Letter Blistering of MCrAlY-coatings in H 2 /H 2 O-atmospheres M. Subanovic a , D. Naumenko a, * , M. Kamruddin b , G. Meier c , L. Singheiser a , W.J. Quadakkers a a Forschungszentrum Jülich GmbH, IEF-2, Leo-Brandt Street, 52425, Jülich, Germany b Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, India c Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA article info Article history: Received 18 December 2008 Accepted 21 January 2009 Available online 25 January 2009 Keywords: Oxidation Hydrogen embrittlement Hydrogen absorption High-temperature corrosion Rare earth elements abstract The oxidation behavior of vacuum plasma sprayed, free-standing MCrAlY-coatings was studied in Ar– 20%O 2 and Ar–4%H 2 –2%H 2 O. During exposure in Ar–4%H 2 –2%H 2 O the coating formed large ‘‘blisters”. This phenomenon was attributed to H 2 -gas evolution within the coating, creating high pressures suffi- cient for coating plastic deformation as a result of atomic hydrogen recombination on coating defects. It is suggested that the hydrogen is produced at the scale/metal interface by reaction of H 2 O-vapor with yttrium and aluminium present in the coating. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction MCrAlY-coatings (M = Ni, Co) are extensively used to protect superalloy gas-turbine components against high-temperature cor- rosion environments by forming a slowly-growing and adherent alumina surface scale [1]. The MCrAlY-coatings have been also applied for decades as bondcoats for ceramic thermal barrier coatings (TBC) [1,2]. In both applications the oxidation behavior of MCrAlY-coatings (bondcoats), which is determined by the growth rate and adherence of the alumina scale, is a crucial factor for the component lifetime. Failure of the TBCs has, in many cases, been related to a critical thickness of the alumina scale formed on the MCrAlY surfaces [3]. The scale growth rate and especially adherence are substantially affected by minor (<1 wt.%) additions of reactive elements (RE), mostly Y [4]. The oxidation behavior of MCrAlY-coatings and bondcoats has been extensively studied in laboratory air. A number of studies is also available on the effect of water vapor on the oxidation. Sev- eral authors found the water vapor addition to result in an enhancement of the scale spallation during cyclic oxidation [5– 7]. This effect was related [7] to hydrogen embrittlement of the interface, whereby the hydrogen was claimed to be produced by reaction between Al and water vapor. Therefore, the effect only occurs after the scale damage cracking and delamination has been initiated by thermal cycling, allowing H 2 O-vapor access to the scale/metal interface. Furthermore, in NiCrAlY model alloys and coatings with low Cr and/or Al contents the presence of water va- por in the test atmosphere was shown to extend the transient oxidation stage prior to formation of a continuous alumina scale [8,9]. Only a few literature sources exist on the oxidation of MCrAlY- materials in H 2 O-containing atmospheres with a low oxygen par- tial pressure (pO 2 ). Leyens et al. [10] studied an EB-PVD NiCoC- rAlY-coating in various H 2 /H 2 O-mixtures and found poorer alumina scale adherence than found during exposure in dry air. Toscano et al. [11] performed cyclic oxidation tests of an EB- PVD-TBC system with a NiCoCrAlY-bondcoat on an IN738 super- alloy substrate in air and Ar–4%H 2 –2%H 2 O at 1100 °C. The TBC- lifetime was extended in the latter, low pO 2 gas, which was attributed to a slower growth rate of the alumina scale. The effect of pO 2 on the growth rate of alumina scales had been described previously for FeCrAl-alloys [12]. It was suggested that reduction of the pO 2 in the environment reduces the gradient in oxygen chemical potential across the scale, thereby reducing the inward oxygen flux across the scale. Recently, the effect of H 2 O-rich atmospheres in application of MCrAlY-coatings in new power plant designs with CO 2 -capture has been considered. One such design concept involves partial oxi- dation of the fuel to produce a syngas, from which hydrogen is sep- arated and burned in a ‘‘hydrogen-turbine”. Another design (oxyfuel) is based on a conventional combined cycle with exhaust gas recirculation. In both cases the environment of the combustion chamber and the turbine are going to be different from that of con- ventional plants, i.e. it contains considerably larger amounts of H 2 O and possibly lower O 2 contents [13,14]. 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.01.008 * Corresponding author. Tel.: +49 2461 613066; fax: +49 2461 613687. E-mail address: d.naumenko@fz-juelich.de (D. Naumenko). Corrosion Science 51 (2009) 446–450 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci