  In situ Electro-Chemical Scanning Tunneling Microscopy (ECSTM) and Electron
Back-Scatter Diffraction analysis of the same local microstructural region were
combined to study the relationships between grain boundary (GB) type and
structure and passivation on microcrystalline copper in 0.1 M NaOH aqueous
solution. The results show that, for high angle random boundaries, passivation
in the Cu(I) oxidation range is characterized by a decrease of the depth of GB
edge region due to the formation of a passive film locally thicker than on the
adjacent grains and thus to anodic oxidation being locally less efficient since
it consumes more copper at the grain boundaries. For sigma3 CSL boundaries, the
Cu(I) passivation efficiency was observed to be dependent on the local
structure of the boundaries. A transition from more to less efficient
passivation was observed for a deviation angle of 0.4-0.5{\deg} of the {111} GB
plane with respect to the perfect geometry of an ideal coherent twin boundary.
This transition would result from the effect on the local anodic oxidation
properties of an increased density of misfit dislocations accommodating the
increased deviation of the boundary plane from the exact CSL plane.
