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.     

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.     

In-Situ Electrochemical Corrosion Behavior of Nickel-Base 718 Alloy Under Various CO<Subscript>2</Subscript> Partial Pressures at 150 and 205 °C in NaCl Solution

The electrochemical corrosion behavior of nickel-base alloy 718 was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization techniques at various partial pressures of CO₂ (\(P_{{{\text{CO}}_{2} }}\)s) in a 25 wt% NaCl solution at 150 and 205 °C. The passive films composed of FeCO₃ exhibit good corrosion resistance with a feature of Warburg impedance, Tafel plots show a complete passivation and the anodic reactions was dominated by a diffusion process at low \(P_{{{\text{CO}}_{2} }}\)s (1.8–9.8 MPa) at 150 °C. While numerous dented corrosion areas appeared on the sample surface for the \(P_{{{\text{CO}}_{2} }}\) of 11.6 MPa at 205 °C, the Tafel plot with three anodic peaks and the Nyquist diagram with an atrophied impedance arc were present. This dented corrosion attribute to the synergistic effects of stress, temperature, \(P_{{{\text{CO}}_{2} }}\) and Cl^?, the temperature and stress could play crucial roles on the corrosion of the alloy 718.     

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.     

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.     

Water Vapour Effect on the Oxidation Mechanism of a Cobalt-Based Alloy at High Temperatures (800–1,100 °C)

A cobalt-based Phynox alloy was oxidized in the 800–1,100 °C temperature range. The alloy oxidation was consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Water vapour enhanced the alloy oxidation rate and scale porosity. Thermal cycling tests at 900 and 1,000 °C showed that water vapour reduces the outer Mn_1.5Cr_1.5O₄ subscale adherence, but the chromia scale adherence was not affected. These temperatures permited a rapid chromium supply from the substrate to form a continuous chromia scale. At 1,100 °C thermal cycling conditions led to scale spallation and chromium depletion in the alloy. In dry air, weight losses were recorded due to cobalt and molybdenum oxidation, giving CoCr₂O₄ and CoMoO₄. In wet air, the initial porous chromia scale permited nickel and cobalt oxidation, leading to Ni₅Co₃O₈ and CoCr₂O₄ formation and resulting in bad adherence during thermal cycling.     

Oxidation Behavior of K4750 Alloy at Temperatures Between 750 ℃and 1000 ℃

This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750 ℃-1000 ℃ in air for 100 h-1000 h by isothermal oxidation tests.The oxidation-kinetic curves were plotted by the static discontinuous weight gain method.Observation and identification of oxidation products were conducted using scanning electron microscopy(SEM),energy dispersive spectroscopy (EDS),electron probe micro-analysis (EPMA) and X-ray diffraction (XRD).X-ray photoelectron spectrometer (XPS) was also used to analyze the chemical state of elements and the distribution in depth.The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law.The average oxidation rate below 900 ℃ was less than 0.1 g/m2.h which meant the alloy was at a complete anti-oxidation grade,and the alloy was at an anti-oxidation grade at 1000 ℃.The predominant surface oxide was Cr2O3,and a double layer structure of the oxide scale was observed at all tested temperatures as time increased.The outer oxide layer contained continuous Cr2O3 and a small amount of oxides mixed TiO2 and NiCr2O4,while the inner oxide layer was composed with Al2O3.The oxidation process could be interpreted by the competitive oxidation of different elements.The diffusion rate of Ti through Cr2O3 layer increased with increasing temperature,and thus the generation of TiO2 was advantageous.The dispersed TiO2 reaching a certain amount destroyed the continuity of the surface oxide layer,which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.     

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.     

Effect of Yttrium as Alloying Element on a Model Alumina-Forming Alloy Oxidation at 1100 °C

In order to study the effect of yttrium as alloying element on the high-temperature oxidation of an alumina-forming alloy, 0.093 wt% yttrium was incorporated into a model FeCrAl alloy. Yttrium has a beneficial effect on the isothermal oxidation behavior in air at 1100 °C. Glancing angle X-ray diffraction made on a sample oxidized for 1000 h under thermal cycling conditions indicated that yttrium is located at the internal interface as Y₃Al₅O₁₂. Secondary neutral mass spectrometry results showed that the diffusion mechanism is modified by the presence of yttrium as an alloying element. Moreover, the beneficial effect of yttrium on the alloy oxidation is also related to a reduced metallic grain size. The growth of metal grains during oxidation was especially observed on the yttrium-free FeCrAl alloy. It is also well established that the diffusion mechanism in the oxide scale is modified by yttrium. The aim of the present work was to show that yttrium also plays a role on the aluminum diffusion in the metallic substrate and has a strong influence on the kinetic transient stage during the FeCrAl–0.1Y oxidation.     

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.     

Oxidation of Slurry Aluminide Coatings on Cast Stainless Steel Alloy CF8C-Plus at 800 °C in Water Vapor

A new, cast austenitic stainless steel, CF8C-Plus (CF8C-P), has been developed for a wide range of high temperature applications, including diesel exhaust components, turbine casings and turbocharger housings. CF8C-P offers significant improvements in creep rupture life and creep rupture strength over standard CF8C steel. However, at higher temperatures and in aggressive environments such as those containing significant water vapor, an oxidation-resistant protective coating will be necessary to extend service life. The oxidation behavior of alloys CF8C and CF8C-P with various aluminide coatings were compared at 800 °C in air plus 10 vol% water vapor. Due to their affordability, slurry aluminides were the primary coating system of interest, although chemical vapor deposition and pack cementation coatings were also compared. Additionally, a preliminary study of the low-cycle fatigue (LCF) behavior of aluminized CF8C-P was conducted at 800 °C. Each type of coating provided substantial improvements in oxidation behavior, with simple slurry aluminides exhibiting very good oxidation resistance after 3,000 h testing in water vapor. Preliminary LCF results indicated that thicker aluminide coatings degraded high temperature fatigue properties of CF8C-P, whereas thinner coatings did not. Results suggest that appropriately designed slurry aluminide coatings are a viable option for economical, long-term oxidation protection of austenitic stainless steels in water vapor.     

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.     

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.     

Hot Corrosion Behavior of Some Superalloys in a Simulated Incinerator Environment at 900 °C

Incinerators are being used to burn solid waste of all types. This burning of waste creates a very aggressive environment at extremely high temperature. This environment attacks the various components of the incinerators. Some studies have been reported regarding behavior of steels in simulated incinerator environment at 550 °C. In present work superalloys Superco 605, Superni 600, and Superni 718 have been subjected to cyclic oxidation in 40 wt.% K₂SO₄ + 40 wt.% Na₂SO₄ + 10 wt.% KCl + 10 wt.% NaCl environment at 900 °C under cyclic condition. Weight change measurements have been done and weight change has been plotted against the numbers of cycles. The oxide scales formed on the surface of the corroded superalloys have been characterize by FESEM, EDS, XRD, cross-sectional analysis, and x-ray mapping. The nickel-based superalloys Superni 600 and Superni 718 indicated better resistance to corrosion in the above environment whereas Superco 605 lead to massive weight gain.     

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.     

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.     

Hot Corrosion of Yttria-Stabilized Zirconia Coating,in a Mixture of Sodium Sulfate and Vanadium Oxide at 950 °C

Yttria-stabilized zirconia thermal barrier coating along with CoNiCrAlY bondcoat was deposited using air plasma spray on Inconel-X750 superalloy. The coated samples were exposed at 950 °C in a mixture of Na₂SO₄ and V₂O₅. The exposed specimens were investigated using XRD and SEM. The formation of spinel and perovskite structures was revealed at the interface of topcoat and the bondcoat. Further, the chemical composition profile of all samples helped to analyze the diffusion behavior of different constituent elements of bondcoat and substrate. XRD analyses of the samples confirmed the phase transformation of the tetragonal zirconia into monoclinic zirconia and yttrium vanadate. The shift of high angle peaks indicated lattice distortion, which was directly related to the stresses in the coating.