Chromia Formers Research Articles

The Effects of Water Vapor on the Oxidation Behavior of Alumina Forming Austenitic Stainless Steels

The isothermal oxidation behavior of three alumina forming austenitic (AFA) stainless steels with varying composition was studied at 650 and 800 °C in dry air and gases which contained water vapor. The AFA alloys exhibited better oxidation resistance than a “good chromia former” at 650 °C, particularly in H2O-containing atmospheres by virtue of alumina-scale formation. Although the AFA alloys were more resistant than chromia formers, their oxidation resistance was degraded at 650 °C in the presence of water vapor. In dry air the AFA alloys formed, thin continuous alumina scales, whereas in Ar–4%H2–3%H2O the areas of continuous alumina were reduced and Fe oxide-rich nodules and regions of Cr, Mn-rich oxides formed. In some regions internal oxidation of the aluminum occurred in the H2O-containing gas. The alloy OC8 had slightly better resistance than OC4 or OC5 in this atmosphere. The alumina-forming capability of the AFA alloys decreases with increasing temperature and, at 800 °C, they are borderline alumina formers, even in dry air. The oxidation resistance of all three alloys was degraded at 800 °C in atmospheres, which contained water vapor (Air–10%H2O, Ar–3%H2O and Ar–4%H2–3%H2O). The areas, which formed continuous alumina, were reduced in these atmospheres and areas of internal oxidation occurred. However, as a result of the borderline alumina-forming capability of the AFA alloys it was not possible to determine which of the H2O-containing atmospheres was more severe or to rank the alloys in terms of their performance. The experimental results indicate that the initial microstructure of the AFA alloys also plays a role in their oxidation performance. Less protective oxides formed at 800 °C when alloy OC8 was equilibrated before exposure rather than being exposed in the as-processed condition. The reason for this is the presence of different phases in the bulk of the two specimens.

The Prospects In Designing New Generation Of High Temperature Coatings In Automobile Engines

AbstractThe influence of the chromium layer with the thickness of 1 micrometer sputter-deposited on the X33CrNiMn23-8 and X50CrMnNiNbN21-9 steel surfaces on the oxidation behavior of these steels has been studied at 1173 K in air, using the microthermogravimetric technique. It has been found that coated materials show very good oxidation resistance under isothermal conditions, comparable with that of chromia formers, due to the formation of Cr2O3scales on their surfaces. It has been also demonstrated that the positive effect of chromium addition on the oxidation resistance of investigated steels is observed during a much longer period of time than the life-time of the chromium coating.

Oxidation of nickel-based alloys in dry and water vapour containing air

Ni-based alloys are widely used in power generation and their oxidation behaviour in the uncoated state is of interest, especially the impact of water vapour in the air on the formation of a protective underlying alumina scale. High-temperature X-ray diffraction was applied to investigate in situ the oxide scale formation in the initial state on the alumina formers CM247DS and CMSX4 in comparison to the chromia formers IN792 and SCA425+. Post-oxidation analysis was performed by field emission scanning electron microscopy. The samples were oxidised for 100 h at 950°C in dry air and in air with 20% relative moisture in the high temperature device on the X-ray diffractometer. In dry air, CM247 and CMSX-4 form α-Al2O3 from the beginning simultaneously with spinels and nickel oxide. The alumina scale underlies the spinels which spall partially on cooling. When adding water vapour, the same oxides were formed simultaneously resulting in a comparable oxide scale. IN792 forms in dry air mainly NiAl2O4 and transitionally CrO2 under laid by an alumina scale. With water vapour, Cr2O3 forms and the underlying alumina scale shows a non coherent dendritic structure. SCA425+ forms in dry air Cr2O3 and Cr containing mixed oxides and with water vapour a more coherent alumina scale than IN792.

Effects of Water Vapour on the Oxidation of Chromia Formers

Water vapour interacts with growing chromia scales in several different ways. Formation and volatilisation of Cr2O2(OH)2 is shown to account quantitatively for chromium loss from thin alloy foils reacted with air-steam mixtures over periods of 103 h. In the shorter term, water vapour is shown to refine the grain structure of Cr2O3 scales grown on Ni-25Cr. Scaling kinetics are at the same time accelerated by an additional, larger contribution to diffusion by a grain boundary species, either OH- or H2O. A slight increase in scaling rate observed at low water vapour partial pressures in H2/H2O gases is thought to be due to hydrogen doping.

Effects of Chrome Contamination on the Performance of La0.6Sr0.4Co0.2Fe0.8O3 Cathode Used in Solid Oxide Fuel Cells

Chrome poisoning effects of various Cr-containing metal sources on the electrochemical performance of the (LSCF6428) cathode have been investigated. It was found that chromia-forming metals caused significant fade in power density due to the chrome poisoning, while alumina-forming alloys exhibited no influence on the cell degradation. This degradation caused by the chrome poisoning was accelerated at higher operating temperatures. Microstructural analysis conducted on the cells tested with chromia formers exhibited the formation of a strontium chromate phase in the entire cathode, leading to the homogeneous distribution of Cr in the cathode. Although the Mn-containing chromia former such as Crofer22 formed a continuous layer of the oxide scale on the mesh surface, it was not effective enough to prevent Cr poisoning of the cathode. In contrast, alumina formers such as Haynes214 and Kanthal formed a continuous layer of the alumina scale, resulting in no Cr contamination in the LSCF6428 cathode.

High temperature carbon corrosion in solid oxide fuel cells

Operating conditions in a current design for a planar geometry oxide fuel cell plant are briefly reviewed and the danger of encountering “metal dusting” conditions identified. Laboratory tests were designed to produce accelerated metal dusting by exposing heat resisting alloys to a CO–26 H2–6 H2O (vol. pct) gas mixture at 680°C under thermal cycling conditions. The hot gas composition corresponded to ac = 2.9 and an oxygen potential high enough to oxidise chromium and aluminium, but not iron or nickel. The alloys tested included ferritic and austenitic chromia formers and two ferritic alumina formers, all with electropolished surfaces. Thermal cycling of the chromia formers led to oxide scale damage followed by internal carburisation, metal dusting and coking. This failure occurred very rapidly on most austenitic materials (Alloy 800, Inconel 601, 690, 693, Alloy 602CA), but did not commence until after approximately 50 one-hour cycles for the ferritic steel Fe–27Cr–0.001Y (wt %). The alloy with the best performance was Inconel 625, which was still protected by its Cr2O3 scale after 500 cycles. The alumina forming alloys showed superior performance, with no damage apparent after 1200 cycles. Additional tests using ground metal surfaces showed that they were more resistant to dusting in the case of chromia formers, but more susceptible in the case of alumina formers, metal dusting.

High temperature carbon corrosion in solid oxide fuel cells

Operating conditions in a current design for a planar geometry oxide fuel cell plant are briefly reviewed and the danger of encountering “metal dusting” conditions identified. Laboratory tests were designed to produce accelerated metal dusting by exposing heat resisting alloys to a CO–26 H2–6 H2O (vol. pct) gas mixture at 680°C under thermal cycling conditions. The hot gas composition corresponded to ac = 2.9 and an oxygen potential high enough to oxidise chromium and aluminium, but not iron or nickel. The alloys tested included ferritic and austenitic chromia formers and two ferritic alumina formers, all with electropolished surfaces. Thermal cycling of the chromia formers led to oxide scale damage followed by internal carburisation, metal dusting and coking. This failure occurred very rapidly on most austenitic materials (Alloy 800, Inconel 601, 690, 693, Alloy 602CA), but did not commence until after approximately 50 one-hour cycles for the ferritic steel Fe–27Cr–0.001Y (wt %). The alloy with the best performance was Inconel 625, which was still protected by its Cr2O3 scale after 500 cycles. The alumina forming alloys showed superior performance, with no damage apparent after 1200 cycles. Additional tests using ground metal surfaces showed that they were more resistant to dusting in the case of chromia formers, but more susceptible in the case of alumina formers, metal dusting.

Crevice corrosion of Cr2O3 and Al2O3 forming alloys in mixed nitrogen-oxygen environments

When exposed freely to low potential bioxidant environments at elevated temperatures, either Cr2O3 or Al2O3 oxide scales can afford long-term protection of commercial alloys, even if such environments contain another potentially corrosive gaseous constituent. Circumstances arise, however, in many technological applications where specific geometry factors, such for example, as crevices/cracks or narrow holes, could result in the oxygen supply to internal regions becoming the dominant control parameter. This has been demonstrated previously with carbon bearing bioxidants, where carburisation of Cr2O3 formers then can ensue, often with disastrous consequences, controlling component integrity or life.To demonstrate that these crevice corrosion principles apply also to other mixed gas bioxidants, and for Al2O3 formers as well as Cr2O3 formers, the corrosion behaviours of two chromia formers, Nimonic 86 and Hastelloy X, at 950°C and of the Al2O3 forming FeCrAlRE alloy, PM 2000, at 1200°C have been examined in separate studies, using blended nitrogen–oxygen environments. The overall objectives were to confirm that protective scales are formed on all alloys in freely exposed conditions, but that with geometries restricting gas access, oxidant depletion led to internal nitridation, with the formation of CrN on Nimonic 86 and Hastelloy X and of TiN and AlN on PM 2000.

Evidence for Chromium Evaporation Influencing the Oxidation of 304L: The Effect of Temperature and Flow Rate

The influence of temperature and flow rate on the oxidation of 304L steel in O2/H2O mixtures was investigated. Polished samples were isothermally exposed to dry O2 and O2+40% H2O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M2O3 film, while the parts experiencing breakaway corrosion form a 10–30-μm thick Fe-rich M2O3/M3O4 scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O2/H2O mixtures.

The corrosion behavior of several heat resistant materials in air + 2% Cl2 at 300 to 800 °C. Part 3 - Alumina formers and intermetallics

The corrosion behavior of Alloy 214 and the intermetallics Fe3Al, TiAl and MoSi2 was investigated in dry air and air with 2% Cl2 at temperatures of 300 to 800 °C. The results show that corrosion resistance of Alloy 214 and the two aluminides very much depend on the test temperature. For TiAl massive corrosion starts at 500 °C. Alloy 214 shows corrosion rates similar to chromia formers at temperatures of 650 and 800 °C while the best corrosion resistance of all alumina formers tested is revealed by Fe3Al. For MoSi2 only some little (“internal”) oxidation is observed even at 800 °C. Das Korrosionsverhalten verschiedener hitzebeständiger Werkstoffe in Luft mit 2% Cl2 bei Temperaturen von 300 bis 800 °C. Teil 3 – Aluminiumoxidbildner und intermetallische Legierungen Das Korrosionsverhalten von Alloy 214 und den intermetallischen Legierungen Fe3Al, TiAl und MoSi2 wurde in getrockneter Luft sowie Luft mit 2% Cl2 bei Temperaturen von 300 bis 800 °C untersucht. Die Ergebnisse zeigen, daß die Korrosionsbeständigkeit von Alloy 214 und den Aluminiden stark von der Temperatur abhängt. Für TiAl wird bereits bei 500 °C ein massiver Korrosionsangriff beobachtet. Die Korrosionsraten von Alloy 214 entsprechen bei 650 und 800 °C denen von chromoxidbildenden Nickelbasislegierungen, während Fe3Al von allen aluminiumoxidbildenden Legierungen die besten Korrosionseigenschaften zeigt. Für MoSi2 wird selbst bei 800 °C nur eine leichte „innere“ Oxidation gefunden.

Influence of Water Vapor and Flow Rate on the High-Temperature Oxidation of 304L; Effect of Chromium Oxide Hydroxide Evaporation

The effect of roman PH 2 O and flow rate on the oxidation of 304Lat 873 K in oxygen is reported. High concentrations of water vapor and highflow rates result in breakaway corrosion. The mass gain after 168 hrincreased by four to five times, compared to oxidation in dry O2. Inthe presence of H2O, the corrosion products consisted of arelatively thin (Cr,Fe)2O3 oxide plus large oxide islandsconsisting mainly of Fe2O3. A mechanism explaining theeffect of water vapor on marginal chromia formers is proposed.

Sulphide formation after pre-oxidation of chromia formers

Pure chromium and the alloys Fe-28Cr, Co-28Cr and Ni-28Cr (wt%) were pre-oxidized in CO-CO 2-N2 at 1173K and then reacted with CO-CO 2-SO 2-N 2 mixtures. All gases had the same oxygen and carbon activities. The SO 2-containing gases had sulphur activities higher than the chromium-chromium sulphide equilibrium value, but Cr 2O 3 was always predicted to be stable with respect to sulphide. Pre-oxidation led to the growth of apparently compact, adherent Cr 2O 3 scales on all materials, plus sublayers containing carbide and nitride in the case of chromium. Addition of SO 2 to the gas led to subsequent nucleation and growth of Cr 3S 4 on top of the pre-formed Cr 2O 3 in all cases where p(S 2) was relatively high. Sulphide formation beneath the oxide occurred only after extended periods of reaction. The formation of metastable sulphide in contact with the gas is attributed to preferential adsorption of sulphur onto the external surface. The presence of other alloy metals plays no significant role in the early stages of sulphide formation, and the rate of Cr 3S 4 growth is controlled by diffusion of chromium through the pre-formed oxide. Base metal sulphides develop at the scale exterior after longer periods.The gas permeability of pre-formed Cr 2O 2 on chromium was greater than for the alloy oxide scales. The addition of sulphur made the scale on chromium impermeable to nitrogen, but left it permeable to carbon. Sulphur made the preformed oxide on Fe-28Cr permeable to carbon (but not nitrogen), but did not affect the impermeability of Cr 2O 3 on Co-28Cr or Ni-28Cr for carbon and nitrogen, until mechanical disruption of the scale on Ni-28Cr resulted from extensive sulphidation. Sulphur adsorption on internal surfaces within the Cr 2O 3 is thought to block nitrogen access.

Emerging power technologies and oxide scale spallation

There is now a shift towards combined cycle for large scale power generation. In these plants, the main spallation issue will be that of combustor can alloys. Units using reheat will add a new dimension to this problem. Spallation of highly cooled turbine blading as a possible issue should not be neglected, however.Combined cycle will be linked in the longer term to coal gasification processes. Where the gasifiers are of the entrained or fluidised bed type, HC1 induced spallation of downstream heat exchangers is a neglected aspect of much experimental work. This form of attack is likely to occur over a wide range of temperatures.In the UK and Northern Europe CHP (Combined Heat and Power), using natural gas as a fuel, will take an increasing share of the market. The most advanced concepts will use recuperative gas turbines, as these have many advantages at low to medium outputs. Again, scale spallation will be an important design consideration.Household or Domestic CHP could well be a new entry into the field of environmentally beneficial, localised power generation. Many of the prime movers under consideration for this sector will require a high temperature air preheater. Here the need to avoid excessive oxidations and scale spallation will be paramount.The paper calls for more efforts on alloys other than chromia formers; alumina formers offer much better high temperature behaviour, apart from their spallation behaviour. There is also a strong economic case for work on alloys reliant on silica. Although these are normally viewed as having poor spallation resistance, they are used in quantity by the Automotive Industry in cyclic applications.A brief review is given of spallation and related phenomena on alumina forming ODS alloys intended for advanced indirect fired gas turbine cycles. Emphasis is given to the statistically variable nature of spallation, and to the fact that, in practice, spallation failures involve more than one mechanism.

The sulphidation and oxidation behaviour of sputter-deposited amorphous AlMo alloys at high temperatures

The sulphidation behaviour of sputter-deposited amorphous AlMo alloys has been studied as a function of temperature (973–1273 K) and alloy composition (34–46 at%Mo) in pure sulphur vapour at 10 3 Pa pressure. It has been shown that under these conditions the sulphidation process follows parabolic kinetics, being diffusion controlled. No influence of the alloy composition on the reaction rate has been observed. The scales were heterogeneous and composed of two layers. The outer scale layer was built of aluminium sulphide, while the inner layer was heterogeneous and composed of a mixture of Al 0.5Mo 2S 4, Al 2S 3 and MoS 2 phases, the latter being the major component. It is believed that the slowest step, determining the overall reaction rate, is the inward diffusion of sulphur through the inner barrier layer of the scale. Over the whole temperature range studied the alloys showed excellent resistance to sulphide corrosion, their sulphidation rates being comparable with, or even lower than the oxidation rates of chromia forming materials. The better protective properties of the sulphide scale on AlMo alloys in comparison with those of the MoS 2 scale on pure molybdenum result probably from lower defect concentration in aluminium-doped MoS 2 phase, constituting the major part of the inner barrier layer. Preliminary results indicate that the alloys under investigation show satisfactory oxidation resistance in the temperature range not exceeding about 1123 K. Under these conditions a protective oxide scale is formed composed virtually only of Al 2O 3 and the oxidation rates are lower than those of chromia formers. At higher temperatures aluminium activity in the alloy is too low to suppress the formation and evaporation of molybdenum oxides.

The influence of certain reactive elements on the oxidation behaviour of chromia- and alumina-forming alloys

The influence of implanted yttrium and lanthanum on the oxidation behaviour of Co25Cr1Al chromia former as well as of β-NiAl alumina former has been studied under isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr 2O 3 scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results, on the one hand, from promotion of fine-grained scale formation and on the other, from changing the mechanism of scale growth from the outward diffusion of cations to the inward diffusion of anions. The influence of implanted yttrium (but not lanthanum) on the growth mechanism and adherence of alumina scale is analogous: the scale adherence is greatly improved but its growth rate is only slightly reduced. Lanthanum, on the other hand, greatly improves the protective properties of the scale but does not, in practice, affect its adherence to the substrate. As in the case of chromia formers, it has been found that the presence of reactive element atoms in the surface layer of the substrate considerably influences the mechanism of alumina scale growth by changing it from predominant inward diffusion of anions to prevailing outward diffusion of cations. It is believed that the influence of an implanted reactive element on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and in various interfacial phenomena.

The effect of preoxidation of some Ni, Fe, and Co-Base alloys on subsequent sulfidation at 982�C in sulfur vapor

The effect of preoxidation was studied on the subsequent sulfidation in sulfur vapor at a pressure of 0.1 atm at 982°C on numerous iron, nickel, and cobalt-base alloys which were either chromia or alumina formers. In general, alumina films were much more protective than chromia films, but the efficacy of preoxidation in reducing sulfidation rates depended more upon perfection of the films and whether cracking and/or spatting occurred. Increasing oxidefilm thickness had a beneficial effect until either penetration of the films by sulfur or cracking occurred, after which sulfidation rates were sometimes greater than for nonpreoxidized samples. The enhanced sulfidation rates are attributed to sulfidation of a solute-depleted substrate, the solute having been selectively removed by oxide formation. One alloy, MA 956, containing 0.5 Y2O3 as fine dispersions which normally provide spatting resistance, still exhibited extensive cracking and spalling of the oxide and was not much better than alloys without dispersoids or reactive-metal additions. The use of preoxidation to reduce sulfidation rates is not viable under the extreme conditions used. Preoxidation is conceptually a good method for inhibiting sulfidation at lower temperatures and much lower sulfur pressures.

The importance of interfacial chemistry in protective oxide scale adherence

The results of studies involving both alumina and chromia formers have demonstrated that segregation of low levels of indigenous impurity elements commonly found in metals and alloys can segregate to the scale-metal interface. Such segregation markedly affects protective-oxide-scale adherence to produce scale exfoliation. The most important element to cause exfoliation effects is sulfur, which is not uncommonly present in metals and alloys to levels of ∼50 ppm. The reduction of such sulfur to the 1–2 ppm range strongly increases oxide scale adherence without requiring additions of “active” elements, such as yttrium. The results of experiments that led to this conclusion are reiterated.

The effect of certain reactive elements on the oxidation behaviour of chromia‐ and aluminaforming alloys

AbstractThe influence of implanted yttrium and lanthanum on the oxidation behaviour of Co‐45Cr and Co‐25Cr‐1Al chromia formers as well as of β‐NiAl and Co‐25Cr‐9Al alumina formers has been studied in isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr2O3‐scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results from the promotion of fine grained scale formation and consequently the elimination of outward diffusion of chromium. The influence of implanted yttrium (but not lanthanum) on the growth rate and adherence of alumina scale is analogous. In this case, however, both these effects result from elimination by reactive element addition of the inward diffusion of oxygen. It may be then concluded that the influence of implanted reactive elements on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and not in various interfacial phenomena.It has been found also that the protective properties of alumina scale are considerably improved by elimination of grain boundaries from the substrate, this effect being stronger than that resulting from reactive element addition.Beneficial influence of implanted reactive elements should be expected only when the protective scale layer is formed from the very beginning of the reaction, directly on the initial surface of the material exposed to the oxidizing atmosphere.