Electrochemistry of Steel Electrodes: A Combined Study by Linear Reflectivity Measurements and Second Harmonic Generation

Steel electrodes were investigated under electrochemically controlled conditions at pH 13. The linear reflectivity at 1064 nm and the second harmonic signal (SHG) at 532 nm generated in reflection were measured for different polarizations and recorded simultaneously with cyclovoltammograms. The potential was varied between −1.4 V and 0.5 V vs. SCE and the optical signals were recorded after repetitive cycling. The reflectivity which probes the whole thickness of the electrochemically active layer and the non-linear optical signal which, in contrast, only probes the electrode surface and its immediately vicinity, exhibit very different behavior with respect to their potential dependence. In a cathodic scan the increase of the respective optical signals is associated with different peaks of the cyclovoltammogram. Furthermore, the SHG signal is strongly dependent on the polarization. The pronounced changes around −1.25/-1.1 V in the cathodic/anodic scan direction, which are observed for pp-polarization, are absent in sp- and ms-polarization. The sp-signal shows features which are paralleled by a decrease in the linear reflectivity. Contrary to the other polarizations, the ms-polarized signal does not show any particular features correlated with peaks of the cyclovoltammogram. The potential dependence of the surface charge expressed by a simple parabolic model describes the SHG signal over extended parts of the potential range. The differences between the potential dependence of the SHG signal and the change of the linear reflectivity fully agree with the two layer model of electrochemically formed iron oxides.