Parabolic Rate Law Research Articles

Microstructure, high-temperature oxidation and molten salt corrosion behaviour of CoCrFeNiAl0.1-RE HEA

In this study, CoCrFeNiAl0.1-RE HEA was melted using a magnetic levitation melting furnace, resulting in a single-phase FCC solid solution structure with a coarse irregular equiaxed grain morphology in the period. The oxidation experiment results showed that the samples' oxidation kinetics followed the parabolic rate law, and the oxidation product species increased with increasing temperature. High temperatures primarily revealed Cr2O3, (Cr, Fe)2O3, and other spinel phases such as NiFe2O4, CoCr2O4, and CoFe2O4. Under the oxide film, Al-rich striped raised phases were observed. According to the elemental divisions, the oxide film will be damaged to expose the FCC matrix in molten salt corrosion, and the film can be divided into four layers: peeling layer, newly oxidised-corroded outer layer, transition layer, and matrix. As molten salt is destroyed, cavities and channels appear in the film, allowing molten salt and oxygen to infiltrate and accelerate the oxidation and corrosion of the sample.

High temperature oxidation behavior of aluminide coatings applied on HP-MA heat resistant steel using a gas-phase aluminizing process

Aluminide diffusion coatings were applied on a high-performance micro-alloyed (HP-MA) heat resistance steel, using a single step gas-phase aluminizing process. Powder mixtures with different compositions were employed for the coating process to achieve low-Al and high-Al coatings. This paper investigates the phase composition, growth mechanism, as well as high temperature oxidation behavior of aluminide-coated HP-MA steels. X-ray diffraction (XRD), scanning electron microscope (SEM), and field emission scanning electron microscopy (FESEM) were used to investigate characteristics of coated specimens. To evaluate the oxidation behavior, specimens were heated to 1100 °C for 100 h in the air atmosphere. Weight measurements were conducted at regular ten-hour time intervals. Furthermore, the parabolic rate constant (kp) values were obtained from the (ΔW/A) vs. oxidation time graphs. Moreover, stresses generated in oxide scale, formed on the aluminide coatings, were theoretically calculated following the first cycle of oxidation. Results showed that the stresses created in the oxide scale exceed the compressive strength of alumina, resulting in its spallation. After 100 h of cyclic oxidation test, the thickness of the oxide scale for the HP-MA steel, low-Al, and high-Al aluminide coated specimens was 22, 14, and 6 μm, respectively. The oxidation behavior of all specimens followed the parabolic rate law. The value of kp was the lowest at all oxidation time intervals for the coatings with higher Al content. This was due to the presence of more Al and Cr elements in the high-Al aluminide coating, improving the ability to form an outer dense alumina layer.

Corrosion Behavior of Surface Nanostructured IN718 Superalloy at 650°C

Abstract In the present study, the process of ultrasonic shot peening (USP) was adopted for developing nanostructures with a grain size of 62 ± 10 nm on the surface of the alloy IN718. The effect of nanostructures was investigated on high-temperature corrosion behavior under cyclic exposure at 650°C for 100 h in different salty environments (weight percent): 100NaCl (T1), 75Na2SO4 + 25NaCl (T2), and 90Na2SO4 + 5NaCl + 5V2O5 (T3). A near parabolic rate law was observed in both non-treated and treated conditions; however, the parabolic rate constant (kp) was reduced for the USP-treated specimens coated with Salt T1 and Salt Mixtures T2 and T3 by nearly 41 %, 52 %, and 50 %, respectively, with respect to the non-treated specimens. Several oxides and spinels such as TiO2, Fe2O3, Fe3O4, Cr2O3, FeVO4, NbO2, NiFe2O4, NiCr2O4, NiVO3, FeNbO4, FeCr2O4, Na2Cr2O7, and Cr2(MoO4)3 and sulfides NiS and FeS and chlorides CrCl3 and NiCl2 were observed in both treated as well as non-treated samples.

Isothermal Oxidation Behaviour of 800H Nickel-Based Alloy

The behaviour of 800H nickel (Ni)-based alloy under isothermal oxidation condition was study in this paper. The isothermal oxidation behaviour was examined in terms of fine grained and coarse grained 800H Ni-based alloy obtained from heat treatment operation at 1000 °C and 1150 °C, respectively. The heat treated samples were undergo an isothermal oxidation attempt at 500 °C in laboratory air for 500 hours exposure duration. The effects on the oxidation kinetics was examined in terms of weight change measurement. In addition, the effects on the oxide scale growth was observed in terms of phase analysis using XRD technique and surface morphology analysis using FESEM outfitted with EDX spectrometer. As a results, both fine grained and coarse grained 800H Ni-based alloy were obey a parabolic rate law showing that the oxide growth rate was followed a diffusion controlled mechanism. Additionally, fine grained 800H Ni-based alloy sample exhibited a inferior oxidation rate compared to coarse grained sample. The XRD analysis exhibited that the oxides scale composed of Cr2O3, TiO2 and MnCr2O4. The observation on the oxide scale morphology indicate that uniform oxide scales were formed on the alloy surface of both samples. However, coarse grained 800H Ni-based alloy recorded an oxide exfoliation which indicate poor oxidation protection.

Performance of air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 alloy subjected to high temperature corrosion environment

Thermal barrier coating plays a vital role in protecting materials’ surfaces from high-temperature environment conditions. This work compares the demeanour of uncoated and air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 substrates subjected to air oxidation and molten salt (Na2SO4 + 60%V2O5) environment condition at 900 °C for 50 cycles. Coating characteristics have been analyzed through microstructure, thickness, porosity, hardness, and bond strength. SEM, EDS and XRD analysis were used to analyze corrosion’s product at the end of the 50th cycle. Coating microstructures showed a uniform laminar structure that is adherent and denser with a coating thickness of 150 ± 20 μm and porosity less than 3.5%. The Microhardness of both the coated substrates were higher than that of the bare substrate. Cr3C2–25NiCr and NiCrMoNb coating bond strength was 38.9 MPa and 42.5 MPa. Thermogravimetric analysis showed the parabolic rate law of oxidation for all the substrates in both environments. In the molten salt environment, all the substrates exhibited higher weight gain compared to the air oxidation environment. In both environmental conditions, the uncoated X8CrNiMoVNb16–13 alloy exhibited higher weight gain than the coated substrates. The formation of Cr2O3, NiO and spinel oxide NiCr2O4 offers good resistance to corrosion to all the substrates in both the environmental condition. However, the presence of Mo and Nb significantly accelerated the corrosion of the substrate, thereby increasing the weight of the NiCrMoNb substrate. It is observed that Cr3C2–25NiCr and NiCrMoNb coating over the X8CrNiMoVNb16–13 substrate significantly protected the substrate against the hot corrosion than the bare alloy exposed to similar environmental conditions.

Long-term oxidation behaviour and thermal stability of heat-resistant stainless steel claddings deposited on AISI 316 stainless steel by the GTAW process

In the present study, the microstructure, oxidation behaviour, and thermal stability of the ER310 and ER446 weld overlay claddings deposited on AISI 316 stainless steel by the gas tungsten arc welding (GTAW) process were investigated. In this regard, the chemical composition, microstructure, and phase composition of the weld metals were characterized by optical microscopy (OM), field-emission scanning electron microscopy (FESEM-EDS), and X-ray diffraction (XRD) analysis. The oxidation behaviour and the bonding state of the clad plates were evaluated and compared after 1500 h of oxidation at 1000 °C using the isothermal oxidation and shear strength tests. Cross-sectional microstructural observations revealed that the substrates and weld overlay claddings were well bonded, and dense and uniform cladding layers without pores, cracks, or discontinuities were achieved. The weld metal produced by ER310 filler wire exhibited a completely austenitic microstructure, while the weld overlay produced by ER446 filler wire showed a ferritic-martensitic two-phase microstructure. The shear strength test results indicated that the bond strength of the ER446/AISI 316 and ER310/AISI 316 interfaces in the as-welded and oxidized conditions was higher than the minimum value (140 MPa) stated in the ASTM A264 standard. It was found that the oxidation process did not significantly reduce the bond strength of the interfaces. The weight gain data showed that both weld overlay claddings follow the parabolic rate law of oxidation, improving the oxidation behaviour of the AISI 316. The ER446 weld overlay had a lower parabolic rate constant (k p ) than the ER310 weld overlay. The SEM images showed that the oxide scales formed on the weld overlay claddings mainly consisted of three layers; Si-rich oxide at the innermost layer, Cr-rich oxide in the middle layer, and spinel oxide at the outer part of the scale. The oxidation behaviour of the clad samples was explained by the diffusion coefficients of elements, thermal stresses, and growth stresses, which are all associated with the matrix structure. • Heat-resistant stainless steel claddings were deposited on the surface of AISI 316 stainless steel using the GTAW process. • Thermal stability and oxidation behavior of the claddings after 1500 h of isothermal oxidation at 1000 °C were investigated. • The ER446 overlay displayed a ferritic-martensitic, while the ER310 overlay exhibited a fully austenitic microstructure. • The clad plates in the as-welded and oxidized conditions had a strength higher than the minimum value stated in ASTM A264. • Both ER446 and ER310 weld overlay claddings were improved the oxidation behaviour of the substrate.

High-temperature air-oxidation of NiCoCrAlx medium-entropy alloys

The oxidation behaviour of four NiCoCrAlx medium-entropy alloys (x = 0, 0.1, 0.3, or 0.5 mol.) was investigated in dry air at 600–900 °C. The oxidation kinetics of all the alloys followed the parabolic rate law at T> 700 °C, and the oxidation rates increased with an increase in temperature and a decrease in Al content. Discontinuous oxide granules were formed on all the alloys at 600 °C, while complex scales were detected, strongly depending on the Al content and temperature. Internal attack of AlN precipitates was also observed for Al-containing alloys at T> 700 °C.

Oxidation mechanism of refractory high entropy alloys Ta-Mo-Cr-Ti-Al with varying Ta content

The effect of Ta on the oxidation resistance of the alloys Tax-(Mo-Cr-Ti-Al)1−x (x=0; 5; 10; 15; 20 at%) in air at 1200 °C was examined. The oxidation behavior improves continuously with the increasing Ta concentration. Alloys with Ta concentrations Ta≤10 at% exhibit poor oxidation. Severe evaporation of MoO3 occurred which was quantitatively estimated. The evaporation of MoO3 is drastically reduced in alloys with the higher Ta concentrations. At 15 and 20 at% Ta, protective CrTaO4 scales are found. The CrTaO4 scale formed on Ta20-(Mo-Cr-Ti-Al)80 grows according to the parabolic rate law as a result of oxygen inward diffusion.

Diverse oxidation behaviors of metal carbide solutions in high-temperature water vapor

Carbide solid solutions show diverse oxidation behaviors at 1000–1400 °C in water vapor. (Ti,Nb,Ta,Hf,Zr)C and (Ti,Nb,Ta,Hf)C exhibit parabolic or near-parabolic oxidation kinetics, while the oxidations of (Ti,Nb,Ta) and (Ti,Nb)C are mainly controlled by linear oxidation kinetics. The oxidation kinetics of (Ti,Nb,Ta,Hf,Zr)C shows a tendency of transition from parabolic to linear oxidation rate law with increasing temperature. On the contrary, a tendency of transition from linear to parabolic rate law with increasing temperature was observed in (Ti,Nb)C, which exhibits the highest oxidation resistance at investigated temperatures. The presence of dense Nb-doped TiO2 oxidation layer is responsible for the high oxidation resistance.

High Temperature Oxidation Behavior of an Equimolar Cr-Mn-Fe-Co High-Entropy Alloy.

The oxidation behavior of an equimolar Cr-Mn-Fe-Co high-entropy alloy (HEA) processed by 3D laser printing was investigated at 700 °C and 900 °C. The oxidation kinetics of the alloy followed the parabolic rate law, and the oxidation rate constant increased with the rising of the temperature. Inward diffusion of oxygen and outward diffusion of cations took place during the high-temperature oxidation process. A spinel-type oxide was formed on the surface, and the thickness of the oxide layer increased with the rising of experimental temperature or time. The exfoliation of the oxide layer took place when the test was operated at 900 °C over 12 h. During oxidation tests, the matrix was propped open by oxides and was segmented into small pieces. The formation of loose structures had great effects on the high-temperature oxidation resistance of the HEA.

Dense-film preparation of zirconium oxide by self-oxidation in air

For the formation of a dense oxide film for use as an anticorrosion film with self-diagnosis and self-healing functions on the surface of structural materials, liquid blankets, and liquid divertors of thermonuclear fusion reactors, we investigated the self-oxidation of zirconium (Zr) at 500°C in air. The microstructure, composition (O/Zr ratio), crystal structure, and formation mechanism of the film were clarified through scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction analysis. It was demonstrated that the dense oxide film could be prepared, and its film exhibited a continuous and layered structure that consists of multiple phases. The outermost surface was unstrained monoclinic ZrO2. The growth of the oxide film adheres to a parabolic rate law.

Physicochemical properties of Mn1.45Co1.45Cu0.1O4 spinel coating deposited on the Crofer 22 H ferritic steel and exposed to high-temperature oxidation under thermal cycling conditions

The Crofer 22 H ferritic steel substrate was coated with an Mn1.45Co1.45Cu0.1O4 spinel by means of electrophoresis. After high-temperature oxidation under thermal cycling conditions, the physicochemical properties of the obtained system were evaluated. During 48-h cycles that involved heating the samples up to temperatures of either 750 or 800 °C, the oxidation kinetics of both coated and unmodified steel approximately obeyed the parabolic rate law. The unmodified steel was oxidized at a higher rate than the system consisting of the substrate and the coating. In its bulk form, the spinel consisted entirely of the cubic phase and it exhibited high electrical conductivity. The Mn1.45Co1.45Cu0.1O4 coating, on the other hand, was compact and consisted of two phases—the cubic and the tetragonal one—and it was characterized by good adhesion to the metallic substrate. After cyclic oxidation studies conducted for the two investigated temperatures (750 or 800 °C), the coating was determined to provide a considerable improvement in the electrical properties of the Crofer 22 H ferritic steel, as demonstrated by the area-specific resistance values measured for the steel/coating system. The evaporation rate of chromium measured for these samples likewise indicates that the coating is capable of acting as an effective barrier against the formation of volatile compounds of chromium. The Mn1.45Co1.45Cu0.1O4 spinel can therefore be considered a suitable material for a coating on the Crofer 22 H ferritic steel, with intermediate-temperature solid oxide electrolyzer cells as the target application.

Oxidation behavior of Al-Cr-Nb-Si-Zr high entropy nitride thin films at 850 °C

Oxidation behavior of Al-Cr-Nb-Si-Zr high entropy nitride (HEN) thin films was investigated in air atmosphere at 850 °C. The thermo-gravimetric analyzer (TGA) results show that oxidation kinetics of HEN films followed the parabolic rate law. A triple oxide layer, Al2O3/Cr2O3/Al2O3, was found with nanocrystalline ZrO2 dispersed in the oxide scales. Through Au nanoparticles as diffusion markers, the formation of the scales was mainly due to the outward diffusion of cations in the beginning, and the dominant diffusion species were changed to inward diffusion of anions in longer oxidation periods.

Effect of oxygen pressure on the oxidation behavior of NiCoCr medium-entropy alloy at 800 °C

The oxidation behavior of NiCoCr medium-entropy alloy was investigated at 800 °C in various oxygen pressures from 10 to 105 Pa. The results showed that the oxidation kinetics of NiCoCr followed the parabolic rate law, regardless of oxygen pressure. The oxidation rate constants of the alloy steadily increased with increasing oxygen pressure. The oxidation mechanism of the MEA was mainly controlled by outward cation diffusion to form an exclusive Cr2O3 layer although minor amounts of Co- or Ni-Cr spinels were observed at PO2 = 105 Pa under TEM analyses. The main diffusing species were doubly- and triply-ionized cation vacancies.

High temperature oxidation of non equi-atomic Al5Cr12Fe35Mn28Ni20 high entropy alloy

The Oxidation behavior of the Al5Cr12Fe35Mn28Ni20 high entropy alloy was investigated in air at temperatures 900 °C, 1000 °C and 1100 °C for 100 h isothermally. The oxidation kinetics at all temperatures obeys the parabolic rate law with an activation energy 100.6 kJ mol−1. The alloy was externally oxidized forming duplex layer consisted of manganese oxides that overlays Mn-Cr-Fe spinel oxides. While, it was observed that at 1000 °C and 1100 °C the samples were also oxidized internally forming Al2O3 deep in the alloy. The initial oxidation behavior of the alloy oxidized at 1000 °C for 1 and 24 h producing external scale of Mn3O4 overlaying Mn-Cr-Fe spinel oxide and Al2O3 precipitates.

Microstructure and oxidation behavior of an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating on low-carbon steel

In the present study, an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating with varying elemental compositions was deposited using the mechanical alloying technique. The composite coating structure before and after an oxidation test was studied using X-ray diffraction (XRD), three-dimensional optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). A cyclic oxidation test was conducted in an 800 °C air atmosphere for 10 cycles (200 h). Based on the XRD, SEM, and EDX analyses, the coating comprised composite-refined Al, Ni, Cr, and Cu particles as well as coarse SiO2 particles. The composite coating had a thickness in the range of 36–59 μm, with good and continuous adherence to the substrate. The cyclic oxidation curve of the composite coating followed a parabolic rate law. The sample that contained 5.3 wt% MoO3 showed the best oxidation resistance after 10 oxidation cycles at 800 °C. This sample had a lower mass gain than the other samples. Moreover, this sample had a Kp of 1.2 × 10−3 mg2/cm2·s. Protective Al2O3 oxides formed on the composite coating surface with a thickness of 0.75–1.89 μm. This oxide scale was dense and fully adhered to the coating, thereby improving oxidation resistance.

High-Temperature Air and Steam Oxidation and Oxide Layer Characteristics of Alloy 617

Nickel-based superalloys are the candidate materials for many high-temperature applications, especially in advanced ultra-supercritical coal-fired power plants and in the next-generation nuclear power plants. This paper reports the results of a comprehensive study on chemical composition, structure and kinetics of oxide film formation of a nickel-based superalloy, alloy 617, in ambient air and steam at 750 °C for a duration of 500 h. The morphology and nature of oxide scales have been characterized by scanning electron microscope with energy-dispersive x-ray attachment, grazing incidence x-ray diffraction technique and glow discharge optical emission spectrometry (GDOES). The mass gain of alloy 617 in steam was higher than that in air, and followed a parabolic rate law for both atmospheres. The depth profiles of the oxide films obtained by GDOES showed the presence of an outer chromium oxide scale with mixed oxides of nickel and aluminum at the interface. The oxidation kinetics of alloy 617 and the nature of oxide scales formed during the steam and air oxidation are discussed.

High temperature oxidation behaviour of non-equimolar AlCoCrFeNi high entropy alloys

Oxidation properties of AlCoCrFeNi high entropy alloys, i.e. 5%Al–35%Co–25%Cr–15%Fe–20%Ni (Al5Co35Cr25Fe15Ni20) and 5%Al–25%Co–20%Cr–15%Fe–35%Ni (Al5Co25Cr20Fe15Ni35), were investigated in air atmosphere at 1273 K. Thermogravimetry (TG) investigations determined that, under these conditions, the samples demonstrate scale growth according to the parabolic rate law with rate constants the order of 10−11 g2cm−4s−1. Combined scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) surface analysis and X-ray diffraction (XRD) studies revealed that the formed protective scales consisted primarily of chromium oxide. Underneath the external Cr2O3 scale, a thin layer of the metallic substrate can be found. Below that, an internal chromium oxide scale with discontinuous Al2O3 can be observed. Al-rich precipitates can also be found in the internal layers, as well as the metallic substrates after oxidation. XRD analysis also indicated that the initial materials are single FCC phase solid solutions. From all these studies it can be concluded that both Co and Ni-rich non-equimolar AlCoCrFeNi high entropy alloys exhibit similar high temperature oxidation behaviour. Furthermore, each of the alloys has promising anti-corrosion properties that can possibly be further improved on, e.g. with higher aluminium content inside the materials.

Oxidation of molten AZ91D Mg alloy in hexafluoropropylene/air atmospheres

The oxidation behavior of molten AZ91D Mg alloy in the mixture of 0.05–5% hexafluoropropylene and air at 670–800 °C was investigated by TG, XRD, SEM, EDS and XPS. Results show that the oxidation of the melt in 0.05 % hexafluoropropylene/air atmosphere at 700 °C obeyed a linear rate rule, while under other conditions it followed a parabolic rate law. The apparent activation energy was 82.78 kJ mo1−1 for the reaction of the melt in 0.1 % hexafluoropropylene/air atmosphere at 670–800 °C. The film formed on the melt surface mainly consisted of MgF2, MgO, C and small amounts of AlF3.

Preparation and oxidation kinetics behavior of bulk Cr3C2-20 wt % Ni cermets

In this study, bulk Cr3C2-20 wt % Ni cermets were successfully fabricated by high-energy milling and pressureless sintering in a vacuum furnace. Microstructures, elements distribution, and high temperature oxidation mechanism were researched by SEM, EPMA, and differential thermal analyzer (DTA), respectively. Oxidation kinetics regularity of bulk Cr3C2-20 wt % Ni cermets was investigated at 600–800 °C for the first time. Isothermal cyclic oxidation experiments were studied using the heat-treatment furnace for 100 h. The results indicated that the porosity decreased, while the hardness, bending strength, and fracture toughness increased with an improvement in the vacuum degree. Cr3C2-20 wt % Ni cermets displayed outstanding oxidation resistance and the dynamic oxidation curves followed the parabolic rate law. Besides, the oxidation rate constants increased three orders of magnitudes with an increase in the oxidation temperatures from 600 °C to 800 °C. The mechanism of the oxidation resistance was the generation of the protective and dense oxide layers on the sub-surface of the oxidation specimens, which hindered the diffusion of Cr3+, Ni2+, O2 and effectively protected the substrate from further oxidation.