international journal of hydrogen energy 40 (2015) 3697e3707 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he 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 bene- ficial 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 tech- nique, 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