Electro-dissolution of 30Nb-Ti alloys in methanolic sulfuric acid-Optimal conditions for electropolishing

Highlights: → Electro-dissolution behaviour of Nb-Ti alloy in methanolic sulfuric acid. → Electro-dissolution process is mass transport limited in all concentrations. → Activation energy (Ea) of dissolution is 16.1 kJ mol-1. → Influence of sulfuric acid hints dissolving ions as rate limiting species. → Electropolishing in methanolic-3 M sulfuric acid renders smooth surface. - Abstract: The electro-dissolution behaviour of a (30 at.%) Niobium-Titanium (NbTi) alloy in non-aqueous methanolic sulfuric acid solution using the rotating disc electrode (RDE) was ascertained. The optimal condition for electropolishing and the mechanism were proposed. The influence of the rotation rate, process temperature and sulfuric acid concentration on the dissolution kinetics was investigated. The dissolution rate (limiting current) increases linearly with increase in rotation rate and follows a Levich behaviour confirming a mass transport controlled process. The temperature dependence in terms of Arrhenius plot renders an activation energy value of Ea = 16.1 kJ mol-1 for the process. The dissolution rate shows a strong dependence on the sulfuric acid concentration (1 M, 3 M and 5 M). Higher sulfuric acid concentrations lead to decreased dissolution rates (limiting current). The dissolution process is mass transport controlled in all concentrations of sulfuric acid. From an electrochemical perspective, a 3 M sulfuric acid was chosen as optimum owing to better controllability of the material removal rate. The dissolving ions are the probable rate limiting species, indicating a compact salt-film mechanism. The average root mean square (RMS) roughness value for an electropolished surface was approximately 10 nm, which is significantly lower than a mechanically polished surface.