A Study On The Corrosion Behavior Of Nanocrystalline Nickel In Aqueous Environments

Nanocrystalline materials possess considerably enhanced properties arising from their small grain size. Significant interest has been paid in the past to their mechanical and physical properties. Very little attention has been attracted by their electrochemical behavior, which is expected to be affected by the significant volume fraction of grain boundaries. In the present study, corrosion experiments were conducted in nanocrystalline nickel in two entirely different solutions in order to obtain a better understanding of its corrosion behavior. This is of particular interest to MEMS and NEMS devices where electroplated nanocrystalline nickel is the metal of choice. The corrosion behavior of bulk (microcrystalline) and nanocrystalline Ni were studied in 3.5% NaCl and 0.1N H₂SO₄ aerated and deaerated solutions. Potentiodynamic polarization and open circuit potential vs time experiments were conducted to observe the corrosion behavior in a pitting environment (3.5% NaCl solution) and a passivating environment (0.1N H₂SO₄). The surface morphology after corrosion testing was analyzed using scanning electron microscopy (SEM) and optical profilometry. The bulk and nano Ni were characterized using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) to determine the grain size and composition, respectively. Nano Ni was found to exhibit a more noble potential in both NaCl and H₂SO₄ solutions compared to its microcrystalline counterpart. In aerated environments, the corrosion rate of nano Ni was significantly higher than that of bulk Ni. This was attributed to the catalytic properties exhibited by the large volume fraction of grain boundaries present in nano Ni in terms of facilitating the oxygen reduction action. In deaerated solutions, nano Ni exhibits either a significantly lower (i.e., NaCl solution) or comparable (i.e., 0.1N H₂SO₄ solution) corrosion rate to that of its microcrystalline counterpart. In terms of surface morphology pits in nano Ni tested in NaCl solution, were found to be uniform in depth ~42 μm, whereas microcrystalline Ni showed a large variation extending to a depth of 59 μm). In the diluted H₂SO₄ solution, bulk Ni exhibited passivation behavior whereas nano Ni was not passivated but, corrosion found to be rather uniform. Nanocrystalline Ni was not able to passivate due to its high density of surface defects inhibiting stable oxide formation. A bimodal pit distribution was observed in nano Ni tested in NaCl solution consisting of numerous submicron pits and a small number of dispersed larger pits. The formation of the latter pits was found to involve both vertical and lateral growth in view of the high density of grain boundaries that provide sensitive sites.