Cyclic oxidation behavior of TiCrN coating deposited on a steel substrate by the Arc-ion plating method

The cyclic oxidation behavior of a TiCrN coating deposited on a steel substrate was studied at 700, 800, 900 and 1000°C in atmospheric air, by repetitively exposing the specimen to thermal cycles of 2 hr-heating and subsequent quenching to room temperature. The coating displayed good cyclic oxidation resistance up to 800°C, with relatively small weight gains, but above 900°C, the coating displayed a drastic diminishment in oxidation resistance, accompanied by large weight gains. Cracks were observed particularly above 900°C throughout the oxide scale that consists of TiO₂, Cr₂O₃ and/or Fe₂O₃.     

Oxidation Resistance of a Cr0.50Al0.50N Coating Prepared by Magnetron Sputtering on Alloy K38G

A Cr_0.50Al_0.50N coating has been prepared by a reactive-magnetron-sputtering method on alloy K38G. The coating possesses mainly the B1 type with a small amount of B4-type crystal structure phase. Isothermal oxidation tests were performed at 900–1,100 °C for 20 h by thermogravimetric analysis (TGA) in air. The results reveal that the coated samples have much lower mass gain than that of the bare alloy. The parabolic rate constants of the coated samples decrease by 2 orders of magnitude compared with the bare alloy at 1,000 and 1,100 °C. During the oxidation of the coated samples below 1,000 °C, the main oxide is Cr₂O₃, but above 1,000 °C, the scale changes to α-Al₂O₃. The observed oxidation behaviors demonstrate that the Cr_0.50Al_0.50N coating can provide good protection against corrosion over a wide temperature range.     

The oxidation behaviour of an aligned Ni-Al-Cr3-C2 eutectic alloy under isothermal and thermal cycling conditions

The isothermal and thermal cycling oxidation behaviour of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy at temperatures from 800° to 1200 °C in flowing air have been investigated using several physical techniques. At all temperatures an initial, protective, external layer of α-Al₂O₃ develops on the alloy surface. However, this breaks down mechanically during thermal cycling, enabling a less protective Cr₂O₃-rich scale to form. The time of retention of the α-Al₂O₃ layer at temperature decreases with increasing temperature, failing after between 30 min and 2 h at 1100° and 1200 °C. However, if platinum metal is introduced into the hot zone at these temperatures, this period is increased to about 48 h. Following formation of the external Cr₂O₃ scale, internal oxide penetration into the alloy can be considerable, involving preferential oxide penetration down the alloy/carbide fibre interfaces. Thermal cycling does not influence markedly the oxidation behaviour, although it does result in formation of a greater quantity of nickel-rich oxide nodules on the scale surface following crack development in the Cr₂O₃-rich scale. This crack development is assisted by differential thermal contraction stresses.     

Oxidation and electrical behavior of AISI 430 coated with cobalt spinels for SOFC interconnect applications

Stainless steel can be used as interconnect plates in solid oxide fuel cells (SOFCs) below operating temperature of 800 °C. Unwanted reactions between the alloy and other SOFC components decrease the efficiency of these energy convertors. One approach to improving interconnect properties is to apply a surface coating to them. In this study, AISI 430 ferritic stainless steel interconnect is coated in a cobalt-base pack mixture using the pack cementation method. Isothermal oxidation, cyclic oxidation and oxidation at different temperatures (400-900 °C) are applied to evaluate the role of the coating layer during oxidation. Area-specific resistance (ASR) of the coated substrates has also been tested as a function of temperature and time. The surface morphology was examined by SEM, the chemical composition and structure of oxide formed were analysed by EDS and XRD. Results showed that the coating layer transforms into MnCo₂O₄, CoCr₂O₄ and CoFe₂O₄ spinels during isothermal oxidation. This scale is protective and acts as an effective barrier against chromium migration into the outer oxide layer and prevents weight gain. The mass gain and spallation indicated that the formation of spinel significantly improved the high temperature oxidation. These spinels also cause a reduction in ASR for coated substrates (9.7 mΩcm²) as compared to uncoated substrates (36.1 mΩcm²) after 200 h of isothermal oxidation at 800 °C.     

Comparison of Oxidation Behavior and Electrical Properties of Doped NiO- and Cr2O3-Forming Alloys for Solid-Oxide, Fuel-Cell Metallic Interconnects

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr₂O₃-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y₂O₃-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr₂O₃-forming alloys were evaluated. The addition of Y₂O₃ and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H₂O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y₂O₃, Ni+Y₂O₃, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr₂O₃-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Effect of Chromium Content on the Oxidation Behaviour of Ferritic Steels for Applications in Steam Atmospheres at High Temperatures

Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800 °C, in 100% H₂O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650 °C, a scale composed of Fe₃O₄, Fe₂O₃ and (Fe,Cr)₃O₄ is formed on P-91, although at 800 °C the scale is mainly composed of Fe₃O₄ and (Fe,Cr)₃O₄. On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr)₂O₃ at 650 °C, and at 800 °C a layer of Cr₂O₃ is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650 °C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.     

Influence of a Coating on Oxidation Resistance and Resistivity of a Chromia Former Alloy for High Temperature Vapor Electrolysis Application

Alloy 230 was studied as possible interconnect material for high temperature vapor electrolysis (HTVE) application. For such a use, the main important requirements are good corrosion resistance and a low area specific electrical resistance (ASR). The samples were oxidized at 800 °C for 250 h under cathodic (Ar–1 %H₂–9 %H₂O) and anodic (dry air) HTVE atmospheres. In situ ASR evolution of alloy 230 in both conditions was determined using a specific device. The influence of a LaCrO₃ coating is also discussed in this study. It is shown that alloy 230 offers good corrosion resistance, improved by the presence of the coating. A calculation of the theoretical ASR is in good agreement with the experimental value obtained in anodic environment. For the cathodic condition, the ASR value is two orders of magnitude higher than the expected one, and this could be explained by the presence of hydrogen in the oxide layer. The LaCrO₃ coating effect on ASR values remains unclear, but it appears to have at least an influence on its evolution over the test duration.     

Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe₂O₃ and (Fe,Co,Cr)₃O₄ spinel with a small amount of Cr. However, (Fe,Co,Cr)₃O₄ spinel and Fe₂O₃ were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co₃O₄ and (Fe,Co)₃O₄ with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)₃O₄ spinel layer and an inner Cr₂O₃ layer was developed on the sputtered coating.     

Indirect evaluation of the long-term oxidation properties of Al−21Ti−23Cr and Al−37Ti−12Cr coating materials for TiAl alloy

The most pertinent coating materials in the Al−Ti−Cr alloy system to improve the high temperature oxidation resistance of a TiAl alloy, with respect to oxidation properties, resistance to thermal stress, and chemical compatibility, are the two-phase alloys of Al−21Ti−23Cr (L1₂+Cr₂Al) and Al−37Ti−12Cr (γ+TiAlCr). In this study, cyclic oxidation tests at 1000 °C and 1200 °C were performed for the specimens coated with both materials of 10 im thickness. Furthermore, breakaway oxidation caused by the formation of a rutile TiO₂ scale was observed, though both bulk alloys showed very stable oxidation behavior. This phenomenon was resulted from the depleted Al content in the coating layer due to Al₂O₃ oxide growth and interdiffusion with the substrate. Considering the decrease of Al content due to oxide growth, the Al−21Ti−23Cr coating with the initial higher Al content was more effective for long-term oxidation protection of the TiAl alloy. On the other hand, when the Al content changes due to the interdiffusion with the substrate, the Al−37Ti−12Cr coating with a smaller compositional gradient with the TiAl substrate was more effective than the Al−21Ti−23Cr coating. Cyclic oxidation tests at 1000 °C and 1200 °C confirmed that for the longer lifetime of coating materials the initial Al content was more important than the smaller compositional gradient with the substrate. Consequently, the Al−21Ti−23Cr coating was considered as more effective one for the long-term oxidation resistance of TiAl alloys.     

Cyclic oxidation of AISI 316L stainless steel – influence of water vapour between 800 and 1000°C

At 800 and 900°C, 10 vol.-%H₂O in air has little effect on the AISI 316L stainless steel oxidation under isothermal and cyclic conditions. The oxide scale is composed of Cr₂O₃ with a small amount of Mn_1·5Cr_1·5O₄ at the external interface. Results show that water molecules or protons can modify the diffusion process in the scale and lower the oxidation rate. At 1000°C, a deleterious effect of water vapour on the scale structure is observed. In situ X-ray diffraction was used to analyse the oxide formation on AISI 316L specimens during isothermal oxidation at 1000°C in moist air. Results show that the breakaway oxidation is due to the iron oxidation starting after 31 h oxidation. This leads to an external Fe₂O₃ scale growth and an internal multilayered FeCr₂O₄ scale formation. In wet air, thermal cycling conditions lead to continuous weight losses at 1000°C, whereas the scale remains adherent in dry air.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Effect of Pre-Oxidation at 800?��C on the Pitting Corrosion Resistance of the AISI 316L Stainless Steel

In situ X-ray diffraction was used to identify the oxides formed on the AISI 316L (containing 2% Mo) stainless steel during isothermal oxidation at 800 °C, in air. Good oxidation behavior was observed on this steel when considering kinetics, structural characteristics and scale adherence. It was demonstrated that molybdenum plays a protective role in that it hinders the outward iron diffusion and leads to the lower growth rate and the better scale adherence. The oxide scale was then composed of Cr₂O₃ with a small amount of Mn_1,5Cr_1,5O₄ at the external interface. Pre-oxidation of the AISI 316L also improved its aqueous corrosion resistance. No pitting corrosion occured during the corrosion test. Aqueous corrosion testing also showed that the oxide scale formed at 800 °C is crack-free and still adherent after cooling to room temperature.     

Metallic interconnects for solid oxide fuel cell: Performance of reactive element oxide coating during long time exposure

One of challenges in improving the performance and cost‐effectiveness of SOFCs (solid oxide fuel cells) is the development of suitable interconnects materials. Chromia‐forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia‐forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR – area specific resistance – value). Such degradation are unacceptable regarding the long‐term operation (>40 000 h). A previous study 1 demonstrated that in air, the addition of a nanometric reactive element oxide (La₂O₃) layer applied by metal organic chemical vapor deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU and Haynes230 alloys for 100 h at 800 °C. In this present study coating performances were checked after 10 months (7500 h) and 20 months (15 000 h) at 800 °C in air. The corrosion products were carefully analyzed by SEM, EDX, and XRD. ASR measurements are realized after long time exposure. This study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 15 000 h in air but this alloy has an ASR value equal to 0.370 Ω cm². The coatings composed of a thin reactive element oxide such as La₂O₃ resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.154 Ω cm². Haynes230 alloy has a better oxidation resistance but the formation of an insulating Al₂O₃/SiO₂ layer could be detrimental.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.     

High-Temperature Oxidation Behavior of Sputtered IN 738 Nanocrystalline Coating

A sputtered nanocrystalline coating of IN 738 alloy was obtained by means of magnetron sputtering. The isothermal oxidation behavior at 800, 900, and 1000°C and the cyclic oxidation behavior at 950°C of the coating were studied in comparison with IN 738 cast alloy. The results indicated that a double external oxide scale was formed on the nanocrystalline coating at 900, 950, and 1000°C without internal oxide and nitride. The scale consisted in an outer mixture of Cr₂O₃, TiO₂, and NiCr₂O₄ and an inner, continuous Al₂O₃ layer, which offered good adhesive and protectiveness. However, at 800°C a continuous Al₂O₃ scale could not be formed during oxidation of nanocrystalline coating and aluminum was still oxidized internally.     

Metallic Interconnects for Solid Oxide Fuel Cell: Performance of Reactive Element Oxide Coating During 10, 20 and 30?Months Exposure

One of challenges in improving the performance and cost-effectiveness of SOFCs (Solid Oxide Fuel Cells) is the development of suitable interconnect materials. Chromia-forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia-forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR—area specific resistance—value). Such degradation is unacceptable regarding the long-term operation (>40,000?h). A previous study demonstrated that in air, the addition of a nanometric reactive element oxide (La₂O₃) layer applied by Metal Organic Chemical Vapour Deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU for 100?h at 800?°C. In the present study, coating performances were checked after 10?months (7,700?h), 20?months (15,400?h) and 30?months (23,100?h) exposure in air at 800?°C. The corrosion products were characterized by SEM, EDX and XRD. ASR measurements are realized after long time exposure. The study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 23,100?h in air but this alloy has a high ASR value equal to 0.410?Ω?cm². The coatings composed of thin reactive element oxides made of La₂O₃ or Y₂O₃ resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.180 and 0.160?Ω?cm² respectively.     

Water Vapour Effect on the Oxidation Mechanism of a Cobalt-Based Alloy at High Temperatures (800–1,100 °C)

A cobalt-based Phynox alloy was oxidized in the 800–1,100 °C temperature range. The alloy oxidation was consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Water vapour enhanced the alloy oxidation rate and scale porosity. Thermal cycling tests at 900 and 1,000 °C showed that water vapour reduces the outer Mn_1.5Cr_1.5O₄ subscale adherence, but the chromia scale adherence was not affected. These temperatures permited a rapid chromium supply from the substrate to form a continuous chromia scale. At 1,100 °C thermal cycling conditions led to scale spallation and chromium depletion in the alloy. In dry air, weight losses were recorded due to cobalt and molybdenum oxidation, giving CoCr₂O₄ and CoMoO₄. In wet air, the initial porous chromia scale permited nickel and cobalt oxidation, leading to Ni₅Co₃O₈ and CoCr₂O₄ formation and resulting in bad adherence during thermal cycling.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Improvement of the Oxidation Resistance of the Single-Crystal Ni-Based TMS-82+ Superalloy by Ni–Al Coatings with/without the Diffusion Barrier

Oxidation behavior of the uncoated base, Ni–Al coated and Re–Cr-Ni plus Ni–Al coated single-crystal (SC) Ni-based TMS-82+ superalloy is studied under cyclic air at 900 °C for 200 h to assess the oxidation resistance. Regardless of the coating processing, Ni–Al coating is effective in improving the oxidation resistance due to the formation of a continuous α-Al₂O₃ layer in the scale. For the uncoated base superalloy, the mass-gain curves are fitted by a subparabolic relationship, and complex oxide products including predominately NiO, some CrTaO₄, α-Al₂O₃, Cr₂O₃, a minor of spinels of (Ni, Co)Al₂O₄, AlTaO₄ and θ-Al₂O₃ are detected. Time-dependence of the oxide growth rate for both coated superalloy with/without the diffusion barrier is explained by the parabolic relationship. The oxide scales consist predominately of α-Al₂O₃ and a minor of θ-Al₂O₃. The diffusion barrier of σ-phase plays a negligible effect on the oxidation resistance during the cyclic exposure environment. The amount of detrimental γ′-phase and topologically close-packed (TCP) phases in the interdiffusion zone in the coated superalloy with the diffusion barrier is greatly reduced compared with that without the diffusion barrier due to the distinct barrier effect limiting diffusion of elements between the bond-coat and the substrate.