Influence of precipitated carbides near grain boundaries on the electrochemical dissolution behaviour of nickel-based superalloys

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The electrochemical dissolution behaviours of single-crystals, equiaxed-crystals, and columnar-crystal structures in a NaNO 3 solution were studied, with a focus on the role of grain boundary carbides in passivation film formation and their impact on the electrochemical dissolution process . The experimental results demonstrate that polycrystalline superalloys develop a less dense, more porous passivation film and exhibit poorer surface quality after dissolution compared to single-crystal counterparts. Electrochemical dissolution experiments, passivation film characterization, and point defect model (PDM) analysis reveal that grain boundary carbides disrupt the continuous growth of the passivation film, creating localized weak points at grain boundaries. Additionally, these carbides induce excessive cation vacancy accumulation within the passivation film, leading to pore formation at these vulnerable regions. The study concludes by proposing qualitative electrochemical machining (ECM) models for the three superalloys with distinct crystal structures.

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