Initial stages of intergranular corrosion as studied locally on copper by electrochemical scanning tunnelling microscopy

摘要

Intergranular corrosion is a major form of degradation of polycrystalline metallic materials that has been shown to be intimately related to the type and energy of the grain boundaries. Coincidence site lattice (CSL) grain boundaries (also called "special" grain boundaries) are reported to have better corrosion resistance than random grain boundaries (which do not have a CSL configuration). For fcc materials such as copper, only ∑3 boundaries among CSL's would have better corrosion resistance than random grain boundaries and other CSL boundaries, and among 13 boundaries, only coherent twins (having a {111} grain boundary plane) would resist intergranular corrosion as opposed to incoherent twins. Electrochemical scanning tunnelling microscopy (EC-STM) allows investigating in situ and locally the topographical alterations of a metallic surface under electrochemical control. Its application to metallic polycrystals can be insightful for better understanding the initial stages of intergranular corrosion and the relationships between local structure at a grain boundary emerging at the solid/liquid interface and corrosion properties. A prerequisite to discuss the effect of the crystallographic characteristics of the grain boundaries is the production of a high purity material, allowing to exclude the segregation of impurities or alloying elements at grain boundaries, which could impact the intergranular corrosion behaviour. This is achieved in this work using high purity cast electrolytic tough pitch (ETP-) Cu, cryogenic rolled and post-annealed at relatively low temperature (200°C). This procedure also produces a grain size compatible with the limited field ofview 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 grains boundaries show that performing cycles of anodic dissolution in the absence of any surface oxide reveals a grain boundary type-dependent corrosion behaviour. Localised attack is initiated at grains boundaries that can be assigned to either random-type boundaries or non-coherent CSL's based on EBSD while, in the same conditions, coherent twin grain boundaries do not corrode. Direct coupling of EC-STM and EBSD, using are-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 boundary at the solid/liquid interface is dependent of 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.