  The initiation of intergranular corrosion at various types of grain
boundaries (GBs) was studied at the nanometer scale on microcrystalline copper
in 1 mM HCl aqueous solution. In situ Electrochemical Scanning Tunneling
Microscopy (ECSTM) and Electron Back-Scatter Diffraction analysis of the same
local microstructural region were combined using an innovative methodology
including micro marking performed with the STM tip. The results demonstrate
that electrochemically-induced intergranular dissolution, at the surface
termination of GBs, is dependent on the grain boundary character. It is found
that random high angle boundaries as well as sigma9 coincidence site lattice
(CSL) boundaries are susceptible to nanoscale initiation of intergranular
corrosion while for sigma3 CSL boundaries the behavior is dependent on the
deviation angle of the GB plane from the exact orientation. For the sigma3
twins, a transition from resistance to susceptibility occurs between 1{\deg}
and 1.7{\deg} of deviation as a result of the increase of the density of steps
(i.e. misorientation dislocations) in the coincidence boundary plane. The work
emphasizes the precision needed in the design of the grain boundary network in
applications where intergranular corrosion or its initiation must be controlled
at the nanoscale.
