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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 9 coincidence site lattice (CSL) boundaries are susceptible to nanoscale initiation of intergranular corrosion while for 3 CSL boundaries the behavior is dependent on the deviation angle of the GB plane from the exact orientation. For the 3 twins, a transition from resistance to susceptibility occurs between 1° and 1.7° 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.

In situ electrochemical scanning tunneling microscopy (ECSTM) was applied to analyze the local susceptibility to corrosion at different grains of cryogenically rolled microcrystalline copper in a HCl solution, and combined with electron backscatter diffraction (EBSD) and field emission scanning electron miscroscopy (FE-SEM) to discuss the relationship between nanometer scale corrosion resistance and crystallographic orientation. The results show that the thickness of the air-formed oxide layer is grain-dependent with the largest values exceeding locally by a factor of 2 the macroscopic value (2.8 nm) measured electrochemically. Anodic dissolution is also grain-dependent with dissolving grains observed to neighbor corrosion-resistant grains. A nearly random texture prevented an EBSD-based assignment of the crystallographic orientation of the grains observed by ECSTM. However, comparison of the etched surface morphology measured in situ by ECSTM and ex situ by FE-SEM suggested that the faster dissolving grains were oriented closer to ⟨111⟩//ND or in between ⟨111⟩//ND and ⟨110⟩//ND while the neighboring corrosion-resistant grains were oriented closer to ⟨001⟩//ND. The higher step density measured by ECSTM on the grains corroding faster despite possibly ⟨111⟩//ND oriented terraces confirms the role of surface defects related to misorientation on the corrosion susceptibility.

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