Elucidating the Electrochemical Polishing Mechanism of Wrought Ni Superalloy Hastelloy X in a NaCl-Glycol Solution.

Electrochemical polishing (ECP) has been conducted in a novel, environmentally friendly deep eutectic solvent (DES) electrolyte of NaCl-glycol to remarkably improve the polishing efficiency and significantly reduce localized corrosion regions of the wrought nickel (Ni)-based superalloy Hastelloy X (HX). Critical secondary passivation behavior has been systematically investigated in the DES electrolyte, combining advanced characterization techniques of electrochemical polarization, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The ECP performance in the DES electrolyte was compared to that of the NaCl aqueous solution. The samples' surface roughness (Ra) decreased from 2.40 ± 0.10 μm to 0.23 ± 0.10 μm (at low current density) and 0.36 ± 0.10 μm (at high current density) in DES solution following ECP, respectively. The surface hardness slightly changed from 234.10 ± 2.30 HV to 226.70 ± 2.50 HV and 224.20 ± 2.50 HV in DES solution after ECP at corresponding low and high current density, primarily attributed to residual stress relief. A qualitative model was developed to relate electrolyte characteristics to polishing surface evolution, providing insights into the design of green ECP processes for aerospace-grade superalloys.