Tuesday, 3 October 2017: 16:20
Camellia 3 (Gaylord National Resort and Convention Center)
Electrochemical scanning tunnelling microscopy (EC-STM) was used to investigate in situ
and locally the topographical alterations of copper surfaces under electrochemical control. The aim of this work was to better understand the early stages of intergranular corrosion and the relationship between local structure of grain boundaries emerging at the solid/liquid interface and corrosion properties. A prerequisite for this approach is the use of a high purity material, in order to exclude the segregation of impurities, such as sulphur, at grain boundaries, which would strongly affect the intergranular corrosion behaviour and prevent us from studying the intrinsic effect of grain boundary structure This was achieved in this work by using high purity cast electrolytic tough pitch (ETP-) Cu, cryogenic rolled and post-annealed at relatively low temperature (200°C). This process also produces a grain size compatible with the limited field of view of STM, as confirmed by electron back scatter diffraction (EBSD) analysis.
EC-STM data obtained in situ after a surface preparation producing oxide-free surfaces with emerging grain boundaries show that performing cycles of anodic dissolution in the absence of any surface oxide reveals a grain boundary structure-dependent corrosion behaviour. Localized attack is initiated at grain boundaries that can be assigned to either random-type boundaries or non-coherent coincidence site lattice (CSL) grain boundaries whereas, under the same conditions, coherent twin grain boundaries are not attacked. Direct coupling of EC-STM and EBSD, using a re-positioning procedure allowing the analysis of the same local surface area by both techniques, show that even for the corrosion resistant coherent twin grain boundaries, the intergranular corrosion behavior measured at the emergence of the grain boundaries at the solid/liquid interface is dependent on the local surface structure. Further combined EC-STM and EBSD analysis will be discussed to link grain boundary type, local surface structure and susceptibility to intergranular corrosion.