Preferential corrosion of coherent twin boundaries in pure nickel under cathodic charging

We describe an investigation of microstructure in pure nickel (Ni) after cathodic charging in an aqueous electrolyte. Intergranular corrosion occurs on the sample surface and takes the form of long trenches along coherent twin boundaries (CTBs): sites that have often been considered especially resistant to corrosion. Integrating electron backscatter diffraction and x-ray computed tomography, we show that the trenches are formed by the growth and eventual overlap of isolated conical cavities, which, in turn, are aligned with (110)-type directions within CTB planes. The (110)-type orientation is consistent with cavity initiation at high-energy, symmetric incoherent twin boundary facets within CTB planes. Our observations show that, far from being always corrosion resistant, CTBs are especially susceptible to intergranular corrosion under some conditions.