?+?30%H₂O gases. Rapidly growing iron oxides appear to be porous and gas permeable. Transition from non-protective to protective oxidation occurs on alloys with higher Cr contents between 13.5 and 22?wt% in H₂O. Excess oxygen, usually found in the actual oxyfuel combustion environments, disrupts the selective oxidation of Fe–Cr alloys by accelerating vaporization of early-formed Cr₂O₃ in combination with accelerated chromia growth induced by the H₂O. Rapid Cr consumption leads to the nucleation and rapid growth of iron oxides. On the contrary, Ni–Cr alloys are less affected by the presence of H₂O and excess O₂. The difference between Fe–Cr and Ni–Cr alloys is not clear but is postulated to involve less acceleration of chromia growth by water vapor for the latter group of alloys.     

On the Pre-oxidation Treatments of Four Commercial Ni-Based Superalloys in Air and in Ar–H<Subscript>2</Subscript>O at 950 °C

The short-term oxidation behaviour of RA 602CA, Inconel 693, Manaurite 40XO and Sumitomo 696, which are four alloys recommended for hydrocarbon processing, was studied both in air and in Ar–H₂O to determine the conditions of pre-oxidation treatments. Regardless of the considered material, the oxidation rate at 950 °C was systematically higher in Ar–H₂O than in dry air. Surface examination of the Al-containing alloys indicated that they were not uniformly oxidized all over the surface. All Al-containing alloys (from 1.6 to 3.2% wt) formed an external protective alumina scale and behaved as alumina-forming alloys in dry air at 950 °C. In contrast, these alloys developed a rate-controlling chromia scale and severe internal oxidation in a H₂O-containing atmosphere. Compared with their oxidation behaviour in air + H₂O, the phenomenon was significantly enhanced in the atmosphere that coupled water vapour with a low oxygen pressure. Consequently, the Al-containing alloys should not be pre-oxidized in a water vapour atmosphere prior to long exposure to a corrosive atmosphere. In contrast, the chromia-forming Al-free 696 alloy exhibited identical oxidation behaviour in both atmospheres, demonstrating that the degradation of this alloy was not significantly affected by water vapour.     

Effect of water vapor on high-temperature oxidation of FeCr alloys

The suppression of protective chromia scale formation in water vapor containing service environments limits in many cases the upper application temperature of high-Cr martensitic and ferritic steels. The present paper discusses the mechanisms which are responsible for this technologically important effect, using results of oxidation tests with two types of FeCr model alloys in Ar-O₂, Ar-O₂-H₂O, and Ar(-H₂)-H₂O mixtures. The data shows that in atmospheres with a high ratio of water vapor to oxygen, Cr exhibits a higher tendency to become internally oxidized than in dry Ar-O₂, or e.g. air. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale and/or to enhance formation of volatile Cr species, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy, likely by incorporation of hydrogen.     

Protective and non-protective scale formation of NiCr alloys in water vapour containing high- and low-pO2 gases

The oxidation behaviour of NiCr alloys with Cr contents of 10, 20 and 25 wt.%, respectively, were studied in Ar-O₂, Ar-O₂-H₂O and Ar-H₂O mixtures. TG and SEM analysis revealed that the chromia scales formed on Ni-25Cr in the wet gases did not differ substantially from those formed in Ar-O₂. For the two “borderline” alloys Ni-10Cr and Ni-20Cr, addition of water vapour to Ar-O₂ hampered the formation of a protective chromia scale which, especially for Ni-10Cr, resulted in substantially increased scale growth rates compared to exposures in dry gas. Different from numerous observations described in the literature for “borderline” FeCr alloys with intermediate Cr contents of 10-20%, the corresponding NiCr alloys showed in Ar-H₂O a smaller tendency for non-protective scale formation than in Ar-O₂-H₂O. This is caused by the decreasing growth rate of NiO with decreasing pO₂ of the test gas, with the secondary effect that external chromia scale formation is promoted in low-pO₂ gases such as Ar-H₂O. Even if the alloy Cr content was too low to obtain external chromia scale formation, the oxidation rate in Ar-H₂O would, in contrast to low-Cr FeCr alloys, be quite small due to the very slow growth rate of NiO in this low-pO₂ gas.     

The Effect of Water-Vapor Content and Gas Flow Rate on the Oxidation Mechanism of a 10%Cr-Ferritic Steel in Ar-H2O Mixtures

The oxidation behavior of a Ferritic 10%Cr steel in Ar-H₂O mixtures was investigated at temperatures between 550 and 650°C. The studies aimed at elucidating the effect of water-vapor content as well as the gas flow rate on the mechanisms of oxide-scale formation. It was found that, the oxide types and morphologies depend on time and exposure conditions. An important observation is that H₂ produced by the reaction of water vapor with the steel can play a significant role in the oxidation process. First, it affects the possibility to form an external hematite layer and if the latter is not present, it tends to decrease the growth rate of the magnetite-base oxide scale. The extend by which the H₂ affects the oxidation behavior depends on the gas-flow conditions, the water-vapor content and the exposure time.     

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O₂, CO₂, and H₂O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

High temperature oxidation of Fe13CrxAl alloys in airH2O vapour mixtures

High-temperature oxidation in air of Fe13CrxAl alloys containing up to 4.5 Al has been studied in the temperature range 680–980°C. A primary aim was to study the oxidation as a function of the Al concentration in the alloys and the water vapour content (from 0.03 to 2.3 vol. % H₂O) in the oxidizing gas.The Fe13Cr alloy exhibits an initial protective behaviour due to formation of protective Cr₂O₃ and $\text{Cr}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ films. This protective stage is succeeded by breakaway oxidation due to depletion of chromium in the alloy beneath the oxide scale; double-layered, porous scales develop and chromium is internally oxidized. Under these conditions the oxidation rate increases significantly with increased water vapour content.Additions of aluminium modify the oxidation behaviour. At sufficiently high Al concentrations protective scales of AL₂O₃ (α-AL₂O₃) are found. The critical Al concentration necessary for selective Al oxidation increases as the temperature increases. Thus at 980°C 4Al is necessary while at 680°C 1Al provides excellent oxidation resistance. When continuous Al₂O₃ scales are formed the oxidation is not significantly dependent on the water vapour content in the air.At Al contents below the critical Al concentration, porous, multilayered scales are formed enriched with Al₂O₃ in the innermost layer. Under these conditions the oxidation rate increases significantly with water vapour content.The results strongly suggest that the increased rate of oxidation with water vapour content is due to gaseous transport across voids and pores with H₂O, H₂, and O₂ as carrier gases.     

Influence of nitrogen and water vapor on the oxidation of thorium between 400 and 1000° C

The reaction of thorium with air, oxygen, oxygen-nitrogen, and oxygen-water vapor atmospheres has been studied in the temperature range of 400–1000°C using thermogravimetric analysis and metallographic and x-ray diffraction examination. It was demonstrated that H₂O and N₂ as well as O₂ are involved in the reaction of thorium with air. Oxidation in pure oxygen followed a relatively slow rate. Faster rates and nonisothermal conditions were observed under certain conditions with mixed atmospheres. Above 600° C,N₂ caused accelerated oxidation and scale color changes. The effects of H₂O were similar but occurred below 600°C. The rate laws followed for reaction with air are complex combinations of cubic, parabolic, and linear laws.     

Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H₂O was investigated. The oxidation kinetics of Ni and the alloys in air + H₂O were very similar, but the mass gains of Ni and each alloy were smaller in air + H₂O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al₂O₃ precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H₂O. The NiO scale formed in air + H₂O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H₂O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H₂O.     

The Effect of Water Vapor on Selective Oxidation of Fe–Cr Alloys

Binary Fe–Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 °C in Ar–20%O₂, Ar–7%H₂O and Ar–4%H₂−7%H₂O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar–O₂ gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an “easy” explanation of the frequently observed effect that Fe–Cr alloys with intermediate Cr contents (e.g. 10–20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.     

Sulfidation behaviour of nickel aluminides

The sulfidation behaviour of four nickel aluminium alloys containing 25 to 45 at.% Al was studied over the temperature range of 750 to 950°C in a gas mixture of H₂-H₂S (0.1 to 10 vol.%). The sulfidation kinetics were determined using a continous weight gain system. The corrosion products were examined by SEM, EDX and XRD. Sulfidation in H₂-H₂S gas mixtures formed bilayered scales consisting of an outer layer of Ni₃S₂ and an inner layer of NiAl_3,5S_5,5 on all alloys regardless of the different aluminium contents. In H₂-H₂S gas mixtures the sulfidation kinetic generally followed the parabolic rate law for all alloys. The influence of aluminium content on corrosion rate was relatively low. The influence of low oxygen partial pressure on sulfidation was investigated in H₂-H₂S-H₂O mixtures. In these atmospheres the corrosion mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The internal oxidation zone consisted of a mixture of γ-Ni₃Al and Al₂O₃. On the alloys containing 36 and 45 at.% Al local attack occurred, fast growing pocks were observed after an incubation period. Nickel aluminides show this corrosion phenomena only in H₂-H₂S-H₂O mixtures. An interruption of the H₂S gas flow stops the running internal oxidation. In flowing H₂-H₂O atmospheres no internal oxidation was observed. These facts prove that H₂S is necessary for starting and maintaining the internal oxidation of the nickel aluminides.     

Oxidation of molten Al-Mg alloy in air,air-SO2, and air-H2S atmospheres

Rates of oxidation of 8 g samples of molten Al-Mg alloys in air, air-SO₂, and air-H₂S atmospheres were determined at 750°C by gravimetry. Weight gains in air containing 10 or more volume percent SO₂ averaged 0.2 % in up to 100 hr, whereas samples heated in air alone gained over 5% weight in 20 hr and 10% in 70 hr. Heating for only 1 hr in SO₂ concentrations of 10% or greater prevented extensive oxidation during additional heating in air alone. H₂S was more effective than SO₂ heating for as little as 1/2hr in 0.25 vol. % H₂S prevented extensive oxidation during subsequent heating for at least 90 hr in air alone. The inhibiting effect of either SO₂ or H₂S probably involves oxidation to SO₃. This reacts with portions of the initial protective amorphous MgO film to form MgSO₄, which has a high volume quotient and maintains protection as nonprotective crystalline MgO forms at the end of an induction period.     

Effect of Water Vapor on the Oxidation Behavior of Fe–1.5Si in Air at 1073 and 1273 K

The oxidation behavior of Fe–1.5Si was investigated at 1073 and 1273 K in air, air–H₂O, Ar–H₂O, O₂–H₂O, and O₂ atmospheres. The extent of corrosion in atmospheres containing H₂O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O₂–H₂O, air–H₂O, and Ar–H₂O. With increasing H₂O contents in air–H₂O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker was located between the external Fe₂O₃ (Fe₃O₄ included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO+Fe₂SiO₄ layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO+Fe₂SiO₄ layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO+Fe₂SiO₄ mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner FeO+Fe₂SiO₄ layer.     

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700–900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr₂O₄ over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.     

The oxidation and decarburizing of Fe-C alloys in air and the influence of relative humidity

Isothermal oxidation treatments were carried out on an Fe-C alloy (0.4 % C): (a) in almost dry air around A₁, and also with an Fe-C alloy (0.5% C) and IRSID pure iron; (b) in dry air ( nm Hg); (c) in almost dry air(1–2% water vapor) at 700° C; and (d) in moist air (31% water vapor). Theresults are as follows: The rate of oxidation at a temperature below A₁depends chiefly on alloy structure, i.e., on thermal history of the sample.The water vapor content of the air strongly influences the scale adherenceas well as the rate of oxidation of the Fe-C alloys below A₁, but has virtuallyno effect on the rate of oxidation of pure iron. Under the same conditions, avery light decarburization of metal occurs in air, whereas no decarburizationoccurred in air with 13% water vapor.     

Influence of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ni–Al Alloys

Ni–Al coating alloys, which are commonly used in gas turbine engines operating in marine environments, are highly susceptible to hot corrosion attack. The effect of alloy composition and exposure conditions on the development of a protective alumina scale, which is important for the hot corrosion resistance of the alloy, and how they affect the transition of alumina from the θ to the α polymorph have been evaluated. A series of Ni–Al model alloys with a base composition of Ni–36 at.% Al, and 5 at.% additions of Cr, Pt and Si were exposed in dry air and in air–10%H₂O at 900 °C. The presence of water vapor in the gas led to higher oxidation rates and retarded the θ- to the α-Al₂O₃ transformation. The oxidation behavior of the alloys and the alumina polymorph which formed differed depending on the alloying element considered. Additions of Cr accelerated the θ to α transformation, while Pt and Si retarded it.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C‐5, P‐11 and P‐22 steel samples were tested in two different environments: air and CO₂ + 2H₂O + 7.52N₂, a gas composition which simulates the combustion products of natural gas, at 450 and 500 °C. P‐22 steel had the best oxidation resistance at both temperatures in air. In CO₂ + 2H₂O + 7.52N₂ environment, the oxidations of all the steels were accelerated and C‐5 exhibited better oxidation resistance than P‐22 and P‐11. Analyses of oxidation products by optical microscopy, SEM‐EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P‐22 in air was explained with reference to the formation of a protective Cr‐containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO₂ + 2H₂O + 7.52N₂ environment were attributed to the depletion of protective Cr‐containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe? Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P‐22 (2.25%) even at 450 and 500 °C.     

Effect of Water Vapor on the High-Temperature Oxidation of Pure Ni

The high-temperature oxidation behavior of pure Ni in air and Ar with and without 30?vol%H₂O at 1,000?°C was investigated to understand the effects of water–vapor on the resulting oxidation kinetics and scale structures. It was found that water–vapor significantly affected the morphology and scale structure of NiO. A duplex NiO scale with a powder-like outer and dense inner NiO layer developed when the Ni was oxidized in atmospheres containing water–vapor. The grain size of the dense inner NiO layer was much smaller than that formed in dry atmospheres. The growth of the powder-like NiO required outward diffusion of Ni and its continued formation occurred at the interface between the powder and dense NiO layers. The dense inner NiO layer grew outward and incorporated the powder-like NiO particles and the resulting grain size of the inner layer was smaller in the presence of water–vapor. The water–vapor is speculated to have prevented sintering of NiO particles during growth of the NiO scale.