Electropolishing of NiTi shape memory alloys in methanolic H2SO4

The electropolishing of NiTi shape memory alloys was surveyed electrochemically. Anodic polarization of NiTi up to 8 V was performed in various aqueous and methanolic H2SO4 solutions. The passivity could be overcome in methanolic solutions with 0.1 mol dm-3 ≤CH2SO4 ≤7 mol dm-3. The dissolution kinetics was studied in dependence of the polarization potential, the H2SO4-concentration, the water concentration and the temperature. For lower concentrations of sulfuric acids (CH2SO4 ≤0.3 mol dm-3) electropolishing conditions were not observed for potentials up to 8 V. The dissolution remained under Ohmic control. In the concentration range from 1 to 7 mol dm-3 a potential independent limiting current was registered depending linearly on the logarithm of concentration. The best results were obtained with a 3 mol dm-3 methanolic sulfuric acid at 263 K which yielded an electropolishing current of 500 A m-2 at a potential of 8 V. Surface roughness as well as current efficiency showed an optimum under these conditions.