Electrochemical characteristics of Fe-Cr alloy thin films prepared by ion-beam-sputter deposition

A series of Fe-Cr alloy thin films having various Cr contents (11-34 mass%) and thicknesses (60-550 nm) were prepared by ion-beam-sputter (IBS) deposition. The structure of the films was examined by transmission electron microscopy (TEM), electron diffraction (ED), glancing angle incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM). The anodic polarization curves of IBS-Fe-Cr alloy films were measured in 1 kmol · m^-3 Na₂SO₄(pH 3.0), 1 kmol · m^-3 HCl(pH 0.0) and 1 kmol · m^-3 NaCl(pH 5.8) at 298 K, and then compared with those of vacuum-induction-melted (VIM) Fe-Cr alloys containing 10-25 mass%Cr. TEM images and ED patterns of IBS-Fe-Cr alloy films indicated that the films consist of equiaxial bcc microcrystals with an average grain size of approximately 30 nm. The root-mean-square roughness value estimated from AFM images of IBS-Fe-Cr alloy films increases from 0.36 nm for a film 60 nm thick to 0.77 nm for a film 550 nm thick with increasing film thickness. This suggests that the surface microroughness becomes large as the film thickness increases. The critical passivation current density, i_crit, of IBS-Fe-Cr alloy films in 1 kmol · m^-3 Na₂SO₄ (pH 3.0) is lower than that of VIM-Fe-Cr alloys: the value of i_crit for an IBS-Fe-12Cr alloy film 140 nm thick is less than that for a VIM-Fe-14Cr alloy by a factor of about 100. The value of i_crit for IBS-Fe-18Cr alloy films increases with increasing film thickness in the range of 60-550 nm, while it never exceeds that for a VIM-Fe-21Cr alloy. The IBS-Fe-21Cr alloy film 90 nm thick covered with a native oxide film does not show spontaneous activation in 1 kmol · m_-3 HCl under the open circuit condition. The alloy film never develops pitting under the subsequent anodic polarization in 1 kmol · m^-3 HCl and 1 kmol · m^-3 NaCl.