Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review

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Review

Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review Xin Xiang, Xiaolin Wang*, Guikai Zhang, Tao Tang, Xinchun Lai China Academy of Engineering Physics, Mianyang 621900, PR China

article info

abstract

Article history:

An aluminide coating typically FeAl/Al2O3 composite coating is one of the most promising

Received 18 August 2014

candidates for the tritium permeation barrier (TPB) in the tritium breeding blanket and

Received in revised form

auxiliary tritium handling system in fusion reactors. The preparation process of the alu-

24 December 2014

minide coating generally involves two steps of aluminization and oxidation. Interdiffusion

Accepted 10 January 2015

occurs between Al atoms and Fe atoms on the substrate surface to form (Fe, Al) solid so-

Available online xxx

lution or FeeAl intermetallic transition layer in the aluminization step. In the oxidation process, the aluminide layer surface is selectively oxidized to form an Al2O3 film. The

Keywords:

aluminide coating can be prepared by the technique of physical vapor deposition (PVD),

Aluminide coating

chemical vapor deposition (CVD), hot-dipping aluminization (HDA), electro-chemical

Tritium permeation barrier

deposition (ECD), packing cementation (PC), plasma sputtering (PS) and solegel etc. CVD,

Preparation technique

HDA and PC technique have potentials to be selected as the candidate engineering prep-

Alloying effect

aration technique of the aluminide TPB coating in fusion reactors. Meanwhile, ECD tech-

Influence factor

nique is rather appealing for the preparation of the aluminide TPB coating because of its easy process controlling, stable coat performance and availability of coating complexgeometry structure. However, compared with the predictions based on the material bulk properties, the aluminide TPB coating often exhibits lower efficiency than anticipated. One important reason is that alloying elements from the coating substrate materials and aluminum sources exert a significant influence on the composition, microstructure, and performance of the aluminide coatings, that is, an alloying effect exists in the aluminide coatings. Based on the source of alloying elements, the alloying effect can be classified as the substrate effect and doping effect. In view of the influence efficacy, the effect of alloying elements on the aluminide coating can also be identified as three types of beneficial effect, adverse effect, and nearly no effect, which can be converted to each other under certain conditions. On the other hand, the alloying effect in aluminide coatings depends on the element species, concentration, temperature, coating preparation technique, medium environment, and other factors. Therefore, in the practical preparation and

* Corresponding author. China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan, PR China. Tel.: þ86 816 3626720; fax: þ86 816 3625900. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.01.052 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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application of the aluminide TPB coatings, the alloying effect must be comprehensively analyzed, so as to obtain the best coating performance under certain conditions. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The ITER test blanket module (TBM) will perform the most important functions that will involve testing the feasibility of tritium production for the fuel self-sufficiency and the energy net output from the ITER fusion reactor. Implemented in the ITER TBMs, the concepts will be tested during the D-T high duty phase. One of the key issues of the TBM operation is the controlling over tritium permeation, to reduce the radiological hazard and to optimize the tritium balance in the reactor. Uncontrolled tritium permeation in fusion reactors can result in tritium inventory buildup in the reactor, tritiumcontaminated wastes, high tritium concentrations in operation areas, hydrogen embrittlement of structural materials and more difficult tritium processing [1]. Apart from the high reliability of the structural design of tritium confinement and tritium handling systems, coating, with a low permeability for tritium, named tritium permeation barrier (TPB) is one of the most effective methods to minimize tritium permeation through structural materials to the environment and other systems [2e4]. The application of TPBs is thus very necessary and helpful for the tritium self-sufficiency and environmental safety in ITER like fusion reactors. The common used TPB coatings can be classified as oxide coatings (eg. Al2O3, Cr2O3, Y2O3, SiO2, Er2O3, ZrO2) [4e13], nonoxide coatings (eg. TiC, TiN, SiC, Si3N4) [14e20], and their composites (eg. Cr2O3/SiO2, Al2O3/SiO2, TiC/TiN, FeAl/Al2O3, Al2O3/SiC, Er2O3/Fe) [14,21e31]. In the practical applications, the non-oxide coatings encounter some unsolvable problems. For example, SiC coatings can interact with hydrogen and can be cracked and even flaked with a high thickness [18,32], and the titanium base ceramic coating is prone to be oxidized and performance degraded above 450  C [33]. On the other hand, oxides especially Al2O3 with excellent comprehensive properties, attract much interest as typical candidate TPB materials for their high melting point, chemical stability, low hydrogen solubility and permeability [5,34]. However, the thermal expansion coefficient shows great difference between the metal substrate and oxide ceramics, and thus significant thermal mismatch exists, leading to the failure of the coatings. The common adopted solution method is to form a functional gradient transition layer between the substrate and the coating. Generally, the technique of thermal treatment followed by high temperature oxidation is employed to form aluminide coatings typically FeAl/Al2O3 after Al deposition on stainless steels [30,31]. At present, the aluminide coating has been selected as one of the prior developed TPBs for the TBMs by Europe, China, United States and India for its high permeation reduction factor (PRF), low thermal mismatch,

metallurgical bonding, excellent compatibility, and selfhealing [35e37]. The preparation of aluminide TPB commonly involves two steps of aluminization and oxidation [30,31]. Aluminization is to form a transition layer of (Fe, Al) solid solution or FeeAl metallic compounds on the substrate surface via interdiffusion between Al atoms from a certain Al source and Fe atoms form the steel substrate. Oxidation is to form an Al2O3 film on the transition layer by selective oxidation. In view of the way of Al introduction, the preparation technique of aluminide coatings can be classified as physical vapor deposition (PVD), chemical vapor deposition (CVD), hot-dipping aluminization (HDA), electro-chemical deposition (ECD), pack cementation (PC), plasma spraying (PS), and solegel etc. The techniques mentioned above all have their own features, and consequently the quality and tritium resistant performance of the corresponding prepared aluminide coatings differs a lot. In fusion reactors, TPB coatings usually need to be prepared on the surface or inner wall of structural containers or pipes with large size and complex shapes. Therefore, it is necessary to choose appropriate techniques to prepare aluminide TPBs based on the working conditions. Presently, the aluminide TPB related studies focus on the preparation technique and performance optimization, so as to improve the coating quality and integrity, and also strive for the engineering application. However, the performance of aluminide TPBs often exhibit lower efficiency than anticipated based on the bulk coating material properties [38,39]. The possible reason can be defects in the barrier coating or higher hydrogen permeability of the defect free barrier coating than desired, or a combination of both [40]. It is obvious that the integrity and microstructure (defect, impurity, etc.) of the aluminide coating will exert some influence on the hydrogen permeation. Defects like voids in TPBs can be reduced or eliminated by a hot isostatic pressing (HIP) [41] or chemically densified coating (CDC) [23] method, and can also by the technology optimizing to densify the coating so as to improve the tritium resistant performance of the barrier coating. By contrast, impurities i.e. alloying elements have much more significant influences on the aluminide coatings. On one hand, the phase and microstructure features (topography, interface, coating thickness and defect configuration etc.) of aluminide coatings forming on different steel substrates with diverse alloying element species and quantities exhibit great difference [3,31,42e44], and thus display different coating performances such as hydrogen permeability, corrosion and high temperature oxidation properties [45e47]. On the other hand, during the formation of aluminide coatings, alloying elements from the Al sources or doping species can also have remarkable influences on the coating formation and performance

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[39,48e50]. The former could be named as the substrate effect, and the latter as doping effect, and both can be called as alloying effect. Nevertheless, compared with the coating preparation techniques, studies on the alloying effect in aluminide TPB coatings relatively lag, and do not reveal a systematic and appropriate theory, which make the preparation technology optimization and coating structure design cannot be fulfilled simultaneously, and thus restrict the development and application of TPB coatings, while it is absolutely an important fundamental scientifical issue in the practical preparation and application of aluminide coatings. In this work, the research progress of the preparation technique and alloying effect of aluminide TPB coatings is reviewed so as to give some references to other TPB teams or researchers.

Preparation technique With the development of modern science and technology, the coating technique develops rapidly. Great achievements have been made in both the area of coating materials, species, preparation techniques, characterization methods etc. and that of protection mechanisms and applications. As a type of coatings, the preparation technique of TPB coatings can transplant or take reference for the existing coating preparation techniques on the premise of TPB peculiarities. The coating preparation technique is crucial for the service performance of TPB coatings. However, studies on TPB coatings are mostly focused on regular lab-scale samples to densify and compound the coatings so as to improve their integrity, and consequently the TPB coatings are far from the engineering applications. Up to now, the used preparation techniques of aluminide coatings can be classified as PVD, CVD, HDA, ECD, PC, PS and solegel etc., while each technique has its own special features. On one hand, the TPB performance of aluminide coatings prepared by different techniques deviates, and the hydrogen (or deuterium, tritium) PRF ranges from tens to thousands even tens of thousands, whereas the general obtained hydrogen PRF in the gas phase condition is lower than 1000, which is not satisfying for the tritium permeation resistance [51]. On the other hand, TPB coatings of high quality have to be prepared on both outer surfaces and inner walls of components with large sizes or/and complex geometries in fusion reactors. However, some techniques such as PVD can prepare TPB coatings of high tritium permeation resistance, yet cannot be used to coat on complex-shaped tritium confining containers. For these reasons, different preparation techniques of aluminide coatings are reviewed in the following sections, so as to propose some techniques suitable for the engineering applications of aluminide TPB coatings in the future fusion reactors.

Physical vapor deposition (PVD) The PVD technique is a physical process of evaporation, sputtering or ionization through which the source material is converted to gaseous atoms, molecules or ions under the vacuum condition, and then deposited on the substrate to form a coating or film. Serra et al. [38] reported that a 1.5 mm thick Al2O3 coating was deposited on MANET II steel by

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closed-field unbalanced magnetron sputtering, and the maximum deuterium permeability of Al2O3 coated steel at 300e500  C was lowered down around 4 orders of magnitude, compared to the bare MANET steel. A newly developed double glow plasma (DGP) technique can also be considered as a type of PVD technique [52]. Liu et al. [52] showed that the Al2O3 coating could be prepared by the DGP method. Firstly, an aluminization layer composed of mainly FeAl3 and Al formed on 316L stainless steels by DGP alumetizing; Secondly, the aluminization layer was oxidized at 600  C to form a a-Al2O3 scale with little q-Al2O3 and g-Al2O3. The DGP Al2O3 coating showed excellent deuterium permeation resistance, which could reduce the deuterium permeability to 3 orders of magnitude at 600  C. It is thus clear that PVD is available for aluminide TPB coatings. However, there are some disadvantages for PVD, such as nonuniformly coating, difficult to deposit on complex-geometry surfaces, poor bonding of the coating with the substrate, and easy to fall off. Therefore, the PVD technique is relatively limited employed for TPB preparations, mainly used in the early TPB studies.

Chemical vapor deposition (CVD) CVD is a deposition process that the film composing element containing elementary substance or compound is firstly provided to the substrate, and then a solid film forms via gas phase or chemical reactions. The CVD technique is determined to be the TPB coating preparation technique for the US DCLL TBM for its advantages of simple facility required, continuous and controllable film composition, uniform and dense film surface, and availability of coating complexgeometry structure [37]. Generally, the deposition temperature of the CVD technique is rather high, which is unfavorable for the mechanical properties of structural components. Therefore, advanced CVD techniques such as metal organic chemical vapor deposition (MOCVD) and chemical vapor deposition in fluidized bed reactors (CVD-FBR) have been developed so as to coat at lower temperatures [53e56]. Natali et al. [53] prepared an Al2O3 scale with some carbon and hydrogen impurities on stainless steels in an atmosphere of water and oxygen at 653 K by the MOCVD method. These impurities can be eliminated successfully by altering the metal organic Al source and using the way of gas carrying [54]. An aluminide coating about 8 mm thick was prepared by CVDFBR on P-92 ferritic steels, which improved the vapor oxidation tolerance of the substrate [55]. It is reported that the growth rate and composition of the CVD-FBR deposited aluminide coating depended on the reactive gas reagent and deposition time [56]. An FeAl/Al2O3 coating was prepared on a one-end-closed Eurofer tube by a two-step process of CVD by CEA, and the obtained PRF at 280e420  C in the air was 6, while in liquid PbeLi alloys was 15 [3], which were far below the expected values according to the advice given by European tritium permeation barrier work group [51]. Therefore, the MOCVD and CVD-FBR techniques are promising in the preparation of aluminide TPB coatings, yet there is still a long way to realize the engineering application of CVD to prepare aluminide TPB coatings.

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Hot-dipping aluminization (HDA) The HDA TPB preparation technique was originally developed by FZK [38,57]: the stainless steel substrate was firstly dipped into an Ar gas protected Al melt at 700  C for 30 s to form a layer of an intermetallic Fe2Al5 phase with a thickness of 20e30 mm; and then thermal oxidized at 950e1075  C to form a three-layered structure composed of a-Fe(Al), FeAl and Al2O3. The maximum deuterium PRF of 260@743 K and 1000@573 K could be obtained after the lab-scale regular MANET II steel samples treated by HDA [38]. For the MANET II steel container (F29  1.5 mm, 100 mm long), the hydrogen PRF in the air at 300e450  C was only 140; while the hydrogen PRF in liquid PbeLi alloys even lowered to 45, since voids and delaminations emerged in the coatings, resulting in the corrosion of the coatings [58]. It is clear that great deviation of the hydrogen PRF of the HDA coating exists when measured in the air and liquid PbeLi alloys [59]. Moreover, voids or void bands can easily form in aluminide coatings because of the Kirkendal effect in the HDA process, leading to the degradation of the tritium permeation resistance of coatings [48]. The voids in the HDA coatings can be suppressed or eliminated by HIP or doping alloying elements [41,60]. The doped rare earth elements in the Al melt can effectively suppress the formation and growth of voids at the interface of the substrate and coating [60]. Therefore, the HDA technique is promising for the engineering preparation of aluminide TPB coatings, provided that the stability, homogeneity and compactness are readily solved.

containers. The deuterium PRF of the aluminide coatings in the air was 3000 at 500  C, ~100 at 740  C, and the number of thermal cycling (room temperature ~750  C) exceeded 20 times when prepared on a 321 workpiece (F80  2 mm, 150 mm long) [31]. Therefore, the ECX technique is very promising for the engineering preparation of aluminide TPB coatings in future fusion reactors.

Pack cementation (PC) The PC technique has a potential to be selected as a candidate engineering preparation technique for TPB coatings for its simple facility and procedure required and high platability. The typical PC technique is a two-step process of alumetization and oxidation. The hydrogen PRF of 103e104 can be obtained if the thickness of Al2O3 outer layer reaches several mm for the PC treated 316L and DIN1.4914 steel regular samples [43,62]. By contrast, the tritium resistance performance of aluminide coatings prepared by PC on structure components drops dramatically. The hydrogen PRF of the aluminide coating on the inner wall of a 316L tube (F10  1 mm, 250 mm long) was only 34 at 235  C in the air [63]; and the deuterium PRF of the coating on the outer surface of a 316L tube (F10  0.55 mm, 150 mm long) was not so high at 350e550  C, in the range of 30e70 [64]. On the other hand, chlorides are often used as the activator during the PC coating, which can cause severe stress corrosion of the nuclear energy components [43]. For this reason, non-chloride activators have to be developed so as to realize the engineering TPB preparation by the PC technique.

Elecrochemical deposition (ECD) Plasma spraying (PS) The ECD technique i.e. plating is widely used in the surface protection in the industry. Since the facility required is very simple and easy to operate, and the thickness and composition of the coating is controllable as well as be capable of coating complex-geometry structures, the ECD technique is promising in the TPB preparation. However, the Al deposition can only be conducted in water free or aprotic electrolytes for its high electronegativity of 1.6 eV. Accordingly, the electrodepositon of Al can be classified as ECA and ECX (X ¼ Al, W, Ta, etc.) [51]. The former is conducted in organic aprotic electrolytes, while the latter in ionic liquids (ILs). Since the ILs possess the following features [51]: very thermal and chemical stable, low vapor pressure, high electrical conductivity, high variability of chemical structure, good miscibility with inorganic metal salts, and mostly liquid at “room temperature” (100  C), the ECX process is much more favored. A dense and closely adherent FeAl coating with a 20 mm thickness was prepared on mild steel samples by the ECX technique [61]. However, the performances of such coatings are not satisfying. The ECX has to be combined with processes of thermal oxidation, annealing or HIP treatment to make the surface of FeAl coating to form a layer of Al2O3 ceramic film. Consequently, the hydrogen PRF of the coatings can be increased at least one order of magnitude [41]. Therefore, a technical approach of “ECX followed by selective oxidation” was proposed almost at the same time by FZK and CAEP [30,42]. With this technique, Zhang et al. [31,42] prepared FeAl/Al2O3 TPB coatings of high quality on stainless steel samples and

The PS technique was initially employed to prepare aluminide TPB coatings by ENEA in the 1980s [65], and the typical procedure was that the fused or half-fused Al powders were sprayed on the steel substrate in the convenient or inert atmosphere, and then heat treated to form aluminide coatings. According to the atmosphere used, PS can be classified as atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS). The former seems to be less appropriate for the TPB preparation since the oxidation taken place during the spraying process decreases the coating quality [44]; while the latter is also not satisfying in regard of its low tritium PRFs because of residual stresses in the scale [44,66]. Generally speaking, compared with CVD, HAD, ECD and PC, the PS technique has no superiority for the preparation of aluminide TPB coatings.

Solegel Solegel is a commonly used wet chemical method of material preparations for its following features: low temperature synthesis, availability of coating on various geometries and substrates, controllability of the coating composition and microstructures. Therefore, it should have potentials for the TPB preparation. It is reported that uniform in thickness, crack-free and well adhered Al2O3 coatings were prepared by solegel on FeeCreAl alloys [5]. These coatings could be effective for the tritium permeation resistance, but have not

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been confirmed. Therefore, the solegel technique could be employed to prepare aluminide TPB coatings, but there is still much work to do.

Alloying effect Substrate effect The substrate effect of materials derives from crystal types (single crystal, polycrystal, amorphous crystal, etc.), microstructures (grain size, configuration, defect, secondary phase, inclusion, etc.) and compositions (alloying element, impurity, etc.). The crystal type and microstructure of materials depend on the preparation techniques, and thus are adjustable and controllable. Relatively speaking, the effect of composition on material properties is more significant. Actually, in metallurgical practices, great changes of material physical, chemical and mechanical properties may occur when a small amount of certain alloying elements are added. For example, Fedorov et al. [67] found that the hydrogen permeability of proton irradiated EP-838 steels was reduced by almost 1 order of magnitude after Ce addition. In the tritium breeding blanket and auxiliary tritium handling system in fusion reactors, the mainly used structural materials are reduced activation ferritic/martensitic steels (RAFM: F82H, CLAM, Eurofer97), austenitic steels (AS: 316L, HR-2), ferritic steels (FS: P-91, HCM12A) and martensitic steels (MS: F82H-mod, MANET) [41,54,55,68e74]. The main alloying elements in these types of steels are Cr (7e22 wt.%), Ni (0e15 wt.%), Mn (0e9 wt.%), Mo (0e3 wt.%), W (0e2 wt.%), and other elements like Al, Ti and C. The content and species of alloying elements vary with steel types. It is thus anticipated that the formation and tritium permeation resistance of aluminide TPB coatings prepared on these steels should be significantly different. The reason is that other alloying elements in the substrate materials besides Fe will also diffuse at high temperatures during aluminizing, exerting influences on the FeAl transition layer. It has been observed experimentally that alloying elements have prominent influences on the mechanical property, hydrogen permeability and corrosion resistance of FeAl alloys [75e78]. Tensile test results [75] of heat-treated FeAl alloys showed that the addition of C was effective in improving the yield strength without affecting the ductility, and C seemed to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, since the fracture mode changed from predominantly intergranular in the low C (0.05 wt.%) alloy to predominantly transgranular in the high C (0.2 wt.%) alloy. Similarly, in Fe3Al-based alloys, the added C led to a precipitation of perovskite Fe3AlC0.5 carbide phase, resulting in a decrease of hydrogen permeability and diffusivity by a factor of 2 without a significant change of the hydrogen solubility
lez-Rodrı´guez et al. [76] reported that the effect of [77]. Gonza the different alloying elements on the hot salt corrosion performance of FeAl-base alloys depended upon the salt used. In NaVO3, the FeAl þ Ce þ Ni alloy exhibited the lowest corrosion rate, whereas the FeAl þ Ce þ Li alloy showed the highest one; while in Na2SO4, the highest corrosion rate was FeAl þ Ce þ Ni, and most of other FeAl-base alloys doped with Ce and/or Ni

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showed similar corrosion rate. On the other hand, the formation, topography, compactness and crystal type of the Al2O3 film formed on the FeAl layer depend on the alloying elements. Just as argued by Kitajima et al. [78], Ti, Fe and Cr could promote the transformation from q-Al2O3 to a-Al2O3 on Fee50Al alloys because their oxides could act as heterogeneous nucleation sites for a-Al2O3; while Ni almost had no apparent effect. Since the typical aluminide coatings prepared on steels are composed of a FeAl transition layer and an outer Al2O3 film, it can be inferred from above statements that the alloying elements in the steel substrates must have some influences on the coatings. For example, Soliman et al. [45] found that the increase of the C content from 0.22 wt.% to 0.44 wt.% led to a decrease in the rate constant of high temperature oxidation by nearly an order of magnitude in alu
nchez et al. [47] also revealed that the minide carbon steels. Sa resulting mass gain of the aluminized HCM12A (12 wt.% Cr) steel sample by the CVD-FBR technique was about 3 times lower than that of P-91 sample after being oxidized at 800  C for 1000 h, that is the high temperature steam oxidation resistance of the aluminized HCM12A is better than P-91, which is caused by the different Cr concentrations in the coatings. Therefore, alloying elements in steel substrates will play an important role in the formation and performance of aluminide TPB coatings, either positive or negative, relying on the factors of concentration, temperature, environmental medium etc.

Doping effect Apart from the alloying elements in the stainless steel substrates, alloying elements from the Al source or pre-deposited layer on the substrate will also play a role in aluminide TPB coatings. It is known that the typical aluminide TPB coatings are composed of FeAl transition layers and Al2O3 scales. The FeAl transition layer in aluminide coatings forms by the interdiffusion of Fe atoms from the substrate and Al atoms from the Al source. There is no doubt that other alloying elements doped in Al sources or pre-deposited on the substrate surfaces will take part in diffusion as well as the substrate ones under the high temperature condition. Cheng et al. [49] reported that only two layers of FeAl3 and Fe2Al5 formed on mild steels after being hot-dipped in pure Al and Al-0.5Si melts; while Al7Fe2Si layers and Al2Fe3Si3 particles also formed besides FeAl3 and Fe2Al5 when the Si concentration in Al melts was in the range of 2.5e10 wt.%, that is the Si element in Al melts is involved in the formation of coating phases. During the isothermal oxidation of HDA aluminide coatings on the Ni pre-plated mild stainless steels at 750  C, the phase constitution of the aluminide layer was observed to transform from high aluminum into low aluminum NieAl intermetallic phases because of the interdiffusion of aluminide layers and nickel layers [79]. The doped alloying elements can also exert an influence on the formation and performance of the FeAl transition layer as well as the phase constitution. In the HDA process, with the increase of the Si concentration in Al melts, the coating interface transformed from an irregular finger-like morphology to a smooth flat one, and the coating thickness dropped gradually [49]. Meanwhile, void bands and cracks in HDA coatings can be effectively suppressed by the Si doping

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and Ni pre-plating, and eventually improved the oxidation resistance of aluminide coatings [39,79]. Glasbrenner et al. [48] also found that the thickness of FeAl intermetallic layer forming on HDA-treated MANET steels decreased with the addition of transition elements of Mo, W and Nb in Al melts, and the internal oxidation of the coatings at high temperatures was suppressed as well, leading to the formation of more dense and close oxide scales. In the process of PC aluminizing, the rare elements as Y, Ce and Hf were found to be able to improve the mechanical property and chemical stability of aluminide coatings prominently [50,80,81]. Specifically, the Y doping was beneficial for the wear resistance of aluminide coatings in the corrosive environment, the critical load of Ycontaining aluminide coatings was 33.6 ± 2.1 g, while the load was only 25.7 ± 1.9 g for the Y-free coatings [50]. Besides, the Y and Ce dopings can significantly increase the resistance to both corrosive erosion and dry sand erosion of aluminide coatings on 1030 steel surfaces [80,81]. On the other hand, the FeAl transition layer has to be oxidized at high temperatures to form a dense oxide scale on the surface so as to achieve better coating performances. During the high temperature oxidation, Al atoms in the FeAl transition layer are oxidized to form Al2O3 films. Nevertheless, Fe atoms and other doped alloying elements are also likely to be oxidized to form the corresponding oxides. For example, apart from Al2O3, NiO and TiO2 formed on the Ni and Ti predeposited FeAl surface after oxidizing at 900  C, respectively [78]. Zhan et al. [82] also found that a little CeO2 existed in the Al2O3 film formed on CLAM steels with the addition of Ce during the PC aluminization. The existence of such oxides (Al2O3 not included) not only can affect the coating compactness, but also can degrade the coating performance. For example, Cr was found to be not beneficial to oxidation of Fe3Al alloys, and the oxide scales formed were nonadherent and fragile [83]. Therefore, for the aluminized coatings, the FeAl transition layer needs to be selectively oxidized at a certain temperature and oxygen potential to suppress the oxidation of non-Al elements (i.e. internal oxidation) so as to improve the comprehensive coating performance. During the HDA aluminization, the transition elements Mo, W and Nb were observed to effectively restrain the internal oxidation of FeAl transition layers, resulting in the formation of more dense coatings [48]. Therefore, it can be concluded that the doped alloying elements have a significant influence on the formation and microstructures of the FeAl transition layer and Al2O3 film formed on stainless steels, and eventually affect the comprehensive performance of aluminide TPB coatings.

Efficacy of alloying effect In view of the influence on the structure and performance of aluminide coatings, the efficacy of alloying effect can be identified as three categories of beneficial effect, adverse effect and nearly no effect. Generally speaking, the types of structural materials used in the tritium handling system or tritium related components are relatively fixed under certain conditions. 316L stainless steels are mainly used as tubes or valves steels in tritium handling systems [3,4,18,31], while RAFM steels are commonly recommended in the tritium

breeding blanket in fusion reactors [35e37]. Therefore, in terms of TPB coatings on such types of steels under certain conditions, the substrate effect should not be very prominent. The method of doping alloying elements is usually considered to adjust and control the structures and performance of TPB coatings. It is obvious that the desired alloying effect is beneficial effect. At present, studies on beneficial alloying elements of aluminide coatings are mainly focused on rare earth elements such as Y, Ce and Hf, which can improve the wear, high temperature oxidation and corrosion resistance of aluminide coatings [81,82,84e86]. The reason can be attributed to the oxides formed by such elements, leading to the enhancement of the coating adherence, and the reduction of the grow stress, interface porosity and structural roughness of the coatings [82,84]. However, the beneficial effect of rare earth elements depends on their contents. Xiao et al. [84] pointed out that the addition of 2e5 wt.% CeO2 could improve the sulfidation resistance of FeAl coatings on carbon steels, while the addition of 8 wt.% CeO2 would induced the thermal crack of FeAl coatings, reducing the coating sulfidation resistance instead. For the HDA aluminide coatings, the presence of Si not only can flat the tongue-shape FeAl intermetallic layer, but also can suppress the voids and cracks in the coatings, and thus improve the coating performance like the cyclic oxidation [39,87,88]. Moreover, some transition metal elements are beneficial for the aluminide coatings [48,89], for instance, Cr and Pt can protect the aluminide coatings against the thermal corrosion [89], and W, Mo, Nb elements can suppress the internal oxidation of aluminide coatings during the high temperature oxidation, resulting in denser Al2O3 films forming on MANET steels [48]. According to the above statement, when the Ce content exceeded a critical value, the beneficial effect of Ce on aluminide coatings converted to an adverse effect, because the excessive Ce induced the thermal crack of the coatings [84]. Zhang et al. [90] also found that high N content in alloy steels was undesirable for the adherence of aluminide coatings, because it could result in the formation of AlN precipitates; and lower N concentration could make the coatings more clean with fewer precipitates and voids; while on the other hand the substrate N could increase the coating microhardness [91]. In CreMo steels, when the Cr content is lower than 2.25 wt.% or higher than 5 wt.%, Cr nearly has no effect on the formation of the intermetallic layer of HDA aluminide coatings, but can retard the interdiffusion of steel and Al, resulting in the thickness reduction of intermetallic layer and interface flatness of steel and intermetallic layer [92]. Moreover, the trace lanthanide elements (<0.025%) such as Ce, La and Nd have minor effect on the Al diffusion layer or coating growth dynamics in 430Y stainless steels [93]. The phase structure of the FeAl transition layer in HDA aluminide coatings was also found to be independent on the doped W, Mo and Nb elements in MANET steels [48]. It can be seen that the beneficial or adverse effect of alloying elements in aluminide coatings depends on the element species, concentrations etc., which can be converted under certain conditions. Therefore, in the applications of aluminide TPB coatings, the coating used condition such as temperatures and environmental media should be considered so as to obtain the best comprehensive coating performance.

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Impact factor of alloying effect Element species In the preparation process of aluminide coatings, the interdiffusion aluminization of Fe and Al atoms occurs to form a FeeAl intermetallic layer, and subsequently Al atoms in the FeeAl layer are selectively oxidized to form an Al2O3 outer layer. The presence of alloying elements will have some influences on these two processes. It is reported that the addition of Si in Al melts not only affected the phase constitution in the HDA process, resulting in the formation of Si containing phases, but also changed the interface structures and thickness of formed aluminide coatings [49]. Janda et al. [94] also pointed out that the transformation temperature of g-Al2O3 to a-Al2O3 on the surfaces of FeAl base alloys changed from 950  C to 750  C with the addition of Zr and Nb elements. According to the material science point that the property is determined by the composition and structure, it is obvious that different alloying elements will exert different influences on the formation and performance of aluminide coatings. In the HDA process, the addition of W, Mo and Nb in Al melts was observed to have influences on the process of aluminization and subsequent high temperature oxidation, and the maximum thickness of aluminide coatings depended on the added alloying element, for which it was the thinnest in the case of AleW, and thickness in the AleMo case, yet both lower
nchez et al. [95] found that the than the pure Al case [48]. Sa vapor oxidation resistance of aluminide coatings formed on HCM12 A steels by the CVD-FBR technique deviated with the addition of Ce or La, and the former was superior than the latter. Moreover, since the species of alloying elements in the steel substrates are different, the formed intermetallic aluminide phases in Al melts are accordingly diverse; for 1.4914 steels, the intermetallic phase was (Fe,Cr)2Al5/(Fe,Cr)Al3, while that was (Fe,Cr,Ni)2Al5/(Fe,Cr,Ni)Al3 for 316L steels [96]. It can be seen that during the formation of aluminide coatings, different alloying elements in the steel substrate and Al source will have different influence on the formed coatings. The reason could be attributed to the obvious difference of atomic structures and electrical properties of different alloying elements, resulting in the significant different physical or chemical properties in materials, and thus have dissimilar influences on the diffusion behaviors of Fe and Al atoms in the aluminization process and subsequent oxidation process, and eventually affect the formation and performance of aluminide TPB coatings.

Concentration The phase constitution and microstructures of the FeAl transition layer in aluminide coatings are affected by the concentration of alloying elements. Cheng et al. [49] found that in the HDA process, with the increase of Si concentration in Al melts, the topography of formed intermetallic layers became flat from finger-shaped, and the thickness dropped nonlinearly; but when the Si concentration exceeded 10 wt.%, the thickness would no more decreased significantly [87]. Meanwhile, when the Si concentration reached 2.5 wt.%, an Al7Fe2Si layer and an Al2Fe3Si3 particle layer also formed in the intermetallic layer apart from FeAl3 and Fe2Al5, and the thickness of each layer differed from the Si content [26,49]. Cheng et al.

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[92] also reported that when the Cr content was lower than 2.25 wt.%, Cr had no significant influence on the phase constitution of the intermetallic layer in aluminide coatings formed on mild steels by hot-dipping in Al-10 wt.% melts, the intermetallic layer was composed of a t5(H)eAl7(Fe,Cr)2Si outer layer with a FeAl3/t1-(Al,Si)5Fe3/Fe2Al5 inner layer; and when the Cr content was higher than 5 wt.%, the intermetallic layer transformed to a t5(H)eAl7(Fe,Cr)2Si single phase with Cr stabilized t5(C)eAl7(Fe,Cr)2Si particles forming on the AleSi topcoats, and the interdiffusion of steels and AleSi increased with the Cr content. On the other hand, the oxidation behaviors of FeAl base alloys are also concentration related. It was observed that under the oxidation condition of relative lower temperature of 900  C, the increase of the Y2O3 content in Fe40Al alloys accelerated the formation of a-Al2O3, leading to the significant decrease of the oxidation rate; while at 1000  C, much denser oxidation layers would form with the increase of the Y2O3 content [97]. Cr was also considered to be not beneficial for the oxidation of Fe3Al alloys at 1000  C, and the oxidation rate increase with the Cr concentration [83]. It thus can be inferred that the alloying effect of aluminide coatings composed of a FeAl transition layer and an Al2O3 outer layer is definitely associated with the concentration of alloying elements. Comparisons revealed that the relatively higher N content in commercial steels such as Fee9Cre1Mo and 304L would result in the formation of AlN precipitates in aluminide coatings, which were undesirable for the coating adherence [90]. Especially for 304L steels, the relatively lower N content could make the coating cleaner with less precipitates and Kirkendal voids [90]. Furthermore, the microhardness of the aluminide 316L steels would increase with the substrate N content [98]. Therefore, the concentration of alloying elements has great influences on the formation and performance of aluminide TPB coatings.

Temperature The process of both aluminization and oxidation has to be conducted under the condition of much higher temperatures than room temperature. Generally, the interdiffusion rate of Fe and Al atoms increases with temperatures; and the oxidation process need to be implemented at certain high temperatures and oxygen potentials to realize the selective oxidation of Al and constrain the oxidation of Fe and other alloying elements so as to increase the concentration and compactness of Al2O3. Therefore, temperature is a critical controlling factor in the preparation process of aluminide coatings, and the alloying effect on coatings is inevitable dependent on temperatures. Sundar et al. [75] found that heat treated at 1100  C, the addition of C could effectively improve the yield strength of FeAl base alloys without changing the ductility significantly; while heat treated at 1300  C, the yield strength decreased significantly and the ductility lowered as well, which could be attributed to the synergetic effect of retained vacancies and fine carbide precipitates formed at higher temperatures. The oxidation behaviors of Cr containing FeAl alloys were also observed to be obviously diverse at different temperatures; the oxidation rate at 1100  C was lower than that at 900  C, since a stable a-Al2O3 layer formed at 1100  C, while a metastable q-Al2O3 layer formed at 900  C [99]. Generally speaking, for all the crystal types of Al2O3, the

Please cite this article in press as: Xiang X, et al., Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.052

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tritium permeation resistance of a-Al2O3 is best [100]. However, the formation temperature of a-Al2O3 on the surface of FeAl alloys is often up to 900e1000  C [101,102], which could be unfavorable for the mechanical properties of substrate materials. Experimentally, the formation temperature of a-Al2O3 can be lowered down by doping alloying elements. It is found that the transformation temperature of a-Al2O3 from g-Al2O3 decreased from 950  C to 900  C after the addition of 0.05 at.% Zr in Fe3Al [103]; the transformation temperature could be further decreased to 750  C when co-doped by Zr, Nb, C and B elements [93]. It is obvious that the alloying effect in aluminide coatings composed of a FeAl transition layer and an Al2O3 film is closely dependent on the temperature. Therefore, it would be of an important engineering worthware for the actual applications of aluminide TPB coatings once the formation temperature of a-Al2O3 can be lowered down by doping alloying elements.

Other factors The alloying effect of aluminide coatings is also dependent on the coating preparation technique and environmental medium apart from the factors of element species, concentration and temperature. It is reported [89] that when Cr was predeposited on the IN 100 nickel-base superalloy, the subsequent aluminide coatings prepared by the PC technique showed a typical three-layer structure, and the outer layer was enriched with Cr; while the coatings prepared by the CVD technique was a pure NiAl layer, and Cr was enriched beneath. The reason is that in the two preparation processes, the activity of Al differs, resulting in different distributions and existing forms of Cr in the aluminide coatings. On the other hand, the environmental medium has an important influence on the alloying effect of aluminide coating. Zhang et al. [104] found that the corrosion erosion resistance of Y containing aluminide coatings on 1030 steels was absolutely different in the media of dry sand, NaCl or H2SO4 containing silicon dioxide slurry, respectively. Mudali et al. [91] also found that the corrosion resistance of Al coated steels with different N contents varied, in the solution of 0.5 M H2SO4, the order of corrosion resistance was: 0.100% N > 0.015% N > 0.200% N > 0.560% N; while in the 0.5 M NaCl solution, the order changed as follows: 0.100% N > 0.560% N > 0.200% N > 0.015% N.

Summary The aluminide coating has been considered as the first choice for TBM TPBs by most ITER partners for its high PRF, low thermal mismatch, metallurgical bonding, excellent compatibility and self-healing. Nowadays, studies on aluminide TPB coatings are mainly focused on lab scales, far from the engineering applications. The preparation of coatings especially the engineering preparation becomes a bottleneck for their applications in fusion reactors. Each of the existing preparation techniques such as PVD, CVD, HDA, ECD, PC, PS and solegel etc. has their own advantages and disadvantages. On the other hand, an alloying effect exists in aluminide coatings, and has significant influences on the formation, composite, microstructure and performance of the coatings, for which

the influence extent is dependent on the factors of element species, concentration, temperature, coating preparation technique and environmental medium. At present, researches on the alloying effect in aluminide coatings are mainly concentrated in the field of high temperature applications, yet are limited involved in TPB coatings. However, the alloying effect is an important fundamental issue for the actual preparation and application of aluminide coatings, and is also a scientific basis to propose the key controlling technique of the practical preparation and application of aluminide coatings on various structural steels. Therefore, the authors think that the following points should be considered for the studies on the preparation technique and alloying effect in aluminide coatings: (1) The actual working conditions such as high temperatures, intense irradiation, liquid metal corrosion and long time aging and the geometry of structural components like large area and complex shape should be taken into account for the selection and development of aluminide TPB coatings. (2) New preparation techniques including composite preparation techniques of aluminide TPB coatings should be developed according to the actual engineering requirements based on the existing preparation techniques. (3) The substrate effect of various structural steels and the doping effect of aluminide coatings on certain types of steels should be systematic studied so as to select an optimum preparation technique to obtain excellent aluminide TPB coating performances. (4) The nature and mechanism of the alloying effect in aluminide coatings should be revealed by the combination of theoretic and experimental methods to provide the scientific basis of the key controlling technique for the preparation and application of aluminide coatings on structural steels.

Acknowledgment This work is supported by National Magnetic Confinement Fusion Science Program (No. 2013GB110006) and National Natural Science Foundation of China (No. 21471137).

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Please cite this article in press as: Xiang X, et al., Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.052