Two-step electropolishing of 316L stainless steel in a sulfuric acid-free electrolyte

The study is to electropolish 316 L stainless steel in a sulfuric acid-free electrolyte, consisting of phosphoric acid, glycerol and distilled water instead of the widely used sulfuric acid-based electrolyte. The influence of water concentration in the electrolyte on the electropolishing characteristics was investigated from the polarization curve, material removal rate and surface roughness Ra. It is confirmed that the limiting current density increases with increasing the water concentration, resulting in a high material removal rate. However, the surface roughness Ra increases because the electropolishing effect is lowered. The surface characteristics were analysed by X-ray photoelectron spectroscopy, and the mechanical properties were studied by nanoindentation. A two-step electropolishing method was proposed to enhance the electropolishing efficiency significantly by utilizing the electropolishing characterizations with different water concentrations. The method contains two electropolishing processes in high and low water concentration electrolyte, respectively. The experimental results show that the material removal rate is increased significantly compared with the one-step electropolishing method with a low water concentration, and the surface roughness is decreased obviously compared with the one-step electropolishing method with a high water concentration.