ABSTRACT
Given that the grain boundaries and triple junctions (i.e., high-energy regions with a much greater degree of disorder) are nearly always anodic (Maurer et al. 1984, Yamashita et al. 1991, Kirchheim et al. 1992, Barbucci et al. 1999, Aledresse and Alfantazi 2004), the corrosion rate of nc materials in most simplistic terms would be expected to be considerably higher than that of mc materials of similar chemical composition, and this view is oen supported in the literature (Rofagha et al. 1991, 1993, Kirchheim et al. 1992, Barbucci et al. 1999, Kim et al. 2003, Aledresse and Alfantazi 2004). However, the corrosion resistance of some other materials is reported to be superior in their nc form. Such contrasting observations of the inuence of nanostructure may depend on the coupled environmental-materials interactions for that system in specic environments. In the case of electrochemical corrosion of the pure metals, the anodic behavior of grain boundaries and the associated anode-cathode area ratio are the governing factors, whereas the eect of alloying elements would be an additional compounding factor for the case of nc alloys. Early fundamental studies on the role of nanostructure in electrochemical corrosion were carried out on Co (Palumbo et al. 1997, Vinogradov et al. 1999, Jung and Alfantazi 2006, Wang et al. 2007), Cu (orpe et al. 1988, Vinogradov et al. 1999, Cheng et al. 2001, Jung and Alfantazi 2006), Ni (Rofagha et al. 1991, Tang et al. 1995, Lu et al. 2000, Liu et al. 2007), and Ni-based binaries (Rofagha et al. 1993, Gonzalez et al. 1996, Kim et al. 2003, Ghosh et al. 2006). e extent of localized corrosion at grain boundaries is reported to decrease in the case of some nc metals (viz., Cu and Co [Vinogradov et al. 1999, Kim et al. 2003, Jung and Alfantazi 2006]), in comparison with their mc counterparts; these observations can be attributed to either (1) decrease in the dierence in the electrochemical potentials of grains and grain boundaries or (2) the greater anode-to-cathode area ratio in nc materials.
