In Situ Scanning Tunneling Microscopy Study of Grain-Dependent Corrosion on Microcrystalline Copper

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.