New Insights on the Role of Chloride During the Onset of Local Corrosion: TEM, APT, Surface Energy, and Morphological Instability

The role of chloride in aqueous corrosion during local oxide destabilization as a precursor to breakdown is investigated by combining transmission electron microscopy, atom probe tomography, and density functional theory (DFT). The observations show that cube-cube epitaxy of rock-salt structure oxide initiates at the metal surface in the sodium sulfate electrolyte, which is not the “conventional” nickel oxide as a large number of solute Cr atoms are captured. The rock-salt oxide thin film roughens and a Cr-rich oxide with corundum structure grows out in chloride containing media. DFT calculations reveal the competitive chemisorption between hydroxide and chloride on hydroxylated NiO (111) and Cr2O3 (001) surfaces. The collective results relate the surface energy reduction to morphological instability and roughening due to chloride chemisorption in aqueous corrosion.