Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800?°C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe–20Ni–14Cr–2.5Al and Fe–18Cr–25Ni–3Al base. At 800?°C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe–14Cr–20Ni–2.5Al. At 900?°C in air with 10?% water vapor, however, additions of Mn in these AFA steels based on Fe–18Cr–25Ni–3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn_1.5O₄ and Cr₂O₃ oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.     

Long-Time Oxidation of Cr2AlC Between 700 and 1,000?°C in Air

Cr₂AlC compounds were synthesized via a powder metallurgical route and their long-term oxidation behavior studied. Oxidation at temperatures between 700 and 1,000?°C for up to 360?days in air resulted in formation of a thin, adherent Al₂O₃ surface layer and a narrow Cr₇C₃ sublayer, accompanied by evaporation of carbon from the Cr₂AlC. Preferential oxidation of Al on the surface suppressed oxidation of the less mobile Cr in Cr₂AlC. In the Al₂O₃ layer, (0.7–8.3)?at.% Cr was incorporated. In the Cr₇C₃ sublayer, Al was either absent or incorporated. When Cr₂AlC oxidized at 850 and 1,000?°C for 30–360?days, metastable θ-Al₂O₃ blades formed on the α-Al₂O₃ layer. However, such blades were scarcely visible when the oxidation was carried out above 1,100?°C, because of the fast θ?→?α-transition. Moreover, the θ-Al₂O₃ were not noticeable during oxidation at 700?°C for 30–360?days, due to a small extent of oxidation.     

Effect of Steam Flow Rate and Sample Orientation on Steam Oxidation of Ferritic and Austenitic Steels at 650 and 700 °C

Steam oxidation of heat exchanger tubing is of growing interest as increasing the efficiencies of conventional pulverised fuel fired power plants requires higher steam temperatures and pressures. These new, more severe steam conditions result in faster steam oxidation reactions, which can significantly reduce the lifetime of boiler components. This paper reports some results from an investigation into the impact of steam flow rates and sample orientation on the steam oxidation of surface ground superheater tube materials. The results show that an increased steam flow rate not only causes faster oxidation rates but also a change in oxide scale morphology. In case of T23, it triggers formation of micro-layered inner oxide, whereas for T92 it promotes the formation of an outer haematite layer. For austenitic steels, the faster steam flow increases the formation of initially protective oxide scales, but also accelerates the growth of oxide nodules with prolonged exposure times.     

Pack Aluminide Coatings Formed at 650 °C for Enhancing Oxidation Resistance of Low Alloy Steels

THE COMMERCIAL ALLOY STEELS containingtypically9-12wt.%Cr and1wt.%Mo are low costmaterials developed for critical strength engineeringapplications at temperatures up to700°C[1-2].Thecomposition and microstructure of these materials arenormally optimised to provide the required mechanicalstrength and creep/fatigue resistance.However,thehigh temperature oxidation resistance is often notsufficient to prevent premature failure caused byoxidation degradation,particularly in high temperat…     

Steam Oxidation Behavior of Advanced Steels and Ni-Based Alloys at 800 °C

This publication studies the steam oxidation behavior of advanced steels (309S, 310S and HR3C) and Ni-based alloys (Haynes^® 230^®, alloy 263, alloy 617 and Haynes^® 282^®) exposed at 800 °C for 2000 h under 1 bar pressure, in a pure water steam system. The results revealed that all exposed materials showed relatively low weight gain, with no spallation of the oxide scale within the 2000 h of exposure. XRD analysis showed that Ni-based alloys developed an oxide scale consisting of four main phases: Cr₂O₃ (alloy 617, Haynes^® 282^®, alloy 263 and Haynes^® 230^®), MnCr₂O₄ (alloy 617, Haynes^® 282^® and Haynes^® 230^®), NiCr₂O₄ (alloy 617) and TiO₂ (alloy 263, Haynes^® 282^®). In contrast, advanced steels showed the development of Cr₂O₃, MnCr₂O₄, Mn₇SiO₁₂, FeMn(SiO₄) and SiO₂ phases. The steel with the highest Cr content showed the formation of Fe₃O₄ and the thickest oxide scale.     

Hot Corrosion Behavior of Superalloy IN718 at 550 and 650 °C

This investigation was undertaken to evaluate oxidation and hot corrosion behavior of the Fe-Ni-based superalloy IN718, at 550 and 650 °C, to explore its performance as turbine engine components under marine environment. Uncoated and different salt-coated samples (100 wt.% NaCl, 75 wt.% Na₂SO₄ + 25 wt.% NaCl, and 90 wt.% Na₂SO₄ + 5 wt.% NaCl + 5 wt.% V₂O₅) were exposed in air at 550 and 650 °C under cyclic heating and cooling for 100 h. Weight gain was studied for both uncoated and salt-coated samples. X-ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy were used to characterize the oxidation and corrosion products. A possible mechanism of corrosion, based on the corrosion compounds, is discussed. The variation in weight gain with time showed a parabolic growth of oxides. Coating with NaCl was found to be detrimental both at 550 °C as well as 650 °C. On the other hand, the salt mixture of NaCl and Na₂SO₄ had no effect at 550 °C; however, it was detrimental at higher temperature of 650 °C. Coatings of salt mixture of Na₂SO₄, NaCl, and V₂O₅ caused very slow oxidation at both the temperatures. Increase in thickness of salt coating was observed to enhance the rate of hot corrosion. Among the three types of salt coatings, the coating of NaCl was found to be most damaging both at 550 and 650 °C.     

Pack Aluminide Coatings Formed at 650℃ for Enhancing Oxidation Resistance of Low Alloy Steels

This study aims to investigate the feasibility of forming iron aluminide coatings on a commercial 9Cr-1Mo (wt.%) alloy steel by pack cementation at 650℃ in an attempt to improve its high temperature oxidation resistance. Pack powders containing Al, Al2O3 and a series of halide salts were used to carry out the coating deposition experiments, which enabled identification of the most suitable activator for the pack aluminising process at the intended temperature. The effect of pack aluminium content on the growth kinetics and microstructure of the coatings was then studied by keeping deposition conditions and pack activator content constant while increasing the pack aluminium content from 1.4 wt.% to 6 wt.%. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the phases and microstructures of the coatings formed and to determine depth profiles of coating elements in the coating layer. Oxidation resistance of the coating was studied at 650 ~C in air by intermittent weight measurement at room temperature. It was observed that the coating could substantially enhance the oxidation resistance of the steel under these testing conditions, which was attributed to the capability of the iron aluminide phases to form alumina scale on the coating surface through preferential A1 oxidation.     

Internal Oxidation of Fe–Mn–Cr Steels,Simulations and Experiments

A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H₂ and dew points of ? 30, ? 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr₂O₄ are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.     

Steam Oxidation Resistance of Advanced Steels and Ni-Based Alloys at 700 °C for 1000 h

The aim of the work was to investigate corrosion resistance of highly alloyed steels and Ni-based alloys in a steam atmosphere for 1000 h at 700 °C. In these steam oxidation experiments, two solid solution strengthened alloys; Haynes^® 230^®, 617 alloy, two gamma-prime (γ′) strengthened alloys; 263 and Haynes^® 282^® and three Cr+Ni- rich stainless steels: 309S, 310S and HR3C austenitic steels were exposed. The study showed that the materials exposed commonly developed thin oxide scales; in Ni-based alloys, these consisted of mainly MnCr₂O₄ spinels and Cr₂O₃, with the exception of 617 alloy where NiCr₂O₄ spinels and Cr₂O₃ were found. In Fe-based alloys, Cr₂O₃, MnCr₂O₄ spinels, Fe,Mn(SiO)₄, and finally Fe₃O₄ developed. No evaporation of chromia has been found within 1000 h test period. Furthermore, the development of TiO₂ was not observed into a large extent in Haynes^® 282^® and 263 alloy, in contrast to the study performed at 800 °C under the same steam environment conditions.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe₂O₃,Fe₃O₄, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO₃. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe₂O₃,Fe₃O₄, andY₂O₃, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.     

Cyclic Oxidation Behaviour of 1Cr–0.5Mo (T11) Boiler Tube Steel and Its Weldments in Air at 900 °C

Oxidation behaviour of weldments at elevated temperature has become an object of scientific investigation. Weldments were prepared using shielded metal arc welding and tungsten inert gas processes to weld together 1Cr–0.5Mo (T11) boiler tube steels. This paper reports the oxidation behaviour of welded and unwelded 1Cr–0.5Mo (T11) boiler tube steel specimens after exposure to air at 900 °C under cyclic condition. The thermogravimetric technique was used to establish kinetics of oxidation. X-ray diffraction and scanning electron microscopy/energy-dispersive analysis techniques were used to analyse the oxidation products. The unwelded steel showed a higher oxidation rate (in terms of weight gain) than that of welded steels.     

The Effects of Molybdenum Addition on High Temperature Oxidation Behavior at 1,000?°C of Type 444 Ferritic Stainless Steel

The influence of molybdenum addition on the oxidation behavior of ferritic stainless steel (FSS) was studied at 1,000?°C up to 100?h in air under isothermal conditions. The results were compared with molybdenum-free FSS in order to explain the role of molybdenum on the oxidation of Type 444 FSS. It is shown that molybdenum plays a similar protective role as the one observed with silicon by promoting the precipitation of Laves phase. Moreover, the Fe₂(Nb, Mo) Laves phases with a small amount of silicon are found nearby the oxide–metal interface and deep in the metallic matrix along the grain boundaries. These highly protective Laves phases hinder the external diffusion of iron, chromium, and manganese cations from the matrix and prevent the internal diffusion of oxygen which leads to the lower oxidation rate and the better scale adherence.     

The 600?°C and 700?°C Isothermal Sections of the Zn-Fe-Bi Ternary System

The 600 and 700?°C isothermal sections of the Zn-Fe-Bi phase diagram were determined by scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray powder diffraction. Two three-phase regions have been confirmed in the 600?°C isothermal section, and one three-phase region has been identified in the 700?°C isothermal section. Bi is almost insoluble in the Fe-Zn binary compounds and ??-Fe, and the solubility of Fe in the L phase is limited.     

Pre-oxidation Effect on Oxidation Behavior of F91 in Carbon Dioxide at 550 °C

Upon exposure to CO₂ at 550 °C, F91 tends to form rapidly growing scales consisting of an outer Fe oxide and an inner Fe–Cr spinel oxide. A comparative study has been carried out between the pre-oxidized and non-pre-oxidized F91, to determine the influence of pre-oxidation upon the oxidation behavior of F91 in CO₂. Formation of a rapidly growing scale and carburization could be inhibited by a pre-oxidation treatment in air prior to oxidation in CO₂. Although during exposure to CO₂, a fast growing scale still would form locally, pre-oxidation changed its structure. Effects of pre-oxidation time on the oxidation resistance in CO₂ are investigated.     

Oxidation Behaviour of Sanicro 25 (42Fe22Cr25NiWCuNbN) in O2/H2O Mixture at 600?°C

The present study investigates oxidation at 600?°C of alloy Sanicro 25 (42Fe22Cr25NiWCuNbN) in dry and wet O₂ environments. The exposure time was 1–168?h. The oxidized samples were analyzed by grazing incidence X-ray diffraction, glow discharge optical emission spectroscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. Alloy Sanicro 25 showed protective oxidation behaviour under the present conditions. Initially, a thin and smooth corundum-type single layer base oxide formed, featuring a Cr-rich bottom part and a Fe-rich top. With time, double-layered oxide nodules form consisting of inward- and outward-growing parts. Below the oxide scale a 100–200?nm thick oxidation-affected zone formed in the alloy, which was depleted in Cr and enriched in Ni. In this region the chromium carbides and copper-rich particles present in the bulk alloy were dissolved. In O₂?+?H₂O environment, chromium volatilized from the surface, causing the chromium content of the oxide to be lower than after oxidation in dry O₂. However, under present experimental conditions, the Cr depletion of the scale was not enough to trigger accelerated corrosion of the alloy.     

Oxidation Comparison of Alumina-Forming and Chromia-Forming Commercial Alloys at 1100 and 1200 °C

21 Commercial alumina-forming and chromia-forming high-temperature alloys were tested at 1100 and 1200 °C for up to 1000 h. Exposure was performed in air using 100-h cycles on ~2-mm-thick test coupons placed in highly sintered alumina crucibles. Visual appearance, gross mass gain and amount of spallation were monitored after every 100 h. Investigation of topography and cross sections was performed in LOM and SEM with EDS at breakaway oxidation or at 1000 h depending on what occurred first. In order to investigate the earlier part of the oxidation process in more detail, separate samples from all materials were exposed under the same conditions for a single 100-h cycle and were then investigated in the same manor.     

High-Temperature Oxidation Behavior of CrMoV,F91 and Mar-M247 Superalloys Exposed to Laboratory Air at 550 °C

The oxidation behavior of three commercial superalloys, CrMoV, F91 and Mar-M247, was studied at 550 °C in laboratory air for 1000 h. Mar-M247 superalloy showed the best oxidation resistance, which is attributed to the formation of a scale rich in Cr₂O₃ and Al₂O₃, followed by F91 and CrMoV. A thick duplex oxide formed on CrMoV alloy and spallation was observed. The results for CrMoV alloy showed that calculated Fe diffusion in magnetite was 200 times faster than literature values for Fe diffusion in Fe₃O₄, which is attributed to grain-boundary diffusion and the effect of impurity on diffusion. F91 initially formed a protective chromium-rich oxide layer followed by formation nodules, leading breakaway oxidation. The oxide nodules consisted of a duplex structure with different morphologies and oxide phases from duplex oxide scale in CrMoV.