The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E-oc), potentiodynamic polarization and ac impedance ( EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy Surface whose passivation performance depends on the halide nature, its concentration and temperature. E-oc shifts positively with time until attaining a steady (E,,) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F- > I- > Br- > Cl-. This sequence is consistent with that for surface film resistance (R-T) and its relative thickness (1/C-T). Nevertheless, in F- medium each of the above parameters increases with [F-] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br- and I- solutions, as well as the lower Cl-concentrations (<= 0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E-corr) to the knee point (E-pt) in the anodic scan, at which passivity breakdown Occurs with rapid increase in the anodic Current and hydrogen gas reaction. At Cl- concentrations > 0.01 M the negative difference effect (NDE) Occurs under cathodic polarization where E-pt lies negative to E-corr. Addition of F- to the Cl- solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection. (C) 2008 Elsevier Ltd. All rights reserved.