Ni Corrosion Product Layer During Immersion in a 3.5% NaCl Solution: Electrochemical and XPS Characterization

Long term wet corrosion resistance of metals depends on the stability of their corrosion product layer. With immersion corrosion tests, such stability can be predicted. EIS and potentiodynamic polarization were complemented with XPS to investigate the characteristics of Ni corrosion product layer formed after 1 hr. and 72 hr. immersion in 3.5% NaCl solution. Two time constants with decreasing Nyquist semi-circle size and phase angle maxima, based on EIS characterization during the immersion times, indicated the formation of an increasingly porous and less adherent corrosion product layer. The product formation shifted the Ni corrosion potential more negatively and increased cathodic and anodic current densities, during potentiodynamic polarization. XPS characterization suggested that a rapid nucleation of NiO could increase H2O adsorption, subsequently triggering the formation of different forms of Ni(OH)(2) in the corrosion product layer. Consequently, the corrosion resistance of the Ni coating decreased after 72 hr. immersion in 3.5% NaCl solution.