Steam generator tubing used in the Pickering Nuclear Plant is comprised of Monel 400, a Ni-Cu alloy. In 2016, five tubes excavated from unit 4 of Pickering exhibited intergranular attack and dealloying of nickel on their outer diameter. Open circuit and potentiostatic corrosion experiments were conducted to evaluate the corrosion performance of commercially available Monel 400 and model Ni-Cu alloys in simulated steam generator environments. It was found that Monel 400 will undergo dealloying in occluded regions, where Cu2+ can accumulate to sufficient concentration that stabilizes elemental copper. Characterization by electron microscopy revealed the presence of Ni3B at Monel 400 grain boundaries which was proposed to be the cause of intergranular attack in this material. Gas chromatography and potentiodynamic polarization were carried out to evaluate the inhibiting effect of HCOO− which occurs at an open circuit.