Study on hot corrosion behavior of Yb2Zr2O7 ceramic against Na2SO4 + V2O5 molten salts at temperatures of 900–1200 °C in air

Yb₂Zr₂O₇ ceramic powders synthesized by chemical‐coprecipitation and calcination method were pressureless‐sintered at 1700 °C for 10 h in air to fabricate dense bulk materials. Hot corrosion studies were performed on Yb₂Zr₂O₇ against Na₂SO₄ and Na₂SO₄ + V₂O₅ (molar ratio = 1:1) molten salts in a temperature range of 900–1200 °C for 8 h in air, respectively. Chemical reactions were investigated using X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The Yb₂Zr₂O₇ ceramic was severely corroded by Na₂SO₄ + V₂O₅ molten salt, however, no chemical reaction was found between individual Na₂SO₄ and Yb₂Zr₂O₇. Yb₂Zr₂O₇ reacted with Na₂SO₄ + V₂O₅ molten salt to form YbVO₄ and m‐ZrO₂. The thickness of hot corrosion scales formed at different temperatures was investigated to evaluate hot corrosion behavior based on fluxing mechanism. The introduction of vanadium into sulfate led to subsequent formation of NaVO₃, which was acidic enough to dissolve Yb₂Zr₂O₇ by acidic fluxing.     

Oxidation of FeCrAl alloys at 500–900°C in dry O2

This paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500–900°C. The samples are exposed isothermally in dry oxygen for up to 72 h using a thermo‐balance. In addition, 168 h exposures are carried out in a tube furnace. The exposed samples are investigated by grazing angle X‐ray diffraction (XRD), scanning electron microscopy/energy dispersive X‐ray analysis (SEM/EDX), and auger electron spectroscopy (AES). The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700–900°C. At all exposure temperatures, most of the sample surface is covered by a thin smooth base oxide. In addition, RE‐rich particles, with a typical size of 1–3 μm form. At 800 and 900°C patches of thick oxide appear, featuring needle‐formed crystallites situated on top of the base oxide. The thick oxide usually forms around Y‐rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of α‐Al₂O₃ already at 700°C. The low temperature of formation of α‐Al₂O₃ is attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr₂O₃). After exposure at 900°C AES shows large amounts of Mg in the outer part of the oxide, MgAl₂O₄ being detected by XRD together with γ‐ and γ‐Al₂O₃.     

Oxidation of Cr–C electroplating between 400 and 900 °C in air

The high temperature oxidation of Cr–C electroplated on a steel substrate was studied at 400–900 °C in air. Before oxidation, the deposit consisted primarily of C‐supersaturated, amorphous Cr grains. During oxidation, oxygen diffused inward, while Cr and the substrate elements such as Fe diffused outward. Carbon tended to escape, but some of it remained and existed mainly as graphite throughout the oxide layer and the Cr–C electrodeposit. A Cr₂O₃ oxide layer having dissolved Fe ions formed on the unoxidized, crystalline Cr layer. The Cr–C electrodeposit oxidized approximately parabolically, with a faster oxidation rate than bulk Cr.     

High‐temperature oxidation of Ti3Al0.7Si0.3C2 compound at 900 and 1000 °C in air

The compound, Ti₃Al_0.7Si_0.3C₂, was synthesized by hot pressing a powder mixture of TiC_X (x = 0.6), Al and Si. Its oxidation at 900 and 1000 °C in air for up to 50 h resulted in the formation of rutile‐TiO₂, α‐Al₂O₃ and amorphous SiO₂. The oxide scales formed consisted of triple layers, viz, an outer TiO₂ layer containing Al₂O₃ particles, an intermediate Al₂O₃ layer, and an inner mixed layer that was rich in TiO₂, but deficient in Al₂O₃ and SiO₂. During oxidation, Ti diffused outwards to form the outer TiO₂ layer, and oxygen was transported inwards to form the inner mixed layer. At the same time, carbon was liberated from Ti₃Al_0.7Si_0.3C₂. Ti₃Al_0.7Si_0.3C₂ oxidized slower than the TiO₂‐forming kinetics, but faster than the (α‐Al₂O₃ or SiO₂)‐forming kinetics.     

Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: Ti‐6Al‐4V

Hexagonal boron nitride (h‐BN) coatings on Ti‐6Al‐4V substrates undergo complete decomposition in air at 900 °C. This fate is similar to that of this ceramic material on chromia‐former alloys, and unlike that of a mass of powder treated in isolation. As the ceramic and alloy oxidize concurrently, outwardly diffusing aluminum (III) ions but not the predominant titanium (IV) ions react with the boron trioxide that forms around the h‐BN basal plane peripheries. Resultant aluminum borate is incorporated into the growing scale and the boron trioxide diffusion barrier is depleted. By this mechanism, the oxidation of h‐BN is maintained at an enhanced rate, until both this material and its oxide completely decompose. Liberated nitrogen from the oxidation of h‐BN can enter the underlying scale as a randomly distributed solute in rutile solid solution. The post‐coating oxide‐atmosphere interface comprises elongated aluminum borate crystallites protruding through at the boundaries between 3–5 at% nitrogen‐doped rutile grains. It differs significantly from that of oxidized, uncoated Ti‐6Al‐4V, which is occupied by a thin α‐alumina layer atop rutile. This interface does not change with an additional 72 h of heat‐treatment.     

Scale growth process at 1473 K on unmodified and yttrium‐ or chromium‐implanted β‐NiAl

Early oxidation of unmodified and yttrium‐implanted or chromium‐implanted β‐NiAl intermetallic compound at 1473 K was studied using a combination of two‐stage‐oxidation exposure with ¹⁸O₂ as a tracer, SIMS elemental distribution analysis (depth profiling and imaging modes) and photoluminescence spectroscopy analysis of the scale phase composition. It was found that phase transformation of transient aluminium oxides, represented by θ‐ Al₂O₃ into stable and protective α‐Al₂O₃ occurs locally and is affected by implanted additions: Yttrium retarded while chromium appeared to accelerate it. Typical patch‐ and/or web‐like scale morphology of the growing scales was observed.     

Corrosion of stainless steel grades in H2O/KOH 50% at 120 °C: AISI304 austenitic and 2205 duplex

We report on the corrosion of austenitic (AISI304) and duplex (2205) stainless steels in H₂O/KOH 50% at 120 °C. The research is based on a combination of electrochemical, structural and compositional analyses, aimed at assessing the surface modifications resulting from anodic attack and their impact on corrosion resistance. Linear sweep voltammetry and electrochemical impedance spectrometry measurements were carried out in an air‐tight high‐temperature cell. In‐plane and cross‐sectional SEM micrography, X‐ray diffractometry and EDX profiling were used to characterise samples attacked under electrochemically controlled conditions. Electrochemical results have shown that AISI304 exhibits a complex passivating behaviour, while the anodic electrokinetics of the duplex is characterised by mixed kinetic control. AISI304 was found to fail by intergranular corrosion and to be covered: in passive conditions by acicular compounds and in transpassive conditions by a compact layer of corrosion products. Duplex samples, instead, exhibit an uniform form of corrosion morphology and bear a compact layer of corrosion products both in passive and in transpassive conditions.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.