In-situ Analysis of Passive Films on High Purity Fe-Cr Alloys Using Ellipsometry and Potential-Modulated Reflection Spectroscopy

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Passive films on high purity Fe-Cr alloys (13-24 mass% Cr), which were prepared by an Ar plasma melting method using high purity Fe produced by a method combining anion exchange and zone-refining processes and high purity Cr produced by an iodide process, have been investigated in-situ in pH 6.0, 1 kmol·m–3 Na2SO4 using ellipsometry and potential-modulated reflection spectroscopy. The thickness, d, optical constants, N2, and cationic mass fractons of Cr3+ ions, XCr, of the films were determined as a function of potential and Cr content of the alloy. The same analyses were performed on 400 series ferritic stainless steels (13-24 mass% Cr) to elucidate the changes in the film properties with the change in the purity of alloy. It was found that for the passive films on high purity Fe-Cr alloys the value of d increased and, at the same time, that of XCr decreased with increasing potential. At a given potential, the values of d and N2 decreased with increasing Cr content of the alloy and those of XCr increased correspondingly. Similar results were obtained on commercial ferritic stainless steels. The values of d, N2 and XCr changed in dependence on the purity of alloy. That is, the values of d and N2 decreased and those of XCr increased with increasing purity. The increase in XCr of the films should contribute the increase in the corrosion resistance of the high purity alloys.