The application of scanning tunnelling microscopy to study in situ passivation at metal grain boundaries

摘要

The application of Electrochemical Scanning Tunnelling Microscopy (ECSTM) to investigate in situ local passivation properties at metal grain boundaries will be discussed. The procedure to obtain data on the local topography at various types of grain boundaries will be presented using microcrystalline copper in 0.1 M NaOH(aq) solution as a model system, and the data will be discussed using a model of variation of the surface and interface levels at grains and grain boundaries in the studied metallic, passivated and reduced surface states. The results show specific Cu(I) passivation properties of the grain boundaries, and a grain boundary-type dependency of the thickness but not of the composition (i.e. copper content) of the passive film between coherent twins and random (or other special) boundaries. The Cu(I) passive film is found systematically thicker on the grains boundaries than on the grains. The content in reversibly consumed copper is also larger so that the composition of the passive film does not significantly differ from that on grains. The GB depth decrease was found larger after passivation at random boundaries, suggesting the formation of a thicker Cu(I) passive film in this type of boundaries. No metal is preferentially consumed by transient dissolution at grains boundaries during Cu(I) passivation. The coupled application of Electron BackScatter Diffraction (EBSD) for grain boundary characterization will be discussed.