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

Abstract

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.