Nanoscale Intergranular Corrosion and Relation with Grain Boundary Character as Studied In Situ on Copper

Abstract

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.

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure–activity-corrosion potential relationships

“…[3][4][5][6][7] However, a limitation of many experimental approaches is that the electrochemical perturbation (and measurement) is applied globally at a macroscopic electrode immersed in a bulk solution, 8 but most corrosion processes are initiated and perpetuated at (sub)microscopic surface sites (e.g., grain boundaries, inclusions, microscratches etc.). [9][10][11][12][13][14] Mismatch between the scale of key corrosion phenomena and conventional electrochemical methods makes it difficult to unambiguously identify the key anodic/cathodic sites driving corrosion. This issue is compounded for the case of atmospheric corrosion, 15 or corrosion in certain automotive/industrial environments (vide infra), 16,17 which take place due to the action of small droplets on the surface in a confined system.…”

Combined in situ microstructural study of the relationships between local grain boundary structure and passivation on microcrystalline copper

“…Thus, for grain boundaries that are classified of the same  CSL type by the EBSD IQ mapping analysis, the efficiency of the passivation varies as revealed by ECSTM analysis of the increased, decreased or unchanged depths of the GB regions. Clearly these results suggest that although of the same  CSL type, the local GB structure may vary between different boundaries and also along the same boundary, modifying the local reactivity towards passivation similarly to what was observed in the initial stages of active dissolution [26].…”

“…The transition was explained by the presence of misfit dislocations (i.e. steps) accommodating the deviation of the boundary plane from the exact CSL plane and by the increased density of step regions introduced in the boundary plane with increasing deviation [26]. The present data obtained for copper passivation by formation of a Cu(I) oxide film show a transition from more efficient to less efficient passivation for a deviation angle  of 0.4-0.5°.…”

“…The repositioning was achieved thanks to micro marks made on the sample surface using the tip as a nano indenter after recording the last STM image. The applied procedure is described in detail elsewhere [26]. A digital scanning electron microscope including the micro marks.…”

“…This is due to the difficulty 3 of applying analytical methods sensitive to the electrochemically-induced local alterations of the topmost surface. Recently, ECSTM (Electro-Chemical Scanning Tunneling Microscopy) has been applied to microcrystalline copper to investigate in situ the local corrosion resistance at the surface termination of grain boundaries in metals [22][23][24][25][26]. It was found that dissolution in the active state, i.e.…”