Copper and copper-nickel alloys are known to form partially passive films in marine conditions, both naturally and under positive potential bias. Here, the anodic passivation behaviour of copper and of constantan (Cu54Ni45Mn1) as a model for a copper-nickel alloy are investigated and compared at high positive overpotentials and in 0.5 M NaCl(aq). Abrupt potential-dependent passive film breakdown is observed for both Cu and Cu-Ni alloys during voltammetry and during chrono-amperometry experiments. For Cu a single transitions occurs at 0.2 V vs. SCE consistent with a Cu(II/I) process leading to interfacial stress and breaking of a passive CuCl film. For Cu-Ni alloy, two stages are observed at 0.3 V vs. SCE due to a Cu(II/I) process and at 1.7 V vs. SCE due to a sub-interfacial Ni(IV/II) process. A breakdown mechanism is proposed based on redox processes at the buried interface at the metallic conductor | passive ion conductor junction.