Investigation of corrosion mechanism of NiTi modified by carbon plasma immersion ion implantation (C-PIII) by electrochemical impedance spectroscopy

Abstract

Electrochemical impedance spectroscopy (EIS) is conducted to investigate the corrosion behavior and mechanism of NiTi modified by carbon plasma immersion ion implantation (C-PIII) in 3.5% NaCl and simulated body fluids (SBF) solutions. The unstable native oxide formed on the pristine NiTi is easily corroded by aggressive ions leading to delamination of the surface structure. C-PIII enhances the surface stability by forming a smooth and crack-free coating with a thickness of 50 nm. Ni leaching is reduced and EIS reveals significantly improved corrosion resistance. The amounts of nickel ions released to SBF and 3.5% NaCl solution are 1.3724 and 4.1463 mg L−1 cm−2, respectively, and C-PIII decreases the amounts of nickel ions in SBF and 3.5% NaCl solution by 99.13 and 99.31%, respectively, after immersion for 96 h. The corrosion resistance mechanism depends on the inactive protection layer and functional carbon film on the NiTi surface. Formation of a homogeneous and uniform layer on the surface, precipitation of corrosion products from soluble compounds, especially in SBF, and formation of titanium oxide at local defects due to the penetration of corrosive ions such as Cl− and water absorption are the major mechanisms responsible for the improved corrosion resistance of the C-PIII NiTi.