Corrosion behavior and product film formation of Ni-Co-Cu nanocrystalline coatings in neutral salt environments

The corrosion behavior of Ni-Co-Cu nanocrystalline coatings in neutral salt environments was systematically investigated by potentiodynamic polarization, linear polarization, electrochemical impedance spectroscopy, and long-term salt spray tests. The electrodeposited ternary coatings were Ni-based solid solution alloys that contained markedly different contents of Co and Cu. Greater Co and Cu contents strongly reduced the polarization and charge-transfer resistance of the coatings, thus decreasing their corrosion resistance in neutral NaCl solutions. In salt spray tests, the initial porous corrosion product film on the Co-/Cu-poor coating evolved into a compact film that finally grew to cover the coating surface, while such morphological transition was absent on the Co-/Cu-rich coating, where the formed film showed a continuous deterioration in its initial porous structure. Abundant Co-/Cu-substituted alpha-Ni(OH)(2) might grow to form the primary compact films that could coalesce and thicken as corrosion was suppressed. A large amount of Cu(OH)(2) together with high corrosion rates might be detrimental to the formation of compact films. A mechanism of the formation of the corrosion product films was proposed based on the corrosion characteristics of the ternary system and detailed corrosion product film analysis. (C) 2018 Elsevier Ltd. All rights reserved.