Influence of Chloride on Nanoscale Electrochemical Passivation Processes

Nucleation and growth of metastable oxides on polycrystalline, solid solution Ni-22 wt % Cr alloy were studied in slightly acidic chloride solution and compared to sulfate solutions at the same pH. A combination of ac and dc electrochemical methods, photoelectron spectroscopy, and atomic force microscopy was utilized to characterize the passivation and local breakdown of oxides, yielding information on oxide morphology at the nanometer scale, thickness, composition, and global rates of formation. Oxide nucleation occurs in the form of islands, which form on newly passivating surfaces within seconds of the establishment of an electrochemical potential driving force. Oxides tend to grow first as islands and coalesce to form nm-scale coverages on the NiCr alloy surface, which is consistent with the sharp decrease in oxidation rates commensurate with thin film-field-driven passivation. At the same time, it is found that chemical driving forces favor simultaneous oxide dissolution, especially in the case of Ni2+ oxides. The balance between these two processes is argued to govern morphologies and thickness evolution. Roughness and topographical heterogeneity evolve with time and depend critically on anion identity. Cl- is shown to enhance the spread in oxide island radii, heights, and Cr/Ni ratio compared to SO42-. The possible impacts of morphology and chemical composition on oxide instability and breakdown are discussed.