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.     

A modified dual-layer SiC oxidation protective coating for carbon/carbon composites prepared by one-step pack cementation

To improve the oxidation resistance of SiC coating produced by pack cementation for carbon/carbon composites, a modified SiC coating has been produced by one-step pack cementation by adding ferrocene in pack compositions. The as-received coating exhibited a dual-layer dense structure, and oxidation protective ability of SiC coating could be improved by introducing ferrocene. The modified coating could protect C/C composites from oxidation for more than 100 h at 1673 K in air. The weight loss of the coated C/C composites was considered to arise from deflection of penetrating cracks formed in outer layer from inner layer to C/C matrix.     

Microstructural characterization of a Pt-aluminide coated IN738LC

In this paper, the microstructural and chemical characterization of the IN738LC superalloy, coated with a Pt-Cr modified aluminide coating, is presented. The effects of aging in air at 850 °C on superalloy and coating microstructures were investigated. The growth of γ′ precipitates in IN738LC follows the Wagner-Lifshits model. The positive effect of Pt in preventing refractory element diffusion into the outer coating is not influenced by the aging. A moderate precipitation of TCP phases has been noted at the coating-superalloy interface.     

Boride coatings obtained by pack cementation deposited on powder metallurgy and wrought Ti and Ti-6Al-4V

Wear resistance of Ti alloys needs to be improved, and an effective way to achieve this is through surface treatment. Boronizing is a surface treatment in which boron diffuses into the surface of Ti leading to the formation of hard and wear-resistant Ti borides. Boronizing of wrought and/or cast Ti alloys by pack cementation has been studied, while similar coatings on Ti alloys produced by powder metallurgy (PM) have not been reported. Also critical process parameters for boronizing Ti alloys, such as pack cementation powder composition and the process temperature have not been systematically studied and analysed. The present work reports on the surface modification of PM Ti and PM Ti-6Al-4V by boronizing, and presents some important thermodynamic aspects of the process comparing it with similar coatings applied to wrought Ti-6Al-4V. The coatings were characterised using scanning electron microscopy and X-ray diffraction. For both Ti and Ti-6Al-4V alloys the use of amorphous B as a B element supplier in the boriding powder pack led to the formation of a uniform external boride layer, while the use of B₄C as a B element supplier in the pack and under the same boronizing conditions, led to the formation of an external TiN layer and an internal layer containing B. The thermodynamic calculations performed proved successful in determining the appropriate conditions for boride coating deposition and estimating the phases likely to be formed. Finally the effect of surface roughness on the coating quality is discussed.     

镁合金AZ91表面化学镀镍层的制备及性能研究

为了优化镁合金表面化学镀镍工艺,降低施镀过程中的污染,研究了AZ91镁合金表面直接化学镀镍工艺,并应用SEM对镀层表面形貌进行观察,通过EDS及XRD对镀层成分及物相进行了分析,采用划线划格法、静态浸泡法分别对镀层与镁合金基体的结合力、镀层的防腐蚀性能进行了分析.研究结果表明:AZ91镁合金表面可通过直接化学镀镍获得结合力良好、表面硬度较高的镍镀层.镀层的成分分析显示:由于NaH2PO2发生自身氧化还原反应,镀层中存在少量P.在3.5%NaCl溶液中的静态腐蚀试验结果表明:镀层的耐腐蚀性能良好,对合金基体可以起到较好的防护作用.     

A MoSi2-SiC-Si oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a dense coating has been produced by a two-step pack cementation technique. XRD and SEM analysis shows that the as-obtained coating was composed of MoSi₂, SiC and Si with a thickness of 80-100 μm. The MoSi₂-SiC-Si coating has excellent anti-oxidation property, which can protect C/C composites from oxidation at 1773 K in air for 200 h and the corresponding weight loss is only 1.04%. The weight loss of the coated C/C composites is primarily due to the reaction of C/C substrate and oxygen diffusing through the penetration cracks in the coating.     

Determination of the ductile to brittle temperature transition of aluminide coatings and its influence on the mechanical behavior of coated specimens

The ductility of various coatings deposited by chemical vapor deposition, pack cementation and slurry processes on Fe- and Ni-based alloys was characterized by indentation at room temperature. A hot indentation apparatus has also been developed to more rapidly determine the ductile to brittle transition temperature of coated specimens. Creep testing has been conducted on bare and coated alloy 230 (NiCrW) specimens at 800 °C with a significant decrease in creep life observed. Based on the observed failure of coated 230 specimens, the impact of coating ductility on substrate creep properties is discussed.     

FeWAl涂层及其耐高温硫腐蚀性能

通过在碳钢上电镀FeW合金镀层然后固相渗Al,获得了FeWAl涂层.对比研究了在800℃硫压为103Pa,氧压为10-21Pa或10-3～1Pa条件下FeAl和FeWAl涂层的硫化行为.结果表明,在高硫压(103Pa)、低氧压(10-21Pa)环境中,FeAl和FeWAl涂层均发生了灾难性腐蚀,腐蚀产物呈粉状.而适当提高环境中的氧分压(10-3～1Pa)时,FeWAl涂层具有良好的抗硫化性能.当在Fe-WAl涂层表面涂敷Fe2O3+SiO2混合层时,提高了涂层表面的氧分压,在高硫压低氧压的环境中促进了由硫     

Pack Aluminide Coatings Formed at 650 ℃ for Enhancing Oxidation Resistance of Low Alloy Steels

This study aims to investigate the feasibility of forming iron aluminide coatings on a commercial 9Cr-lMo (wt.%)alloy steel by pack cementation at 650 ℃ in an attempt to improve its high temperature oxidation resistance. Pack powders containing Al, Al2O3 and a series of halide salts were used to carry out the coating deposition experiments, which enabled identification of the most suitable activator for the pack aluminising process at the intended temperature. The effect of pack aluminium content on the growth kinetics and microstructure of the coatings was then studied by keeping deposition conditions and pack activator content constant while increasing the pack aluminium content from 1.4 wt.% to 6 wt.%. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the phases and microstructures of the coatings formed and to determine depth profiles of coating elements in the coating layer. Oxidation resistance of the coating was studied at 650 ℃ in air by intermittent weight measurement at room temperature. It was observed that the coating could substantially enhance the oxidation resistance of the steel under these testing conditions, which was attributed to the capability of the iron aluminide phases to form alumina scale on the coating surface through preferential Al oxidation.     

Interdiffusion behavior of Al‐rich oxidation resistant coatings on ferritic–martensitic alloys

Interdiffusion of thin Al‐rich coatings synthesized by chemical vapor deposition (CVD) and pack cementation on 9Cr ferritic–martensitic alloys was investigated in the temperature range of 650–700 °C. The compositional changes after long‐term exposures in laboratory air and air + 10 vol% H₂O were examined experimentally. Interdiffusion was modeled by a modified coating oxidation and substrate interdiffusion model (COSIM) program. The modification enabled the program to directly input the concentration profiles of the as‐deposited coating determined by electron probe microanalysis (EPMA). Reasonable agreement was achieved between the simulated and experimental Al profiles after exposures. The model was also applied to predict coating lifetime at 650–700 °C based on a minimum Al content (C_b) required at the coating surface to re‐form protective oxide scale. In addition to a C_b value established from the failure of a thin CVD coating at 700 °C, values reported for slurry aluminide coatings were also included in lifetime predictions.     

Analyses of the gaseous species in halide-activated cementation coating packs

The partial pressures of equilibrium gaseous species in pack-cementation diffusion processes using pure condensed Al(l), Cr(s), and Si(s) metals activated by various halide salts under Ar or reducing environments which contain different amounts of hydrogen have been calculated. The analyses indicate that the presence of hydrogen gas in the system does not significantly alter the equilibrium partial pressures of gaseous species for a pack containing Al at unit activity. In general, the partial pressures of CrCl₂ and SiCl₂ are higher in a chloride-activated pack than in a fluoride-activated pack. Therefore, the codeposition of Cr or Si with Al is possible when a chloride salt is used as the activator with a masteralloy of composition dilute in Al. The addition of hydrogen to the gas phase decreases the partial pressure of Cl₂ and thereby the partial pressures of CrCl₂ and SiCl₂. However, the presence of hydrogen and the formation of hydrogen halides and metal hydrides could increase the kinetics of the transporting processes, particularly for the coatings of Cr and Si. The study also indicates that the vapor pressures for the volatile chlorides of Fe are significantly higher than those for Ni or Co, so that displacement reactions leading to the loss of Fe from the substrate are expected for coating processes involving high halide activities.     

包渗温度对Ti-Al渗层组织和耐磨性能的影响(英文)

为了提高纯铜表面的耐磨性能,采用电镀/浆料包渗相结合的方法,以TiO2粉为渗Ti源,纯Al粉为还原剂,在Cu表面预镀Ni随后表面浆料包渗Ti-Al,制备Ti-Al共渗层。研究了包渗温度对Ti-Al渗层组织和耐磨性能的影响。采用SEM和XRD分析了渗层表面形貌和结构。结果表明:在800～950°C共渗12 h时,随着温度的升高,渗层组织变化过程为NiAl+Ni3(Ti,Al)→NiAl+Ni3(Ti,Al)+Ni4Ti3→Ni4Ti3+NiAl→NiAl+Ni3(Ti,Al)+NiTi;Ti-Al渗层的摩擦因数随着包渗温度的升高而降低,最小摩擦因数约为纯铜的1/3,最小硬度为纯铜的5倍。     

Morphology, structure and formation mechanism of silicide coating by pack cementation process

The MoSi2 coating on C 103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases＇ transitional layer consists of NbSi2 and a few NbsSi3 and the diffuse layer is composed of NbsSi3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.     

Development and growth of boron-modified and germanium-doped titanium-silicide diffusion coatings by the halide-activated,pack-cementation method

A halide-activated, cementation pack has been developed to codeposit either silicon and boron or else silicon and germanium in a single processing/reaction step to grow Ti-silicide diffusion coatings on commercially pure (CP) titanium, Ti-22Al-27Nb, and Ti-20Al-22Nb. Since boron is nearly insoluble in TiSi₂, a TiB₂ layer is localized at the surface of the B-modified silicide coatings. The thickness of the TiB₂ layer is controlled by the choice of boron activity and halide activator in the pack. Germanium is soluble in the Ti-silicide layers but inhomogeneously distributed in the Ge-doped silicide coating. The germanium content is controlled by choices of the Si-to-Ge ratio and the halide activator in the pack. The growth kinetics for the five-layered B-modified silicide coatings are generally similar to the undoped silicide coatings. The growth mechanism for the five-layered Ge-doped silicide coatings is generally different from the undoped silicides. The growth of dual-layer Ti-boride coatings was also studied.     

Effect of Aluminizing on the Oxidation Behavior of the Titanium Alloy,IMI 834

This article presents the development of a simple and cost-effective coating to protect the titanium alloy, IMI 834, from oxidation and alpha-case formation so that the alloy can be used safely, even at relatively high operating temperatures. A surface-modification technique, pack aluminizing, was employed to coat the alloy. Cyclic-isothermal oxidation studies at 800°C revealed that the plain aluminide coating obtained by pack aluminizing exhibits good oxidation resistance compared to the uncoated alloy. Based on the results obtained by different techniques, it is suggested that the plain aluminide is an economical, simple, and efficient coating for prevention of alpha-case formation and protection against oxidation of components fabricated from IMI 834.     

Oxidation Resistant Ce-modified Silicide Coatings Prepared on Ti–6Al–4V Alloy by Pack Cementation Process

Cerium-modified silicide coatings were prepared on Ti–6Al–4V by pack cementation. The effects of different kinds of activators (NaCl, AlF₃, AlCl_3, and NH₄Cl) and pack CeO₂ concentrations (1, 3, and 5 wt%) on the coating structures were studied. The results show that the coatings were mainly composed of a TiSi₂ outer layer, a TiSi middle layer, a Ti₅Si₄ inner layer and a 1–2 μm thick Ti₅Si₃ interdiffusion zone. NH₄Cl was a more suitable activator for preparing the Ce-modified silicide coating on Ti–6Al–4V, based on the coating microstructure and growth rate. The coating thickness decreased with increasing CeO₂ concentration in the pack. Oxidation tests at 800 °C in air showed that the Ce-modified silicide coating showed improved oxidation resistance compared to both the uncoated alloy and the pure silicide coating. A dense, but thick oxide scale formed that was composed of a TiO₂ outer layer and a SiO₂ inner layer.     

Mo合金高温抗氧化涂层的研究

用粉末固体渗法在钼合金表面制备了MoSi2渗层,研究了铝硅共渗和硼硅共渗对渗层的结构和抗氧化性能的影响.结果表明,与渗硅相比,铝硅共渗层厚度保持不变,渗层由单层结构变成多层复合结构,抗氧化性下降;硼硅共渗层厚度减少,结构没有明显改变,抗氧化性能得到了明显改善.     

Effect of Mo/Si Ratio on the Microstructure and Oxidation Resistance of Si-Mo Coating for C/C Composites

Oxidation protective Si-Mo coatings were prepared by pack cementation on the surface of C/SiC coated carbon/carbon composites. The influences of Mo/Si mass ratio in the pack powders on the microstructure and oxidation resistance of the coating were investigated. It showed that with the Mo/Si ratio increasing from 0.1 to 0.4, the MoSi2 proportion and the dimension of cracks in the coating increased, and the oxidation protective ability of the coated specimens firstly increased and then decreased. The coating...     

Steam oxidation resistance and thermal stability of chromium aluminide/chromium hybrid coating on alloy steels formed at low temperatures

This study investigated the feasibility of forming a hybrid coating with a structure consisting of a top Cr-aluminide layer and an inner Cr layer on alloy steels by using a two step process: electro-Cr plating and then pack aluminising at low temperatures. The oxidation resistance of the coating so formed was tested in ultrasupercritical steam of 650 °C and 30 MPa. The factors affecting the oxidation kinetics of the coating were studied by comparing its oxidation behaviour with that of the pack Fe-aluminide coating tested in the same ultrasupercritical steam. The thermal stability of the coating at 650 °C was investigated by a series of isothermal annealing experiments in argon atmosphere. It was demonstrated that the outer Cr-aluminide layer of the coating can improve the steam oxidation resistance of the steel substrate. The inner Cr layer can function as an effective barrier preventing the outward diffusion of Fe from the steel substrate; it can also act as a buffer zone, substantially reducing the rate of the inward Al diffusion process.     

Nb-Ti-Si 合金表面辉光离子渗 Mo / 包埋渗 Si制备 MoSi2 涂层的研究

目的在Nb-Ti-Si合金表面制备MoSi2涂层。方法先进行辉光离子渗Mo,再进行包埋渗Si。分析温度对渗Mo层表面形貌、厚度、元素互扩散的影响,以及渗Si后涂层的表面形貌和结构。结果在1100℃进行辉光离子渗Mo,渗Mo层与基体形成了显著的互扩散。对渗Mo层进行包埋渗Si后,所形成的涂层组织致密,具有多层结构,由外向内依次为MoSi2层、NbSi2层和Nb5Si3层,在MoSi2和NbSi2层之间存在(Mo,Nb)Si2互扩散区。结论通过辉光离子渗/包埋渗的方法,可以在Nb-Ti-Si合金表面制备Mo Si2涂层,且涂层与基体呈冶金结合,结合较好。