Oxidation Characteristics and Oxide Layer Evolution of Alloy 617 and Haynes 230 at 900 °C and 1100 °C

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr₂O₃ double layer and isolated NiO–NiCr₂O₄–Cr₂O₃ triple layer were observed at a steady-state condition. For Haynes 230, a MnCr₂O₄ layer was formed on top of the Cr₂O₃ layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr₂O₃ and outer MnCr₂O₄ or TiO₂. The spallation of outer layer and subsequent volatilization of the Cr₂O₃ layer produced a rugged surface and interface as well as internal oxidation.     

In Situ Oxide Growth Characterization of Mn-Containing Ni–25Cr (wt%) Model Alloys at 1050 °C

The oxidation behaviour of four model alloys with the composition Ni–25Cr–xMn (with x = 0, 0.5, 1 and 1.5 wt%) was investigated at 1050 °C in air by thermogravimetry and by in situ observations in an environmental scanning electron microscope (ESEM). The addition of manganese modifies the oxidation rate of Ni–25Cr alloys by (1) increasing the parabolic constant k _p compared to that for Ni–25Cr and (2) lowering the short-time oxidation rate. Regardless of the Mn concentration, in situ ESEM observations indicated the formation of spinel crystallites from the very beginning of the oxidation process. The size of the spinel crystallites was directly linked to the initial Mn concentration. The obtained results suggested that the formation of the spinel layer at the top of the chromia oxide scale rather than at the metal–oxide interface as thermodynamically expected must be attributed to the higher diffusion rate of Mn than Cr both in the lattice and at the grain boundary.     

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.     

Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

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.     

High-Temperature Oxidation Behavior of SIMP Steel at 800 °C

In this work, the high-temperature oxidation behavior of SIMP and commercial T91 steels was investigated in air at 800 °C for up to 1008 h. The oxides formed on the two steels were characterized and analyzed by XRD, SEM and EPMA. The results showed that the weight gain and oxide thickness of SIMP steel were rather smaller than those of T91 steel, that flake-like Cr₂O₃ with Mn_1.5Cr_1.5O₄ spinel particles formed on SIMP steel, while double-layer structure consisting of an outer hematite Fe₂O₃ layer and an inner Fe–Cr spinel layer formed on T91 steel, and that the location of the oxide layer spallation was at the inner Fe–Cr spinel after 1008 h, which led the ratio between the outer layer and the inner layer to decrease. The reason that SIMP steel exhibited better high-temperature oxidation resistance than that of T91 steel was analyzed due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Oxidation Kinetics and Oxide Scale Characterization of Nickel-Based Superalloy IN738LC at 900 °C

The high-temperature isothermal oxidation behavior of the polycrystalline nickel-based superalloy IN738LC was investigated at 900 °C in air for up to 1000 h. The results from the tests suggest that the alloy showed single-stage parabolic oxidation behavior during isothermal oxidation. The oxidized samples were characterized using SEM and SEM/EDS, and the results show that the alloy is comprised of an outer dense chromia scale with titania proving Type II oxidation behavior. In addition, the formation of a spinel composition adjacent to the external layer and a discontinuous needle-shaped alumina scale in the alloy subsurface zone were also observed. The depletion of gamma prime (γ′) phase leads to a precipitate-free zone formation in the subscale zone. A JMatPro thermodynamic analysis showed that an increase in titanium content from 1 to 3.44 wt.% increased the chromium activity by 50%. Therefore, the results suggest that the presence of high amounts of titanium (~3.44 wt.%) in IN738LC increased the oxidation kinetics by increasing the chromium scale growth rate and resulting in an oxidation rate constant of 2.79 × 10^?6 mg² cm^?4 s^?1.     

Cyclic and Isothermal Oxidation at 1,100 °C of a CVD Aluminised Directionally Solidified Ni Superalloy

CVD aluminide coatings deposited on a Directionally Solidified (DS) substrate were oxidized at 1,100 °C up to 240 h under isothermal and cyclic oxidation conditions to study the growth mechanisms of the oxide scales and the possible degradation of the coatings. The specimens were investigated using light and scanning electron (SEM) microscopy, energy-dispersive spectrometry (EDS) and X-Ray Diffraction (XRD). The results indicate that the coatings provide a much greater beneficial effect under isothermal conditions than upon cycling. The cycled specimens undergo oxide-scale spallation and increased roughening, which can derive from growth and thermal stresses as well as from the NiAl → Ni₃Al phase transformation associated with Al depletion. Under isothermal conditions, typical oxide scales formed with the appearance of some rumples. However, the origin of rumpling is uncertain from these experimental results.     

Improvement of Oxidation Resistance of γ-TiAl at 900 and 1000 °C Through Hot-dip Aluminizing

Hot-dip aluminizing method and subsequent interdiffusion treatment were used to develop a TiAl₃ coating on Ti–45Al–2Cr–2Nb–0.15B (at.%) alloy. A two-phase coating consisting of an outer pure Al layer and an inner TiAl₃ layer formed on the alloy after the hot-dip and then a single phase TiAl₃ coating was obtained by using interdiffusion treatment. Oxidation of the TiAl₃ coating was conducted at 900 and 1000 °C. Both the interrupt oxidation and the isothermal oxidation tests indicated that the coating provided high protectiveness for the alloy. The coating was stable for at least 300 h during the interrupt oxidation at 900 and 1000 °C, and it was stable for at least 500 h at 1000 °C and 1000 h at 900 °C during the isothermal oxidation. The oxidation behavior of the coating was discussed in detail.     

Influence of Titanium on the High Temperature Oxidation and Chromia Volatilization of Ternary Ni–Cr–C Alloys

Two ternary alloys, Ni–25Cr–0.25C and Ni–25Cr–0.50C (wt%) and three versions containing also titanium (1 and 2 wt%) were cast and submitted to oxidation in dry synthetic air at 1200 °C to observe the effects of titanium on the behaviors of the nickel-based alloys belonging to this category. The mass gains are wholly parabolic in all cases and the values of the parabolic and chromia volatilization constants are typical of a chromia-forming behavior. The mass gains of the Ti-containing alloys are faster than for the Ni–Cr–C alloys, and these kinetic differences are consistent with the differences in chromia thickness and in Cr-impoverishment of the subsurface. In addition, the presence of Ti led to the development of thin a TiO₂ outer scale isolating chromia from hot air, but without benefit for the protection of Cr₂O₃ against volatilization. The obtained results also suggest that Ti may perturb the Cr diffusion in volume but also delay the oxide spallation during cooling.     

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.     

Oxidation Behavior of Sputtered DD98M Nanocrystalline Coating at 1000 °C

Increasing the efficiency of coal fired steam power plants is an important contribution towards clean coal power. In fact, new ferritic steels are expected to withstand 325 bar and 650 °C. Moreover, in order to facilitate CO₂ capture oxygen can be used instead of air for combustion (oxycombustion) so that no NO_X emissions are produced. Boiler components, such as superheater tubes, are exposed to both steam and fireside corrosion and at higher temperatures, ferritic steels corrode at very fast rates under both atmospheres. A solution can be found in the use of protective coatings, a number of which, applied by techniques capable of depositing said coatings both on the inner and outer surfaces of tubes, are being studied within nationally and European funded projects. In particular, two new Ni and Cr modified aluminide coatings deposited on P92 by non-line-of-sight hybrid processes have been produced and the preliminary results of on-going laboratory testing, both under oxycombustion model atmospheres as well as under pure steam at 650 °C are promising, in particular those exhibited by the Cr enriched aluminide coating. Moreover, results obtained in a pilot oxycombustion boiler operated by CIUDEN in Leon, Spain are also shown.     

Kinetics of High Temperature Oxidation and Chromia Volatilization for a Binary Ni–Cr Alloy

The volatilization of the external chromia layer formed by oxidation of a Ni–30Cr alloy was studied at 1000, 1100, 1200 and 1300°C. This was done by analyzing the mass gain curves obtained by thermogravimetry on the basis of the differential equation that describes the oxidation kinetic, taking into account at each step the loss of Cr₂O₃ oxidized again into gaseous CrO₃. The quantification of the parabolic constant and the volatilization constant was done by drawing . This led to the values of both K_p and K_ν for the four tested temperatures. These were compared to chromium balance sheets obtained by measuring the thicknesses of the chromia layer and chromium depletion in the alloy. A good agreement was found between the two methods. They both follow an Arrhenius law and precise values of the two constants are provided.     

Surface Condition of New γ–γ′ Co-Al-Mo-Nb and Co-Al-W Cobalt-Based Superalloys After Oxidation at 800 °C

A new type γ–γ′ Co-Al-Mo-Nb Co-based superalloys were developed due to limitations of basic Co-Al-W superalloys, related to tungsten alloying. The present study aims to characterization of new γ–γ′ Co-10Al-5Mo-2Nb (at.%) cobalt-based superalloy performance in terms of the high-temperature exposure under cyclic conditions, with particular regard to surface condition. Specimens were tested in cycles of high-temperature exposition (25, 50, 75, 100 and 150 h) in air environment at 800 °C. Detailed analysis of oxidized surfaces by scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction was made at various intervals during testing. The cyclic oxidation behavior of new alloy was compared to the basic Co-9Al-9W (at.%) Co-based superalloy.     

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.     

Characterization of Alumina Scales Grown on a 2nd Generation Single Crystal Ni Superalloy During Isothermal Oxidation at 1050, 1100 and 1150 °C

This paper concerns microstructural characterization of alumina scales formed on a 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050, 1100 and 1150 °C for 100 h. Samples for high resolution characterization of Al₂O₃ scales were prepared using a focused ion beam (FIB) method. High resolution TEM and S/TEM techniques were used for a detailed characterization and a direct comparison of the phase composition and chemistry of the oxide scales formed during high temperature oxidation. The growth of transient θ-Al₂O₃ and its transformation to α-alumina is addressed for each oxidation temperature along with the differences in the diffusion of reactive elements, such as Hf, Zr and Y, through grain boundaries of the α phase. The θ to α transformation front was proven to move from the metal-scale to the scale-gas interface. The results presented in this paper indicate that after 100 h of oxidation at 1050 and 1100 °C there are still some θ-alumina grains remaining and even in the regions where the transformation to α was finished the surface retained the whisker-like morphology.     

Oxidation of Zirconium Alloys in Oxygen at High Temperatures up to 1600 °C

The oxidation kinetics of the classic pressurized water reactors (PWR) cladding alloy Zircaloy-4 has been extensively investigated over a wide temperature range. In recent years, new cladding alloys optimized for longer operation and higher burn-up are being increasingly used in Western light water reactors (LWR). These alloys were naturally optimized regarding their corrosion behavior for operational conditions. The publicly available data on high temperature oxidation of the various cladding materials are very scarce. This paper presents the results of a first test series with Zircaloy-4 as reference material, Framatome Duplex cladding, Framatome M5^® and the Russian E110 alloy. The first two are Zr–Sn, the latter two Zr–Nb alloys. All materials were investigated in isothermal and transient tests in a thermal balance under argon–oxygen atmosphere. Strong and varying differences (up to 500%) of oxidation kinetics between the alloys were found till 1000 °C, where the breakaway effect plays a role. Smaller but still significant differences (20–30%) were observed at higher temperatures. Generally, the advanced cladding alloys here studied show also a favorable behavior at high temperatures during accident scenarios.     

A Critical Compressive Stress for Increasing the Oxidation Kinetics of Fe–20Cr Alloy Oxidized at 900 °C

The effects of compressive stresses on the oxide-scale morphologies formed on an Fe–20Cr alloy were investigated by comparison of the oxidation behavior in air under classical conditions, i.e., without any applied mechanical stresses and under static compressive stresses, at 900 °C. The study was carried out mainly by comparisons of oxidation kinetics gained by thermogravimetric analysis (TGA), surface morphologies of oxidized specimens observed by scanning electron microscopy (SEM), oxidized products examined by X-ray diffraction (XRD). It was found that the application of compressive stresses induced an increase in oxidation rate, but a decrease of oxide grain size. When the stresses are in the range of 5–8 MPa, both chromium- and iron-oxides formed but, at other stresses, only chromia was present. In particular, there was a maximum in oxidation rate when the applied stress was 5 MPa. The paper places emphasis on analyzing the cause of this phenomenon.