Corrosion and passivation in nanoscopic and microscopic dimensions: the influence of grains and grain boundaries

In technical applications of polycrystalline metals, the grain structure and role of grain boundaries are of great importance. Electron back scattering diffraction (EBSD) identifies the surface orientation of grains. In combination with microelectrochemical techniques, single crystal experiments can be carried out on polycrystalline metals. Application of AFM (atomic force microscopy), LFM (lateral force microscopy) and dF/ds (force microscopy) can provide complementary information in the nanometre range, e.g. on grain boundaries. As an example, the etching and passivation of Ti (hcp) has been investigated. First, a variety of technical samples from different companies were analysed with respect to grain size, orientation, and twins formation. AFM showed different roughnesses on grains (e.g. (0001) >(xxx0)), while dF/ds demonstrated the greater hardness of films on (0001) compared with (xxx0). Grain boundaries between grains of the same and different orientations must be distinguished. In iron the topography of grain boundaries (steps, hills, trenches) shows a clear dependence on the polishing procedure and on the orientation of neighbouring grains. (100) orientated grains always show the fastest metal removal during both chemical and electrochemical polishing. These effects can be correlated with different surface packing densities of the relevant crystal faces.