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https://doi.org/10.1016/j.electacta.2004.04.032
Copy DOIJournal: Electrochimica Acta | Publication Date: Jun 7, 2004 |
Citations: 262 |
The early stages of passivation in high temperature water of a nickel–chromium–iron alloy (Alloy 600) have been investigated by X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). The samples (polycrystal Ni−16Cr−9Fe (wt. %) and single crystal Ni−17Cr−7Fe (1 1 1)) have been exposed for short time periods (0.4–8.2 min) to high temperature (325 °C) and high pressure water, under controlled hydrogen pressure, in a microautoclave designed to transfer the samples from and to the XPS spectrometer without air exposure. In the early stages of oxidation of the alloy (0.4–4 min), an ultra-thin oxide layer (about 1 nm) is formed, which consists of chromium oxide (Cr 2O 3), according to the Cr 2p 3/2 core level spectrum. An outer layer of Cr(OH) 3 with a very small amount of Ni(OH) 2 is also revealed by the Cr 2p 3/2, Ni 2p 3/2, and O 1s core level spectra. At this early stage, there is a temporary blocking of the growth of Cr 2O 3. For longer exposures (4–8 min), the Cr 2O 3 inner layer becomes thicker, at the expense of the outer Cr(OH) 3 layer. This implies the transport of Cr and Ni through the oxide layer, and release of Ni 2+ in the solution. The structure of the ultra-thin oxide film formed on a single crystal Ni−17Cr−7Fe(1 1 1) alloy was analysed by STM in the constant current mode; STM images reveal that, in the early stages of oxidation, the oxide is crystalline, and the observed structure is consistent with the hexagonal structure of the oxygen sub-lattice in the basal plane (0 0 0 1) of α-Cr 2O 3.
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