TEM Characterization of Corrosion Products Formed on a Stainless Steel-Zirconium Alloy
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583 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
EXPERIMENTAL Specimens of SS-1 5Zr alloy, crushed to 75 to 150 pm size fraction, were immersed in 90'C deionized water for two years. The solution volume was such that the sample surface areato-leachant volume ratio was -2000 m'. On completion of the test, individual particles of the stainless steel and intermetallic phases were selected on the basis of scanning electron microscopy (SEM), embedded in resin, and sectioned by a Reichert-Jung Microtome to yield -50-nm thick samples for TEM examination. Transmission electron microscopy was performed with a JEOL 2000FX unit operating at 200 kV and a PHILIPS CM30 unit operating at 300 kV, both equipped with an energy dispersive x-ray spectrometer. High-resolution TEM was carried out with a JEOL 4000EX microscope operating at 400 kV with a point-to-point resolution of 1.65 A. RESULTS AND DISCUSSION Corrosion Products on an Austenite Particle Figure 1 is a TEM image of corrosion products formed on an austenite particle from the reacted SS-15Zr alloy. The corrosion layer was largely detached from the metal, probably due to the mechanical force applied during ultramicrotomy. However, it was still possible to establish a spatial relationship between the metal surface and the corrosion layer. At least two distinct corrosion products formed on the surface. The corrosion product that appears to have been in immediate contact with the stainless steel surface (i.e., B in Fig. 1) exhibited a relatively dense and uniform microstructure, whereas the corrosion product on the outer surface of the layer (C in Fig. 1) was more porous. Assuming that no materials were lost during sample handling and preparation, the total layer thickness was estimated to be between 0.5 and 1.0 tim.
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Figure 1. Bright-field TEM micrograph of an austenite-corrosion layer interface. At least two corrosion products (B and C) have formed on the stainless steel (A) surface, based on their distinctive microstructural characteristics.
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Electron diffraction analyses indicated that both corrosion products were crystalline. The corrosion product B was indexed to match the cubic trevorite structure (JCPDS-ICDD 10-325, nominal formula NiFe20 4); the outer product C was indexed to match the face-centered cubic maghemite-c structure (JCPDS-ICDD 39-1346, nominal formula Fe20 3). The NiFe 20 4 spinel was observed by Nakayama et al. [4, 5] in the passive film of 18Cr-8Ni austenitic stainless specimen that was heated in 300"C deoxygenated water for 24 h. This spinel structure was unaffected even after the sample was heated for 3 h at 1000"C. The presence of nickel in a spinel-type lattice was also reported by Castle and Clayton [6] in the oxide layer of a stainless steel alloy heated in 200"C water. These studies show that the spinel structure can be stabilized by nickel ions under appropriate experimental conditions. Figure 2 compares the EDS spectra obtained from the austenite phase A, and the corrosion products B and C. The austenite sp
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