## 1 Introduction  
Inconel 718 is a type of nickel-based super alloy that has been widely employed in the aerospace industry because of its high-temperature strength. However, this kind of super alloy has high hardness, high cutting force, large elastic deformation and low heat conductivity coefficient. Thus, it has become one of the most typical difficult-to-cut materials in industry, and is time-consuming and high-cost to machine using traditional mechanical machining [1; 2; 3]. Electrochemical machining (ECM) is an anodic dissolution process, which can be used to remove materials efficiently regardless of the material's hardness. Compared with traditional machining methods, there is virtually no tool wear, residual stress or metallurgical defects [4; 5]. Thus, ECM is especially suitable for the machining of difficult-to-cut materials and has become an important processing technology in the aerospace industry [6; 7]. However, the machining accuracy of ECM is greatly limited by the poor localization effect of anodic dissolution, which is generally known as stray corrosion [4]. Stray corrosion is a common phenomenon that occurs on the non-machined surface adjacent to the machining area. It will cause unwanted material removal and can lead to poor machining accuracy and surface quality to the machining part. Researchers have made many attempts to improve the localization effect of anodic dissolution in ECM. For example, the use of pulsed power has been proved to be an effective method to localize the anodic dissolution and minimize the inter-electrode gap on many occasions [8; 9]. Auxiliary electrodes have been applied to change the electric field distribution. By eliminating the stray currents on the non-machined area, stray corrosion can be effectively reduced [10; 11; 12]. Recently, hybrid machining processes such as vibration-assisted ECM [13], laser-assisted ECM [14] and a combined electrical discharge machining (EDM)-ECM process [15] have also been used to obtain the desired machining accuracy. In particular, the use of a passivating NaNO\({}_{3}\) electrolyte is a widely-used method to improve the machining accuracy in ECM of steel, due to the passivation characteristic [16; 17]. A compact passive film of several nanometers can be formed at low current density in NaNO\({}_{3}\), and thereby hinder the metal dissolution [18]. [PERSON] used a NaNO\({}_{3}\) solution to obtain a smooth and bright surface in a finishing process with ultrasonic and magnetic assistance [19]. [PERSON] et al. used a low concentration of NaNO\({}_{3}\) to localize the anodic dissolution in ECM [20]. [PERSON] and [PERSON] used NaNO\({}_{3}\) solution in a wire electrochemical cutting process to achieve more accurate cuts [21]. However, studies have also verified that the use of NaNO\({}_{3}\) electrolyte may be ineffective in reducing stray corrosion. Due to the prominent effects of chromium, an active metal dissolution was found for the Ni-20%Cr-Co alloys [22]. The polarization curves of cobalt also presented an active cobalt dissolution in NaNO\({}_{3}\) solution [23]. Furthermore, the nickel-based super alloy Inconel 718 has also been found to suffer serious stray corrosion in NaNO\({}_{3}\) solution due to the formation of a porous oxide film on the specimen surface, as reported in our previous studies [24; 25]. As reported in some studies [26; 27], the use of an alkaline solution can help promote the formation of a passive film on the surface during the ECM process. Thus, the use of a mixed solution of NaNO\({}_{3}\) and NaOH was investigated to improve the localization effect during the ECM of Inconel 718 alloy. The potentiodynamic polarization curves and current efficiencies for metal dissolution were measured and showed that the use of an alkaline solution can help promote the formation of a compact passive film, and the anodic metal dissolution can be completely eliminated by adding a certain proportion of NaOH into the NaNO\({}_{3}\) solution.