Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H₂O form a Cr₂O₃ scale in the early stage of oxidation. However, the Cr₂O₃ scale gradually degrades with time. In order to determine the effect of H₂O on the deterioration of a Cr₂O₃ scale, the evaporation behavior of Cr₂O₃ in N₂–O₂–H₂O atmospheres was investigated. The rate of mass loss in an N₂–O₂–H₂O atmosphere was found to be one order of magnitude higher than the rates in N₂–O₂ and N₂–H₂O atmospheres, indicating that deterioration of the Cr₂O₃ scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr₂O₃ is probably based on the following reactions: 1/2Cr₂O₃(s)+3/4O₂(g)+H₂O(g)=CrO₂(OH)₂(g). However, it is also speculated that the reaction, Cr₂O₃(s)+2/3O₂(g)=2CrO₃(g), affects the evaporation of Cr₂O₃ at temperatures higher than 1323 K. The evaporation rate of Cr₂O₃ is roughly comparable to the growth rate of the Cr₂O₃ scale. Therefore, a Cr₂O₃ scale can be degraded by the evaporation of Cr₂O₃.     

Microstructure and analysis of oxide scales formed on Cr-Si-Ni compacts in air and H2O-containing atmosphere

The purpose of this study was to characterize the oxide scales formed on various Cr-Si-Ni compacts at 1273 K in air and H₂O-containing atmosphere by TEM. It was found that CrSi₂-(5-20)mass%Ni compacts form double layer scales, consisting of an outer Cr₂O₃ layer and an inner SiO₂ layer. The oxide scale changed from SiO₂- to Cr₂O₃-based scale with an increase in the Ni concentration. However, it was observed that the oxide scale formed in H₂O-containing atmospheres showed local SiO₂ growth into the substrate. This result suggests that the inward oxidant diffusion promotes the local growth of SiO₂ in the H₂O-containing atmospheres.     

Oxygen Transport during the High Temperature Oxidation of Pure Nickel

The high temperature oxidation of nickel has been investigated in air under atmospheric pressure in the temperature range 600–900°C. The oxidation kinetic curves deviate from the parabolic law for temperatures over 800°C. The observation of scale morphologies and the use of two stage oxidation experiments under ¹⁶O₂/¹⁸O₂ atmospheres showed that oxygen transport through the NiO scale had to be taken into consideration during the oxidation process. Despite the main outward diffusion of Ni species through the oxide scale, the inward oxygen diffusion at lower temperatures (<800°C) or the oxygen transport, probably as molecular species, via pores or micro-cracks were found to play a major role in the formation of duplex oxide scales, made of small equiaxed oxide grains at the metal/oxide interface overgrown by larger columnar grains at the gas/oxide interface. Oxygen diffusion coefficients into thermally grown NiO scales were determined and compared to the values of Ni diffusion coefficients from the literature.     

Microstructural Investigation of Protective and Non-Protective Oxides on 11% Chromium Steel

FIB, SEM and STEM/EDX were used to investigate X20 stainless-steel samples exposed to dry O₂, or O₂ containing 40% H₂O, with a flow velocity of 0.5 cm/s or 5 cm/s, for 168 hr or 336 hr at 600°C. Thin protective Cr-rich (Cr,Fe)₂O₃ was maintained on the samples exposed to dry O₂, even after 336 hr, and on the sample exposed to O₂/H₂O mixture with the low-flow velocity (0.5 cm/s) for 168 hr. The oxide scale formed in the latter environment contained less Cr, due to Cr loss through CrO₂(OH)₂ evaporation. Breakaway oxidation occurred on the samples exposed in high-gas-flow velocity for shorter time (168 hr) or in low-gas-flow velocity (0.5 cm/s) for longer time (336 hr). The breakaway scales featured a two-layered structure: an outward-growing oxide “island” consisting of almost pure hematite (α-Fe₂O₃), and an inward-growing oxide “crater” consisting of (Cr,Fe)₃O₄. The transition from a thin protective (Cr,Fe)₂O₃ scale to a non-protective thick scale on this martensitic/ferritic steel originated locally and was followed by rapid oxide growth, resulting in a thick scale that covered the whole sample surface.     

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.     

Initial oxidation and chromium diffusion. I. Effects of surface working on 9-20% Cr steels

Five chromium steels with different surface finishes (electropolished, polished, ground, sandblasted) were oxidised for 1, 10 or 100 h in three different atmospheres: H₂-2.5%H₂O, N₂-20%H₂-0.49%H₂O and N₂-20.5%O₂ at 600 °C. After oxidation the mass gain was determined and SNMS depth profiles were recorded for determination of the oxide compositions and the diffusion profiles of the selectively oxidised elements in the metal phase. From the diffusion profiles Cr-diffusivities were derived.At low p_O2 strong surface deformation causes fast growth of Cr-rich oxides, whereas on weakly deformed specimens oxide growth is retarded by insufficient Cr supply. At high p_O2, on less deformed steels, Fe-rich scales were observed, whereas after stronger surface deformation Cr-rich scales formed if the Cr content of the steel was high enough.     

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.     

On the effects of surface coatings on the high-temperature oxidation of nickel

Ni and Ni coated with superficial oxide coatings including SiO₂, CeO₂ and La₂O₃ have been oxidized in the temperature range 500-1200 °C in 10⁻⁴-1 atm oxygen. The oxidation kinetics and the surface kinetics have been determined, and the microstructure and the composition of the oxide coatings and the thermally grown NiO scales have been characterized by a range of different microscopy and spectroscopy techniques. At 800-900 °C, the coatings decrease the oxidation rate by approximately one order of magnitude. The oxide coatings increase the relative growth inside the scale compared to uncoated Ni. Still, the major growth mechanism is governed by outward Ni diffusion, which indicates that the reason for the improved oxidation resistance is due to a decrease in the overall outward flux of Ni across the scales.     

Oxidation behaviour of electron beam physical vapour deposition β-NiAlHf coatings at 1100 °C in dry and humid atmospheres

NiAl intermetallic compounds attract increasing attention for high-temperature applications due to their combination of excellent properties, especially as metallic protective coatings on superalloys or as bond coats in thermal barrier coatings (TBCs). Oxidation behaviour of the β-NiAlHf coatings prepared by electron beam physical vapour deposition (EB-PVD) was investigated at 1100 °C for short time and 100 h in 20 % O₂ + Ar and 15 % H₂O + Ar, respectively. The results of mass changes reveal that the addition of minor reactive element Hf significantly improves the cyclic oxidation resistance of the β-NiAl coatings in both dry and humid atmospheres. During the initial oxidation stage, water vapour retards the phase transformation from θ-Al₂O₃ to α-Al₂O₃. Moreover, compared with the case in dry atmosphere, water vapour significantly reduces the surface roughness of the oxide scales formed on EB-PVD β-NiAlHf coatings after 100 h cyclic oxidation, which corresponds with the difference of surface morphologies of the oxide scales.     

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.     

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.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

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.     

The high temperature oxidation of 11% chromium steel: Part I – Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O₂ and O₂ + 10 and 40% H₂O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI‐XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α‐(Cr,Fe)₂O₃ oxide in dry O₂ at 600°C. In mixtures of oxygen and 10 or 40% H₂O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO₂(OH)₂(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron‐rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O₂/H₂O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H₂O. The thick oxide formed in O₂/H₂O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.     

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.     

High temperature oxidation of Mo–Si–B alloys: Effect of low and very low oxygen partial pressures

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10^?19 and 10^?12 bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10^?6–10^?4 bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.     

Synthesis, matching and deconstruction of polarization curves for the active corrosion of zinc in aerated near-neutral NaCl solutions

SYMADEC 1, used previously to synthesise, match and deconstruct polarization curves for the Fe/H₂O/H⁺/O₂(4e⁻) corrosion system, has been updated to SYMADEC Multimetal. The polarization curve of a zinc anode alloy corroding with formation of a non-protective layer of ZnO in fully-aerated, near-neutral NaCl solution was modelled. To test the SYMADEC Multimetal model four published polarization curves of zinc actively corroding in similar salt solutions are deconstructed. The polarization curve current/voltage contribution from the ZnO reduction/oxidation peak overlaid on the polarization curve is estimated by a bi-Gaussian relationship.     

In Situ observation of the effect of temperature on Cu2O scale morphology

The effect of temperature on copper oxide scale morphology was studied during in situ oxidation of OFHC copper in a hot-stage environmental scanning electron microscope (HSESEM). Cuprous oxide scales grown at low temperature [0.2T_m(Cu₂O)] and intermediate temperatures 0.6–0.7T_m(Cu₂O)] were found to be crystallographically oriented. At intermediate temperatures, scales exhibited nonplanar features such as ridges and growth pyramids. At high temperatures [T> 0.8T_m(Cu₂O)], scales had planar morphologies and a few dislocation growth pits. Downquenching and upquenching of the Cu₂O scales from steady-state oxidation temperatures induced morphological changes such as cavity formation and surface reconstruction.     

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.