In this study, we used a cathodic cage plasma system to deposit an oxynitride coating on a nickel–titanium metallic substrate for three different processing gas compositions (60%H2 + 40%N2; NT1, 50%H2 + 50%N2; NT2%, and 40%H2 + 60%N2; NT3). The influence of various gas mixtures on surface morphology, chemical composition, and crystal structure of treated samples (NT1, NT2, and NT3) was investigated. A spectrophotometer is used to determine the amount of nickel ion leaching in a chloride-containing saline electrolyte using atomic absorption spectroscopy. Potentiodynamic polarization curves and electrochemical impedance spectroscopy depict the enhanced corrosion resistance of treated samples as compared to the base Ni–Ti sample. Titanium oxynitride coating in NT3 samples shows an excellent barrier against corrosion behavior and nickel ion leakage in a corrosive environment.