Cyclic Oxidation and Hot Corrosion Behavior of Y/Cr-Modified Aluminide Coatings Prepared by a Hybrid Slurry/Pack Cementation Process

Y/Cr-modified aluminide coatings were prepared on a Ni-base superalloy K417G using a hybrid slurry/pack cementation process. The coatings consisted of a NiAl layer with dissolved Cr and Y. The microstructures and high temperature corrosion behavior of the coatings were characterized using SEM/EDS, XRD, EPMA and SIMS. Cyclic oxidation tests at 1000 °C for 200 h were carried out in air. The results indicated that specimens coated by either the Y/Cr-modified aluminide coatings or the simple aluminide coatings exhibited better oxidation resistances than the cast alloy. The Y/Cr-modified aluminide coatings possessed lower oxidation rates and better degradation resistance than the simple aluminide coatings during the oxidation tests. Furthermore, the alumina scales formed on the Y/Cr-modified aluminide coatings were considerably more adherent than those on the simple aluminide coatings during the thermal cycling. The hot corrosion tests consisted of applying a 25 wt% K₂SO₄ +75 wt% Na₂SO₄ salt mixture to the specimens and exposing at 900 °C. The Y/Cr-modified aluminide coatings showed the longest service life compared with the cast alloy and aluminide coatings, which suffered significant sulfur attack. After 200 h, the Y/Cr-modified aluminide coatings were still protective.     

铬铝涂层及其抗高温腐蚀的行为

本文作者采用粉末包装法制备Cr-Al二元涂层,对Cr-Al二元涂层的制备以及涂层的高温腐蚀性能进行了研究。实验结果表明,在纯Ni上制备Cr含量较高的Cr-Al共渗涂层是困难的,纯Ni上Cr-Al共渗时,Al会同时沉积到纯Ni试样和Cr粉表面上,由于Cr粉表而上铝涂层的形成,大大降低丁Cr的活度,从而抑制了Ni上渗Cr的过程;然而,在纯Ni上用二步法制备Cr-Al二元涂层是可行的,先渗Cr后渗Al能产生理想的Cr-Al二元涂层,涂层具有良好的抗高温氧化和抗热腐蚀性能;而先渗Al后渗Cr则产生有孔洞的Cr-Al二元涂层,涂层的抗高温腐蚀性能很差。     

309不锈钢表面CeO2 改性铝化物涂层的循环氧化行为

以NH₄Cl为活化剂,采用包埋法在309不锈钢上制备了纳米CeO₂改性铝化物涂层。采用X射线衍射分析仪、扫描电子显微镜和能谱仪对涂层以及循环氧化50次后的表面和横截面进行了分析。微观结构研究表明,改性涂层中包含Fe₄Al₁₃相,由于基底金属的向外扩散,改性涂层捕获了少量的CeO₂纳米颗粒。与不添加CeO₂纳米颗粒的普通铝化物涂层相比,在900 ℃的大气环境下,分散CeO₂改性铝化物涂层表现出更好的防氧化剥落性能;在50次循环氧化后,CeO₂改性铝化物涂层上仍然可以发现一些Fe₂Al₅相,并存在向外扩散的Al层、中间的FeAl层和外部的Fe₂Al₅+FeAl混合层,这表明CeO₂纳米颗粒可以延缓铝化物涂层的降解。     

Effect of Y2O3 content in the pack on microstructure and hot corrosion resistance of Y–Co-modified aluminide coating

Y–Co-modified aluminide coatings on nickel base superalloys were prepared by pack cementation method. Effect of Y₂O₃ content in the pack mixture on microstructure and hot corrosion resistance of the coatings was investigated. The results show that with the increase in Y₂O₃ content, the content of Co in the coatings increases. The mass gain of the coatings with Y₂O₃ addition of 1, 2 and 3 wt.% is 0.6, 0.55 and 0.42 mg/cm² after hot corrosion at 1173 K for 100 h, respectively. Y₂O₃ addition accelerates the diffusion of Co and thus increases the hot corrosion resistance of the coating.     

A study on composite coatings of yttrium ion-plating and pack aluminizing for high-temperature applications

Yttrium (Y) was incorporated by an ion-plating method either before or after pack aluminizing to maximize the corrosion resistance of IN 713C. Various combinations of pack aluminizing and yttrium-ion plating were examined with respect to coating sequence, aluminum activity, and corrosive environment. Of all the various coating combinations examined, the best corrosion resistance was obtained from H/A + Y (high-activity aluminizing + Y-ion plating) type composite coatings. Uniformity of the Y deposition was greatly dependent upon the surface condition of the aluminide-coating layer. The high-activity aluminide coating gave better uniformity of Y deposition than did the low-activity-aluminide coating.Improvement of corrosion resistance by the Y-modified-aluminide composite coatings of H/A + Y type occur because the presence of Y between the Al₂O₃ columns improves Al₂O₃ scale adherence and substantially prevents depletion of Al in the aluminide-coating layer.     

Low-temperature formation and oxidation resistance of ultrafine aluminide coatings on Ni-base superalloy

Ultrafine aluminide coatings were successfully produced on Ni-18Fe-17Cr superalloy at 540-600 °C in a modified pack-aluminizing process. Repeated ball-impacts accelerated the formation of the aluminide coatings by a surface refining process, resulting in atomic diffusion occurring at a relatively low temperature. The effects of the operation temperature and the treatment duration on the formation of the coatings have been investigated. The coatings possessed a two-layer structure. The top layer, approximately 5 µm in thickness, exhibited equiaxial coarse grains and was dominated by NiAl₃, with small amounts of Fe₂Al₅ and CrAl₅. The bottom layer showed high density, homogeneous, ultrafine grains with diameters approximately 30-50 nm. High-temperature oxidation tests were carried out at 1000 °C. The oxidation kinetics and microstructure of the oxide scale were studied. The experimental results indicated that the coatings greatly enhanced the high-temperature oxidation resistance of Ni-18Fe-17Cr superalloy.     

Sulphidation Behavior of a Non Harmful Water-Based Al and Al–Si Slurry Coating on CM247 Superalloy

Slurry aluminide coatings are widely applied to protect metallic surfaces from oxidation and corrosion. They are frequently used in gas turbine engine nozzles because of economical advantages and a straight-forward manufacturing route. A variety of commercial slurries are available to aluminize the surfaces of nickel-based superalloys, however, they have two main disadvantages. First, the phosphates and chromates or halides used as binders and to activate the diffusion species are environmentally harmful; second, the conventional systems have to be heat-treated in an inert atmosphere. As an outcome of the PARTICOAT project the variety of slurry derived coatings has been extended by tailoring the particle size of the metallic source. By doing that, environmentally friendly water-based slurries were developed to produce in a one-step process und atmospheric conditions, a thermal barrier system based on an aluminum diffusion layer and an alumina foam layer which serves as bond coat as well as top coat (TC). CM 247 nickel base superalloy was coated and heat-treated in air using newly developed Al and Al–Si slurries. The oxidation behavior was investigated at 1,000 °C and then compared to pack-cemented aluminide coatings. The sulphidation behavior was investigated at 1,000 °C in an atmosphere of 1.5 vol% SO₂ in synthetic air for Al and Al–Si slurry coated samples with and without the alumina foam TC layer. PARTICOAT Al-based slurries,, after the initial stabilization of the TC, showed similar oxidation kinetics as pack cemented aluminides when exposed to air. When the coatings were exposed to sulphide-containing atmospheres, their oxidation rates increased, producing typical type I corrosion damage. Coatings without TC produced more protective oxide scales. The weight gain and coating area affected by corrosion were slightly lower for the Al-based slurries after 1,000 h of exposure than for the Al–Si based ones. The new coating presented here offers unique advantages in comparison to state-of-the-art slurry and pack cemented coatings by opening a potential way to manufacture a complete thermal barrier coating system by a simple, inexpensive and environmentally safe deposition and heat-treatment in air.     

Cyclic oxidation of Pt/Pd-modified aluminide coating on a nickel-based superalloy at 1150 °C

Pt-, Pd-, and Pt/Pd-modified aluminide coatings were prepared on Inconel 738LC by pack aluminizing at 1034 °C. During pack aluminizing, Pt-modified aluminide coating formed a two-phase β-NiAl + PtAl₂ layer and a β-NiAl layer on an interdiffusion zone, whereas Pd- and Pt/Pd-modified aluminide coatings formed only the thicker β-NiAl layer. However, Pd-modified aluminide coating had many pores. During cyclic oxidation, Pt/Pd-modified aluminide coating had a surface that was less rumpled than that of Pt-modified aluminide coating due to its thicker thickness. Pt/Pd-modified aluminide coating had a 22% greater Al-uptake than Pt-modified aluminide coating. Cyclic oxidation tests at 1150 °C showed that Pt/Pd-modified aluminide coating had the best cyclic oxidation resistance. After the cyclic oxidation, an additional γ-Ni phase was seen beneath the outermost alumina scale on the the γ′-Ni₃Al phase in Pt/Pd-modified aluminide coating. The γ-Ni phase, which had a higher Cr content, increased the adhesion and stability of the alumina.     

The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl₂) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.     

Novel processing in inert atmosphere and in air to manufacture high‐activity slurry aluminide coatings modified by Pt and Pt/Ir

Slurry‐derived coatings are an interesting alternative method to pack aluminization of nickel‐base superalloys, which provide similar properties and protection at high temperatures. For highest performance, these aluminide coatings are modified by the addition of Pt or, as recent research suggests, with Pt/Ir. While the combination of Pt and Pt/Ir with an out‐of‐pack process is state of the art, slurry coatings are of special interest as a repair method for turbine blades. In this study, the microstructural evolution of slurry‐derived coatings manufactured on CM 247 in inert atmosphere as well as in air was investigated. Layers of Ni, Pt, and Pt/Ir mixtures were electrodeposited. After annealing, a diffusion heat‐treatment with a slurry containing aluminum or aluminum–silicon powder was applied on the samples. The addition of silicon is well known to be beneficial for hot corrosion environments. The reaction and interdiffusion behavior of aluminum/aluminum–silicon determines the microstructural evolution of the coatings. Depending on the initial electroplated layer on the surface, different microstructures can be obtained, such as the Pt/Ir‐modified beta phase (Ni,Pt)Al or two‐phase layers of PtAl₂ and NiAl. Additionally, the reactivity between the elements at the surface and those from the slurry was shown to determine homogeneity and surface roughness of the diffusion coating, also depending on the atmosphere used during slurry aluminization. Finally, it was demonstrated that iridium has a high influence on the diffusion behavior and especially the distribution of platinum in the coatings. Such new coatings have the potential to overcome some disadvantages of conventionally manufactured high‐activity aluminide coatings, as the combination of Pt/Ir‐electroplating with the slurry process results in less detrimental substrate elements like molybdenum or tungsten close to the surface.     

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM^®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl₃ and TiAl₂ phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl₂ phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.     

分散剂Y2O3对渗铝层的影响

通过改变传统渗剂来研究分散剂（Al2O3，Y2O3）在渗铝过程中的作用和对渗铝层的影响。利用XRD和SEM对渗层相的组成、氧化前后表面形貌进行分析。结果表明：分散剂Y2O3在最初的渗铝过程中参加了渗铝过程，不仅防止铝粉粘结，而且影响渗层相的组成。分散剂为Al2O3时，渗层主要为Al3Ti＋AlTi＋AlTi3相，而分散剂中含有Y2O3时，渗层土要为AlTi3相，这与传统渗铝机理认为分散剂在渗铝过程中不参加渗铝反应相矛盾。同时，分散剂也影响着渗铝层氧化前后的表面形貌及Al2O3的晶格类型。     

Diffusion aluminide coatings for TiAl intermetallic turbine blades

Alloys based on intermetallic phases of a Ti–Al system are materials that, thanks to their resistance characteristics, can be widely used in automotive and aerospace applications. The main restriction for the use of Ti–Al materials is their insufficient oxidation resistance above 850 °C. Oxidation parameters might be improved by aluminide coatings based on TiAl₂ and TiAl₃ phases, which could induce the creation of an Al₂O₃ scale in the oxidation process. This type of aluminide could be deposited on the surface of TiAl alloys by various methods such as pack cementation, plasma spraying or magnetron sputtering. This article presents a new method of aluminide coating deposition on TiAl intermetallic alloys: out of pack technology. The investigated coating was deposited on turbine blades made of a Ti45Al5Nb intermetallic alloy. The surface morphology, structure, phase and chemical composition have been investigated using XRD phase analysis, SEM and EDS. The phase analysis showed that TiAl₃ and TiAl₂ were the main components of the deposited coating. An isothermal oxidation test of the TiAl turbine blades was conducted as well. After 1000 h of testing at 950 °C, the scale formed on the surface of the uncoated blades underwent spallation. The scale on the turbine blade with deposited aluminide coatings was very thin and no spallation was observed.     

Preparation of aluminide coatings at relatively low temperatures

1 Introduction Protective coatings by pack aluminizing are frequently applied to metals to protect them from high temperature oxidation and hot corrosion attack [1, 2]. Pack aluminizing consists of heating the parts to be coated in a closed or vented pac…     

Hot Corrosion Behavior and Microstructural Change of Al-Gradient CoNiCrAlYSi Coatings, Produced by LVPS and Diffusional Processes

Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma spray and an additional step of diffusional over aluminizing (pack cementation) techniques on an Inconel-738 substrate. Hot corrosion of these coatings was investigated using Na₂SO₄–20wt%NaVO₃ molten salt at 880?°C for 800?h. Hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20?h. X-ray diffraction, field emission scanning electron microscopy and electron probe micro analysis techniques were used to determine the nature of phases, investigation of the thermally grown oxide, examination of the surface attack and determination of the elemental distribution. The gradient coating showed better performance by re-healing alumina scale due to its possession of more β phase as Al reservoir. Results indicated that pack cementation process caused an increase in amount of aluminum-rich β phase and better hot corrosion properties of gradient coatings owing to the Al enrichment in the outer layer and rapid formation of protective oxide on the surface.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

简单/改性铝化物涂层的研究现状

航空发动机各部件高温结构材料在苛刻环境下服役时,会遭受严重的高温氧化和热腐蚀.在合金表面施加铝化物涂层后,高温下表面能够生成一层致密且生长缓慢的Al2O3氧化膜,从而隔绝腐蚀介质,以防止合金被快速氧化腐蚀.概述了铝化物涂层的优点,包括制备简单、成本低廉.重点综述了以Ni、Fe、Ti/TiAl为合金基体的铝化物涂层微观结构.涂层的微观结构主要由渗铝工艺、基材成分及后处理工艺等因素决定,渗铝工艺包括渗剂成分、渗铝温度和渗铝时间.在高温下渗铝,Al的活度较低,涂层主要以基体元素向外扩散形成外扩散型涂层为主;在低温下渗铝,Al的活度较高,涂层主要以Al向内扩散形成内扩散型涂层为主.还归纳了不同渗铝涂层在干燥空气和水蒸气环境中的高温氧化行为,阐述了水蒸气对铝化物涂层高温氧化行为的影响,比较了Ni-Al系和Fe-Al系涂层的抗高温氧化性能.同时介绍了Cr-Al、Si-Al和Pt-Al 3种改性铝化物涂层的研究进展,包括制备方法、微观结构及抗高温氧化和腐蚀性能.最后,展望并总结了高温防护涂层的发展趋势.     

CLAM钢表面铝化物涂层的制备与铅液相容性研究

目的探索铝化物涂层的制备工艺,研究其是否能有效抑制铅液对CLAM钢的腐蚀。方法用配制的渗剂对CLAM钢进行包埋渗铝,并通过随后的热扩散和原位氧化处理,在CLAM钢表面制备铝化物涂层,研究不同渗铝时间和热扩散时间对涂层厚度的影响。通过静态氧化试验和铅液腐蚀试验,分别评价铝化物涂层的抗氧化性能及其与铅液的相容性,采用XRD、SEM和EPMA分析涂层的相组成以及铅液腐蚀前后的微观形貌和元素分布。结果包埋渗铝+热扩散+原位氧化处理制备的铝化物涂层主要由约30μm的FeAl相层和约70μm的α-Fe(Al)固溶体层组成。在热处理过程中,由于Al和Fe的互扩散现象,涂层中的Fe-Al相依次经过了Fe₂Al₅、FeAl₂、FeAl、Fe₃Al和α-Fe(Al)的转变。在600℃空气中静态氧化120 h后,铝化物涂层试样氧化质量增量为0.028 mg/cm²,比CLAM钢的氧化质量增量降低了1个数量级,铝化物涂层使CLAM钢的氧化动力学曲线由直线规律转变为抛物线规律。经550℃铅液腐蚀600、1800 h后,铝化物涂层的腐蚀质量增量分别为0.058、0.077 mg/cm²,仅约为CLAM钢的1/120。CLAM钢表面产生了疏松多孔的铁氧化物层,而铝化物涂层没有发生明显的腐蚀,但是腐蚀1800 h后,随着表面铝含量的不断消耗,Al₂O₃层厚度逐渐减小。结论铝化物涂层具有良好的抗氧化性能及与铅液的相容性,能够有效抑制铅液对CLAM钢的腐蚀。     

包埋法Co-Cr-Y改性高温合金GH586渗铝涂层高温氧化行为

通过粉末包埋法运用Co-Cr-Y对高温合金GH586渗铝涂层进行改性,并进行了高温氧化行为的研究。研究结果表明：经Co-Cr-Y改性后的渗铝涂层在1000 °C氧化100h后,其平均增重为0.36毫克/平方厘米,远远低于基体的增重量。经X射线衍射分析,涂层的主要物相为NiAl,在1000 °C氧化过程中生成了连续而致密的氧化膜,主要包含 Al2O3, Cr2O3和 CoCr2O4。通过扫描电镜观察涂层表面和截面的形貌,可以涂层比基体展现出了更为优秀的氧化性能。另外发现,氧化过程中的富Cr（W）在晶界上聚集,有利于为涂层生成连续而致密的氧化膜不断提供Cr元素,从而提高涂层高温氧化性能。     

Structure and cyclic oxidation resistance of Pt, Pt/Pd-modified and simple aluminide coatings on CMSX-4 superalloy

The coatings were prepared by the means of Pt and Pt/Pd galvanizing, followed by vapor phase aluminizing at 1050 °C. Microstructural and phase analysis revealed that all the investigated coatings consisted mainly of β-NiAl phase, however the Pt-modified aluminide coating also contained PtAl₂ phase and pure platinum precipitates. The cross-sectional microstructure of the coatings was zonal and composed of β-NiAl phase zone and the diffusion zone. The Pt modified aluminide coating's cross-section also incorporated an outermost zone consisting of β-NiAl and PtAl₂ phases. The concentration profiles proved that both Pt and Pd contents decrease gradually inwards the modified coatings. Cyclic oxidation tests at 1100 °C proved that Pt/Pd-modified aluminide coatings exhibit the best performance under cyclic conditions. The analysis of oxidation kinetics curves showed that the course of simple aluminide coating's oxidation is slightly different from that of Pt- and Pt/Pd-modified aluminide coatings.