Oxidation and corrosion of silicon nitride ceramics with different sintering additives at 1200 and 1500 °C in air, water vapour, SO2 and HCl environments - A comparative study

The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of silicon nitride based ceramics was investigated. Comparative tests were conducted at 1200 and 1500 °C in air, in water vapour, and with the highly corrosive gases HCl and SO₂. Si₃N₄ was prepared with MgO, Al₂O₃, Y₂O₃ and Al₂O₃ + Y₂O₃ sintering additives. Hot pressed discs were tested for a total time of up to 128 h. The electrically conductive ceramic composites Si₃N₄ + TiN and Si₃N₄ + MoSi₂ were also tested under the same conditions. The effects that the different corrosion environments have on the different ceramics are presented. SEM studies of the oxidised ceramics show the direct transformation of Si₃N₄ grains into SiO₂ through a reaction interface layer.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Investigation of the HCl (g) attack on pre-oxidized pure Fe, Cr, Ni and commercial 304 steel at 400 °C

The paper presents the results from an investigation studying the ability of pre-oxidized metals and alloys to withstand chlorine attack in the form of gaseous HCl. The materials under investigation were pure Fe (s), Cr (s), Ni (s), and a commercial 18Cr-10Ni-Fe (304) alloy. The samples were pre-oxidized in different well defined environments, dry 10 vol.% O₂ (g) + N₂ (bal.), 10 vol.% O₂ (g) + 5 vol.% H₂O (g) + N₂ (bal.) and 10 vol.% O₂ (g) + 250 vppm SO₂ (g) + N₂ (bal.) for 24 h at 400 °C using a horizontal tube furnace. Afterwards the oxide films were characterized by GI-XRD, FEG-SEM, XPS and ToF-SIMS. The samples were then exposed further in 10 vol.% O₂ (g) + 500 vppm HCl (g) + x (x = 5 vol.% H₂O (g), 250 vppm SO₂ (g)) + N₂ (bal.). The exposure time was 100 h and after the exposures during the cool down process the reaction chamber was flushed with dry 10 vol.% O₂ (g) + N₂ (bal.). The corroded samples were then examined by the same techniques mentioned before. HCl (g) showed mainly to be aggressive toward the Fe (s) samples that form a relatively thick and porous oxide scale consisting of layered Fe₂O₃ (s)/Fe₃O₄ (s) during pre-oxidation, and the aggressiveness did not depend on the pre-oxidation conditions. All the other materials formed thin and dense oxides (20-100 nm) during pre-oxidation, and they did not suffer accelerated oxidation caused by HCl (g) during the subsequent exposure. The only exception was Ni (s) that had been pre-oxidized in an atmosphere containing SO₂ (g), in this case Ni sulphides and sulphates were formed during pre-oxidation which in turn caused accelerated oxidation to Ni when subsequently exposed to HCl (g). HCl (g) readily reacts with NiSO₄ (s) and Ni₃S₂ (s) and forms NiCl₂ (s) and SO₃ (g).     

Transient Oxidation Behavior of Fe-5wt.%Al Alloy at 1073 K in Atmospheres Containing Both Oxygen and Water Vapor

The transient oxidation behavior of Fe-5wt.%Al alloy was investigated at 1073 K. Alloys were oxidized in two atmospheres, where gas compositions were changed from an initial N₂-12.2H₂O or O₂-12.2H₂O to N₂-0.9O₂-12.2H₂O and vice versa. When the gas composition was changed from N₂-12.2H₂O to N₂-0.9O₂-12.2H₂O and also from N₂-0.9O₂-12.2H₂O to N₂-12.2H₂O or O₂-12.2H₂O, the oxidation kinetics and scale structures rapidly changed. When the gas composition was changed from O₂-12.2H₂O to N₂-0.9O₂-12.2H₂O, the oxidation kinetics changed after a 1.5-hsec delay. When the alloy was oxidized rapidly, dissociative fissures were generated and grew. The formation and extinction of these dissociative fissures was strongly influenced by the oxygen contents. It is suggested that the oxidation kinetic were affected by dissociation reactions and that this was controlled by the oxygen-activity gradient across the scale.     

Influence of nitridation on the oxidation of a 304 steel at 800 °C

This paper presents a study on the oxide growth on a AISI 304 chromia-forming alloy, in air at 800 °C. After the nitridation treatment was performed on the steel surface, a γ_N solid solution is detected. In this case, no nitride formation in the alloy surface could be observed. In situ X-ray diffraction has been used to follow the oxides evolution at testing temperature. At the beginning of the oxidation test, CrN is formed together with Fe₂O₃. Nevertheless, Cr₂O₃ quickly appears and leads to a protective oxide scale formation growing according to a parabolic rate law. During oxidation in situ X-ray diffraction also shows that Fe₂O₃ is transformed into FeCr₂O₄. Our results show that nitridation increases the high temperature oxidation resistance of 304 steels at 800 °C.     

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr₂O₄ along with FeCr₂O₄ and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Experimental study and modeling of high temperature oxidation of Nb-base in situ composites

The oxidation behavior of Nb-base in situ composites in static air at 1100, 1200 and 1250 °C has been studied. The reaction followed the parabolic rate law. The oxide scales were mainly composed of TiNb₂O₇, TiNb₁₀O₂₉, TiO₂, Nb₂O₅ and SiO₂. The theoretical oxidation model proposed by Gesmundo et al. has been applied to analyze existing oxidation data of Nb-base in situ composites. The result shows that oxidation rate of the Nb-base in situ composites is basically accordance with the analytical results predicted by the theoretical oxidation model to a good approximation.     

Discontinuous oxidation and erosion-oxidation of a CeO2-dispersion-strengthened chromium coating

A CeO₂-dispersion-strengthened chromium coating was developed on a carbon steel using a two-step process: prior electrodeposition of a Ni-CeO₂ nanocomposite film and subsequent chromization using a conventional pack cementation method. Compared to the CeO₂-free coatings prepared on the carbon steel without and with pre-electrodeposition of a pure Ni film, the CeO₂ dispersed chromium coating offered profoundly improved discontinuous oxidation resistance at 900 °C in 5% O₂ + N₂ and in 5% O₂ + 1000 ppm SO₂ + N₂, and erosion-oxidation resistance in a laboratory-scale fluidized-bed combustor (FBC), mainly because of the development of a denser, less wrinkled and more adherent chromia scale.     

Oxidation and interfacial fracture behaviour of NiCrAlY/Al2O3 coatings on an orthorhombic-Ti2AlNb alloy

Al₂O₃ diffusion barriers of various thicknesses have been fabricated by filtered arc ion plating between the NiCrAlY coating and the O-Ti₂AlNb alloy. Isothermal oxidation tests and three-point bend tests have been conducted to investigate the influence of the Al₂O₃ diffusion barriers on the oxidation and interfacial fracture behaviour of the coatings. The results indicate that the Al₂O₃ diffusion barrier defers interdiffusion and gives oxidation resistance of the NiCrAlY coatings. The thickness of the Al₂O₃ interlayer not only influences the oxidation behaviour but also affects the interfacial fracture properties. Additionally, thermal exposure affects the critical load in three-point bend tests.     

Oxidation behavior of silicon carbide at 1200 °C in both air and water–vapor-rich environments

Oxidation of SiC in both air and water–vapor–rich environments was carried out at 1200 °C to examine the effects of different oxidation conditions on the early-stage oxidation behavior of SiC. Two different types of SiC oxidation behavior were found, passive or active, depending on the oxidation environment. All the samples possessed amorphous oxide layers, regardless of the oxidation environment. Three Si oxidation states (SiO, Si₂O₃, and SiO₂) were observed in this layer. The amorphous Si₂O₃ state was dominant, and the ratio of the three different states changed with the test conditions.     

Oxidation of Cr2AlC at 1300 °C in air

High purity, dense Cr₂AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr₂O₃ but Al₂O₃. Since Al was consumed to produce the Al₂O₃, Al-depletion and Cr-enrichment occurred underneath the Al₂O₃ layer. This led to the formation of a Cr₇C₃ layer containing voids. These grew during oxidation, eventually destroying the Cr₇C₃ layer formed on the unoxidized Cr₂AlC matrix.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Corrosion behavior of an alumina forming austenitic steel exposed to supercritical carbon dioxide

Microstructure characterization of corrosion behavior of an alumina forming austenitic (AFA) steel exposed to supercritical carbon dioxide was conducted at 450–650 °C and 20 MPa. At low temperature and short exposure times, the oxidation kinetics were parabolic and the oxide scales were mainly composed of protective and continuous Al₂O₃ and (Cr, Mn)-rich oxide layers. As the temperature and exposure time increased, the AFA steel gradually suffered breakaway oxidation and its oxide scales showed a multilayer structure mainly composed of Fe₃O₄, (Cr, Fe)₃O₄, NiFe/FeCr₂O₄/Cr₂O₃/Al₂O₃, FeCr₂O₄/Al₂O₃, and NiFe/Cr₂O₃/Al₂O₃, in sequence. The corrosion mechanism based on the microstructure evolution is discussed in detail.     

Kinetic conditions for the simultaneous formation of oxide and sulphide in reactions of iron with gases containing sulphur and oxygen or their compounds

The scaling of pure iron has been investigated in N₂O₂SO₂ and COCO₂COS mixtures between 700 and 900°C. Simultaneous formation of FeO and FeS at the scale/gas phase boundary is observed when the diffusion in the aerodynamic boundary layer or the reaction at the scale/gas phase boundary is the rate-controlling step of the oxidation in O₂N₂ mixtures or of the sulphidation in COCOS mixtures. In those cases the addition of the second oxidant (SO₂ to O₂N₂ mixtures, and an increased CO₂ to COCO₂COS mixtures) increases the rate of the oxidation or sulphidation reactions. When, however, the diffusion of iron ions and electrons through the oxide or sulphide layer respectively, or the reaction at the metal/scale phase boundary are rate-determining, the thermodynamically stable phase (oxide or sulphide) is formed exclusively and the addition of the second oxidant has no influence on the scaling rate. These results may be understood from an evaluation of the equilibria prevailing at the scale/gas phase boundary.     

A Cr2O3-deposited Sm(Co0.68Fe0.22Cu0.08Zr0.02)7.5 magnet with increased oxidation resistance at 700 °C

A Sm(Co_0.68Fe_0.22Cu_0.08Zr_0.02)_7.5 alloy was arc-ion-plated with a thin Cr₂O₃ film. It completely prevented the external oxidation and sufficiently suppressed the internal oxidation of the alloy in air at 700 °C for 20 h, causing the alloy to form only a very shallow layer where the Sm oxidation occurred. The mechanism for the effect of the Cr₂O₃ film on the oxidation of the alloy was proposed based on phase characterization of the oxidized layer.     

Study of corrosion resistance properties of nitrided carbon steel using radiofrequency N2/H2 cold plasma process

C38 carbon steel have been plasma-nitrided using a radiofrequency cold plasma discharge treatment in order to investigate the influence of gas composition on corrosion behaviour of nitrided substrates. The investigated C38 steel was nitrided by a RF plasma discharge treatment using two different gas mixtures (75% N₂/25% H₂ and 25% N₂/75% H₂) at different times of plasma-treatment on non-heated substrates. Electron Probe Microanalysis (EPMA) showed that the nitrided layer formed using 75% N₂/25% H₂ gas mixture was thicker compared to those formed in the case of 25% N₂/75% H₂ or pure N₂. The modifications of the corrosion resistance characteristics of plasma-nitrided C38 steel in 1 M HCl solution were investigated by weight loss measurements and ac impedance technique. The results obtained from these two evaluation methods were in good agreement. It was shown that the nitriding treatment in both cases (75% N₂/25% H₂ and 25% N₂/75% H₂) improves the corrosion resistance of investigated carbon steel, while the better performance is obtained for the 75% N₂/25% H₂ gas mixture. X-ray photoelectron spectroscopy (XPS) was carried out before and after immersion in corrosive medium in order to establish the mechanism of corrosion inhibition using N₂/H₂ cold plasma nitriding process.     

Thermal barrier coating bonded by (Al2O3–Y2O3)/(Y2O3-stabilized ZrO2) laminated composite coating prepared by two-step cyclic spray pyrolysis

Thermal barrier coating bonded by (Al₂O₃–Y₂O₃)/(Y₂O₃-stabilized ZrO₂) (YSZ) laminated coating has been developed on Ni-based superalloy by two-step cyclic pyrolysis. It is demonstrated, from cyclic oxidation tests at 1100 °C, that YSZ top coat and alloy substrate can be bonded together effectively by the (Al₂O₃–Y₂O₃)/YSZ laminated coating, showing good resistance to oxidation, cracking, spallation and buckling. These beneficial effects can be attributed to the sealing effect of the designed multi-sealed compact bond coat with α-Al₂O₃ layers, the decrease of thermal stresses, the increase of fracture toughness in such bond coat and no interdiffusion between the substrate and bond coat.     

Oxidation inhibition of γ-TiAl alloy at 900 °C by inorganic silicate composite coatings

Inorganic silicate composite coatings on γ-TiAl were fabricated by air spraying. The oxidation behaviour of the alloy was investigated at 900 °C. The results indicated that rapid oxidation occurred in the γ-TiAl, and multilayered non-protective TiO₂ and Al₂O₃ scales formed. For coated γ-TiAl alloy, the oxidation was markedly inhibited; a thin Al₂O₃ layer was detected, which improved the oxidation resistance of the alloy. The low oxygen partial pressure at the interface of the coatings and the alloy promotes the preferentially oxidation of Al in the γ-TiAl substrate, and the outward diffusion of Ti to form TiO₂ was retarded.