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.     

Selective Oxidation of Chromium by O2 Impurities in CO2 During Initial Stages of Oxidation

This study shows that the corrosion behaviour of 12 wt% Cr steel in CO₂ at 550 °C is determined in the first stage of oxidation by reaction with O₂ impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9–12 wt% chromium steels from fast corrosion rate have emerged.     

Effect of H2O and CO2 on the Oxidation Behavior and Durability at High Temperature of ODS-FeCrAl

Cyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O₂, H₂O and CO₂. Compared to 1 h cycles in dry O₂, exposure in air + 10 vol.% H₂O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO₂ + 50 % H₂O + 0.75 % O₂ had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO₂ + 50 % H₂O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O₂ was added to the CO₂/H₂O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000.     

Corrosion of Binary Fe–Si Alloys in Reducing Oxidizing and Sulfidizing–Oxidizing Atmospheres at 700 °C

The corrosion behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si was studied at 700 °C in an H₂–CO₂ gas mixture providing 10^?20 atm O₂ as well as in an H₂–H₂S–CO₂ gas mixture providing the same oxygen pressure coupled to an S₂ pressure of 10^?8 atm. All the alloys followed complex kinetics which were mostly linear for Fe–5Si, but showed one or two parabolic stages for the other two alloys. Simple oxidation produced essentially two-layered scales in which Si was confined to the alloy consumption zone in the form of silicon oxide and iron-silicon double oxide. Corrosion in the oxidizing–sulfidizing gas mixture produced scales composed of a thick external zone of pure FeS followed by an internal region containing complex mixtures of FeS with Si and Fe oxides. Internal oxidation of silicon was only observed for the oxidation of Fe–5Si in both environments. The extent of corrosion decreased in both gas mixtures with an increase in the Si content of the alloys. Finally, the addition of sulfur produced a significant increase of the overall mass gains for each alloy.     

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.     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.     

Effects of Corrosion in Supercritical CO<Subscript>2</Subscript> on the Microstructural Evolution in 800H Alloy

This study investigates corrosion of Fe–Ni-based alloy 800H that were exposed to supercritical CO₂ (sCO₂), ambient air and argon gas at pressures up to 20 MPa, at 650 and 750 °C for up to 1000 h. This alloy and other comparable metal alloys are expected to be used in sCO₂ heat exchanger cycles as proposed in the DOE Advanced Ultra-Supercritical program. Alloy 800H is considered for this application, because it meets the high-temperature strength and creep rupture requirements and is a lower cost alternative to other Ni-based alloys. The oxidation performance and microstructural changes due to exposure in sCO₂ have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO₂ as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO₂. No such zone was observed after exposure to Ar, suggesting this recrystallization was associated with the oxidation process and not simply an effect of surface finishing. A wider recrystallized zone was observed underneath the oxide formed in sCO₂ than in air. The effect of air and sCO₂ on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO₂. It was concluded that the carbon species provided by the sCO₂ atmosphere in conjunction with the increased grain boundary density in the recrystallized zone allowed for more ingress of carbon into the base metal, which resulted in a higher densities of carbides beneath the oxide scale.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.     

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.     

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.     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

An XPS Study of Native Oxide and Isothermal Oxidation Kinetics at 300 °C of AZ31 Twin Roll Cast Magnesium Alloy

Magnesium alloys are very important for lightweight applications. Industrially, these alloys cannot be used without some necessary processing to improve their corrosion properties. The most widely used methods include coating, surface treatments and cladding. In these processes, the magnesium oxide scale plays an important role in the bonding mechanism between the substrate and the coating or the cladding materials. The aim of this study is to understand the spontaneous oxide formation and the initial oxidation kinetics of the TRC AZ31 magnesium alloy. The results are important for the understanding of the subsequent surface treatment processes of that alloy. Therefore, the study comprises: first, the analysis of the native oxide which forms spontaneously after Twin Roll Casting of an AZ31 magnesium sheet, and second, the investigation of the oxidation behavior of the AZ31 magnesium alloy heated in air at 300 °C with different exposure times ranging from 1 to 180 min. The results showed that the thickness of the native oxide is 5 nm and the oxide surface mainly comprises of magnesium, oxygen, and carbon compounds. The oxide film contains magnesium oxide in the form of MgO as the main oxide compound with a very thin layer of MgCO₃ and Mg(OH)₂. The X-ray photoelectron spectroscopy results revealed two stages of oxidation kinetics during exposure to 300 °C. Rapid growth represents the first stage, which lasts for about 30 min. After that period, the oxide growth slowed, indicating a steady state character, where the oxide film growth approaches a limit value. This slow growth is due to the lack of diffusion of oxygen into a dense oxide layer possessing a low concentration of defects. The oxidation kinetics follows an inverse logarithmic law.     

Influence of the Oxygen Partial Pressure on the High Temperature Corrosion of 38Ni–34Fe–25Cr Alloy in Presence of NaCl Deposit

In biomass gasification processes, some molten salts formed during the process can promite high temperature corrosion. In this study the chromia-forming austenitic alloy Haynes^® HR-120 was oxidized with a deposit of sodium chloride for 96 h at 825 and 900 °C. Two different atmospheres were selected; one with a high oxygen partial pressure (Ar/O₂ 90/10 %vol.) and one, named syngas, with a low oxygen partial pressure (CO/H₂/CO₂ 45/45/10 %vol.). While at 900 °C the behaviour of the alloy in presence of sodium chloride was catastrophic in high oxidizing conditions, the impact of sodium chloride was insignificant in the syngas atmosphere. When exposed to the Ar/O₂ mixture, the catastrophic oxidation was attributed to the setting up of an active oxidation. At 900 °C under the syngas atmosphere, the protective behaviour of the alloy seems linked to the association of a faster evaporation of the salt and a very low oxygen partial pressure. At 825 °C a catastrophic behaviour is observed under the syngas atmosphere as the NaCl evaporation rate is much slower.     

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).     

SECM Study of Effect of Chromium Content on the Localized Corrosion Behavior of Low-Alloy Steels in Chloride Environment

This paper investigates the effect of chromium (Cr) content (0, 1, 3 and 5% Cr) in epoxy-coated alloy steel against corrosion using in situ electrochemical techniques such as EIS and SECM in a 3% NaCl solution. The EIS results revealed that the epoxy-coated Cr steel exhibited higher impedance values than carbon steel, which is attributed to the greater resistance of Cr steel toward corrosion. Based on the cyclic voltammogram results, the tip potentials were set at ?0.7, 0.04 and 0.60 V for determining the concentration of dissolved oxygen at cathodic region, and oxidation of Cr²⁺ and Fe²⁺ at anodic region, respectively. The SECM measurements showed that, the tip current in the anodic region has decreased with increase in Cr content of the sample, which indicates that the oxidation of Fe²⁺ and Cr²⁺ decreases (corrosion is reduced) with the increase in Cr content of the steel. Besides, 5% Cr steel can maintain the highest corrosion resistance, and 1 and 3% Cr steels have higher corrosion resistance than the 0% Cr steel. This higher corrosion resistance of Cr steel samples could be due to the formation of Cr-rich hydro-oxide layers [Cr(OH)₃ as a corrosion product] on the surface of the samples. Thus, the epoxy-coated Cr alloy steel has greater corrosion resistance in a chloride-containing environment than the carbon steel. Hence, epoxy-coated Cr alloy steel can be successfully used as a construction material in structures.     

The influence of bromine on the gaseous oxidation of an austenitic stainless steel

The effects of bromine on the oxidation behaviour of an austenitic stainless steel have been studied in oxygen and a CO₂/10%CO mixture at 750°C and 1 atm total pressure. Supporting work has also been carried out on chromium and nickel with similar results. Bromine at 0·06 atm enhanced under certain circumstances the formation of oxide scale in oxygen, but led to the formation of volatile halides in the CO₂/CO mixture. The former effect was only obtained when the bromine was present at the commencement of oxidation, or preformed protective oxide films were fractured by thermal cycling.     

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.     

Simulation of Fe-Cr-X Alloy Exposed to an Oxyfuel Combustion Atmosphere at 600 °C

In coal-fired power plants using oxyfuel combustion process with carbon capture and sequestration, instead of air, a mixture of oxygen and recirculated flue gas is injected in the boiler. A series of steels were exposed to CO₂-SO₂-Ar-H₂O gas mixtures at 600 °C for 1000 h to compare their high temperature corrosion behavior. During the corrosion process, carburization, decarburization and recrystallization were observed underneath the oxide scale depending on the gas mixture and alloy composition. The conditions that lead to carburization are not yet completely understood, but decarburization can be simulated using thermodynamic and kinetic models. In this work, the results of these simulations are compared with measured values for one of the alloys that displayed a decarburized region. Since the mobility of carbon in the scale is not known, two strategies were adopted: simulation of alloy-atmosphere contact; and estimation of the carbon flux to produce the observed decarburization. The second approach might give an insight on how permeable to carbon the scale is.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

The oxidation behavior of an austenitic steel, type 1.4841, with a Cr content of 25 wt% and a high-Si content of 2.8 wt% was studied during isothermal oxidation at 1,286 K in air. A thick, crystalline Cr₂O₃ layer, on top of a much thinner, amorphous SiO₂ layer, developed on the alloy substrate. After formation of a closed Cr₂O₃ scale, parabolic growth kinetics prevailed as long as the associated constant, steady-state Cr concentration in the alloy at the substrate/oxide interface of about 13 ± 1 wt% was maintained. Upon prolonged oxidation, successive cracking and spallation of the thickening oxide scale eventually led to breakaway oxidation, because the “bulk-”Cr concentration in the interior of the alloy dropped below the critical value required to ‘heal’ the protective oxide layer after oxide spallation. Application of a lifetime prediction model of the alloy substrate under isothermal oxidation conditions allowed determination of the breakaway-oxidation time as a function of alloy-sheet thickness, by employing the Cr volume-diffusion coefficient in the alloy and the parabolic growth-rate constant, both determined in the present study by fitting calculated to experimental Cr-depletion profiles for various oxidation times.