ORIGINAL PAPER Microstructure Evolution During Steam Oxidation of a Nb Stabilized Austenitic Stainless Steel A. N. Hansson • K. Pantleon • F. B. Grumsen • M. A. J. Somers Received: 6 May 2009 / Revised: 23 September 2009 / Published online: 24 October 2009 Ó Springer Science+Business Media, LLC 2009 Abstract The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated at 500, 600, and 700 °C for a fixed oxidation time of 336 h. The samples were characterised using X-ray diffraction, reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500 °C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700 °C. The major part of the inner oxide layer developed within the former alloy grains, whereas a Fe–Cr spinel formed along the former alloy grain boundaries. Trans- mission electron microscopy and electron energy loss spectroscopy investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe–Cr spinel embedded in a metallic Fe–Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. The thickness of the inner oxide zone at high humidity (46%) is not significantly affected by the type of carrier gas used, whereas this thickness at low humidity (8% H 2 O) is sensitive for the carrier gas and increases in the following order: air \ Ar?7% H 2 \ Ar, indi- cating that the presence of oxygen or hydrogen in addition to a relatively low content of water vapour counteracts the effect of water vapour on the development of the inner oxide zone. Keywords Water vapour oxidation Á Internal oxidation Á Austenitic stainless steel Á Oxide scale Á Diffusion Á Grain boundaries A. N. Hansson Á K. Pantleon Á F. B. Grumsen Á M. A. J. Somers (&) Department of Mechanical Engineering, Technical University of Denmark, Kemitorvet b. 204, 2800 Kgs. Lyngby, Denmark e-mail: somers@mek.dtu.dk Present Address: A. N. Hansson DONG Energy, A. C. Meyers Vænge 9, 2450 Copenhagen SV, Denmark 123 Oxid Met (2010) 73:289–309 DOI 10.1007/s11085-009-9182-x