Electrochemical machining (ECM) is a proven processing technique for fabricating difficultto-cut nickel-based superalloys with complex shapes using the principle of anodic dissolution. However, the metallic surface is susceptible to stray corrosion under conditions of low current density, which increases the difficulty of using ECM on nickel-based superalloy such as Hastelloy X (HX). In this study the electrochemical dissolution behavior of wrought HX at low current density was systematically analyzed. The results revealed that M 23 C 6 carbides were irregularly distributed on the grain boundaries. The polarization curves and open-circuit potential measurements showed that an appropriate temperature (35 • C) and concentration (10 wt.%) aided in the formation of efficient and stable dissolution in NaNO 3 solution. The findings also revealed that selective corrosion occurred preferentially on the grain boundary or near the M 23 C 6 precipitations after passivation film polarization. After careful investigation of the different-stage dissolution microstructures and the solid black block-shape products, M 23 C 6 precipitation was found to play a key role in the dissolution of HX alloy at low current density. A qualitative model was established to demonstrate the electrochemical dissolution behavior of wrought HX alloy in NaNO 3 solution. This model offers a new insight into the suppression of stray corrosion of Ni-based superalloys in aerospace applications.

Ni-based GH625 superalloy has been widely employed in the aerospace industry due to its high strength, outstanding corrosion resistance and high temperature resistance. A novel hybrid processing method of ultrasonic-assisted electrochemical grinding (UAECG) can achieve the high-effective removal rate for difficult-to-process materials and thereby avoids the stray corrosion during the process. This study systematically investigated the electrochemical dissolution behaviors of GH625 alloy at low current density and effects of processing parameters on its surface roughness. A qualitative model was proposed to further reveal its removal mechanism for GH625 alloy during UAECG process. Polarization curves depicted that an efficient and stable electrochemical dissolution was achieved at an appropriate temperature (20℃) and concentration (10 wt.%) of NaNO3 electrolyte. The findings also revealed that selective corrosion preferentially occurred on the grain boundary or near the NbC carbides under different current density corrosion circumstance. Compared with ECG process, the excellent surface quality (Ra = 0.37 μm) and taper of the small holes (taper = 0.04 ± 0.005°) are obtained at the optimized condition of pulse voltage of 5.8V, feed rate of 0.6mm /min, cathode speed of 12kr /min and ultrasonic drive amplitude of 60 % by UAECG technology.

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