TB2 titanium alloy is a typical near-β titanium alloy. It has high specific strength, toughness, and elasticity. Sheet parts are commonly used for the skin of aircraft missiles and satellite-connecting belts. TB2 is highly sensitive to various surface defects, such as microcracks and scratches, which significantly affect the fatigue life of parts [1–3]. Surface defects affect both the high-cycle fatigue life and low-cycle fatigue life, whereas the surface roughness affects the high-cycle fatigue life [4–6]. Polishing is a common method for improving the surface integrity and fatigue life of titanium alloy parts. Shahzad et al. found that the surface morphology plays a key role in the initiation of fatigue cracks, accounting for 90% of the fatigue life [7, 8]. Novovic et al.  found that without considering the residual stress, when the surface roughness Ra > 0.1 µm, the surface morphology has an effect on fatigue. When the surface roughness Ra < 0.1 µm, the surface morphology has no effect on fatigue. Owing to the high hardness and low thermal conductivity of TB2 titanium alloy, significant grain wear and surface burns can occur during grinding . A common surface treatment is manual grinding, which causes dust pollution, and is labor-intensive. Because of its good processability and consistency, electrochemical machining is suitable for polishing parts with complex shapes, slender parts, and thin plates, as it has good reproducibility [11–13].
The electropolished surface quality is affected by the electrolyte flow status and the dissolution state of different materials . Because most of the materials are multiphase, the formation of the surface during electropolishing is affected by different mechanisms. Yi et al.  found that when the volume ratio of the electrolyte is 1 : 10 (H2SO4 : CH3OH), the etching is isotropic. This electrolyte can be used for general metal polishing. In addition, these authors found that the surface roughness of TA2 decreases sharply from 64.1 nm to 1.2 nm. When electropolishing a titanium alloy, the type of electrolyte can fundamentally change the pitting characteristics and surface morphology during the entire electrochemical machining process [16–18]. Moreover, an environment-friendly electrolyte should be considered .
An ethylene glycol-based electrolyte is especially conducive to the formation of a polishing film owing to its high viscosity. It can produce a bright and pit-free finish . Fushimi and Habazaki  studied the dissolution behaviour of pure titanium in NaCl-containing ethylene glycol. They found that the mass transfer is controlled by titanium species, such as the TiCl4 adhesive layer. Kim et al.  further studied the effect of the surface adhesive layer on surface dissolution. They added 20% ethanol in a 1 mol/L NaCl-containing ethylene glycol solution to regulate the TiCl4 adhesive layer, and then obtained a surface with Ra = 2.341 nm on pure titanium. Ferreri et al.  used an ethanol-ethylene glycol-NaCl electrolyte to prepare the EBSD samples, and collected high-quality EBSD datasets. Moreover, Huang et al.  placed the anode in translational motion to control the surface adhesive layer, obtaining a surface roughness Ra of 1.9 nm on a Ti-6Al-4V alloy surface. NaCl-containing ethylene glycol is an environment-friendly electrolyte with a good polishing surface quality and simple equipment, which has a promising future in production. However, the majority of previous research has been conducted in a beaker with a small sample size, and the mechanical stirring and translational motion are not adequate for polishing a large surface.
In addition, because the surface oxide layer hinders the anodic dissolution in the electropolishing process, most researchers have used sandpaper for mechanical polishing to improve the surface reactivity in the pre-treatment stage. [21–23, 25] However, different surface treatment processes affect the surface roughness, microstructure, and stress state of the materials, which can change their sensitivity to corrosion . Zuo et al.  noted that the nucleation rate of metastable pitting increases with an increase in the surface roughness of polished stainless steel. Furthermore, appropriate surface treatment can also improve the fatigue life of the surfaces. [28, 29] Lopez-Ruiz et al.  found that the surface obtained by conventional shot peening as a pre-treatment is smoother than that obtained by severe shot peening, because the severe shot peening produces obvious dimples and surface defects. Therefore, to find the ideal pre-treatment method, it is necessary to study the influence of the surface state on the corrosion behaviour during electropolishing. The importance of surface pre-treatment has been neglected in previous studies using NaCl-containing ethylene glycol.
In this study, mechanical grinding was used to remove the thick oxide layer on the surface of TB2 titanium alloy sheets to improve the reactivity of electropolishing and remove large surface defects. The effect of different surface states on electropolishing was studied through surface roughness, surface topography, and 3D profile. Thereafter, an appropriate grinding method was found. Therefore, to electropolish a large surface, flushing electropolishing was adopted to improve the inconsistent surface quality owing to stirring. This is because electrolyte flushing quickly refreshes the electrolyte in the processing area and removes heat and reaction products. The flow field is even and controllable. A good surface quality was obtained under the optimised parameters, which verifies the possibility of large surface polishing in the flushing electrolyte.