Dealloying and Intergranular Attack of Monel 400 in Nuclear Steam Generators

Abstract

Steam generator (SG) tubing used in the Pickering Nuclear Plant is comprised of Monel 400 (Monel), a Ni-Cu alloy. In 2016, five tubes excavated from unit 4 of Pickering exhibited intergranular attack (IGA) and dealloying of Ni on their outer diameter. Open circuit and potentiostatic corrosion experiments were conducted to evaluate the corrosion performance of commercially available Monel and of model Ni-Cu alloys in simulated steam generator environments. It was found that Monel will undergo dealloying in occluded regions, where Cu²⁺ can accumulate to sufficient concentration that it stabilizes elemental copper. Characterization by electron microscopy revealed the presence of Ni₃B at Monel 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 open circuit.