Isotropic Tuning of Electrochemical Etching for the Nanometric Finishing of Metals

Abstract

Isotropic etching polishing (IEP) based on the merging of isotropic etch pits has been proposed as a generic metal finishing approach. In this work, the tuning of the etching isotropy of various metals, which is the key to realizing the finishing effect of IEP, is studied by theoretical analysis and etching experiments. The isotropic etching of various metals can be realized through mass transfer polarization by adjusting the electrochemical parameters. The addition of sulfuric acid in the electrolyte is the most effective for tuning the isotropy of electrochemical etching. It can decrease the diffusion coefficient of metal ions, thereby increasing the resistance of mass transfer and transforming the electrochemical dissolution of metal into mass transfer polarization. In this study, the atomic and close-to-atomic scale surface finishing of various metals and alloys has been successfully achieved through isotropic etching. After etching at a current of 1.5 A for 3 min, the surface Sa roughness of TA2 is drastically reduced from 242 to 3.98 nm. After etching for 1 min at a current of 3 A, the surface Sa roughness of pure tungsten, NiTi, and CoCrNi decreases from 9.33, 76.4, and 37.6 nm, respectively, to 1.16, 2.01, and 2.51 nm, respectively.