Preparation Technique and Alloying Effect of Aluminide Coatings As Tritium Permeation Barriers: a Review

international journal of hydrogen energy 40 (2015) 3697e3707

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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 7 February 2015 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 complex- geometry 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. 3698 international journal of hydrogen energy 40 (2015) 3697e3707

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.

metallurgical bonding, excellent compatibility, and self- Introduction healing [35e37]. The preparation of aluminide TPB commonly involves two The ITER test blanket module (TBM) will perform the most steps of aluminization and oxidation [30,31]. Aluminization is important functions that will involve testing the feasibility of to form a transition layer of (Fe, Al) solid solution or FeeAl tritium production for the fuel self-sufficiency and the energy metallic compounds on the substrate surface via interdiffu- net output from the ITER fusion reactor. Implemented in the sion between Al atoms from a certain Al source and Fe atoms ITER TBMs, the concepts will be tested during the D-T high form the steel substrate. Oxidation is to form an Al2O3 film on duty phase. One of the key issues of the TBM operation is the the transition layer by selective oxidation. In view of the way controlling over tritium permeation, to reduce the radiological of Al introduction, the preparation technique of aluminide hazard and to optimize the tritium balance in the reactor. coatings can be classified as physical vapor deposition (PVD), Uncontrolled tritium permeation in fusion reactors can result chemical vapor deposition (CVD), hot-dipping aluminization in tritium inventory buildup in the reactor, tritium- (HDA), electro-chemical deposition (ECD), pack cementation contaminated wastes, high tritium concentrations in opera- (PC), plasma spraying (PS), and solegel etc. The techniques tion areas, hydrogen embrittlement of structural materials mentioned above all have their own features, and conse- and more difficult tritium processing [1]. Apart from the high quently the quality and tritium resistant performance of the reliability of the structural design of tritium confinement and corresponding prepared aluminide coatings differs a lot. In tritium handling systems, coating, with a low permeability for fusion reactors, TPB coatings usually need to be prepared on tritium, named tritium permeation barrier (TPB) is one of the the surface or inner wall of structural containers or pipes with most effective methods to minimize tritium permeation large size and complex shapes. Therefore, it is necessary to through structural materials to the environment and other choose appropriate techniques to prepare aluminide TPBs e systems [2 4]. The application of TPBs is thus very necessary based on the working conditions. Presently, the aluminide TPB and helpful for the tritium self-sufficiency and environmental related studies focus on the preparation technique and per- safety in ITER like fusion reactors. formance optimization, so as to improve the coating quality The common used TPB coatings can be classified as oxide and integrity, and also strive for the engineering application. e coatings (eg. Al2O3,Cr2O3,Y2O3, SiO2,Er2O3, ZrO2) [4 13], non- However, the performance of aluminide TPBs often exhibit e oxide coatings (eg. TiC, TiN, SiC, Si3N4) [14 20], and their lower efficiency than anticipated based on the bulk coating composites (eg. Cr2O3/SiO2,Al2O3/SiO2, TiC/TiN, FeAl/Al2O3, material properties [38,39]. The possible reason can be defects e Al2O3/SiC, Er2O3/Fe) [14,21 31]. In the practical applications, in the barrier coating or higher hydrogen permeability of the the non-oxide coatings encounter some unsolvable problems. defect free barrier coating than desired, or a combination of For example, SiC coatings can interact with hydrogen and can both [40]. It is obvious that the integrity and microstructure be cracked and even flaked with a high thickness [18,32], and (defect, impurity, etc.) of the aluminide coating will exert the titanium base ceramic coating is prone to be oxidized and some influence on the hydrogen permeation. Defects like performance degraded above 450 C [33]. On the other hand, voids in TPBs can be reduced or eliminated by a hot isostatic oxides especially Al2O3 with excellent comprehensive prop- pressing (HIP) [41] or chemically densified coating (CDC) [23] erties, attract much interest as typical candidate TPB mate- method, and can also by the technology optimizing to rials for their high melting point, chemical stability, low densify the coating so as to improve the tritium resistant hydrogen solubility and permeability [5,34]. However, the performance of the barrier coating. By contrast, impurities i.e. thermal expansion coefficient shows great difference between alloying elements have much more significant influences on the metal substrate and oxide ceramics, and thus significant the aluminide coatings. On one hand, the phase and micro- thermal mismatch exists, leading to the failure of the coat- structure features (topography, interface, coating thickness ings. The common adopted solution method is to form a and defect configuration etc.) of aluminide coatings forming functional gradient transition layer between the substrate and on different steel substrates with diverse alloying element the coating. Generally, the technique of thermal treatment species and quantities exhibit great difference [3,31,42e44], followed by high temperature oxidation is employed to form and thus display different coating performances such as aluminide coatings typically FeAl/Al2O3 after Al deposition on hydrogen permeability, corrosion and high temperature stainless steels [30,31]. At present, the aluminide coating has oxidation properties [45e47]. On the other hand, during the been selected as one of the prior developed TPBs for the TBMs formation of aluminide coatings, alloying elements from the by Europe, China, United States and India for its high perme- Al sources or doping species can also have remarkable in- ation reduction factor (PRF), low thermal mismatch, fluences on the coating formation and performance international journal of hydrogen energy 40 (2015) 3697e3707 3699

[39,48e50]. The former could be named as the substrate effect, closed-field unbalanced magnetron sputtering, and the and the latter as doping effect, and both can be called as maximum deuterium permeability of Al2O3 coated steel at alloying effect. Nevertheless, compared with the coating 300e500 C was lowered down around 4 orders of magni- preparation techniques, studies on the alloying effect in alu- tude, compared to the bare MANET steel. A newly developed minide TPB coatings relatively lag, and do not reveal a sys- double glow plasma (DGP) technique can also be considered tematic and appropriate theory, which make the preparation as a type of PVD technique [52]. Liu et al. [52] showed that technology optimization and coating structure design cannot the Al2O3 coating could be prepared by the DGP method. be fulfilled simultaneously, and thus restrict the development Firstly, an aluminization layer composed of mainly FeAl3 and application of TPB coatings, while it is absolutely an and Al formed on 316L stainless steels by DGP alumetizing; important fundamental scientifical issue in the practical Secondly, the aluminization layer was oxidized at 600 Cto preparation and application of aluminide coatings. In this form a a-Al2O3 scale with little q-Al2O3 and g-Al2O3. The DGP work, the research progress of the preparation technique and Al2O3 coating showed excellent deuterium permeation alloying effect of aluminide TPB coatings is reviewed so as to resistance, which could reduce the deuterium permeability give some references to other TPB teams or researchers. 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, Preparation technique difficult to deposit on complex-geometry surfaces, poor bonding of the coating with the substrate, and easy to fall With the development of modern science and technology, the off. Therefore, the PVD technique is relatively limited coating technique develops rapidly. Great achievements have employed for TPB preparations, mainly used in the early TPB been made in both the area of coating materials, species, studies. preparation techniques, characterization methods etc. and that of protection mechanisms and applications. As a type of coatings, the preparation technique of TPB coatings can Chemical vapor deposition (CVD) transplant or take reference for the existing coating preparation techniques on the premise of TPB peculiarities. The CVD is a deposition process that the film composing element coating preparation technique is crucial for the service per- containing elementary substance or compound is firstly pro- formance of TPB coatings. However, studies on TPB coatings vided to the substrate, and then a solid film forms via gas are mostly focused on regular lab-scale samples to densify phase or chemical reactions. The CVD technique is deter- and compound the coatings so as to improve their integrity, mined to be the TPB coating preparation technique for the US and consequently the TPB coatings are far from the engi- DCLL TBM for its advantages of simple facility required, neering applications. Up to now, the used preparation tech- continuous and controllable film composition, uniform and niques of aluminide coatings can be classified as PVD, CVD, dense film surface, and availability of coating complex- HDA, ECD, PC, PS and solegel etc., while each technique has geometry structure [37]. Generally, the deposition tempera- its own special features. On one hand, the TPB performance of ture of the CVD technique is rather high, which is unfavorable aluminide coatings prepared by different techniques deviates, for the mechanical properties of structural components. and the hydrogen (or deuterium, tritium) PRF ranges from tens Therefore, advanced CVD techniques such as metal organic to thousands even tens of thousands, whereas the general chemical vapor deposition (MOCVD) and chemical vapor obtained hydrogen PRF in the gas phase condition is lower deposition in fluidized bed reactors (CVD-FBR) have been than 1000, which is not satisfying for the tritium permeation developed so as to coat at lower temperatures [53e56]. Natali resistance [51]. On the other hand, TPB coatings of high quality et al. [53] prepared an Al2O3 scale with some carbon and have to be prepared on both outer surfaces and inner walls of hydrogen impurities on stainless steels in an atmosphere of components with large sizes or/and complex geometries in water and oxygen at 653 K by the MOCVD method. These fusion reactors. However, some techniques such as PVD can impurities can be eliminated successfully by altering the prepare TPB coatings of high tritium permeation resistance, metal organic Al source and using the way of gas carrying [54]. yet cannot be used to coat on complex-shaped tritium An aluminide coating about 8 mm thick was prepared by CVD- confining containers. For these reasons, different preparation FBR on P-92 ferritic steels, which improved the vapor oxida- techniques of aluminide coatings are reviewed in the tion tolerance of the substrate [55]. It is reported that the following sections, so as to propose some techniques suitable growth rate and composition of the CVD-FBR deposited alu- for the engineering applications of aluminide TPB coatings in minide coating depended on the reactive gas reagent and the future fusion reactors. deposition time [56]. An FeAl/Al2O3 coating was prepared on a one-end-closed Eurofer tube by a two-step process of CVD by Physical vapor deposition (PVD) 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 The PVD technique is a physical process of evaporation, expected values according to the advice given by European sputtering or ionization through which the source material tritium permeation barrier work group [51]. Therefore, the is converted to gaseous atoms, molecules or ions under the MOCVD and CVD-FBR techniques are promising in the prep- vacuum condition, and then deposited on the substrate to aration of aluminide TPB coatings, yet there is still a long way form a coating or film. Serra et al. [38] reported that a 1.5 mm to realize the engineering application of CVD to prepare alu- thick Al2O3 coating was deposited on MANET II steel by minide TPB coatings. 3700 international journal of hydrogen energy 40 (2015) 3697e3707

Hot-dipping aluminization (HDA) containers. The deuterium PRF of the aluminide coatings in the air was 3000 at 500 C, ~100 at 740 C, and the number of The HDA TPB preparation technique was originally developed thermal cycling (room temperature ~750 C) exceeded 20 by FZK [38,57]: the stainless steel substrate was firstly dipped times when prepared on a 321 workpiece (F80 2 mm, into an Ar gas protected Al melt at 700 C for 30 s to form a 150 mm long) [31]. Therefore, the ECX technique is very layer of an intermetallic Fe2Al5 phase with a thickness of promising for the engineering preparation of aluminide TPB 20e30 mm; and then thermal oxidized at 950e1075 C to form a coatings in future fusion reactors. three-layered structure composed of a-Fe(Al), FeAl and Al2O3. The maximum deuterium PRF of 260@743 K and 1000@573 K Pack cementation (PC) could be obtained after the lab-scale regular MANET II steel samples treated by HDA [38]. For the MANET II steel container The PC technique has a potential to be selected as a candidate (F29 1.5 mm, 100 mm long), the hydrogen PRF in the air at engineering preparation technique for TPB coatings for its 300e450 C was only 140; while the hydrogen PRF in liquid simple facility and procedure required and high platability. PbeLi alloys even lowered to 45, since voids and de- The typical PC technique is a two-step process of alumetiza- laminations emerged in the coatings, resulting in the corro- tion and oxidation. The hydrogen PRF of 103e104 can be ob- sion of the coatings [58]. It is clear that great deviation of the tained if the thickness of Al2O3 outer layer reaches several mm hydrogen PRF of the HDA coating exists when measured in the for the PC treated 316L and DIN1.4914 steel regular samples air and liquid PbeLi alloys [59]. Moreover, voids or void bands [43,62]. By contrast, the tritium resistance performance of can easily form in aluminide coatings because of the Kirken- aluminide coatings prepared by PC on structure components dal effect in the HDA process, leading to the degradation of the drops dramatically. The hydrogen PRF of the aluminide tritium permeation resistance of coatings [48]. The voids in coating on the inner wall of a 316L tube (F10 1 mm, 250 mm the HDA coatings can be suppressed or eliminated by HIP or long) was only 34 at 235 C in the air [63]; and the deuterium doping alloying elements [41,60]. The doped rare earth ele- PRF of the coating on the outer surface of a 316L tube ments in the Al melt can effectively suppress the formation (F10 0.55 mm, 150 mm long) was not so high at 350e550 C, and growth of voids at the interface of the substrate and in the range of 30e70 [64]. On the other hand, chlorides are coating [60]. Therefore, the HDA technique is promising for often used as the activator during the PC coating, which can the engineering preparation of aluminide TPB coatings, pro- cause severe stress corrosion of the nuclear energy compo- vided that the stability, homogeneity and compactness are nents [43]. For this reason, non-chloride activators have to be readily solved. 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 The PS technique was initially employed to prepare aluminide simple and easy to operate, and the thickness and composi- TPB coatings by ENEA in the 1980s [65], and the typical pro- tion of the coating is controllable as well as be capable of cedure was that the fused or half-fused Al powders were coating complex-geometry structures, the ECD technique is sprayed on the steel substrate in the convenient or inert at- promising in the TPB preparation. However, the Al deposition mosphere, and then heat treated to form aluminide coatings. can only be conducted in water free or aprotic electrolytes for According to the atmosphere used, PS can be classified as at- its high electronegativity of 1.6 eV. Accordingly, the elec- mospheric plasma spraying (APS) and vacuum plasma trodepositon of Al can be classified as ECA and ECX (X ¼ Al, W, spraying (VPS). The former seems to be less appropriate for Ta, etc.) [51]. The former is conducted in organic aprotic the TPB preparation since the oxidation taken place during the electrolytes, while the latter in ionic liquids (ILs). Since the ILs spraying process decreases the coating quality [44]; while the possess the following features [51]: very thermal and chemical latter is also not satisfying in regard of its low tritium PRFs stable, low vapor pressure, high electrical conductivity, high because of residual stresses in the scale [44,66]. Generally variability of chemical structure, good miscibility with inor- speaking, compared with CVD, HAD, ECD and PC, the PS ganic metal salts, and mostly liquid at “room temperature” technique has no superiority for the preparation of aluminide (100 C), the ECX process is much more favored. A dense and TPB coatings. closely adherent FeAl coating with a 20 mm thickness was prepared on mild steel samples by the ECX technique [61]. Solegel However, the performances of such coatings are not satisfying. The ECX has to be combined with processes of thermal Solegel is a commonly used wet chemical method of material oxidation, annealing or HIP treatment to make the surface of preparations for its following features: low temperature syn-

FeAl coating to form a layer of Al2O3 ceramic ﬁlm. Conse- thesis, availability of coating on various geometries and sub- quently, the hydrogen PRF of the coatings can be increased at strates, controllability of the coating composition and least one order of magnitude [41]. Therefore, a technical microstructures. Therefore, it should have potentials for the approach of “ECX followed by selective oxidation” was pro- TPB preparation. It is reported that uniform in thickness, posed almost at the same time by FZK and CAEP [30,42]. With crack-free and well adhered Al2O3 coatings were prepared by e e e this technique, Zhang et al. [31,42] prepared FeAl/Al2O3 TPB sol gel on Fe Cr Al alloys [5]. These coatings could be coatings of high quality on stainless steel samples and effective for the tritium permeation resistance, but have not international journal of hydrogen energy 40 (2015) 3697e3707 3701

been conﬁrmed. Therefore, the solegel technique could be showed similar corrosion rate. On the other hand, the for- employed to prepare aluminide TPB coatings, but there is still mation, topography, compactness and crystal type of the much work to do. 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 Alloying effect Fee50Al alloys because their oxides could act as heteroge-

neous nucleation sites for a-Al2O3; while Ni almost had no Substrate effect apparent effect. Since the typical aluminide coatings prepared on steels are composed of a FeAl transition layer and an outer

The substrate effect of materials derives from crystal types Al2O3 film, it can be inferred from above statements that the (single crystal, polycrystal, amorphous crystal, etc.), micro- alloying elements in the steel substrates must have some in- structures (grain size, configuration, defect, secondary phase, fluences on the coatings. For example, Soliman et al. [45] inclusion, etc.) and compositions (alloying element, impurity, found that the increase of the C content from 0.22 wt.% to etc.). The crystal type and microstructure of materials depend 0.44 wt.% led to a decrease in the rate constant of high tem- on the preparation techniques, and thus are adjustable and perature oxidation by nearly an order of magnitude in alu- controllable. Relatively speaking, the effect of composition on minide carbon steels. Sanchez et al. [47] also revealed that the material properties is more significant. Actually, in metallur- resulting mass gain of the aluminized HCM12A (12 wt.% Cr) gical practices, great changes of material physical, chemical steel sample by the CVD-FBR technique was about 3 times and mechanical properties may occur when a small amount lower than that of P-91 sample after being oxidized at 800 C of certain alloying elements are added. For example, Fedorov for 1000 h, that is the high temperature steam oxidation et al. [67] found that the hydrogen permeability of proton resistance of the aluminized HCM12A is better than P-91, irradiated EP-838 steels was reduced by almost 1 order of which is caused by the different Cr concentrations in the magnitude after Ce addition. coatings. Therefore, alloying elements in steel substrates will In the tritium breeding blanket and auxiliary tritium play an important role in the formation and performance of handling system in fusion reactors, the mainly used structural aluminide TPB coatings, either positive or negative, relying on materials are reduced activation ferritic/martensitic steels the factors of concentration, temperature, environmental (RAFM: F82H, CLAM, Eurofer97), austenitic steels (AS: 316L, medium etc. HR-2), ferritic steels (FS: P-91, HCM12A) and martensitic steels (MS: F82H-mod, MANET) [41,54,55,68e74]. The main alloying Doping effect elements in these types of steels are Cr (7e22 wt.%), Ni (0e15 wt.%), Mn (0e9 wt.%), Mo (0e3 wt.%), W (0e2 wt.%), and Apart from the alloying elements in the stainless steel sub- other elements like Al, Ti and C. The content and species of strates, alloying elements from the Al source or pre-deposited alloying elements vary with steel types. It is thus anticipated layer on the substrate will also play a role in aluminide TPB that the formation and tritium permeation resistance of alu- coatings. It is known that the typical aluminide TPB coatings minide TPB coatings prepared on these steels should be are composed of FeAl transition layers and Al2O3 scales. The significantly different. The reason is that other alloying ele- FeAl transition layer in aluminide coatings forms by the ments in the substrate materials besides Fe will also diffuse at interdiffusion of Fe atoms from the substrate and Al atoms high temperatures during aluminizing, exerting influences on from the Al source. There is no doubt that other alloying ele- the FeAl transition layer. ments doped in Al sources or pre-deposited on the substrate It has been observed experimentally that alloying elements surfaces will take part in diffusion as well as the substrate have prominent influences on the mechanical property, ones under the high temperature condition. Cheng et al. [49] hydrogen permeability and corrosion resistance of FeAl alloys reported that only two layers of FeAl3 and Fe2Al5 formed on [75e78]. Tensile test results [75] of heat-treated FeAl alloys mild steels after being hot-dipped in pure Al and Al-0.5Si showed that the addition of C was effective in improving the melts; while Al7Fe2Si layers and Al2Fe3Si3 particles also yield strength without affecting the ductility, and C seemed to formed besides FeAl3 and Fe2Al5 when the Si concentration in be beneficial in suppressing the hydrogen embrittlement at Al melts was in the range of 2.5e10 wt.%, that is the Si element the grain boundary, since the fracture mode changed from in Al melts is involved in the formation of coating phases. predominantly intergranular in the low C (0.05 wt.%) alloy to During the isothermal oxidation of HDA aluminide coatings predominantly transgranular in the high C (0.2 wt.%) alloy. on the Ni pre-plated mild stainless steels at 750 C, the phase

Similarly, in Fe3Al-based alloys, the added C led to a precipi- constitution of the aluminide layer was observed to transform e tation of perovskite Fe3AlC0.5 carbide phase, resulting in a from high aluminum into low aluminum Ni Al intermetallic decrease of hydrogen permeability and diffusivity by a factor phases because of the interdiffusion of aluminide layers and of 2 without a significant change of the hydrogen solubility nickel layers [79]. The doped alloying elements can also exert [77]. Gonzalez-Rodrı ´guez et al. [76] reported that the effect of an influence on the formation and performance of the FeAl the different alloying elements on the hot salt corrosion per- transition layer as well as the phase constitution. In the HDA formance of FeAl-base alloys depended upon the salt used. In process, with the increase of the Si concentration in Al melts, þ þ NaVO3, the FeAl Ce Ni alloy exhibited the lowest corrosion the coating interface transformed from an irregular finger-like rate, whereas the FeAl þ Ce þ Li alloy showed the highest one; morphology to a smooth flat one, and the coating thickness þ þ while in Na2SO4, the highest corrosion rate was FeAl Ce Ni, dropped gradually [49]. Meanwhile, void bands and cracks in and most of other FeAl-base alloys doped with Ce and/or Ni HDA coatings can be effectively suppressed by the Si doping 3702 international journal of hydrogen energy 40 (2015) 3697e3707

and Ni pre-plating, and eventually improved the oxidation breeding blanket in fusion reactors [35e37]. Therefore, in resistance of aluminide coatings [39,79]. Glasbrenner et al. [48] terms of TPB coatings on such types of steels under certain also found that the thickness of FeAl intermetallic layer conditions, the substrate effect should not be very prominent. forming on HDA-treated MANET steels decreased with the The method of doping alloying elements is usually considered addition of transition elements of Mo, W and Nb in Al melts, to adjust and control the structures and performance of TPB and the internal oxidation of the coatings at high tempera- coatings. It is obvious that the desired alloying effect is tures was suppressed as well, leading to the formation of more beneficial effect. At present, studies on beneficial alloying el- dense and close oxide scales. In the process of PC aluminizing, ements of aluminide coatings are mainly focused on rare the rare elements as Y, Ce and Hf were found to be able to earth elements such as Y, Ce and Hf, which can improve the improve the mechanical property and chemical stability of wear, high temperature oxidation and corrosion resistance of aluminide coatings prominently [50,80,81]. Specifically, the Y aluminide coatings [81,82,84e86]. The reason can be attrib- doping was beneficial for the wear resistance of aluminide uted to the oxides formed by such elements, leading to the coatings in the corrosive environment, the critical load of Y- enhancement of the coating adherence, and the reduction of containing aluminide coatings was 33.6 ± 2.1 g, while the load the grow stress, interface porosity and structural roughness of was only 25.7 ± 1.9 g for the Y-free coatings [50]. Besides, the Y the coatings [82,84]. However, the beneficial effect of rare and Ce dopings can significantly increase the resistance to earth elements depends on their contents. Xiao et al. [84] e both corrosive erosion and dry sand erosion of aluminide pointed out that the addition of 2 5 wt.% CeO2 could coatings on 1030 steel surfaces [80,81]. improve the sulfidation resistance of FeAl coatings on carbon

On the other hand, the FeAl transition layer has to be steels, while the addition of 8 wt.% CeO2 would induced the oxidized at high temperatures to form a dense oxide scale on thermal crack of FeAl coatings, reducing the coating sulfida- the surface so as to achieve better coating performances. tion resistance instead. For the HDA aluminide coatings, the During the high temperature oxidation, Al atoms in the FeAl presence of Si not only can flat the tongue-shape FeAl inter- transition layer are oxidized to form Al2O3 films. Nevertheless, metallic layer, but also can suppress the voids and cracks in Fe atoms and other doped alloying elements are also likely to the coatings, and thus improve the coating performance like be oxidized to form the corresponding oxides. For example, the cyclic oxidation [39,87,88]. Moreover, some transition apart from Al2O3, NiO and TiO2 formed on the Ni and Ti pre- metal elements are beneficial for the aluminide coatings deposited FeAl surface after oxidizing at 900 C, respectively [48,89], for instance, Cr and Pt can protect the aluminide

[78]. Zhan et al. [82] also found that a little CeO2 existed in the coatings against the thermal corrosion [89], and W, Mo, Nb

Al2O3 ﬁlm formed on CLAM steels with the addition of Ce elements can suppress the internal oxidation of aluminide during the PC aluminization. The existence of such oxides coatings during the high temperature oxidation, resulting in

(Al2O3 not included) not only can affect the coating compact- denser Al2O3 films forming on MANET steels [48]. ness, but also can degrade the coating performance. For According to the above statement, when the Ce content example, Cr was found to be not beneficial to oxidation of exceeded a critical value, the beneficial effect of Ce on alu-

Fe3Al alloys, and the oxide scales formed were nonadherent minide coatings converted to an adverse effect, because the and fragile [83]. Therefore, for the aluminized coatings, the excessive Ce induced the thermal crack of the coatings [84]. FeAl transition layer needs to be selectively oxidized at a Zhang et al. [90] also found that high N content in alloy steels certain temperature and oxygen potential to suppress the was undesirable for the adherence of aluminide coatings, oxidation of non-Al elements (i.e. internal oxidation) so as to because it could result in the formation of AlN precipitates; improve the comprehensive coating performance. During the and lower N concentration could make the coatings more HDA aluminization, the transition elements Mo, W and Nb clean with fewer precipitates and voids; while on the other were observed to effectively restrain the internal oxidation of hand the substrate N could increase the coating microhard- FeAl transition layers, resulting in the formation of more ness [91].InCreMo steels, when the Cr content is lower than dense coatings [48]. Therefore, it can be concluded that the 2.25 wt.% or higher than 5 wt.%, Cr nearly has no effect on the doped alloying elements have a significant influence on the formation of the intermetallic layer of HDA aluminide coat- formation and microstructures of the FeAl transition layer ings, but can retard the interdiffusion of steel and Al, resulting and Al2O3 film formed on stainless steels, and eventually in the thickness reduction of intermetallic layer and interface affect the comprehensive performance of aluminide TPB flatness of steel and intermetallic layer [92]. Moreover, the coatings. trace lanthanide elements (<0.025%) such as Ce, La and Nd have minor effect on the Al diffusion layer or coating growth Efficacy of alloying effect dynamics in 430Y stainless steels [93]. The phase structure of the FeAl transition layer in HDA aluminide coatings was also In view of the influence on the structure and performance of found to be independent on the doped W, Mo and Nb elements aluminide coatings, the efficacy of alloying effect can be in MANET steels [48]. It can be seen that the beneficial or identified as three categories of beneficial effect, adverse ef- adverse effect of alloying elements in aluminide coatings defect and nearly no effect. Generally speaking, the types of pends on the element species, concentrations etc., which can structural materials used in the tritium handling system or be converted under certain conditions. Therefore, in the ap- tritium related components are relatively fixed under certain plications of aluminide TPB coatings, the coating used condi- conditions. 316L stainless steels are mainly used as tubes or tion such as temperatures and environmental media should valves steels in tritium handling systems [3,4,18,31], while be considered so as to obtain the best comprehensive coating RAFM steels are commonly recommended in the tritium performance. international journal of hydrogen energy 40 (2015) 3697e3707 3703

Impact factor of alloying effect [92] also reported that when the Cr content was lower than 2.25 wt.%, Cr had no signiﬁcant inﬂuence on the phase Element species constitution of the intermetallic layer in aluminide coatings In the preparation process of aluminide coatings, the inter- formed on mild steels by hot-dipping in Al-10 wt.% melts, the e diffusion aluminization of Fe and Al atoms occurs to form a intermetallic layer was composed of a t5(H) Al7(Fe,Cr)2Si outer

FeeAl intermetallic layer, and subsequently Al atoms in the layer with a FeAl3/t1-(Al,Si)5Fe3/Fe2Al5 inner layer; and when e Fe Al layer are selectively oxidized to form an Al2O3 outer the Cr content was higher than 5 wt.%, the intermetallic layer e layer. The presence of alloying elements will have some in- transformed to a t5(H) Al7(Fe,Cr)2Si single phase with Cr sta- e e fluences on these two processes. It is reported that the addi- bilized t5(C) Al7(Fe,Cr)2Si particles forming on the Al Si top- tion of Si in Al melts not only affected the phase constitution coats, and the interdiffusion of steels and AleSi increased in the HDA process, resulting in the formation of Si containing with the Cr content. On the other hand, the oxidation be- phases, but also changed the interface structures and thick- haviors of FeAl base alloys are also concentration related. It ness of formed aluminide coatings [49]. Janda et al. [94] also was observed that under the oxidation condition of relative pointed out that the transformation temperature of g-Al2O3 to lower temperature of 900 C, the increase of the Y2O3 content a-Al2O3 on the surfaces of FeAl base alloys changed from in Fe40Al alloys accelerated the formation of a-Al2O3, leading 950 C to 750 C with the addition of Zr and Nb elements. to the significant decrease of the oxidation rate; while at According to the material science point that the property is 1000 C, much denser oxidation layers would form with the determined by the composition and structure, it is obvious increase of the Y2O3 content [97]. Cr was also considered to be that different alloying elements will exert different influences not beneficial for the oxidation of Fe3Al alloys at 1000 C, and on the formation and performance of aluminide coatings. In the oxidation rate increase with the Cr concentration [83].It the HDA process, the addition of W, Mo and Nb in Al melts was thus can be inferred that the alloying effect of aluminide observed to have influences on the process of aluminization coatings composed of a FeAl transition layer and an Al2O3 and subsequent high temperature oxidation, and the outer layer is definitely associated with the concentration of maximum thickness of aluminide coatings depended on the alloying elements. Comparisons revealed that the relatively added alloying element, for which it was the thinnest in the higher N content in commercial steels such as Fee9Cre1Mo case of AleW, and thickness in the AleMo case, yet both lower and 304L would result in the formation of AlN precipitates in than the pure Al case [48].Sanchez et al. [95] found that the aluminide coatings, which were undesirable for the coating vapor oxidation resistance of aluminide coatings formed on adherence [90]. Especially for 304L steels, the relatively lower HCM12 A steels by the CVD-FBR technique deviated with the N content could make the coating cleaner with less pre- addition of Ce or La, and the former was superior than the cipitates and Kirkendal voids [90]. Furthermore, the micro- latter. Moreover, since the species of alloying elements in the hardness of the aluminide 316L steels would increase with the steel substrates are different, the formed intermetallic alu- substrate N content [98]. Therefore, the concentration of minide phases in Al melts are accordingly diverse; for 1.4914 alloying elements has great influences on the formation and steels, the intermetallic phase was (Fe,Cr)2Al5/(Fe,Cr)Al3, while performance of aluminide TPB coatings. 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, Temperature different alloying elements in the steel substrate and Al The process of both aluminization and oxidation has to be source will have different influence on the formed coatings. conducted under the condition of much higher temperatures The reason could be attributed to the obvious difference of than room temperature. Generally, the interdiffusion rate of atomic structures and electrical properties of different alloy- Fe and Al atoms increases with temperatures; and the ing elements, resulting in the significant different physical or oxidation process need to be implemented at certain high chemical properties in materials, and thus have dissimilar temperatures and oxygen potentials to realize the selective influences on the diffusion behaviors of Fe and Al atoms in the oxidation of Al and constrain the oxidation of Fe and other aluminization process and subsequent oxidation process, and alloying elements so as to increase the concentration and eventually affect the formation and performance of aluminide compactness of Al2O3. Therefore, temperature is a critical TPB coatings. controlling factor in the preparation process of aluminide coatings, and the alloying effect on coatings is inevitable Concentration dependent on temperatures. Sundar et al. [75] found that heat The phase constitution and microstructures of the FeAl tran- treated at 1100 C, the addition of C could effectively improve sition layer in aluminide coatings are affected by the con- the yield strength of FeAl base alloys without changing the centration of alloying elements. Cheng et al. [49] found that in ductility significantly; while heat treated at 1300 C, the yield the HDA process, with the increase of Si concentration in Al strength decreased significantly and the ductility lowered as melts, the topography of formed intermetallic layers became well, which could be attributed to the synergetic effect of flat from finger-shaped, and the thickness dropped non- retained vacancies and fine carbide precipitates formed at linearly; but when the Si concentration exceeded 10 wt.%, the higher temperatures. The oxidation behaviors of Cr contain- thickness would no more decreased significantly [87]. Mean- ing FeAl alloys were also observed to be obviously diverse at while, when the Si concentration reached 2.5 wt.%, an Al7Fe2Si different temperatures; the oxidation rate at 1100 C was layer and an Al2Fe3Si3 particle layer also formed in the inter- lower than that at 900 C, since a stable a-Al2O3 layer formed metallic layer apart from FeAl3 and Fe2Al5, and the thickness at 1100 C, while a metastable q-Al2O3 layer formed at 900 C of each layer differed from the Si content [26,49]. Cheng et al. [99]. Generally speaking, for all the crystal types of Al2O3, the 3704 international journal of hydrogen energy 40 (2015) 3697e3707

a tritium permeation resistance of -Al2O3 is best [100]. How- the inﬂuence extent is dependent on the factors of element a ever, the formation temperature of -Al2O3 on the surface of species, concentration, temperature, coating preparation FeAl alloys is often up to 900e1000 C [101,102], which could be technique and environmental medium. At present, researches unfavorable for the mechanical properties of substrate ma- on the alloying effect in aluminide coatings are mainly terials. Experimentally, the formation temperature of a-Al2O3 concentrated in the ﬁeld of high temperature applications, yet can be lowered down by doping alloying elements. It is found are limited involved in TPB coatings. However, the alloying that the transformation temperature of a-Al2O3 from g-Al2O3 effect is an important fundamental issue for the actual prep- decreased from 950 C to 900 C after the addition of 0.05 at.% aration and application of aluminide coatings, and is also a

Zr in Fe3Al [103]; the transformation temperature could be scientific basis to propose the key controlling technique of the further decreased to 750 C when co-doped by Zr, Nb, C and B practical preparation and application of aluminide coatings elements [93]. It is obvious that the alloying effect in alumi- on various structural steels. Therefore, the authors think that nide coatings composed of a FeAl transition layer and an Al2O3 the following points should be considered for the studies on film is closely dependent on the temperature. Therefore, it the preparation technique and alloying effect in aluminide would be of an important engineering worthware for the coatings: actual applications of aluminide TPB coatings once the for- a mation temperature of -Al2O3 can be lowered down by (1) The actual working conditions such as high tempera- doping alloying elements. tures, intense irradiation, liquid metal corrosion and long time aging and the geometry of structural compo- Other factors nents like large area and complex shape should be The alloying effect of aluminide coatings is also dependent on taken into account for the selection and development of the coating preparation technique and environmental me- aluminide TPB coatings. dium apart from the factors of element species, concentration (2) New preparation techniques including composite and temperature. It is reported [89] that when Cr was pre- preparation techniques of aluminide TPB coatings deposited on the IN 100 nickel-base superalloy, the subse- should be developed according to the actual engineer- quent aluminide coatings prepared by the PC technique ing requirements based on the existing preparation showed a typical three-layer structure, and the outer layer techniques. was enriched with Cr; while the coatings prepared by the CVD (3) The substrate effect of various structural steels and the technique was a pure NiAl layer, and Cr was enriched doping effect of aluminide coatings on certain types of beneath. The reason is that in the two preparation processes, steels should be systematic studied so as to select an the activity of Al differs, resulting in different distributions optimum preparation technique to obtain excellent and existing forms of Cr in the aluminide coatings. On the aluminide TPB coating performances. other hand, the environmental medium has an important (4) The nature and mechanism of the alloying effect in influence on the alloying effect of aluminide coating. Zhang aluminide coatings should be revealed by the combi- et al. [104] found that the corrosion erosion resistance of Y nation of theoretic and experimental methods to pro- containing aluminide coatings on 1030 steels was absolutely vide the scientific basis of the key controlling technique different in the media of dry sand, NaCl or H2SO4 containing for the preparation and application of aluminide coat- silicon dioxide slurry, respectively. Mudali et al. [91] also ings on structural steels. 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% Acknowledgment N > 0.015% N.

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

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