Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

The Effects of KCl,NaCl and K<Subscript>2</Subscript>CO<Subscript>3</Subscript> on the High-Temperature Oxidation Onset of Sanicro 28 Steel

The present study investigates the early stages in the oxidation process of Sanicro 28 (Fe31Cr27Ni) stainless steel when exposed to an alkali salt (KCl, NaCl or K₂CO₃) for 2 h at 450 and 535 °C. After the exposure, the oxidized samples were analyzed with a combinatory method (CA, XPS and SEM–EDX). It was found that all three salts were corrosive, and the overall oxidation reaction rate was much higher at 535 °C than at 450 °C. There were clear differences in terms of the impact of cations (Na⁺, K⁺) and anions (Cl^?, CO₃ ^2?) on the initial corrosion process at both temperatures. When focusing on the cations, the presence of potassium ions resulted in a higher rate of chromate formation than in the presence of sodium ions. When studying the effect of anions, the oxidation of iron and chromium occurred at higher rates in the presence of both chloride salts than in the presence of the carbonate salt, and chloride salts seemed to possess higher diffusion rate in the gas phase and along the surface than carbonate salts. Moreover, at the higher temperature of 535 °C, the formed chromate reacted further to chromium oxide, and an ongoing oxidation process of iron and chromium was identified with a significantly higher reaction rate than at 450 °C.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

Oxidation of Stainless Steel Powder

To understand the corrosion behavior of a model 304L(p)–ZrO₂(s) composite, a 304L stainless steel powder was studied under oxygen at high temperature. Oxidation tests were performed with thermogravimetry. The so-called jumps method, which involves a sudden change of the temperature, was also applied to propose a kinetic model. Two periods with different rate-determining steps could be distinguished for short (<12 h) and long time experiments (12–20 h). SEM observations of oxidized particles revealed an oxide layer structure similar to that of alloy plates of same composition: during the first ten hours period, the external scale surrounding stainless steel particles was found to be chromium oxide; for the second oxidation period, the outer oxide layer was enriched in iron. Considering the relatively short-term oxidation period, a kinetic model based on an outward growth of chromia from oxidation of Cr in solution in the spherical alloy particles was successfully compared to the experimental mass gain curve. The k_p value deduced from this modeling was found to be in agreement with the literature data. The diffusion of interstitial chromium ions is the rate-determining step in agreement with the absence of influence of the oxygen partial pressure.     

High temperature Corrosion behaviour of iron aluminides and iron-aluminium-chromium alloys

The high temperature corrosion of different iron aluminides and iron-aluminium-chromium alloys containing between 6 and 17 wt% aluminium, 2 and 10 wt% chromium and additions of mischmetal has been investigated in air as well as in carburising, chlorinating and sulphidising environments. It was found that all alloys showed excellent corrosion resistance to both oxidation in air and carburisation in CH₄/H₂ up to at least 1100°C and to sulphidation in SO₂/air up to at least 850°C. In these environments the corrosion behaviour is not influenced by the concentrations of aluminium and chromium. In oxygen deficient H₂S-atmospheres, however, the corrosion behaviour depends sensitively on the aluminium and chromium concentration. At least 12 wt% aluminium in chromium-free alloys or 10 wt% aluminium in alloys containing 10 wt% chromium are required to provide sulphidation resistance at 550°C. The chlorination resistance of iron-aluminium-chromium alloys is low due to their formation of volatile aluminium chlorides.     

Effect of Cobalt Content on the Oxidation and Corrosion Behavior of Ni–Fe-Based Superalloy for Ultra-Supercritical Boiler Applications

The oxidation and corrosion behavior of three model alloys with different cobalt contents (6–20 wt%) were investigated in static air and a simulated coal ash/gas environment at 750 °C. The model alloys follow a parabolic law approximately during the oxidation in static air. High cobalt level improves the oxidation resistance, however, without noticeable improvement in coal ash/gas corrosion resistance. The sample with the highest cobalt content grows the thinnest oxide layer and the fewest internal oxidation products in the oxidation test. Cobalt in the model alloys promotes the establishment of a protective chromium oxide scale during the oxidation test, but did not show much difference in restraining the inward diffusion of sulfur by increasing its content. The oxidation and corrosion products formed on the sample surface consist mainly of a protective chromium oxide film. Internal aluminum oxide particles have been found especially along the grain boundaries of the base alloy.     

Influence of SO2 on the Oxidation of 304L Steel in O2 + 40%H2O at 600 °C

The effect of sulphur dioxide on the oxidation of alloy 304L in O₂ + 40%H₂O has been investigated at 600 °C. A protective chromium-rich corundum-type oxide forms in clean dry O₂. Exposure to O₂ + 40%H₂O environment results in chromium vaporization in the form of CrO₂(OH)₂. This causes local failure of the protective oxide and the formation of 10 μm thick oxide islands on the alloy grain centers. The oxide islands are layered, the outer part consisting of hematite while the inner part is FeCrNi spinel oxide. The addition of 100 ppm SO₂ to O₂ + 40%H₂O reduces the corrosion rate compared to O₂ + 40%H₂O. SO₂ is suggested to influence oxidation by two separate effects. Firstly, SO₂ forms surface sulfate on the oxide surface that impedes the vaporization of chromium from the protective oxide. This slows down the breakdown of the protective oxide. Secondly, SO₂ also influences the rapid oxidation that ensues once the protective oxide has been destroyed. In this case, the presence of surface sulfate interferes with the surface reactions involved in oxidation. In this way, SO₂ slows down the growth of the oxide islands.     

Corrosion Behavior of 9Cr-1Mo Steel in Sulfur Dioxide Environment

Corrosion behavior of annealed 9Cr-1Mo steel was studied in SO₂ environment at 1173 K, at flow rates from 8.33 × 10^?7 to 33.33 × 10^?7 m³/s, and parabolic rate law was followed. The rate constants were found to be independent of flow rate, within the range of flow rate investigated. Corrosion at temperatures from 973 to 1173 K, at a constant flow rate of 16.66 × 10^?7 m³/s, at 1 atmospheric pressure, for 6 h also exhibited parabolic law, however, the rate constants were observed to increase significantly with rise in temperature. The outer layer of the scale formed at 973 K was essentially of iron oxide, with small amount of chromium oxide whereas the inner layer was predominantly of chromium sulphide and chromium oxide. The scale formed at 1173 K was multilayered, in contrast to double layered formed at 973 K and 1073 K. The outer thick layer of the scale formed at 1173 K, consisted of iron oxide followed by thin substrate of chromium sulphide, iron sulphide/iron oxide, and chromium sulphide/chromium oxide toward the substrate. A model is proposed for the process of corrosion of 9Cr-1Mo steel in SO₂ environment, based on the present investigation.     

Corrosion of Alloy Haynes 230 in High Temperature Supercritical Carbon Dioxide with Oxygen Impurity Additions

The corrosion of Ni-based alloy Haynes 230 in supercritical carbon dioxide at temperatures of 650 and 750 °C at a pressure of 20 MPa was investigated. In high-purity research grade CO₂, the corrosion performance of the alloy was excellent with a thin, uniform, protective chromium-rich oxide layer forming on the surface. Introduction of 10 and 100 ppm O₂ impurity in the CO₂ environment noticeably enhanced oxidation with evidence of oxide spallation and nodule formation. In these oxygen impurity added tests, increased oxidation led to subsurface voids due to the more rapid outward diffusion of chromium as well as intergranular alumina and chromia. The oxygen concentration at the inlet and the outlet of the autoclave was measured and used to support the results of characterization of the surface oxide to develop a more holistic understanding of the role of oxygen impurity on the corrosion process. In all cases, there some carbon was observed, which manifested as slightly higher concentration of chromium–carbide phase at the grain boundaries compared to the unexposed alloy.     

Influence of the Oxygen Partial Pressure on the Oxidation of Inconel 617 Alloy at High Temperature

Ni-based superalloy Inconel 617 (IN617) is one of the main candidate structural materials for high temperature components (heat exchanger) of the gas-cooled fast reactor (GFR), a possible candidate for generation IV nuclear reactor. The material in operating conditions will be exposed to impure He at a temperature of around 850 °C. The impurities are expected to be oxidizing (such as O₂, H₂O) but since no feedback experience is available for this type of reactor, the level of impurities is completely unknown. Hence, an attempt has been made to understand the influence of oxygen partial pressure on oxide composition and on the oxidation mechanisms of IN617 at 850 °C. To achieve this, oxidation tests were performed at 3 different range of partial pressure: 10^?5, 0.2 and 200 mbar. Tests were performed from 1 h to 28 days and the obtained oxide layers were characterized using MEB, EDX, XPS, XRD and GD-OES. The oxide layers were mainly composed of chromia containing TiO₂ and thickening with time. Aluminium oxide formed internally. Other oxides were detected in the scale, such as NiO, CoO, MoO₃ and MnO₂, except for the lowest oxygen partial pressure experiments, where a selective oxidation took place. The scale-growth mechanism was cationic for low and medium oxygen partial pressure conditions. A growth following a transient oxidation mechanism was observed for high oxygen partial pressure.     

Paralinear Oxidation of Chromium in O2 + H2O Environment at 600–700 °C

The oxidation of chromium in dry O₂ and in O₂ + 10%H₂O at 600 and 700 °C is studied. Scale morphology is investigated by several methods, including scanning electron microscopy (SEM) of cross sections prepared by focussed ion beam milling (FIB). In O₂ + H₂O at 600 and 700 °C, chromium forms a duplex scale consisting of an inner barrier oxide and a discontinuous outer oxide made up of blade-shaped crystals. Thermogravimetric (TG) measurements show that water vapour influences chromium oxidation by causing vaporization of the protective oxide, resulting in paralinear oxidation kinetics. An extension of the original treatment by Tedmon is deduced, which allows for the determination of the evaporation rate constant k _s and the parabolic oxidation rate constant k _d from TG data acquired during short exposures. The results show that k _d is the same in dry O₂ and in O₂ + 10%H₂O. Equivalently, the transport properties of chromia are the same in the two environments. The equilibrium constant of CrO₂(OH)₂ formation from chromia is reported. The activation enthalpy of the vaporization reaction is determined.     

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O₂ + 40% H₂O. Prior to exposure the samples were sprayed with KCl, K₂CO₃ or K₂SO₄, the amount of salt corresponding to 1.35 ??mol K⁺/cm². For reference, salt-free samples were exposed in 5% O₂ + 40% H₂O and in 5% O₂ (N₂ was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K₂CO₃ strongly accelerate the corrosion of 304L while K₂SO₄ has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K₂CO₃ react with the chromium-rich oxide on the sample surface, forming K₂CrO₄. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO₂ and SO₃).     

Oxidation Behavior of UHP Fe-Based Model Alloy and SUS310S Steel in Superheated Steam and Supercritical Water Conditions

The quest for improvements in cycle efficiency of energy production plants leads to progressively higher operating temperatures and pressures in steam boiler. For this reason, the oxidation resistance of structural materials is the key issue, and hence, the consequences of increased growth rate of the oxides pose serious concern, such as oxide exfoliation. The oxidation behavior of ultra-high-purity (UHP) Fe–23Cr–23Ni–2Mo–0.6Nb wt% model alloy and SUS310S in steam and super critical water at atmospheric pressure and 25 MPa, respectively, and at 973 K (700 °C) is investigated. To elucidate the oxidation behavior, weigh gain measurement and characterization of the resulting oxide were performed. The critical chromium content to form and maintain protective oxide increased with increasing the pressure, 23 wt% in steam and 26 wt% in SCW conditions. Grain refinement was efficient for enhancing the oxidation resistance of SUS310S.     

Effect of Shot-peening on the Oxidation Behaviour of Boiler Steels

The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study.     

Hot Corrosion Behavior of a Ni3Al-Based IC21 Alloy in a Molten Salt Environment

Ni₃Al-based alloys have become important candidates for hot components in turbine engines, owing to their low densities and outstanding mechanical properties in service environments. The hot corrosion behavior of a Ni₃Al-based IC21 alloy in a molten salt environment of 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C was studied, via oxidation kinetics analyses, scanning electron microscope observations and energy dispersive as well as diffraction analyses by X-ray. A multilayer corrosion oxide scale and dendritic morphology internal corrosion zone formed after hot corrosion, and inter-phase selective corrosion phenomena were also observed. Salt fluxing and oxidation-sulfidation processes were inferred to be the essential hot corrosion mechanisms of the alloy. Moreover, additions of Cr and Y proved to be beneficial to the hot corrosion resistance of the IC21 alloy, while the Mo content should be strictly controlled.     

Thermogravimetric Study of Oxidation-Resistant Alloys for High-Temperature Solar Receivers

Three special alloys likely to be suitable for high-temperature solar receivers were studied for their resistance to oxidation up to a temperature of 1050°C in dry atmospheres of CO₂ and air. The alloys were Haynes HR160, Hastelloy X, and Haynes 230, all nickel-based alloys with greater than 20% chromium content. The oxidation rate of specimens cut from sample master alloys was followed by thermogravimetry by continuously monitoring the weight change with a microbalance for a test duration of 10 h. The corrosion resistance was deduced from the total weight increase of the specimens and the morphology of the oxide scale. The surface oxide layer formed (scale) was characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy and in all cases was found to be chromia. Oxidation was analyzed by means of parabolic rate law, albeit in some instances linear breakaway corrosion was also observed. For the temperature range investigated, all alloys corroded more in CO₂ than in air due to the formation of a stronger and more protective oxide scale in the presence of air. At 1000°C, the most resistant alloy to corrosion in CO₂ was Haynes 230. Alloy Haynes HR160 was the most oxidized alloy at 1000°C in both CO₂ and air. Hastelloy X oxidized to a similar extent in CO₂ at both 900°C and 1000°C, but in air, it resisted oxidation better at 1000°C than either at 900°C or 1000°C.     

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-m 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.     

Kinetics of Internal Oxidation of Mn-Steel Alloys

Internal oxidation of three Mn-steel alloys with 1.7, 3.5 and 7.0 wt% Mn concentration at 950 °C in a gas mixture composed of nitrogen, hydrogen and water vapor with a dew point of +10°C was evaluated. For these alloys, the kinetics of internal oxidation are diffusion-controlled and obey parabolic growth rate law. The diffusion coefficient of oxygen and manganese determined from the observed internal oxidation kinetics are 3.35 × 10^?7 and 4.14 × 10^?12 cm²/s at 950 °C, respectively. The formed internal oxide precipitates are mainly composed of MnO. The solubility product of MnO in an austenitic iron matrix is estimated to be (7.66 ± 0.18) × 10^?9 mol fraction² at 950 °C. The numerical simulation of concentration depth profiles of precipitated oxygen is in agreement with depth profiles determined with image analysis and X-ray microanalysis. Validity of the numerical simulation in case of the phase transformation was also tested. When a 1.7 wt% Mn-steel alloy is oxidized at 850 °C (instead of 950 °C) with a dew point of +12 °C partial phase transformation from austenite to ferrite takes place due to the Mn depletion. The associated precipitated oxygen concentration depth profile can be predicted accurately with numerical simulation.     

Dependence on the Chromium Content of the High-Temperature Oxidation Behavior of Ta-Rich Nickel-Based Cast Alloys

The high-temperature stability of primary tantalum carbides is a problem of importance for chromium-rich cast alloys, based on cobalt or nickel. The focus of this study was nickel-based alloys, as these alloys are particularly sensitive to a lack of TaC in the as-cast state and by dissolution due to high-temperature exposure. In this work, a possible way for promoting the formation of many TaC precipitates by changing from the usual 30 wt% chromium content was investigated. Five alloys with Cr content varying from 10 to 50 wt% were prepared and then subjected to microstructure characterization and to oxidation tests. In contrast with what was expected, decreasing the Cr content in comparison with the Ni–30Cr–0.4C–6Ta reference alloy did not succeed in obtaining more TaC precipitates, but instead had the opposite effect. Concerning the high-temperature oxidation behavior at 1127 and 1237 °C, loss of resistance was observed only for a Cr content at the lower level of 10 wt%. It was noticed that a subscale CrTaO₄ developed during oxidation and seemed to promote oxide spallation during cooling.     

High-Temperature Corrosion Behavior of Different Regions of Weldment of 2.25Cr-1Mo Steel in SO<Subscript>2</Subscript> + O<Subscript>2</Subscript> Atmosphere

This paper investigates the corrosion behavior of different regions of weldment of 2.25Cr-1Mo steel exposed in mixed oxidation and sulfidation (SO₂ + O₂) environment up to 500 h at 773 K. Microstructural investigation and characterization of oxide scales are done using SEM, TEM, and XRD. The obtained results infer that heat-affected zone corrodes faster than both base and weld metal. The reaction kinetics follows a parabolic growth rate for all regions. The higher corrosion rate of heat-affected zone is attributed to the formation of Cr₂₃C₆ secondary precipitates leading to depletion of protective inner scale of the Cr-rich oxide during welding.