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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P390
The Effect of Crystal Orientation on the Oxidation Behavior of Iron Substrates
Claudia Juricic 1 , Haroldo Pinto 2 , Thomas Wroblewski 3 , Anke Pyzalla 2
1 Vienna University of Tecnology, Institute of Material Science and Tecnology, Karlsplatz
13, 1040 Vienna, Austria – 2 Max-Planck Institute for Iron research, Max-Planck
Str.1, 40237 Düsseldorf, Germany – 3 HASYLAB at DESY, Notkestr. 85, 22603 Hamburg,
Germany
Dense and adherent oxide layers protect metallic substrates by barring the diffusion
between the corrosive environment and the reactant and, thus, by decreasing the corrosion
rate. The integrity of protective oxide films is, however, strongly influenced by
strain/stress generation in the oxides at high temperatures as well as at room temperature
after cooling.
The Pilling-Bedworth-Ratio (PBR), which is the ratio between the volume of the
oxide and of the metal necessary for the oxide formation, has been often used to
estimate strains in oxide layers. However, not even a qualitative connection exists
between the PBR and the internal stresses in oxide layers. This is due to other factors
affecting the strain state, such as cation diffusion mechanisms, lattice misfit between
substrate and oxide in epitaxial layers, the formation of new oxides and gradients in
the oxide composition of the layer. After oxidation and cooling the strain/stress state
is also modified by additional stresses formed as a result of differences in the thermal
properties of the oxide layer and metallic substrate. These cooling stresses are usually
related to oxide spalling.
The present work deals with the oxidation behavior of pure iron single and polycrystals.
A systematic in-situ synchrotron diffraction study of the influence of crystal orientation,
layer epitaxy and texture on the phase specific strain/stress state is carried out at
450 ◦ C and 650 ◦ C using the LIBAD-method. Above 570 ◦ C the third iron oxide phase,
wuestite, becomes stable and grows between the substrate and magnetite. The effect
of wuestite and higher oxidation rates at 650 ◦ C on layer morphology, texture and
strain/stress state is also presented. Ex-situ diffraction experiments reveal the residual
stresses in the epitaxial oxide layers after cooling.