Cyclic oxidation and mechanical behaviour of slurry aluminide coatings for steam turbine components

The excellent steam oxidation resistance of iron aluminide coatings on ferritic steels at 650 °C has been demonstrated both by laboratory tests and field exposure. These coatings are formed by the application of an Al slurry followed by diffusion heat treatment at 700 °C for 10 h. The resulting microstructure is mostly composed of Fe₂Al₅ on top of a much thinner FeAl layer. This coating exhibits perpendicular cracks due to thermal expansion mismatch between coating and substrate. However, these stress relieving cracks do not seem to have an effect on the mechanical properties of the substrate. Cyclic oxidation, creep resistance and TMF testing of these coatings at 650 °C indicate excellent performance.     

Long term oxidation resistance and thermal stability of Ni-aluminide/Fe-aluminide duplex diffusion coatings formed on ferritic steels at low temperatures

A coating with a duplex structure consisting of an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer was formed on a commercial type of ferritic steel P92 using a two step process of electroless Ni/B plating followed by pack aluminising at 650 °C. Nearly 11,000 h oxidation test in air and more than 13,000 h isothermal annealing test in argon atmosphere were carried out to assess the long term oxidation resistance and thermal stability at 650 °C. The amount of oxidation was only about 0.66 mg/cm² for the coating as compared to 10.3 mg/cm² for the uncoated steel after nearly 11,000 h oxidation test. Inward Al diffusion took place during oxidation test, which led to the transformation of the outer Ni₂Al₃ layer to NiAl and increase in the Al diffusion depth. However, once the outer Ni₂Al₃ layer was completely transformed to NiAl, it stayed stable during the remaining period of oxidation test, providing long term oxidation resistance. Kirkendall voids formed and grew and then finally disappeared in the coating layers due to interdiffusion processes taking place at the oxidizing temperature at the interfaces between different layers of the duplex coating. No spallation was observed in the coating during the entire period of the oxidation or isothermal annealing tests.     

Steam oxidation resistance of two-layered Ni-Cr and Al APS coating for USC boiler applications

Thermal spray of Ni-Cr and Al coatings was attempted on modified 9Cr-1Mo steel, to evaluate their steam oxidation resistance. Atmospheric plasma spray (APS) coatings of 50Ni-50Cr as undercoat and Al topcoat were attempted with the aim that the pores produced by 50Ni-50Cr undercoat can be filled with Al topcoat during the steam oxidation. The steam oxidized samples evinced the Ni and Cr diffusion towards the Al coating structure and changed the topcoat in to the Ni-Al intermetallics. Though the two-layered coating exhibited an excellent performance against the steam oxidation for the base steel substrate till 3000 h of test, the top layers of the coating underwent significant internal oxidation.     

Identification of degradation mechanisms in coatings for supercritical steam applications

The development and qualification of coatings for materials used in modern steam power plants stems from the increased demand for higher efficiency, and hence higher operating temperatures. Within the EU funded project ‘SUPERCOAT’, several coatings, both overlay and diffusion type, were investigated. Seven different coatings are presented in this work. They included two commercially available HVOF coatings (Ni–20Cr and Ni–50Cr), an aluminium‐based slurry coating (IPCOTE), together with two further variations of this slurry coating containing sputter‐coated inter‐layers. An overlay slurry coating consisting of silica particles embedded in a matrix of alumina and chromia was also examined. The final coating to be investigated was a pack‐aluminised sample of P92. All the coating systems examined showed superior oxidation resistance compared to the 9%Cr steel substrate (P91 or P92) in extended exposures to a steam environment at 650 °C. However, in service component lifetime will be limited by degradation of the coating, therefore it is essential that the mechanisms controlling this behaviour are understood. This paper reviews several degradation mechanisms that have been observed during long‐term exposure of these coatings. The mechanisms that have been observed include depletion of active alloying elements, diffusion of aluminium into the substrate from the coating, formation of Kirkendall porosity and mechanical failure of the coatings. Examples of each of these mechanisms will be presented. Possible processing routes to avoid these degradation mechanisms will also be discussed.     

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

Steam oxidation resistance of Ni-aluminide/Fe-aluminide duplex coatings formed on creep resistant ferritic steels by low temperature pack cementation process

Steam oxidation resistance and thermal stability were studied at 650 °C for a coating with an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer formed on P92 steel surface. The parabolic rate law of oxidation was obeyed only in less than 2000 h with positive deviations occurring at longer oxidation times. The outer layer of the coating was transformed to NiAl during oxidation, but it remained stable once it was formed. The mechanisms for the enhanced thermal stability were discussed and a simple approach to enhancing the lifetime of the coating was proposed.     

喷丸对马氏体耐热钢高温蒸汽氧化行为的影响

利用高温蒸汽氧化试验装置对比研究P92、G115钢以及喷丸的G115钢的高温蒸汽氧化行为,结果表明,这3个试样的650℃蒸汽氧化动力学曲线符合ΔW=ktn规律.相同蒸汽氧化时间下G115钢的氧化质量增加远小于P92钢,主要是因为G115钢中富Cr层的形成和富Cu相在氧化层界面的析出.经喷丸处理的G115钢氧化质量增加小...     

An investigation of pack-aluminide coating on steel

Aluminide coatings are known to protect steels from oxidation and corrosion in hydrocarbon and sulfur-bearing atmospheres. Pack cementation is ideally suited for forming these coatings on small intricate components, wherein a diffused layer is formed which is well bonded to the substrate. Even though pack aluminide coated steels are being commercially used, there has not been any systematic investigation of the factors that control the coating formation. The present investigation has been carried out to define the boundary conditions under which diffusion in the solid phase determine the coating kinetics. The effect of pack activity and temperature on the structure and kinetics of aluminde layer formation on EN-3 steel has been investigated. The coating characteristics were evaluated by metallography, EPMA, X-ray diffraction, and scanning electron microscope (SEM). Oxidation resistance of the coated samples were compared to that of 304 stainless steel after heating in air at 900°C for 72 h. The surface aluminum composition was found to be about 20% by weight which remained constant with time in the temperature range of 750°C–900°C. Weight gains and layer thicknesses obeyed parabolic relationship with time at all temperatures. Under these conditions, the system constitutes a vapor-solid diffusion couple. Interdiffusion coefficient values in the Fe-Al system have been determined, and the activation energy has been calculated to be 57 Kcals/mole, which agrees well with the literature values.     

Oxidation pre-treatment to improve the mechanical property and oxidation resistance of Si–Mo–Cr coated C/C composites

A transition layer with an interlocking structure was obtained at the interface between C/C composites and Si–Mo–Cr coating by oxidation pre-treatment and pack cementation process. The effects of pre-oxidation time on the microstructures, mechanical property and oxidation behavior of the coated C/C samples were investigated. After oxidation pre-treatment at 1173 K in air for 6 min, the flexural strength of the coated samples increases by 71.7% and the mass loss rate decreases by 77.4%. The improvements of the mechanical property and oxidation resistance are primarily attributed to the formation of the moderate transition layer between C/C and Si–Mo–Cr coating.     

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800-1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)₃Si₅Al₂ phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)₂Al layer formed in the coating between the (Nb,X)₅Si₃ layer and the substrate, but when aluminizing was performed at 900-1000 °C, the new layer formed was (Nb,Ti)Al₃. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000-1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)₃Si₅Al₂, (Nb,X)Si₂ and (Nb,Ti)Al₃ was obtained by co-depositing Si and Al at 1050 °C.     

Effects of Cr–Si Diffusion Barrier Layer on the Oxidation Resistance of NiCrAlY Coating System with Aluminized Top Layer

A multilayer coating, which consisted of a Cr–Si co-deposited layer as the diffusion barrier, a plasma sprayed NiCrAlY middle layer, and an aluminized top layer, was developed. During the aluminizing treatment, Cr in the NiCrAlY layer was released as the γ/γ′ structure of this layer transformed to the β phase. The released Cr was inhibited by the inner Cr–Si layer to diffuse into the substrate, and a Cr layer eventually formed over the Cr–Si layer. The Cr layer impeded the inward diffusion of Al due to the low solubility of Al in the Cr layer so that more Al atoms remain in the coating and contributed to the oxidation resistance of the coating. The multilayer coating exhibited better oxidation and spallation resistance than coatings without a Cr–Si layer, at least at 1050 °C for up to 1000 h.     

Simultaneous Aluminizing and Chromizing of Steels to Form (Fe,Cr)3Al Coatings

A cementation pack involving halide activatorsand elemental Al and Cr powders has been used to achievethe codeposition and diffusion of aluminum and chromiuminto low-alloy steels. A two-step treatment at 925°C and 1150°C yields dense anduniform ferrite coatings of about 400-m thickness,with surface compositions of approximatelyFe₃Al plus several percent Cr. The two stepheating schedule prevents the formation of a blocking chromium carbide atthe substrate surface. An attempt was made to add atrace of Ce to the Al + Cr content of the coating byintroducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Uponcyclic oxidation in air at 700°C, the coated steelexhibits a negligible 0.085 mg/cm² weightgain for 1900 one-hour cycles. Virtually no attack wasobserved on coated steels tested in a simulated boileratmosphere at 500°C for 500 hr. But coatings with asurface composition of only 8 wt.% Al and 6 wt.% Crsuffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than500°C for 1000 hr.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram     

Formation of a Rhenium-base Diffusion-barrier-coating System on Ni-base Single Crystal Superalloy and its Stability at 1,423 K

A diffusion-barrier-coating system having a duplex structure comprised of an inner Re(W)–Cr–Ni layer and an outer Ni-aluminide layer was formed on a fourth generation, single-crystal Ni-base superalloy by using electroplating of Re(Ni) and Ni(W) films, Al- and Cr- (high-Cr and low-Cr) pack cementations, and a combination of the two treatments. With the ReW-high-Cr coating, fine needle- or plate-like precipitates formed in the alloy substrate below the inner Re(W, Cr, Ni) layer, while there was little of this precipitate with the ReW-low-Cr pack-cementation coating. The inner, Re-base alloy layer in the ReW-high-Cr coating was identified to be a σ-(Re,Cr,W,Ni) phase, while the inner layer of the ReW-low-Cr was a mixture of σ-(Re,Cr,W,Ni) and δ-Re(Cr,W,Ni) phases. After heating the coated alloys at 1,150 °C for 100 h in air, the outer Al reservoir layer became β-NiAl containing (31–33)Al with α-Cr particles and fine precipitates of γ′-Ni₃Al with both the ReW-high-Cr and ReW-low-Cr treatments. In the case of the ReW-high-Cr coating there were numerous light-colored, needle-like precipitates formed deep in the alloy substrate under the inner layer, while in the case of the ReW-low-Cr coating γ′ appeared near the inner layer. It was found that the inner, Re-base alloy layer acted as a diffusion barrier, and that its structure was maintained with little change in composition after 100 h of oxidation at 1,150 °C. K. Z. Thosin is from Indonesian Institute of Sciences, LIPI.     

SiC/SiC–YAG–YSZ oxidation protective coatings for carbon/carbon composites

SiC/SiC–YAG–YSZ coatings were prepared by pack cementation, chemical vapor deposition and slurry painting on carbon/carbon (C/C) composites. The microstructures and oxidation behavior of coatings were investigated. The results show that the coatings displayed good oxidation and thermal shock resistance due to a dense glassy layer with silicates formed on the coating of SiC–YAG–YSZ. The weight gain rate of coated C/C composites was 1.77% after oxidation for 150 h at 1773 K. SiC in outer coating can promote the formation of oxygen diffusion barrier and lead to the optimum oxidation resistance for the coatings, compared with YSZ and YAG.     

Development of Re‐based diffusion barrier coatings on nickel based superalloys

A diffusion barrier type coating with a duplex layer structure, an inner σ‐(Re, W, Cr, Ni) as a diffusion barrier and outer Ni‐aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS‐82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room temperature and 1423 K for up to 360 ks. The inner σ layer with a composition (at%) of (35–40) Re, (15–20) W, (15–25) Cr and (15–25) Ni was produced by electrodeposition of Ni‐70Re and Ni‐20W films from aqueous solutions followed by Cr‐pack cementation at temperatures between 1473 and 1573 K, and the outer Ni‐aluminides of β‐(Ni,Cr)Al + γ′‐(Ni,Cr)₃Al was formed by electrodeposition of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both σ layer and alloy substrate were retained with little change. Furthermore, there was little Al in the σ layer. It could be concluded that the Re‐based alloys such as σ (Re(W),Cr,Ni) are very promising candidates as a diffusion barrier between the outer Al‐reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)‐Cr‐Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.     

A study on the microstructure and cyclic oxidation behavior of the pack aluminized Hastelloy X at 1100 °C

A pack aluminizing process at 950 °C for 9 h has been employed on the nickel-base superalloy Hastelloy X to deposit a 75 μm thick β-NiAl aluminide layer on the surface. A nanoscale dendritic structure is observed on the surface of the aluminide coating. A finger-like interdiffusion zone is found between the aluminide layer and the substrate. Fine precipitates with complex phases are distributed in the NiAl layer. The cyclic oxidation tests of aluminized alloys and untreated substrates were conducted at 1100 °C for 196 h. It was observed that the aluminizing process greatly enhances the cyclic oxidation resistance of Hastelloy X at 1100 °C due to a dense and protective alumina layer formed on the surface. Complex phase transformation reactions occurred in the aluminide layer. Owing to the oxidation and interdiffusion reactions at high temperature, the Al content of the NiAl layer was depleted to form some low Al containing γ-substrate grains on the surface and a continuous γ layer between the aluminide layer and substrate. Thermal stress induced, transverse cracks in the interdiffusion zone, were observed possibly due to the difference of thermal expansion coefficients among the substrate, aluminide layer and interdiffusion zone.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

Microstructure and oxidation resistance of reactive plasma clad Cr7C3 /γ-Fe ceramic composite coating

A new type oxidation resistance in situ Cr7C3/γ-Fe ceramic composite coating was fabricated on hardened and tempered grade C steel by reactive plasma clad with Fe-Cr-C alloy powders. The oxidation resistance of the ceramic composite coating was investigated under the test condition of 900℃ and 50 hours. The results indicate that the coating has a rapidly solidified microstructure consisting of blocky primary Cr7C3 and the inter-blocky Cr7C3/γ-Fe eutectics and is metallurgically bonded to the hardened and tempered grade C steel substrate. The high temperature oxidation resistance of the coating is up to 1.9 times higher than that of grade C steel. The oxidation kinetics curve of the coating is conforming to the parabolic-rate law equation. The excellent oxidation resistance of the coating is mainly attributed to the continuous oxide films which consist of Cr2O3 and Fe2O3. The continuous oxide films can prevent the inner part of the coating from being further oxidized.     

Improvement in oxidation resistance of a Ni3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni₃Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni₂Al₃ and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al₂O₃ scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.