Temperature and water vapour effects on the cyclic oxidation behaviour of Fe-Cr alloys

Binary Fe-Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar-20O₂ and Ar-20O₂-5H₂O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chromium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation.     

Pourbaix diagrams for chromium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Cr-Br⁻-H₂O system at 25 °C were developed in 400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solutions, common concentrations in different parts of absorption devices. The diagrams were compared with the simple Cr-H₂O system at 25 °C. Equilibria for the Cr-Br⁻-H₂O system at 25 °C were determined for bromide ion activities of 15.61, 194.77, 650.06, and 2042.65, which corresponded to the 400, 700, 850, and 992 g/L LiBr solutions, respectively. Activities of all the dissolved species containing chromium were plotted for 10⁻⁶, 10⁻⁴, 10⁻², and 10⁰. Comparison of the simple Cr-H₂O system at 25 °C with the diagrams for Cr-Br⁻-H₂O system at 25 °C showed that the dominant aqueous Cr(III) species in acid solutions was Cr⁺³ for Br⁻ activities of 15.61, 194.77, and 650.06, whereas it was CrBr⁺² for Br^- activity of 2042.65. Aqueous CrBr⁺² formed at a Br⁻ activity higher than 943.05. The chromium solubility range in the acid area of the diagrams extended slightly to higher pH values with increasing Br⁻ activity and decreasing water activity, as a result of destabilization of Cr₂O₃.     

Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

Influence of the applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels under conditions of cathodic hydrogen charging

Stress corrosion cracking (SCC) of as-quenched 4340 and 3.5NiCrMoV steels was studied under hydrogen charging conditions, with a cathodic current applied to the gauge length of specimens subjected to Linearly Increasing Stress Test (LIST) in 0.5 M H₂SO₄ solution containing 2 g/l arsenic trioxide (As₂O₃) at 30 °C. Applied stress rates were varied from 20.8 to 6 × 10⁻⁴ MPa s⁻¹. Both the fracture and threshold stress decreased with decreasing applied stress rate and were substantially lower than corresponding values measured in distilled water at 30 °C at the open circuit potential. The threshold stress values correspond to 0.03–0.08 σ_y for 4340 and 0.03–0.2 σ_y for the 3.5NiCrMoV steel. SCC velocities, at the same applied stress rate, were an order of magnitude greater than those in distilled water. However, the plots of the crack velocity versus applied stress rate had similar slopes, suggesting the same rate-limiting step. The fracture surface morphology was mostly intergranular, with quasi-cleavage features.     

Oxidation behavior of Ti3AlC2 at 1000-1400 °C in air

The isothermal oxidation behavior of bulk Ti₃AlC₂ has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti₃AlC₂ has excellent oxidation resistance. The oxidation of Ti₃AlC₂ generally followed a parabolic rate law with parabolic rate constants, k_p that increased from 4.1×10⁻¹¹ to 1.7×10⁻⁸ kg² m⁻⁴ s⁻¹ as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al₂O₃. The outer layer changed from rutile TiO₂ at temperatures below 1200 °C to a mixture of Al₂TiO₅ and TiO₂ at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al₂TiO₅ and, predominantly, α-Al₂O₃, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al₂O₃ layers.     

Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe-25Cr-35Ni-1Nb, wt.%) was oxidised and carburised in CO-CO₂ corresponding to a_C = 0.1 and p_O2 = 3 × 10⁻¹⁶ atm at 1080 °C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised, solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation, assuming the scale-metal interface to be fixed. The pores are concluded to be Kirkendall voids.     

Characterization of microstructures and oxidation behaviour of Nb–20Si–20Cr–5Al alloy

Static oxidation in air was performed on Nb–20Cr–20Si–5Al alloy at high temperatures ranging from 700 to 1400 °C. Pesting occurred at 700 °C while internal oxidation took place at 1300 and 1400 °C where Al₂O₃ initiated at the interface between NbCr₂ and Nb₉Cr₃Si₂ phases. Phases present were Nb₅Si₃, NbCr₂, Nb solid solution and Nb₉Cr₃Si₂ depending on the temperature. The aluminium content on each of the phases was analyzed. Al content in Nb₉Si₂Cr₃ has been found to be as high as 5–6 atomic percent. SEM, EDS and XRD techniques were utilized in order to characterize the specimens.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

Pourbaix diagrams for titanium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Ti–Br⁻–H₂O system at 25 °C in the absence and presence of titanium hydrides were developed in 400, 700, 850, and 992 g/L LiBr solutions. The diagrams were compared with the simple Ti–H₂O system at 25 °C. Comparison of the simple Ti–H₂O system with the diagrams of the Ti–Br⁻–H₂O system at 25 °C showed that the titanium solubility range in the acid, neutral, and weak alkaline areas of the diagrams extended slightly to both higher pH values and potentials with increasing bromide ion activity and decreasing water activity.     

Unusual oxidation behaviour of Zr50Cu50 alloy at high temperatures

The oxidation of Zr₅₀Cu₅₀ alloy at 500-700 °C is characterized by preferential oxidation of zirconium, while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₁₄Cu₅₁ phase. The strong reaction at 800 and 850 °C resulted in the total corrosion of the specimens in 21 and 15 h, respectively. The oxidation at elevated temperatures showed an anomalous decrease of the oxygen consumption rate in the temperature range 930-1000 °C, corresponding to the preferentially oriented crystallites of ZrO₂ in the oxide scale at 900 and 1000 °C. The oxide layer consists of ZrO₂ and CuO in the whole temperature interval of the oxidation. The reaction kinetics obeys a parabolic rate law. An activation energy of 92.0 ± 0.3 kJ/mol has been estimated.     

Electrochemical pitting corrosion behaviour of α-brass in LiBr containing solutions

The potentiodynamic anodic polarization curve of α-brass (70% Cu-30% Zn) in 1 M LiBr solution showed an initial active region of the alloy dissolution followed by two well defined anodic current peaks then a narrow passivation region before the pitting potential (E_pit) is reached. The initial active anodic region exhibited Tafel slope with 90 mV dec⁻¹ attributed to the formation of CuBr₂⁻ complexes. The anodic current peaks were attributed to the formation of CuBr and Cu²⁺ ions, respectively. The change of pH values of LiBr solution did not affect the anodic polarization curves of α-brass. Increasing the solution temperature from 30 to 90 °C changed the corrosion type from pitting to general one. The addition of 10⁻² M benzotriazole (BTAH) to 1 M LiBr solution is completely inhibited the pitting corrosion at 30 °C while it did not inhibit the pitting at 90 °C. The inhibition effect was attributed to the adsorption of BTAH molecules on the alloy surface, which obeys Langmuir isotherm. The presence or absence of pitting corrosion was confirmed by using SEM.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Hot corrosion behaviour of Yb2Zr2O7 ceramic coated with V2O5 at temperatures of 600-800 °C in air

To better understand the hot corrosion behaviour of Yb₂Zr₂O₇ ceramic in molten V₂O₅, hot corrosion experiments were performed in a temperature range of 600-800 °C in air. Different reaction products of ZrV₂O₇, YbVO₄ and m-ZrO₂ were identified depending upon the hot corrosion conditions, for example, ZrV₂O₇ and YbVO₄ at 600 °C for 2 h and 8 h; ZrV₂O₇, m-ZrO₂ and YbVO₄ at 700 °C for 2 h; m-ZrO₂ and YbVO₄ either at 800 °C for 2 h or at 700-800 °C for 8 h. The hot corrosion reaction mechanisms were further discussed based on the thermal instability of ZrV₂O₇ at elevated temperatures.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

Gamma-radiation-induced corrosion of carbon steel in neutral and mildly basic water at 150 °C

The effect of γ-radiation on the kinetics of carbon steel corrosion has been investigated by characterizing the oxide films formed on steel coupons at 150 °C and at two pH values. Results show that continuous irradiation enhances surface oxide formation with the type of oxide formed dependant on the solution pH. For experiments at 150 °C and a [OH⁻] equivalent to that for pH_{25 °C} = 10.6, the surface oxide on carbon steel after γ-irradiation was non-porous and uniform, and no localized corrosion was observed. This oxide, however, appears to be susceptible to brittle fracture during cooling. Raman spectroscopy of the surface film indicates that it is a mixture of the phases of Fe₃O₄ and γ-Fe₂O₃. In contrast, at 150 °C with [OH⁻] equivalent to neutral pH_{25 °C}, metal dissolution is significant and the surface oxide film is very porous. Raman spectra show that this oxide film is also composed of a mixture of Fe₃O₄ and γ-Fe₂O_3. The results from this work combined with previously reported electrochemical studies of the same system as a function of pH and temperature can be used to deconvolute the effects of radiation, pH and temperature on the nature of the corrosion process.     

Carburisation of ferritic Fe–Cr alloys by low carbon activity gases

Model Fe–Cr alloys were exposed to Ar–CO₂–H₂O gas mixtures at 650 and 800 °C. At equilibrium, these atmospheres are oxidising to the alloys, but decarburising (a_C ≈ 10⁻¹⁵ to 10⁻¹³). In addition to developing external oxide scales, however, the alloys also carburised. Carbon supersaturation at the scale/alloy interface relative to the gas reflects local equilibrium: a low oxygen potential corresponds to a high p_CO/p_CO2 ratio, and hence to a high carbon activity. Interfacial carbon activities calculated on the basis of scale–alloy equilibrium are shown to be in good agreement with measured carburisation rates and precipitate volume fractions, providing support for the validity of the thermodynamic model.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Effects of the nickel-coated ferritic stainless steel for solid oxide fuel cells interconnects

In this study, a protective nickel layer was prepared on the SUS430 alloy substrate by the atmospheric plasma spraying technology (APS). Oxidation kinetics, area specific resistance (ASR) and interfacial microstructure of the SUS430 alloy with nickel layers as well as its elemental composition were studied under the flowing humidified hydrogen atmosphere at 800 °C to evaluate the effectiveness of the protective nickel layer. The current collector between the interconnect and the anode was optimized in this paper. Results showed that the oxide growth rate constant of the SUS430 alloy with the APS nickel layer was 1.99 × 10⁻¹⁴ g² cm⁻⁴ s⁻¹, which was only one fiftieth of that of the alloy without a protective layer. The ASR of the SUS430 alloy with the APS nickel layer was 12 mΩ cm² after being oxidized under the simulated SOFC anode operating atmosphere at 800 °C for 250 h. With the optimized structure of current collector, the contact ASR between the nickel-coated interconnect and the Ni-YSZ anode was only 3 mΩ cm² after being oxidized at 800 °C for 100 h.     

Conductivity and stability of cobalt pyrovanadate

Cobalt pyrovanadate was successfully synthesised by a solid state route and the conductivity in both oxidising and reducing environments was determined for the first time. Impedance measurements between 300 °C and 700 °C in air determined that Co₂V₂O₇ is an intrinsic semiconductor with activation energy of 1.16(3) eV. The conductivity in air reached a maximum of 4 × 10⁻⁴ S cm⁻¹ at 700 °C. Semiconducting behaviour was also observed in 5% H₂/Ar, albeit with a much smaller activation energy of 0.04(4) eV. Between 300 °C and 700 °C the conductivity ranged from 2.45 S cm⁻¹ to 2.68 S cm⁻¹, which is approaching the magnitude required for SOFC anode materials. Thermogravimetric analysis found a significant weight loss upon reduction of the compound. X-ray diffraction analysis, coupled with data from previous research, suggested compound degradation into Co_2−xV_1+xO₄, CoO and VO. The redox instability and the low conductivity lead us to the conclusion that cobalt pyrovanadate is unsuitable for utilisation as an anode material for SOFCs although the conductivity is reasonable in a reducing atmosphere.