Low-voltage plasma electrolytic oxidation as a tool for corrosion resistance control of the NiTi alloy

This study investigates plasma electrolytic oxidation (PEO) of Nitinol and its effect on surface characteristics and corrosion resistance. Two electrolytes were employed: concentrated phosphoric acid (V) and phosphoric acid (V) modified with ethylene glycol. Oxide layers were produced at applied voltages ranging from 60 to 90 V and characterized in terms of morphology, composition, wettability, and corrosion behavior. The influence of the applied voltage was examined to identify processing conditions leading to oxide layers with favorable characteristics, and the mechanism of oxide layer formation was also analyzed. Scanning electron microscopy (SEM) and 3D surface mapping revealed porous coatings with good homogeneity. Higher voltages promoted increased porosity, while the addition of ethylene glycol resulted in smaller and more uniform pores. Energy-dispersive X-ray spectroscopy confirmed a reduction in surface nickel content and the incorporation of phosphorus into the oxide layers. All PEO-treated surfaces exhibited enhanced hydrophilicity, particularly for coatings formed in ethylene glycol-containing electrolytes. Raman and X-ray photoelectron spectroscopy identified TiO<inf>2</inf> and Ti<inf>2</inf>O<inf>3</inf> as the dominant phases, with minor amounts of nickel phosphates and organic species. Electrochemical and immersion corrosion tests demonstrated a significant improvement in corrosion resistance, accompanied by low nickel release and detectable phosphorus release, which may be beneficial for osseointegration. Process conducted in a concentrated phosphoric acid bath yielded coatings characterized by gradual biodegradation in 0.9 wt% NaCl, releasing modest amounts phosphorus species into the corrosion medium. The ethylene glycol-modified bath produced more stable coatings. The process voltage was found to determine the coating thickness (SEM) and barrier properties (electrochemical impedance spectroscopy and potentiodynamic polarization). Among the investigated conditions, samples treated at 90 V in phosphoric acid and at 60 V in phosphoric acid with ethylene glycol exhibited the most favorable overall properties.