Electrochemical dissolution behavior of nickel-based Hastelloy X superalloy at low current densities

Electrochemical machining (ECM) is a proven processing technique for fabricating difficult-to-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 M23C6 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 NaNO3 solution. The findings also revealed that selective corrosion occurred preferentially on the grain boundary or near the M23C6 precipitations after passivation film polarization. After careful investigation of the different-stage dissolution microstructures and the solid black block-shape products, M23C6 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 NaNO3 solution. This model offers a new insight into the suppression of stray corrosion of Ni-based superalloys in aerospace applications.