Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.     

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the presence of Na₂SO₄ deposit and water vapor at 900°C and 1000°C. The mass gain per unit area of the samples superficially coated with Na₂SO₄ exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases on the sample surface were identified by XRD as TiO₂, Na₂Si₂O₅ and Na₂TiO₃. After Ti₃SiC₂ corroded in the presence of the Na₂SO₄ deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti₃SiC₂ beneath the Na₂SO₄ film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO₂ to form volatile Si(OH)₄, an intermediate porous and TiO₂-enriched layer formed in the corrosion layer.     

Oxidation of Ti3SiC2 between 900 and 1200 °C in Air

Ti₃SiC₂ materials were synthesized by hot pressing using a new starting-material system consisting of a TiC_x(x=0.6)/Si powder mixture. The oxidation of Ti₃SiC₂ at temperatures between 900 and 1200 °C in air for up to 100 h resulted in the formation of an outer TiO₂ layer, an intermediate SiO₂-rich layer and an inner (TiO₂ + SiO₂) mixed layer. During oxidation, Ti diffused outwards to form the outer TiO₂ layer, and oxygen transported inwards to form the inner (TiO₂ + SiO₂) mixed layer. At the same time, the carbon in Ti₃SiC₂ escaped into the air. Below the scale, there was a narrow oxygen-affected zone, The oxidation at the scale-matrix interface proceeded by the disintegration of the lamellar Ti₃SiC₂ grains to form crystallites with a size of a few tens of nanometers containing oxygen. The detailed scale characteristics and oxidation mechanism are described.     

High-temperature SO2-gas corrosion of TiN–Ti5Si3 composites prepared by polymer pyrolysis

By pyrolyzing a mixture of Si-containing pre-ceramic polymers and TiH₂ powders in a N₂ atmosphere, a TiN–Ti₅Si₃ composite was synthesized. The composite was then corroded between 700 °C and 1000 °C for 20 h in an Ar–0.2% SO₂ atmosphere. TiN was mainly oxidized to rutile TiO₂. Ti₅Si₃ was oxidized to TiO₂ supersaturated with Si ions, and sulfidized to Ti₂S supersaturated with Si ions. At initial stage of corrosion, oxidation dominated sulfidation. As corrosion proceeded, sulfidation progressively occurred underneath the oxide scale based on the decreased oxygen potential and increased sulfur potential near the scale/matrix interface.     

Improving the Na2SO4-induced corrosion resistance of Ti3AlC2 by pre-oxidation in air

Ti₃AlC₂ suffers severe Na₂SO₄-induced corrosion attacks at temperatures higher than 800 °C in air. A convenient and efficient pre-oxidation method is proposed to enhance the corrosion resistance of Ti₃AlC₂. The corrosion weight-changes of the pre-oxidized samples were decreased by about four orders of magnitude compared with those of the untreated specimens. The mechanism on improvement of corrosion resistance was investigated by means of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy/energy-dispersive spectroscopy. A continuous and adherent α-Al₂O₃ scale was prepared by high-temperature pre-oxidation treatment in air. The preformed dense Al₂O₃ scale has good compatibility with the Ti₃AlC₂ substrate, and consequently, can act as an efficient barrier against corrosion. Long-time corrosion tests demonstrate that the Al₂O₃ scale conserves after corrosion attack and is capable of long-term stability.     

Influence of Al-La Cocementation on the Oxidation Behavior of Ti3SiC2-Base Ceramic

Al–La thermal diffusing was conducted on Ti₃SiC₂-base ceramic by the pack-cementation method. The microstructure, phase and oxidation resistance of the diffusion layer were characterized. The complete aluminide coatings have not been obtained. Al and La penetrated into the Ti₃SiC₂ substrate quickly, distributing into the whole interior of the specimen after cementation at 1100°C for 4 hr. Al existed as a solid solution and dispersed particles of AlLa₃. Oxidation of cemented Ti₃SiC₂ at 1100°C in air for 20 hr formed a single continuous Al₂O₃ layer with a small amount of TiO₂ grains on the outer layer. Compared with Ti₃SiC₂, the parabolic rate constant of the cemented Ti₃SiC₂ was decreased by two orders of magnitude, which means that the cementation treatment remarkably improves the oxidation resistance of Ti₃SiC₂. Al distributed in Ti₃SiC₂ acted as a reservoir to supply enough Al for the formation of a continuous Al₂O₃ scale during the oxidation process.     

Hot corrosion of modified Ti3Al-based alloy coated with thin Na2SO4 film at 910 and 950 °C in air

The hot corrosion behaviour of a modified Ti₃Al-based alloy under thin Na₂SO₄ deposit film was investigated at 910 and 950 °C in air. The corrosion product was identified by XRD and its morphologies on the surface and cross-section were observed by SEM. The alloy suffered from considerable hot corrosion attack. The mass gain versus time curves obtained by TGA exhibited two regions of different kinetics. The whole corroded layer consisted of loose and porous mixture oxides of TiO₂, Nb₂O₅ and Al₂O₃. Numerous small nodules of corrosion product were observed. An illustrative schematic was established to describe the formation process of such nodules. It seemed that the refractory oxides played a significantly important role in determining the development of hot corrosion attack.     

Improving the surface hardness and wear resistance of Ti3SiC2 by boronizing treatment

In order to modify surface properties of Ti₃SiC₂, boronizing was carried out through powder pack cementation in the 1100-1400 °C temperature range. After boronizing treatment, one mixture layer, composed of TiB₂ and β-SiC, forms on the surface of Ti₃SiC₂. The growth of the coating is processed by inward diffusion of boron and obeys a linear rule. The boronizing increases the hardness of Ti₃SiC₂ from 3.7 GPa to a maximal 9.3 GPa and also significantly improves its wear resistance.     

Cyclic-Oxidation Behavior of Ti3SiC2-Base Material at 1100°C

The cyclic-oxidation behavior of Ti₃SiC₂-base material was studied at 1100°C in air. Scale spallation and weight loss were not observed in the present tests and the weight gain would just continue if the experiments were not interrupted. The present results demonstrated that the scale growth on Ti₃SiC₂-base material obeyed a parabolic rate law up to 20 cycles. It then changed to a linear rate with further increasing cycles. The scales formed on the Ti₃SiC₂-base material were composed of an inward-growing, fine-grain mixture of TiO₂+SiO₂ and an outward-growing, coarse-grain TiO₂. Theoretical calculations show that the mismatch in thermal expansion coefficients between the inner scale and Ti₃SiC₂-base matrix is small. The outer TiO₂ layer was under very low compressive stress, while the inner TiO₂+SiO₂ layer was under tensile stress during cooling. Scale spallation is, therefore, not expected and the scale formed on Ti₃SiC₂-base material is adherent and resistant to cyclic oxidation.     

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

Rapid fabrication of the Ti3SiC2 bonded diamond composite by spark plasma sintering

A mixture of Ti/Si/TiC/diamond powders was employed to fabricate the Ti₃SiC₂ bonded diamond composite using the spark plasma sintering-reactive synthesis method. The addition of diamond does not inhibit the synthesis of Ti₃SiC₂ in the sintered product. In the matrix Ti₃SiC₂ grains developed lamellar morphology with an average length size of 5-10 μm. Ti₃SiC₂ matrix displays good pullout strength with diamond, and the Ti₃SiC₂ bonded diamond material exhibits good wear resistance.     

The corrosion behavior of Ti3SiC2 in common acids and dilute NaOH

Hot isostatically processed bulk, fine (3-5 μm) grained samples of Ti₃SiC₂ were immersed in concentrated and dilute hydrochloric, HCl, sulphuric, H₂SO₄, nitric, HNO₃, dilute hydrofluoric, HF, acids and sodium hydroxide, NaOH, solutions at room temperature. Based on six-months weight changes the dissolution rates of Ti₃SiC₂ in concentrated and dilute HCl, H₂SO₄ and dilute NaOH were found to be negligible (<2 μm/yr). In dilute HF and concentrated HNO₃ the corrosion rates were, respectively, ≈5 μm/yr and 13 μm/yr respectively. In contrast to Ti metal, the weight losses of Ti₃SiC₂ in dilute HNO₃ were higher (≈250-320 μm/yr) and depended on concentration. Post-immersion scanning electron microscopic micrographs of samples immersed in HNO₃ indicated that an oxygen rich Si-based layer forms on the surface of the samples. This implies that the Ti atoms are leached out into the HNO₃ solution, leaving behind a Si-rich layer that is ultimately oxidized. Cyclic polarization and potentiostatic i-t transients in dilute HCl and H₂SO₄ acids, strongly suggest that a thin irreversible electrically insulating layer forms on the surface of Ti₃SiC₂. Exposing a sample to a constant current density of 0.6 mA/cm² for two days resulted in the formation of a 5 μm thick SiO₂-based layer on the surface. The presence of such a layer would explain the excellent corrosion resistance of Ti₃SiC₂ in these acids.     

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.     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

Synthesis of Ti3SiC2 by infiltration of molten Si

High-purity Ti₃SiC₂ compounds have been fabricated by infiltration of molten Si into a precursor, a partially sintered TiC_x (x = 0.67) preform. The Si source and the TiC_x preform were placed side by side on carbon cloth, and the system was heated to 1550 °C. Molten Si infiltrated the preform through the carbon cloth, and a direct reaction between TiC_x and molten Si immediately occurred at the reaction temperature to yield pure Ti₃SiC₂. We could observe phase formation and the microstructure of the bulk products with time, which were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Pure Ti₃SiC₂ compounds were formed on the exterior of the TiC_x preform at 1550 °C when the sintered TiC_x:Si ingot molar ratio was 3:1.4. At 1550 °C, no other minor phases were detected for any of the sintering time ranges.     

Oxidation behavior of porous Ti3SiC2 prepared by reactive synthesis

High-purity porous Ti₃SiC₂ with a porosity of 54.3% was prepared by reactive synthesis and its oxidation behavior was evaluated under air in the temperature range from 400 to 1000 °C. Thermogravimetric analysis and differential scanning calorimetry (TG-DSC), scanning electron microscope (SEM), X-ray diffractometometry (XRD), energy dispersive spectrometer (EDS), Raman spectrum, BET surface area analysis, and pore-parameter testing were applied to the studies of the oxidation kinetics, phase composition, micro morphology, and porous structure parameters of porous Ti₃SiC₂ before and after oxidation. The results showed that the formation of TiO₂ oxidized products with different modifications was the primary factor influencing the oxidation resistance and structural stability of porous Ti₃SiC₂. Cracks were observed in the samples oxidized in the full temperature range of 400-1000 °C because of the growth stress and thermal stress. At 400-600 °C, anomalous oxidation with higher kinetics and the aberrant decrement in pore size and permeability were attributed to the occurrence of severe cracking caused by the formation of anatase TiO₂. At raised temperatures over 600 °C, the cracking phenomena were alleviated by the formation of rutile TiO₂, but the outward growth of the oxide scales detrimentally decreased the connectivity of porous Ti₃SiC₂.     

Effect of Ti3SiC2 content on the corrosion behavior of Al/Ti3SiC2 composite coatings

The purpose of this study was to investigate the electrochemical corrosion properties of flame-sprayed Al and Al/(5, 10, 15)% Ti₃SiC₂ coatings in a 3.5% NaCl solution. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used for analyzing the microstructural characteristics of the coatings. For examining the corrosion behavior of the coatings, a potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) were used. After the potentiodynamic polarization test, the SEM micrograph of coatings indicated that Ti₃SiC₂ particles played a significant role in pitting corrosion. The results of potentiodynamic polarization tests revealed that Al/Ti₃SiC₂ coating is nobler than that of the pure aluminum coating. On the contrary, the addition of Ti₃SiC₂ particles reduced the process of thickening the passive layer. The results of the EIS tests demonstrated that the presence of Ti₃SiC₂ particles significantly enhances the corrosion resistance of the coatings.     

Corrosion of Ti3AlC2 at 800–1100 °C in Ar–0.2% SO2 gas atmosphere

Ti₃AlC₂ was corroded between 800 and 1100 °C in an Ar–0.2% SO₂ gas atmosphere according to the equation: Ti₃AlC₂ + O₂ → rutile-TiO₂ + α-Al₂O₃ + (CO or CO₂). The scales that formed on the Ti₃AlC₂ were thin and rich in α-Al₂O₃, whose growth rate was exceedingly slow. The TiO₂ was present either as the outermost surface scale or a mixture inside the α-Al₂O₃-rich scale. In the Ti₃AlC₂, the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti₃AlC₂ over TiAl.     

Investigation by fractography of the high-temperature oxidation of the 20%Cr-25%Ni-Nb stainless steel

Simple procedures have been developed for the preparation, by fracture, of transverse corroded metal sections for subsequent examination of the microstructure and microchemistry of the corrosion attack by scanning electron microscopy. Examination of 20%Cr-25%Ni-Nb (20/25/Nb) stainless steel following 120-hr oxidation in carbon dioxide at 1000°C confirmed that the essentially three-dimensional picture revealed by fracture provided additional microstructural information not apparent from optical microscopy of standard metallographically mounted transverse sections. The size and morphology of the oxide grains constituting the various layers of the uniform and the pitting types of external scale were established. Nearest to the underlying steel, a band of the inner Cr₂O₃ layer contained a high but variable silicon concentration as well as substantial voidage. Beneath the scale, silica intrusions dramatically influenced the mechanical properties of the 20/25/Nb stainless steel to the depth of this intergranular attack, as fracture at 78 K occurred by a brittle mode. Steel unaffected by oxidation, however, continued to remain ductile.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.